Thomas Cheviet

Isabelle Lefebvre-Tournier

Sharon Wein

Suzanne Peyrottes

PLASMODIUM PURINE METABOLISM AND ITS INHIBITION BY NUCLEOSIDE AND NUCLEOTIDE ANALOGUES

Malaria still affects around 200 million people and is responsible for more than 400,000 deaths per year, mostly children in subequatorial areas. This disease is caused by parasites of the Plasmodium genus.

Only a few WHO-recommended treatments are available to prevent or cure plasmodial infections, but genetic mutations in the causal parasites have led to onset of resistance against all commercial antimalarial drugs. New drugs and targets are being investigated to cope with this emerging problem, including enzymes belonging to the main metabolic pathways, while nucleoside and nucleotide analogues are also a promising class of potential drugs. This review highlights the main metabolic pathways targeted for the development of potential antiplasmodial therapies based on nucleos(t)ide analogues, as well as the different series of purine-containing nucleoside and nucleotide derivatives designed to inhibit Plasmodium falciparum purine metabolism.

INTRODUCTION

WHO, in 2018, reported more than 435,000 deaths associated with malaria, and at least 219 million people-mostly children-are still affected by this disease in 91 countries located in Africa and Asia. 1 The disease, which represents an endemic in tropical areas, is caused by protozoal parasites of the Plasmodium genus in the Apicomplexa phylum. Among the six human Plasmodium species (P. falciparum, P. vivax, P. malariae, P. ovale curtisi, P. ovale wallikeri [START_REF] Sutherland | Two Nonrecombining Sympatric Forms of the Human Malaria Parasite Plasmodium Ovale Occur Globally[END_REF][START_REF] Calderaro | Accurate Identification of the Six Human Plasmodium Spp. Causing Imported Malaria, Including Plasmodium Ovale Wallikeri and Plasmodium Knowlesi[END_REF] and P. knowlesi originally known to cause simian malaria), P. falciparum is the most widespread and lethal. The parasite is transmitted to humans by female mosquitos of the Anopheles genus (Figure 1). Plasmodium begins its development cycle in humans by multiplication in hepatocytes. After 10-12 days, the parasites are released in the blood and invade the erythrocytes (asexual multiplication). This intraerythrocytic phase constitutes the symptomatic phase of the disease, with massive destruction of erythrocytes and sometimes adherence to blood vessels of large organs like the brain, thus restricting the blood flow with serious consequences. A small proportion of the blood stages develop into sexual forms (so-called gametocytes) that are ingested by a feeding mosquito, therefore completing the cycle. Parasites of the P. vivax and P. ovale species can persist for years as dormant stages in the liver (so-called hypnozoites) and can cause relapses.

Figure 1 : Plasmodium falciparum life-cycle

Since the historical use of quinine 400 years ago, particularly as of the late 19 th century, several treatments of synthetic or natural origins have been developed (Figure 2) and have been successfully used to cure malarial diseases in infected human hosts. The various classes of antimalarial drugs belong to a few families according to their chemical structures, such as: aryl amino alcohols (quinine, mefloquine, halofantrine, etc.); 4-aminoquinolines (chloroquine, amodiaquine, piperaquine, etc.); 8-aminoquinolines (primaquine, tafenoquine), antifolates (proguanyl, pyrimethamine, sulfadoxine, dapsone, etc.); naphthoquinones (atovaquone); peroxides and sesquiterpene derivatives (artemisinin, artesunate, artemether, etc.); antimicrobials (tetracycline, azithromycin, fluoroquinolones, etc.). In addition to chemotherapy, an efficient vaccine would be a desirable tool to control and eventually eradicate malaria, but such a vaccine has yet to be validated despite clinical assays. [START_REF] Coelho | Advances in Malaria Vaccine Development: Report from the 2017 Malaria Vaccine Symposium[END_REF][START_REF] Matuschewski | Vaccines against Malaria-Still a Long Way to Go[END_REF]

Figure 2: Timeline of main antimalarials and occurring resistance

Blue pills: first used as antimalarial; red circles: first reported resistance; black circles: no longer marketed AS/AQ, artesunate/amodiaquine; DHA-PPQ, dihydroartemisinin-piperaquine; A/P, atovaquone/proguanil; A/L, artemether/lumefantrine; ASSP, artesunate sufadoxine pyrimethamine; S/P, sulfadoxine/pyrimethamine; AS/MQ, arestunate/mefloquine; ACTs, artemisinin combination therapies; and C/D, chlorproguanil/dapsone. Mutations in the parasite genome have appeared over the last century, inducing fast spreading resistance against all commercial drugs (Figure 2). To address these issues, and in the absence of an available efficient vaccine, new drugs with original modes of action are crucial as resistance to front-line antimalarial drugs (essentially artemisinin-based combination therapies, ACTs) is spreading in Southeast Asia. A number of collaborative drug discovery programs have thus been initiated worldwide and a few drug candidates are currently under clinical development or have recently been approved for clinical use.

Among them, most of the new drugs correspond to compounds that are structurally related to previously used drugs or artemisinin-based combinations, and some molecules with a novel mechanism of action have been proposed. To our knowledge, none of them belong to the nucleos(t)ide analogue family, targeting proteins involved in purine or pyrimidine metabolism, except DSM265 (currently in phase IIa clinical trials). [START_REF] Llanos-Cuentas | Antimalarial Activity of Single-Dose DSM265, a Novel Plasmodium Dihydroorotate Dehydrogenase Inhibitor, in Patients with Uncomplicated Plasmodium Falciparum or Plasmodium Vivax Malaria Infection: A Proof-of-Concept, Open-Label, Phase 2a Study[END_REF] The latter is a selective inhibitor of Plasmodium dihydroorotate dehydrogenase (PfDHODH), which plays a central role within the de novo pyrimidine pathway. However, target mutations were found in patient's parasites (already observed in earlier in vitro studies) and concerned a few patients with recurrence at day 28 after receiving a single oral dose of DSM265. Once again, this data highlights the need to use combination therapies.

Herein, we will focus on nucleobase, nucleoside and nucleotide analogues, which so far have been reported to target and inhibit proteins involved in the purine salvage pathway of Plasmodium falciparum.

This family of derivatives is of special interest as they can mimic the endogenous substrates of these proteins, so the design of analogues and their study may result in a new class of antimalarial drugs. In addition, as Plasmodium falciparum relies solely on the de novo pathway for pyrimidine nucleotide biosynthesis and on the salvage pathway for purine nucleotides, both metabolic processes appear to be attractive chemotherapeutic targets. The development of these analogues as inhibitors of plasmodial purine metabolism is relatively recent and somehow associated with the discovery of the antiviral and antitumoral potential of such compounds. The main nucleic acid biosynthesis pathways of Plasmodium will be described as well as the wide collection of analogues targeting the corresponding proteins.

I/ NUCLEIC ACID METABOLISM IN PLASMODIUM FALCIPARUM

Mammalian cells are able to synthesize nucleic acids from small molecules through the de novo pathway but requiring 6 ATP equivalents for the synthesis of IMP from ribose-5-phosphate and a dozen steps. In contrast, the salvage pathway is based on the recycling of related metabolites, which is less energy-demanding (a single step from hypoxanthine to IMP and 1 ATP equivalent is required for phosphoribosylpyrophosphate formation). The human host has both pathways to produce purine and pyrimidine nucleotides, whereas protozoan parasites such as Plasmodium only possess the salvage pathway for purine containing derivatives and de novo synthesis of pyrimidine derivatives. The latter are therefore essential for parasite growth, which requires a huge amount of purine and pyrimidine nucleoside triphosphates for its nucleic acid biosynthesis. A few reviews have provided detailed accounts of purine and pyrimidine pathways as chemotherapeutic targets. [START_REF] Cassera | Purine and Pyrimidine Pathways as Targets in Plasmodium Falciparum[END_REF][START_REF] El Kouni | Potential Chemotherapeutic Targets in the Purine Metabolism of Parasites[END_REF][START_REF] Hyde | Targeting Purine and Pyrimidine Metabolism in Human Apicomplexan Parasites[END_REF]

1) Pyrimidine Metabolism

Protozoan parasites rely solely on the de novo pathway to produce their pyrimidine derivatives, and they are not very capable of salvaging pyrimidines from erythrocytes. Briefly, Pf pyrimidine biosynthesis involves six enzymatic steps (Figure 3) as in the human host producing a central metabolite, i.e. uridine-5'-monophosphate (UMP), which is then used as precursor for all pyrimidine nucleotides (dUMP, dCTP, dTTP, UTP and CTP) required for DNA/RNA biosynthesis. The related enzymes can be divided in two groups of monofunctional and multifunctional proteins, i.e. the first and last three, respectively. Little attention has been focused on carbamoyl phosphate synthetase (CPS II) and aspartate transcarbamoylase (ATC), whereas Pf dihydroorotase (DHOase) has received much attention. The latter has been characterized and found to share many properties with mammalian proteins. [START_REF] Krungkrai | Dihydroorotase of Human Malarial Parasite Plasmodium Falciparum Differs from Host Enzyme[END_REF] The three following enzymes, i.e. orotate phosphoribosyltransferase (OPRT), dihydroorotate hydrogenase (DHODH) and dihydroorotase (DHOase), are considered as key enzymes to target the human parasite, and DHODH is one of the most widely studied. [START_REF] Phillips | Plasmodium Dihydroorotate Dehydrogenase: A Promising Target for Novel Anti-Malarial Chemotherapy[END_REF][START_REF] Singh | Dihydroorotate Dehydrogenase: A Drug Target for the Development of Antimalarials[END_REF] Their 3D crystal structures have been solved, their characteristics have been studied in detail and compared to their human counterpart. This review is not intended to provide a detailed account of these proteins and their inhibitors, but these data may be gleaned from the literature [START_REF] Cassera | Purine and Pyrimidine Pathways as Targets in Plasmodium Falciparum[END_REF][START_REF] Krungkrai | Insights into the Pyrimidine Biosynthetic Pathway of Human Malaria Parasite Plasmodium Falciparum as Chemotherapeutic Target[END_REF] Metabolites: UDP, uridine-5'-diphosphate; UTP, uridine-5'-triphosphate; dUDP, 2'-deoxyuridine-5'-diphosphate; dUMP, 2'-deoxyuridine-5'-monophosphate; CTP, cytidine-5'-triphosphate; dTMP, 2'-deoxythymidine-5'monophosphate; dTTP, 2'-deoxythymidine-5'-triphosphate; dCTP, 2'-deoxycytidine-5'-triphosphate; and CoQ, co-enzyme Q.

2/ Purine Metabolism

Plasmodium species are purine auxotroph parasites that depend exclusively on purine materials derived from erythrocytes. Furthermore, as the Plasmodium genome has an extremely high adenine content (over 80% AT content), the parasite requires a considerable amount of adenine nucleotides (dATP and ATP) to maintain its high replication rate. Erythrocytes are unable to perform de novo purine synthesis alone, so the corresponding nucleobases and nucleosides are taken up from the plasma by the erythrocytes and then by the parasites. [START_REF] Quashie | Uptake of Purines in Plasmodium Falciparum-Infected Human Erythrocytes Is Mostly Mediated by the Human Equilibrative Nucleoside Transporter and the Human Facilitative Nucleobase Transporter[END_REF][START_REF] Quashie | A Comprehensive Model of Purine Uptake by the Malaria Parasite Plasmodium Falciparum: Identification of Four Purine Transport Activities in Intraerythrocytic Parasites[END_REF] Purine import from the host cell is mediated by membrane transporters such as equilibrative nucleoside transporters (ENTs, Figure 5), Pf ENT1 is the main one from the four transporters (Pf ENT1-4) reported to date and supplying the purine salvage pathway. [START_REF] Quashie | A Comprehensive Model of Purine Uptake by the Malaria Parasite Plasmodium Falciparum: Identification of Four Purine Transport Activities in Intraerythrocytic Parasites[END_REF][START_REF] Frame | Purine Import into Malaria Parasites as a Target for Antimalarial Drug Development[END_REF] Although the transport of purine and pyrimidine derivatives in mammalian cells is well-documented, [START_REF] De Koning | Nucleobase Transporters (Review)[END_REF][START_REF] Boswell-Casteel | Equilibrative Nucleoside Transporters-A Review[END_REF] few studies have been performed on the parasite transporters in the last decade and mainly concerned Pf ENT1.

