A phase Ib trial of isatuximab, bendamustine, and prednisone in relapsed/refractory multiple myeloma

Patients with triple-class refractory (TCR) multiple myeloma (MM) often need cytoreductive chemotherapy for rapid disease control. Bendamustine is an outpatient-administered, bifunctional alkylator and isatuximab is an anti-CD38 monoclonal antibody with unique cytotoxicity characteristics. We hypothesized that isatuximab-bendamustine-prednisone would be well-tolerated regimen in TCR MM, and conducted single-center, phase Ib, investigator-initiated study.

Patients had TCR MM and last daratumumab exposure ≥ 6 weeks. This study was conducted as a 3 + 3 design to establish the maximally tolerated dose (MTD) and/or recommended phase 2 dose (RP2D). Isatuximab 10 mg/kg IV was administered weekly (cycle 1), and every 2 weeks thereafter. Bendamustine was administered on days 1 and 2 at 3 dose levels (DL): 50, 75, and 100 mg/m². Methylprednisolone was administered as 125 mg on day 1 and prednisone 60 mg days 2-4. Common definitions were used for DLTs, adverse events (CTCAE v 5.0), and disease response.

Fifteen patients were treated (3 DL1, 6 DL2, 6 DL3). Median age was 71, 53% had high-risk cytogenetics, and 34% had prior BCMA-targeting therapy. One DLT was observed at DL2 (Grade 3 thrombocytopenia plus bleeding). There were no Grade 5 treatment-related AEs. The MTD was not reached. The overall response rate was 20% (3/15) including one stringent complete response. The median PFS was 2.5 months (95% CI 0.9-4.1 months).

We demonstrated the safety and tolerability of isatuximab-bendamustine-prednisone. Toxicities were mild and manageable with limited intervention. The study was discontinued due to slow accrual. However, we observed responses even among highly refractory patients.

This study was registered on clinicaltrials.gov as NCT04083898 on 9/6/2019.