Fixed Duration Mosunetuzumab Plus Polatuzumab Vedotin Has Promising Efficacy and a Manageable Safety Profile in Patients with BTKi Relapsed/Refractory Mantle Cell Lymphoma: Initial Results from a Phase Ib/II Study

Background: New treatment options are needed for patients (pts) with relapsed/refractory (R/R) mantle cell lymphoma (MCL), especially for pts with progressive disease (PD) after Bruton's tyrosine kinase inhibitor (BTKi) therapy. Outcomes for R/R MCL pts with PD after the BTKi ibrutinib are very poor, demonstrated by a median overall survival of 2.9 months (Martin et al. Blood 2016). While chimeric antigen receptor (CAR) T-cell therapies are approved for pts with R/R MCL; treatment is associated with high toxicity and difficulty with accessibility. Mosunetuzumab (Mosun) is a CD20xCD3 T-cell engaging bispecific antibody that engages and redirects T cells to eliminate malignant B cells. Polatuzumab vedotin (Pola) is a CD79b-targeted antibody-drug conjugate that inhibits cell division and induces apoptosis in B cells. This ongoing Phase Ib/II study (NCT03671018) is investigating Mosun in combination with Pola (M-Pola) in pts with R/R B cell non-Hodgkin lymphoma. Here, we report initial safety and efficacy data from an ongoing Phase II expansion cohort of pts with R/R MCL who had received prior BTKi therapy. Methods: Eligiblepts had histologically confirmed R/R MCL and had received ≥2 prior regimens (including an anti-CD20 agent, BTKi, and anthracycline- or bendamustine-based therapy). Mosun was administered subcutaneously (SC) by step-up dosing on Days (D) 1 (5mg), 8 (45mg), and 15 (45mg) of Cycle (C) 1, then 45mg on D1 of every cycle from C2D1, every 3 weeks for a total of 17 cycles. Pola (1.8mg/kg intravenous [IV] infusion) was administered on D1 of C1-6. All pts received corticosteroid premedication prior to C1 of treatment. From C2, premedication was optional for pts who did not experience cytokine release syndrome (CRS) in the previous cycle. Response was assessed by investigators using Lugano 2014 criteria (Cheson et al. J Clin Oncol 2014). Results: As of October 27, 2022, 20 pts had received M-Pola (n=10 active on treatment, n=1 completed treatment, n=9 discontinued Mosun treatment). Median age of all pts was 68.0 (range: 44-82) years, 75% were male, 95% had Ann Arbor stage III/IV disease, and 40% had a MIPI score ≥6 at study entry. Median number of prior lines of therapy (LOT) was 3 (range: 2-9). All pts had received prior BTKi therapy; 7 pts (35%) had received prior CAR T-cell therapy; and 85% (17/20) of pts were refractory to their last LOT. Median time since last LOT was 1.6 (range: 0-39) months. The proportions of pts with high-risk MCL factors at baseline were: 65% of pts with Ki-67 proliferation index ≥50%; 50% with blastoid/pleomorphic variants; and 20% with TP53 mutation. The overall median time on study was 7.2 (range: 0.4-17.5) months. The most common (≥25%) Mosun and/or Pola-related adverse events (AEs) in all pts (n=20) were CRS (50%), injection site reaction (50%), fatigue (45%), dyspnea (35%), paresthesia (30%), diarrhea (30%), myalgia (30%), infusion-related reaction (25%), and nausea (25%). Two (10%) Grade (Gr) 5 AEs occurred, both of which were due to COVID-19 pneumonia; neither were deemed treatment-related by the investigator. Ten pts (50%) experienced CRS events; 9 pts experienced Gr 1 events, and 1 pt experienced a Gr 2 event, which was managed with tocilizumab and low-flow oxygen. One pt was managed with corticosteroids and no pts required pressors or ICU admission. All CRS events resolved by the data cut-off. Treatment-related neurologic AEs, potentially consistent with ICANs, occurred in 3 pts (15%; Gr 1 confusional state and Gr 2 amnesia, 1 pt; Gr 1 agitation, 1 pt; and Gr 2 memory impairment, 1 pt). Neuropathy occurred in 3 pts (15%); all events were Gr 1. Four pts (20%) had AEs leading to treatment discontinuation: 1 pt with Gr 3 uveitis (related to M-Pola); 1 pt with Gr 3 pneumonitis (related to M-Pola); and 2 pts with Gr 5 COVID-19 pneumonia (both unrelated to M-Pola). Overall response rate (ORR) and complete response (CR) rates were 75% and 70%, respectively (Figure). Best ORR and CR rates were generally consistent in high-risk MCL subgroups (Figure). Median duration of CR was not evaluable (NE; 95% confidence interval: 3.8-NE). Conclusions: Fixed-duration Mosun SC in combination with Pola IV induced high CR rates in pts with R/R MCL who had previously received BTKi therapy, including those in high-risk subgroups and those who had received prior CAR T-cell therapy. CRs were observed early and persisted during the follow up period. M-Pola had a manageable safety profile, and all CRS events were low grade.

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