Further investigations are thus required for rational design of purine transport inhibitors as antimalarial drugs.

A few of the enzymes involved in Pf purine metabolism have attracted special attention, namely purine phosphoribosyl transferases (PRT), adenosine deaminases (ADA) and purine nucleoside phosphorylases (PNP). Phosphoribosyl transferases induce the ribophosphorylation of purine bases in a single step and are present in many parasites, as well as in mammalian cells. In Plasmodium, hypoxanthine-guanine-xanthine-phosphoribosyl transferase (HGXPRT) converts hypoxanthine, guanine and xanthine into inosine-5'-monophosphate (IMP), guanosine-5'-monophosphate (GMP) and xanthosine-5'-monophosphate (XMP), respectively (Figure 6). Purine nucleoside phosphorylases degrade nucleosides into their corresponding nucleobases and sugar counterparts (Figure 7). Adenosine deaminases convert the exocyclic amino group of purine nucleos(t)ides into their oxo-derivatives (Figure 8). Moreover, HGXPRT, ADA and PNP enzymes are highly expressed and key players in purine metabolism, but other enzymes are also involved, such as guanosine-5'-monophosphate synthase (GMPS), adenylosuccinate synthetase (AdSS), adenosine-5'-monophosphate deaminase (AMPDA) and inosine-5'monophosphate dehydrogenase (IMPDH). This metabolism is remarkable because of the few enzymes involved even though it has a crucial role in parasite survival. In addition, these enzymes are also present in the metabolic pathway of other plasmodial parasites and may have a high degree of similarity. As example PvENT1 shares ∼75% amino acid sequence identity with PfENT1. However, differences between P. vivax and P. falciparum were reported and resulted in modifications of the 3D protein structure and/or conformation. [START_REF] Das | Plasmodium Falciparum and Plasmodium Vivax: So Similar, yet Very Different[END_REF] As example, structural differences between Pv and Pf ADA have been highlighted by computational studies and crystal structure analysis, and mainly concern the ligand recognition domain. [START_REF] Ivanov | The Adenosine Deaminases of Plasmodium Vivax and Plasmodium Falciparum Exhibit Surprising Differences in Ligand Specificity[END_REF] The parasite salvages adenosine from the erythrocytes and the combined action of ADA and PNP successively leads to the formation of inosine and hypoxanthine (Figure 4). As very little adenosine is available from the erythrocyte (due to the activity of erythrocytic adenosine kinases), the parasite essentially relies on hypoxanthine, which is also salvaged from the host. The key purine metabolism precursor is consequently hypoxanthine, and therefore malarial cultures are often enriched with this nucleobase, which serves as a nutrient to promote parasite growth. Hypoxanthine is then converted into IMP by HGXPRT (Figure 4). IMP is thus the nucleotidic precursor of all other purine nucleotides required for nucleic acid biosynthesis, especially adenosine 5'-monophosphate (AMP). As Pf does not have the genes coding for nucleoside kinases [START_REF] Ward | Protein Kinases of the Human Malaria Parasite Plasmodium Falciparum: The Kinome of a Divergent Eukaryote[END_REF][START_REF] Cassera | Erythrocytic Adenosine Monophosphate as an Alternative Purine Source in Plasmodium Falciparum[END_REF] (AMP synthesis directly from adenosine), there is no other alternative to obtain AMP, and IMP is the sole adenosine 5'-monophosphate metabolite available. To a lesser extent, it has been shown that by using high concentrations of adenosine in Pf cultures, AMP synthesized by RBC could induce erythrocytic AMP uptake into the parasite. [START_REF] Cassera | Erythrocytic Adenosine Monophosphate as an Alternative Purine Source in Plasmodium Falciparum[END_REF] IMP is also transformed into XMP via IMPDH and then into GMP via GMPS. GMP is also produced via a second pathway involving guanosine and guanine uptake, with the latter being PNP and HGXPRT substrates, respectively. Similarly, XMP is produced through HGXPRT activity following xanthine uptake. 7 To date, the main strategy developed to inhibit Plasmodium falciparum growth involved targeting PNP and ADA Pf enzymes, because of their importance in the hypoxanthine pathway in the parasite.

Uptake of purine bases and nucleosides into the parasite a. Erythrocyte transport pathways

As for many human cells, mature erythrocytes have a profusion of transport systems (transporters and channels) located at the plasma membrane and responsible for the uptake of ions, hexoses, amino acids and other nutrients (Figure 5). Human nucleoside transporters (hNTs) thus include various transport system families, such as hENT1 and hENT2 (both equilibrative nucleoside transporters and responsible for purine and pyrimidine uptake, with nucleosides being the preferred substrates) [START_REF] Boswell-Casteel | Equilibrative Nucleoside Transporters-A Review[END_REF] , hFNT1 (facilitated nucleobase transporter, which transports hypoxanthine and adenine) and hCNTs (concentrative ioncoupled nucleoside transporters). [START_REF] Frame | Purine Import into Malaria Parasites as a Target for Antimalarial Drug Development[END_REF][START_REF] Quashie | Uptake of Purines in Plasmodium Falciparum-Infected Human Erythrocytes Is Mostly Mediated by the Human Equilibrative Nucleoside Transporter and the Human Facilitative Nucleobase Transporter[END_REF] It has been demonstrated that hENT1 and hFNT1 are the most important source of purine-containing derivatives in Pf-infected RBC, while parasite-induced new permeability pathways (NPP, an alteration in membrane permeability) has also been mentioned. [START_REF] Ginsburg | New Permeability Pathways Induced in Membranes of Plasmodium Falciparum Infected Erythrocytes[END_REF][START_REF] Quashie | Uptake of Purines in Plasmodium Falciparum-Infected Human Erythrocytes Is Mostly Mediated by the Human Equilibrative Nucleoside Transporter and the Human Facilitative Nucleobase Transporter[END_REF] NPP are designed to facilitate the uptake of low molecular weight metabolites required for the parasite growth, such as sugar, sugar-alcohols, polyamine, nucleosides, nucleobases, ions, amino-acids [START_REF] Liu | Plasmodium Falciparum Ensures Its Amino Acid Supply with Multiple Acquisition Pathways and Redundant Proteolytic Enzyme Systems[END_REF] , etc. [START_REF] Gero | New Malaria Chemotherapy Developed by Utilization of a Unique Parasite Transport System[END_REF][START_REF] Kirk | Membrane Transport in the Malaria Parasite and Its Host Erythrocyte[END_REF] b. Parasite transporters Four equilibrative nucleoside transporters (Pf ENT1-4) have been identified to date (Figure 5)-but they have yet to be fully characterized-and are expressed during the intraerythrocytic stage of the Plasmodium life cycle. 202 According to genetic studies, Pf ENT1 has only 17% identical sequence homology with hENT1 and only shares 15 to 22% identical amino acid sequence with other parasite ENT transporters, and all four share only 2% similarity between them. [START_REF] Frame | Purine Import into Malaria Parasites as a Target for Antimalarial Drug Development[END_REF] Among them, Pf ENT1 (also called Pf NT1) is mainly responsible for the import of most purine nucleosides and nucleobases inside the parasite. Its characteristics (affinity and substrate specificity) have been the focus of some debate with regard to the model used for transport studies and the effects of purine metabolism upon import. [START_REF] Kirk | Purine Uptake in Plasmodium: Transport versus Metabolism[END_REF] In 2008, Quashie et al. [START_REF] Quashie | A Comprehensive Model of Purine Uptake by the Malaria Parasite Plasmodium Falciparum: Identification of Four Purine Transport Activities in Intraerythrocytic Parasites[END_REF] suggested that hypoxanthine and purine nucleoside uptake was mediated by a high-affinity and broad-specificity transporter, i.e. Pf NT1 (located at the parasite plasma membrane [START_REF] Rager | Localization of the Plasmodium Falciparum PfNT1 Nucleoside Transporter to the Parasite Plasma Membrane[END_REF] ), and the existence of another high-affinity transporter and two low-affinity systems. Downie and co-workers proposed an alternative model, taking into account the fact that highaffinity proteins of purine metabolism readily transform Pf ENT substrates into nucleotides. [START_REF] Kirk | Purine Uptake in Plasmodium: Transport versus Metabolism[END_REF] They thus reported a fast, equilibrative and low-affinity mechanism for Pf NT1. [START_REF] Downie | Purine Salvage Pathways in the Intraerythrocytic Malaria Parasite Plasmodium Falciparum[END_REF] Pf ENT1 is a proton-dependent transporter [START_REF] Hyde | The ENT Family of Eukaryote Nucleoside and Nucleobase Transporters: Recent Advances in the Investigation of Structure/Function Relationships and the Identification of Novel Isoforms[END_REF] accepting inosine, hypoxanthine, guanine, adenosine and guanosine as substrates, but with higher affinities for hypoxanthine and adenosine (Ki 300-700µM range). [START_REF] Riegelhaupt | Transport of Purines and Purine Salvage Pathway Inhibitors by the Plasmodium Falciparum Equilibrative Nucleoside Transporter PfENT1[END_REF] They are reported to be 11 transmembrane helix segment proteins, with intracellular N-ter and extracellular C-ter, which is the common structure for all protozoan and mammalian ENT1. Helices are mostly linked by short hydrophobic connectors, except for two large loops (one extracellular and one intracellular). [START_REF] Sundaram | Topology of a Human Equilibrative, Nitrobenzylthioinosine (NBMPR)-Sensitive Nucleoside Transporter (HENT1) Implicated in the Cellular Uptake of Adenosine and Anti-Cancer Drugs[END_REF] As its human homologue, Pf ENT1 may also accept pyrimidine nucleosides as substrate. [START_REF] Jiang | Potent and Selective Activity of a Combination of Thymidine and 1843U89, a Folate-Based Thymidylate Synthase Inhibitor, against Plasmodium Falciparum[END_REF] Pf ENT2 and ENT3 have received much less attention. [START_REF] Frame | Purine Import into Malaria Parasites as a Target for Antimalarial Drug Development[END_REF] Pf ENT2 was shown to be located at the parasitic endoplasmic reticulum membrane and thus may not be involved in purine uptake. [START_REF] Downie | PfNT2, a Permease of the Equilibrative Nucleoside Transporter Family in the Endoplasmic Reticulum of Plasmodium Falciparum[END_REF] Pf ENT3 has yet to be localized or shown to be involved in purine or pyrimidine transport.

Frame et al. have shown that Pf ENT4 is a low-affinity (in the millimolar range) purine transporter able to transport both nucleosides and bases such as adenine, adenosine and 2'-deoxyadenosine with low affinity. [START_REF] Frame | Malaria Parasite Type 4 Equilibrative Nucleoside Transporters (ENT4) Are Purine Transporters with Distinct Substrate Specificity[END_REF] Pf ENT4 does not transport AMP or hypoxanthine (the main metabolite in the purine pathway), thus presenting a different substrate profile from that of Pf ENT1.

