Dr Renata Ursu
email: renata.ursu@aphp.fr

; L Thomas

; D Psimaras

; O Chinot

; E Le Rhun

; D Ricard

; M Charissoux

S Cuzzubbo

; F Sejalon

V Quillien

K Hoang-Xuan

; F Ducray

J-J Portal

A Tibi

E Mandonnet

C Levy-Piedbois

E Vicaut

A F Carpentier

A randomized multicenter trial (ASTER Trial). An ANOCEF Study

Keywords: Angiotensin II Receptor Blockers, steroids and radiotherapy in glioblastoma - Glioblastoma (GBM), Angiotensin receptor blockers (ARBs), Losartan, steroids, peritumoral edema

INTRODUCTION

Glioblastoma (GBM) is the most common malignant primary brain tumor in adults, with an incidence over three per 100 000 people [1]. The standard of care (SOC), with surgical resection followed by radiotherapy (RT) with concomitant and maintenance temozolomide (TMZ) chemotherapy, generally leads to a median overall survival of about 15 months [START_REF] Stupp | Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma[END_REF]. The tumor-treating fields device represents an additional treatment option for glioblastoma [START_REF] Stupp | Maintenance Therapy With Tumor-Treating Fields Plus Temozolomide vs Temozolomide Alone for Glioblastoma: A Randomized Clinical Trial[END_REF].

Malignant brain tumor patients develop peritumoral vasogenic edema, which further increases neurological deficits and intracranial pressure [START_REF] Dubois | Gliomas and the vascular fragility of the blood brain barrier[END_REF]. Steroids reduce brain tumor-related edema and are thus required in almost all patients [START_REF] Marantidou | Steroid requirements during radiotherapy for malignant gliomas[END_REF]. However, steroids are associated with numerous side effects that not only impact the quality of life but can also become life-threatening [START_REF] Roth | Corticosteroid use in neuro-oncology: an update[END_REF][START_REF] Pitter | Corticosteroids compromise survival in glioblastoma[END_REF]. Because of these side effects, steroid-sparing drugs represent an unmet medical need. Peritumoral edema results from blood-brain barrier alterations and leakage of plasma fluid and proteins into the surrounding tissue, a process in which vascular endothelial growth factor (VEGF) is reported to play a key role [START_REF] Pitter | Corticosteroids compromise survival in glioblastoma[END_REF].

Bevacizumab, a monoclonal antibody targeting VEGF, has thus shown a striking steroid-sparing effect. However, this drug requires intravenous infusions, can be associated with significant side effects and approval in this clinical setting is lacking [START_REF] Gerstner | VEGF inhibitors in the treatment of cerebral edema in patients with brain cancer[END_REF][START_REF] Chinot | Bevacizumab plus radiotherapy-temozolomide for newly diagnosed glioblastoma[END_REF]. Corticorelin acetate, a synthetic analog of human corticotropin-releasing factor, reduced dexamethasone requirement when compared to controltreated patients but this reduction did not reach statistical significance [START_REF] Recht | Steroid-sparing effect of corticorelin acetate in peritumoral cerebral edema is associated with improvement in steroid-induced myopathy[END_REF]. In a randomized study, although Boswellia serrata slightly reduced cerebral edema on MRI, no reduction of dexamethasone dosage was seen in these patients [START_REF] Kirste | Boswellia serrata acts on cerebral edema in patients irradiated for brain tumors: a prospective, randomized, placebo-controlled, double-blind pilot trial[END_REF].

In 2012, a reduced steroid requirement in patients taking angiotensin-converting enzyme inhibitors or angiotensin receptor blockers (ARBs) for high blood pressure was reported in a retrospective series of patients undergoing cerebral radiotherapy [START_REF] Carpentier | Steroid-sparing effects of angiotensin-II inhibitors in glioblastoma patients[END_REF]. This observation was further extended in MRI analysis of newly diagnosed GBM patients, treated or not with ARBs. The volumes of peritumoral hyper T2-Fluid Attenuated Inversion Recovery (FLAIR) signal were significantly lower in patients treated with ARBs when compared to the non ARB-treated group, suggesting that ARBs might be associated with reduced peritumoral edema [START_REF] Kourilsky | Impact of Angiotensin-II receptor blockers on vasogenic edema in glioblastoma patients[END_REF]. These observations were supported by studies reporting local expression of angiotensinogen and angiotensin receptors in human glioblastoma [START_REF] Fogarty | Multiple angiotensin receptor subtypes in normal and tumor astrocytes in vitro[END_REF][START_REF] Juillerat-Jeanneret | Renin and angiotensinogen expression and functions in growth and apoptosis of human glioblastoma[END_REF], and reduction of angiogenesis and VEGF expression by ARBs in several tumor models [START_REF] Fujita | Blockade of angiotensin AT1a receptor signaling reduces tumor growth, angiogenesis, and metastasis[END_REF][START_REF] Egami | Role of host angiotensin II type 1 receptor in tumor angiogenesis and growth[END_REF][START_REF] Arrieta | Blockage of angiotensin II type I receptor decreases the synthesis of growth factors and induces apoptosis in C6 cultured cells and C6 rat glioma[END_REF], including glioma [START_REF] Rivera | AT1 receptor is present in glioma cells; its blockage reduces the growth of rat glioma[END_REF].

The ASTER study was thus designed as a randomized, placebo-controlled, clinical trial to assess whether or not the addition of Losartan, an ARBs that cross the blood-brain barrier [START_REF] Arrieta | Blockage of angiotensin II type I receptor decreases the synthesis of growth factors and induces apoptosis in C6 cultured cells and C6 rat glioma[END_REF][START_REF] Rivera | AT1 receptor is present in glioma cells; its blockage reduces the growth of rat glioma[END_REF] to standard of care (SOC) can reduce steroid requirement during RT in patients with newly diagnosed GBM.

PATIENTS AND METHODS

Trial design

ASTER was a multi-center (7 academic centers in France), prospective, double-blinded, placebocontrolled, randomized (1:1), phase III trial. The study was compliant with the Declaration of Helsinki and Good Clinical Practice guidelines. The full trial protocol can be found in Appendix 1.

The trial was registered with ClinicalTrials.gov, number NCT01805453.

Patient Eligibility criteria

The study was open to patients aged 18 years and older, with newly diagnosed GBM (World Health Organization (WHO) grade IV GBM), and with a Karnofsky performance status (KPS) score of 50% or higher. Patients had to be eligible for standard RT with concomitant TMZ [START_REF] Stupp | Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma[END_REF], with radiotherapy starting within 10 weeks after surgery.

Exclusion criteria were the absence of residual tumor left on the screening MRI (complete surgical resection), any prior treatment of glioblastoma including any local therapy during surgical resection, any treatment for high blood pressure (whatever the therapeutic class of drugs), systolic blood pressure <110 mmHg, relative or definite contra-indication to Losartan, pregnant or breast feeding women and MRI contra-indication.

Before being included in the study, patients signed an informed consent form, which was approved by the institutional review board.

Treatment and drug administration

After biopsy or partial surgical resection, patients with histologically confirmed glioblastoma and planned for standard of care (SOC) with RT and TMZ were randomized between Losartan or placebo.

Randomization was stratified by center and by type of intervention (biopsy vs surgical resection). The treatment (Losartan/placebo) was started 7-10 days before the beginning of concomitant RT and TMZ, and was maintained during the total study duration.

A progressive escalation in Losartan 50mg/placebo treatment was done, starting with 1 tablet/day during the first 7-10 days, then, twice a day (100mg/day) until halting for any reason (end of the study period, unacceptable toxicity, withdrawal of patient consent or death). The treatment by Losartan/placebo was stopped one month after completion of RT. Treatment with any other approved or investigational chemotherapeutic agents was not allowed until tumor recurrence. If a patient experienced tumor progression, second-line chemotherapy was offered per local practice.

Patient Surveillance and Follow-up

A complete physical examination with collection of laboratory parameters was performed within 1 week before treatment initiation. At baseline, end of RT and each follow-up visits, neurological and general examinations, complete blood count, serum biochemistry, liver function tests and steroid dosage adaptation were performed.

The KPS was assessed by the treating clinician at baseline, at the end of RT and at each follow-up visits. Magnetic resonance imaging (MRI) were performed at the end of RT, one month after completion of RT and then every other month until radiological progression Toxicity was graded according to the NCI expanded Common Toxicity Criteria (NCI CTC 3.0). Radiologic progressions were assessed by the local investigators, following RANO criteria.

