Overcoming resistance in CD44-overexpressing myeloma through combination therapy with ATRA, bortezomib, and NK cells

CD44 is a cell-surface glycoprotein frequently overexpressed in cancers and associated with poor prognosis. We evaluated the prognostic significance of CD44 overexpression in multiple myeloma (MM) and investigated the therapeutic efficacy of combining natural killer (NK) cell therapy with all-trans retinoic acid (ATRA) and bortezomib (Bor) against CD44-overexpressing MM. Clinical data from the CoMMpass database were analyzed to assess survival outcomes relative to CD44 expression. NK cells were expanded from healthy donors using K562-OX40L-mbIL-18/21 feeder cells supplemented with interleukin (IL)-2 and IL-15. Functional assays were performed using CD44-high myeloma cell lines treated with ATRA and Bor, individually or in combination with NK cells. Therapeutic efficacy was evaluated based on tumor growth, systemic dissemination, and survival in an intravenous U266-green fluorescent protein-firefly luciferase xenograft NOD/SCID IL-2Rγnull mouse model. Clinical data showed CD44 overexpression correlated with inferior overall survival (P < 0.0001) in all three stages I, II, and III of the revised International Stage System. In vitro, ATRA and Bor co-treatment downregulated β-catenin and CD44 expression, inhibited proliferation, migration, and invasion, and enhanced NK cell-mediated cytotoxicity via upregulation of MICA/B, Fas, TRAIL-R2, and intercellular adhesion molecule-1. In vivo, combination therapy with NK cells, ATRA, and Bor significantly suppressed CD44 expression, reduced extramedullary spread, and prolonged survival without notable toxicity. These preclinical findings support CD44 overexpression as a marker associated with inferior survival in MM and provide a rationale for pharmacologic tumor priming with ATRA and bortezomib to enhance NK cell-mediated cytotoxicity. However, the therapeutic strategy requires further validation in heterogeneous patient-derived models and prospective clinical studies before clinical efficacy can be inferred.