Abstract 1598: Preclinical efficacy of a bispecific NK cell engager studied using a humanized multiple myeloma mouse model

Abstract Natural killer (NK) cells are an essential part of the innate immune system as they can recognize and eliminate various cells in distress, such as virus-infected and tumor cells. Several promising therapeutics based on NK cell function are currently being developed and under preclinical and clinical investigation. This study aimed to evaluate the efficacy of an affibody-derived bispecific NK cell engager engineered to target CD16a on NK cells and BCMA on multiple myeloma (MM) cells, both in vitro and in vivo. In vitro, efficient lysis of MM cells was observed across a panel of cell lines with varying BCMA expression using NK cells from healthy donors. The bispecific engager also achieved MM cell clearance in a 48-hour real-time cytotoxicity assay at a 2:1 effector-to-target ratio when combined with ADAPT-NK cells—ex vivo expanded adaptive NK cells from healthy donors (Haroun-Izquierdo A et al., J Immunother Cancer 2022). The engager did not induce IL-6 release measured in the presence of PBMCs and target cells. In vivo, a humanized mouse model of MM was used to assess preclinical efficacy of the engager. In this model, human MM.1S cells - tagged with GFP and luciferase (BPS Bioscience) and xenograft-derived from mouse bone marrow - were combined with ADAPT-NK cells. The NSG-Tg(huIL-15) mice (The Jackson Laboratory) were given an intravenous injection of 0.5 million bone marrow-derived MM.1S cells, and tumor growth was followed by bioluminescence imaging (BLI). At study day 4, the dosing of the engager was started and continued daily until study day 18. Belantamab was used as a positive control and administered every fourth day, for a total of four doses, beginning on study day 4. ADAPT-NK cells were administered on study days 4, 8 and 13. Blood samples were obtained for flow cytometry throughout the study, and the mice were sacrificed on study day 33 when bone marrow was harvested for flow cytometry analysis. Body weight developed similarly in all groups. Inhibition of tumor growth was observed by BLI from day 18 onward in the engager-treated group in comparison to vehicle controls. In the ex vivo BLI, lower tumor burden was observed in bones, liver, lungs and spleen in the engager treated group compared to the control. Flow cytometry of bone marrow showed fewer BCMA+CD138+ cells, indicating MM cells, in the engager- and belantamab-treated groups compared to vehicle controls. These findings support the potential of the bispecific NK cell engager combined with ADAPT-NK cells as a treatment for MM. The study also demonstrates the utility of the humanized MM mouse model for evaluating NK cell engagers using BLI and flow cytometry. Citation Format: Mari I. Suominen, Katja M. Fagerlund, Mervi Ristola, Justyna Zdrojewska, Yumei Diao, Carina Norström, Kristina Witt-Mulder, Ellen Santangelo, Thorstein Boxaspen, Hannah Cuthbertson Husbyn, Kim A. Giang, Caterina Heinz, Silje Z. Krokeide, Fredrik Schjesvold, Per-Åke Nygren, Ebba Sohlberg, Karl-Johan Malmberg, Stefan Svensson Gelius, Jukka P. Rissanen, Jenni H. Mäki-Jouppila. Preclinical efficacy of a bispecific NK cell engager studied using a humanized multiple myeloma mouse model abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1598.