Steroids dosage management

The steroid dosage (always converted to equivalent prednisone) was assessed by the clinician on charge of the patient on a weekly basis. The corticosteroids had to be prescribed at the minimal dose required to control the mass effect or symptoms of cerebral edema. The indicative recommendation was to tapper steroids by 2-10mg/day every week (eq prednisone) if the patient was clinically stable.

In case of clinical worsening, the recommendation was to increase dosage by 30mg/day (eq prednisone) if the patient experienced headaches and nausea/vomiting, and by 10-20mg/day (eq prednisone) in other cases.

Peritumoral edema assessment

Evolution of cerebral edema on MRI (based on MRI at the beginning, at the end and 1 month after the

MGMT methylation status

The MGMT status of glioblastoma patients was determined using pyrosequencing (PSQ). A 10 µm FFPE section of tumoral tissue with histologically estimated tumor cell content above 50% was first processed with the EpiTect Fast FFPE Bisulfite Kit (Qiagen, France), following manufacturer's recommendation. PSQ was then performed on this Bisulfite treated DNA as previously described [START_REF] Quillien | Comparative assessment of 5 methods (methylation-specific polymerase chain reaction, MethyLight, pyrosequencing, methylationsensitive high-resolution melting, and immunohistochemistry) to analyze O6-methylguanine-DNA-methyltranferase in a series of 100 glioblastoma patients[END_REF][START_REF] Quillien | Validation of the high-performance of pyrosequencing for clinical MGMT testing on a cohort of glioblastoma patients from a prospective dedicated multicentric trial[END_REF] using the PyroMark Q24 MGMT kit on a PyroMark Q24 system (Qiagen, France). All assays were performed in duplicate and each sample was averaged together. The average percentage of the 5 CpGs tested was considered and a cut-off of 8% was used to classify the patients [START_REF] Quillien | Comparative assessment of 5 methods (methylation-specific polymerase chain reaction, MethyLight, pyrosequencing, methylationsensitive high-resolution melting, and immunohistochemistry) to analyze O6-methylguanine-DNA-methyltranferase in a series of 100 glioblastoma patients[END_REF].

IDH1R132H expression status

IHC staining for IDH1 R132H mutant protein was carried out on 4-µm paraffin sections of formalinfixed tumor samples using mouse monoclonal anti-R132H-IDH1 antibody culture supernatant, as previously described [START_REF] Camelo-Piragua | Mutant IDH1-specific immunohistochemistry distinguishes diffuse astrocytoma from astrocytosis[END_REF].

Statistical considerations and outcomes

The primary objective was the median steroid dosage required to control brain edema on the last day of radiotherapy in each arm. The secondary outcomes were: steroids dosage 1 month after the end of RT, assessment of cerebral edema on MRI (MRI at the beginning, at the end and 1 month after the end of RT), tolerance (NCI-CTCAE v3.0), blood pressure, HbA1C, glycemia, body weight and sideeffects of steroids, progression free survival (PFS) and OS.

According to the database of our previous patients corresponding to those to be included in the trial [START_REF] Carpentier | Steroid-sparing effects of angiotensin-II inhibitors in glioblastoma patients[END_REF], the standard deviation of the dose (corresponding to our primary criterion) was around 46 mg/day (eq prednisone), thus a sample size of N= 39 patients / group will allow a 80% power to detect a difference between groups around 30 mg/day (eq prednisone) corresponding to an effect size (∆/δ) = 0.65. We considered a two-sided 5% significance level. To consider the possibility of one patient lost for follow-up in each arm, we plan to include 40 patients in each arm.

The intent to teat (ITT) population was limited to patients that received at least one day of concomitant treatment. The ITT analysis on the primary endpoint as well as quantitative secondary criteria were carried out by mixed model of ANOVA (random factor patient, fixed factor treatment). A Log transformation was used when distribution was not Gaussian. Incidence of steroids side-effects were analyzed by Chi-2 or Fisher's exact probability test. Tolerance (NCI-CTCAE v3.0) at the beginning, at the end and 1 month after the end of RT were compared by Man-Whitney test. All tests will be twosided with a significance level at 5%.

RESULTS

Patient characteristics

Eighty patients (80) were included and randomized between March 2013 and August 2014. Five [START_REF] Marantidou | Steroid requirements during radiotherapy for malignant gliomas[END_REF] patients, who did not even start radiotherapy because of fast clinical deterioration (n=4), or treatment with placebo/Losartan because of low blood pressure (n=1), were not considered in the intent-to treat analysis (Figure 1). The characteristics of the 75 remaining patients are shown in table 1. The two groups were well balanced at baseline except for the MGMT status (Table 1).

The compliance to the treatment was good in 68/75 patients (91%). Two (2) patients had a reduced RT regimen (3 weeks) due to poor clinical conditions; 3 patients died before completion of RT; one patient stopped losartan/placebo after 4 weeks due to low blood pressure and one patient stopped Losartan/placebo after 6 days because of a Lyell syndrome, probably related to lamotrigine.

Steroids and performance status (KPS)

Within the ITT population, the steroid dosage required to control brain edema either on the last day of radiotherapy (primary endpoint), or one month after completion of RT, was not impacted by Losartan (table 2). At the time of the enrollment, 9 patients (25%) were free of steroids in losartan arm and 7 patients (19%) in placebo arm.

When steroids were required, patients were treated with methylprednisolone, prednisolone or prednisone (dexamethasone was not given to any patients).

Similarly, the performance status (KPS), either on the last day of RT, or one month after completion of RT, was not different between both arms (table 2).

Peritumoral edema

In 60 patients, the MRI were available and centrally reviewed. Two (2) of these patients died before completion of RT, leaving 58 patients (32 in the placebo arm; 26 in the Losartan arm) for this analysis.

No reduction of peritumoral edema was seen (Figure 2).

Tolerance

At the end of radiotherapy, mean blood pressure decrease from baseline was significantly higher in Losartan arm, compared to placebo; sitting systolic blood pressure (-9,9 mmHg vs -1,3 mmHg, p = 0,009), sitting diastolic blood pressure (-5,5 mmHg vs -0,9 mmHg, p = 0,036), in Losartan arm and placebo arm respectively.

The incidence of adverse events (AE) was similar in both arms (84,2% in control arm vs 83,8% in Losartan arm). In addition, 27 patients reported at least one serious adverse event (SAE), 17 (44,7%) in control arm vs 10 (27,0%) in Losartan arm (p = 0,11). Among them, 6 SAEs were considered as possibly related to the protocol: 1 in the Losartan arm (grade 4 thrombopenia) and 5 (2 grade 4 thrombopenia, 1 hypertemia, 1 headaches and 1 venous thombosis) in the control arm. In addition five

(5) SAEs were considered as possibly related to the treatment received: 1 in the Losartan arm (grade 4 thrombopenia) and 4 (1 grade 4 thrombopenia, 1 fever and 2 grade 4 alanine aminotransferase increased) in the control arm.

Survival and patient follow-up

At time of analysis, 64 pts had died, and 70 had progressed after the initial treatment. When disease progression occurred, further treatments were left at the investigator's choice. A few patients underwent a second surgery (n= 4), or a second course of RT (n=2), but most of them had one or several lines of chemotherapy. Chemotherapy consisted of TMZ (n= 10), nitrosoureas (n=31), platinium-based chemotherapy (n=10), Bevacizumab (n=38) metronomic cyclophosphamide (n=2).

