Glofitamab monotherapy in patients with heavily pretreated relapsed/refractory (R/R) mantle cell lymphoma (MCL): Updated analysis from a phase I/II study.

7008 Background: Glofitamab, a bispecific antibody with a novel 2:1 (CD20:CD3) format, engages and redirects T cells to eliminate malignant B cells. In a Phase I/II study (NCT03075696), fixed-duration glofitamab monotherapy showed high, durable complete responses (CR) and manageable safety in patients (pts) with heavily pretreated R/R MCL (Phillips et al. ASH 2022). We report updated efficacy and safety data for pts with R/R MCL. Methods: Pts received obinutuzumab pretreatment (Gpt; 1000mg or 2000mg) 7 days before their first glofitamab dose. Glofitamab step-up dosing was given on Cycle (C) 1 Day (D) 8 (2.5mg) and D15 (10mg), and target dose (16mg or 30mg) from C2D1 to C12 as a fixed-duration therapy. Efficacy endpoints were CR, overall response rate (ORR), duration of CR (DOCR), duration of response (DOR) and progression-free survival (PFS). Post-hoc PFS and overall survival (OS) landmark analyses were performed in pts with a CR at end of treatment (EOT). Results: As of Sept 4, 2023, 61 pts were enrolled; 60 received study treatment (1000mg Gpt, n=16; 2000mg Gpt, n=44). Median number of prior lines of therapy was 2 (range: 1–5); 73.3% were refractory to the last line of prior therapy. Median duration of glofitamab therapy was 7.4 months; median number of cycles was 12. Of the 31 pts (51.7%) who received prior Bruton’s tyrosine kinase inhibitor (BTKi) therapy, 29 (93.5%) were BTKi refractory; median number of prior lines of therapy was 3 (range: 1–5); 90.0% were refractory to the last line of prior therapy. At a median PFS follow-up of 17.2 months, ORR and CR rate were 85.0% and 78.3%, respectively. Median DOCR was 15.4 months; the majority of CRs (28/47; 59.6%) were ongoing at data cut-off. Of 19 pts with a CR, 9 had disease progression and 10 died (fatal adverse event [AE], n=8; COVID, n=7; septic shock, n=1). Estimated 12-month DOCR and DOR rates were 71.0% and 66.6%, respectively. Median PFS was 16.8 months. Landmark analyses indicated that most pts with a CR at EOT were alive and progression free 15 months post-EOT (OS rate, 72.7%; PFS rate, 59.2%). Observed efficacy data were consistent across both Gpt cohorts. In post-BTKi pts after a median PFS follow-up of 26.1 months, ORR and CR rate were 74.2% and 71.0%, respectively. Median DOCR was 12.6 months; median PFS was 8.6 months. No new safety signals were observed. Cytokine release syndrome (CRS) remained the most common AE (42/60, 70.0%; Grade 1/2, 58.3%) and was lower in the 2000mg (63.6%) vs the 1000mg (87.5%) Gpt cohort. Glofitamab monotherapy (2000mg Gpt) is currently under investigation in the Phase III GLOBRYTE study (NCT06084936). Conclusions: Fixed-duration glofitamab continues to demonstrate compelling response rates that are maintained beyond EOT, with long-term durability observed in heavily pretreated pts with R/R MCL, including pts with prior BTKi therapy. The safety profile was manageable and CRS mainly low grade. Clinical trial information: NCT03075696 .