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Summary Lack of preclinical patient-derived xenograft cancer models in which to conduct large-scale molecular studies seriously impairs the development of effective personalized therapies. We report here an in vivo concept consisting of implanting human tumor cells in targeted tissues of an avian embryo, delivering therapeutics, evaluating their efficacy by measuring tumors using light sheet confocal microscopy, and conducting large-scale RNA-seq analysis to characterize therapeutic-induced changes in gene expression. The model was established to recapitulate triple-negative breast cancer (TNBC) and validated using TNBC standards of care and an investigational therapeutic agent., Graphical abstract, Highlights • Human TNBC cells implanted in the avian embryo survive and efficiently form tumors • Anti-tumoral therapies intravenously injected are assessed by 3D imaging • Post-graft large-scale analyses allow studying the mechanism of action of drugs • The AVI-PDX allows preclinical evaluation of therapies and patient stratification, Biological sciences research methodologies; Cancer; Model organism

Lack of preclinical patient-derived xenograft (PDX) cancer models in which to conduct large scale molecular studies seriously impairs the development of effective personalized therapies. We report here on an in vivo concept consisting of implanting human tumor cells in targeted tissues of an avian embryo, delivering therapeutics, evaluating their efficacy by measuring tumors using light sheet confocal microscopy, and conducting large scale RNAseq analysis to characterize therapeutic-induced changes in gene expression. The model was established to recapitulate triple negative breast cancer (TNBC) and validated using TNBC standards of care (SOCs) and an investigational therapeutic agent.

Background : Anti-PD-1 immunotherapy, although efficient in several cancer indications; has shown some limitations due to the appearance of resistance mechanisms. More specifically, the overexpression of A2A receptor (adenosine receptor) on the surface of infiltrated CD8 T cells following anti-PD-1 treatment blunts the immune response in an adenosine-rich tumor microenvironment (TME). In addition, increased evidence of immune escape has been described through gene hypermethylation processes. Erymethionase (methionine-gamma-lyase encapsulated into red blood cells) is an innovative experimental therapy designed to directly reduce systemic levels of L-methionine, an amino-acid nutrient needed for tumor growth and metastasis. Erymethionase is expected to indirectly decrease adenosine levels in TME and to reduce hypermethylation reactions. This bimodal action makes of erymethionase a promising agent to combine with immune checkpoint inhibitors. The benefit of such combination (erymethionase and PD-1 blockade agent) was investigated in a TNBC-like syngeneic mouse model. Methods : Mice bearing orthotopic EMT-6 syngeneic breast carcinoma were intravenously injected once weekly for 4 consecutive weeks with vehicle or mouse erymethionase at 30 or 60 U/kg alone or in combination with anti-mouse PD-1 antibody (RMP1-14 clone, intraperitoneal route, 10 mg/kg, twice weekly for 3 consecutive weeks) from D7 (D0 referring to injection of tumor cells). Erymethionase treatment was associated to daily oral vitamin B6 (pyridoxine, PN) uptake to maintain enzyme activity. The body weight, the length and width of the tumor were measured twice a week. Animal behavior was checked every day. Satellite animals receiving 60 U/kg of erymethionase or vehicle were sacrificed to collect 500-1000 m3 size-tumors to perform immunophenotyping, measure metabolites and/or assess biomarkers. Results : Analysis of health parameters revealed that all treatments were well tolerated along the experiment. A delayed start of exponential growth was reported for the combination at the highest dose of erymethionase vs single agents leading to a significant tumor growth inhibition and an increased survival (median survival time of 35 days for the combination vs 23 days for anti-PD-1 or erymethionase 60 U/kg alone; statistically significant). The antitumor effects were less pronounced at the lower dose level of erymethionase in combination with anti-PD1. Analysis of collected tumors to investigate on the mechanism(s) of action is ongoing. Conclusion : To our knowledge, this is the first in vivo demonstration of anti-PD-1 therapy potentiation using a L-methionine-restricting agent. The data are supportive of the planned erymethionase first-in-human study. Citation Format: Karine Sénéchal, Sylvie Maubant, Marie Leblanc, Séverine Ciré, Fanny Gallix, Aurély Andrivon, Olivier Duchamp, Fabrice Viviani, Françoise Horand, Alexander Scheer, Vanessa BOURGEAUX. Erymethionase (methionine-gamma-lyase encapsulated into red blood cells) potentiates anti-PD-1 therapy in TNBC syngeneic mouse model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2258.

