Leticia M Lazinski

Guy Royal

Maxime Robin

Marc Maresca

Romain Haudecoeur

Bioactive Aurones, Indanones and Other Hemiindigoid Scaffolds: Medicinal Chemistry and Photopharmacology Perspectives

Hemiindigoids comprise a range of natural and synthetic scaffolds that share the same aromatic hydrocarbon backbone as well as promising biological and optical properties. The encouraging therapeutic potential of these scaffolds has been unraveled by many studies over the past years and uncovered representants with inspiring pharmacophoric features such as the acetylcholinesterase inhibitor donezepil and the tubulin polymerization inhibitor indanocine. In this review, we summarize the last advances in the medicinal potential of hemiindigoids, with a special attention to molecular design, structure-activity relationship, ligand-target interactions, and mechanistic explanations covering their effects. As their strong fluorogenic potential and photoswitch behavior recently started to be highlighted and explored in biology, giving rise to the development of novel Page 1 of 96 ACS Paragon Plus Environment Journal of Medicinal Chemistry 2 fluorescent probes and photopharmacological agents, we also discuss these properties in a medicinal chemistry perspective.

INTRODUCTION

Hemiindigoids are aromatic scaffolds derived from the better-known indigoids, themselves termed in reference to natural indigo, a famous chromophore used as a dye since the antiquity and whose structure was described by Baeyer in 1883. [START_REF] Baeyer | Ueber die Verbindungen der Indigogruppe[END_REF] Unlike indigoids, which are made of two symmetrical bicyclic aromatic units linked by an exocyclic carbon-carbon double bond, hemiindigoids display an "indigoid part" connected to a "stilbene part" through the double bond (Figure 1).

Hemiindigoids are thus asymmetrical compounds, but they retain the existence of two isomers, called Z and E, instead of the cis and trans forms for indigoids. They consist of four main scaffolds, depending on the nature of the intracyclic atom embedded in the five-membered ring. The oxo scaffold, called aurone (in red in Figure 1), is certainly the most studied, as it occurs from natural sources, mainly plants. Indeed, over a hundred of natural aurones are known, [START_REF] Boucherle | Occurrences, biosynthesis and properties of aurones as high-end evolutionary products[END_REF] especially with polyphenolic substitution patterns, such as sulfuretin, maritimetin or hispidol (Figure 2), and numerous synthetic derivatives were developed for medicinal chemistry purposes. [START_REF] Haudecoeur | Recent advances in the medicinal chemistry of aurones[END_REF] Carbon-based counterparts are often called 2-arylideneindanones, or more simply indanones (in green in Figure 1). The aza and thio versions are known as hemiindigos and hemithioindigos (sometimes azaaurones and thioaurones, in blue and yellow in Figure 1, respectively). While these subclasses have been known for a long time (hemithioindigos were reported for the first time by Friedländer in 1906, 4 aurones by Geissman and Heaton in 1943) [START_REF] Geissman | Anthochlor pigments. IV. The pigments of Coreopsis grandiflora, Nutt. I[END_REF] , their medicinal properties and unique photophysical behaviors have been studied only recently. Especially, the past ten years have witnessed a great interest for hemiindigoids, aurones and indanones in the foreground, in several pharmaceutical domains such as cancer, neurodegenerative disorders, infectious diseases or inflammation. Meanwhile, the hemiindigo and hemithioindigo scaffolds emerged as reliable photoswitchable platforms for various applications, often allowing a reversible Z-to-E conversion using visible light with high bistability and favorable photophysical parameters. Recently, they even made a timid incursion in biology, paving the way for future exciting opportunities in photopharmacology, a field expanding rapidly. Thus, we propose herein to take stock of the advances in the medicinal chemistry and photochemistry of hemiindigoids achieved within the past ten years, and to identify the most promising perspectives these fascinating scaffolds can offer.

MAIN SYNTHETIC METHODS OF HEMIINDIGOIDS

Numerous straightforward synthetic pathways were developed for preparing hemiindigoids over the years, and aurones received the closest attention, due to their natural origin. However, a large proportion of these methods followed pure synthetic chemistry objectives, and appears as not easy transferrable to pharmacomodulation endeavors. Thus, only synthetic routes allowing to reach hemiindigoids with various functionalization patterns are presented herein. Most of the available methods comprise the synthesis of the widely described aurone scaffold, but, with some adaptations, they can be extended to indanones, HI and HTI. In most cases, aldol condensation between the "indigo" part and a benzaldehyde constitutes the main path. The synthesis of the "indigo" part through intramolecular acylation most frequently precedes the condensation for indanones and HTI when functionalization is desired, and the starting material is not commercially available. In the case of HI, functionalized indoxyl heterocycles are less described and conventional synthesis usually starts from 3-indolyl acetate or 1-acetyl-3-acetoxyindole, which are directly employed in the condensation step under basic conditions (Scheme 1). [START_REF] Daisley | Reaction of indol-3-(2H)-one derivatives with some active methylene compounds[END_REF][START_REF] Chalkha | synthesis and characterization of functionalized pyrazole derivatives bearing amide and sulfonamide moieties from aza-aurones[END_REF] Typically, for thioindoxyls, the synthesis starts from the conversion of a thiophenol into 2-(phenylthiol)-acetic acid, followed by the transformation into the corresponding acid chloride using SOCl 2 and, finally, ring closure through Friedel-Crafts in the presence of AlCl 3 . [START_REF] Wiedbrauk | Hemithioindigo-an emerging photoswitch[END_REF] An alternative method employed very often for the synthesis of thioindoxyls is the lithium diisopropylamide (LDA)-mediated cyclization of 2-(methylthio)benzamides. [START_REF] Mukherjee | Application of directed metalation in synthesis, part 2: an expedient synthesis of methoxybenzo[b]thiophenes[END_REF] For indanones, the synthesis usually starts from arylpropionic acids and cyclization has been reported to occur in the presence of sulfuric acid, Tb(OTf) 3 , P 2 O 5 and methane sulfonic acid. [START_REF] Huang | Synthesis of 1-indanones from benzoic acids[END_REF][START_REF] Cui | Synthesis of 1-indanones by intramolecular Friedel-Crafts reaction of 3-arylpropionic acids catalyzed by Tb(OTf) 3[END_REF][START_REF] Premasagar | Methanesulfonic acid catalyzed cyclization of 3-arylpropanoic and 4-arylbutanoic acids to 1-indanones and 1-tetralones[END_REF] Then, as already stated, the last step of the synthesis is usually performed through aldol condensation in the presence of bases (e.g., KOH, NaOH, NaH, piperidine) or acids (e.g., AcOH, HCl, H 2 SO 4 ), both of which are frequently used for indanones, aurones and HTI. [START_REF] Wiedbrauk | Hemithioindigo-an emerging photoswitch[END_REF][START_REF] Sheng | synthesis and AChE inhibitory activity of indanone and aurone derivatives[END_REF][START_REF] Jung | In vitro and in silico insights into tyrosinase inhibitors with (E)-benzylidene-1-indanone derivatives[END_REF][START_REF] Haudecoeur | Discovery of naturally occurring aurones that are potent allosteric inhibitors of hepatitis C virus RNA-dependent RNA polymerase[END_REF] For aurones, neutral Al 2 O 3 or "on-water" synthesis in refluxed H 2 O are also employed as milder conditions for fragile substituents, but both methods include limitations. [START_REF] Varma | Alumina-mediated condensation. A simple synthesis of aurones[END_REF][START_REF] Venkateswarlu | On water" synthesis of aurones: first synthesis of 4,5,3',4',5'-pentamethoxy-6-hydroxyaurone from Smilax riparia[END_REF] Alternatively, for aurones and HTI, cyclization of 2'-hydroxychalcones [START_REF] Narsinghani | Synthesis, docking studies and antioxidant activity of some chalcone and aurone derivatives[END_REF][START_REF] Regenass | Discovery of a locally and orally active CXCL12 neutraligand (LIT-927) with anti-inflammatory effect in a murine model of allergic airway hypereosinophilia[END_REF] and 2'-nitrochalcones [START_REF] Nguyen | DIPEA-promoted reaction of 2-nitrochalcones with elemental sulfur: an unusual approach to 2-benzoylbenzothiophenones[END_REF] respectively are frequently used methods that affords derivatives with good yields and functional group tolerance (Scheme 1). Overall, several other ingenious methods have been reported so far, such as the 5-exo-dig cyclization of substituted acetylene derivatives using Pd-, Ag-or Au-based catalysis, [START_REF] Harkat | Versatile and expeditious synthesis of aurones via Au Icatalyzed cyclization[END_REF] or the Suzuki coupling of arylboronic acids with 2-bromomethylene derivatives, [START_REF] Kraus | Divergent approach to flavones and aurones via dihaloacrylic acids. Unexpected dependence on the halogen atom[END_REF] but they globally appear as more difficult to implement in a medicinal chemistry context, due to the poor accessibility of variously substituted starting materials and/or the cost of reagents and catalysts.

BIOLOGICAL ACTIVITIES OF HEMIINDIGOIDS

Anticancer activities

Inhibition of tubulin polymerization. The targeting of microtubules is an attractive strategy for inducing cycle arrest of cancer cells, and several antitubulin drugs are already in clinical use. [START_REF] Kaur | Recent developments in tubulin polymerization inhibitors: an overview[END_REF] In 2000, the National Cancer Institute (USA) discovered indanocine (8, Figure 3), an indanone derivative with strong antiproliferative activity (IC 50 = 2.0-25 nM against seven multidrugresistant lines). It acted as a cytostatic and cytotoxic agent, blocking tubulin polymerization at the colchicine binding-site (IC 50 = 1.7 µM) while selectively causing apoptosis of multidrug-resistant cancer cells. [START_REF] Leoni | a microtubule-binding indanone and a selective inducer of apoptosis in multidrug-resistant cancer cells[END_REF] As a consequence, in the last decade several studies reported new analogues, based on the indanone scaffold with colchicine- (7) and/or combretastatin-inspired substitution patterns -especially including the aromatic trimethoxy moiety, at positions 4, 5 and 6. One good illustration of this design strategy is compound 9, which was found as a tubulin polymerization inhibitor with IC 50 = 7.56 µM, able to cause a disruption of the microtubule network in a HeLa cells assay. Therefore, 9 acted as an efficient antiproliferative agent in an evaluation involving five human cancer cell lines (HeLa, A549, Bel-7402, PC-3, MDA-MB-231), yielding IC 50 values of 0.17-0.6 µM. [START_REF] Hu | Synthesis, biological evaluation and mechanism study of a class of benzylideneindanone derivatives as novel anticancer agents[END_REF] Fused-indole derivative 10 exerted stronger effects in both tubulin polymerization (IC 50 = 1.99 µM) and antiproliferative assays (IC 50 = 0.03-0.04 µM against HeLa, A549, Bel-7402, PC-3 and K562 cell lines). [START_REF] Chen | Synthesis, biological evaluation and mechanism study of chalcone analogues as novel anti-cancer agents[END_REF] A series of aurone-based derivatives was globally found as equally potent, as illustrated by the comparable tubulin polymerization inhibition displayed by 11, a close analogue of 9 (IC 50 = 8.6 µM vs. 7.56 µM). [START_REF] Lewin | Influence of the skeleton on the cytotoxicity of flavonoids[END_REF] The introduction of a trimethoxylated aryl group at position 3 of the indanone scaffold afforded additional series of antimitotic agents. Among them, compound 12, despite its modest antiproliferative action (IC 50 = 4.0-10.0 µM against THP-1, HCT-116, MCF-7, T-47D and A549 cells), revealed a slightly improved action on tubulin (IC 50 = 1.5 µM). [START_REF] Singh | Anticancer activity of gallic acid template-based benzylidene indanone derivative as microtubule destabilizer[END_REF] A molecular dynamics study and conformational modes analysis proposed a crucial role for the trimethoxylated ring A of 12 in the binding process. Indeed, the ring would be embedded between S8/S9 β-sheets, H8 α-helix and T7 loop of colchicine binding-site of the αβ-tubulin dimer, triggering robust hydrophobic interactions and hydrogen bonds, and causing perturbations in lateral contacts with microtubule. [START_REF] Tripathi | Insight into microtubule destabilization mechanism of 3,4,5-trimethoxyphenyl indanone derivatives using molecular dynamics simulation and conformational modes analysis[END_REF] Compound 13 is another example with submicromolar antipolymerization activity (IC 50 = 0.63 µM), but it showed disparate results in the antiproliferative test, exhibiting either a good (IC 50 = 0.01-0.10 µM for MCF-7 and HCT cells) or a weak profile (IC 50 = 4.9-11.0 µM for THP-1 and A549 cells), depending on the cell lines. [START_REF] Prakasham | Synthesis and anticancer activity of 2-benzylidene indanones through inhibiting tubulin polymerization[END_REF] Regarding in vivo potency, 13 induced a reduction of Ehrlich ascite carcinoma by 45.5% at 20 mg/kg (day 12) in Swiss-albino mice. [START_REF] Singh | Anticancer activity and toxicity profiles of 2-benzylidene indanone lead molecule[END_REF] Analogue 14 was further investigated for its pharmacological properties but was found to exert only moderate anticancer activity. [START_REF] Srivastava | Fluorinated benzylidene indanone exhibits antiproliferative activity through modulation of microtubule dynamics and antiangiogenic activity[END_REF] All these molecules act through a similar mechanism, leading to cell cycle arrest at the G2/M phase and cell apoptosis. Targeting of collateral sensitivity in multidrug-resistance cancer cells. In the frame of multidrug resistance in cancer, it was found that specific compounds were able to target specifically tumors with resistant phenotypes, a counter-intuitive phenomenon that was termed collateral sensitivity.

