P1126: MOSUNETUZUMAB IS EFFICACIOUS AND WELL TOLERATED IN PATIENTS AGED <65 AND ≥65 YEARS WITH RELAPSED/REFRACTORY FOLLICULAR LYMPHOMA AND ≥2 PRIOR THERAPIES: SUBGROUP ANALYSIS OF A PIVOTAL PHASE II STUDY

Abstract

Background: Advancing age is associated with a decline in immune function and an increase in the incidence of comorbidity (Castellino et al. 2017). Vulnerability to treatment-related toxicities may also increase with age. Mosunetuzumab is a T-cell engaging CD20xCD3 bispecific monoclonal antibody (Ab) that redirects T cells to eliminate malignant B cells. In a pivotal Phase II study (NCT02500407), mosunetuzumab induced deep and durable remissions and had a favorable safety profile in patients (pts) with relapsed/refractory (R/R) follicular lymphoma (FL) and ≥2 prior therapies (Budde et al. ASH 2021). Aims: In the current study, we evaluated the efficacy and safety of mosunetuzumab in pts aged <65 and ≥65 years (age stratification per ESMO Clinical Practice Guidelines for management of newly diagnosed and R/R FL; Dreyling et al. 2021) in the pivotal Phase II study. Methods: All pts had Grade (Gr) 1–3a FL and ECOG performance status (PS) 0–1, and were R/R to ≥2 prior therapies, including an anti-CD20 Ab and an alkylating agent. Intravenous mosunetuzumab was given in 21-day cycles with Cycle (C) 1 step-up dosing: C1 Day (D) 1: 1mg; C1D8 2mg; C1D15 and C2D1: 60mg; D1 of C3+: 30mg. Pts with a complete response (CR) by C8 completed therapy; pts with a partial response or stable disease continued treatment for up to 17 cycles, unless disease progression or unacceptable toxicity occurred. The primary endpoint was CR (as best response) rate by PET/CT assessed by Independent Review Committee using standard response criteria (Cheson et al. 2007). Cytokine release syndrome (CRS) was graded using ASTCT criteria (Lee et al. 2019). All pts provided informed consent. Results: As of August 27, 2021, 90 pts had received mosunetuzumab; 67% were aged <65 years and 33% were aged ≥65 years. Of 30 pts aged ≥65 years, median age was 71 years (range: 65–90) and 43% had ECOG PS 1 at entry, 53% had FLIPI 3–5, and 70% had Ann Arbor stage III/IV disease. Median number of prior therapies in the older age group was 3 (range: 2–8); 77% were refractory to a prior anti-CD20 Ab and 43% were double-refractory to a prior anti-CD20 Ab and an alkylating agent. Compared with younger pts, those aged ≥65 years had a numerically higher objective response rate (87% [95% CI: 69–96] vs 77% [95% CI: 64–87]) and CR rate (70% [95% CI: 51–85] vs 55% [95% CI: 42–68]). The 18-month event-free rate for duration of response was 54% (95% CI: 31–76) in pts aged ≥65 years and 59% (95% CI: 43–74) in those aged <65 years. The rate of Gr 3–4 adverse events (AEs) was comparable in the older and younger age groups (73% vs 68%, respectively). A lower rate of serious AEs of any Gr was observed in pts aged ≥65 years (37% vs 52%). One patient in each age group discontinued treatment due to mosunetuzumab-related AEs. CRS occurred less frequently in pts aged ≥65 years than in those aged <65 years (30% vs 52%), while CRS events were predominantly low-Gr in both groups (age ≥65 vs <65 years: Gr 1, 20% vs 28%; Gr 2, 7% vs 22%); high-Gr CRS was uncommon (one patient in each group). All CRS events resolved. No cases of aphasia, seizures, encephalopathy or cerebral edema occurred. Rates of serious AEs of infection (any Gr) were comparable in pts aged ≥65 and <65 years (17% vs 22%, respectively). The PK disposition of mosunetuzumab was also comparable in pts aged ≥65 and <65 years. Summary/Conclusion: Mosunetuzumab is efficacious and well tolerated in older and younger pts with R/R FL and ≥2 prior therapies. In older pts (aged ≥65 years), numerically lower rates of CRS and serious AEs were observed.