GlycoVariant Anti-CD37 Small Modular Immuno-Pharmaceutical Exhibits Superior Natural Killer Cell Mediated Cytotoxicity Against Chronic Lymphocytic Leukemia Cells at Low Concentrations and Low Antigen Density

Abstract 1847

CD37 is a tetraspanin transmembrane family protein that is strongly expressed on the surface of mature human B cells and transformed mature B cell lymphoma and leukemia cells, including CLL cells. It is absent or minimally expressed on normal T cells, NK cells, monocytes, and granulocytes. Predominant expression of CD37 on CLL cells makes it an ideal candidate to target with potential agents for treatment of CLL. TRU-016, a small modular immunopharmaceutical protein (SMIP) that specifically binds to an extracellular region of CD37, is presently in clinical trials in CLL patients. TRU-016 includes humanized immunoglobulin variable regions (scFv) fused to a human IgG1 Fc region. We have previously reported that SMIP-016, the chimeric version of the humanized TRU-016, induced apoptosis in CLL B cells in the presence of goat anti-human Fc antibody cross-linker through a novel, caspase-independent pathway. Furthermore, SMIP-016 showed potent in vivo activity in a SCID xenograft mouse model. Aside from direct cytotoxicity, SMIP-016 mediates antibody-dependent cellular cytotoxicity (ADCC) by NK cells both in vitro and in vivo. In an attempt to enhance its ADCC function, a new variant of SMIP-016, SMIP-016^GV, was designed with a modification of the glycosylation of the Fc portion of the molecule. SMIP-016^GV exhibits enhanced binding to both low- and high-affinity molecular variants of human CD16 (FcγRIII) and augments ADCC potency when compared to SMIP-016. In this study, we compared SMIP-016^GV and SMIP-016 in direct cytotoxicity and ADCC against CLL B cells. While SMIP-016 and SMIP-016^GV mediated comparable direct cytotoxicity at 48 hrs in the presence of goat anti-human Fc crosslinker, the SMIP-016^GV resulted in 2 to 4 fold increase in NK cell mediated ADCC function at all effector to target ratios tested. This increased ADCC with SMIP-016^GV was observed using NK cell effectors derived from both normal as well as CLL-affected individuals. In addition, this enhanced cytotoxicity was sustained at concentrations of SMIP-016^GV as low at 5E^-6 μg/ml. These low concentrations of SMIP-016^GV were also able to mediate superior ADCC in 697 cells expressing as few as 10,000 molecules of surface CD37 antigen. Furthermore, NK cells stimulated with the glycovariant were potently activated and released 3 to 4 fold more IFNγ compared to SMIP-016. Ongoing studies are aimed at defining other effector cells which may interact with SMIP-016^GV via different Fcγ Receptors. Collectively, these results suggest potential use of the SMIP-016^GV with enhanced ADCC function as an alternate for TRU-016 in B cell malignancies including CLL therapy.