Several medicinal chemistry approaches were tried for exploiting such properties, through various mechanisms often involving efflux pumps. Ferrocene-based flavonoids, including aurone derivatives, were especially found to induce glutathione efflux in MRP1-transfected BHK-21 cells.

Compound 15 (Figure 4) caused a significant net glutathione efflux in these cells after correction from the effect detected on wild-type BHK-21 cells (52% at 5 µM). This observation could be the source of the difference seen in terms of cytotoxic action for 15 against NCI-H69 cancer cells and the resistant MRP1-overexpressing counterpart H69AR (EC 50 >100 µM and 12 µM respectively, a >8.2 selectivity ratio). [START_REF] Pérès | Ferrocene-embedded flavonoids targeting the Achilles heel of multidrug-resistant cancer cells through collateral sensitivity[END_REF] Thus, the presence of MRP1 in the cells, initially providing defense against anticancer agents, enabled the rise of a collateral sensitivity phenomenon. Very recently, a series of hemiindigo derivatives was reported. Evaluated against both the MES-SA sensitive and multidrug-resistant cancer cell lines, most of the compounds selectively targeted resistant cells.

Compound 16 was the most promising analogue, with antiproliferative IC 50 values of 18 µM and 3.7 µM against MES-SA and MES-SA/Dx5 (a doxorubicine-induced resistant line) respectively, providing a selectivity ratio of 4.8. By contrast, aurone derivatives were far less active, although they shared similar selectivity ratios. Higher IC 50 values of 60 µM and 15.0 µM were recorded for compound 17, one of the best aurones in the series. [START_REF] Toth | Synthesis and anticancer cytotoxicity of azaaurones overcoming multidrug resistance[END_REF] Surely, more work is needed for improving the modest observed ratios and to further assess the true potential of the hemiindigoid family at targeting collateral sensitivity.

Inhibition of poly(ADP-ribose)polymerases (PARPs).

The PARP-1 protein plays an important role in DNA damage repair, and thus constitutes a promising anticancer target for inhibitor development. Especially, targeted therapeutic strategies rely on the concept of synthetic lethality, [START_REF] Lord | PARP inhibitors: synthetic lethality in the clinic[END_REF] for example in the treatment of cancers with specific mutations, such as breast cancers (with mutated genes BRCA1 or BRCA2). Several agents and preclinical and clinical studies have been reported to date, and among them aurone derivatives were found as potent inhibitors of PARP-1.

Starting from an initial lead based on a dihydrobenzofuran scaffold (IC 50 = 9.5 µM against PARP-1), modulation endeavor of Patel et al. led to the production of a first aurone series. While the sole presence of a naked B-ring did not improve the activity, better results were obtained upon phenol group insertion at position 4' (e.g., IC 50 = 0.75 µM for 18, Figure 5). Crystallographic data of a 18-PARP-1 complex (PDB code 4OQA, Figure 6A) showed that the resorcinol group was directed toward a hydrophilic pocket including side chains from Asp766, Asp770, Ser864, Asn868 and Arg878, without filling it however. Extended substituents were thus anchored to position 4' of the aurone scaffold. The piperazine moiety from compound 19 afforded improved IC 50 (0.114 µM), among other piperazine-or morpholine-containing derivatives with submicromolar potencies. Indeed, the crystal structure of a morpholino counterpart of 19 bound to PARP-1 revealed the extent of additional interactions made with the hydrophilic pocket (PDB code 4OQB, Figure 6B). [START_REF] Patel | Discovery and structure-activity relationship of novel 2,3dihydrobenzofuran-7-carboxamide and 2,3-dihydrobenzofuran-3(2H)-one-7-carboxamide derivatives as poly(ADP-ribose)polymerase-1 inhibitors[END_REF] Switching the flexible piperazine arm for a more rigid 5,6,7,8-tetrahydro-[1,2,4]triazolo [4,3a]pyrazine heterocycle -among other tested scaffolds-gave compounds with two-digit nanomolar anti-PARP-1 activity. Compounds 20 and 21, the most promising ones, were found active both against PARP-1 (IC 50 = 30 nM and 27 nM, respectively) and PARP-2 (IC 50 = 2 nM and 1.9 nM, respectively), but a wide range of derivatives with alternative heterocyclic substituentsin place of CF 3 or cyclopropyl-shared very similar values (IC 50 = 30-67 nM against PARP-1 and 2.1-4.6 nM against PARP-2). The co-crystallization of 21 with a CATΔHD construct of PARP-1 -the catalytic domain with deletion of the autoinhibitory folded helical domain-revealed key interactions between the cyclopropyl part and hot spot residues from the adenosine binding pocket, such as Asn868 and Arg878. Upon further replacing the substituent at position 4', this time with benzimidazole derivatives, the IC 50 values decreased, hitting single-digit nanomolar range with compound 22 (IC 50 = 4 nM against PARP-1 and 0.7 nM against PARP-2). Structural data obtained from another analogue, 23 (IC 50 = 18 nM and 4 nM against PARP-1 and PARP-2, respectively), in complex with the CATΔHD construct, showed that the benzimidazole moiety pointed toward Arg878. Interestingly, the selectivity profile of compounds 20-23 against several PARP isoforms (PARP- 1 and2, TNKS1 and2 Inhibition of tyrosinase. Human tyrosinase catalyzes the first two steps of melanogenesis, and is a key target for reducing melanin formation in vivo. Besides the implications in dermocosmetics, the inhibition of human tyrosinase could mitigate the melanin-induced resistances of melanoma against a wide range of classical anticancer therapies, such as chemotherapy, immunotherapy or radiotherapy. [START_REF] Buitrago | Are human tyrosinase and related proteins suitable targets for melanoma therapy?[END_REF] However, only few human tyrosinase inhibitors are known, as most studies focus on assays against tyrosinase from the mushroom Agaricus bisporus, which eventually appears as a deceptive model. [START_REF] Roulier | Advances in the design of genuine human tyrosinase inhibitors for targeting melanogenesis and related pigmentations[END_REF] After a first report in 2006 focusing on cellular melanogenesis inhibition, [START_REF] Okombi | Discovery of benzylidenebenzofuran-3(2H)-one (aurones) as inhibitors of tyrosinase derived from human melanocytes[END_REF] polyhydroxylated aurones were also described as chemical tools able to adopt versatile behaviors against both Agaricus bisporus mushroom and bacterial tyrosinases. Indeed, while monophenol and catechol substitutions at the B-ring led to the production of either substrates (4'-hydroxy and 3',4'-dihydroxy, such as 24, Figure 7) or activators (2'-hydroxy and 3'-hydroxy, such as 25) of tyrosinases, the resorcinol counterpart allowed to reach efficient inhibition of the isolated enzymes. 41,[START_REF] Haudecoeur | Investigation of the binding-site homology of mushroom and bacterial tyrosinases by aurones as effectors[END_REF] Compound 26 was especially associated with IC 50 values of 4-9 µM against mushroom and bacterial enzymes, and was found to inhibit another tyrosinase, from Polyporus arcularius (IC 50 = 34 µM). [START_REF] Markova | Recombinant tyrosinase from Polyporus arcularius: overproduction in Escherichia coli, characterization, and use in a study of aurones as tyrosinase effectors[END_REF] A resorcinol-based indanone counterpart, 27, was also identified with very similar potency (IC 50 = 12.3 µM, K i = 10.3 µM) against mushroom tyrosinase, 44 and was even highlighted as a more potent inhibitor in another study (IC 50 = 0.034 µM and 1.39 µM using Ltyrosine and L-DOPA as a substrate, respectively). [START_REF] Jung | In vitro and in silico insights into tyrosinase inhibitors with (E)-benzylidene-1-indanone derivatives[END_REF] Compound 28, a close derivative of 27 with an additional fused phenyl ring, also showed some interesting activity (IC 50 = 0.15 µM). [START_REF] Choi | Identification of (Z)-2-benzylidene-dihydroimidazothiazolone derivatives as tyrosinase inhibitors: anti-melanogenic effects and in silico studies[END_REF] Alternatively, 4'-dimethylamino and 4'-diethoxymethyl groups introduced on the indanone scaffold yielded promising results (IC 50 = 6-8 µM), as illustrated by compound 29. [START_REF] Bukhari | Biological activity and molecular docking studies of curcumin-related α,β-unsaturated carbonyl-based synthetic compounds as anticancer agents and mushroom tyrosinase inhibitors[END_REF] In addition, embedding a hydroxypyridine-N-oxide (HOPNO) moiety on the aurone scaffold afforded a series of three compounds with high activity against human tyrosinase, bearing either a 4-hydroxy, a 6hydroxy, or a 4,6-dihydroxy substitution pattern. Compound 30 reached a K i of 350 nM and exhibited a similar potential to thiamidol, a reference molecule in the field of human tyrosinase targeting. QM/MM dynamics were performed in order to rationalize the binding of this compound to the dicopper center. As expected, the HOPNO moiety would interact with the metals while the rest of the aurone structure would provide contacts with various residues of the second coordination sphere, such as Ile368 and the flexible 302-310 loop, especially His304, Lys306 and Arg308.

However, cell-based assays provided underwhelming results, and 30 seemed to suffer from its low solubility and membrane-crossing issues (IC 50 = 85.3 µM in a whole-cell context using human melanoma cells). Inhibition of proto-oncogene serine/threonine-protein (PIM) kinases. PIM kinases are seen as important therapeutic targets in the frame of anticancer therapy since several years. Overexpressed in hematologic cancers and solid tumors, the three isoforms PIM1, PIM2 and PIM3 are both involved in numerous cancer-related disorders, such as cell proliferation or apoptosis. Thus, the development of PIM inhibitors recently attracted attention. [START_REF] Asati | PIM kinase inhibitors: structural and pharmacological perspectives[END_REF] Initially, a virtual screening involving more than 5,000,000 compounds yielded 532 candidates evaluated against PIM1 kinase, eventually affording five active molecules at 5 µM. Among them, the indolaurone derivative 31 (Figure 8) was found as a promising submicromolar 'hit' inhibitor (IC 50 = 410 nM against PIM1), [START_REF] Tsuganezawa | A novel Pim-1 kinase inhibitor targeting residues that bind the substrate peptide[END_REF] interacting with PIM1 ATP-binding cleft (PDB code 3UMX, Figure 9A), [START_REF] Parker | Flexibility of the P-loop of Pim-1 kinase: observation of a novel conformation induced by interaction with an inhibitor[END_REF] but suffered from poor kinase selectivity and metabolic stability. Thus, modulation efforts were undertaken in order to improve the profile of 31. The 7-amino and 6-hydroxy groups, and the indole B-ring were all modulated independently for identifying the most beneficial features. It clearly appeared that a piperazinyl moiety was favorable at position 7, while a methoxy group gave the best results at position 6. Thus, variations of the B-heterocyclic ring of an otherwise optimized scaffold afforded two compounds with low nanomolar activity, 32 and 33 (IC 50 = 3 nM and 2 nM, respectively).

Compound 32 was especially co-crystallized with PIM1 kinase (PDB code 3UMW, Figure 9B), and seemed to bind also at the ATP-binding site. While the piperazine ring formed two crucial hydrogen bonds with Asp128 and Glu171, the extra methyl group introduced at position 6 was found to be set in a hydrophobic pocket, endorsing its higher potency, as compared with the hydroxy group of the parent compound 31. Many other hydrogen bonds were observed, especially between the aurone carbonyl and Asp186 and Lys67, and between the heterocyclic B-ring and Glu121. In addition, 32 showed a far better kinase-selective profile, as revealed by tests performed against a large panel of kinases. Contrariwise, 33, which shared a similar PIM1 inhibition potency, was associated with a global lower selectivity, while exhibiting an identical interaction pattern, just as its 6-azaindole derivative (PDB codes 5VUC and 5VUA, Figures 9C and 9D respectively). Cellbased assays confirmed the ability of 32 to inhibit the phosphorylation of BAD, a PIM1 substrate, and to cause apoptosis with a cell cycle arrest at the G1 phase, consistent with PIM1 inhibition effects already documented. [START_REF] Nakano | Rational evolution of a novel type of potent and selective proviral integration site in moloney murine leukemia virus kinase 1 (PIM1) inhibitor from a screening-hit compound[END_REF] Then these compounds were modulated for reaching a better activity against PIM3. Indeed, 32 was found to be less active against PIM3 than PIM1 (IC 50 = 25 nM vs. 3 nM). By combining the heterocyclic moieties at the B-rings of 25 and 26, and by introducing an additional methyl group on the methylene bridge between the aurone scaffold and the piperazine group, the authors designed compound 34, which displayed a better anti-PIM3 activity (IC 50 = 2.5 nM) while retaining a good PIM1 inhibitor profile (IC 50 = 1.9 nM) and in vivo efficacy against pancreatic cancers in mice. [START_REF] Nakano | Design and synthesis of an in vivo-efficacious PIM3 kinase inhibitor as a candidate anti-pancreatic cancer agent[END_REF] A further exploration of the impact of a substituent at position 6 of X3 B-ring heterocycle afforded a series of compounds with very high activity against PIM1.

