Mosunetuzumab Shows Promising Efficacy in Patients with Multiply Relapsed Follicular Lymphoma: Updated Clinical Experience from a Phase I Dose-Escalation Trial

Sarit Assouline and Won Seog Kim contributed equally.

Introduction: Follicular lymphoma (FL) is considered an indolent yet incurable disease characterized by recurrent relapses: disease-free intervals shorten, and refractoriness increases with each relapse. Patients (pts) with FL who have received at least two prior systemic therapies typically have a poor prognosis. This is particularly true for those who have progression of disease within 24 months of front-line treatment (POD24), or are refractory to multiple agent classes; such patients are left with limited treatment options. Mosunetuzumab is a full-length, fully humanized immunoglobulin G1 CD20/CD3 bispecific antibody that redirects T cells to engage and eliminate malignant B cells. GO29781 (NCT02500407) is an ongoing open-label, multicenter, Phase I/Ib, dose-escalation and expansion study evaluating the safety, efficacy and pharmacokinetics of mosunetuzumab in pts with relapsed/refractory (R/R) B-cell lymphoma. Here, we present updated clinical data from pts with R/R FL treated with mosunetuzumab after at least two prior systemic therapies.

Methods: Data are presented from Group B, in which pts received intravenous mosunetuzumab monotherapy as step-up doses in Cycle 1 on Days 1 and 8 and the target dose administered on Day 15. Mosunetuzumab was given on Day 1 of each subsequent 21-day cycle for 8 cycles in pts with a complete response (CR), and up to 17 cycles in those with a partial response (PR) or stable disease (SD).

Results: As of January 21, 2020, 62 pts with FL (with at least two prior systemic therapies), received mosunetuzumab at dose levels between 0.4/1.0/2.8mg and 1/2/13.5mg (Cycle 1 Day 1/8/15 dose levels). The median age was 59 (range 27–85) years, and median number of prior therapies was 3 (range 2–11). Thirty-three pts (53%) were refractory to both a prior anti-CD20 antibody and an alkylating agent (double refractory), 30 (48%) had POD24, and four (6%) had received prior chimeric antigen receptor T-cell (CAR-T) therapy.

The overall response rate (ORR) was 68% (42/62), with 31 pts (50%) achieving CR (Figure). Consistent CR rates were observed in high-risk pt populations, including those with double refractory disease (18/33 [55%]), POD24 (16/30 [53%]), PI3Ki refractory (7/9 [78%]), and those who received prior CAR-T therapy (2/4 [50%]). With a median time on study of 14.4 months, 26 pts (62% of all responders; including 74% of pts who achieved CR) remained in remission at the data cut-off. The median duration of response (DOR) was 20.4 months (95% CI: 11.7 months, upper limit not reached) for all 42 responders. The median PFS was 11.8 months (95% CI: 7.3–21.9 months).

Adverse events (AEs) were reported in 60 pts (97%); serious adverse events (SAE) were reported in 22 pts (35%). The most frequently reported (>10% of pts) grade (Gr) 3 or higher AEs included hypophosphatemia (23%; transient and clinically asymptomatic) and neutropenia (21%; with a low rate of febrile neutropenia [2%]). Overall, 14 pts (23%) experienced CRS; in four pts, CRS was classified as a SAE. CRS events were reversible, mostly of Gr 1 or 2 (Gr 1, n=11; Gr 2, n=2; Gr 3, n=1; Lee, et al. Blood 2014), and predominantly occurred during Cycle 1. No patient required tocilizumab, intensive care unit admission or use of vasopressors for CRS management. Neurologic AEs (NAEs; defined by any AEs reported as Preferred Terms in SOC Nervous System Disorders and SOC Psychiatric Disorders) were observed in 28 pts (45%); all were Gr 1 (n=18) or 2 (n=10). The most commonly reported NAEs were headache (24%), insomnia (15%) and dizziness (11%). No Gr ≥3 NAEs or serious NAEs were reported.

Conclusions: Fixed-duration mosunetuzumab monotherapy results in high response rates and durable disease control with a tolerable safety profile in heavily pretreated patients with FL, including known high-risk subgroups. Updated pharmacodynamics and biomarker data will be presented at the meeting.