Effects of IL-21, KIR Blockade, and CD137 Agonism on the Non-Clinical Activity of Elotuzumab

Elotuzumab is a humanized monoclonal antibody (mAb) that binds specifically to Signaling Lymphocytic Activation Molecule F7 (SLAMF7, also known as CS1), a glycoprotein expressed on the surface of multiple myeloma (MM) tumor cells. SLAMF7 has not been detected on hematopoietic stem cells or on other normal tissues, but is expressed on some normal leukocytes, including natural killer (NK) cells. Elotuzumab enhances NK cell activation directly via SLAMF7 and also enhances NK cell-mediated antibody-dependent cell-mediated cytotoxicity (ADCC) of SLAMF7-expressing MM cells. NK cell-mediated ADCC can be enhanced by stimulating activating NK receptors as well as by blocking inhibitory NK receptors. IL-21 is a pleiotropic cytokine that has been shown to augment the ability of NK cells to mediate ADCC. CD137 (4-1BB) agonism has been shown to enhance both NK cell ADCC in vitro and the efficacy of a tumor-specific monoclonal antibody in a syngeneic mouse tumor model. NK cell-mediated ADCC can be reduced by inhibitory signaling via killer immunoglobulin-like receptors (KIRs) on NK cells, which recognize MHC class I molecules as ligands. Lirilumab is a human IgG4 mAb that binds inhibitory KIR2DL1/2/3, blocking interaction with HLA-C ligands and promoting NK cell activity. We investigated the ability of IL-21, agonistic CD137 mAb, and lirilumab to augment elotuzumab activity in vitro and in mouse models to support and guide clinical evaluation. Standard cytotoxicity assays were used to assess in vitro ADCC with human peripheral blood mononuclear cells or purified NK cells as effectors and SLAMF7-expressing tumor cells as targets. Flow cytometry was used to evaluate CD107a expression on NK cells. Efficacy was assessed in immunodeficient mice using a SLAMF7-expressing OPM2 human MM subcutaneous xenograft model. IL-21 was able to increase elotuzumab-mediated ADCC in vitro, but showed little or no enhancement of elotuzumab activity in the OPM2 tumor model. CD137 agonism showed minimal enhancement of elotuzumab ADCC in vitro, but was able to synergize with elotuzumab in vivo to mediate potent anti-tumor activity in the OPM2 xenograft tumor model. Tumors progressed in all eight of the mice treated with either buffer control or with anti-CD137 mAb alone. Of the eight mice treated with elotuzumab alone, one achieved a complete regression by the end of the study. In contrast, seven of eight mice in the group treated with both elotuzumab and anti-CD137 mAb achieved complete regression. Blocking the inhibitory KIR pathway with lirilumab augmented elotuzumab-mediated ADCC in vitro and synergized with elotuzumab to mediate potent anti-tumor activity using a mouse model expressing a KIR2DL3 transgene on a RAG-deficient background. Tumors progressed in all ten mice treated with either isotype control antibody or with lirilumab alone. Of the ten mice treated with elotuzumab alone, two achieved partial regressions and another mouse achieved a complete regression by the end of the study. Six of the ten mice in the group treated with both elotuzumab and lirilumab achieved complete regression and one mouse achieved a partial regression. Together, these approaches demonstrate that targeting activating and inhibitory receptors on NK cells can enhance or synergize with elotuzumab to increase NK cell ADCC towards myeloma cells.

This study is being funded by Bristol-Myers Squibb.

Editorial assistance was provided by Caudex Medical and was funded by Bristol-Myers Squibb.