Glycovariant CD37 Small Modular Immuno-Pharmaceutical (TruADhanCe™ SMIP) Promotes Enhanced Natural Killer Cell Mediated Cytotoxicity against Primary Chronic Lymphocytic Leukemia Cells.

Abstract 1744

Poster Board I-770

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 expressed minimally or is absent on normal T-cells, natural killer 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) targeted towards the extracellular region of CD37, is presently in clinical trials in CLL patients. TRU-016 consists of variable regions (scFv) and engineered constant regions encoding the human IgG1 domains. We have previously reported that SMIP-016, the chimeric precursor of the fully humanized TRU-016, induced apoptosis in CLL B cells in the presence of goat anti-human Fc ab 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. Recently, in an attempt to enhance the ADCC function, a new variant of SMIP-016, Tru-ADhanCe SMIP-016, has been created with a modification of the glycosylation of the Fc portion of the molecule. TRU-ADhanCe SMIP-016 has been shown to exhibit enhanced binding to both low- and high-affinity molecular variants of human CD16 (FcRIII) and augmented ADCC potency when compared to SMIP-016. In this study, we compared TruADhanCe SMIP-016 and SMIP-016 in direct cytotoxicity and ADCC experiments using CLL B-cells. While SMIP-016, and TruADhanCe SMIP-016 mediated comparable direct cytotoxicity at 24, 48 and 72 hrs in the presence of anti-human Fc crosslinker, the TruADhanCe SMIP-016 resulted in 2 to 4 fold increased NK cell mediated ADCC function. Consistent with the comparable direct cytotoxic effects, the early phosphorylation patterns were similar in cells treated with TruADhanCe SMIP-016 or SMIP-016 in the presence of anti-human Fc cross linker. Ongoing studies are aimed to define the mechanistic basis of the enhanced ADCC function by TruADhanCe SMIP-016 and to determine if use of soluble CD16.Fc as a cross-linker, an in vitro model of in vivo Fc receptor binding, may reveal enhanced apoptotic-signaling of TruADhanCe SMIP-016. These results suggest potential use of TruADhanCe versions of TRU-016 with enhanced ADCC function as an alternate for TRU-016 in B cell malignancies including CLL therapy. [This work was supported by D. Warren Brown Foundation, Leukemia and Lymphoma Society and National Cancer Institute.]