Median OS was 14,2 months [10,1-21,3] in the Losartan arm and 16,7 months [START_REF] Kirste | Boswellia serrata acts on cerebral edema in patients irradiated for brain tumors: a prospective, randomized, placebo-controlled, double-blind pilot trial[END_REF][START_REF] Dubois | Gliomas and the vascular fragility of the blood brain barrier[END_REF][START_REF] Marantidou | Steroid requirements during radiotherapy for malignant gliomas[END_REF][START_REF] Roth | Corticosteroid use in neuro-oncology: an update[END_REF][START_REF] Pitter | Corticosteroids compromise survival in glioblastoma[END_REF][START_REF] Gerstner | VEGF inhibitors in the treatment of cerebral edema in patients with brain cancer[END_REF][START_REF] Chinot | Bevacizumab plus radiotherapy-temozolomide for newly diagnosed glioblastoma[END_REF][START_REF] Recht | Steroid-sparing effect of corticorelin acetate in peritumoral cerebral edema is associated with improvement in steroid-induced myopathy[END_REF][START_REF] Kirste | Boswellia serrata acts on cerebral edema in patients irradiated for brain tumors: a prospective, randomized, placebo-controlled, double-blind pilot trial[END_REF][START_REF] Carpentier | Steroid-sparing effects of angiotensin-II inhibitors in glioblastoma patients[END_REF][START_REF] Kourilsky | Impact of Angiotensin-II receptor blockers on vasogenic edema in glioblastoma patients[END_REF][START_REF] Fogarty | Multiple angiotensin receptor subtypes in normal and tumor astrocytes in vitro[END_REF][START_REF] Juillerat-Jeanneret | Renin and angiotensinogen expression and functions in growth and apoptosis of human glioblastoma[END_REF][START_REF] Fujita | Blockade of angiotensin AT1a receptor signaling reduces tumor growth, angiogenesis, and metastasis[END_REF][START_REF] Egami | Role of host angiotensin II type 1 receptor in tumor angiogenesis and growth[END_REF][START_REF] Arrieta | Blockage of angiotensin II type I receptor decreases the synthesis of growth factors and induces apoptosis in C6 cultured cells and C6 rat glioma[END_REF][START_REF] Rivera | AT1 receptor is present in glioma cells; its blockage reduces the growth of rat glioma[END_REF][START_REF] Quillien | Comparative assessment of 5 methods (methylation-specific polymerase chain reaction, MethyLight, pyrosequencing, methylationsensitive high-resolution melting, and immunohistochemistry) to analyze O6-methylguanine-DNA-methyltranferase in a series of 100 glioblastoma patients[END_REF][START_REF] Quillien | Validation of the high-performance of pyrosequencing for clinical MGMT testing on a cohort of glioblastoma patients from a prospective dedicated multicentric trial[END_REF][START_REF] Marantidou | Steroid requirements during radiotherapy for malignant gliomas[END_REF] in the placebo arm (hazard ratio (HR) = 1,14 [0,70-1,87]) (Figure 3).The median PFS was 6,6 months [5,1-9,9] in the Losartan arm and 9,5 months [START_REF] Marantidou | Steroid requirements during radiotherapy for malignant gliomas[END_REF][START_REF] Dubois | Gliomas and the vascular fragility of the blood brain barrier[END_REF][START_REF] Marantidou | Steroid requirements during radiotherapy for malignant gliomas[END_REF][START_REF] Roth | Corticosteroid use in neuro-oncology: an update[END_REF][START_REF] Pitter | Corticosteroids compromise survival in glioblastoma[END_REF][START_REF] Gerstner | VEGF inhibitors in the treatment of cerebral edema in patients with brain cancer[END_REF][START_REF] Chinot | Bevacizumab plus radiotherapy-temozolomide for newly diagnosed glioblastoma[END_REF][START_REF] Recht | Steroid-sparing effect of corticorelin acetate in peritumoral cerebral edema is associated with improvement in steroid-induced myopathy[END_REF][START_REF] Kirste | Boswellia serrata acts on cerebral edema in patients irradiated for brain tumors: a prospective, randomized, placebo-controlled, double-blind pilot trial[END_REF][START_REF] Carpentier | Steroid-sparing effects of angiotensin-II inhibitors in glioblastoma patients[END_REF][START_REF] Kourilsky | Impact of Angiotensin-II receptor blockers on vasogenic edema in glioblastoma patients[END_REF][START_REF] Stupp | Maintenance Therapy With Tumor-Treating Fields Plus Temozolomide vs Temozolomide Alone for Glioblastoma: A Randomized Clinical Trial[END_REF] in the placebo arm (log rank test, p=0,3879).

MGMT methylation status was obtained in 67 patients. Thirty-three ( 33) patients (49%) were MGMTmethylated, and 34 (51%) were not. MGMT methylation was significantly associated with overall survival: median OS was 23,8 months [START_REF] Rivera | AT1 receptor is present in glioma cells; its blockage reduces the growth of rat glioma[END_REF][START_REF] Gerstner | VEGF inhibitors in the treatment of cerebral edema in patients with brain cancer[END_REF] for methylated patients and 9,5 months [START_REF] Roth | Corticosteroid use in neuro-oncology: an update[END_REF][START_REF] Pitter | Corticosteroids compromise survival in glioblastoma[END_REF][START_REF] Gerstner | VEGF inhibitors in the treatment of cerebral edema in patients with brain cancer[END_REF][START_REF] Chinot | Bevacizumab plus radiotherapy-temozolomide for newly diagnosed glioblastoma[END_REF][START_REF] Recht | Steroid-sparing effect of corticorelin acetate in peritumoral cerebral edema is associated with improvement in steroid-induced myopathy[END_REF][START_REF] Kirste | Boswellia serrata acts on cerebral edema in patients irradiated for brain tumors: a prospective, randomized, placebo-controlled, double-blind pilot trial[END_REF][START_REF] Carpentier | Steroid-sparing effects of angiotensin-II inhibitors in glioblastoma patients[END_REF][START_REF] Kourilsky | Impact of Angiotensin-II receptor blockers on vasogenic edema in glioblastoma patients[END_REF][START_REF] Stupp | Maintenance Therapy With Tumor-Treating Fields Plus Temozolomide vs Temozolomide Alone for Glioblastoma: A Randomized Clinical Trial[END_REF] for unmethylated patients (HR = 0,28 [0,16-0,49], p-value < 0,0001). Treatment by Losartan had no impact on survival within each MGMT status subgroups of patients.

DISCUSSION

Management of cerebral edema for glioblastoma patients remains a major challenge in neurooncology. Chronic steroid administration reduces vasogenic cerebral edema but results in many adverse effects that impairs quality of life, underscoring the need for corticoid-sparing drugs [START_REF] Roth | Corticosteroid use in neuro-oncology: an update[END_REF][START_REF] Pitter | Corticosteroids compromise survival in glioblastoma[END_REF][START_REF] Galicich | Use of dexamethasone in treatment of cerebral edema associated with brain tumors[END_REF].

Because several studies have suggested that ARBs might reduce VEGF secretion [START_REF] Jain | Antiangiogenesis strategies revisited: from starving tumors to alleviating hypoxia[END_REF][START_REF] Ager | The renin-angiotensin system and malignancy[END_REF] and vasogenic peritumoral edema in GBM [START_REF] Carpentier | Steroid-sparing effects of angiotensin-II inhibitors in glioblastoma patients[END_REF][START_REF] Kourilsky | Impact of Angiotensin-II receptor blockers on vasogenic edema in glioblastoma patients[END_REF][START_REF] Jain | Antiangiogenesis strategies revisited: from starving tumors to alleviating hypoxia[END_REF], this randomized trial was designed to assess the impact of ARB (Losartan) in patients with newly-diagnosed GBM. The steroid dosage required to control brain edema on the last day of radiotherapy in losartan arm was the primary objective of our trial. No significant differences were observed between the study groups. This endpoint was probably showing reductions of both steroid dosage and peritumoral FLAIR images on MRI [START_REF] Carpentier | Steroid-sparing effects of angiotensin-II inhibitors in glioblastoma patients[END_REF][START_REF] Kourilsky | Impact of Angiotensin-II receptor blockers on vasogenic edema in glioblastoma patients[END_REF]. These discrepancies might be explained by the retrospective nature of these studies, and/or association with other drugs taken by ARBs-treated patients in these series. However, we cannot exclude that the time exposure to Losartan in this trial (<3 months) was too short to mediate significant biological effects. A limited time exposure to Losartan/placebo was selected for this trial because the primary endpoint was focused on steroid requirement during the period of radiotherapy.

Over the recent years, a considerable interest was raised about the potential antitumor properties of Angiotensin-II inhibitors. The scientific basis behind this interest relies on the fact that Angiotensin II/ angiotensin II receptor type 1(AngII/AT1R) signaling promotes VEGF-mediated angiogenesis [START_REF] George | The renin-angiotensin system and cancer: old dog, new tricks[END_REF],

