L-Asparaginase Sensitivity and Asparagine Synthetase Expression In Primary Tumor Cells From AML Patients

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 and from 0% to 31% 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.

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.

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.

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.

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.