Phase 1b Study of PD-1 Blockade with Pembrolizumab in Patients with Relapsed/Refractory Primary Mediastinal Large B-Cell Lymphoma (PMBCL)

Introduction: Like classical Hodgkin lymphoma (cHL), PMBCL frequently harbors genetic alterations of the 9p24.1 locus, leading to overexpression of the PD-1 ligands, PD-L1 and PD-L2. This provides a possible mechanism of immune evasion and suggests that PMBCL could have a genetically determined vulnerability to PD-1 blockade. Pembrolizumab is a humanized monoclonal antibody that blocks the interaction between PD-1 and its ligands. Pembrolizumab has already demonstrated robust antitumor activity in advanced solid tumors and in cHL. KEYNOTE-013 (NCT01953692) is a multicenter, multicohort phase 1b trial testing the safety and preliminary efficacy of pembrolizumab in patients with hematologic malignancies. Based on its genetics, PMBCL was included as an independent cohort in this trial. Here we report the preliminary results in this patient population.

Methods: The PMBCL cohort of KEYNOTE-013 is enrolling patients with relapsed/refractory (R/R) disease who have relapsed after or are ineligible for autologous stem cell transplant (ASCT). Pembrolizumab is administered intravenously at a dose of 10 mg/kg every 2 weeks for up to 2 years or until confirmed disease progression or unacceptable toxicity. The primary end points are safety and antitumor activity. Response is being evaluated using computed tomography (CT) and positron emission tomography (PET) at week 12 and every 8 weeks thereafter, using IHP 2007 criteria. Other end points include complete remission (CR) rate, duration of response (DOR), and exploratory biomarker analyses.

Results: As of July 23, 2015, 10 patients with R/R PMBCL with a median age of 28 (23-62) years have been enrolled in this cohort. Patients were heavily pretreated: 40% had ≥4 prior lines of therapy, and 60% had prior radiation. Six patients (60%) experienced at least 1 adverse event (AE) of any grade related to study treatment. These treatment-related AEs, all grade 1/2, were: hypothyroidism and decreased appetite (2 patients each), and diarrhea, nausea, vomiting, fatigue, edema, weight loss, and arthralgia (1 patient each). There were no grade 3-5 treatment-related AEs. Two patients experienced a serious AE (grade 3 infectious pneumonia) unrelated to study drug. No patient discontinued for toxicity. 

Nine patients were evaluable for response (1 discontinued treatment based on clinical progression before week 12). The objective response rate (ORR) was 44% (4/9), with 1 patient achieving a CR and 3 patients achieving a partial response. The intent-to-treat ORR was 40%. With a median follow-up of 144 days, the median DOR has not been reached (1+ to 291+) days, with all 4 responses ongoing at the time of data cutoff. Six of 10 patients have discontinued study treatment because of disease progression, and 4 patients remain on study.

Conclusion: The preliminary results of KEYNOTE-013 indicate that PD-1 blockade with pembrolizumab is associated with a tolerable safety profile and a promising response rate in heavily pretreated patients with R/R PMBCL. Those patients often have a very poor outcome with conventional therapy, justifying further studies of pembrolizumab in this population.