Pirtobrutinib in Covalent BTK Inhibitor Pre-Treated Mantle Cell Lymphoma: Updated Results and Subgroup Analysis from the Phase 1/2 BRUIN Study With >3 Years Follow-Up from Start of Enrollment

Topic: 18. Indolent and mantle-cell non-Hodgkin lymphoma - Clinical Background: Pirtobrutinib is a highly selective, non-covalent (reversible) BTK-inhibitor (BTKi). Aims: Here, we report updated results of pirtobrutinib in patients (pts) with cBTKi pre-treated relapsed/refractory (R/R) mantle cell lymphoma (MCL) and more than 3 years follow-up from start of enrollment. Methods: Pts with cBTKi pre-treated R/R MCL received pirtobrutinib monotherapy in a multicenter phase 1/2 BRUIN trial (NCT03740529). Efficacy was assessed in the prespecified primary efficacy cohort that comprised the first 90 enrolled pts who had measurable disease, had received a prior cBTKi, and had no known central nervous system involvement. The primary endpoint was overall response rate (ORR) as assessed by independent review committee. Secondary endpoints included duration of response (DOR) and safety. A data cut of 29 July 2022 was utilized. Results: Among MCL pts who received a prior cBTKi (n=90), median age was 70 years (range, 46-87), median prior lines of therapy were 3 (range, 1-8), 82% discontinued a prior cBTKi due to disease progression, and 78% had intermediate/high risk sMIPI score. Of samples available, 17/36 (47%) had TP53 mutations and 25/34 (74%) had Ki67 ≥30%. The ORR was 57% (95% CI, 46-67), including 19% complete responses (n=17) and 38% partial responses (n=34). At a median follow-up time of 13 months, the median DOR among the 51 responding pts was 17.6 months (95% CI, 7.3-27.2). The 12- and 18-month estimated DOR rates were 58% (95% CI, 41-72) and 45% (95% CI, 27-61), respectively. ORR and DOR by subgroups are shown in the Table. The median progression-free survival was 7.4 months (95% CI, 5.3–13.3). The median overall survival was 23.5 months (95% CI, 15.9-NE). In the MCL safety cohort (n=166), the most frequent treatment-emergent adverse events (TEAE) were fatigue (31%), diarrhea (22%), and anemia (17%). The most common Grade ≥3 TEAE was neutropenia (15%). Grade ≥3 TEAE of hemorrhage (3%) and atrial fibrillation/flutter (2%) were infrequent. Only 5 (3%) pts discontinued due to a treatment-related AE. Summary/Conclusion: Pirtobrutinib continues to show durable efficacy and a favorable safety profile in heavily pre-treated R/R MCL pts with prior cBTKi therapy. Responses were observed in pts with high-risk disease features including pts with blastoid/pleomorphic variants, elevated Ki67 index, and TP53 mutations.© 2023 American Society of Clinical Oncology, Inc. Reused with permission. This abstract was accepted and previously presented at the 2023 ASCO Annual Meeting. All rights reserved. Keywords: Bruton’s tyrosine kinase inhibitor (BTKi), Mantle cell lymphoma