Like Pf, Pv parasites also present four equilibrative nucleoside transporters. Pv ENT1 shares an almost identical amino acid sequence with Pf ENT1, although small differences have been observed with variable affinities of both transporters for the same substrate. [START_REF] Deniskin | Targeting the Plasmodium Vivax Equilibrative Nucleoside Transporter 1 (PvENT1) for Antimalarial Drug Development[END_REF] The substrate and inhibitor specificity of P. berghei ENT1 has been reported recently. PfENT3 is not yet localized; "?" sign, unknown transporters.

HGXPRT

HGXPRT is the most targeted purine metabolism enzyme in P. falciparum due to its major and multiple implications in the metabolism of this parasite (Figure 4). The crucial role of HGXPRT in purine metabolism has also been observed in P. vivax. [START_REF] Keough | Plasmodium Vivax Hypoxanthine-Guanine Phosphoribosyltransferase: A Target for Anti-Malarial Chemotherapy[END_REF] Indeed, it provides the sole PRT activity for three substrates, i.e. converting hypoxanthine, xanthine and guanine into the corresponding nucleoside 5'monophosphates (IMP, GMP and XMP). Briefly, HG(X)PRT facilitates transfer of the phosphoribosyl moiety from PRib-PP (alpha-D-5-phopshoribosyl-1-pyrophosphate) to 6-oxopurines in the presence of divalent metal ions, i.e. usually Mg 2+ in vivo (Figure 6). Reyes Few differences between the human and Pf HGXPRT were revealed by Keough et al. [START_REF] Keough | Purification and Characterization of Plasmodium Falciparum Hypoxanthine-Guanine-Xanthine Phosphoribosyltransferase and Comparison with the Human Enzyme[END_REF] when comparing the specific activity (greater for human enzymes), kinetic parameters and substrate selectivity (the human enzyme does not accept xanthine as substrate, contrary to the parasite protein). Both enzymes are active as a homotetrameric form and share 44% sequence identity and 76% similarity. Superposition of their structures (in the presence of substrates of the catalytic reaction) generates a good root-meansquare deviation (rmsd) value (nearly 1,7 Å). [START_REF] Raman | Plasmodium Falciparum Hypoxanthine Guanine Phosphoribosyltransferase[END_REF] Considering these structural similarities between the two enzymes, the challenge is to selectively target and inhibit the parasite phosphoribosyl transferases so as to avoid any toxicity for human cells.

The crystal structures of the human 6-oxopurinephosphoribosyltransferase (Hs HGPRT) in the presence of nucleoside analogues have been often described. The human enzyme has three important binding sites hosting the nucleobase (through hydrogen bonds with residues), the 5'-phosphate (binding pocket is located between residues 137-141) and the pyrophosphate moiety (of Prib-PP or only PPi).

In humans, PRib-PP first binds to the metal ion and then the required nucleobase associates with the complex to form the intermediate. Then the pyrophosphate dissociates from the ribooxocarbenium complex and nucleoside 5'-monophosphate is released from the enzyme catalytic site. [START_REF] Xu | Kinetic Mechanism of Human Hypoxanthine-Guanine Phosphoribosyltransferase: Rapid Phosphoribosyl Transfer Chemistry[END_REF] The initial proposed mechanism for PfHGXPRT was slightly different and consisted of a ping-pong exchange, where PRib-PP first binds to the enzyme, followed by PPi release and then nucleobase entry [START_REF] Sarkar | Biochemical Characterization of Plasmodium Falciparum Hypoxanthine-Guanine-Xanthine Phosphorybosyltransferase: Role of Histidine Residue in Substrate Selectivity[END_REF] . These results have been contested by the reported crystal structure of PfHGXPRT complexed with hypoxanthine.PPi.Mg 2+ and a few immucillin transition state analogues. 175 It has been shown that nucleobase binding is conditional to PRib-PP entry, suggesting that PfHGXPRT proceeds through a steady-state-ordered mechanism. 176

PNP

PNP are also important enzymes present in both mammals and protozoan parasites. Like HGXPRT, PNP inhibition leads to death of the parasite by blocking the formation of hypoxanthine, a key metabolite of the Pf purine metabolism (Figure 4). [START_REF] Madrid | Plasmodium Falciparum Purine Nucleoside Phosphorylase Is Critical for Viability of Malaria Parasites[END_REF][START_REF] Kicska | Purine-Less Death in Plasmodium Falciparum Induced by Immucillin-H, a Transition State Analogue of Purine Nucleoside Phosphorylase[END_REF] Pf PNP is structurally different from the human protein (hexamer versus homotrimer and low sequence homology, ~20% identity of the primary amino acid sequence) and exhibits different catalytic rates with inosine as substrate (56 s -1 for hPNP and 0.34s -1 for Pf PNP). [START_REF] Kicska | Transition State Analogue Inhibitors of Purine Nucleoside Phosphorylase from Plasmodium Falciparum[END_REF][START_REF] Shi | Plasmodium Falciparum Purine Nucleoside Phosphorylase: Crystal Structures, Immucillin Inhibitors, and Dual Catalytic Function[END_REF][START_REF] Schnick | Structures of Plasmodium Falciparum Purine Nucleoside Phosphorylase Complexed with Sulfate and Its Natural Substrate Inosine[END_REF] [START_REF] Kline | Pre-Steady-State Transition State Analysis of the Hydrolytic Reaction Catalyzed by Purine Nucleoside Phosphorylase[END_REF][START_REF] Lewandowicz | Transition State Analysis for Human and Plasmodium Falciparum Purine Nucleoside Phosphorylases[END_REF] the nucleoside and phosphate (as nucleophilic participant) first bind to the active site of the enzyme and the reaction transforms the inosine/enzyme complex (located on a single active site of the homotrimer), into a hypoxanthine/ribose/enzyme complex (Figure 7). Then the enzyme catalyzes cleavage of the glycosidic bond to release the ribose-1'-phosphorylated moiety, giving rise to a tight binding hypoxanthine/enzyme complex. The latter finally dissociates within the corresponding nucleobase and the D-ribose phosphorylated moiety. Both enzymes have similar transition states, with the active site environment being more constrained in the parasite enzyme than in the human one (on the basis of different interactions between the active site and substrate).

ADA

ADA is a zinc-dependent hydrolase that catalyzes the irreversible hydrolysis of adenosine into inosine, its corresponding 6-oxopurine derivative. The reaction is triggered by the deprotonation of a water molecule which then reacts with the substrate, giving rise to 6-amino-6-hydroxypurine as intermediate, which provides the corresponding oxopurine by releasing one ammonia molecule (Figure 8). [START_REF] Luo | Transition state Variation in Human, Bovine, and Plasmodium Falciparum Adenosine Deaminases[END_REF]

Figure 8: Mechanism and transition state of the Pf adenosine deaminase catalyzed reaction

Human and Pf ADAs are functionally similar and have high sequence homology, but they differ slightly in terms of substrate specificity and catalytic turnover. Pf ADA thus accepts both adenosine and 5'-methylthioadenosine (MTA, a polyamine biosynthesis metabolite) as substrate, which is not the case for its human homologue. [START_REF] Ting | Targeting a Novel Plasmodium Falciparum Purine Recycling Pathway with Specific Immucillins[END_REF] Pv ADA has been crystallized in a complex with three different ligands (adenosine, guanosine and 2'deoxycoformycin, i.e. a known human ADA inhibitor) [START_REF] Larson | Structures of Substrate-and Inhibitor-Bound Adenosine Deaminase from a Human Malaria Parasite Show a Dramatic Conformational Change and Shed Light on Drug Selectivity[END_REF] . A parasite-specific conformation was observed, which closed the substrate binding site within a solvent-less cavity, whereas the human enzyme does not close its active site. This difference may explain the recognition of MTA, as the solvent-less cavity hosts the hydrophobic thiomethyl group. Amino-acid changes in the active sites of Pv and Pf ADAs have been highlighted via computational studies and comparison with a Pf ADA homology model. [START_REF] Ivanov | The Adenosine Deaminases of Plasmodium Vivax and Plasmodium Falciparum Exhibit Surprising Differences in Ligand Specificity[END_REF] In addition, P. knowlesi growth (in infected primates) was found to be inhibited by 2'-deoxycoformycin, indicating that parasite survival was dependent on adenosine deaminase. [START_REF] Webster | Adenosine Deaminase in Malaria Infection: Effect of 2'-Deoxycoformycin in Vivo[END_REF] In 2017, the Pf proteome was studied to identify commercial drugs which could target and inhibit Pf enzymes. Dipyridamole (a nitrogen heterocycle derivative used as blood vessel dilatator) was thus reported to be a Pf ADA inhibitor with an IC50 of 30 nM. [START_REF] Mogire | Target-Similarity Search Using Plasmodium Falciparum Proteome Identifies Approved Drugs with Anti-Malarial Activity and Their Possible Targets[END_REF]

IMPDH

Inosine 5'-monophosphate dehydrogenase catalyzes XMP formation from IMP through a covalent mechanism (Figure 9) and serves as an alternative/complementary pathway for providing GMP. As GMP synthesis mediated by HGXPRT is not substantial enough to provide the overall amount of GMP required for nucleic acid biosynthesis in the parasite, IMPDH inhibition could indirectly and partially block GMP formation from XMP. [START_REF] Hedstrom | IMP Dehydrogenase: Structure, Mechanism and Inhibition[END_REF][START_REF] Fotie | Inosine 5'-Monophosphate Dehydrogenase (IMPDH) as a Potential Target for the Development of a New Generation of Antiprotozoan Agents[END_REF] IMPDH is a homotetramer protein. The active site is located at the C-terminal chain, whereas the cofactor (NAD + ) binds at another site. [START_REF] Raza | In Silico 3-D Structure Prediction and Molecular Docking Studies of Inosine Monophosphate Dehydrogenase from Plasmodium Falciparum[END_REF] Consequently, two classes of IMPDH inhibitors can be developed, i.e. one targeting the substrate binding site and the other targeting the cofactor binding loops via an uncompetitive mechanism (e.g. mycophenolic acid and phenyl-oxazole urea, respectively). [START_REF] Cuny | Inosine-5'-Monophosphate Dehydrogenase (IMPDH) Inhibitors: A Patent and Scientific Literature Review (2002-2016)[END_REF] Human and Pf IMPDH genome sequences shown only 48% of similar amino acid sequences and exhibit differences in the structure of the active site. [START_REF] Raza | In Silico 3-D Structure Prediction and Molecular Docking Studies of Inosine Monophosphate Dehydrogenase from Plasmodium Falciparum[END_REF] That could possibly help to design selective inhibitors of the plasmodial enzyme.

GMPS

Pf GMP synthetase (belonging to the glutamine amidotransferase family) is a dimeric protein catalyzing the irreversible formation of GMP through XMP amination. GMP synthesis via HGXPRT is not enough to maintain optimal GMP levels in the parasite, [START_REF] Bhat | Kinetic and Biochemical Characterization of Plasmodium Falciparum GMP Synthetase[END_REF] thus blocking this enzyme may be a valuable approach to inhibit nucleic acid biosynthesis. Pf and human GMPS share 20-30% common chain sequence and both contain two domains, i.e. one catalyzing formation of the XMP-adenyl complex (ATPPase), and the other providing the required ammonia from glutamine hydrolysis (GATase, glutamine amidotransferase). The ammonia molecule is channeled to the other domain in order to generate the GMP product 63 (Figure 10), which is a common amidotransferase reaction step (ping-pong mechanism). [START_REF] Bhat | Kinetic and Biochemical Characterization of Plasmodium Falciparum GMP Synthetase[END_REF] Both ATPPase and GATase sites can be targeted in competitive and uncompetitive ways, respectively. For instance, glutamine analogues, such as acivicin and 6-diazo-5-oxo-norleucine (use as antibiotics and known hGMPS inhibitors), are able to inactivate the enzyme by covalent modification of the GAT domain. [START_REF] Bhat | Kinetic and Biochemical Characterization of Plasmodium Falciparum GMP Synthetase[END_REF] Pf GMPS also differs with regard to other plasmodial species associated with a single 20 amino-acid long sequence located in its GAT domain. 63

domains II/ TARGETING PURINE METABOLISM WITH NUCLEOS(T)IDE ANALOGUES

Because of the importance of purine metabolism for plasmodial parasites, the inhibition of related enzymes has been considered since 1984. [START_REF] Wang | Parasite Enzymes as Potential Targets for Antiparasitic Chemotherapy[END_REF] Nucleoside analogues are well-known inhibitors of the nucleic acids biosynthesis, so they may also be viewed as potential candidates for targeting of purine metabolism enzymes due to their structural similarities with their substrate. Series of purine-based heterocycles, nucleoside and nucleotide analogues have been studied with the aim of targeting these enzymes and related transporters. A few of them were designed on the basis of transition state studies. This part of the review concerns such compounds as inhibitors of transporters and/or enzymes involved in plasmodial purine metabolism.

