CLL-039: Pirtobrutinib (LOXO-305), a Next-Generation, Highly Selective, Non-Covalent BTK Inhibitor in Previously Treated CLL/SLL: Results from the Phase 1/2 BRUIN Study

Context: Despite the marked efficacy of covalent BTK inhibitors (BTKi) in CLL/SLL, the development of resistance and discontinuation for adverse events can lead to treatment failure. Low oral bioavailability or short half-life of these agents can lead to suboptimal BTK target coverage and ultimately result in acquired resistance in some patients (pts). Pirtobrutinib (LOXO-305) is a highly selective, non-covalent BTKi that inhibits both WT and C481-mutated BTK with equal, low nM potency. Objective: To evaluate pirtobrutinib safety and efficacy in pts with CLL/SLL. Design: BRUIN is an ongoing multi-center phase 1/2 trial (NCT03740529). Enrollment was initiated 21 March 2019. Setting: Global: community hospitals, and academic medical centers. Patients: As of 27 September 2020, 323 previously treated pts with B-cell malignancies (170 CLL/SLL, 61 MCL, 26 WM, and 66 other B-cell lymphomas) were enrolled. Interventions: Oral pirtobrutinib (7 dose escalation levels: 25–300mg once daily) in 28-day cycles. Main Outcomes Measures: Determining the maximum tolerated dose/recommended phase 2 dose (RP2D), safety profile, and efficacy based on response assessment using disease-specific criteria (iwCLL2018 for CLL/SLL) per protocol. Results: Pirtobrutinib demonstrated high oral exposures, with doses ≥100mg QD exceeding BTK IC90 for the entirety of the dosing interval. No DLTs occurred. The only treatment-emergent adverse events, regardless of attribution or grade seen in ≥10% of pts (n=323), were fatigue (20%), diarrhea (17%), and contusion (13%). An RP2D of 200 mg QD was selected for future studies. The ORR (per iwCLL 2018) was 63%, with 69 PRs, 19 PR-Ls, 45 SDs, 1 PD, and 5 discontinued prior to first response assessment. Responses deepened over time among pts with ≥10 months of follow-up (n=29; 86% ORR). ORR was not influenced by the reason for prior BTKi discontinuation (i.e., progression vs intolerance) or other classes of prior therapy received (including a covalent BTK and a BCL2 inhibitor). The longest followed responding pt continues on treatment for 17.8+ months. Conclusions: Pirtobrutinib showed promising efficacy in pre-treated CLL/SLL pts, was well-tolerated, and exhibited a wide therapeutic index.