Targeting and Depletion of Acute Myeloid Leukemia Blasts By MEN1112, a Novel Humanized Defucosylated Monoclonal Antibodies with Specificity for Bst1/CD157 Antigen

MEN1112 is a new humanized, defucosylated, monoclonal antibody (mAb) with high specific affinity for Bst1/CD157 antigen. Bst1/CD157 antigen expression on blood cells of acute myeloid leukemia (AML) patients and healthy donors was investigated by flow cytometry using a PE-labeled MEN1112 mAb. Twenty three patients affected with AML have been tested, 18 at diagnosis, 4 at relapse, 1 resistant. In 16 out of 23 patients both bone marrow (BM) and peripheral blood (PB) specimens were evaluated. PB and BM samples from healthy donors (N=2) were also assessed. In healthy donors and AML patients, PB and BM lymphocytes were Bst1/CD157 antigen negative whereas monocytes and neutrophils showed a distinct pattern of MEN1112 mean fluorescence intensity (MIF), with monocytes having the brightest expression. In the stem cell compartment, an intermediate level of MFI was observed (p<0.001). Prevalence of expression of the antigen on patients’ samples was over 90%. On AML blast cells from each single patient, MEN1112 expression was heterogeneous; indeed the antigen was expressed on 50%±29% and 47% ±39% of blasts in BM and PB, respectively. The anti-leukemia activity of MEN1112 on AML cell lines was tested, in vitro, by a flow cytometry-based cell depletion assay in the presence of lymphokine activated immune effector cells: a strong depletion of leukemia cells was demonstrated suggesting that MEN1112 might exert anti-leukemia activity through antibody dependent cell-mediated cytotoxicty (ADCC). The activity of MEN1112 was also tested ex vivo on whole PB showing that the antibody was able to deplete AML blasts in 9 out of 23 patients (47.4 %) with a percentage of AML blast depletion ranging between 4.3 - 66 %. In whole BM from 2 out of 11 evaluable patients MEN1112 induced 68% and 23% of AML blast depletion. Bst1/CD157 shedding assessment showed that, in the sera from AML samples, the concentration of Bst1/CD157 antigen was comparable to that measured in healthy donors. Moreover, since Fcγ receptor (CD16) genotype might be a factor contributing to the antitumor activity of the antibody, the polymorphism CD16-158Phe/Val was analyzed. Five out of 19 samples were homozygous for CD16-158 Phe; 5 were homozygous for CD16- 158 Val and 9 were heterozygous for CD16-158. MEN1112-induced blast depletion was observed for each genotype. Moreover, in an attempt to identify the determinant of MEN1112 activity, % in PB of blast (antigen positive), NK cells or residual normal cells were evaluated. Altogether, these results are promising suggesting the potential for an ADCC-mediated MEN1112 antileukemic effect and they support the clinical development of MEN1112.