Low-Frequency PPM1D Gene Mutations Affect Treatment Response to BCMA-Targeted CAR T-Cell Therapy in Multiple Myeloma

Background: BCMA-targeted Chimeric Antigen Receptor (CAR) T-cell therapy has revolutionized the treatment of Relapsed/Refractory Multiple Myeloma (RRMM). However, the disease is not curable and progression after CAR T-cell treatment remains a challenge. Clonal hematopoiesis, specifically mutations in the DNA damage response gene PPM1D, has been linked to therapy resistance and inferior survival in lymphoma patients undergoing cellular therapy. The impact of PPM1D mutations on MM patient outcome after CAR T-cell therapy remains undefined. Methods: We conducted a retrospective single-center study of 83 patients with RRMM patients treated with idecabtagene vicleucel or ciltacabtagene autoleucel between 2022 and 2025. Next-generation sequencing was performed on peripheral blood mononuclear cells collected prior to CAR T-cell infusion to identify PPM1D exon 6 mutations (variant allele frequency > 0.01). We analyzed associations between mutational status, clinical characteristics, toxicity, and survival. Results: PPM1D mutations were detected in 14.5% (12/83) of patients. PPM1D-mutated patients had fewer prior autologous stem cell transplantation compared to wild-type patients (50% vs. 82%, p = 0.02) and presented more advanced disease burden and adverse prognostic features (R-ISS stage III 58% vs. 20%, p = 0.05). Notably, PPM1D status did not impact initial efficacy; complete remission rates were comparable between groups (67% vs. 69%). However, PPM1D mutations were significantly associated with inferior progression-free survival (PFS) (median PFS: 6 months vs. 16 months, p = 0.04). Regarding toxicity, the mutated subgroup exhibited significantly higher rates of grade ≥2 cytokine release syndrome and a trend toward increased neurotoxicity (25% vs. 7%). Conclusions: PPM1D clonal hematopoiesis is frequent in RRMM and despite deep initial responses, patients harboring PPM1D mutations face a significantly higher risk of early relapse. PPM1D mutations may serve as a biomarker for poor durability of response and should be further evaluated in larger, prospective trials.