Purine-based Analogues

In 2006, Keough et al. studied the effects of purine nucleobase skeleton modifications and the ability of these derivatives to bind to human and Pf HGXPRT, and the hypothetical discrimination between these two enzymes. [START_REF] Keough | Lead Compounds for Antimalarial Chemotherapy: Purine Base Analogs Discriminate between Human and P. Falciparum 6-Oxopurine Phosphoribosyltransferases[END_REF] By analogy with 6-mercaptopurines used in cancer chemotherapy [START_REF] Erb | Pharmacokinetics and Metabolism of Thiopurines in Children with Acute Lymphoblastic Leukemia Receiving 6-Thioguanine versus 6-Mercaptopurine[END_REF] , the proposed mechanism of action consists of their conversion-mediated by 6-oxopurinePRTases-into their corresponding nucleoside 5'-monophosphate derivatives, thereafter incorporated into DNA or RNA and thus interfering with cell replication. Determination of the Km for these derivatives revealed that the presence of an exocyclic oxygen-containing group at position-2, and/or a chlorine atom at position-6, and/or the replacement of the carbon atom at position-8 by a nitrogen allowed strong discrimination between human and Pf HGXPRT. In addition, these nucleobases are able to inhibit P. falciparum in vitro growth. The best IC50 values were 1.3 µM and 6.6 µM for 6-mercaptopurine and 8-azaguanine, respectively.

Nucleoside analogues

Since natural substrates of the targeted enzymes are mostly nucleosides and derivatives, the easiest and most promising way to selectively inhibit them is to design analogues. In the field of antiviral and antitumoral chemotherapy, the search for nucleoside analogues has led to the discovery of the active compounds which are nowadays used as drugs. Some of these derivatives were tested against the Pf purine salvage pathway, leading to the identification of lead compounds which served as a starting point for the synthesis of novel derivatives.

a. Cordycepin

Cordycepin (Table 1), an antibiotic and anti-tumoral nucleoside extracted from Cordyceps militari, was tested in vitro, by Trigg et al. in 1971 against P. knowlesi and in vivo in P. berghei-infected mice. [START_REF] Trigg | The Effects of Cordycepin on Malaria Parasites[END_REF] Effects were noted on in vitro parasite growth at micromolar concentrations, but a full cure was not obtained in mice even at high concentration. The fact that nucleic acid biosynthesis in the parasite was affected suggested that the purine metabolism of the parasite was targeted.

b. Mizoribine and ribavirin derivatives

Mizoribine (Table 1) is an immunosuppressive drug that was isolated from Eupenicillium brefeldianum in Japan in 1971. It is clinically used for renal transplantation [START_REF] Kawasaki | A New Approach in the Treatment of Renal Disease[END_REF] as well as an inhibitor of Hs IMPDH in leukemia and viral diseases. Its proposed mechanism of action involves IMPDH inhibition by the mizoribine-5'-monophosphate, thus limiting both XMP formation and GMP synthesis mediated by GMPS. [START_REF] Yokota | Mode of Action and Effects in Clinical Use[END_REF] When tested as an antimalarial by Webster and Whaun in 1982, mizoribine appeared to be active against drug-resistant Pf strains at 50 µM. [START_REF] Webster | Antimalarial Properties of Bredinin. Prediction Based on Identification of Differences in Human Host-Parasite Purine Metabolism[END_REF] Variations in the concentration of several purine-containing nucleosides and nucleotides in infected RBCs have been studied and a significant decrease was observed in the presence of mizoribine.

Recently, Raza et al. performed virtual screening of a library of ribavirin analogues (Table 1, a base modified nucleoside analogue known as a competitive inhibitor of human IMPDH [START_REF] Streeter | Mechanism of Action of 1-β-D-Ribofuranosyl-1,2,4-Triazole-3-Carboxamide (Virazole), A New Broad-Spectrum Antiviral Agent[END_REF][START_REF] Thomas | The Application and Mechanism of Action of Ribavirin in Therapy of Hepatitis C[END_REF] ) using a 3Dhomology model of Pf IMPDH. [START_REF] Raza | In Silico 3-D Structure Prediction and Molecular Docking Studies of Inosine Monophosphate Dehydrogenase from Plasmodium Falciparum[END_REF] Five promising ribavirin derivatives were identified as potential ligands, which should be able to selectively target the parasite enzyme by interacting with the active site (hydrogen bonds and hydrophobic pockets). To date, these molecules have not yet been tested in vitro against P. falciparum growth.

Table 1. Representative nucleoside analogues as potential purine metabolism inhibitors

Cordycepin (3-deoxy-adenosine) [START_REF] Trigg | The Effects of Cordycepin on Malaria Parasites[END_REF] Inhibitors of Pf IMPDH [START_REF] Raza | In Silico 3-D Structure Prediction and Molecular Docking Studies of Inosine Monophosphate Dehydrogenase from Plasmodium Falciparum[END_REF][START_REF] Webster | Antimalarial Properties of Bredinin. Prediction Based on Identification of Differences in Human Host-Parasite Purine Metabolism[END_REF] Ribavirin derivatives screened by Raza et al. [START_REF] Raza | In Silico 3-D Structure Prediction and Molecular Docking Studies of Inosine Monophosphate Dehydrogenase from Plasmodium Falciparum[END_REF] Adenosine analogues tested against Pf GMPS by Bhat et al. [START_REF] Bhat | Kinetic and Biochemical Characterization of Plasmodium Falciparum GMP Synthetase[END_REF] Pyrrolidine nucleoside mono-and bisphosphonates [START_REF] Keough | Design of Plasmodium Vivax Hypoxanthine-Guanine Phosphoribosyltransferase Inhibitors as Potential Antimalarial Therapeutics[END_REF] Tubercidin and derivatives 74

Nitrobenzylthionucleosides [START_REF] Gero | Antimalarial Action of Nitrobenzylthioinosine in Combination with Purine Nucleoside Antimetabolites[END_REF] c. Psicofuranine and decoyinine

As psicofuranine and decoyinine (Table 1) were reported to inhibit hGMPS (with 17.3 and 46.5 µM IC50 values), [START_REF] Nakamura | Biochemical Characterization of Human GMP Synthetase[END_REF] Baht et al. evaluated them on purified Pf GMPS. They reported a weak inhibitory effect of psicofuranine (25% inhibition at 0.5 mM) and no effect of decoynine. [START_REF] Bhat | Kinetic and Biochemical Characterization of Plasmodium Falciparum GMP Synthetase[END_REF] Only psicofuranine had an IC50 of 0.3 mM when tested against Pf cultures (3D7 strain). [START_REF] Mcconkey | Plasmodium Falciparum: Isolation and Characterisation of a Gene Encoding Protozoan GMP Synthase[END_REF] d. PNBPs -Pyrrolidine nucleoside bisphosphonates Initially synthesized and tested for their ability to inhibit E. coli, 6-oxopurine phosphoribosyltransferases, [START_REF] Keough | Inhibition of the Escherichia Coli 6-Oxopurine Phosphoribosyltransferases by Nucleoside Phosphonates: Potential for New Antibacterial Agents[END_REF] pyrrolidine nucleoside phosphonates (Table 1) were subsequently assessed as potential inhibitors of PvHGPRT and PfHGXPRT. The design of such compounds was based on a central 5-membered ring skeleton, bearing three substituents anchored through free rotation bonds, thus allowing to adopt the best conformation at the active site of the enzyme. [START_REF] Keough | Design of Plasmodium Vivax Hypoxanthine-Guanine Phosphoribosyltransferase Inhibitors as Potential Antimalarial Therapeutics[END_REF] In comparison with compound 3, the addition of a second phosphonate tail to the pyrrolidine ring markedly decreased the Ki values (from millimolar to nanomolar range) for all three enzymes but to the detriment of selectivity. It should be noted that: (i) the nature of the nucleobase did not seem to have a real impact on the activity, (ii) the addition of the hydroxyl group on the second phosphonate tail enhanced the inhibitory activity of these derivatives and the stereochemistry had an influence on the affinity of the studied enzyme. For instance, the (R)-isomer of compound 1 exhibited a 7.5-fold higher Ki value than that of its (S)-isomer on Pv HGPRT. Studies of the crystal structures of the Pv HGPRT-compound 1 complex revealed that the oxygen-linked phosphonate tail located within the 5'-phosphate binding pocket could explain the difference in Ki observed between the two stereoisomers. Additional interactions also occurred between water molecules, the hydroxyl group and the active site for the (S)-isomer of compound 1.

Structural data regarding the Pf protein are currently not available.

e. Tubercidins

The successful use of adenosine analogues in cancer chemotherapy led Coomber et al. to test some of them against Plasmodium falciparum. In vitro, the most efficient ones were the 7-deaza-adenosine derivatives tubercidin and sangivamycin (Table 1), with IC37 of 0.7 and 0.3 µM (IC37 values are quite unusual compared to IC50, but it was used by the authors to compare their results with previously reported ones). Their mechanism of action was suggested to be related to purine metabolism, especially with a decrease of IMP content in treated Pf-infected erythrocytes. [START_REF] Coomber | Adenosine Analogues as Antimetabolites against Plasmodium Falciparum Malaria[END_REF] In addition, 5-iodotubercidin (a known human adenosine kinase inhibitor) has been shown to inhibit Pf growth in vitro (IC50 of 2 µM) [START_REF] Cassera | Erythrocytic Adenosine Monophosphate as an Alternative Purine Source in Plasmodium Falciparum[END_REF] and several tubercidin derivatives appeared to be Pf ENT1 substrates. [START_REF] Riegelhaupt | Transport of Purines and Purine Salvage Pathway Inhibitors by the Plasmodium Falciparum Equilibrative Nucleoside Transporter PfENT1[END_REF] However, no further studies have been recently reported in the literature concerning this family of compounds.

f. NBMPR -nitrobenzylthioinosine Nitrobenzylthioinosine (NBMPR) and its guanine analogue (NBTGR) were first assayed as antimalarial drugs in 1989 (Table 1). [START_REF] Gero | Antimalarial Action of Nitrobenzylthioinosine in Combination with Purine Nucleoside Antimetabolites[END_REF] In addition to their micromolar IC50 values in Pf-K1 strains (multidrug resistant)-infected erythrocytes, they were successfully used in an original approach based on the alteration of infected erythrocyte membrane permeability. Briefly, the two derivatives were used to facilitate the uptake of cytotoxic nucleosides (presumably through permeability pathways induced by the parasite) into infected erythrocytes, while their ability to selectively inhibit the human transporters (hENT1) [START_REF] Young | Nucleoside Transport in Animal Cells[END_REF] prevented the uptake and further toxic effects in uninfected cells. Later on, it was also proven that Pf ENT1 and Pf ENT4 were insensitive to NBMPR. [START_REF] Parker | Identification of a Nucleoside/Nucleobase Transporter from Plasmodium Falciparum, a Novel Target for Anti-Malarial Chemotherapy[END_REF]

Transition state analogues as Pf purine metabolism inhibitors

In theory, a perfect and chemically stable transition state analogue will bind tighter to the enzyme active site than the substrate. The protein will enclose the compound with a more thermodynamically stable and static conformation, thus preventing natural substrate binding until release of the transition state analogue.

