MEN1112/OBT357, an Anti Bst1/CD157 Humanized Antibody Inducing Acute Myelogenous Leukemia (AML) Blast Depletion in an Autologous Ex Vivo Assay: A Potential New Targeted Therapy for AML

Acute myeloid leukemia (AML) is a disease with a poor outcome and novel approaches are needed to improve survival and decrease toxicity of current therapies. Bst1/CD157 is a protein belonging to the ADP-ribosyl-cyclase family expressed on monocytes and neutrophils. This antigen was shown to be also expressed in peripheral blood (PB) and bone marrow (BM) blasts of acute myeloid leukemia (AML) patients either at primary diagnosis or at relapse(1,2,3). MEN1112/OBT357 is a humanized, de-fucosylated antibody targeting Bst1/CD157 with high affinity and developed to generate antibody dependent cell-mediated cytotoxicity (ADCC) response against AML blasts. Peripheral blood (PB) and bone marrow (BM) samples of 38 AML patients (29 at diagnosis, 6 at relapse, 3 resistant), have been analyzed for the expression of Bst1/CD157 on AML blast  cells by fluorescence-activated cell sorting (FACS) using a PE conjugated form of MEN1112/OBT357. Bst1/CD157 expression has been confirmed in 91% and 96% of PB and BM AML samples, respectively. Furthermore, statistical analysis demonstrated that monocyte-oriented blasts are characterized by a brighter expression of Bst1/CD157 compared to blasts of non-monocytic lineage. The efficacy of MEN1112/OBT357 in depleting AML blasts was evaluated through FACS analysis in an autologous ex vivo assay performed on whole blood.  The assay was set up using blood from healthy donors exposed to 10 μg/ml Rituximab for 18 hours to induce B cell depletion. In the same conditions, the ability of 10 μg/ml MEN1112/OBT357 to induce blasts depletion was tested. In whole PB, MEN1112/OBT357 was able to deplete AML blasts in 15/32 evaluable cases (46%). In BM, MEN1112/OBT357 induced blast depletion in 9/24 evaluable cases (36%). Interestingly, higher depletion rate was observed in relapse/refractory patients. When CD16A-158Phe/Val polymorphisms were analyzed utilizing a sequence based typing (SBT) assay, it was demonstrated that AML blast depletion was independent by FcRg polymorphism. Furthermore, no significant shedding of Bst1/CD157 antigen was observed in sera from AML patients, compared to the sera from patients with other hematologic diseases or healthy donors.

In summary, we confirmed the frequent expression of Bst1/CD157 on blasts from AML patients, with the brightest pattern of positivity observed in cases belonging to monocytic differentiation lineage. MEN1112/OBT357 also induced a promising ADCC against AML blasts in an autologous setting, which is independent from FcR g  phenotype. Since in vivo the exposure of AML blasts to MEN1112/OBT357 largely exceeds the incubation time of the depletion assay, we expect a further improvement of its anti-leukemic effect in the clinical setting. Based on these results, a phase I study in patients with relapsed or refractory AML has been initiated in December 2014.