L-asparaginase (ASNase), an enzyme currently used in combination with chemotherapy for Acute Lymphoblastic Leukemia (ALL) treatment, hydrolyzes L-asparagine (Asn) - a non-essential amino acid - into aspartic acid (Asp) and ammonia, leading to Asn removal from the circulation. Similarly, restriction to exogenous L-methionine (Met) - an essential, sulfur-containing proteinogenic amino acid - blocks cell cycle in the late S or G2 phase in Met-dependent cancer (Hoffman and Jacobsen 1980; Guo et al., 1993; Kokkinakis et al., 1997, 2004, 2005 and 2006; Pavillard et al., 2006; Yano et al., 2014). ERYTECH Pharma's strategy was to combine Met depletion, by using Methionine-γ-lyase (MGL) (a pyridoxal-dependent enzyme) with Asn depletion (via ASNase activity), and investigate its in vitro and in vivo therapeutic potential. In vitro sensitivity to MGL combined with ASNase was assessed on 7 cell lines representative of different cancers (glioblastoma, AML, gastric and pancreatic adenocarcinoma or carcinoma). Fixed enzyme concentrations (IC50), determined previously in dose-response studies using each enzyme alone, were used in a sequential therapeutic scheme adding MGL first and ASNase 3 or 4 days later. Tumor cells sensitivity was evaluated by measuring cell viability. In vivo combination study, in a human gastric xenograft mouse model, was performed using ASNase and MGL encapsulated into red blood cells, respectively named erymethionase and eryaspase. The scheme of administration consisted in erymethionase treatment on Day 7 and 15 followed by eryaspase treatment on Day 21. Control arms received vehicle or single therapy with 3 repeated administrations. In vitro study results showed enhanced mortality with MGL-ASNase sequential therapy vs single therapies (ASNase and MGL tested alone) in 72% (5/7) of the tested cellular models (AML, glioblastoma, gastric and pancreatic cancers models). Those data were confirmed in vivo with an enhanced tumor inhibition when erymethionase was administered twice before eryaspase, compared to vehicle or erymethionase-alone treatments (respectively 57% and 36% on Day 37). The result significance is supported by a p-value inferior to 0.0001. To conclude, our in vitro results showed promising potential for enzymatic bi-therapy treatment on several cancer models and that intrinsic tumor cells properties determined its efficiency. In vivo data confirmed, in a gastric cancer model, the therapeutic efficacy of this sequential erymethionase-eryaspase treatment. However, this enzymatic bi-therapy still requires further investigation. The identification of involved mechanisms of action should allow to optimize the therapeutic design and select best responders. A preclinical program is currently ongoing on specific biomarkers, investigation on tumor cell synchronization with ASNase therapy and optimal therapeutic enzyme sequence for treatment. Citation Format: Karine Aguera, Fanny Gallix, Fabien Gay, Karine Senechal, Severine Cire, Françoise Horand, Alexander Scheer, Vanessa Bourgeaux. Enzymatic combination investigation in cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5827.