Moreover, an ideal transition state analogue will be highly selective of the targeted enzyme, avoiding toxicity and unwanted side-effects.

Natural transition state species have a very short longevity and consequently are not directly observable, but they can be predicted, or hypothesized, by studying the reaction process mechanisms and determining the intrinsic kinetic effects of isotopic compounds. A chemical structure could then be proposed on the basis of in silico calculations. Transition state analogues of three parasite enzymes have thus been reported

to date, i.e. Pf ADA, Pf PNP and Pf HGXPRT. [START_REF] Schramm | Enzymatic Transition State Theory and Transition State Analogue Design[END_REF] a. Pf ADA inhibition

 Coformycin and derivatives

Coformycin and 2'-deoxycoformycin (namely pentostatin) were both known as mammalian ADA inhibitors. In 2007, Tyler et al. studied the affinity of these derivatives toward human, bovine and Pf ADAs (Table 2) and inhibition constants in the low nanomolar range were observed, but with no specific selectivity. [START_REF] Tyler | Synthesis of 5'-Methylthio Coformycins: Specific Inhibitors for Malarial Adenosine Deaminase[END_REF] On the basis of previous results showing that 5'methylthioadenosine or inosine [START_REF] Ting | Targeting a Novel Plasmodium Falciparum Purine Recycling Pathway with Specific Immucillins[END_REF] were Pf ADA substrates, 5'-alkylthio and 5'-phenylthio analogues of coformycin and 2'-deoxycoformycin were synthesized. [START_REF] Tyler | Synthesis of 5'-Methylthio Coformycins: Specific Inhibitors for Malarial Adenosine Deaminase[END_REF] Among them, 5'-methylthio derivatives appeared to be highly selective of the parasite enzyme, with Ki of less than 1 nM (Table 2), especially 5'-methylthiocoformycin, which showed >20,000fold selectivity for Pf versus Hs ADA. However, no further studies have been reported.

 L-nucleoside analogues

In 1995, Upston et al. showed that L-adenosine was significantly transported (via a unique parasiteinduced pathway) and selectively metabolized (through ADA activity) within parasites infectederythrocytes. [START_REF] Upston | Parasite-Induced Permeation of Nucleosides in Plasmodium Falciparum Malaria[END_REF] These preliminary results led the same team to investigate series of L-nucleoside analogues (Table 2). [START_REF] Gero | L-Purine Nucleosides as Selective Antimalarials[END_REF][START_REF] Brown | L-Nucleoside Analogues as Potential Antimalarials That Selectively Target Plasmodium Falciparum Adenosine Deaminase[END_REF] Among them, L-coformycin and L-isocoformycin were shown to specifically inhibit Pf ADA activity in the picomolar range (contrary to their D-isomer, especially 2'-deoxycoformycin, which interacted with both human and parasite enzymes). [START_REF] Brown | L-Nucleoside Analogues as Potential Antimalarials That Selectively Target Plasmodium Falciparum Adenosine Deaminase[END_REF] L-coformycin, L-isocoformycin and L-sangivamycin inhibited the in vitro growth of Pf with an ID50 in the micromolar range. 84

Table 2. Nucleoside analogues as Pf ADA inhibitors

Coformycin and derivatives as inhibitors of human and plasmodic adenosine deaminases [START_REF] Tyler | Synthesis of 5'-Methylthio Coformycins: Specific Inhibitors for Malarial Adenosine Deaminase[END_REF] Ki* values correspond to the equilibrium dissociation constant upon completion, after a slow onset, for tight binding inhibition of the enzyme (consequent upon a time-dependent conformational change in the enzyme that improve the binding after a moment, thus Ki* is lower than Ki) reported for transition state-analogues. Compounds with Ki* values are called slow-onset tight binding inhibitors. Ki values of >10,000 indicate that assays with 10 µM concentration of the compound showed no inhibition under the assay conditions L-nucleoside derivatives as PfADA inhibitors [START_REF] Gero | L-Purine Nucleosides as Selective Antimalarials[END_REF][START_REF] Brown | L-Nucleoside Analogues as Potential Antimalarials That Selectively Target Plasmodium Falciparum Adenosine Deaminase[END_REF] Dimers with L-nucleosides as carrier for cytotoxic D-nucleosides [START_REF] Gero | New Malaria Chemotherapy Developed by Utilization of a Unique Parasite Transport System[END_REF] Gero et al. took advantage of the ability of some L-nucleosides to be selectively transported into infected erythrocytes and proposed dinucleoside monophosphate prodrugs (Table 2) that combined a Lnucleoside as carrier and a cytotoxic nucleoside as antimalarial agent. [START_REF] Gero | New Malaria Chemotherapy Developed by Utilization of a Unique Parasite Transport System[END_REF] The release mechanism involved the cleavage of the 3'-5' phosphodiester linkage, delivering the free 3'-OH L-nucleoside and the 5'- 3) [START_REF] Kicska | Transition State Analogue Inhibitors of Purine Nucleoside Phosphorylase from Plasmodium Falciparum[END_REF] and Imm-H was also shown to be able to kill parasites in culture experiments, with an IC50 of 63 nM (3D7 strain). [START_REF] Ting | Targeting a Novel Plasmodium Falciparum Purine Recycling Pathway with Specific Immucillins[END_REF] But these immucillin derivatives were first designed as substrates of human PNP and exhibited low to modest selectivity for parasite PNPs, which is a crucial point for the development of antimalarial immucillins. As the structures of the Pf PNP-ImmH complex revealed that the enzyme could accept 5'-substitued nucleosides, [START_REF] Shi | Plasmodium Falciparum Purine Nucleoside Phosphorylase: Crystal Structures, Immucillin Inhibitors, and Dual Catalytic Function[END_REF] 5'-methylthioimmucillin (MT-ImmH) was designed to selectively target the parasite enzyme. 5'-MT-ImmH inhibited both human and Pf PNP in infected erythrocyte cultures and selectively bound to the Pf enzyme according to the Ki values (Table 3). Later, it was also demonstrated that MT-ImmH was able to kill Pf in cultures, with an IC50 of 50 nM (3D7 strain). [START_REF] Ting | Targeting a Novel Plasmodium Falciparum Purine Recycling Pathway with Specific Immucillins[END_REF] Other immucillin analogues (e.g. 2'-deoxy-Imm, 8-aza or fluorinated purine, 6-methoxy or thiopurine, 5'-acid or amide-Imm, ara-Imm, etc.) were reported by Lewandowicz et al., but they all exhibited higher binding affinity for human protein. [START_REF] Lewandowicz | Energetic Mapping of Transition State Analogue Interactions with Human and Plasmodium Falciparum Purine Nucleoside Phosphorylases[END_REF] Overall, these studies highlighted that the 5'-methylthiosubstitution resulted in a unique Pf PNP substrate specificity, as observed for MT-ImmH.

Table 3. Transition state analogues as Pf PNP inhibitors

Immucillins and methylthioimmucillin (ND: not determined) [START_REF] Kicska | Transition State Analogue Inhibitors of Purine Nucleoside Phosphorylase from Plasmodium Falciparum[END_REF][START_REF] Shi | Plasmodium Falciparum Purine Nucleoside Phosphorylase: Crystal Structures, Immucillin Inhibitors, and Dual Catalytic Function[END_REF] Second-generation immucillin derivatives [START_REF] Lewandowicz | Energetic Mapping of Transition State Analogue Interactions with Human and Plasmodium Falciparum Purine Nucleoside Phosphorylases[END_REF] These compounds are represented in neutral form, but the nitrogen is protonated at physiological pH thus mimicking the cation of the transition state. No slow-onset tight binding inhibition was observed for Pf PNP

Fluorinated second-generation immucillins 89

Third-generation immucillin derivatives [START_REF] Taylor | Acyclic Ribooxacarbenium Ion Mimics as Transition State Analogues of Human and Malarial Purine Nucleoside Phosphorylases[END_REF][START_REF] Clinch | Third-Generation Immucillins: Syntheses and Bioactivities of Acyclic Immucillin Inhibitors of Human Purine Nucleoside Phosphorylase[END_REF] The complete iminoribitol ring shown in red

The intact iminoribitol ring is shown in red L-enantiomers of potent immucillins [START_REF] Clinch | Syntheses and Bio-Activities of the L-Enantiomers of Two Potent Transition State Analogue Inhibitors of Purine Nucleoside Phosphorylases[END_REF]

 Second generation immucillins: N1-branched analogues

To overcome the expensive and complex synthesis of these immucillins, the development of a second generation of Imm derivatives was considered while focusing on easier synthetic pathways. The latter initiatives generally involved the displacement nitrogen close to the C1' position on the iminoribitol ring to mimic the PNP transition state. This modification nevertheless requires removal of the 2'-hydroxy group to avoid the formation of an unstable hemiaminal scaffold and to eliminate one asymmetric carbon.

On the basis of a structural analysis and theoretical calculations, Schramm's group proposed DADMeimmucillin-H (4'-deaza-1'-aza-2'-deoxy-1'-(9-methylene)-immucillin-H, derivatives (Table 3), where the introduction of a methylene bridge between the sugar residue and 9-deazahypoxanthine allows the leaving group to be more distant from the oxacarbenium ion mimic than in Imm-H. [START_REF] Lewandowicz | Transition State Analysis for Human and Plasmodium Falciparum Purine Nucleoside Phosphorylases[END_REF] DADMe-immucillin-H (BCX-3408) and DADMe-immucillin-G (4'-deaza-1'-aza-2'-deoxy-1'-(9methylene)-immucillin-G, BCX4945) (Table 3) have been identified as potent human PNP inhibitors (with picomolar Ki * values) [START_REF] Lewandowicz | Achieving the Ultimate Physiological Goal in Transition State Analogue Inhibitors for Purine Nucleoside Phosphorylase[END_REF] since 2003 while exhibiting lower affinities for Pf PNP. [START_REF] Lewandowicz | Energetic Mapping of Transition State Analogue Interactions with Human and Plasmodium Falciparum Purine Nucleoside Phosphorylases[END_REF] Despite this, DADMe-immucillin-G has been shown to completely inhibit Pf PNP and suppress P. falciparum development in Aotus primates, [START_REF] Cassera | Plasmodium Falciparum Parasites Are Killed by a Transition State Analogue of Purine Nucleoside Phosphorylase in a Primate Animal Model[END_REF] but no further development has been reported to date.

Many immucillin derivatives have been proposed and studied (fluorinated, N-alkylated, 5'H, 5'thiophenol, 5'-carboxylic acids, etc.) to revert the selectivity of these derivatives toward the Pf enzyme, but they preferentially bound to Hs PNP rather than Pf PNP (Table 3). [START_REF] Lewandowicz | Energetic Mapping of Transition State Analogue Interactions with Human and Plasmodium Falciparum Purine Nucleoside Phosphorylases[END_REF][START_REF] Mason | A Beta-Fluoroamine Inhibitor of Purine Nucleoside Phosphorylase[END_REF] Second and first generation immucillins and transition state studies on bovine, human and malarial PNPs were reviewed by Schramm in 2005. [START_REF] Taylor Ringia | Transition States and Inhibitors of the Purine Nucleoside Phosphorylase Family[END_REF] In 2010, Cui et al. proposed novel N-branched immucillin derivatives where the purine base was replaced by uracil, as uridine was reported to be a PNP substrate, including variations in the linker length between the base and the pyrrolidine ring. [START_REF] Cui | Exploring New Inhibitors of Plasmodium Falciparum Purine Nucleoside Phosphorylase[END_REF] Modest affinities for Pf PNP were determined for uracil-based transition state analogues (Ki in the micromolar range), and no significant antiparasitic activity was observed in vitro.