Among the studied molecules, 6'-fluoro and 6'-chloro derivatives were found to be particularly promising, both in the 6-methoxy and 6-hydroxy series. Compound 35 especially combined an excellent anti-PIM1 activity (IC 50 = 1.4 nM) with a decent effect against PIM2 (IC 50 = 140 nM) and good ADMET properties. The presence of the extra chlorine group allowed to hamper offtarget interactions, including with other kinases, while improving the affinity for PIM2, thereby opening a way for reaching selective pan-PIM inhibitors. [START_REF] Nakano | Design and synthesis of potent and selective PIM kinase inhibitors by targeting unique structure of ATP-binding pocket[END_REF] A very recent study focused on PIM1 inhibitors discovery based on the aurone scaffold with hydroxy substitution at the A-ring. While Targeting of other kinases, cancer-related proteins and pathways. Hemiindigoid derivatives were also associated with sporadic hit discovery, targeting kinases or other proteins and signaling pathways linked to cancer. In the Wnt pathway, the protein-protein interaction between the Frizzled 7 receptor and the PDZ subdomain of Dishevelled protein (Dvl) could contribute significantly to tumorigenesis. [START_REF] Fujii | An antagonist of dishevelled protein-protein interaction suppresses βcatenin-dependent tumor cell growth[END_REF] Thus, compound 37 (Figure 10) was identified among eighteen other potential inhibitors from a 7-million structure virtual screening. Evaluated through SPR analysis, 37 was the most promising candidate for inhibiting the interaction (K d = 11.2 µM). [START_REF] Choi | Identification of small-molecule compounds targeting the dishevelled PDZ domain by virtual screening and binding studies[END_REF] Another target, retinoic acid receptor (RAR) was successfully modulated in the treatment of acute promyelocytic leukaemia. [START_REF] Altucci | RAR and RXR modulation in cancer and metabolic disease[END_REF] Therefore, efforts were undertaken for developing active ligands for anti-cancer therapy. An adamantyl aurone derivative, 38, was discovered among other flavonoids as a RARα antagonist, with an IC 50 value of 4.42 µM. However, the selectivity of compound 38 for RARα was not assessed, although it constitutes a crucial indicator of its potential as anticancer agent. [START_REF] Lorenzo | Adamantyl arotinoids that inhibit IκB kinase α and IκB kinase β[END_REF] For kinases, protein phosphorylation is a crucial process for cellular activities, that is known to be deregulated in cancer and linked to protein mutations, and thus these enzymes are often considered as relevant targets for cancer therapy, and several of them included hemiindigoids in their respective inhibitor portfolio. Compound 39 was discovered as a potent allosteric inhibitor of Akt, a kinase involved in the regulation of p53 activation in several cancers, with an IC 50 value of 207 nM. [START_REF] Rajagopalan | Biological evaluation of 2-arylidene-4, 7-dimethyl indan-1-one (FXY-1): a novel Akt inhibitor with potent activity in lung cancer[END_REF] This aurone derivative demonstrated anti-proliferative and anti-metastatic effects in vivo against lung cancer models, and was thus further chemically modulated for affording 40, a close analogue with enhanced properties. Compound 40 especially showed an improved inhibition of isolated Akt (IC 50 = 66 nM) and was associated with an extension of the pre-G 0 /G 1 cell-cycle phase in HL-60 leukemia cells and a pro-apoptosis effect. [START_REF] Balasubramaniam | FCX-146, a potent allosteric inhibitor of Akt kinase in cancer cells: Lead optimization of the second-generation arylidene indanone scaffold[END_REF] Compound 41 was identified as a submicromolar inhibitor (IC 50 = 0.735 µM) of Aurora B kinase (AURKB), an enzyme overexpressed in cancer cells and involved in mitosis and tumorigenesis. Exhibiting a good selectivity for AURKB among a panel of kinases, 41 managed to target the enzyme in a cellular context, as attested by the observed accumulation of cancer cells in the G 2 /M phase, and induced a global significant reduction of a human liver tumor weight in vivo. [START_REF] Xie | In vitro and in vivo characterization of a benzofuran derivative, a potential anticancer agent, as a novel Aurora B kinase inhibitor[END_REF] In addition, aurone compounds 42 and 43 emerged from virtual screenings conducted respectively against CK2 and c-KIT, two protein kinases involved in cancer. In each case these "hits" are active at the low micromolar level, with an IC 50 value of 6.0 µM for 42 against CK2 (a crystal structure of the 42-CK2 complex is depicted in Figure 11, corresponding to PDB code 3WIL), [START_REF] Nakanishi | Identification of protein kinase CK2 inhibitors using solvent dipole ordering virtual screening[END_REF] and IC 50 values of 15.6 µM and 1.4 µM for 43 against WT and D816V mutant c-KIT kinases, respectively. [START_REF] Park | Discovery of dual inhibitors for wild type and D816V mutant of c-KIT kinase through virtual and biochemical screening of natural products[END_REF] A further exploration of anti-CK2 properties of aurones afforded numerous compounds with nanomolar activities, with a clear beneficial effect of substituents at position 3'. Compound 44 was one of the most active aurones of the series (IC 50 = 3.3 nM). [START_REF] Protopopov | Flavone inspired discovery of benzylidenebenzofuran-3(2H)-ones (aurones) as potent inhibitors of human protein kinase CK2[END_REF] Altogether, these studies constitute promising starting points for further modulation endeavors. Besides kinases, histone deacetylases (HDACs) are also widely associated with cancer development. A series of aurones, tested against HDAC 1, 2 and 6, afforded compound 42 as the most promising compound with moderate micromolar potency (IC 50 = 5.1-27 µM), making this molecule a potential anticancer multitarget agent. [START_REF] Zwick | Aurones as histone deacetylase inhibitors: identification of key features[END_REF] These results were contested by another study which hypothesized a false-positive result caused by fluorescence interferences during the test. [START_REF] Itoh | False HDAC inhibition by aurone compound[END_REF] However, aureusidin (Figure 2), already identified by Zwick et al., was confirmed to induced HDAC inhibition in a more recent phytochemical work. Aureusidin was especially found as an efficient inhibitor of HDAC4 (K i = 0.80 µM), but delivered far less activity against other subtypes (K i = 1.66-4.55 µM against HDAC 2, 7, 8). [START_REF] Kumboonma | New histone deacetylase inhibitors from the twigs of Melanorrhoea usitata[END_REF] A set of interactions was predicted through molecular docking with the zinc via the catechol group, and with His158, Gly330 and Pro298. A small series of aurones was also evaluated against a class III HDAC, sirtuin 1 (SIRT-1), and yielded compound 38 as the best inhibitor (IC 50 = 1 µM). [START_REF] Manjulatha | Ethylenediamine diacetate (EDDA) mediated synthesis of aurones under ultrasound: their evaluation as inhibitors of SIRT1[END_REF] Another enzyme, tyrosyl-DNA phosphodiesterase (TDP1), known for its capacity to repair DNA lesions induced by anticancer drugs, was targeted by extended aurone derivatives. Compound 46 combined a good activity (IC 50 = 63 nM against TDP1) with a high selectivity index and a sensitizer profile for topotecan cytotoxic action in A-549 and HEK-293 cell lines. [START_REF] Zakharova | Synthesis and evaluation of aryliden-and hetarylidenfuranone derivatives of usnic acid as highly potent Tdp1 inhibitors[END_REF] Finally, the targeting of G-quadruplex DNA has recently emerged as a new approach for anticancer agent development. A small series of indanones with indole-based B-rings was proposed. Compound 47 appeared as particularly interesting, as it provided a high level of stabilization for oncogene-related G-quadruplex topologies (ΔT 1/2 = 12.4-24.3 °C for c-myc, c-kit1 and c-kit2) while not targeting telomeric quadruplex (ΔT 1/2 = -0.4 °C) and classical duplex DNA (ΔT 1/2 = 2.8 °C). [START_REF] Diveshkumar | Specific stabilization of c-MYC and c-KIT G-quadruplex DNA structures by indolylmethyleneindanone scaffolds[END_REF] Such a specificity appears as mandatory for the development of anticancer G-quadruplex ligands that would not trigger genomic instability. However, some compounds were found as micromolar or submicromolar cytotoxic or antiproliferative agents. For example, a pharmacomodulation effort around 3'-substituted indanones afforded compound 48 (Figure 12) as the most active analogue in a cytotoxicity assay, but with prohibitive action against normal cells as well, leading to a very low selectivity index shared by all the derivatives of the series. [START_REF] Tugrak | Synthesis of mono Mannich bases of 2-(4hydroxybenzylidene)-2,3-dihydroinden-1-one and evaluation of their cytotoxicities[END_REF][START_REF] Tugrak | Synthesis and anticancer properties of mono Mannich bases containing vanillin moiety[END_REF][START_REF] Gul | New phenolic Mannich bases with piperazines and their bioactivities[END_REF][START_REF] Tugrak | Synthesis and biological evaluation of some new mono Mannich bases with piperazines as possible anticancer agents and carbonic anhydrase inhibitors[END_REF] A similar lack of selectivity was recorded regarding ferrocene derivatives. [START_REF] Tiwari | In vitro inhibitory properties of ferrocene-substituted chalcones and aurones on bacterial and human cell cultures[END_REF] The indanone scaffold, associated with an heterocyclic B-ring, also provided a series of extended cytotoxic compounds in the micromolar range, such as compound 49, which showed IC 50 values between 0.23 and 2.89 µM against the growth of a panel of sixty cancer cell lines. [START_REF] Shaik | synthesis and biological evaluation of novel pyrazolochalcones as potential modulators of PI3K/Akt/mTOR pathway and inducers of apoptosis in breast cancer cells[END_REF] Disparate results were also reported through recent phytochemistry works that led to the isolation of new aurone structures from plants. Besides moderate activities recorded from classical polyhydroxyaurones, compound 50 and its 4'-prenylated analogue showed low micromolar values in some cases (IC 50 = 3.4 µM against NB4 cells and 3.5 µM against SH-SY5Y cells, respectively). [START_REF] Gao | Aurones and isoaurones from the flowers of Rosa damascene and their biological activities[END_REF][START_REF] Hu | Flavonoids from Garcinia paucinervis and their biological activities[END_REF] The isoprenylated aurone 51, extracted from Rosa damascena, showed IC 50 values in the same range in a cytotoxicity assay (IC 50 = 3.4 µM and 4.8 µM against SHS-SY5Y and NB4 cell lines, respectively). [START_REF] Wang | A new isoprenylated aurone from the flowers of Rosa damascene and Its cytotoxicities[END_REF] Unsurprisingly, linking an aurone derivative to the well-known antimitotic compound podophyllotoxin boosted the cytotoxicity of the resulting hybrid agents, especially for 52 (IC 50 = 0.10-0.97 µM against MCF-7, A549, DU-145 and MDA MB-231 cells).

The aurone scaffold was found to contribute to the activity however, as demonstrated by the results obtained with podophyllotoxin analogue etoposide (IC 50 = 1.9-3.1 µM in the same conditions). [START_REF] Paidakula | Design and synthesis of 4βacetamidobenzofuranone-podophyllotoxin hybrids and their anti-cancer evaluation[END_REF] Antiproliferative assays also led to promising results in some cases. Furanoaurone 53 was evaluated against sixty cancer cell lines and afforded a median IC 50 of 4.1 µM. [START_REF] Ragab | synthesis and structure-activity relationship of novel semisynthetic flavonoids as antiproliferative agents[END_REF] A series of 10hydroxyindanones with various B-rings gave inhibition values in the same order of magnitude, with compound 54 reaching the low micromolar range (IC 50 = 2.5 µM against U-251 cells). The low pro-apoptotic action of this compound suggests a different mechanism that was not unraveled. [START_REF] Lozano-Gonzalez | Anticarcinogenic and metal chelation properties of novel hydroxybenzylidene-1-indanone derivatives in the U-251 glioblastoma cell line[END_REF] Finally, the 7-acetoxyaurone 55 was the only compound with submicromolar potency against PC-3 prostate cancer cells (IC 50 = 0.74 µM). [START_REF] Popova | Efficient synthesis of aurone Mannich bases and evaluation of their antineoplastic activity in PC-3 prostate cancer cells[END_REF]

