Pembrolizumab in Japanese patients with primary mediastinal large B-cell lymphoma: results from the KEYNOTE-A33 study

KEYNOTE-A33 (NCT04317066) is an open-label, single-arm, phase 1 trial designed to evaluate the safety and efficacy of pembrolizumab in Japanese patients with relapsed or refractory (R/R) primary mediastinal large B-cell lymphoma (PMBCL).

Patients received pembrolizumab 200 mg every 3 weeks for up to 35 cycles. The primary endpoints were safety and objective response rate (ORR) per International Working Group 2007 criteria by independent central review. The secondary endpoint was disease control rate (DCR). Duration of response (DOR), progression-free survival (PFS), and overall survival (OS) were exploratory.

Seven patients were enrolled and treated; the median age was 32 years (range 26-43) and 86% were female. The median time from the first dose to data cutoff (April 12, 2022) was 5.6 months (range 2.4-21.2). Grade 3-5 treatment-related adverse events (AEs) occurred in 2 patients (29%; 2 grade 4 neutropenia, 1 grade 3 febrile neutropenia); however, no patient discontinued pembrolizumab or died because of treatment-related AEs. The ORR was 43% [95% confidence interval (CI) 10-82]. DCR was 57% (95% CI 18-90). Median DOR was not reached (NR). Four (57%) patients had a reduction in target lesion size of ≥ 50%. The median PFS was 2.9 months (95% CI 2.6-NR). The median OS was 17.5 months (95% CI NE-NE), and the 12 months OS rate was 100%.

Overall, pembrolizumab had manageable safety and clinically meaningful antitumor activity in Japanese patients with R/R PMBCL, results that were consistent with those observed in prior global studies.

Registry and the Registration No. of the study/trial: Clinicaltrials.gov: NCT04317066.