BCMA-Directed MRD Detection as a Predictor of Relapse after BCMA CAR T in Multiple Myeloma.

Background: Recent approvals of chimeric antigen receptor T-cells (CAR T) and bispecific antibody therapies offer new hope for relapsed refractory multiple myeloma (RRMM) patients, with superior efficacy over standard regimens observed in clinical trials. However, relapse after BCMA-directed therapy is common and requires further investigation.

Patients and methods: We conducted a retrospective cohort study on 57 RRMM patients treated with BCMA-directed CAR T. Only the patients who had an initial response and lost BCMA-expressing identified PC following CAR T infusion at Day 30 were included in the analysis. Multicolor flow cytometry (MFC) to detect BCMA + plasma cell (PC) re-emergence was performed on bone marrow samples at defined intervals and clinical responses were assessed using International Myeloma Working Group criteria.

Results: The majority of patients achieved undetectable BCMA on MFC postinfusion, with subsequent BCMA+ PC re-emergence observed in 55% of cases. Notably, 91% of patients experiencing clinical relapse showed BCMA+ PC re-emergence, often preceding relapse. Early relapse (<6 months) was associated with earlier BCMA re-emergence.

Conclusion: Early BCMA+ PC re-emergence may serve as a prognostic marker for clinical relapse post-BCMA CAR T therapy. Monitoring BCMA+ PC levels via MFC offers potential for early relapse detection and informed treatment decisions. Further studies, including novel BCMA-directed minimal residual disease (MRD) detection technologies, are warranted to validate these findings and refine RRMM management strategies.