ACS Paragon Plus Environment

Journal of Medicinal Chemistry . [START_REF] Huang | Multitarget-directed benzylideneindanone derivatives: anti-β-amyloid (Aβ) aggregation, antioxidant, metal chelation, and monoamine oxidase B (MAO-B) inhibition properties against Alzheimer's disease[END_REF] The study of a small series of aurones against MAO-A and MAO-B afforded results similarly advocating for a beneficial effect of a N-dialkyl group at position 4'. Indeed, while interesting values were obtained for a 4'-chloro analogue, the best results were provided by 57 and, to a lesser extent, by its 6-hydroxy derivative (IC 50 = 99 nM vs. 240 nM against MAO-B, and 270 nM vs. 980 nM against MAO-A, respectively). [START_REF] Geldenhuys | A scaffold hopping approach to identify novel monoamine oxidase B inhibitors[END_REF] A diversification of the 4dialkylamino group yielded more active analogues, reminiscently of the previous work involving indanones. N-Methylethyl compound 58 reached an IC 50 of 28 nM against MAO-A (IC 50 = 810 µM against MAO-B), a value significantly different from other tested dialkylamino substituents, including pyrrolidine, piperidine, morpholine and piperazine moieties. Interestingly, a 6-methoxy derivative bearing a 4'-piperidino group provided the better inhibition of MAO-B, an observation relevant with the previously described tolerance for an additional methyl group at this position in the indanone series. [START_REF] Li | Synthesis and evaluation of 4-hydroxyl aurone derivatives as multifunctional agents for the treatment of Alzheimer's disease[END_REF] However, the presence of a 4'-dialkylamino group did not appear as mandatory for anti-MAO activity. Even simple aurones, such as naturally occurring sulfuretin and hispidol (Figure 2), managed to potently inhibit the action of MAO-A (IC 50 = 4.16 µM vs. 0.26 µM). The latter was also identified as a low micromolar inhibitor of MAO-B (IC 50 = 2.5 µM). [START_REF] Baek | Selective inhibition of monoamine oxidase A by hispidol[END_REF] Other B-substituted aurones mostly failed to significantly improve MAO-A and MAO-B inhibition potency, but some individual compounds were interesting for their ability to target both enzymes. MAO-A selectivity (>600-fold ratio between the measured IC 50 values). [START_REF] Nel | 2-Benzylidene-1-indanone derivatives as inhibitors of monoamine oxidase[END_REF] A further functionalization of substituents at the A-and B-rings of the indanone scaffold led to three extended molecules, without a clear benefit both in terms of anti-MAO-B inhibition and selectivity ratio however. Compound 62 was found as the most promising ligand of the series (IC 50 = 262 nM against MAO-B, selectivity ratio >36). [START_REF] Affini | Novel indanone derivatives as MAO B/H 3 R dual-targeting ligands for treatment of Parkinson's disease[END_REF] A ratio of 180 was reached by hemithioindigo 63 (IC 50 = 0.35 µM against MAO-B, 63 µM against MAO-A), the best value among a series of 10-hydroxy derivatives. [START_REF] Guglielmi | thiophen-3-ol derivatives as effective inhibitors of human monoamine oxidase: design, synthesis, and biological activity[END_REF] A recent study focusing on the B-ring modulation of 5,6-dimethoxyindanones did not afford more active molecules against MAO-B, even if compound 64 displayed an exquisite selectivity for the enzyme over MAO-A (IC 50 = 0.02 µM vs. 58 µM, respectively, a selectivity ratio >2500), eventually highlighting once more the beneficial effect of 4'-amino derivatives. [START_REF] Wang | Multifunctional indanone-chalcone hybrid compounds with anti-β-amyloid (Aβ) aggregation, monoamine oxidase B (MAO-B) inhibition and neuroprotective properties against Alzheimer's disease[END_REF] However, the higher selectivity was obtained with indolaurone 65, with a ratio >38,000. The molecule was one of the best known hemiindigoid inhibitor of MAO-B (IC 50 = 2.6 nM) while being completely inactive against MAO-A (IC 50 >100 µM). 94

Neurochemical activities.

Inhibition of monoamine oxidases (MAO-A and MAO-B). Monoamine oxidases (MAO

Inhibition of cholinesterases (AChE and BuChE).

Cholinesterases catalyze the hydrolysis of cholinergic neurotransmitters, assuring a crucial role for cholinergic neurons deactivation and various neurological functions, such as memory or cognition. However, in the AD context, a depletion of cholinergic neurotransmitters occurs, and inhibitors of cholinesterases are able to mitigate the symptoms of the disease. A selective inhibition of acetylcholinesterase (AChE) or butyrylcholinesterase (BuChE) is often required, depending on the stage of AD progression. Thus, two drugs were put on the market, including the well-known donepezil (66, Figure 14), whose structure is closely related to hemiindigoids. Starting from donepezil, a first study focused in 2012 on the modulation of the A-ring with the introduction of various alkylpiperidine substituents and the swapping of the initial piperidinyl fragment of donepezil with aromatic groups, eventually affording indanones. Compound 67 (Figure 14) was found as the most potent AChE inhibitor, more efficient than the parent compound donepezil in in vitro assays (IC 50 = 1.8 nM), with a good selectivity (IC 50 = 9.5 µM against BuChE, a >5,000 ratio). Substituting the nitrogen atom afforded a series of benzylpyridinium derivatives, reminiscently of the donepezil structure. Compound 68 especially shared a similar profile than 67 (IC 50 = 1.9 nM and AChE-vs.-BuChE selectivity ratio >5,000), with a very high activity recorded against human AChE (IC 50 = 0.8 nM). [START_REF] Lan | synthesis and biological activity of novel donepezil derivatives bearing N-benzyl pyridinium moiety as potent and dual binding site acetylcholinesterase inhibitors[END_REF] Transpositions into an aurone scaffold led to less active compounds with decreased selectivity, as witnessed by a direct comparison between compounds 69 (IC 50 = 52 nM and 1,620 nM against AChE and BuChE, respectively) and 68. [START_REF] Nadri | 5,6-Dimethoxybenzofuran-3-one derivatives: a novel series of dual acetylcholinesterase/butyrylcholinesterase inhibitors bearing benzyl pyridinium moiety[END_REF] Introducing an extra carbonyl linker between the pyridinium moiety and the phenyl ring only produced far less active molecules, in the micromolar range. [START_REF] Alipour | Synthesis of some new 3-coumaranone and coumarin derivatives as dual inhibitors of acetyl-and butyrylcholinesterase[END_REF] Replacing the pyridinyl ring of the indanone scaffold by a phenyl counterpart overall maintained the inhibition potency (IC 50 = 9.1 nM against AChE), but modifying the alkyl linker length or the nature of the amino group at position 6 of the indanone scaffold led to an activity drop. [START_REF] Meng | synthesis, and evaluation of indanone derivatives as acetylcholinesterase inhibitors and metal-chelating agents[END_REF] Activities related to Aβ, tau and α-synuclein peptides. Neurological amyloid diseases are characterized by the aggregation of particular proteins into the brain, especially Aβ and tau peptides in Alzheimer's disease (AD) and α-synuclein in Parkinson's disease (PD). Indeed, these events lead to the formation of amyloid plaques and neurofibrillary tangles (NFTs) that provoke neuronal dysfunction and degeneration. Thus, anti-aggregation agents could reduce the progression of the disease, and numerous studies are dedicated to the identification of new molecules of interest. A significant part of hemiindigoid-based compounds evaluated for their ability to prevent Aβ peptides aggregation originated from so-called multitarget studies. However, in most cases, they actually reported formerly identified anti-MAO and/or anti-ChE agents secondly tested for their aggregation inhibition properties. Thereby, many hemiindigoids from the literature only provided moderate to weak activity, with IC 50 values higher than 10 µM or merely inhibition percentages at 20 or 25 µM, without further characterization. Several previously mentioned neurochemical agents fall into this category, including 56, 58, 64 (Figure 13), 73, 74 or 77 (Figure 14). [START_REF] Huang | Multitarget-directed benzylideneindanone derivatives: anti-β-amyloid (Aβ) aggregation, antioxidant, metal chelation, and monoamine oxidase B (MAO-B) inhibition properties against Alzheimer's disease[END_REF][START_REF] Li | Synthesis and evaluation of 4-hydroxyl aurone derivatives as multifunctional agents for the treatment of Alzheimer's disease[END_REF][START_REF] Wang | Multifunctional indanone-chalcone hybrid compounds with anti-β-amyloid (Aβ) aggregation, monoamine oxidase B (MAO-B) inhibition and neuroprotective properties against Alzheimer's disease[END_REF][START_REF] Huang | Discovery of indanone derivatives as multi-targetdirected ligands against Alzheimer's disease[END_REF]103,105,109 However, a more active analogue was discovered by Wang et al. Indeed, the 4'-nitro analogue of 64, compound 78 (Figure 15), displayed a higher potency for self-induced Aβ 1-42 aggregation inhibition (IC 50 = 1.8 µM), while retaining the anti-MAO-B profile already observed in the series (IC 50 = 0.03 µM). Interestingly, 78 was also able to moderately induce the disaggregation of Aβ fibrils (IC 50 = 7.9 µM). [START_REF] Wang | Multifunctional indanone-chalcone hybrid compounds with anti-β-amyloid (Aβ) aggregation, monoamine oxidase B (MAO-B) inhibition and neuroprotective properties against Alzheimer's disease[END_REF] A survey involving many polyhydroxylated flavonoids identified the aurone maritimetin (Figure 2) as the most potent compound against Aβ aggregation in a Thioflavin T assay (IC 50 = 2.7 µM). However, maritimetin showed actually a weak action against fibrils formation in electron microscopy experiments. 110 As the Aβ peptides result from a sequential cleavage of APP by β-site APP cleaving enzyme 1 (BACE1), this enzyme is also a Aβ-related target of interest for AD treatments. A first indanone series provided extended BACE1 inhibitor 79 as the lead compound, with only micromolar activity however (IC 50 = 2.5 µM). 111 The further study by Gabr et al., which afforded compound 76 (Figure 14) as an efficient inhibitor of AChE, revealed the nanomolar potency of the molecule against BACE1 as well (IC 50 = 13.1 nM). 108 The impact of hemiindigoids on tau peptide aggregation was far less studied, but a series of aurones showed promising results. Several polyhydroxylated derivatives still inhibited more than 50% of the aggregation of AcPHF6, a model of tau, at a concentration of 1 µM. Aureusidin (Figure 2) was especially found as one of the most promising molecules (62% inhibition of AcPHF6 aggregation at 1 µM), able to prevent fiber formation and elongation through atomic force microscopy analyses. 112 A further, recent study confirmed the general potential of hemiindigoids by identifying indanone-based inhibitors of AcPHF6. The indanone bracteatin (Figure 2) counterpart especially led to a ~90% inhibition of fiber formation at 10 µM. 113 In addition, compounds 80 and 81 were found as potent GSK3β inhibitors (IC 50 = 310 nM and 450 nM, respectively), a kinase involved in tau hyperphosphorylation in AD pathological context. 114,115 Besides, the detection of β-amyloid plaques and neurofibrillary tangles, which compose the senile plaques that characterize AD from aggregated Aβ and tau peptides, is a crucial element for a diagnosis of the disease as early as possible. The hemiindigoid scaffolds allowed the development of such probes, revealing a very high affinity for their targets. Hence, radioiodinated hemiindigo compound 82 was found to bind both tau and Aβ aggregates with exquisite affinities (K i = 17 nM and 31 nM, respectively). In vivo experiments confirmed the potency of this molecule to successfully stain NFTs in specific brain sections related to AD. 116 The scaffold hopping into indanones retained partially the activity: the indanone counterpart of 82 showed a K i of 240 nM for brain homogenates from an AD patient.

However, the introduction of the 135 I substituent at position 5 yielded compound 83, which displayed a better affinity (K i = 100 nM). 117 The further insertion of a methoxy group at position 6 improved the affinity values to low nanomolar levels, as observed for compound 84 (K i = 12 nM against Aβ 1-42 and 29 nM against AD patients brain homogenates). The radiolabeled molecule was found to stain β-amyloid plaques, and showed enhanced uptake in brain compared to previously described indanones. 118 Indanones also succeeded to label α-synuclein fibrils from PD brain tissues. Compound 85 especially showed a very good affinity (K D = 39 nM) while being fairly selective versus Aβ fibrils (K D = 390 nM, a ten-fold ratio). 119 No data were reported regarding the activity of hemiindigoids against other amyloid-type proteins, such as prions, human islet amyloid polypeptide or systemic amyloid A. Hence, the evaluation of their aggregation inhibition properties against these targets could provide new treatments for various diseases.

Neuroprotection and other neurochemical effects. Several naturally occurring aurones were identified as neuroprotective agents through various degeneration and neurotoxicity models.

Aureusidin (Figure 2) was found among other flavonoids as an inhibitor of CDK5/p25 (IC 50 = 3.5 µM), a deregulated variation of physiological p35 or p39-regulated CDK5, involved in abnormal phosphorylation events related to neuronal cell death in the frame of AD. 120 This activity could be responsible for the attenuation of glutamate-induced neurotoxicity measured in HT22 neuronal cells, moderate at micromolar concentrations (EC 50 = 11.9 µM) but reaching 97% at 40 µM. 121 The analogue sulfuretin (Figure 2) also showed a significant effect on the same model, leading to a 93% improvement of cell viability at 10 µM, a potency shared with other flavonoids, such as flavanones and chalcones. In addition, sulfuretin allowed a 1.8-fold attenuation of glutamate-induced ROS generation at 5 µM. 122 A further study provided an EC 50 value of 3.5 µM against glutamate-induced HT22 cell death. 123 In addition, sulfuretin exerted neuroprotection effects by inhibiting 6hydroxydopamine-induced neuronal cell death, 124 and by attenuating Aβ-induced neurotoxicity. 125 Through an activation of Akt/GSK3β and ERK signaling pathways, the compound also prevented MPP + -induced neuronal cell death, highlighting a potential beneficial role in protection against PD. 126 Potent antioxidant properties could be partly responsible of this multiple neuroprotective action. Damaurone D (Figure 2) shared a favorable effect in MPP + -induced neurotoxicity. In addition, it showed an attenuation of α-synuclein expression, accumulation and fibrillation, a promising profile for the development of an anti-PD agent. 127 Naturally occurring aurones 86 (Figure 15) and (Z)-7,4'-dimethoxy-6-hydroxy-aurone-4-O-β-glucopyranoside also provided interesting results respectively on 6-hydroxydopamine-induced (IC 50 = 2.9 µM) 128 and H 2 O 2induced neurotoxicity 129 models. The latter compound was further identified as an anxiolytic agent through an elevated plus maze test on rats. Finally, hispidol (Figure 2) was associated with antidepressant properties, as it allowed a dose-dependent decrease in immobility times in forced swimming and tail suspension tests performed on mice.

