Serum soluble BCMA can be used to monitor relapse of multiple myeloma patients after chimeric antigen receptor T-cell immunotherapy

Chimeric antigen receptor T-cell (CAR-T) therapy has been proven very effective in treating hematologic malignancies. Ciltacabtagene autoleucel (cilta-cel), a second-generation CAR-T cell with double B cell maturation antigen (BCMA) targeting binding domains, showed an 88% overall response rate (ORR) in patients with relapsed/refractory multiple myeloma (MM), which were carried out in our institute. This study aimed to assess the prognostic potential of soluble BCMA (sBCMA) in serum as a biomarker in MM after CAR-T therapy. Serum samples (n = 44) from MM patients were collected before and after CAR-T therapy. The level of sBCMA was analyzed by enzyme-linked immunosorbent assay (ELISA). Additionally, three patients' long-term longitudinal analysis were performed. Serum sBCMA level was correlated with the percentage of malignant plasma cells in bone marrow (r = 0.613). After CAR-T infusion, the sBCMA level in serum of MM patients decreased markedly (median: 508,513 pg/mL before CAR-T infusion, 89,198 pg/mL in the first month, 8448 pg/mL in the second months, and 6010 pg/mL in the third month after CAR-T infusion). In patients who obtained objective response (≥ PR), re-elevated sBCMA indicated the possibility of disease recurrence. At a cutoff 69,326.27 pg/mL, sBCMA shows high sensitivity (87.5%) and specificity (88.5%) for identifying relapse of MM after CAR-T therapy. Our results suggested that serum sBCMA level changes in response to the clinical status of MM patients after anti-BCMA CAR-T therapy. Furthermore, sBCMA may be a auxiliary biomarker for disease monitoring in MM patients after CAR-T therapy.