and that tumor growth is inhibited by angiotensin-II inhibitors in several tumor models [16 -18]. In addition, Losartan can improve tumor perfusion, hence exposure to chemotherapy, through vascular decompression, doing so by reducing matrix components (CAFs, hyaluronan and collagen) synthesis, responsible for high intra-tumoral pressure [START_REF] Chauhan | Angiotensin inhibition enhances drug delivery and potentiates chemotherapy by decompressing tumour blood vessels[END_REF]. Finally, several retrospective studies have shown improved survival in patients taking angiotensin system inhibitors, in several cancers such as pancreatic cancer [START_REF] Nakai | Inhibition of renin-angiotensin system affects prognosis of advanced pancreatic cancer receiving gemcitabine[END_REF], lung cancer [START_REF] Wilop | Impact of angiotensin I converting enzyme inhibitors and angiotensin II type 1 receptor blockers on survival in patients with advanced non-smallcell lung cancer undergoing first-line platinum-based chemotherapy[END_REF][START_REF] Menter | Effect of angiotensin system inhibitors on survival in patients receiving chemotherapy for advanced non-small-cell lung cancer[END_REF], renal cancer [START_REF] Keizman | Angiotensin system inhibitors and outcome of sunitinib treatment in patients with metastatic renal cell carcinoma: a retrospective examination[END_REF][START_REF] Izzedine | Hypertension and angiotensin system inhibitors: impact on outcome in sunitinib-treated patients for metastatic renal cell carcinoma[END_REF] and even glioblastomas [START_REF] Januel | Impact of renin-angiotensin system blockade on clinical outcome in glioblastoma[END_REF][START_REF] Levin | Effect of angiotensin system inhibitors on survival in newly diagnosed glioma patients and recurrent glioblastoma patients receiving chemotherapy and/or bevacizumab[END_REF]. One recent retrospective analysis did not support this hypothesis [START_REF] Happold | Do statins, ACE inhibitors or sartans improve outcome in primary glioblastoma?[END_REF]. In our prospective randomized trial, no significant difference in survival between Losartan and placebo treated patients was seen. Yet, no definite conclusion can be drawn, because patients were exposed to Losartan for less than 3 months, over a 15-month median survival time. In addition, even though the patients were randomly assigned, the treatment groups were misbalanced on MGMT status (36% vs 61% of methylated MGMT in patients with available MGMT status in the Losartan and placebo-treated groups, respectively). Tumor expression of MGMT is a major prognosis factor in GBM patients [START_REF] Hegi | MGMT gene silencing and benefit from temozolomide in glioblastoma[END_REF] and in our trial, MGMT methylation was indeed associated with median overall survival (23.8 vs 9.5 months for methylated and unmethylated MGMT status, respectively, p< 0,0001).

In conclusion, despite the documented role of the AngII/ATR1 axis in VEGF-mediated angiogenesis, angiotensin-II inhibitors did not show any impact on steroid requirements during radiotherapy in this trial.

TABLE CAPTIONS

Table 1: Patients characteristics.

Table 2: Steroid dosage required to control brain edema and the performance status (KPS) at inclusions, on the last day of RT and one month after completion of RT.

FIGURE CAPTIONS

  end of RT), by the extraction of [T2-FLAIR volume] -[gadolinium-enhanced volume] from MRI images, was done retrospectively by 2 investigators. The T2-FLAIR and T1 gadolinium-enhanced volumes were calculated from MRI using the Philips' Pinnacle treatment planning system (TPS) with the Pinnacle version P16.02 (C. L-P; AF.C.).

  not an optimal one, because tapering steroids in a given patient closely depends on subjective and repeated assessments by the physicians, who might leave the steroid dosage unchanged during radiotherapy. Studies of peritumoral edema on MRI might be more accurate, although modifications of steroid dosages over time and inherent difficulty to measure edema on MRI raise other issues. On MRI, GBM are classically described as heterogeneously contrast enhancement surrounded by peritumoral hyper T2-FLAIR signal that reflects a combination of edema and tumor infiltration. In this trial, the evolution of cerebral edema on MRI at the end and 1 month after the end of RT, was arbitrary evaluated by the formula [T2-FLAIR volume] -[gadolinium-enhanced volume]. No significant reduction of peritumoral edema was seen in patients taking ARBs, one month after RT .These disappointing results are in sharp contrast with previous studies made in series of glioblastoma patients

Figure 1 :Figure 2 :

 12 Figure 1: Flow chart

Figure 3 :

 3 Figure 3: Kaplan Meier survival curve of patients treated within the protocol. Overall survival (upper left) and Progression free survival (upper right) in the ITT population. Overall survival in MGMTunmethylated patients (lower left) and in methylated MGMT patients (lower right).

Fig 2 :

 2 Fig 2: Estimation of the peritumoral edema [T2-FLAIR volume] -[gadolinium-enhanced volume] variations from baseline, (median [Q1-Q3] min max in cm3).

Acknowledgments: Study was supported by the Association Oligocyte and the Association pour le développement des neurosciences à Avicenne (ADNA).

Funding: This work was supported by the Assistance Publique -Hôpitaux de Paris (Département de la Recherche Clinique et du Développement) -sponsor of the trial. The study was funded by a grant from Programme Hospitalier de Recherche Clinique -PHRC 2012 (Ministère de la Santé).

Conflict of Interest Statement:

OC is consultant for Hoffman -La Roche, Ipsen and Abbvie. ELR has received research grants from Mundipharma and Amgen and honoraria for lectures or advisory boards from Abbvie, Daiichi-Sankyo, Mundipharma, Novartis. FD has received honoraria from Hoffman -La Roche, UCB Pharma, GSK, BMS, Abbvie. Other authors have nothing to declare.

Karnofsky performance score (KPS)

Median (Q1-Q3) 80,0 (70,0-90,0) 80,0 (70,0-90,0)

Steroid dosage at inclusion, (mg/day) (eq prednisone)

Median (Q1-Q3) 40,0 (0,0-60,0) 40,0 (20,0-60,0)

Table 2

Steroid dosage required to control brain edema and the performance status (KPS) at inclusions, on the last day of RT and one month after completion of RT.


Delhia Alby

Fabrice Salles

Amine Geneste

Bénédicte Prélot

Jerzy Zajac

Clarence Charnay
email: clarence.charnay@umontpellier.fr

Microwave-assisted hydrothermal synthesis of manganate nanoflowers for selective retention of strontium

Keywords: manganate nanoflowers, hydrothermal synthesis, microwave-assisted synthesis, adsorption selectivity, strontium, cesium, multicomponent aqueous solutions, radioactive remediation

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Two dimensional metal oxides have attracted increasing research interest due to their multiple functionalities, such as electronic behavior, optical and charge transfer properties, surface reactivity, large specific surface area, mechanical flexibility [START_REF] Yang | Advances and challenges in chemistry of two-dimensional nanosheets[END_REF][START_REF] Niu | From two-dimensional materials to heterostructures[END_REF][START_REF] Kalantar-Zadeh | Two dimensional and layered transition metal oxides[END_REF]. Various stacking nanostructures were elaborated with the 2D nanosheets in order to design advanced materials with tailored properties [START_REF] Li | Heterostructures based on two-dimensional layered materials and their potential applications[END_REF]. Therefore, the development of single or multilayer materials has gained a broad attention and has demonstrated its value in a wide range of applications like in catalysis and electrocatalysis [START_REF] Roberts | Birnessite nanotubes for electrochemical supercapacitor electrodes[END_REF][START_REF] Zhang | Growth of manganese oxide nanoflowers on vertically-aligned carbon nanotube arrays for high-rate electrochemical capacitive energy storage[END_REF][START_REF] Ni | Low-temperature synthesis of monodisperse 3D manganese oxide nanoflowers and their pseudocapacitance properties[END_REF][START_REF] And | Synthesis of Monodisperse Nanocrystals of Manganese Oxides[END_REF][START_REF] Zhang | Shape-Controllable Synthesis and Electrochemical Properties of Nanostructured Manganese Oxides[END_REF][START_REF] Cheng | High-Power Alkaline Zn-MnO2 Batteries Using γ-MnO2 Nanowires/Nanotubes and Electrolytic Zinc Powder[END_REF], nanoelectronics [START_REF] Elshof | Electronic band structure and electron transfer properties of twodimensional metal oxide nanosheets and nanosheet films[END_REF][START_REF] Osada | Exfoliated oxide nanosheets: new solution to nanoelectronics[END_REF], fuel cells or energy storage devices [START_REF] Chen | Microwave-Hydrothermal Crystallization of Polymorphic MnO 2 for Electrochemical Energy Storage[END_REF], gas sensors [START_REF] Tian | Trace level detection of hydrogen gas using birnessite-type manganese oxide[END_REF] and biosensors, photovoltaic devices [START_REF] Furchi | Photovoltaic Effect in an Electrically Tunable van der Waals Heterojunction[END_REF], biomedical research [START_REF] Yang | Advances and challenges in chemistry of two-dimensional nanosheets[END_REF], gas barrier layers, and many others [START_REF] Wang | Titanium oxide nanosheets: graphene analogues with versatile functionalities[END_REF]. Layered materials are also known to be efficient adsorbents for the removal of radioactive and heavy metal cations [START_REF] Roth | Layer like porous materials with hierarchical structure[END_REF][START_REF] Alby | Recent developments in nanostructured inorganic materials for sorption of cesium and strontium: Synthesis and shaping, sorption capacity, mechanisms, and selectivity-A review[END_REF][START_REF] Galamboš | Sorption of anthropogenic radionuclides on natural and synthetic inorganic sorbents[END_REF][START_REF] Filipowicz | Adsorption of 137Cs on titanate nanostructures[END_REF][START_REF] Sarina | Separate or simultaneous removal of radioactive cations and anions from water by layered sodium vanadate-based sorbents[END_REF]. Indeed, the interlayer space of a lamellar structure can fit to the dimension of the inserted ions or molecules through a swelling/shrinking process as the interaction between the layers is weak and easily breakable. These properties have been largely exploited during the exfoliation of inorganic nanosheets or intercalation of molecules and ions, thus resulting in the modification of numerous material properties (flexibility, layer rigidity, reactivity, porosity, hydrophiliclipophilic balance, etc.) [START_REF] Roth | Layer like porous materials with hierarchical structure[END_REF][START_REF] Mcnulty | Optimizing the structure and yield of vanadium oxide nanotubes by periodic 2D layer scrolling[END_REF][START_REF] Krumeich | Morphology and Topochemical Reactions of Novel Vanadium Oxide Nanotubes[END_REF][START_REF] Reinoso | Controlled Uptake and Release of Metal Cations by Vanadium Oxide Nanotubes[END_REF][START_REF] Li | Synthesis of Single Crystal Manganese Oxide Octahedral Molecular Sieve (OMS) Nanostructures with Tunable Tunnels and Shapes[END_REF]]. As a typical example of potential uses, the swelling clays were tested for the selective removal of heavy metals [START_REF] Uddin | A review on the adsorption of heavy metals by clay minerals, with special focus on the past decade[END_REF].

