Impact of Gamma-Secretase Inhibition on Outcomes Following BCMA CAR-T Therapy in Multiple Myeloma: A Comparison of Two Phase 1 Trials.

BACKGROUND: BCMA-directed chimeric antigen receptor (CAR)-T cell therapy represents a major therapeutic breakthrough for relapsed/refractory multiple myeloma (RRMM), offering deep and durable responses in heavily pretreated patients. However, a subset of patients experiences early relapse or fail to respond, highlighting the need for strategies to enhance efficacy. Gamma-secretase inhibitors (GSIs) have been shown to increase surface BCMA expression on malignant plasma cells and may potentiate the activity of BCMA CAR-T cells, particularly in patients with low baseline BCMA antigen density. Two contemporaneous phase 1 trials (FH9952 and FH9762) evaluated the fully human BCMA-targeted CAR-T product FCARH143, with FH9952 incorporating GSI co-administration.

OBJECTIVE: To determine whether GSI use improves clinical outcomes following FCARH143 CAR-T therapy in RRMM, with particular focus on overall survival (OS), progression-free survival (PFS), and subgroup differences based on prior BCMA-targeted therapy and baseline tumor BCMA levels.

STUDY DESIGN: This retrospective analysis compared outcomes from two single-arm, single-center phase 1 trials of the fully human BCMA-targeted CAR-T cell product FCARH143 in RRMM: FH9952 (n = 18), which incorporated the GSI crenigacestat, and FH9762 (n = 25), which did not. Both studies had extended follow-up beyond previously published reports. Eligible patients had measurable RRMM with ≥10% bone marrow plasma cell involvement and confirmed BCMA expression. BCMA-naïve patients were defined as those without prior exposure to BCMA-targeted therapies (e.g., BCMA CAR-T, bispecific antibodies, or antibody-drug conjugates); BCMA-exposed patients had received at least one such therapy. All participants received lymphodepletion with fludarabine and cyclophosphamide followed by infusion of FCARH143. In FH9952, crenigacestat (25 mg orally, three times weekly) was administered from day 0 through day +18. Outcomes included response rates, adverse events, OS, and PFS. Cox proportional hazards models were used for survival analysis.

RESULTS: With extended follow-up (median 5.8 years), the overall cohort (n = 43) had a median OS of 2.7 years (95% CI 1.9-4.8) and median PFS of 1.1 years (95% CI 0.72-2.2). Baseline characteristics were generally similar, though BCMA-exposed status was more frequent in FH9952 (39% versus 8%). Rates of cytokine release syndrome, immune effector cell-associated neurotoxicity, and early immune effector cell-associated hematotoxicity were comparable between trials. Among BCMA-naïve patients (n = 34), GSI use was associated with improved OS (not reached versus 2.3 years; adjusted HR 0.30, 95% CI 0.10-0.88, P = .028) and a trend toward improved PFS (2.6 versus 1.3 years; adjusted HR 0.47, 95% CI 0.22-1.04, P = .062). No benefit was observed among BCMA-exposed patients. Exploratory spline modeling suggested GSI mitigated the adverse impact of low tumor BCMA levels, with inferior outcomes at low BCMA expression observed only in the non-GSI cohort.

CONCLUSIONS: Co-administration of a GSI with BCMA CAR-T therapy was associated with improved survival in BCMA-naïve RRMM patients, particularly those with low baseline tumor BCMA levels. These findings suggest GSI modulation of BCMA surface expression may enhance CAR-T efficacy in select patients and support further prospective investigation.