Final Analysis of Keynote-170: Pembrolizumab in Relapsed or Refractory Primary Mediastinal Large B-Cell Lymphoma (PMBCL)

Introduction: Similar to classical Hodgkin lymphoma, primary mediastinal large B-cell lymphoma (PMBCL) expresses high level of the programmed cell death 1 (PD-1) ligands: PD-L1 and PD-L2. Prior analyses of the phase 2 KEYNOTE-170 (NCT02576990) study demonstrated that monotherapy with the PD-1 inhibitor pembrolizumab had effective antitumor activity and acceptable safety in relapsed/refractory (R/R) PMBCL, leading to FDA approval for adult patients with R/R PMBCL after ≥2 prior therapies. Long-term durability of response with PD-1 blockade in patients with PMBCL, especially those who achieve complete remission (CR), remains a critical clinical question. Here we present updated efficacy and safety of patients with R/R PMBCL in KEYNOTE-170 after ~4 years of follow-up.

Methods: Patients with R/R PMBCL who progressed after or were ineligible for autologous stem cell transplant and received ≥2 prior therapies received pembrolizumab 200 mg every 3 weeks (Q3W) for up to 35 cycles (~2 years). Response assessments were Q12W per 2007 Revised Response Criteria for Malignant Lymphomas. End points included overall response rate (ORR), progression-free survival (PFS), and duration of response (DOR) by investigator assessment; as well as overall survival (OS) and safety. Data cutoff date was October 23, 2020.

Results: At data cutoff, the median duration of follow-up was 48.7 months (range,41.2-56.2). Baseline characteristics of the patients have been previously described (Armand P, et al. J Clin Oncol. 2019; 37(34): 3291-3299). Among all treated patients (N=53), 13 completed 2 years of treatment and 40 patients discontinued, primarily due to progressive disease (n=18). ORR per investigator assessment was 41.5% (95% CI, 30.0-53.7), with 20.8% complete response rate and 20.8% partial response rate. Median DOR was not reached (NR) and 80.6% of patients had a response ≥48 months. Median PFS was 4.3 months (95% CI, 2.8-13.8) and the 48-month PFS rate was 33.0%. Median OS was 22.3 months (95% CI, 7.3-NR) and the 48-month OS rate was 45.3%. At data cutoff, treatment-related adverse events (AEs) of any grade occurred in 30 patients (56.6%); the most commonly reported (incidence >5%) were neutropenia (18.9%), asthenia (9.4%), hypothyroidism (7.5%), and fatigue and pyrexia (5.7% each). Grade 3-4 treatment-related AEs occurred in 22.6% of patients; only neutropenia (n=7) occurred in ≥2 patients. No grade 5 treatment-related AEs occurred.

Conclusions: With 48.7 months of follow-up in heavily pretreated participants with R/R PMBCL, treatment with pembrolizumab monotherapy continued to demonstrate sustained antitumor activity. PFS and OS demonstrated promising trends for long-term survival with no new safety signals identified.