Nevertheless, the development of new ion-exchange materials remains an important goal for the purification of the radioactive aqueous effluents produced from the activities of the nuclear power stations, research centers or military sites, or arising from nuclear accidents or decontamination processes [START_REF] Ghaly | Evaluation of synthetic Birnessite utilization as a sorbent for cobalt and strontium removal from aqueous solution[END_REF]. Such inorganic materials as zeolites and clays have been fully investigated as an alternative to organic resins [START_REF] Faghihian | Evaluation of a new magnetic zeolite composite for removal of Cs+ and Sr2+ from aqueous solutions: Kinetic, equilibrium and thermodynamic studies[END_REF][START_REF] Wahlberg | Adsorption of Cesium on Clay Minerals -Geological Survey Bulletin 1140-A[END_REF]. More recently, layered titanates or manganates have been proposed as plausible candidates for remediation purposes on account of their high sorption capacity and selectivity towards some monovalent or divalent cations [START_REF] Dyer | Sorption characteristics of radionuclides on synthetic birnessite-type layered manganese oxides[END_REF][START_REF] Kasap | Titanate nanotubes: preparation, characterization and application in adsorption of strontium ion from aqueous solution[END_REF]. Titanate and manganate nanomaterials have been largely studied over the years regarding their different dimensional structures (1D, 2D or 3D) [START_REF] Li | Synthesis of Single Crystal Manganese Oxide Octahedral Molecular Sieve (OMS) Nanostructures with Tunable Tunnels and Shapes[END_REF][START_REF] Wang | Removal of multi-heavy metals using biogenic manganese oxides generated by a deep-sea sedimentary bacterium -Brachybacterium sp[END_REF] in decontamination uses [START_REF] Lopano | Cs-exchange in birnessite: reaction mechanisms inferred from time-resolved X-ray diffraction and transmission electron microscopy[END_REF] and green chemistry [START_REF] Oh | Layered nanomaterials for green materials[END_REF]. Birnessite-like phases, the most investigated synthetic manganate nanostructures, are related to natural phyllomanganates which exhibit a basal spacing of 7 Å for one layer of structured water in the case of the birnessite and 10 Å for two layers of water in the case of the buserite structure. This layered structure presents edge sharing MnO6 octahedra with a net negative layer charge due to the intra-framework substitution of Mn 3+ for Mn 4+ , balanced by various interlayer cations such as Na + , Ca 2+ or K + [START_REF] Lanson | Structure of synthetic Na-birnessite: evidence for a triclinic one-layer unit cell[END_REF]. This material is especially interesting as ion exchanger, since it can fix cations very strongly [START_REF] Ghaly | Evaluation of synthetic Birnessite utilization as a sorbent for cobalt and strontium removal from aqueous solution[END_REF][START_REF] Lopano | Cs-exchange in birnessite: reaction mechanisms inferred from time-resolved X-ray diffraction and transmission electron microscopy[END_REF][START_REF] Vir | Studies on the adsorption of cesium and strontium radionuclides on hydrated manganese oxide[END_REF][START_REF] Mckenzie | The Synthesis of Birnessite, Cryptomelane, and Some Other Oxides and Hydroxides of Manganese[END_REF]. Several synthesis pathways were investigated to optimize its structure and properties useful for many different applications [START_REF] Ni | Low-temperature synthesis of monodisperse 3D manganese oxide nanoflowers and their pseudocapacitance properties[END_REF]. Since then, the efforts have focused on various nanostructures that this material can produce (i.e., nanotubes, nanobelts, nanoflowers…) [START_REF] Zhang | Growth of manganese oxide nanoflowers on vertically-aligned carbon nanotube arrays for high-rate electrochemical capacitive energy storage[END_REF][START_REF] Chen | Microwave-Hydrothermal Crystallization of Polymorphic MnO 2 for Electrochemical Energy Storage[END_REF][START_REF] Li | Synthesis of Single Crystal Manganese Oxide Octahedral Molecular Sieve (OMS) Nanostructures with Tunable Tunnels and Shapes[END_REF]. It was shown that their optical and electronic properties were dependent on the shape, size, and crystallographic structure [START_REF] Zhang | Shape-Controllable Synthesis and Electrochemical Properties of Nanostructured Manganese Oxides[END_REF]. Moreover, these manganese oxides exhibited notable surface charge due to their redox properties (versatility of the manganese oxidation number) leading to noticeable cation-exchange properties and thus a high reactivity in sorption phenomena [START_REF] Lanson | Structure of synthetic Na-birnessite: evidence for a triclinic one-layer unit cell[END_REF]. It was reported that the cation exchange in these materials occurred within the interlayer space being accompanied by a layer swelling mechanism [START_REF] Lopano | Cs-exchange in birnessite: reaction mechanisms inferred from time-resolved X-ray diffraction and transmission electron microscopy[END_REF][START_REF] Kuma | Arrhen1us, Crystal structures of synthetic 7 A and 10 A manganates substituted by mono-and divalent cations[END_REF][START_REF] Golden | Iion exchange, thermal transformations, and oxidizing properties of birnessite[END_REF]. However, the manganate nanoflowers or manganate nanospheres achieved by a hydrothermal synthesis route in an acid medium were demonstrated to exhibit a poor performance for Sr 2+ retention [START_REF] Kim | Selective sorption of strontium using two different types of nanostructured manganese oxides[END_REF].

In the present study, a microwave assisted hydrothermal synthesis in basic conditions was proposed to obtain homogenous manganate nanoflowers. In regards with cesium and strontium isotopes representing the predominant radionuclides released into the environment after a nuclear accident, the sorption performances of such nanoflowers were tested in different aqueous media so as to monitor various competitive effects occurring during the adsorption process carried out under conditions close to real uses. The combination between the results of Density Functional Theory (DFT) calculations and isothermal titration calorimetry measurements contributed to shed light on the ion-exchange mechanism within manganate nanoflowers.

Experimental 1) Materials

Potassium permanganate was obtained from Prolabo, hexadecylamine (>94%) from Merck Millipore, whereas cesium nitrate (99%), strontium nitrate (98%) and calcium nitrate tetrahydrate (99%) were Sigma-Aldrich products. Ultrapure water was obtained with a Purelab purification system (18 µS cm -1 ). Regarding the adsorption isotherms, the different compositions of mineral water used as mean solvents are reported in Table 1. The magnetron delivered a puissance up to 1200 W (2400 Hz) and the temperature was controlled by an optical fiber system.

The nanoflower synthesis was adapted from a conventional hydrothermal synthesis of manganate nanotubes in a classical oven [START_REF] Roberts | Birnessite nanotubes for electrochemical supercapacitor electrodes[END_REF]. Different parameters were investigated, including the temperature and duration of the hydrothermal treatment under microwave radiation. The duration of the first step corresponding to the addition of the amine template into the manganate solution was also an adjustable parameter (not investigated here with a Perkin Elmer Optima 7000 DV. This method was also used to measure the amount adsorbed of strontium. The material was first dissolved in acid media (nitric and perchloric acids) and the measurement was realized by axial view for manganese and radial view for strontium.