 Third generation immucillins: Acyclic analogues

As a continuation of their research program on transition state analogues as PNP inhibitors, Schramm's group designed third generation acyclic immucillin derivatives by cleaving the iminoribitol ring, which led to iminoalcohol chains linked to 9-deazapurine [START_REF] Taylor | Acyclic Ribooxacarbenium Ion Mimics as Transition State Analogues of Human and Malarial Purine Nucleoside Phosphorylases[END_REF] , or by replacing the pyrrolidine ring by a serinol chain. [START_REF] Clinch | Third-Generation Immucillins: Syntheses and Bioactivities of Acyclic Immucillin Inhibitors of Human Purine Nucleoside Phosphorylase[END_REF] The flexibility of the acyclic scaffold allowed the chain to rotate freely inside the active site, possibly enhancing the positioning of the compound. In addition, these compounds were easier to synthesized while using cheaper precursors. Only the acyclic iminoalcohol series was assessed against both human and Pf PNPs, and all compounds exhibited better selectivity for human protein. The best Kd value for the human enzyme was reported for the imino-tri-alcohol DATMe-Imm-H (Table 3). Pf PNP thus seemed to be more selective for the ribose ring than its human homologue, and consequently DADMe-immucillins and immucillins are still the best compounds in this series to date.

 L-nucleosides

Several studies carried out on L-enantiomers of known biologically active nucleoside analogues have highlighted the importance of isometry and the potential of L-nucleosides. [START_REF] Gumina | Antiviral Activity and Molecular Mechanism[END_REF][START_REF] Gumina | L-Nucleosides as Chemotherapeutic Agents[END_REF][START_REF] Mathé | L-Nucleoside Enantiomers as Antivirals Drugs: A Mini-Review[END_REF] Based on these observations, Clinch et al. synthesized and compared D-immucillins (Table 3) to their L-enantiomers against human and Pf PNPs (Table 3). [START_REF] Clinch | Syntheses and Bio-Activities of the L-Enantiomers of Two Potent Transition State Analogue Inhibitors of Purine Nucleoside Phosphorylases[END_REF] All L-analogues were less potent than their D-enantiomers against malarial and human protein, while showing nanomolar range inhibition. No selectivity towards Pf enzymes was reported.

c. Pf HGXPRT Inhibition

In 1999, Li et al. hypothesized that known transition state inhibitors of PNPs could also mimic HGPRT (human enzyme) and HGXPRT ribooxocarbenium species after phosphorylation of the 5'hydroxyl of the nucleoside analogue. 100 The nitrogen atom replacing the intracyclic oxygen of ribose in immucillins can provide a cation which may serve as the ribocation transition state of the enzyme. 5'-monophosphate derivatives of immucillinH and immucillinG (Figure 12: Comparative affinities of 5'phosphate immucillin derivatives for Hs HGPRT and Pf HGXPRT) were thus synthesized and evaluated towards Pf and human HG(X)PRT, and the two compounds were found to exhibit similar affinities to both proteins in the low nanomolar range. Immucillin 5'-monophosphate derivatives were not further investigated due to their susceptibility to phosphates present in biological media.

d. Conclusion on immucillins and transition state analogues

The use of transition state analogues has been successful in inhibiting PNP, ADA and HGXPRT. A wide variety of structures, mainly based on the immucillin skeleton (four generations developed through the years), have been synthesized along with studies on the purified enzymes. One of their limitations is their lack of selectivity regarding human and parasite enzymes, and their entry into the parasite may also be problematic due to their high polarity. To our knowledge, the mechanism of their uptake into parasites does not involve Pf ENT1 transporters. [START_REF] Riegelhaupt | Transport of Purines and Purine Salvage Pathway Inhibitors by the Plasmodium Falciparum Equilibrative Nucleoside Transporter PfENT1[END_REF] phosphonomethoxy-propyl)adenine) (Figure 13). The latter was described as a nucleotide analogue resistant to phosphoesterase hydrolysis due to the presence of the phosphonate moiety (P-C bond) and an acyclic chain replacing the sugar ring, and it was found to be active against anti-DNA viruses. 106 Meanwhile, the antiviral activity of 2-(phosphonomethoxy)ethyl-adenine (PMEA) was also reported. 107 (S)-HPMPC (cidofovir) was the first ANP marketed (in 1996) for cytomegalovirus retinitis treatment.

Over the last three decades, many ANPs have been designed, synthesized and evaluated against viruses (cytomegalovirus, hepatitis B and HIV), and they still represent a promising and efficient family of drugs. 108,109 In this respect, (S)-HPMPA, (S)-3-deaza HPMPA and (S)-HPMPADAP (2,6-diaminopurine) were first tested in 1991 against Pf and P. berghei infected RBCs (Figure 14). S-HPMPA is active in vitro against Pf at nanomolar concentration, which is more than 1,000-fold less than the cytotoxic dose for human cells. 115 When evaluated in P. berghei infected mice, four injections of 20 mg/kg HPMPA at 3-day intervals decreases parasitemia to low detectable levels but does not lead to a complete cure. 115 Using extracted plasmodial DNA polymerases, the inhibitory activity of these compounds was also observed and interference with the replication process was hypothesized. Unfortunately, the effects of (S)-HPMPA in Pf infected mice were hampered by nephrotoxicity and hepatotoxicity as well as the limited half-life in plasma. 116 In 1999, Smeijsters et al. conducted an in vitro study of 48 ANPs belonging to five main structural groups with the aim of inhibiting plasmodial DNA polymerase activity (Figure 14). 117 All HPMPA derivatives (except (S)-HPMPA and (S)-3-deaza-HPMPA) exhibited antiplasmodial activity in the micromolar range.

(S)-HPMPA was found to be less potent (0.18 µM) than reported by de Vries et al. due to the use of a different IC50 determination method. One of the most interesting results was the difference in IC50 between the two isomers of HPMPA and 3-deaza-HPMPA. Indeed, (R)-enantiomers of both compounds were less potent than the (S)-enantiomers. This was attributed to the fact that kinases may not be able to recognize (R)-enantiomers of phosphorylated substrates.

Conversely, PMEA derivatives had no significant activity, presumably due to the lack of the hydroxyl group present in the HPMPA derivatives. Studies on hydrogenated and fluorinated ((S)-FPMPA) derivatives revealed no inhibition, thus confirming the previous hypothesis. However, the difference in antimalarial activity observed between PMEA (IC50> 250 µM) and its prodrugs bis(POM)PMEA (IC50 = 37.3 µM) was significant. As the prodrug is more lipophilic and neutral than PMEA, the improved activity could be explained by the efficient prodrug uptake through the host cell and parasite membranes.

Modification of the nucleobase counterpart with similar phosphonate tail was then envisaged. (S)-HPMPG exhibited a similar antimalarial activity of about 4 µM, as well as 2-chloroadenine, 2,6-diaminopurine, 7deazaadenine HPMP compounds, whereas 2-aminomethyladenine, 2-aminomethylhypoxanthine or 2methylthioadenine derivatives did not exhibit any activity at 100 µM. 117 The nature of the nucleobase and the presence of the phosphonate moiety thus seemed to be crucial as non-phosphorylated compounds (acyclic nucleoside) were inactive. For the latter, either the lack of appropriate kinases in Plasmodium limits their transformation into active form and/or their polarity prevents their passive passage through the membranes. In conclusion, this pioneer work highlighted the antimalarial potency of ANPs and the importance of the nature of the nucleobase and the stereochemistry for in vitro antiplasmodial activity, as well as the possible passive uptake of these drugs when the corresponding prodrugs were used.

In comparison of several PME analogues revealed that the nature of the nucleobase had a crucial effect on binding to the human HGPR catalytic site. The best tradeoff in terms of affinity (Ki in the low micromolar range) and selectivity was found to be the combination of a PEE tail and a 8-substituted guanine as nucleobase (i.e. 8-azaguanine, etc.). Selected derivatives were tested in vitro and were found to be able to inhibit the Pf growth, with IC50 values in the micromolar range (Figure 15). 118 In the absence of any crystallized Pf HGXPRT with the studied ANPs, human HGPRT complexes were used as models in structural studies and highlighted that the loop hosting the phosphonate or phosphate group seemed to be identical for both enzymes. 118 The three oxygen atoms of the phosphonate group makes hydrogen bonds with the protein, whereas the natural substrate (GMP) has fewer hydrogen bonds, indicating that the phosphonate group is essential for tight binding at the active site while not being involved in any selectivity contribution. According to docking studies, the purine ring of ANPs binds at the nucleobase binding site of the enzyme. These results pave the way for the development of ANPs targeting HGXPRT, based on the fact that the potency of this family was found to inhibit Pf growth in vitro and on findings regarding their interaction within the active site of the enzyme.

Meanwhile, these authors also reported the synthesis of 43 α-and β-substituted PEE derivatives bearing guanine and hypoxanthine (Figure 16: Structure of α-and β-branched 2-(phosphonoethoxy)ethyl ANPs) and their inhibitory activity against Pf and human HG(X)PRT was determined. 119 Overall, αsubstituted (R1=methyl or ethyl) phosphonate derivatives were either non-active or weakly active against both enzymes and it was suggested that these results were due to steric hindrance. or more being highly selective for human HGPRT. This latter enzyme appears to be able to host longer derivatives than the parasite enzyme. The presence of an oxygen atom within the four-atom linker was mandatory for interfering with parasite proteins, indicating that the oxygen in the ribose ring may be responsible for important binding site interactions. As previously observed, guanine is apparently the preferred nucleobase and all modified nucleobases (2-amino-6-chloropurine, 6-bromopurine, etc.) lead to derivatives with weaker inhibitory activities (Ki values above 100 µM). This indicates that efficient binding of both the phosphonate group and the nucleobase is essential at the catalytic site to obtain optimal Ki values. In conclusion, at least a four carbon atom chain length and a guanine as nucleobase were required for significant inhibition of these proteins, but with low or no selectivity towards parasite proteins, while the presence of an oxygen atom in the linker was somewhat favorable. to the nucleobase (Figure 19). 121 These new derivatives, which incorporated a small polar or anionic side chain, were less efficient but presented some selectivity for Pf and Pv enzymes. These derivatives were all tested as racemic mixtures, so the impacts of stereoisomery could not be determined. the fact that a five-atom chain is optimal for tight interactions at the active site. 123,124 Concerning HPEP derivatives, again the nature of the nucleobase was found to be crucial as only the 8-bromoguanine, guanine and hypoxanthine analogues showed some affinity (with Ki in the low micromolar range), but only the guanine HPEP ANP presented a 6-fold better selectivity for the parasite enzyme. CPEE derivatives exhibited similar activities against both enzymes, and in this case only the hypoxanthine derivatives appeared to be more selective for Pf HGXPRT than for human HGPRT. Finally, the presence of an acidic, ester or hydroxyl group as side chain did not lead to a significant difference between these three series of compounds. The crystal structures of HPEPG and HPEPHx revealed that residues located in the vicinity of the active site were flexible and mobile, and the activity enhancement was hypothesized to be associated with the 'locking' of these residues (tighter binding). In order to facilitate cell uptake, phosphorobisamidate prodrugs incorporating L-phenylalanyl moieties (Figure 20: HPEP and CPEE ANPs and their corresponding prodrugs ) were prepared and assessed in vitro in Pf strains W2 and D6. 122 Mono-substituted phosphorobisamidates did not exhibit significant antimalarial activity, probably due to the fact that the free amino group may not be easily hydrolyzed in cells. IC50 values of di-substituted phosphoramidate prodrugs ranged from 22 to 103 µM.

c. Bisphosphonates ANPs -ANbPs

On the basis of an analysis of the crystal structure of human HGPRT complexed with PEEG analogues, new series of ANPs containing a second oxymethylphosphonate group and increased flexibility of the two phosphonate tails were designed (Figure 21). 125 Most of the compounds had Ki values in the sub-micromolar range, but little selectivity towards Pf HG(X)PRT was observed. Inhibitory activity against the parasite enzyme was noted when the oxygen atom of the phosphonate tail was moved from position 4 to 3. The position of the oxygen atom may thus modify the chain conformation while enhancing its binding at the active site. Studies on the crystal structure of human HGPRT complexed with the first derivative of Figure 21 revealed that the second phosphonate group interacted with amino acid residues via Mg 2+ ion, thus highlighting the contribution of this group in comparison to PEEG analogues. Docking studies also revealed that the second phosphonate group was located near the PPi binding site but did not share interactions inside this pocket.

