Patients with relapsed or refractory multiple myeloma who were treated with the two highest doses of REGN5459, a bispecific antibody targeting BCMA and CD3, experienced a 90.5% overall response rate, according to results of a phase I/II clinical trial presented at the AACR Annual Meeting 2023, held April 14-19.

REGN5459 binds to BCMA on malignant plasma B cells that constitute multiple myeloma and CD3 on T cells, bringing the two cell types together so that the latter can attack the former.

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The overall response rate in the study population was 65.1%. Among the 21 patients treated at the higher doses (480 mg and 900 mg), the response rate was 90.5%, of which 61.9% were complete responses or better, including 38.1% that were stringent complete responses, a deeper response category characterized by the absence of clonal myeloma cells in the bone marrow and a normal blood test result for free light chains.

Responses occurred early and deepened with time, with a projected 78.1% of patients continuing to respond at a year. “In comparison to the average lifespan of heavily pretreated patients at this stage, which is six to nine months, that the one-year progression-free survival may be more than 70% is very promising,” Suvannasankha said.

In terms of adverse events, across all dose levels, 53.5% of patients experienced CRS, of which none were grade 4 or 5 and 87% were grade 1. In all cases, Suvannasankha said, the condition did not lead to discontinuation and the patients were able to escalate treatment to the full planned dose.