3) Adsorption

In order to evaluate the retention performance (capacity and selectivity) of the manganate nanomaterials towards cesium and strontium ions, their adsorbed amount Γads and the adsorption isotherms were determined according to the depletion method with the following procedure. A solid sample (0.020 g) was first placed into a Nalgene™ reactor and a 20 mL solution of cesium nitrate (from 0.05 mmol L -1 to 4 mmol L -1 ) and/or strontium nitrate (from 0.03 mmol L -1 to 2.5 mmol L -1 ) was added. To reach the adsorption equilibrium, the tubes were stirred on a rotary shaker overnight at 298 K. To separate the solid phase from the liquid supernatant, the tubes were centrifuged at 11000 rpm for 10 minutes. After filtration (porosity of 0.22 µm), the supernatant was analyzed by Ionic Chromatography (IC). The cationic composition was thus determined with a Shimadzu device composed by a Shim-pack IC-C1 column and a CDD-6A conductivity detector at 313 K. The volume of injection was of 45 μL and the flow rate of 1.5 mL min -1 . The mobile phase used to measure the cation content was composed of nitric (5 mmol L -1 ) for cesium analysis and a mixture tartaric acid (4 mmol L -1 ) and ethylenediamine (1.4 mmol L -1 ) for strontium and calcium analysis.

The enthalpy changes accompanying adsorption were measured in a differential titration nanocalorimeter in a heat flow mode and installed in a TAM III thermostat working at a constant temperature kept at 298 K within ±0.0001. The operating procedures and data processing were detailed previously [START_REF] Prelot | On the real performance of cation exchange resins in wastewater treatment under conditions of cation competition: the case of heavy metal pollution[END_REF]. A 4 mg solid sample was introduced into the calorimeter cell containing 0.8 mL of ultrapure water and maintained under constant stirring for 2 hours to attain the thermal equilibrium (the reference cell was only filled with water). A syringe system was used to perform 10 sec injections of 10 µL of the appropriate solution (cesium or strontium nitrate). The thermal peaks were recorded after an equilibrium of 45 min between two injections and the area under each peak was determined to calculate the corresponding enthalpy change. A similar procedure was employed to evaluate the effect of dilution [START_REF] Prelot | On the real performance of cation exchange resins in wastewater treatment under conditions of cation competition: the case of heavy metal pollution[END_REF]. Each calorimetric run was repeated 3 times.

4) Computational part

Structure simulation and parameters for manganese oxide

The computational effort was made to determine a correct model of the structure for manganese oxide saturated with K + as interlayer cations. The initial atomic coordinates of the framework were taken from the literature after comparison with different birnessite structures.

The structure built from Post and Veblen was used [START_REF] Post | Crystal structure determinations of synthetic sodium, magnesium, and potassium birnessite using TEM and the Rietveld method[END_REF]. Then, the unit cell parameters were fixed based on the XRD data obtained in this work as a function of the nature of the interlayer cation and using a C2/C symmetry. Finally, a plausible crystallographic structure was proposed with the imposed symmetry.

Theoretical diffractograms were thus generated with Reflex software using the simulated structures and further comparison with the experimental data allowed the nature of the synthesized solids to be confirmed.

Determination of accessible surface area and pore volume

Following Düren et al. [START_REF] Düren | Calculating Geometric Surface Areas as a Characterization Tool for Metal -Organic Frameworks[END_REF], both the accessible surface area and the pore volume of the simulated structure models were calculated. The surface was thus calculated using the center of a nitrogen probe molecule rolling across the surface. In this case, the diameter of the nitrogen probe molecule was fixed at 3.68,1 pm, while the Universal Force Field (UFF) was used for the framework atoms [START_REF] Rappé | UFF, a full periodic table force field for molecular mechanics and molecular dynamics simulations[END_REF].

Exchange energy determination

In order to estimate the energy required to replace the interlayer cation, Density Functional Theory (DFT) calculations were performed on the structures containing interlayer cations. For this purpose, the previous structures validated by comparison with the experimental XRD patterns were considered as the starting point. The so-obtained models were therefore geometry optimized based on DFT calculations assuming fixed unit cell parameters. In complement, calculations were also performed on both the cations (K + , Ca 2+ , Sr 2+ , Cs + ) alone and the corresponding structures without interlayer cations so as to determine the adsorption energy using the following equation: Eads = ECox -EOx -EC where ECox, EOx and EC correspond to the energy of the oxide structure with C + cations, the one without C + cations and the intrinsic energy of C + . The base of this calculation is summarized in the scheme 1.

Scheme 1. Evaluation of the adsorption energy for the cation (in blue) between manganate layers (in black). This energy obtained from DFT calculation corresponds to the difference between the energy of (a) and the sum of (b) and (c).

To ensure consistency in the comparison of data, the calculations were done on the double cell (i.e., according to the b axis if c corresponds to the axis of the stacking). These DFT calculations were performed using a DFT approach with a double numerical radial function basis set (DMol 3 software package) as well as the PW91 GGA density functional containing polarization functions on hydrogen atoms (DNP).

Results and discussion

Manganate nanoflowers were initially reported by direct electrodeposition of manganese oxide on a substrate consisting of a film of carbon nanotubes [START_REF] Zhang | Growth of manganese oxide nanoflowers on vertically-aligned carbon nanotube arrays for high-rate electrochemical capacitive energy storage[END_REF]. A synthesis route at low temperature was also described by mixing potassium permanganate and formamide for 8 hours [START_REF] Ni | Low-temperature synthesis of monodisperse 3D manganese oxide nanoflowers and their pseudocapacitance properties[END_REF]. Microwave-assisted synthesis methods have been used for several years to synthesize various porous adsorbents because the microwave energy can lead to much higher heating rates, thereby improving their preparation and decreasing significantly the synthesis time from days to hours [START_REF] Wu | Ordered mesoporous Znbased supported sorbent synthesized by a new method for high-efficiency desulfurization of hot coal gas[END_REF][START_REF] Cao | Advances in microwave assisted synthesis of ordered mesoporous materials[END_REF]. Manganate nanoflowers were achieved in this way by hydrothermal method in acid medium at 373 K performed for 25 minutes under microwave irradiation [START_REF] Li | Facile controlled synthesis and growth mechanisms of flower-like and tubular MnO 2 nanostructures by microwave-assisted hydrothermal method[END_REF]. An alternative procedure with a microwave-assisted hydrothermal treatment in basic conditions is proposed in the present work.

The morphology of the structure was observed by Scanning Electron Microscope (SEM) and Transmission Electronic Microscopy (TEM) where distinct nanoflower structures appeared well formed and homogeneous (Figure 1). Elemental analysis confirmed the absence of the amine template in the final material, since no nitrogen was detected whereas only carbon traces (0.60 wt%) were measured. This evidences that the amine has been totally removed throughout the washing step. Moreover, the material is mainly composed of Mn (IV) with some traces of Mn (III), as inferred from the XPS analysis (Fig. S1 in Supporting Information). The chemical formula corresponds to KMn4O8.

Taking this formula for simulation of the nanostructure, it was possible to compare the simulated diffraction patterns with the experimental ones. As reported in Fig. 2a, a very good agreement was found, thus indicating that the theoretically generated structure coincided with that of the synthesized solid. Moreover, the concordance between the simulated and experimental X-ray diffraction patterns confirms that no other crystalline phase containing manganese is present in the sample. The so-obtained material therefore presents a monoclinic structure (C2/m) with a = 514.9 pm, b = 568.6 pm, c = 731.0 Å and β=100.76°. Using this structure, such textural properties as specific surface area and pore volume could be estimated theoretically. Such data could not be obtained experimentally from the results of gas nitrogen adsorption at 77 K since a thermal pre-treatment under vacuum is required before the measurement of the adsorption isotherm. Such a step leads to the closure of the interlayer space of the solid due to the dehydration. However, the lamellar materials keep their structure as already observed in the case of the swelling clays [START_REF] Salles | Hydration sequence of swelling clays: Evolutions of specific surface area and hydration energy[END_REF]. Moreover, these conclusions are well supported by the Fig. S2 in Supporting Information. When the temperature is increased to 373K under vacuum, a shift of the (00l) peak to the wide angles is observed on X-ray diffraction diagram in agreement with the collapse of the interlayer space. In consequence, the molecular simulation was the only way to evaluate these parameters (see Table 2). It is important to note however that the compound is stable up to 873 K, as evidenced by XRD in Fig. 2b. This thermal stability is of high importance in view of potential uses of the material in the high level waste disposal or thermal catalytic degradation in which the temperature can reach 400-500 K [START_REF] Tian | Trace level detection of hydrogen gas using birnessite-type manganese oxide[END_REF][START_REF] Jullien | Physicochemical Reactivity in Clay-Rich Materials: Tools for Safety Assessment[END_REF][START_REF] Eren | Performance of layered birnessite-type manganese oxide in the thermal-catalytic degradation of polyamide 6[END_REF]. It follows that the structure slightly evolves upon ion exchange between K + and Cs + , which leads to comparable textural properties. The range of specific surface area is in good agreement with the values generally obtained in the case of swelling clays such as montmorillonites: their specific surface areas can vary from 300-1200 m² g -1 as a function of the nature of the interlayer cation [START_REF] Salles | Hydration sequence of swelling clays: Evolutions of specific surface area and hydration energy[END_REF]. In contrast, the solid saturated with Sr 2+

undergoes shrinkage of the structure probably due to the higher cation charge compared to those of alkali cations.

