Trispecific Antibodies for Selective CD16A-Directed NK-Cell Engagement in Multiple Myeloma

Development of antibody scaffolds to directly engage cytotoxic effector cells such as T-cells for therapeutic applications is limited by the scarcity of surface antigens which are expressed exclusively on tumor cells and show limited or no expression on non-malignant cells. We have therefore designed a novel antibody format to selectively retarget effector cell cytotoxicity to tumor cells co-expressing two surface antigens. NK-cells play an important role in the innate immune response to multiple myeloma (MM) and are known to contribute to the efficacy of novel therapeutics. We, therefore, utilized a MM-based model system to generate proof-of concept data demonstrating antibody-mediated NK-cell retargeting to cell lines co-expressing two MM-expressed surface antigens with increased selectivity ('dual-targeting').

B-cell maturation antigen (BCMA/CD269) is widely considered to be a promising target antigen for antibody-based therapies of MM due to its almost universal expression on patient myeloma cells and its restricted surface expression on cells outside of the haematological lineage. However, low levels of expression on healthy tissue, including skin, has been reported, which could account for potential side effects associated with BCMA-targeted antibody therapies due to effector cell activation in these organs.

To increase selectivity of antibody-induced, effector cell-mediated cytotoxicity towards malignant tissue, we developed a trispecific antibody format capable of selectively engaging NK-cells through bivalent binding to CD16A (FcγRIIIa) and monovalent binding to both BCMA and CD200, a second MM-expressed surface antigen found in the majority of MM patients. Using an in vitro model system, we demonstrated that binding to BCMA⁺/CD200⁺ cell lines and the resulting increase in avidity leads to preferential lysis of antigen double-positive cells compared with antigen single-positive cells. These data suggest that dual-targeting may increase the therapeutic window compared to approaches targeting only one antigen, thereby improving safety of BCMA-directed antibody therapeutics for MM. In addition to the MM-based model system used here, the novel trispecific antibody scaffolds we have developed may be adapted to alternative target combinations within MM or in other tumor indications. Moreover, they could be used to target phenotypically distinct tumor cell clones to induce deeper and more prolonged antitumor responses. Consequently, dual-targeting of effector cells to tumors using the described antibody technology could also be applied to increase safety of T-cell engaging antibodies in the absence of exclusively tumor-expressed target antigens.