Figure 21: Bisphosphonate ANPs tested on human and malarial 6-oxopurinePRTs

Two types of prodrug were proposed to increase the cell permeability of parent ANPs. The negative charges of PEEG (Figure 15) were either masked by one or two lipophilic chains (Figure 22), or as a bis phosphoramidate including an alanine ethyl ester. In comparison to the parent drug (PEEG, IC50 242 µM on Pf strain D6), the presence of two substituents decreased the biological activity (IC50 56 µM) and the best result was obtained for the prodrug that included a single hexadecyloxypropyl chain (7.6 µM) 125 , a behavior that had already been noted with antiviral compounds. 126 For ANbPs (Figure 22), ethyl L-phenylalanyl (instead of alanyl) moieties were envisaged for phosphorobisamidate prodrugs with the aim of further increasing the lipophilicity of the final compounds. In vitro evaluation of these three prodrugs on Pf strains D6 and W2 led to similar results, with IC50 values in the low micromolar range. 125 Note that some cytotoxicity on mammalian cells was observed for the lipophilic monoester PEEG prodrug (~36-61 µM CC50 on all three human cell lines tested) as well as for bisphosphonate prodrugs (~41-130 µM CC50). 125 HGPRT and based on molecular docking studies, suggesting that increasing the length of the phosphonate tail by one atom could allow its location in the PPi binding pocket. 127 Various nucleobase modifications showed that hypoxanthine (compound 7), 8-bromoguanine (compounds 5 and 8) and 7-deazapurine (compound 6) derivatives were equally or less active than the guanine derivative (compound 4, Figure 23). Adding one carbon atom into the phosphonate chain (Figure 23, compound 4) led to a 10-fold decrease in the Ki values on the human enzyme (0.006 µM to 0.03 µM) but had no effect on Pf HGXPRT (Ki = 0.07 µM). For this latter derivative, a crystal structure was obtained in a complex with human HGPRT and the affinity gain was associated with the location of the second phosphonate chain within the PPi binding site, whereas the phosphonate tail of the previous derivative was too short to reach this site and share interactions. They also showed that this new compound bonded strongly in the active site through a network of hydrogen bonds between protein residues, phosphonyl oxygen atoms and magnesium ions. Finally, phenylalanine phosphoramidate prodrugs were synthesized and their antimalarial and cytotoxicity activities were determined. Most of them exhibited IC50 values in the micromolar range against D6 and W2 Pf strains, and prodrugs of the best ANbPs in this series exhibited low cytotoxicity compared to the others (CC50 212 µM on human lung carcinoma cells). derivatives, which were obtained as pure isomers (Figure 24). 124

Figure 24: Fluorinated ANPs and their biological activities

Briefly, FPMP derivatives were found to be very weak inhibitors (Ki >50 µM) of the three enzymes, except for the (S)-isomer incorporating hypoxanthine which exhibited a Ki of 13 µM toward Pf HGXPRT.

Among the FPEP series, the guanine analogues exhibited inhibition constants in the low micromolar range for the three enzymes, with no significant selectivity for the parasite enzymes (Figure 24). No FPEP and FPMP derivatives were more active than PEEHx and PEEG ANPs.

Molecular docking demonstrated that the fluoromethyl group was too small to interact significantly with protein residues at the active site, thus not really reflecting any effect of the fluorinated group on the activity. One interesting finding was the effect of stereoisomery, (S)-and (R)-isomers providing different profiles, as previously noted by other authors. Docking studies revealed different positioning of the nucleobase for each isomer, with one being closest to the aromatic ring of residue F186. The stereochemistry also influenced the position of the phosphonate group within the 5'-phosphate group loop, thus affecting the activity. Indeed, the binding of this group is one of the most crucial for the affinity. 124 e. Acyclic immucillin phosphonates (AIP)

In 2012, Schramm and co-workers concluded from their preliminary studies on immucillin 5'monophosphate derivatives that they were unable to act efficiently as antimalarials in vitro due to hydrolysis of the 5'-phosphate group by cellular phosphatases. 101 As an alternative and inspired by the work done on ANPs, they designed acyclic immucillin phosphonate derivatives (AIPs, f. Aza-ANPs

In 2012, novel ANP analogues, where a nitrogen atom replaced the oxygen atom within the parent PEE scaffold, were compared for their ability to inhibit Pf HGXPRT, with the aim of introducing a novel class of nitrogen-containing ANPs (Figure 26) 128 This modification led to less active derivatives and increased the affinity for the human enzyme (reversed selectivity). These negative effects on the activity and selectivity may be attributed to the ability of the secondary amino group to protonate at physiological pH, thus modifying the chain conformation (confirmed by docking studies). 128 All compounds showed weak potency towards parasite enzymes. Finally, promising results were obtained when the second tail had a length of three carbon atoms and included a carboxylic acid or a methyl ester group (compounds 19-20).

Docking studies on these compounds were also performed with human HGPRT to support the biological results. The functional group is supposed to be located in a hydrophobic pocket differing from the PPi binding site. In the case of the hydroxyl compound, the -OH group is supposedly bound through hydrogen bonds to the internal phosphonate, thus hampering it from occupying the 5'-phosphate bonding pocket.

The nature of the linker has a marked impact on the location of the second group within the active site, e.g. in the case of the two cyano compounds 15 and 16, thus lowering the affinity. prodrugs led to a potency gain in comparison to the parent compounds. However, some cytotoxicity was observed for the guanine derivatives, presumably due to the higher affinity of this ANP for the human protein, contrary to the hypoxanthine-containing prodrug. In theory, these compounds should form a tight drug-enzyme complex. Indeed, the presence of the nitrogen atom allows triple anchoring of the compound through the nucleobase and the formation of two phosphonate chains of different lengths. One phosphonate tail was designed to fill the Pf HGXPRT pyrophosphate binding site and the other one the phosphate binding pocket. The main chain included five atoms, which was found to be the optimum length for this target. Studies on their affinity towards human, Pf and Pv PRTases showed that the addition of a second phosphonate group seemed to create new interactions at the active site with the human enzyme, which may have been responsible for the improvement in all Ki values compared to their monophosphonate analogues (Figure 29). These compounds however remained poorly selective for the parasite enzymes. Modification of the length of the side chain and incorporation of an oxygen atom were then envisaged (Figure 30). 129 Regardless of the nature of the second phosphonate tail, guanine derivatives showed similar or better affinity for human HGPRT, with Ki values in the sub-micromolar range. The increased length of the side to eight atoms and the presence of oxygen atoms led in all cases to more potent inhibitors for all three enzymes. In this case, selectivity was in favor of the parasite enzymes for hypoxanthine and 8-bromoguanine derivatives.

Due to the polyanionic nature of the parent compounds, only the corresponding phosphoramidate prodrugs were tested in cell experiments (Pf D6 and W2 strains), and cytotoxicity was assessed on human lung carcinoma A549 cells. The prodrugs exhibited similar IC50 values, in the micromolar range, against both Pf W2 and D6 strains, with weak cytotoxicity against mammalian cells (SI for prodrugs ≥ 15).

Extensive structural studies revealed that the main phosphonate chain was located in the 5'phosphate binding pocket, whereas the other chain was hosted by the PPi binding site. The importance of coordination of the magnesium ion with the second phosphonate moiety through a water molecule for tight binding was highlighted. It was also found that 8-bromoguanine induced inversion of the common positioning of the two phosphonate chains and led to occupation of the 5'-phosphate binding site by the second phosphonate group, not by the main one. The longer chain can enter deeply into the binding site, and forming strong interactions. Finally, the hydrogen bond interactions of amino acid residues in the active site with the purine ring have been described and the most important ones were identified (hydrogen bond between V187 and the N 1 of the purine ring is present in every case). The authors also observed a hydrogen bond between the N 3 of the purine ring and water. 129 Recently, ANPs containing a triazole ring instead of a ribose ring and a phosphonic acid group at position 4 were reported (Figure 31). 130 Both guanine enantiomers exhibited the same Ki on purified Pf and human HG(X)PRT, which surprisingly indicated that stereoisomery had no effect on affinity. Also surprisingly, the xanthine compound showed the best selectivity for Pf enzymes among others (SI > 250). HGXPRT and Pv HGPRT, leading to studies on the impact of the nucleobase nature, linker length, the presence of side-chains and their functionalities on the activity and selectivity. Docking studies were also performed to enhance their affinity and/or selectivity. However, the unknown structure of Pf HGXPRT is a major limitation which could hamper rational design of these derivatives based on docking studies. All computational studies have been performed with human and/or Pv HGPRT, which is not entirely comparable to the Pf HGPRT. In addition, the use of prodrug approaches is mandatory for such compounds since phosphonic acids do not readily cross biological membranes-an aspect that could complicate matters (i.e. synthesis and cost).

The overall findings of all of these studies show us that nucleos(t)ide analogues are far from revealing their full potential. Two decades have been enough to develop a broad range of compounds effectively and selectively targeting different enzymatic steps of the Plasmodium purine salvage pathway, and it is hoped that the future of this class of compounds will meet with the success it deserves.

Figure 4 :

 4 Figure 4: Representative scheme of Pf purine metabolism

37

 37

Figure 5 :

 5 Figure 5: Main transporters involved in the uptake of purines and derivatives into RBC and the

Figure 6 :

 6 Figure 6: Mechanism and transition state of the 6-oxopurinephosphoribosyltransferase catalyzed

Figure 7 :

 7 Figure 7: Mechanism and transition state of the Pf purine nucleoside phosphorylase catalyzed

Figure 9 :

 9 Figure 9: Pf inosine 5'-monophosphate deshydrogenase catalyzed reaction

Figure 10 :

 10 Figure 10: Formation of GMP from XMP, catalyzed by Pf GMPS from its GAT and ATPPase

  monophosphate D-nucleoside (the position of the phosphate moiety, 3'-or 5'-inside the targeted cell, was shown to be crucial for the activity. The best results were obtained for dinucleotides containing 5-fluoro-2'-D-deoxyuridine (FUdR) as cytotoxic nucleoside and 2'-deoxy-L-adenosine or L-adenosine as carriers, with in vitro antimalarial activities in the low micromolar range. No further information is currently available.b. Pf PNP Inhibition First generation: immucillinsIn 1966, Reist et al. first synthesized an adenosine analogue in which the sugar ring oxygen was replaced by a nitrogen.[START_REF] Reist | Synthesis of 4'-Acetamidoadenosine and Other Derivatives of 4-Amino-4-Deoxy-D-Ribose[END_REF] In 1998, studies on bovine PNP transition states led Miles et al. to design immucillin-H (Imm-H) and immucillin-G (Imm-G), as analogues of the proposed riboxocarbenium ion transition state.[START_REF] Miles | One-Third-the-Sites Transition State Inhibitors for Purine Nucleoside Phosphorylase[END_REF] These compounds are iminoribitol C-glycoside derivatives incorporating an iminoribitol sugar scaffold and 9-deazapurines. Replacement of the labile glycosidic bond by a C-N link led to a stable analogue with increased pKa values for the nucleobase N 7 atom (Figure11). Tight binding constants, in the picomolar range, were determined for these two immucillins on bovine and human PNP.