Based on the asparaginase paradigm, several arginine-catabolizing enzymes have been developed for the treatment of arginine-dependent cancers (Wheatley, 2004). The arginine deiminase (ADI) enzyme catalyzes the hydrolysis of arginine (Arg) (Sugimura, 1990). ADI purified from Mycoplasma has a short half-life (≈4h) in the circulation and was found to be highly immunogenic (Holstberg, 2002). In order to increase half-life and to limit immunogenicity, a pegylated form of the enzyme (ADI-PEG-20) was developed. Phase I/II clinical studies in hepatocarcinoma and in advanced melanoma concluded on its limited efficacy at the tested doses. Notably, reduction of the duration of Arg depletion was linked to the emergence of ADI-PEG-20 antibodies (Ascierto, 2006). Encapsulation of ADI into red blood cells (RBC) is a promising alternative to improve the half-life and reduce the immunogenicity of the protein. Argininosuccinate synthase (ASS1) is the key enzyme in arginine biosynthesis (Haines, 2011). ASS1 expression varies in tumors and ASS1 loss is associated with poor prognosis in different cancers (Qiu, 2015). All these data strengthen the importance of selecting ASS1-negative patients for Arg-depletion based enzymatic therapy. Using a scalable, standardized production process, we synthesized an ADI enzyme from an optimized M. arginini coding sequence. ADI was encapsulated into RBC by hypotonic dialysis (ERY-ADI) and PK-PD parameters were evaluated in CD1 mice, in comparison with free ADI. Sensitivity to ADI was assessed in vitro by measuring the cell viability of 3 cancer cell lines with different ASS1 expression levels. ASS1 expression was screened by immunohistochemistry (IHC) in a large panel of tissue microarrays (TMA) from 16 human cancer types. Administration of ERY-ADI (5.5IU/mL) reduced mouse plasmatic Arg level to 30% of control values and led to a maintained depletion for 5 days. The same dose of free ADI strongly depleted Arg ( All these results highlight that arginine depletion through ADI treatment is effective against ASS1-negative cancer cells. ERY-ADI represents an innovative product with an improved efficacy for sustained Arg depletion and suitable for the treatment of ASS1 deficient cancers. Citation Format: Fabien Gay, Karine Aguera, Karine Senechal, Julie Bes, Anne-Marie Chevrier, Fanny Gallix, Christine Guicher, Philip Lorenzi, Vanessa Bourgeaux, Willy Berlier, Françoise Horand, Yann Godfrin. Arginine deiminase loaded in erythrocytes: a promising formulation for L-arginine deprivation therapy in cancers. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4812.

L-asparaginase (L-ASPA) is included in current chemotherapy regimen for the treatment of acute lymphoblastic leukemia (ALL). The sensitivity of ALL to L-ASPA has been linked to the incapacity of leukemic cells to produce their own asparagine, since they are deficient in a specific enzyme, asparagine synthetase (ASNS). Thus, ASNS-negative leukemic cells rely on plasmatic asparagine and can be starved to death by L-ASPA treatment. Several studies evidenced the potential of asparagine depletion to treat lymphomas. Indeed, L-ASPA is routinely administered to treat canine lymphomas and its adjunction in chemotherapy regimens significantly improves the outcome of patients with NK/T cell lymphoma. Some studies suggest its benefit for the treatment of B and T-cell non-Hodgkin lymphomas (NHL). In this study, we assessed the in vitro sensitivity to L-ASPA of 11 lymphoma cell lines, including diffuse large B-cell lymphoma (DLBCL), peripheral T-cell lymphoma (PTCL) and other NHL cell lines. We analyzed ASNS expression in tissue microarrays and biopsies from 226 cases of lymphomas, including 158 DLBCL, 15 PTCL, as well as other NHL and Hodgkin lymphomas. Sensitivity to L-ASPA (expressed as an IC50) was assessed by measuring the cell viability in the presence of various concentrations of L-ASPA. ASNS expression was assessed with a validated immunohistochemistry method attributing a score based on labeling intensity from 0 (no expression) to 3 (strong expression). Tumors expressing no/low ASNS (scores 0 and 1) were considered potentially sensitive to asparagine depletion. All lymphoma cell lines were proved to be sensitive to L-ASPA. Their in vitro sensitivity usually exceeded well-known sensitive cell lines (MOLT-4 and HL-60) representative of acute leukemia. ASNS expression was null/low in 171/226 (76%) total cases of lymphomas. 