Anti-infective activities.

Anti-bacterial activities. The growing concern of bacterial resistance has put back the quest for new anti-bacterial agents to the forefront of the pharmaceutical scene. As standard antibiotics chemical classes are progressively dismissed, the development of new scaffolds appears as a crucial need for treating bacterial infections in the next decades. Among these new chemical agents, several hemiindigoid-based series emerged. First, aurone derivatives with bicyclic B-rings were developed for their antibacterial activities. Quinoline-based analogues were especially investigated, and afforded compounds with weak effects. The N-allyl compound 87 (Figure 16) was identified as one of the most promising agents in a first study, exhibiting a MIC value of 33 µM against Pseudomonas aeruginosa, while being almost inactive against a panel of other Gram positive (Staphylococcus aureus, Bacillus subtilis, Clostridium tetani) and Gram negative (Escherichia coli, Vibrio cholerae) strains. The activities provided by the rest of the series were only marginal. 131 A second report focused on 2-chloroquinolin-3-yl derivatives. Again, most of the results obtained were underwhelming, even if some analogues showed a modest selective activity against B. subtilis over S. aureus. 132 Switching to bicyclic chromone B-rings afforded far more active agents. Especially, the dimethyl analogue 88 reached the low micromolar range against P. aeruginosa and S. aureus. The measured MIC value of 1.6 µM was notably lower than the MIC of ciprofloxacin and gentamicin. 133 Aurones with other various structures were also identified. The evaluation of a series of furoaurones against several Gram positive and Gram negative bacteria revealed a good potency of several derivatives. Compound 89 especially showed MIC values between 4.4 and 8.8 µM against B. subtilis, S. aureus, E. coli and P. vulgaris and could be an inhibitor of cyclooxygenase-II, according to docking studies. 134 Several studies only reported inhibition zones in mm, identifying compounds with structural diversity, such as 90, which displayed a similar result than ampicillin, but with no MIC values measured. 135 However, in all of these studies, no data regarding a potential general cytotoxicity was included, and the reported putative antibacterial compounds could actually be cytotoxic agents, as exemplified by the study of Tiwari et al. which showed both antibacterial activity and cytotoxic activity ferrocenyl aurone analogues. [START_REF] Tiwari | In vitro inhibitory properties of ferrocene-substituted chalcones and aurones on bacterial and human cell cultures[END_REF] Thus, a recent study examined the antibacterial activity of a series of substituted aurones, with a particular attention to their potential cytotoxic action. Among forty compounds, 91 was found as the most promising analogue with a specific activity against Gram positive strains, especially nisin-resistant B. subtilis (MIC of 3.9 µM), through a hypothesized alteration of the membrane. Surprisingly, 91 was also able to inhibit Helicobacter pylori and Vibrio alginolyticus selectively among other Gram negative bacteria (MIC of 15.6 and 3.9 µM, vs. >500 µM for thirteen other strains). The molecule did not exhibit any cytotoxic effect at these concentrations (IC 50 = 190-340 µM against BEAS-2B and IMR-90 cells). 136 Aurones were also identified as antitubercular compounds in a high-throughput screening. Among a large diversity of chemical structures, three aurones emerged, including compound 92 which showed the best effect against Mycobacterium tuberculosis (IC 90 = 4.8 µM) together with a low cytotoxicity (selectivity index of 17). 137 Further exploring the aurone and hemiindigo chemical spaces, Campaniço et al. discovered a series of compounds with very high activity against tuberculosis. Indeed, some N-acetyl hemiindigo analogues reached MIC 90 and MIC 99 values below 1 µM. Among them, compound 93 combined a good activity (MIC 90 of 0.65 µM, MIC 99 of 0.74 µM) with a low toxicity at these concentrations (CC 50 = 19 µM against HEK293T cells). Several analogues were even able to inhibit multidrug-resistant and extensively drug-resistant clinical M. tuberculosis isolates. 138 The de novo design of a HtrA (High-temperature requirement A) inhibitor, a protease from H. pylori presumed to be a potential therapeutic target, finally afforded one hemithioindigo derivative, compound 94, with a K D value of 12 µM. Antifungal activities. In a similar way, pathogenic fungi are becoming increasingly resistant to widely used antifungals. The corresponding infections thus pose a serious threat to the global health, especially considering Candida, Aspergillus, Pneumocystis and Cryptococcus species, that cause more than 90% of fungi-related deaths. In the literature, a significant portion of antifungal results were produced through general antimicrobial screenings, performed preliminary against bacterial strains. Indeed, several hemiindigoid series already mentioned in the previous section yielded low to absent activities against a variety of fungus, especially Candida albicans, Aspergillus niger or Fusarium oxysporum. 131,132,136 However, some derivatives showed actual activity. Compound 88 (Figure 16) afforded MIC values of 14-17 µM against A. niger and A.

flavus, and comparable results were obtained with the antifungal reference nystatin. 133 A link was established between a high affinity for this RNA tertiary structure and an antifungal potency against Candida parapsilosis. Bracteatin (Figure 2) constituted the starting point of this study (K i = 2.1 µM against the ai5γ intron and MIC of 53 µM), and compound 98 was identified as a potent analogue by removing phenol groups from the A-ring (K i = 5.8 µM against the ai5γ intron and MIC of 30 µM). However, furan derivatives outside the hemiindigoid chemical space were found as more efficient both in terms of ai5γ intron targeting and antifungal activity. Anti-parasite activities. As parasitic diseases are responsible for almost one million of deaths annually (with almost one half being caused by Plasmodium species in low-income countries), the quest for efficient and accessible anti-parasite compounds still appears as a sanitary emergency. In addition, many parasitic diseases are contained in tropical areas, and have been largely neglected by the pharmaceutical efforts of developed nations. Addressing these pathologies is an urgent need.

Regarding hemiindigoids, most of the anti-parasite activities were reported against protozoan parasites, such as Plasmodium, Leishmania or Trypanosoma species. Hence, antimalarial aurones were first found, leading to a series of compounds with simple structure and activities in the micromolar range. If B-substituted derivatives were sometimes encouraging, compound 99 (Figure 18) was identified as the best inhibitor of the chloroquine-resistant Plasmodium falciparum W2 strain (IC 50 = 1.18 µM), without any cytotoxicity measured in human cells (EC 50 >100 µM against HEK293T cells). However, this IC 50 value remained one order of magnitude higher than that of chloroquine. 143 Another series of various methoxy-or methyl-substituted aurones only produced inactive compounds, with the exception of two derivatives exhibiting IC 50 values in the 10-20 µM range. 144 The transition to a hemiindigo scaffold was far more successful. The substitution of positions 3' and 4' yielded dozens of molecules with submicromolar activity against chloroquineresistant P. falciparum. Compound 100 even reached the hundred nanomolar range (EC 50 = 0.11 µM) with no toxicity reported against HEK293T cells (EC 50 >100 µM). Interestingly, the positions 3' and 4' appeared as very tolerant for a wide variety of substituents, especially phenyl groups. The series was also active against P. berghei (EC 50 = 0.81 µM for 100). However, the intracyclic NH acetylation led to a dramatic drop of activity in all cases, highlighting the crucial role of the unsubstituted group. 145 QSAR studies were conducted in order to extract the crucial structural elements from the reported hemiindigoid series. Hence, the presence of bulky groups at positions 4 and/or 6 was pinpointed as beneficial, and overall the roles of electron-withdrawing substituents at the B-ring and of electron-donating groups at the A-ring were underlined, a statement partially consistent with the aforementioned series, including 99 and 100. 146 A further study focusing on hemiindigos endorsed the introduction of a bulky substituent at the meta position of the B-ring, and proposed four new structures derived from 100, with heterocyclic 3'-aryl groups. 147 Antileshmanial aurones were also identified. If several methyl-and methoxy-substituted compounds only afforded weak inhibitors against Leishmania donovani (IC 50 = 10-50 µM for the most active derivatives), 144 a set of molecules with closely-related structures gave more promising results against L. infantum. Indeed, some analogues reached micromolar values against intracellular amastigotes, especially compound 101 (IC 50 = 1.3 µM) which in addition revealed a low cytotoxicity against THP1differentiated macrophages, leading to a good selectivity index (57.5). 148 Aurones were also pinpointed for their ability to inhibit the ABC transporter ABCI4, responsible for the efflux of antimony, a first-line drug against leishmaniasis. Compound 102 was able to restore the susceptibility of a ABCI4-expressing L. major line to Sb III , allowing to divide its EC 50 value by 6, and to reach a similar level of activity than against a control L. major line without ABCI4. The parasites were sensitized even for control, although no significant cytotoxicity was recorded either on L. major (EC 50 = 190 µM) or THP-1 cells (EC 50 = 350 µM), indicating a probable intrinsic antiparasitic effect of 102. 149 Finally, aurone 103 showed some activity against Toxoplasma gondii proliferation (IC 50 = 10 µM) without causing cytotoxicity at this concentration. 150 The protozoan Entamoeba histolytica was also the target of an indanone derivative, that was identified through a virtual screening as a potential inhibitor of E. histolytica hexokinase 1, a crucial enzyme for glycolysis regulation. However, the compound was actually a weak inhibitor, with an IC 50 value of 96 µM. 151 Hemiindigoid compounds were developed for other parasite species. Among them, Schistosoma worms are responsible for schistosomiasis, a neglected disease causing a high morbidity in Africa, South America and Asia. A natural aurone, isolated from a plant traditionally used in Chinese medicine, was evaluated against adult S. japonicum worms, but did not cause any worm mortality even at higher concentrations. 152 The introduction of a furane-and an indole-based (compound 104) B-ring on the aurone scaffold afforded two compounds with an EC 50 of 34 µM against S. mansoni worms. 104 especially appeared as promising, due to its lack of general cytotoxicity against macrophages (CC 50 >150 µM). 153 In addition, the thiophene counterpart 105 killed about 50% of S. mansani adult worms at a concentration of 25 µM, although no EC 50 value was measured. The compound was still non-cytotoxic (CC 50 >150 µM). 154 Antiviral activities. The recent (and still ongoing) COVID-19 pandemic has underlined the crucial importance of specific antiviral development, as the efficacy of vaccines globally appeared as suboptimal, an observation that also applies to some other viral infections. Indeed, new antiviral agents are still needed for a wide range of pathologies in order to reach a decent treatment response.

In the hemiindigoid chemical space, aurone analogues were discovered as anti-influenza A virus (IAV) agents. First, derivatives of usnic acid were evaluated for their ability to inhibit the viral reproduction. Among more than forty tested molecules, aurone-usnic acid hybrids were found as potent compounds, especially 107 (Figure 19) which was one of the best antiviral agents of the series (ED 50 = 4.7 µM). The contribution of the aurone scaffold was clearly visible upon comparison with usnic acid itself (ED 50 = 52 µM). However, it revealed a quite high level of cytotoxicity, leading to an insufficient selectivity index (8.3). The four aurones tested afforded similar results. 156 More classical aurones delivered far more efficient activities in a further study. Surprisingly, while initially developed for targeting IAV neuraminidase, the compounds exhibited only moderate action against the enzyme (IC 50 = 37-45 µM) but managed to induce significant viral reduction at concentrations one thousand times lower (EC 50 = 30-50 nM). Although less active than flavone counterparts, compound 108 provided a very potent profile by combining an exquisite anti-IAV activity (EC 50 = 35 nM) with a good therapeutic index value of ~5000. 157 An additional study tried to retrieve IAV neuraminidase inhibitors from a docking-based virtual screening. The approach was overall successful, as several aurone analogues presented IC 50 values below 50 nM in the series. Compound 109 was the most active aurone found, showing a higher affinity for IAV neuraminidase (IC 50 = 20 nM) than the reference oseltamivir carboxylate (IC 50 = 40 nM). 158 Some aurones were also tested in anti-HIV assays. First, Gao et al. performed an antiviral evaluation in their phytochemical report involving five natural aurone analogues. Among them, compound 50 (Figure 12) showed an interesting effect (EC 50 = 2.4 µM) while being devoid of cytotoxicity at 200 µM, leading to a selectivity index >82. Maritimetin (Figure 2) was also moderately active (EC 50 = 12 µM), but in the study the recorded values were globally far higher than the reference azidothymidine. [START_REF] Gao | Aurones and isoaurones from the flowers of Rosa damascene and their biological activities[END_REF] Variously substituted synthetic aurones showed very weak efficacy at inhibiting HIV-1 infection at 10 µM in another report, especially when compared with more active analogous chalcones and flavonols. 159 Regarding hepatitis viruses, a unique aurone compound was tested as an erythrocentaurin derivative against hepatitis B virus, with only millimolar activity. 160 Other series of analogues tested against hepatitis C were more successful.

