Abstract 64: Depleting CD38 leads to an aggressive phenotype of multiple myeloma

Abstract Introduction: Multiple myeloma (MM), the second most common hematological cancer in the United States, is characterized by the proliferation of malignant clonal plasma cells within the bone marrow. Since the FDA approval of Daratumumab in 2015, CD38-targeted immunotherapies have significantly advanced the treatment of MM, improving patient response. However, due to the multifunctional role of CD38 in the complex bone microenvironment, its role in MM remains under investigation. This study examined the effect of modulating CD38 expression by using a CD38 knock-out (KO) clone of a human MM cell line xenografted into immunodeficient mice. Methods: CRISPR/Cas9 gene editing was used to KO CD38 expression in the MM.1S human MM cell line, confirmed with RNA sequencing and flow cytometry. After tumor implantation in mice, bioluminescence imaging (BLI) monitored tumor burden. PET was performed using [18F]-FDG to detect metabolic activity and [89Zr]-Daratumumab to analyze human CD38 expression throughout the body. Post-PET tissue biodistribution and select bone autoradiography were performed to evaluate the specificity of tracer uptake. CT, dual-energy X-ray (DEXA), and immunohistochemistry were used to quantify changes in bone mineral density and osteolytic disease. The imaging results were corroborated with changes in blood composition and cytokine production in the microenvironment. Results: After the cells were implanted into mice, BLI showed a significant tumor burden increase in CD38 KO MM models compared to WT MM models at 4 weeks. CT and DEXA imaging revealed that the KO mice had significantly decreased bone density, and similarly, planar X-ray imaging indicated severe myeloma bone disease characterized by cortical and trabecular osteolytic lesions in the femur and tibia. [89Zr]-Daratumumab accumulated in the spine, femur, and tibia in WT MM mice, with minimal bone uptake in CD38 KO MM mice. [18F]-FDG PET revealed an increased metabolic activity of CD38 KO MM, with an aggressive kinetic uptake of the tracer. Blood analysis indicated elevated markers of disease progression and renal dysfunction, with reduced leukocyte and thrombocyte production in CD38 KO MM xenografts. Cytokine profiling revealed altered levels of pro-inflammatory cytokines, including IL-6, RANKL, and VEGF-α in the bone microenvironment of MM with depleted CD38. RNA sequencing showed significant transcriptional changes in CD38 KO cells, particularly in cell adhesion and inflammatory pathways. Conclusions: The findings underscore CD38’s critical role in MM progression, where its depletion drives an aggressive phenotype marked by widespread metastasis, extensive bone degradation, renal involvement, and heightened immunosuppression. These results showcase the critical importance of CD38 expression levels and suggest a potential role for CD38 and related transcriptional changes in contributing to the aggressive phenotype of MM. Citation Format: Michael Riley Dyer, Alexander Zheleznyak, Rui Tang, Julie Prior, Erin Teubner, Brad Manion, Mikhail Y. Berezin, Junwei Du, Julie O'Neal, Monica Shokeen. Depleting CD38 leads to an aggressive phenotype of multiple myeloma [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 64.