Multiple Myeloma (MM) Highly Expresses CGEN-928 and the Anti-CGEN-928 TM21 Antibody Markedly Inhibits the Growth of MM Cells and Enhances the Anti-Myeloma Effects of Dexamethasone, Melphalan and Bortezomib

Abstract 4067

Although patients with multiple myeloma (MM) initially respond to current treatment modalities, it remains an incurable disease. Many new therapeutic options have become available during the past several years but nearly all patients develop resistance to currently available therapeutic options. In addition, there is no tumor marker that is uniformly expressed in all MM cells although CD138 is considered to be present on the surface of tumor cells in most cases of MM but generally is only present in a subset of the patients' tumor population and may be absent in the most resistant part of the tumor clone. In order to address these unmet clinical needs, we queried Compugen's MED Platform, an expression database which covers over 40,000 microarray experiments, for genes overexpressed in B cell-derived malignancies including MM and that exhibit low expression levels in normal cells and tissues. One of the most prominent candidates was CGEN-928, which was validated as over-expressed in MM at the mRNA level using an independent panel of both hematological malignancies and normal tissues. In this study, we first investigated whether the previously unidentified membrane antigen CGEN-928 is expressed in cells from human MM cell lines, human MM xenografts and fresh bone marrow (BM) aspirates derived from MM patients using flow cytometric analysis and immunohistochemical staining with the anti-CGEN-928 TM21 polyclonal antibody (Compugen Ltd, Tel Aviv, Israel). Using this antibody, we found that CGEN-928 was highly expressed in cells from the MM1s, U266 and RPMI8226 MM cell lines. Next, we examined CGEN-928 antigen expression in fresh tumor cells from BM aspirates from 17 MM patients and also showed high expression of CGEN-928. Notably, expression of this antigen was not only found on CD138⁺ MM cells but also on MM tumor cells lacking CD138 expression. We also examined the expression of CGEN-928 using our human MM xenograft models LAGκ-1A (bortezomib-sensitive), LAGκ-1B (bortezomib-resistant) and LAGλ-1 (melphalan-resistant). The bortezomib-sensitive MM tumor LAGκ-1A expresses CD138 whereas the bortezomib-resistant version LAGκ-1B developed from the same patient after the patient developed bortezomib reisistance does not express CD138. Cells from all three tumor types showed high levels of reactivity with the TM21 antibody. Similar to the fresh MM BM samples, CGEN-928 expression was not only found on CD138⁺ MM cells but also on CD138^- tumor cells derived from these human MM xenografts. Because this molecule is highly expressed on MM cells, we hypothesized that the anti-CGEN-928 antibody may show anti-MM effects and enhance the anti-MM effects of other anti-MM drugs. To evaluate this_, we examined the effect of the TM21 antibody alone and in combination with dexamethasone, melphalan and bortezomib in vitro using cell proliferation MTT assays. Anti-TM21 polyclonal antibody (100 mg/ml) decreased MM tumor cell proliferation and increased apoptosis in cells from the MM1s, RPMI8226 and U266 cell lines. Next, we determined the effects of combining the anti-CGEN-928 antibody with bortezomib, melphalan or dexamethasone on MM1s cells. Cell proliferation assays demonstrated marked enhanced anti-proliferative effects when CGEN-928 antibody at concentrations of 5, 10, 50 and 100 mg/ml was combined with bortezomib, melphalan or dexamethasone. Further investigations are defining the mechanisms and signal transduction pathways that produce the anti-MM effects of CGEN-928. These preliminary studies suggest that the CGEN-928 antigen is highly expressed in MM and treatment with an anti-CGEN-928 polyclonal antibody produces anti-MM effects alone and in combination with other anti-MM agents; and thus, this antigen may be a target for the treatment of multiple myeloma. Currently, a monoclonal anti-CGEN-928 antibody is in development that will be used by our group to evaluate its anti-MM effects both in vitro and in vivo using our SCID-hu models of human MM.