Figure 11 :

 11 Figure 11: Common structure of first-generation immucillins

Figure 12 :

 12 Figure 12: Comparative affinities of 5'-phosphate immucillin derivatives for Hs HGPRT and Pf

  In 2012, Frame et al. demonstrated that immucillin and DADMeimmucillins were recognized by Pf ENT4, 35 suggesting the possible role of this transporter in the uptake of these compounds into the parasite The fourth generation of immucillins designed as human PNP transition state analogues were reviewed by Ho et al. in 2007. 102 DADMe-immucillin-G was under preclinical development for malaria treatment in 2011, 94 but the data are currently unavailable. Finally, as for all known antimalarial treatments, Plasmodium falciparum has developed genetic drug resistance (gene amplification and drug-binding mutations of Pf PNP gene) to PNP inhibition by DADMe-Imm-G, as recently demonstrated by Ducati et al. 103 4. Nucleotide analogues: the acyclic nucleoside phosphonate family a. Introduction to acyclic nucleoside phosphonate derivatives Since the 1960s, nucleoside analogues have been proposed as antiproliferative agents based on their ability to interfere (as their 5'-triphosphate derivatives) with the last steps of nucleic acid biosynthesis. Extensive modification of the nucleobase and/or sugar moiety was carried out to enhance their efficacy. In 1978, A. Holy and E. De Clercq described the broad spectrum antiviral activity of a new family of nucleoside analogues consisting of a simple aliphatic chain linked to a nucleobase, i.e. (S)-DHPA ( (S)-9-(2,3-dihydroxypropyl)adenine, also known as Duviragel ® , (Figure 13: First acyclic nucleoside and nucleoside phosphonate analogues designed). 104 A few months earlier, G. Elion had reported a new antiherpetic agent, another acyclic nucleoside analogue, 105 i.e. 9-(2-hydroxyethoxymethyl)guanine, currently known as acyclovir (Figure 13: First acyclic nucleoside and nucleoside phosphonate analogues designed) and in 1988 she shared the Nobel Prize with G. H. Hitchings and Sir J. Black for this discovery. Studies on the antiviral potential of phosphonoacetic acid, with the aim of developing isopolar and catabolically stable nucleotide analogues, gave rise to the design of the first acyclic nucleoside phosphonate (ANP) in 1986 by Holy et al, i.e. (S)-HPMPA or ((S)-9-(3-hydroxy-2-

Figure 13 :

 13 Figure 13: First acyclic nucleoside and nucleoside phosphonate analogues designed

Figure 14 :

 14 Figure 14: IC50 of several ether ANPs in Pf-infected RBCs (from Smeijsters et al.)

  2009, Keough et al. reported, for the first time, novel series of ANPs to target Pf HGXPRT. 118 These authors performed an SAR study to compare the length and position of the oxygen atom within the phosphonate tail (e.g. cyclic-3-hydroxy-2-(phosphonomethoxy)-propyl (cyclic-HPMP), 2-(phosphonoethoxy)ethyl (PEE), 2-(phosphonomethoxy)ethyl (PME), 3-fluoro-2-(phosphonomethoxy)propyl (FPMP), 2-(phosphonomethoxy)propyl (PMP), etc., (Figure 15: Inhibition constants of two series (C4 and C5) of ether ANPs on parasite and human 6-oxopurine PRTs) and the nature of the nucleobase on the affinity for Pf and human enzymes, and selectivity toward the parasite protein. A

Figure 15 :

 15 Figure 15: Inhibition constants of two series (C4 and C5) of ether ANPs on parasite and human 6-

Figure 16 :

 16 Figure 16: Structure of α-and β-2-(phosphonoethoxy)ethyl ANPs

Figure 17 :

 17 Figure 17: Inhibitory activities of β-branched PEE ANPs on human HGPRT and Pf HGXPRT

Figure 18 :

 18 Figure 18: Studies of the impact of the chain length of ANPs on their inhibitory activities against

Figure 19 :

 19 Figure 19: Representatives of PMP ANPs

Figure 20 :

 20 Figure 20: HPEP and CPEE ANPs and their corresponding prodrugs

Figure 22 :

 22 Figure 22: Biological assessment of ANbP prodrugs on Pf strains

Figure 23 :

 23 Figure 23: Novel ANbPs designed by Spacek et al. to study the impact of the phosphonate chain

Figure 25 )Figure 25 :

 2525 Figure 25: First AIPs tested to inhibit Pf HGXPRT and Hs HGPRT activities

Figure 26 :

 26 Figure 26: Comparison the inhibitory activity of oxygen-and nitrogen-containing ANPs

Figure 27 :Prodrugs (Figure 28 )

 2728 Figure 27: Evaluation of N-branched ANP potent inhibitors of malarial and human 6-

Figure 28 :

 28 Figure 28: Cyano-aza-ANPs and prodrugs against Pf HGXPRT and human HGPRT

Figure 29 :

 29 Figure 29: Generic structure of the aza-ANPs bisphosphonates and first derivatives studied

Figure 30 :

 30 Figure 30: Biological activities of asymmetric aza-bisphosphonate-ANPs and their prodrugs

  targets, Pf PNP and Pf HGXPRT inhibition has been intensively investigated, along with Pf ADA and Pf IMPDH to a lesser extent. Molecular modelling approaches have been used to improve potential inhibitors.However, the weak (or the lack) of specificity of many compounds towards human and Pf species has somewhat limited their development as drug candidates. The antimalarial activity of immucillin derivatives and analogues has been demonstrated in vitro and in vivo. Within this class, DADMeimmucillin-G (BCX4945) was identified as one of the most potent Pf PNP inhibitors (with subnanomolar affinity) while acting as a transition state analogue. This compound reached the preclinical phase in 2012, but no further information has been reported since then. The use of ANPs as inhibitors of the plasmodial purine metabolism was proposed in the late 1990s, and investigations are still ongoing. It could represent one of the most promising families of derivatives. Various series of ANPs have been reported to inhibit Pf

Figure 3 : Pf pyrimidine metabolism

	Enzymes: CPSII, carbamoyl phosphate synthetase II; CA, carbonic anhydrase; ATC, aspartate transcarbamoylase;
	DHO, dihydroorotase; DHODH, dihydroorotate dehydrogenase; OPRTase, orotate phosphoribosyl transferase;
	ODC, orotidine 5'-monophosphate decarboxylase; NMPK, nucleoside monophosphate kinase; RNR,
	ribonucleotide reductase; TS, thymidylate synthase; TMPK, thymidylate monophosphate kinase; CTPS, cytosine
	5'-triphosphate synthetase; TK, thymidine kinase; UPRT, uracile phosphoribosyl transferase; and UK, uridine
	kinase.

  et al. reported a detailed enzymatic characterization of Pf HGXPRT, demonstrating that phosphoribosyl transferases are the most important enzymes in purine metabolism.[START_REF] Reyes | Enzymes of Purine and Pyrimidine Metabolism from the Human Malaria Parasite, Plasmodium Falciparum[END_REF]

  methylenehydrofolate dehydrogenase; MTHFR, methylenetetrahydrofolate reductase; MTI, 5'methylthioinosine; MT-Imm, 5'-methylthioimucillin; NAD+, oxidized nicotinamide adenine dinucleotide (coenzyme); NADH, reduced nicotinamide adenine dinucleotide (coenzyme); NBMPR, nitrobenzylthioinosine; NDP, nucleoside diphosphate; NMPK, nucleoside monophosphate kinase; NPP, new permeability pathway; NTP, nucleoside triphosphate; ODC, orotidine 5'-monophosphate decarboxylase; OPRTase, orotate phosphoribosyltransferase; pABA, para-aminobenzoic acid; Pb, Plasmodium berguei; PDB, protein data bank; PEE, 2-(phosphonoethoxy)ethyl; Pf, Plasmodium falciparum; Pf3D7, Plasmodium falciparum strain used in lab assays; Pfcrt, Pf chloroquine-resistant transporter gene; PfCRT, Plasmodium falciparum digestive vacuole chloroquine-resistant transmembrane SHMT, serine hydroxymethyltransferase; SK, shikimate kinase; T, thymine; THF, 5,6,7,8-tetrahydrofolate; TK, thymidine kinase; TMP, thymidine-5'-monophosphate; TMPK, thymidylate monophosphate kinase; tRNA, transfer ARN; TS, thymidylate synthase; UDP, uridine-5'-diphosphate; UK, uridine kinase; UPRT, uracile phosphoribosyltransferase; UTP, uridine-5'-triphosphate; WHO, world health organization; XMP, xanthosine monophosphate.

transporter; PfMDR, Plasmodium falciparum multidrug resistance protein; Pfmdr1, gene of Plasmodium falciparum multidrug resistance protein; Pk, Plasmodium knowlesi; pKa, logarithm of acid dissociation constant Ka; PlasmoDB, the online plasmodic genomic resources http://PlasmoDB.orgPm: Plasmodium malariae; PME, 2-(phosphonomethoxy)ethyl; PMP, 2-(phosphonomethoxy)propyl; PNP, purine nucleoside phosphorylase; Po, Plasmodium ovale; PPi : inorganic pyrophosphate; PRib-PP, phosphoribosyl pyrophosphate; PSAC, plasmodial surface anion channel; PTPS, 6-pyruvoyltetrahydropterin synthase III; Pv, Plasmodium vivax; PVM, parasitophorous vacuole membrane; RBC, red blood cell; RNA, ribonucleic acid; RNR, ribonucleotide reductase; ROS, reactive oxygen species;

Table of Contents graphic

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ACKNOWLEDGMENTS

CNRS (Centre National de la Recherche Scientifique) and University of Montpellier are gratefully acknowledged for their support.

Hypoxanthine compounds were found to be modest inhibitors of Pf and human HG(X)PRT ((R)-isomer Ki values of 3 and 8 µM were observed, respectively) and poorly active against Pv HGXPRT (Ki >100 µM).

CONCLUSION

As the resistance of Plasmodium species to all antimalarial medicines is spreading in Asia and Africa, the search for new treatments is crucial. The main recommendations of WHO to address the drugresistance problem include the use of a single-dose treatment to cure uncomplicated malaria, safety for vulnerable persons such as pregnant women and young children, combining drugs with different targets, activity against several stages of the parasite life cycle and low cost (~ US$1 per adult and US$0.60 for an average child's treatment). Consequently, the scientific community, especially medicinal chemists, biochemists and biologists working in the field of parasitology, is still facing a challenge to discover original drugs and identify novel targets.

Here we have discussed nucleos(t)ide analogues as a class of potential antimalarials and the related metabolic pathway, i.e. enzymes belonging to the purine salvage pathway which have been investigated for chemotherapy. Indeed, Plasmodium parasites behave as purine auxotrophs and inhibition of the corresponding enzymes would lead to death of the parasites through purine starvation. Among the possible