A Phase 2, Open-Label, Multicenter Study of Tazemetostat in Combination with Rituximab for the Treatment of Relapsed or Refractory Follicular Lymphoma

Background: Patients with follicular lymphoma (FL) who do not achieve a response after 2 or more prior lines of systemic therapy have a poor prognosis when treated with salvage chemotherapy alone or PI3K inhibitors. Enhancer of zeste homolog 2 (EZH2) is a histone methyltransferase that supports germinal center (GC) formation and suspends differentiation during B-cell development by silencing genes that limit proliferation and promote exit from the GC. Throughout the pathogenesis of FL, EZH2 plays a consistent underlying role in maintaining the GC and B-cell proliferation, regardless of the oncogenic drivers. Dysregulation of EZH2 or gain-of-function mutations in EZH2 can lead to accumulation of malignant B cells and may contribute to the development of FL. Tazemetostat, a first-in-class oral EZH2 inhibitor, recently gained approval by the US Food and Drug Administration in patients with relapsed or refractory (R/R) FL after demonstrating single-agent, antitumor activity in patients with mutant (MT) or wild-type (WT) EZH2 R/R FL. Here we examine the efficacy and safety of tazemetostat in combination with rituximab for the treatment of R/R FL.

Study Design and Methods: This trial is a phase 2, single-arm, open-label, multicenter study of tazemetostat in combination with rituximab in patients with R/R FL who have received at least 2 prior lines of systemic therapy, including an anti-CD20-based regimen. Patients eligible for inclusion are aged ≥18 years with grade 1 to 3A FL who have met Groupe d'Etude des Lymphomes Folliculaires (GELF) criteria to receive treatment, have an Eastern Cooperative Oncology Group (ECOG) score ≤2, have >5.0 x 10⁹/L circulating malignant cells, and have not received any prior treatment with EZH2 inhibitors. Patients with transformed FL, a history of clinically significant gastrointestinal conditions, or any uncontrolled illness are excluded from this study. EZH2 mutation testing will be performed during patient screening.

Patients will receive tazemetostat 800 mg by mouth twice daily beginning on cycle 1 day 1 for continuous 28-day cycles until the end of cycle 24, for a total of 24 months of therapy. Patients will also receive rituximab 375 mg/m² intravenously (IV) on cycle 1 day 1, then 375 mg/m² IV or 1400 mg subcutaneously on days 8, 15, and 22 of cycle 1, and on day 1 of cycles 3, 4, 5, and 6, for a total of 8 doses of rituximab.

The primary endpoint is the objective response rate (ORR; the proportion of patients with complete or partial response) in patients with WT EZH2. Key secondary endpoints include median progression free survival (PFS) in patients with WT EZH2; median PFS in all patients, regardless of mutational status; ORR in patients with MT EZH2; efficacy outcomes in rituximab-refractory patients; and incidence of adverse events. Efficacy and safety analyses will be performed on all patients who receive ≥1 dose of study drug. The evaluation of ORR will be based on Lugano 2014 response criteria and will be presented with corresponding 2-sided Clopper-Pearson 95% confidence intervals. The Kaplan-Meier method will be used to estimate PFS. Survival follow-up will be done following the study treatment period every 6 months for 2 years or until disease progression or death for all patients who complete the 24 months of tazemetostat therapy.