Adsorption performance of manganate nanoflowers

Adsorption isotherms for strontium and cesium cations were measured to determine the adsorption capacity and affinity of the solid material towards a given cation by using ultrapure water and two types of mineral water (their compositions are reported in Table 1) to prepare aqueous solutions of strontium and cesium. The results obtained at 298 K are given in Fig. 3. Regarding the adsorption of Sr 2+ , the adsorption isotherms represent high-affinity curves with a quasi-vertical initial portion and a saturation plateau at higher equilibrium concentrations. When this bivalent cation is adsorbed from aqueous solutions in ultrapure water, the initial steep portion of the isotherm can extend up to an amount adsorbed of 1.1 mmol g -1 , whereas the maximum adsorption capacity is about 1.2 mmol g -1 (2.4 meq g -1 ). In this case, the distribution coefficient (Kd in mL g -1 ) can be evaluated in Henry's law region [START_REF] Alby | Recent developments in nanostructured inorganic materials for sorption of cesium and strontium: Synthesis and shaping, sorption capacity, mechanisms, and selectivity-A review[END_REF]. The value of Log(Kd) estimated in this manner is about 6 which indicates that almost all strontium cations available in the solution have been retained by the solid surface, thereby replacing the potassium counter-ions initially present in the interlayer space, as described in the Scheme 2. Moreover, the exchange mechanism thus described is well supported by the exchange ratio between the desorbed K + and adsorbed Sr 2+ which is close to 2 as displayed on Fig. S3 in Supporting Information.

Scheme 2. Schematic diagram for the preferential adsorption of Sr 2+ in manganates

According to the layered structure of manganate nanoflowers, the interlayer space of their lamellar structure can adjust to fit to the large dimension of the inserted Sr 2+ ions (hydrated radius of 0.412 nm) [START_REF] Nightingale | Phenomenological Theory of Ion Solvation. Effective Radii of Hydrated Ions[END_REF] through a swelling/shrinking process. The abovementioned performance parameters of the manganate nanoflowers can be compared with similar parameters reported for other materials in the literature on the subject. Much better sorption performance is unambiguously notified for the present sample when the comparison is made with hydroxyapatite, usually regarded as the reference with a retention capacity being of 0.4 mmol g -1 [START_REF] Wen | Efficient capture of strontium from aqueous solutions using graphene oxide-hydroxyapatite nanocomposites[END_REF], or even when compared with bentonite (0.6 mmol g -1 ) [START_REF] Galamboš | Cesium sorption on bentonites and montmorillonite K10[END_REF]. Note that the same maximum retention capacity has been obtained with a zeolite A sample [START_REF] Faghihian | Evaluation of a new magnetic zeolite composite for removal of Cs+ and Sr2+ from aqueous solutions: Kinetic, equilibrium and thermodynamic studies[END_REF].

When the adsorption is performed under competitive conditions (i.e., mineral water rich either in Na + or in Ca 2+ ), strontium cations may enter into competition with some other ions present in the solution. In aqueous solutions prepared from mineral water rich in sodium, only a very small decrease down to 1.1 mmol g -1 in the retention capacity is observed. On the contrary, with a mineral water rich in Ca 2+ , the amount adsorbed is markedly reduced and it levels off at about 0.4 mmol g -1 . This ultimately points towards a strong competition from the cations accompanying strontium in solutions with different compositions. Logically, calcium should be the main competitor, as it is present in large quantities in the systems studied. Its depressing impact on the strontium adsorption has been reported previously [START_REF]Public health statement[END_REF][START_REF] Mangano | Elevated in Vivo Strontium-90 from Nuclear Weapons Test Fallout among Cancer Decedents: A Case-Control Study of Deciduous Teeth[END_REF][START_REF] Krejci | Selective sequestration of strontium in desmid green algae by biogenic co-precipitation with barite[END_REF]. To better probe the competition between strontium and calcium in the system studied and thus to shed more light on the retention selectivity of the present material, supplementary measurements of individual calcium adsorption have been done in single-solute and bi-solute (in competition with Sr 2+ ) systems. The results are reported in Fig. 4 and compared with the corresponding adsorption isotherms for strontium. When each cation is adsorbed from singlesolute solutions (c.f., Figure 4a), the two adsorption curves are very similar. They appear as having almost the same affinity towards the manganate nanoflower structure. Thus, it is not surprising that they will compete against each other when present together in aqueous solutions. When the adsorption process is studied in aqueous solutions containing both competitors in equimolar 1:1 proportions, their mutual competition results in a quite similar depression of the amount adsorbed down to about 0.5 mmol g -1 , especially at low equilibrium concentrations, as can be seen in Fig. 4b. As far as the adsorption of cesium in similar systems is concerned, the adsorption isotherms represent curves with a low affinity in the initial concentration range (c.f., Fig. 3b).

In view of potential uses in the selective removal of strontium and cesium from nuclear waste water, an important conclusion drawn from the comparison of the adsorption curves in this early stage of individual adsorption in single-solute systems is that the manganate nanoflowers synthesized in the present work are highly selective for strontium against cesium.

It is worth noting that this selectivity is still valid when both solutes are adsorbed from aqueous solutions in mineral water rich in Na + (c.f., Fig. 3b). In the range of higher equilibrium concentrations, the retention capacity of the solid sample appears to strongly depend on the mutual concentrations of cations in the equilibrium solutions. For example, the maximum amount of cesium adsorbed onto manganate nanoflowers within the concentration range studied in the present work reaches about 1.0 mmol g -1 in ultrapure water and 1.1 mmol g -1 in mineral water rich in Na + . This slight upward tendency in the retention capacity can likely be ascribed to the positive effect of the increased ionic strength in the presence of numerous ionic species in the aqueous phase. Anyhow, none of those ions can greatly affect the individual affinity of strontium or cesium species for the manganate structure. On the contrary, the presence of Ca 2+ in great quantities has a detrimental effect also on the retention of cesium. The effect is pronounced at very low equilibrium concentrations where the individual adsorption of this species is strongly decreased (Figure 3b). When the equilibrium concentration of Cs + increases, its adsorption quantity becomes continuously greater up to about 0.6 mmol g -1 at the end of the concentration range studied here.

In further adsorption experiments, strontium and cesium cations have been put in direct competition with each other within the bi-solute aqueous solutions prepared using ultrapure water. The corresponding ratios between the initial concentrations of cesium and strontium in the stock solution were 2:1 mmol L -1 (for the overall ionic strength being comparable to that obtained in the previous experiments) and 4:2 mmol L -1 (for the specific concentrations being comparable with those tested in the previous experiments). The individual adsorption isotherms inferred from these experiments have been plotted in Figures 5a and5b. The comparison between the shapes of the adsorption curves in the very beginning of the concentration range reinforces the hypothesis formulated previously that only strontium is retained by manganate nanoflowers in this initial adsorption range. In the absence of other ionic species in the aqueous phase, cesium cannot compete effectively with strontium at very low concentrations. When the overall concentration of the equilibrium solution increases (at the same initial molar proportions between the two solutes), cesium can be retained by the material but to a much lesser extent attaining a steady value of about 0.4 meq g -1 in the adsorption plateau region, irrespective of the Cs:Sr concentration ratio. For higher strontium concentrations in the equilibrium bulk phase, a saturation plateau is also observed (Fig. 5b)

with an adsorption quantity of 2.2 meq g -1 (1.1 mmol g -1 ). This value is slightly smaller than the one measured in the single-solute systems but equal to that determined when the adsorption phenomenon has been carried out in solutions prepared using mineral water rich in sodium. This indicates again a very small effect on the strontium retention produced by competing monovalent cations, like Na + and Cs + , when present at higher concentrations. The observed differences in the selectivity of manganate nanoflowers towards calcium, strontium, and cesium were rationalized through the DFT-based calculation of the energy of ion adsorption at saturation on the one hand and calorimetric measurements of the enthalpy of displacement accompanying ion adsorption in the single-solute systems on the other hand. The results are reported in Table 3 and in Fig. 6, respectively. )

