PB1864: BRUIN CLL-321: A PHASE 3 OPEN-LABEL, RANDOMIZED STUDY OF PIRTOBRUTINIB VS INVESTIGATOR’S CHOICE OF IDELALISIB+RITUXIMAB OR BENDAMUSTINE+RITUXIMAB IN BTKI PRETREATED CLL/SLL (TRIAL IN PROGRESS)

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Background: Covalent Bruton Tyrosine Kinase (BTK) inhibitors (BTKi) have transformed the management of patients with chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), but these treatments are not curative and many patients will require additional treatment. Covalent BTKi share pharmacologic liabilities (e.g. low oral bioavailability, short half-life) that collectively may lead to suboptimal BTK target coverage especially in rapidly proliferating tumors with high BTK protein turnover such as accelerating CLL/SLL, ultimately manifesting as acquired resistance in some patients. Aims: To address these limitations, pirtobrutinib, a highly selective, non-covalent BTKi that inhibits both wild type (WT) and C481-mutated BTK with equal low nM potency was developed. In the phase 1/2 BRUIN study, pirtobrutinib achieved pharmacokinetic exposures that exceeded its BTK IC96 at trough, was well tolerated and demonstrated promising efficacy in CLL/SLL patients regardless of prior therapy, number of prior lines of therapy, or BTK C481 mutation status. The purpose of this randomized phase 3 study is to demonstrate the superiority of continued BTK pathway inhibition with pirtobrutinib compared to other available therapies in patients with BTKi-treated CLL/SLL. Methods: BRUIN CLL-321 is a randomized, open-label, global phase 3 study comparing pirtobrutinib monotherapy versus investigator’s choice of idelalisib plus rituximab or bendamustine plus rituximab in patients with CLL/SLL who have been treated with a prior covalent BTKi. Prior therapy with venetoclax is permitted. Approximately 250 patients will be randomized 1:1. Randomization will be stratified by 17p deletion (yes/no) and prior venetoclax (yes/no). Patients receiving investigator’s choice are eligible to crossover to pirtobrutinib monotherapy if they experience progressive disease per iwCLL 2018 (determined by IRC). Eligible patients are adults aged ≥18 years with a diagnosis of CLL/SLL who require therapy per iwCLL 2018 criteria and who have received prior covalent BTKi. Unlimited number of lines of prior therapy are allowed. Key exclusion criteria include CNS involvement by CLL/SLL, Richter transformation any time pre-enrollment, a major bleeding event on prior covalent BTKi and history of allogeneic stem cell transplant (SCT) or autologous SCT or chimeric antigen receptor (CAR) T-cell therapy within 60 days of randomization. The primary endpoint is progression-free survival (PFS) per iwCLL assessed by an independent review committee. Secondary endpoints include overall survival (OS), overall response rate (ORR), duration of response (DoR), safety and tolerability, and patient reported outcomes. The global study is currently enrolling patients (NCT04666038). Results: This study is a Trial in Progress. The results will be presented at a later date. Summary/Conclusion: This study is a Trial in Progress. The conclusions will be presented at a later date.

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