Dose-escalation part of Phase I study of single-agent mosunetuzumab in Japanese patients with relapsed/refractory B-cell non-Hodgkin lymphoma

Objective This dose-escalation part of an ongoing Phase I study assessed the tolerability, safety and pharmacokinetics of mosunetuzumab in Japanese patients with relapsed/refractory B-cell non-Hodgkin lymphoma (NHL). Methods Mosunetuzumab was administered intravenously, with step-up dosing in a 3 + 3 design, on Days 1, 8 and 15 of Cycle 1, and Day 1 of each subsequent 21-day cycle for up to 17 cycles to patients across five cohorts with different target doses (2.8, 6.0, 13.5, 27.0 or 60.0 mg). Results As of 5 July 2022, 23 patients had received mosunetuzumab. The median patient age was 63.0 years, 56.5% of patients were male, and 69.6% of patients had diffuse large B-cell lymphoma, 17.4% had transformed follicular lymphoma (FL) and 13.0% had FL. The median number of prior lines of therapy was 4. Mosunetuzumab was well tolerated and there were no deaths. The most common adverse events (any grade) were neutropenia/neutrophil count decreased (47.8%) and cytokine release syndrome (34.8%). Most cytokine release syndrome events were Grade 1/2 (one Grade 3), and most occurred within 24 hours of the first dose of mosunetuzumab. The apparent half-life of mosunetuzumab was 4.1-5.0 days. Two patients achieved a complete response, and 11 patients achieved a partial response. Conclusions This study demonstrated that mosunetuzumab has an acceptable safety profile and antitumor activity in Japanese patients with relapsed/refractory B-cell NHL. The recommended Phase II dose of 1.0/2.0/60.0/60.0/30.0 mg was tolerable and there were no new or different safety signals compared with the global Phase I study. Single-agent mosunetuzumab demonstrates an acceptable safety profile and antitumor activity in Japanese patients with relapsed/refractory B-cell NHL and no new safety signals compared with the global Phase I study.