118/158 (75%) DLBCL cases expressed no/low ASNS, whereas ASNS expression was low/null in 2/15 (13%) PTCL cases. Globally, B-cell NHL displayed a lower expression of ASNS than T-cell NHL, Hodgkin lymphoma and non tumoral lymph nodes. The in vitro sensitivity to L-ASPA of all lymphoma cell lines and the low/null expression of ASNS in the majority of the lymphoma tissues tested suggest that L-ASPA may be effective for the treatment of various lymphomas. As suggested by ASNS expression, L-ASPA treatment may be particularly effective in DLBCL and others B-cell NHL, whereas its efficacy in T-cell NHL is probably more limited. However, L-ASPA has only been used scarcely in the treatment of lymphomas despite promising clinical responses. Its well known serious side-effects (hypersensitivity, coagulation disorders, pancreatitis…) render its use particularly hazardous in older or frail patients. The development of a new formulation of L-ASPA with better safety profile must be considered in order to allow the clinical development of L-ASPA in the treatment of aggressive lymphomas. Citation Format: Willy Berlier, Karine Aguera, Fanny Gallix, Anne-Marie Chevrier, Alexandra Traverse-Glehen, Yann Godfrin. Low asparagine synthetase expression and in vitro sensitivity highlights L-asparaginase potential for the treatment of aggressive lymphomas. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5369. doi:10.1158/1538-7445.AM2015-5369

L-asparaginase (L-ASPA) displays a strong clinical benefit in the treatment of acute lymphoblastic leukemia (ALL), where it is included in most of current chemotherapy regimen. L-ASPA depletes plasmatic asparagine (ASN), an amino acid essential for the proliferation of leukemic cells. Since these cells are deficient in asparagine synthetase (ASNS), they rely on external (plasmatic) source of ASN and can be starved to death by L-ASP treatment. Several studies evidenced the potential of ASN depletion to treat lymphomas. Indeed, many animal and human lymphoma cell lines have been shown to be sensitive to L-ASPA in vitro. In veterinary medicine, L-ASPA is routinely administered to treat effectively both feline and canine lymphomas (Wypig et al ., 2013). L-ASPA regained attention in the treatment of human lymphomas since its adjunction in current chemotherapy regimens significantly improved the outcome of patients with NK/T cell lymphoma (Zou et al ., 2014). Some studies also evidenced its benefit in combined chemo or monotherapy for the treatment of B-cell and T-cell lymphomas (Sun et al ., 2006; Takahashi et al ., 2010). In this study, we assessed the in vitro sensitivity to L-ASPA of 6 lymphoma cell lines and we analyzed ASNS expression in biopsies from 166 cases of lymphomas (130 B-cell lymphomas and 17 T-cell lymphomas). Sensitivity to L-ASPA (expressed as an IC50) was assessed in vitro by measuring the cell viability in the presence of various concentrations of E.coli L-ASPA. ASNS expression in biopsies (TMA, USBiomax, Rockville, MD) was assessed with a validated immunohistochemistry (IHC) method attributing a score to each tumor based on ASNS labeling intensity from 0 (no expression) to 3 (strong expression). Tumors expressing no/low ASNS (scores 0 and 1) were considered potentially sensitive to asparagine depletion. As shown in the following table, all cell lines were proved to be sensitive to L-ASPA. Their in vitro sensitivity exceeded cell lines MOLT-4 (ALL) and HL-60 (AML). | Cell line | Sensitivity to L-ASPA (IC50 in IU/mL) | | ----------------------------------------------- | --------------------------------------------- | | HuT-78 (Peripheral T-cell lymphoma,PTCL) | 0.11 ± 0.02 | | Toledo (Diffuse large B-cell lymphoma, DLBCL) | 0.19 ± 0.03 | | SU-DHL-8(Diffuse large B-cell lymphoma, DLBCL) | 0.10 ± 0.04 | | SU-DHL-10(Diffuse large B-cell lymphoma, DLBCL) | 0.10 ± 0.01 | | REC-1 (Mantle cell lymphoma, MCL) | 0.15 ± 0.03 | | KHYG-1 (NK/T-cell lymphoma) | 0.16 ± 0.06 | | MOLT-4 (acute lymphoid leukemia, ALL) | 0.19 ± 0.07 | | HL-60 (acute myeloid leukemia, AML) | 0.23 ± 0.02 | Table 1 As shown in the following table, ASNS expression was null/low in 85% in the entire population of patients with B-cell lymphomas. Considering DLBCL, 63% of patients displayed no ASNS expression at all. ASNS expression was also null/low in 88% of patients with T-cell lymphomas (n=17). | ASNS expression (IHC score) | Type of lymphoma (% of