Mosunetuzumab monotherapy is active and tolerable in patients with relapsed/refractory diffuse large B-cell lymphoma.

As part of a phase 1/2 study, this single-arm expansion cohort established the efficacy and safety of mosunetuzumab monotherapy in patients with relapsed/refractory (R/R) diffuse large B-cell lymphoma (DLBCL) (received >=2 prior lines of therapy). Intravenous mosunetuzumab was administered with cycle (C)1 step-up dosing for cytokine release syndrome (CRS) mitigation: C1 day (D) 1: 1 mg; C1D8 2 mg; C1D15 and C2D1: 60 mg; C3 + D1: 30 mg. Hospitalization was not mandatory. Patients with complete response (CR) completed treatment after C8; those with partial response or stable disease continued treatment for a total of 17 cycles. The primary endpoint was CR rate (best response) by Independent Review Facility, assessed against a historical control CR rate (20%). Eighty-eight patients (73.9% de-novo DLBCL; 26.1% transformed follicular lymphoma) were enrolled; all had received prior anthracycline and anti-CD20 therapy. Overall response and CR rates were 42.0% (95% confidence interval [CI], 31.6-53.1) and 23.9% (95% CI, 15.4-34.1, respectively; CR rate did not reach statistical significance versus the historical control [P = 0.36]). Median time to first response was 1.4 months. Median progression-free survival was 3.2 months (95% CI: 2.2, 5.3). The CR rate in 26 patients who received prior CAR-T therapy was 12%. CRS was one of the most common adverse events (26.1% of patients); predominantly grade 1 to 2 and primarily in C1. Four patients (4.5%) discontinued mosunetuzumab due to adverse events. Mosunetuzumab demonstrated notable efficacy and a manageable safety profile in patients with R/R DLBCL, including those previously treated with CAR-Ts.