A Phase 2 Study of Pembrolizumab during Lymphodepletion after Autologous Hematopoietic Cell Transplantation for Multiple Myeloma

The programmed death-1 (PD-1) axis can suppress immune surveillance against multiple myeloma (MM). We tested the safety and efficacy of pembrolizumab, an anti-PD-1 antibody, in MM after autologous hematopoietic cell transplantation (AHCT). We enrolled patients with MM who did not achieve a complete response (CR) to induction therapy. The study intervention involved a total of 9 doses of i.v. pembrolizumab, with 1 dose given every 21 days starting on day +14 post-AHCT. The primary endpoint was the rate of CR at end of treatment (EOT) in patients receiving ≥2 pembrolizumab doses. Thirty-two patients were enrolled, but 3 withdrew consent before receiving the first dose. The study was terminated early after failing to meet its interim analysis endpoint to detect a 20% difference in EOT CR rate conversion. The median patient age was 59 years. All but 1 patient received triplet induction for a median of 4 cycles (range, 2 to 7 cycles), with 69% partial response (PR) and 31% very good PR (VGPR). No grade 4/5 toxicities or graft failures occurred. Among 26 evaluable patients, 23 had an EOT evaluation, and 7 of these 23 (31%) achieved CR. Two patients had EOT serologic CR but no bone marrow confirmation (CRu), and 1 patient had no EOT evaluation. Bone marrow was minimal residual disease-negative by flow cytometry in 12 of 16 patients (75%) at day +180. With a median follow-up of 23.7 months (range, 15.1 to 33.5 months), no patient achieving EOT CR/CRu had relapsed, whereas 3 patients progressed before EOT and 1 patient progressed at 8 months after EOT VGPR. The estimated 2-year progression-free rate was 83% (95% confidence interval, 68% to 100%). Our data show that early post-AHCT pembrolizumab with lenalidomide maintenance is feasible; however, the efficacy is uncertain and requires further study. This trial was registered at ClinicalTrials.gov (NCT02331368).