cases) | | ----------------------------------- | ------------------------------------ | ------------------ | | DLBCL (n=110) | Others BCL (n=20) | PTCL (n=3) | Others TCL (n=14) | MCL (n=3) | Hodgkin (n=16) | | Negative (0) | 62,7 | 70,0 | 0,0 | 57,1 | 33,3 | 43,8 | | Low positive (1) | 21,8 | 25,0 | 66,6 | 35,7 | 66,6 | 56,3 | | Positive (2) | 7,3 | 5,0 | 33,3 | 7,1 | 0,0 | 0,0 | | Highly positive (3) | 8,2 | 0,0 | 0,0 | 0,0 | 0,0 | 0,0 | Table 2 Globally, these results suggest that L-ASPA is potentially effective for the treatment of several lymphomas. Indeed, B-cell as well as T-cell lymphoma cell lines are sensitive to L-ASP in vitro and the majority of lymphoma tissues express no/low ASNS. Based on our results on ASNS expression in lymphoma biopsies, L-ASPA therapy may be beneficial for up to 85% of patients with DLBCL. Up to 90% of patients with other B-cell lymphomas or T-cell lymphomas may be sensitive to L-ASPA treatment as well. However, L-ASPA has only been used scarcely in the treatment of lymphomas despite promising clinical responses. Its well known serious side-effects (hypersensitivity, coagulation disorders, pancreatitis, and liver failure) render its use hazardous, particularly in older or frail patients. Therefore, the development of a new formulation of L-ASPA with safer profile has to be considered in order to allow the clinical development of L-ASPA in the treatment of aggressive lymphomas. Disclosures Berlier: ERYTECH: Employment, Equity Ownership. Aguera: ERYTECH: Employment. Chevrier: ERYTECH: Employment. Gallix: ERYTECH: Employment. Godfrin: ERYTECH Pharma: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees.

Due to asparagine synthetase (ASNS) deficiency, tumors cells auxotrophic for L-asparagine lack capability to synthesize this amino acid and depend to circulating L-asparagine for survival. Based on this rational, L-asparaginase (L-Aspa), an enzyme depleting L-asparagine, is an essential drug in the treatment of acute lymphoblastic leukemia (ALL), as these hematological cancer cells express low levels of ASNS. Recent evidence suggests that some solid tumors are also deficient in ASNS (Lorenzi, 2008; Dufour, 2012; Zhang, 2013), providing rationale for testing L-Aspa treatment in other contexts. In this study, we investigated the levels of ASNS expression in tumor tissues from patients with urological cancers (bladder, prostate and kidney) and we investigated the sensitivity to L-Aspa of bladder and prostate cell lines. Material & Method: ASNS expression was evaluated by a validated immunohistochemistry (IHC) method on tissue biopsie microarrays (USBiomax, Rockville, MD) including 384 cases of bladder, 94 cases of prostate and 75 cases of kidney tumors. A score was attributed to each tumor based on ASNS IHC labeling intensity from 0 (no labeling) to 3 (strong labeling). Tumors expressing no/low ASNS (scores 0 and 1) were considered potentially sensitive to asparagine depletion. Sensitivity to L-Aspa (expressed as an IC50) was assessed in vitro by measuring the cell viability of bladder cancer cell lines (RT-4 and UM-UC-3) and prostate cancer cell line PC-3 in the presence of various concentrations of L-Aspa. ASNS expression in cell lines was evaluated by western-blot. Results: ASNS expression was low/null in 77.3% of bladder tumors, 86.2% of prostate tumors and 72% of kidney tumors. In bladder tumors, no correlation was found between the grade of the tumor and its expression of ASNS. Bladder and prostate cancer cell lines were all sensitive to L-Aspa (IC50 = 0.31, 0.12 and 0.22 IU/mL for RT-4, UM-UC-3 and PC-3 cell lines, respectively) at a level similar to the sensitivity observed with the L-Aspa-sensitive MOLT-4 ALL cell line (IC50=0.19 IU/mL). All cell lines displayed low basal expression of ASNS. Conclusion: The sensitivity of bladder and prostate cancer cell lines to L-Aspa and the low/no expression of ASNS in the majority of urological tumor biopsies suggest that an extensive population of patients with urological tumors may respond to asparagine depletion. However, L-Aspa is accompanied with well known serious side-effects (hypersensitivity, coagulation disorders, pancreatitis, liver failure) and this drug should be used very carefully especially in older or frail patients. A new safer formulation is requested to make easier the clinical development of L-Aspa in urological cancer. Citation Format: Willy Berlier, Karine Aguera, Fanny Gallix, Fabien Gay, Gautam Borthakur, Yann Godfrin. Potential of asparagine depletion for the treatment of bladder cancer and other urological tumors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5436. doi:10.1158/1538-7445.AM2014-5436

