CD38-targeted antibody-polymer drug conjugates for enhanced treatment of multiple myeloma.

Multiple myeloma (MM) remains a formidable disease, especially in relapsed or refractory cases when there are limited treatment options. In this study, we introduce two polymer-antibody drug conjugates (pADCs), ISA-P-EPI (U6244-021) and DARA-P-EPI (U6244-031), which contain semitelechelic N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-epirubicin (EPI) conjugate attached to CD38-targeting antibodies Isatuximab (ISA) and Daratumumab (DARA). These pADCs enhance therapeutic efficacy by combining the specificity of ISA and DARA with the cytotoxic potency of EPI while preserving antibody function. The EPI is linked to the HPMA polymer backbone via a tetrapeptide spacer cleavable by lysosomal enzymes, enabling drug release upon endocytosis within tumor cells. This design achieves a higher drug-to-antibody ratio than conventional ADCs for safer delivery of drug payload. In vitro studies demonstrate efficient binding, internalization, and cytotoxic efficacy of these pADCs in MM cell lines. Mechanistic investigations revealed significant therapeutic effects, including cell cycle arrest, immunogenic cell death, and preserved antibody-dependent cellular cytotoxicity (ADCC). In addition, pADCs were effective in 5 out of 8 primary samples, with their efficacy closely correlating with CD38 surface expression levels. To enhance therapeutic outcomes, we employed panobinostat to upregulate CD38 expression, which further improved pADC efficacy. In a preclinical NRG mouse model inoculated with MM.1S-luc cells, pADC treatment significantly delayed tumor progression and prolonged survival, with all treated mice remaining alive at the 100-day endpoint. These findings underscore the potential of CD38-targeted pADCs as a novel approach to combining chemotherapy with immunotherapy for MM treatment, warranting further investigation into their optimization and clinical application.