Indolaurones showed especially good activities against hepatitis C virus (HCV) RNA-dependent RNA polymerase NS5B, reminiscently of a previous work. [START_REF] Haudecoeur | Discovery of naturally occurring aurones that are potent allosteric inhibitors of hepatitis C virus RNA-dependent RNA polymerase[END_REF] Interestingly, the substitution of the indole part appeared as tolerant in terms of size and position, as most of the designed compounds fell into the similar activity range (IC 50 = 2.2-8 µM). Compound 110 was one of the best inhibitors of NS5B, with an IC 50 value of 2.3 µM. 161 In addition, dimeric aurones were developed, leading to potent compounds with micromolar activity against HCV NS5B. Compound 111 showed a promising profile (IC 50 = 1.3 µM), and was associated with a probable binding at the palm pocket I of the enzyme, as testified by mutagenesis experiments (an activity drop was recorded against the H95Q mutant). However, only low effects were seen in a J6/JFH1 cell infection model (EC 50 = 200 µM for 111, 42 µM for the best compound). 162 A last study about the antiviral potential of aurones focused on several flavonoids from a phytochemical work. Among them, sulfuretin (Figure 2) was found as a very weak inhibitor of two fish viruses, infectious hematopoietic necrosis virus and viral hemorrhagic septicemia virus, with activity values in the hundred micromolar range.

Anti-inflammatory and antioxidant activities.

Inflammation and oxidative stress from various sources are responsible for many chronic pathologies. In this context, the control of inflammation could be achieved through the suppression of pro-inflammatory cytokines production, such as tumor necrosis factor-alpha (TNF-α). The targeting of intracellular signaling pathways regulating cytokine expression appeared as a promising strategy. The nuclear factor kappa B (NF-κB) is directly involved in immune response and inflammation-related proteins regulation. A series of aurones managed to inhibit canonical NF-κB signaling by attenuating IκB kinase (IKK) activity, resulting in a lower TNF-α expression in macrophages. In the case of aurone 112 (Figure 20), this expression was halved in presence of 40 µM of compound, and a concomitant reduction of the nitric oxide synthase (iNOS) level was observed (~30% at 50 µM). 164 A similar anti-inflammatory effect was recorded from aureusidin (Figure 2) at similar concentrations in a lipopolysaccharide (LPS)-induced mouse macrophage model, through inhibition of NF-κB nuclear translocation and activation of MAPKs-and ROSdependent Nrf2/HO-1 signaling pathways. 165 Damaurone D (Figure 2) was also found as an inhibitor of the expression of LPS-stimulated pro-inflammatory enzymes iNOS and cyclooxygenase-2 (COX-2). A significant reduction was reached at concentrations of 10-20 µM.

Although a series of analogues bearing various B-rings was synthesized and evaluated, 85 (Figure 15) remained the most potent derivative in the reduction of nitrite production in LPS-stimulated RAW 264.7 cells (IC 50 = 4 µM vs. >5 µM). 166,167 An indanone series was investigated for their interleukin-5 (IL-5) inhibition properties. IL-5 is a cytokine produced by Th2 lymphocytes, involved in the immune and inflammatory response. Among the tested compounds, hydrogenated analogues without the exocyclic double bond were more promising, and analogues with a proper indanone scaffold were globally very weak inhibitors. Compound 113, the best molecule among them, only showed 47% of IL-5 activity inhibition at 30 µM, in contrast to 100% inhibition for its reduced counterpart. 168 Direct enzymatic inhibition is another anti-inflammatory strategy. Indeed, numerous enzymes are involved in the inflammation process, especially in the arachidonic acid metabolism. A report especially focused on the inhibition of phospholipase A 2 (PLA2), cyclooxygenases (COXs) and lipoxygenases (LOXs) by various α,β-unsaturated carbonyls. Among them, three indanone derivatives exhibited moderate activities. Compound 114 especially displayed IC 50 values of 37 µM, 2 µM and 19 µM against PLA2, COX-1 and COX-2, respectively, laying the foundations for promising multitarget anti-inflammatory molecule. 169 In addition, 114 was found as a potential immunosuppressive agent, able to inhibit polymorphonuclear neutrophils cell movement (IC 50 = 7 µM) and ROS production. 170 An enhanced ROS level is indeed a factor of inflammation. As the enzyme NADPH oxidase, which catalyzes the production of superoxide ions, plays a crucial role in the modulation of the macrophage ROS level, indanones were developed as potential inhibitors of NADPH oxidase. Good inhibition properties were recorded, resulting in lowered ROS levels in a LPS-induced inflammatory macrophage model, especially for compound 115 (IC 50 = 1.33 µM). 171 Activities of hemiindigoid derivatives were also reported in specific pathological inflammatory contexts. Hispidol (Figure 2) was found as a promising agent against inflammatory bowel disease (IBD), a chronic gastrointestinal inflammatory disorder, by inhibiting TNF-α-induced adhesion of monocytes to colon epithelial cells (IC 50 = 0.50 µM), TNFα-induced ROS production and LPS-induced TNF-α expression (reduction of TNF-α to the level of the LPS-untreated control at 10 µM). Interestingly, hispidol caused a significant suppression of TNBS-induced rat colitis, an in vivo animal model of IBD, through oral administration of 30 mg/kg of hispidol. A dose of 300 mg/kg of the positive control sulfasalazine was required for reaching a similar effect. 172 Further developments provided compound 116, which globally shared the potential of hispidol (IC 50 = 2 µM for TNF-α-induced adhesion, and significant suppression of TNBS-induced rat colitis at 25 mg/kg). 173 Indanone 117 managed to inhibit the inflammatory response in the frame of acute lung injury both in vitro and in vivo, by interfering with myeloid differentiation 2 (MD2), an attractive target involved in the production of pro-inflammatory cytokines. 174 Aurone glucoside 118 exhibited anti-inflammatory and antioxidant properties in a Rd10 mouse model of retinitis pigmentosa, protecting the retinal morphology potentially through an activation of p38 pathway. 175 Finally, sulfuretin (Figure 2) showed a beneficial antiinflammatory effect in both atopic dermatitis and rheumatoid arthritis contexts. It induced the suppression of Th2 cell activation at 10-20 µM and provided an improvement of atopic dermatitis symptoms in an in vivo mouse model, with a suppression of the immune response, at these concentrations. 176 Sulfuretin triggered the suppression of TNF-α-induced chemokine production through inhibition of the NF-κB signaling pathway, thereby allowing an in vivo reduction of the paw thickness of mice with collagen-induced arthritis. 177 In addition, sulfuretin caused a decrease of ROS level in human hepatic L02 cells, providing hepatoprotection. 178 Besides, the compound was described as an antioxidant, able to scavenge DPPH (89-91%) and ABTS +• (71%) radicals and to inhibit AAPH-induced lipid peroxidation (53%) at high concentrations (100 µM). 179 Other sporadic weak antioxidant behaviors were reported for hemiindigoids, such as compound 97 (Figure 17) 141 or ferrocenyl derivative 119 180 using DPPH-, ABTS +• -or galvinoxyl-based assays.

Anti-diabetic and anti-adipogenic activities.

The incidence of diabetes and obesity is increasing worldwide, and they now constitute one of the major causes of death in developed countries, while potentially leading to numerous health complications. Several biomolecules are potentially involved in type-II diabetes, making them more or less validated therapeutic targets. Some of them were targeted by hemiindigoid derivatives in the recent years. Dual-specificity protein tyrosine phosphatase localized to mitochondrion 1 (PTPMT1), whose knockdown is known to cause an increase of insulin secretion in β-cells, was thus the target of a structure-based virtual screening. Among a wide diversity of molecules, compound 120 (Figure 21) emerged as one of the fifteen active structures retained, showing an activity in the micromolar range (IC 50 = 12 µM). 181 The inhibition of kinase DRAK2 provided promising results regarding islet survival improvement for type-I and type-II diabetes. In this context, aurones, indanones and related scaffold were developed starting from compound 40 (Figure 10), a moderately active compound (IC 50 = 3.15 µM). The introduction of oxygen-based chemical groups at position 5 appeared as overall beneficial, as testified by compound 121, the best inhibitor of the series (IC 50 = 0.25 µM). Beyond DRAK2 in vitro inhibition, 121 was able to induce a partial and dose-dependent recovering of glucose-stimulated insulin secretion after a palmitateinduced impairment in isolated pancreatic islets. 182 α-Glucosidase is another target of interest for antidiabetic strategies. Indeed, the enzyme catalyzes the cleavage of polysaccharides into glucose, and its modulation could thus allow a control of hyperglycemia in the frame of type-II diabetes.

Aurones and benzofuranones showed some activity against this enzyme, with IC 50 values in the micromolar range. Compound 122 was the most active aurone identified in the study (IC 50 = 31 µM). In addition, it promoted glucose consumption in HepG2 cells at 1 µM to a similar level than positive control metformin at 1 mM. 183 The peroxisome proliferator-activated receptor gamma (PPARγ) is involved in the expression of genes regulating lipid management and blood glucose, and was thus targeted by inhibitors for antidiabetic action. While thiazolidinedione derivatives were previously developed, a series of analogous indanones provided valuable results, especially compound 123 which revealed a very good affinity for PPARγ (IC 50 = 0.62 µM). 184 Recently, the protein mitoNEET, a mitochondrial 2Fe-2S iron-sulfur cluster, was discovered along with its important role in both diabetes and obesity processes. Through a screening of mitoNEET potential ligands, three aurones were identified. Compound 124 exhibited a very high affinity for mitoNEET, reaching a K i value of 6 nM. 185 Further studies are required for evaluating the antidiabetic potential of this compound, however. Lastly, sulfuretin (Figure 2) showed anti-adipogenic activities. As obesity is a global health issue and a risk factor for a series of other pathologies, including diabetes and cardiovascular diseases, a pharmacological approach allowing to regulate adipogenesis is necessary for patients with higher body mass index. A high-concentration ( 70

Potential interfering chemical reactivity and properties.

The development of molecular scaffolds dedicated to medicinal applications should avoid to trigger chemical cross-reactivity with various biological elements. In the case of hemiindigoids, the core structure of all aforementioned scaffolds contains an enone-based electrophilic Michael acceptor system, somewhat similar to the closely related chalcones. However, many reports described that the reactivity of this chemical group is particularly low, especially in aurones and hemiindigos, due to the presence of an electrodonor substituent at the α-carbon of the enone that confers to the system a conflicting enamine/enol character. 188,189 Thus, synthetic chemical methods developed for Michael addition on aurones or hemiindigos always involved highly reactive nucleophiles such as N-heterocyclic carbenes. 190 Indeed, among biologically relevant nucleophiles, even electron-rich thiols like glutathione or N-acetylcysteine failed to react with aurones derivatives. 191 Studies are lacking for the two other hemiindigoid scaffolds, but hemithioindigos, and to a lesser extent indanones, should logically undergo similar effects. As a consequence, the impact of hemiindigoids electrophilicity is expected to be minimal in their biological activity, and cross-reactivity with cell nucleophiles does not appear as a major issue. Furthermore, the overall low toxicity of hemiindigoids argue for their stability in physiological conditions. While sporadic cytotoxicity occurs, it usually affects individual compounds among other innocuous, closely-related analogues based on the same scaffold, suggesting an independent mechanism. Nevertheless, if overall hemiindigoids and their substructures have not been flagged in common pan-assay interference compounds (PAINS) libraries, it cannot be excluded that their weak electrophile character could occasionally interfere with reactive nucleophilic species used as bioassay reagents or products. Such a phenomenon was observed, for example, when evaluating aurones as tyrosinase inhibitors, using the highly nucleophilic 3-methyl-2-benzothiazolinone hydrazone (MBTH, Besthorn's hydrazone) as a reagent for quinone capture, 192 leading to the use of alternative methods. 41,[START_REF] Haudecoeur | Investigation of the binding-site homology of mushroom and bacterial tyrosinases by aurones as effectors[END_REF][START_REF] Haudecoeur | 2-Hydroxypyridine-N-oxide-embedded aurones as potent human tyrosinase inhibitors[END_REF] Similarly, a special attention should generally be paid when using a strong nucleophile in an assay mixture. In addition to chemical reactivity, possible PAINS-related interferences in bioassays could originate from compounds intrinsic fluorescence covering the operating wavelengths. 193 Indeed, as described below, hemiindigoids frequently show emission bands between 500 and 600 nm, thus matching output wavelengths in several biochemical assays. One must keep in mind these limitations when biochemically evaluating hemiindigoids, and should implement appropriate control experiments in order to dismiss a potential fluorescence interference.

HEMIINDIGOIDS AS SMART AGENTS FOR BIOLOGICAL AND PHOTOPHARMACOLOGICAL APPLICATIONS

Photophysical properties of hemiindigoids.

