Efficacy and safety of mosunetuzumab monotherapy for Japanese patients with relapsed/refractory follicular lymphoma: FLMOON-1

Background: In a global phase I/II study (GO29781; NCT02500407), single-agent mosunetuzumab had a manageable safety profile and induced durable complete responses in patients with relapsed/refractory (R/R) B-cell non-Hodgkin lymphoma, including in patients with R/R follicular lymphoma (FL). In this analysis, the efficacy and safety of mosunetuzumab monotherapy were evaluated in an expansion cohort, FLMOON-1, in Japanese patients with R/R FL who had received ≥ 2 prior lines of therapy in a phase I study (JO40295, jRCT2080223801). Methods: Mosunetuzumab was administered intravenously at the recommended phase II dose (with cycle 1 step-up dosing) for eight cycles or up to 17 cycles, or until disease progression or unacceptable toxicity. The pre-specified primary endpoint was Independent Review Facility (IRF)-assessed complete response rate (CRR; as best overall response). Secondary objectives included investigator (INV)-assessed CRR, INV- and IRF-assessed objective response rate (ORR), and safety. Results: At the data cutoff (October 13, 2023), 19 patients (median age 72 years) were evaluated. The IRF-assessed CRR and ORR were 68.4% and 78.9%, respectively; the INV-assessed CRR and ORR were 63.2% and 84.2%, respectively. Grade 3–4 adverse events (AEs) were observed in 89.5% of patients, with a low incidence of AEs leading to mosunetuzumab discontinuation (10.5%) and one fatal AE unrelated to mosunetuzumab. Cytokine release syndrome occurred in 47.4% of patients and were mostly Grade 1 in severity. Conclusion: These findings indicate mosunetuzumab has a consistent efficacy and manageable safety profile in Japanese patients with R/R FL compared with previously reported data from the global phase I/II study.