Affibody-based hBCMA x CD16 dual engagers for NK cell-mediated killing of multiple myeloma cells

We describe the development and characterization of the (to date) smallest Natural Killer (NK) cell re-directing human B Cell Maturation Antigen (hBCMA) x CD16 dual engagers for potential treatment of multiple myeloma, based on combinations of small 58 amino acid, non-immunoglobulin, affibody affinity proteins. Affibody molecules to human CD16a were selected from a combinatorial library by phage display resulting in the identification of three unique binders with affinities (K_D) for CD16a in the range of 100 nM-3 µM. The affibody exhibiting the highest affinity demonstrated insensitivity towards the CD16a allotype (158F/V) and did not interfere with IgG (Fc) binding to CD16a. For the construction of hBCMA x CD16 dual engagers, different CD16a binding arms, including bi-paratopic affibody combinations, were genetically fused to a high-affinity hBCMA-specific affibody. Such 15-23 kDa dual engager constructs showed simultaneous hBCMA and CD16a binding ability and could efficiently activate resting primary NK cells and trigger specific lysis of a panel of hBCMA-positive multiple myeloma cell lines. Hence, we report a novel class of uniquely small NK cell engagers with specific binding properties and potent functional profiles.