Hemiindigoids exhibit special optical properties that are conferred by their intrinsically planar structure and a fully conjugated π-π aromatic system. Thus, beyond their aforementioned activities over several biological targets, the ability of these scaffolds to absorb light can give rise to fluorescence emission 194,195 and, in the case of oxo, aza and thio analogues, to photochromism, [START_REF] Wiedbrauk | Hemithioindigo-an emerging photoswitch[END_REF]196 a marked feature especially for hemiindigos (HI) and hemithioindigos (HTI). These light-induced behaviors make hemiindigoids phototools of interest, and their potential use as fluorescent probes and photochromic units has recently started to be addressed in the photobiology and photopharmacology fields. Their fluorogenic applications in cellular medium are facilitated by their light absorption and intense fluorescence emission in the visible region of the electromagnetic spectrum. They exhibit large Stokes shifts and, generally, possess high fluorescence quantum yields in apolar solvents. In addition, changes in their substitution patterns and push-pull effect modulation are known strategies to further increase the energy gap between absorption and emission and to induce bathochromic shifts. 195,197,198 Most hemiindigoids can promptly cross cell membranes due to their lipophilic character, which enables their delivery to transmembrane and intracellular targets. Another interesting characteristic is their pronounced solvatofluorochromism. 195 Indeed, the ability to generate different fluorescence outputs according to the polarity of the environment is an important property for tracing and imaging biological events. Several aurones were especially reported as promising environment-sensitive probes. One In a similar way, the photochromic properties of HI and HTI, especially attractive for biological applications, can be tuned by solvent polarity. 197,199 Their photoisomerization occurs through the excitation of the π-π* electronic transition band with visible light, where one of the de-excitation pathways leads to the conversion of the thermodynamically stable Z isomer into the metastable E isomer. 194,200 For both scaffolds, this conversion occurs upon irradiation with blue or green light, whereas the backward reaction proceeds either photochemically, within the range of green to red light, or thermally, with thermal half-lives reaching the month to year range for the most stable E analogues (e.g., compounds 126 and 127 in Figure 23). 197,201 HI and HTI are thus positive photochroms, with the E form being bathochromically shifted relative to the Z form. Unlike other classes of photochroms well explored in biology, such as azobenzenes, HI and HTI do not require the use of harmful UV light and show considerable band separation between the two isomers, a characteristic that contributes to the quasi-complete Z-to-E and E-to-Z conversions and the subsequent high photostationary state (PSS) ratios commonly observed for these photochroms. 197,202 This feature opens the possibility to selectively address one of the forms in biological medium, a property that perfectly match the photopharmacology basis that consists in exerting a spatiotemporal control over biological activity using light as a non-invasive stimulus, in an attempt to reduce side-effects of drugs. 203 Thus, on-off switching of activity becomes possible when the biological target discriminates between the two isomeric forms, making these compounds very attractive for the development of drugs with localized and selective action. In addition, the replacement of the phenyl-based B-ring of HTI by a hydrogen-bond-donor pyrrole moiety confers a unique combination of high bistability and exquisite interconversion ratios at operable wavelengths, due to the stabilization of E isomers through hydrogen bonding with the intracyclic ketone (e.g., compound 128 in Figure 23). 202 Similarly, substituting the intracyclic nitrogen atom of the HI scaffold provided analogues with very stable Z and E forms, a consequence of the additional steric hindrance between the N-substituent and the aromatic B-ring in the Z configuration (e.g., compound 129 in Figure 23). 197 These studies yielded photoswitchable systems that would in theory perfectly meet the requirements for in vivo photopharmacological applications. Last but not least, HTI are highly resistant to photofatigue and can undergo many photoisomerization cycles without being degraded. 202,204 The photoisomerization mechanism is well described for the naked

Fluorescent biosensors based on hemiindigoid scaffolds.

Hemiindigoids are solvatochromic dyes. In living systems, environmental changes around these chromophores produced by their interaction with proteins, lipids, metal ions and small molecules often trigger significant shifts in fluorescence emission wavelength and/or intensity. As a general tendency, their fluorescence emission increases with decreasing environmental polarity, a phenomenon that theoretical calculations linked to the existence of a charge transfer character of the excited states, e.g., in the context of aurones and HTI. In polar environments, this HOMO-56 LUMO π-π* transition is stabilized, reducing fluorescence quantum yields. To take profit of these effects, some applications of hemiindigoids as sensing probes have been developed in the last ten years.

First, the fluorescent tracking of proteins, peptides and small molecules constitutes a powerful tool to monitor biochemical pathways and pathological processes. Studies providing proof-of-concept principle for off-on fluorescence of hemiindigoids are available and highlight the increments in fluorescence emission of hemiindigoids in contact with hydrophobic sites of biomolecules, in a concentration-dependent manner. The aforementioned aurone 125 (Figure 22) was able to detect the presence of compound-binding biomolecules such as human serum albumin or poly-L-glutamic acid, triggering a ~25-fold increase of fluorescence upon binding, 195 a result that confirmed an earlier work focusing on other aurone derivatives such as 130 (Figure 24). 194 Lipid sensing was also explored, and aurones were incorporated into L-α-dipalmitoylphosphatidylcholine bilayers.

The authors noticed that the fluorescence emission of aurone 125 increased when the molecule was located inside the liposomes. 195 Applying this principle, Wang et al. developed three different solvatochromic aurones (57, 131, 132, Figures 13 and 24) conjugated with dimethylamino, Nethylcarbazoyl and pyrene moieties to monitor the dynamics between lipid droplets (LD) and mitochondria under oxidative stress conditions. 205 LD are fatty acid storage organelles that participate in energy homeostasis and play a role in oxidative stress regulation. An accumulation of these organelles is observed in pathologies such as cancer and obesity. For the first time, their interaction with mitochondria in the presence of an excess of H 2 O 2 were directly monitored through two-photon fluorescence. In this context, the authors verified that the aurones were able to selectively stain LD inside cells with superior photobleaching resistance than BODIPY 493/503, a common probe widely used for fluorescent staining of lipids. In another study, Nan et al evaluated a series of indanones for their binding affinity and fluorescence staining capacity towards βamyloid plaques in AD. In vitro evaluations using brains from AD patients revealed a high staining profile for compound 84 (Figure 15). In addition, in vivo studies performed with the same molecule over normal mice confirmed its ability to cross the blood brain barrier and to undergo brain uptake followed by fast washout. 118 These results attest for the potential application of indanones as βamyloid plaque probes, and more generally for the appropriate physicochemical profile of hemiindigoids to operate in living environments. In addition, chemical reaction sensing allows to specifically detect small molecules in a complex environment, a desirable tool for early diagnosis and disease monitoring. To enable such applications, the intrinsic solvatochromic properties of hemiindigoids could be mitigated by an appropriate design, in order to provide bright fluorescence in aqueous environments. Indeed, by reducing aromatic aggregation, restraining the donor-acceptor character and allowing the formation of a hydrogen-bond network with the solvent, waterfluorescent indanones were obtained. 206 This concept was explored for cysteine detection. This semi-essential amino acid is involved in several biological processes and is associated with metabolic disorders and increased risk for health problems when present at high levels. Fang et al introduced an acrylate group able to react with cysteine at position 5 of indanone 133 (Figure 24). 207 In the presence of the target, the acrylate group was hydrolyzed to the corresponding phenol 134, which, in turn, possesses higher fluorescence quantum yield (0.73) compared to the acrylate (0.13).

Therefore, increments in fluorescence intensity, generated by the formation of 5-hydroxyindanone, were quantitatively correlated to cysteine concentration in the medium. In some cases, even antisolvatochromic behaviors were observed. For example, 135 was able to selectively detect H 2 S, a small molecule generating during the cirrhosis process, through the reduction of the azido group to amino (136), which triggers an increase of the quantum yield, allowing two-photon fluorescence imaging of liver cells (Figure 24). 206 The aminoindanone showed remarkable anti-solvatochromic properties, as the quantum yield increased with the solvent polarity (0.

Biological applications of photoswitchable hemiindigoids.

A Z/E photoisomerization triggers geometric changes immediately accompanied by changes in electronic and physicochemical properties of hemiindigoid photochroms (photoswitches), such as dipolar moment and solubility. As the interactions between small molecules and biological targets can be quite sensitive to minimal structural modifications, photoswitches find a direct application in the development of photocontrolable drugs and probes. Indeed, selectivity constitutes a crucial feature in the development of new drugs and is also a main axe of photopharmacology, where it is directly related to modifications in pharmacokinetic and pharmacodynamic parameters undergone by photochroms upon switching. To date, photopharmacology has been dominated by azobenzenes, but HI and HTI recently started to find their way in this field, with promising results. It is worth noting that these molecules are closely related to aurones and indanones. These last two classes convey numerous biological activities, as previously described in this review, and could guide the elucidation of the biological potential of HI and HTI, still far from being completely known. In the context of photopharmacology, a special interest should be directed to skin and eye-related activities, where light in the blue-to-red range can easily penetrate; but with adaptations, a wider range of biological tissues could be targeted such as the gastrointestinal tract, the throat, the respiratory system and even inner organs. 208 Few examples of HI/HTI photoswitches dedicated to biological applications are available in the literature. Indeed, the scaffolds initially experienced some drawbacks in aqueous media, such as intrinsic low solubility, loss of photoswitching efficiency and reduced thermal stability in water. However recent reports tended to mitigate these issues by providing insightful results. In a study from Berdnikova, new HI with photoswitching properties in water or water-containing medium were identified and their ability to bind HIV-1 RNA was investigated. 209,210 Except for 126, all derivatives presented overall good solubility and ability to isomerize in water or water/co-solvent mixtures upon irradiation at 470 nm for the forward reaction, and 590 nm for the backward reaction. Compound 137 was able to switch in pure water and to associate with HIV-1 RNA with high affinity and photoswitchable fluorescence (Figure 25). an operable activity gap between the Z and E forms (a ~30-fold difference). Indeed, even if both isomers of HTI, and more generally of hemiindigoids, could appear as superimposable, with a lower geometrical change as compared to azobenzenes, an appropriate incorporation of substituents at some key positions (e.g., positions 4 and 7) should be sufficient to break the apparent symmetry.

More recently, HTI 139 was reported as a potent antimitotic in cellulo. 216 Its structure was inspired from colchicine and several indanones previously reported as inhibitors of tubulin polymerization.

Initially intended as a switch-on inhibitor, 139 was finally found more active in its 'dark' form (mainly Z configuration, EC 50 = 1 µM) than in its 'lit' form (mainly E configuration, EC 50 = 4 µM).

However, the study highlighted for the first time the potential of HTI to switch upon irradiation in a cell-based context. Additional insights were provided by compound 140, a close analogue of 139 with a similar activity profile ('lit'/'dark' EC 50 ratio of 4.1). 217 Whereas no photoswitching was observed in aqueous medium, an unequivocal activity drop was measured in an antimitotic cellular assay following sample irradiation at 450 nm (Figure 26). The authors hypothesized that photoswitching can occur in intracellular medium, where water-excluded lipidic membranes and vesicles are present and could favor the photochromic behavior of the HTI inhibitor, even if interconversion fails in pure water. The exact mechanism that explains this difference in activity is uncertain, but it opens new avenues for biological applications of hemiindigoids that did not show optimal photoswitching in aqueous environments. However, compound 140 suffered from its polar protic chemical group at position 4' which led to a far less stable E isomer, reminiscently of azobenzenes. Finally, replacing the B-ring by a pyrrole counterpart afforded a series of alternative tubulin ligands. Among them, compound 141 (Figure 25) showed a reverse selective antimitotic activity: the 'lit' form, mainly E-141, was found more active than the 'dark' form, mainly Z-141

(IC 50 = 4.5 µM vs. >50 µM, respectively). 218 Cell-precise photoisomerization of this compound allowed to block microtubule dynamics only in selected cells with temporal precision, a result that puts HTI and hemiindigoids on the exciting path of in vivo photopharmacological applications.

CONCLUSION AND PROSPECTS

Hemiindigoids have attracted attention in medicinal chemistry within the past ten years, with multiple and increasingly reported biological activities reports in various therapeutic fields, including anticancer, anti-infective, neurochemical or anti-inflammatory. In the frame of cancer, aurone derivatives were especially found as low nanomolar inhibitors of two emerging targets, PARP-1 and PIM1 kinase, and their interaction patterns were rationalized through the resolution of crystal structure of ligand-protein complexes. Indanocine and its indanone-based analogues were developed as colchicine alternatives, for their potent antimitotic action. Acetylcholinesterase inhibition was the most explored neurochemical action, with the aim to transpose donepezil activity in aurone and indanone scaffolds, sometimes with very promising results. In addition, structurally-diverse hemiindigoid compounds were identified for their anti-infective effect against a large set of pathogenic bacteria, fungi, parasites and virus, and sometimes against resistant strains. Thus, a large ensemble of various biological activities appears reachable by an appropriate substitution and/or B-ring heterocycle introduction, even if the recorded effects often remain to be confirmed by in vivo studies involving appropriate selectivity, toxicity and pharmacokinetics assessments.