Introduction The rational of use of L-asparaginase (L-Aspa) is based on asparagine synthetase (ASNS) deficiency in leukemic cells. Its efficacy is due to its capacity to deplete plasmatic asparagine, starving the leukemic cells and subsequently inhibiting protein synthesis. It is a mainstay in the treatment of acute lymphoid leukemia (ALL), where it is established that resistance to treatment is in part related to detectable expression of ASNS (Aslanian et al., 2001; Su et al., 2008). In acute myeloid leukemia (AML), promising results have been obtained in clinical trials (Capizzi et al., 1988), with an improvement of complete remission rates from 18% to 54% in refractory patients 60 year old. More recently, it has been reported that leukemic cells from AML patients with M1, M4 and M5 FAB subtypes were more sensitive to L-Aspa (Okada et al., 2003).The aim of this study was to investigate the sensitivity to L-Aspa and the ASNS expression in an AML cell line (HL-60) and in primary leukemic cells from newly diagnosed AML patients. Materials and methods AML (HL-60) and ALL (MOLT-4) cell lines were grown according to ATCC recommendations. Primary leukemic cells from the bone marrow of 24 AML patients (median age: 69 years; range: 2-83) were harvested on EDTA and isolated by Ficoll density gradient within 72h. ASNS expression was determined by western-blot on isolated leukemic cells and expressed as a ratio ASNS/cyclophilin A. When a sufficient amount of leukemic cells was available, sensitivity to L-Aspa (expressed as an IC50 - concentration inhibiting 50% of cell viability) was evaluated in vitro by incubating various concentrations of L-Aspa (0.001 to 10 IU/mL) with the cells and by measuring the cell viability with a counting kit (CCK-8 viability assay) at day 4. Results Determination of IC50 for the HL-60 cell line demonstrated that these cells were equally sensitive to L-Aspa than the ALL cell line MOLT-4 in vitro (0.23 IU/mL versus 0.19 IU/mL, respectively). The expression of ASNS in the HL-60 cell line was low but higher than in MOLT-4 which is a well known ASNS deficient cell line. IC50 determination was possible on 17/24 patients. Seven displayed a high sensitivity to L-Aspa (IC50 < 0.01 IU/mL) whereas 5 displayed a moderate sensitivity (IC50 < 0.5 IU/mL). Remaining 5 patients were resistant to L-Aspa. The cells from the healthy subject were resistant (IC50 > 10 IU/mL). To date, ASNS expression has been evaluated on 16 patients with ratio ranges from 0 to 2.27: six were negative, 4 low positive (< 0.2), and 6 positive (> 0.5, amongst them 4 were > 1). No correlation was observed between ASNS expression and FAB grade. However, patients with blasts sensitive to L-Aspa had mainly M1 or M5 AML. Conclusion AML cell line HL-60 and 71% of primary cells from AML patients were found sensitive to L-Aspa. The sensitivity of cells from M1 and M5 AML patients is consistent with the findings of Okada et al. (2003). Globally, these results suggest that L-Aspa is effective for killing AML cells. Based on the epidemiology of AML subtypes (Selter et al., 2011) and our results, L-Aspa therapy may be beneficial for 50-70% of patients with AML. However, L-Aspa has only been used scarcely in the treatment of AML, mainly because of the commonly observed adverse effects that impair its use. A new formulation of L-Aspa encapsulated in homologous red blood cells was reported with a better safety profile, allowing its use even in elderly patients (Hunault et al., ASH 2012). A clinical study is currently recruiting patients unfit for intensive chemotherapy in order to evaluate its efficacy in combination with low-dose cytarabine (NCT01810705) in AML. In this study, L-Aspa sensitivity and ASNS expression in primary tumor cells at diagnosis will be explored to investigate the relationship with clinical response. Disclosures: Berlier: ERYTECH Pharma: Employment. Aguera:ERYTECH Pharma: Employment. Gallix:ERYTECH Pharma: Employment. Bertrand:ERYTECH Pharma: Principal Investigator Other. Thomas:ERYTECH Pharma: Principal Investigator Other; Orphan Europe: Consultancy. Godfrin:ERYTECH Pharma: Employment, Equity Ownership, Membership on an entity’s Board of Directors or advisory committees.