Abstract 319: Testing of trispecific antibodies for treating multiple myeloma using the Membrane Proteome Array

Abstract Background: Assessment of off-target antibody reactivity is a regulatory requirement prior to clinical development; however, conventional screening methods are frequently ineffective in screening newer therapeutic modalities including multispecifics and CAR-T cell therapies. This presents an issue for T-cell engaging therapeutics that require exquisite specificity, as even minimal cross-reactivity could result in non-target cell killing accompanied by serious or even life-threatening consequences. Methods: We developed the Membrane Proteome Array (MPA)—containing 6, 000 human proteins—as a comprehensive approach to identify off-target protein interactions. The MPA is a cell-based protein array that assesses binding interactions in native, unfixed cells, allowing for quantitative and high sensitivity detection using flow cytometry. The MPA has been used to test specificity of over 1000 therapeutic molecules, including conventional MAbs and newer biotherapeutic modalities, including scFv, VH/VHH, and bispecific molecules, as well as other protein modalities. Results: We used the MPA to evaluate the specificity of several multispecific antibody candidates targeting the G protein-coupled receptor GPRC5D, a promising target for multiple myeloma. Here we tested a panel of GPRC5D bispecifics using a CD3 targeting arm, as well as trispecific antibodies targeting GPRC5D, BCMA, and CD3. The MPA was successfully able to assess specificity of these molecules, showing compatibility with screening trispecifics. As a result, lead candidates devoid of cross-reactivity were identified for further development. The MPA has also been used to rapidly evaluate the specificity of novel CAR-T cell therapies for cancer and autoimmunity. Conclusions: The MPA is compatible with screening of multispecifics, has been used for lead selection, and has been used in numerous successful regulatory applications for antibody-based therapies. The MPA is currently being developed as an FDA-qualified Drug Development Tool. Citation Format: Lewis Joe Stafford, J. Tabb Sullivan, Carmen T, Navia, Amelia M. Snyder, Diana M. Norden, Benjamin J. Doranz. Testing of trispecific antibodies for treating multiple myeloma using the Membrane Proteome Array [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 319.