Eradication of Tumors in Pre-Clinical Models of Multiple Myeloma by Anti-CS1 Monoclonal Antibody HuLuc63: Mechanism of Action Studies.

Introduction: We have recently shown that CS1 (CD2 subset 1, CRACC, SLAMF7), a cell surface glycoprotein of the CD2 family, is uniformly expressed on myeloma cells from multiple myeloma (MM) patients. Based on its high expression in MM and limited expression in normal cells, we propose CS1 as a novel and specific antibody target for the treatment of MM.

Methods: A panel of monoclonal anti-CS1 antibodies (mAbs) was generated to identify a potential therapeutic candidate. MAb clones MuLuc63 and MuLuc90 were selected for testing in CS1 positive MM xenograft models in vivo in severe combined immunodeficient mice. HuLuc63, a humanized IgG1 version of MuLuc63, was generated as the potential therapeutic candidate for the treatment of MM. HuLuc63 and Fc-modified versions of HuLuc63 were tested for anti-tumor activity in mouse models vivo. In vitro antibody-dependent cellular cytotoxicity (ADCC) assays were performed to define the potential mechanism of action for HuLuc63.

Results: Both MuLuc63 and MuLuc90 exhibited significant in vivo anti-tumor activity compared to isotype control antibodies in the L363 MM xenograft model. MuLuc63 was significantly more potent, resulting in rapid tumor eradication in most of the animals for the length of the study (~4 months). Based on these results, MuLuc63 was humanized to generate HuLuc63, which exhibited similar affinity for CS1 when compared to the mouse parent antibody. In two different MM xenograft models, L363 and OPM2, HuLuc63 exhibited significant anti-tumor activity resulting in tumor eradication in a high proportion of animals. To investigate the mechanism of action, two modified versions of HuLuc63 were tested in xenograft models. One version, HuLuc63-Ala,Ala, exhibits a mutation in the Fc region that decreases the ability to interact with the Fc receptor on natural killer (NK) cells. The second version, HuLuc63-LF, exhibits low levels of fucosylation in the Fc region that would result in increased binding to the Fc receptor. Compared to HuLuc63, the LF version exhibited significantly better in vivo anti-tumor activity towards, while the Ala,Ala mutant exhibited no anti-tumor activity. These data indicate that the Fc region of HuLuc63 is critical for its anti-tumor activity, and suggest ADCC as a possible mechanism of action. In vitro, HuLuc63 exhibits substantial ADCC towards L363 and OPM2 cells. The activity was dose-dependent, with increasing cytotoxicity being observed with concentrations ranging from 0.01μg/mL to10 μg/mL.

Conclusions: These pre-clinical data support HuLuc63 as a new therapeutic for the treatment of MM and suggest that ADCC is part of the mechanism of action. HuLuc63 will be entering a phase I clinical study for multiple myeloma.