real/

Γ ads /Γ max also contributions due to the swelling effect, even though such an energy component should be relatively small on account of small variations of the interlayer distance upon cation exchange depending on the cation nature. In the case of cesium, its energy of adsorption being smaller than that of potassium, the effectiveness of cation exchange is very limited, especially at very low cesium concentrations where the driving force for ion-exchange is dominated mostly by the difference in the adsorption energy. A noticeable increase in the cesium concentration resulting in the increased electrochemical potential of this cation is necessary to push the cation-exchange pathway in the reverse direction. Identical energy values obtained separately for strontium and calcium argue in favor of similar adsorption behavior, as evidenced by almost the same adsorption isotherms reported in Fig. 4a. The same conclusion may be formulated when analyzing the initial portions of the individual adsorption isotherms from bi-solute solutions in Fig. 4b. Beyond this initial concentration region, there is an increase in the amount of strontium retained by the nanoflower sample at the expense of calcium adsorption. This change, even though it remains relatively small, cannot be explained only by the adsorption energy difference. Moreover, the configurations of the interlayer divalent cations inferred from molecular simulations indicate that both types of cations are located on similar sites within the layered structure, which means that the mechanism of adsorption at a microscopic scale cannot be greatly affected by cation positions (see Figure 7).

Therefore, some other contributions to the total energy and entropy balance should be taken into consideration. It is often postulated that cation exchange with the use of various inorganic oxides is a complex displacement process in which the resulting enthalpy of displacement may be governed also by the concomitant hydration effects related to the modification of the hydration shells of adsorbing and desorbing species [START_REF] Prelot | Effect of Nanoscale Pore Space Confinement on Cadmium Adsorption from Aqueous Solution onto Ordered Mesoporous Silica: A Combined Adsorption and Flow Calorimetry Study[END_REF][START_REF] Prelot | What are the main contributions to the total enthalpy of displacement accompanying the adsorption of some multivalent metals at the silica-electrolyte interface?[END_REF]. For this purpose, the values of the enthalpy of cation hydration in aqueous solutions have been collected in Table 3. The experimental enthalpies of displacement recorded for three single-solute systems are plotted as a function of surface coverage by the adsorbing cation in Fig. 6. These curves reveal great differences in the driving force for ion-exchange with potassium ions from the viewpoint of enthalpy. For cesium, the displacement process appears almost athermal. This may be easily justified by including in the difference between the energy of cesium and potassium adsorption also the difference in the hydration enthalpy (with the values given in Table 3). In the case of strontium and calcium, the overall displacement process is endothermic and thus entropy-driven. This endothermic character falls in line with the values of the enthalpy of hydration overcoming that of adsorption, as can be seen in Table 3. With the exception of the first few points, the enthalpy values measured for strontium and calcium are different. The displacement of potassium by calcium is more endothermic than that involving strontium.

This likely explains why there is less calcium retained by the manganate structure in higher concentration regimes when both cations are adsorbed from equimolar bi-solute solutions (see Fig. 4b). Finally, the ion-exchange behavior of the present layered material is in good agreement with that of some swelling clays as montmorillonites for which the effect of both the hydration of the interlayer cation and the partial charge on the ion exchange has already been proven both experimentally and theoretically [START_REF] Salles | Driving force for the hydration of the swelling clays: Case of montmorillonites saturated with alkaline-earth cations[END_REF][START_REF] Salles | Diffusion of Interlayer Cations in Swelling Clays as a Function of Water Content: Case of Montmorillonites Saturated with Alkali Cations[END_REF].

Conclusion

A new synthesis route was developed to obtain homogenous manganate nanoflowers as observed by microscopy. These materials have been proved to be really efficient for the capture of strontium in aqueous media. Indeed, the maximum amount adsorbed was as high as 2.4 meq g -1 in ultrapure water with a value of Log(Kd) = 6, indicating high performances compared to those of other materials reported in the literature. The competition between calcium and strontium could be observed when a large concentration of calcium was present due to their similar interaction energies with the manganate framework. The material remained selective towards strontium against cesium even in multicomponent solutions provided that the concentration of calcium remained low, thereby allowing the manganate nanoflowers to be considered as well suited for nuclear decontamination uses.

Figure 1 :

 1 Figure 1: Manganate nanoflowers observed by (a) TEM and (b) SEM

Figure 2 :

 2 Figure 2: X-Ray diffraction patterns for manganate nanoflowers (a) at ambient temperature : experimental (black line) and simulated data (red line), (b) under vacuum: at 298 K (black), 373 K (red), 473 K (blue), 573 K (orange), 673 K (green), 773 K (purple), 873 K (cyan), back to 298 K (pink)

Figure 3 :

 3 Figure 3: Isotherms of (a) strontium and (b) cesium adsorption onto manganate nanoflowers at 298 K from aqueous solutions in ultrapure water (squares), mineral water rich in Na + (circles), and mineral water rich in Ca 2+ (triangles)

Figure 4 :

 4 Figure 4: Isotherms of strontium (squares) and calcium (stars) adsorption onto manganate nanoflowers from (a) single-solute and (b) bi-solute solutions at 298 K

Figure 5 :

 5 Figure 5: Individual isotherms for cesium (circles) and strontium (squares) adsorption onto manganate nanoflowers at 298 K from bi-solute aqueous solutions. The ratio between the initial concentrations of cesium and strontium in the stock solution was as follows: (a) 2:1 mmol L -1 and (b) 4:2 mmol L -1 .

Figure 7 .

 7 Figure 7. Snapshots showing the positions of the compensating cations inside the managanate layers for Cs + (yellow) (a) and (b) and Sr 2+ (green) (c) and (d). Violet and red elements represent V and O atoms, respectively. The snapshots (a) and (c) correspond to a projection according to the xz plan, while (b) and (d) correspond to a projection according to the xy plan.

Table 1 :

 1 Composition of the two mineral water used to prepare cesium and strontium solutions for the purpose of retention tests

	Ions	Mineral water rich in Ca 2+	Mineral water rich in Na +
		(mmol L -1 )	(mmol L -1 )
		pH = 7.60	pH = 5.85
	Ca 2+	1.57	0.06
	Mg 2+	0.42	0.04

Manganate nanoflowers were obtained through a hydrothermal synthesis under microwave radiation in a Teflon autoclave with a StartSynth ® oven from Milestone Company.

Table 2 :

 2 Unit cell parameter, c, hydrated radius for the cations[START_REF] Nightingale | Phenomenological Theory of Ion Solvation. Effective Radii of Hydrated Ions[END_REF], specific surface area, SSA, and pore volume, PV, as obtained by molecular simulations for XMn4O8 where X= K + ,

			Cs + and Sr 2+
		c (Å)	Hydrated radius (Å)	SSA N2 (m² g -1 )	PV (cm 3 g -1 )
	K +	7.31	3.31	1120	0.16
	Cs +	7.54	3.29	1118	0.18
	Sr 2+	6.92	4.12	892	0.15

Table 3 :

 3 Adsorption energy at saturation for different cations onto manganate nanoflowers as determined by the DFT-based calculation, together with the corresponding enthalpy of cation hydration in aqueous solution taken from Ref.[START_REF] Marcus | A simple empirical model describing the thermodynamics of hydration of ions of widely varying charges, sizes, and shapes[END_REF].

	Cation ∆ Hads (kJ mol -1 ) ∆ Hhyd (kJ mol -1 ) [57]
	Sr	-1096	-1470
	Ca	-1097	-1600

Table 3 )

 3 , it is not surprising that strontium and calcium can displace very easily potassium from the interlayer space. Indeed a strong difference between divalent and monovalent cations confirm the preferential adsorption for Ca2+ and Sr 2+ cations compared to that of K + and Cs + . As a similar theoretical energies of adsorption have been obtained from DFT calculations for Ca2+ and Sr 2+ , it is more difficult to propose a strict differentiation between these two cations. Moreover, it is necessary to note that these energy values include

		20
		18
	g -1	16
	∆ dpl H cum (J	4 6 8 10 12 14
		2
		0
		0.0	0.2	0.4	0.6	0.8	1.0

ACKNOWLEDGMENT

The authors greatly acknowledge the financial support of this work by the RSNR DECLIQ project (ANR-11-RSNR-0004) in the framework of the French ANR Program 'Investments for the Future: Nuclear Safety and Radiation Protection Research'