Clinical Outcomes of Relapsed/Refractory Multiple Myeloma Patients Following Treatment with Bispecific Antibodies (BiAbs)

Background: Bispecific antibodies (BiAbs) are a novel off-the-shelf class of drugs currently being investigated in clinical trials for patients with relapsed/refractory multiple myeloma (RRMM) with promising efficacy in heavily pretreated patients. BiAbs simultaneously bind two antigens, thereby engaging CD3+ T cells with myeloma cells expressing specific antigens such as BCMA, GPRC5D, FcRH5 or CD38. However, the outcome of myeloma patients after relapse on BiAbs is unknown and effective approaches for salvage therapy are needed.

Methods: Demographics, disease characteristics and post-clinical trial outcomes were collected retrospectively on RRMM patients who relapsed after BiAb therapy at the Tisch Cancer Institute (The Mount Sinai Hospital, New York). We identified a total of 116 patients who were enrolled on trials with BiAbs targeting either BCMA or GPRC5D. Of these, 69 patients were no longer enrolled on the trials due to disease progression (including 5 patients who died on the trial). Clinical data was collected up until July of 2021. This retrospective study was approved by the institutional review board (IRB) and follows the Declaration of Helsinki and International Conference on Harmonization Guidelines for Good Clinical Practice (IRB: GCO#: 11-1433). Survival and response duration were calculated by Kaplan-Meier estimation.

Results: The 64 RRMM patients had a median age of 58.5 years (range: 46-82) at time of disease progression following BiAbs therapy, and 48% were male. Median time from diagnosis to initiation of BiAbs therapy was 5 years (range: 1.6-16.3) and patients had a median follow-up of 24.9 months from time of relapse from BiAb therapy. Fifty patients (78%) had high-risk cytogenetics, including gain1q21, del17p, t(4;14), t(14;16) and t(14;20). Most patients were highly pretreated with a median of 7 prior lines (range: 3-17) and 54 patients (84%) had received an autologous stem cell transplant (ASCT) prior to receiving BiAbs. Three patients were treated with chimeric antigen receptor (CAR) T cell therapy prior to BiAb and 5 patients were exposed to a BCMA antibody-drug conjugate prior to the BiAb. Furthermore, 89% of patients were triple-class refractory while 44% were penta-refractory.

Following treatment with a BiAb, 2 patients were lost to follow up, 1 patient decided to be monitored off treatment and 61 patients received a median of 2 lines of therapy (range: 1-8). Most common therapies included a second BiAb (n=20; 33%), CAR T cells (n=15; 26%) or intensive chemotherapy (n=36; 59%) such as melphalan, carmustine or VDPACE with stem cell rescue (n=13) or DCEP (n=23). Best response to initial treatment following the BiAb varied widely and included 12 complete responses, 5 very good partial responses, 17 partial responses, 2 minimal responses, 10 stable disease and 13 progressed disease for an overall response rate (ORR) of 58%. Encouraging responses were seen in 10 patients who directly transitioned from one BiAb to another and 8 patients who directly transitioned to CAR T cell therapy. The progression-free survival of those 18 patients who directly transitioned to a T cell directed therapy was 28.9 months (95% CI: 21.6-NE) and their median overall survival was not reached. Furthermore, the overall survival for the whole cohort of patients was 17.6 months (95% CI: 12.0-NE).

Conclusion: Our data suggests that heavily pretreated, predominantly triple-class refractory, patients relapsing after BiAbs may still have good outcomes when sequentially treating with other immunological/T cell-directed therapeutics such as BiAbs and CAR T cells. Studying the appropriate sequence of these treatments is of paramount importance as BiAbs are expected to become part of the standard of care for RRMM patients.