Overall, hemiindigoids are especially engaging for the rapid access they offer to a full range of substituents at both aromatic rings, and for the facile assembly of the aurone, indanone, HI and HTI scaffolds themselves. Their flatness also enables a precise control of their geometry and interaction patterns inside protein pockets. With the recent rise of photochemical studies dedicated to HI and HTI photochromic properties and early "proof-of-concept" uses in photopharmacological applications, a new avenue is opening up that endorses further investigations about hemiindigoid potential as biological photoswitches. Indeed, hemiindigoids bring a set of advantageous properties for medicinal applications, first and foremost their possibility of thermal bistability and nearquantitative interconversion using visible light. Yet, to date, most examples of bioactive hemiindigoids still rely on the more explored aurone and indanone backbones. HI and HTI still received less attention, but they now appear as light-controllable counterparts whose close structural analogy with aurones and indanones could orientate the elucidation of their biological potential for more ambitious applications in photopharmacology. The replacement of existing bioactive aurones and indanones by HI or HTI versions could even represent a promising strategy for turning simple medicinal agents into photoswitchable entities, able to deliver their activity with a high spatiotemporal precision. As many of the biological effects described within this review could easily trigger undesirable events (e.g., off-target cytotoxic activities, rise of resistances to antibacterial action, collateral neurochemical modifications), the possibility to "turn-off" the activity on demand is often highly attractive. Early results drawn from existing studies tended to demonstrate that the HI/HTI scaffolds are fully compatible with such an objective. While they share some drawbacks with other known scaffolds, the first reported examples have shown that these issues could be circumvented through a careful molecular design. First, despite the apparent symmetry between the Z and E isomers, an appropriate substitution pattern (i.e., substitution at position 7) successfully led to an important (~30-fold) discrimination between their respective activities in the case of compound 138. 215 Second, while several examples of HI/HTI photoswitches in water were reported, their conversion in aqueous environments is often challenging. However, even compounds containing strong protic electron donor substituents at position 4', known to severely jeopardize photoswitching in water and photostability of the E isomer, showed good conversions in cellular contexts, possibly relying on cell water-excluded microenvironments. 217 Third, optimal interconversion wavelengths could be easily tuned by molecular design adjustments, leading to a wide set of possibilities covering the entire visible spectrum, ranging from ~400 to ~600 nm for Z-to-E conversion, and from ~500 to ~750 nm for E-to-Z conversion. These properties were exploited for the conception of practicable HTI photoswitching agents, optimized to be fully compatible with the limited set of fixed-wavelength lasers available in biology, thereby addressing a common issue met with more classical scaffolds like azobenzenes. 218 To date, antimitotic agents

Figure 1 .

 1 Figure 1. Structures of various indigoids and hemiindigoids.

Figure 2 .

 2 Figure 2. Structures of the best-known naturally occurring aurones.

a

  Reagents and conditions: (a) arylaldehyde, KOH (10-50%), MeOH, H 2 O; (b) arylaldehyde, AcOH, HCl, H 2 O; (c) arylaldehyde, piperidine (cat.), EtOH; (d) arylaldehyde, Al 2 O 3 (neutral), CH 2 Cl 2 ; (e) arylaldehyde, H 2 O; (f) arylaldehyde, NaOH, MeOH, H 2 O; (g) Hg(OAc) 2 , DMSO or pyridine; (h) S 8 , DMSO, Et 3 N.

Figure 3 .

 3 Figure 3. Structures and activities of tubulin polymerization inhibitors.

Figure 4 .

 4 Figure 4. Structures and activities of collateral sensitivity inducers.

Figure 5 .Figure 6 .

 56 Figure 5. Structures and activities of PARP inhibitors.

47

 47

Figure 7 .

 7 Figure 7. Structures and activities of tyrosinase inhibitors.

Figure 8 .

 8 Figure 8. Structures and activities of PIM inhibitors.

Figure 9 .

 9 Figure 9. Compared crystal structures of 31 (A, gold, PDB code 3UMX), 32 (B, cyan, PDB code

Figure 10 .

 10 Figure 10. Structures and activities of inhibitors of other cancer-related targets.

Figure 11 .

 11 Figure 11. Crystal structure of 42 in complex with CK2 (PDB code 3WIL).

Figure 12 .

 12 Figure 12. Structures and activities of cytotoxic and antiproliferative compounds.

  ) are responsible for the oxidation of various neurotransmitters, such as dopamine or 5hydroxytryptamine. In the AD context, MAO are involved in the neurodegeneration process via oxidative stress and Aβ indirect production, and MAO inhibitors have shown beneficial effect on cognitive function for AD patients. Selective anti-MAO agents are thus needed, and hemiindigoids have displayed promising profiles. 4'-Dialkylamino indanones were first identified through a multitarget study involving the development of agents with both MAO inhibition, anti-aggregation, antioxidant and metal chelation properties. In this context, only low micromolar inhibitors were discovered. Compound 56 (Figure 13) shared a very similar profile with analogues bearing variously substituted 4'-amino substituents (IC 50 = 35.2-41 µM against MAO-A and 7.5-10.9 µM against MAO-B for 56 and its N-dimethyl, N-diethyl and N-methylethyl derivatives), indicating a good tolerance to alkyl groups length in that position. Replacing the 5-hydroxy group of 56 by a methoxy led to a complete loss of anti-MAO-A activity, but did not significantly change the anti-MAO-B potential (IC 50 = 19 µM)

Anthracenyl derivative 59 (

 59 K i = 0.11 µM and 0.20 µM against MAO-A and MAO-B, respectively)[START_REF] Badavath | synthesis and MAO inhibitory activity of 2-(arylmethylidene)-2,3-dihydro-1-benzofuran-3-one derivatives[END_REF] and furanyl indanone 60 (IC 50 = 0.18 µM and 0.04 µM against MAO-A and MAO-B, respectively)[START_REF] Nel | 2-Heteroarylidene-1-indanone derivatives as inhibitors of monoamine oxidase[END_REF] were overall the best anti-MAO agents among them. Modulation efforts around the indanone scaffold provided another series of compounds variously substituted at the B-ring. If the indanone counterpart of hispidol was less active against MAO-A (IC 50 = 1.67 µM), several analogues reached inhibition values below 200 nM. In addition, compound 61, among others, showed an impressive effect on MAO-B inhibition (IC 50 = 5 nM) together with a good MAO-B vs.

Figure 13 .

 13 Figure 13. Structures and activities of MAO inhibitors.

Figure 14 .

 14 Figure 14. Structures and activities of AChE and BuChE inhibitors.

  130

Figure 15 .

 15 Figure 15. Structures and activities of compounds with various neurochemical activities.

  139

Figure 16 .

 16 Figure 16. Structures and activities of anti-bacterial compounds.

  142

Figure 17 .

 17 Figure 17. Structures and activities of antifungal compounds.

Figure 18 .

 18 Figure 18. Structures and activities of anti-parasite compounds.

  163

Figure 19 .

 19 Figure 19. Structures and activities of antiviral compounds.

Figure 20 .

 20 Figure 20. Structures and activities of anti-inflammatory and antioxidant compounds.

Figure 21 .

 21 Figure 21. Structures and activities of anti-diabetic and anti-adipogenic compounds.

Figure 22 .

 22 Figure 22. Structure and photophysical properties of compound 125.

Figure 23 .

 23 Figure 23. Examples of HI/HTI-based photoswitchable compounds in fundamental studies.

Figure 24 .

 24 Figure 24. Examples of aurone/indanone-based fluorogenic probes.

  photophysical properties remains challenging, and most of the existing examples involve the HTI

Figure 25 .

 25 Figure 25. Structures of HI/HTI-based photoswitchable compounds developed for biological and

Figure 26 .

 26 Figure 26. Confocal microscopy images of immunofluorescently labelled microtubule networks with or

139- 141

 141 photoswitchable agents able to address cutaneous T-cell lymphoma, a 'class 1' target, directly

  The most promising compounds bore NEt 2 , NMePr and NPr 2 groups, showing IC 50 values in the 15-20 nM range against eeAChE, together with 75-125 selectivity ratios. Assays involving human AChE resulted in an activity drop to IC 50 = 400-1,000 nM however.[START_REF] Huang | Discovery of indanone derivatives as multi-targetdirected ligands against Alzheimer's disease[END_REF] Moving the alkylpiperidine moiety at position 6 of an aurone series allowed to partially retain the AChE inhibition potency. Only few compounds were found as submicromolar inhibitors. Among them, compound 70 and its alkylpyrrolidine analogue provided the best measured effects (IC 50 = 0.40 µM and 0.75 µM respectively). Again, introducing a (CH 2 ) 3 linker was overall slightly detrimental.100 Actually, the position of the alkylamino group seemed to exert only a limited impact on the activity. A further study proposed to compare several aurone series, bearing the alkylamino group either at position 6 or 4'. While the latter substitution afforded the best compounds, such as 71 (IC 50 = 0.16 µM and 7.9 µM against AChE and BuChE, respectively), the inhibition values did not differ greatly from the 6-substituted analogues, which provided results in the same range than 70. 101 Similarly, swapping to an indanone scaffold did not cause a significant change in the inhibition potency. As an illustration, compound 72, which resembles to a reverse indanone-based 70, gave nearly identical AChE inhibition values (IC 50 = 0.3 µM). Finally, sporadic studies reported hemiindigoid analogues as AChE/BuChE inhibitors. If the aurone series of Mughal et al. was almost inactive, 107 the 2'-(2aminoquinoline)indanone 76 was associated with an inhibition value in the nanomolar range (IC 50 = 15 nM against AChE), with a predicted high ability to cross the blood-brain barrier. 108 More recently, indanones bearing phenylcarbamate groups at position 3' were evaluated, yielded inhibitors with reversed selectivity, such as compound 77 (IC 50 = 21 µM vs. 0.31 µM against AChE and BuChE, respectively, a selectivity ratio of ~70). 109

	Journal of Medicinal Chemistry	Page 30 of 96
	IC 50 value of 25 nM against AChE (IC 50 = 2.43 µM against BuChE, a ~100 ratio). 105 Positively
	charged piperazinium derivatives were recently reported, leading to a globally weaker inhibition
	effect against AChE, but with a very variable action against BuChE. If compound 75 displayed a
	similar level of activity against AChE and BuChE (IC 50 = 0.32 µM and 0.43 µM, respectively), the
	allylpiperazinium analogue showed a far greater gap between IC 50 values (IC 50 = 0.43 µM against
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These observations

were confirmed by another study, which thus focused on the modification of the B-ring of indanones. Various amino substituents were introduced at position 4', leading to activities in the low nanomolar range, but with no clear improvement in terms of anti-AChE effect or AChE-vs.-BuChE selectivity. 102 However, shortening the linker between the B-ring and the piperidine moiety resulted in the production of aurone 73, with a far better AChE inhibition efficacy (IC 50 = 8.8 nM against eeAChE, 37 nM against the human enzyme), a remarkable exception among several other amino analogues which provided little or no improvement of the previously described activities.

103

Another series of 4'-substituted indanones was first reported in 2016, bearing this time amino groups directly bound to the B-ring. The results were overall underwhelming, with IC 50 values around ten times higher than 71 and 72.

104

However, a further study proposed to persevere with a series of indanones with directly bound extended piperazinyl substituents at position 4'. The other amino group of these piperazines was systematically functionalized with aromatic substituents, yielding significantly longer compounds with a far more promising activity. Analogue 74, bearing a terminal pyrimidine group, reached an AChE vs. 25 µM against BuChE, a ~60 ratio). Hence, these results demonstrated that this series of compounds enables a fine-tuned management of the AChE-vs.-BuChE selectivity, depending on the desired applications.

106

  Candida species. Among several phenyl-based and heterocyclic B-rings, compounds 95 and 96 (Figure17) were identified as the most potent inhibitors, with IC 50 values between 10 µM and 18 µM against three species: C. albicans, C. glabrata and C. tropicalis. Compound 95 was able to prevent C. albicans biofilm formation at these concentrations. 140 Furanoindanones were also evaluated with various heterocyclic B-rings, but the MIC values obtained were globally underwhelming. One of the best compounds, 97, inhibited Rhizophus oryzae, Mucor mucido, A.

Compound 89 showed a weaker profile, with MIC values of

18-35 µM against A. niger and C. albicans, 134

and 90 inhibited A. niger, F. oxysporum and Penicillum italicum with promising inhibition zone values similar than griseofulvin.

135

However, several hemiindigoid molecules were discovered through dedicated antifungal studies. A series of substituted aurones was evaluated against niger and C. albicans only at concentrations of 60-120 µM. The activities observed were far weaker than the reference fluconazole.

141

Several polyhydroxylated aurones were found to target specific RNA structures, the group II intron, absent in vertebrates but crucial for plants and fungi.

  80 in PBS buffer, 0.53 in EtOH, 0.12 in MeCN and 0.02 in CH 2 Cl 2 ). Overall, these promising results offer a glimpse of the versatility of hemiindigoids and could endorse future research about applications of the scaffolds as biosensors.

			NH 2
					N
	O			O	O
	O			O	O
	130  abs 430 nm (H 2 O)  em 560 nm (H 2 O)  F 0.035 (dioxane)	131  abs 457 nm (DMSO)  em 552 nm (DMSO)	132  abs 437 nm (DMSO)  em 549 nm (DMSO)
	O	Cys	HO	N 3	H 2 S	H 2 N
	O
	O			O	O	O
	133  F 0.13 (PBS)		134  abs 400 nm (PBS)  em 505 nm (PBS)  F 0.73 (PBS)	135 negligible fluorescence	136  abs 375 nm (PBS)  em 498 nm (PBS)  F 0.80 (PBS)

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ACKNOWLEDGEMENTS

The authors are grateful to ANR ("Agence Nationale de la Recherche") for financial support through Labex ARCANE and CBH-EUR-GS (ANR-17-EURE-0003) and through ANR project EPIDERMIS (ANR-19-CE44-0002). The authors thank Labex ARCANE for the fellowship grant of L.M.L in the frame of project SHAPESHIFT.

ABBREVIATIONS USED

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