Population Pharmacokinetic Analysis of Pirtobrutinib, a reversible BTK inhibitor, in Patients with Hematological Malignancies from the Phase 1/2 BRUIN Study

Purpose This study aimed to characterize the pharmacokinetics (PK) of pirtobrutinib, a highly selective and noncovalent Bruton’s tyrosine kinase (BTK) inhibitor, in adult patients with hematological malignancies using data from the Phase 1/2 BRUIN study (NCT03740529). Methods Pirtobrutinib was administered as monotherapy at doses ranging from 25-300mg once daily (QD). PK samples were collected at multiple time points, including intensive sampling on Cycle (C) 1 Day (D) 1, C1D8, C2D1, and C4D1 for Phase 1 dose-escalation, and sparse sampling on C1D8 and C4D1 for Phase 1 dose-expansion and Phase 2. Population PK (popPK) analysis used nonlinear mixed-effects modeling. Results Overall, 4487 evaluable pirtobrutinib concentrations obtained from 595 patients were included in the popPK analysis. Pirtobrutinib PK was well characterized by a 2-compartment model with linear clearance and 4-transit compartments for absorption. The mean elimination half-life was estimated to be 18.8 hours. At the recommended Phase 2 dose (200mg QD), the steady-state mean maximum drug concentration was 6460ng/mL, the mean minimum was 2260ng/mL, and the mean area under the concentration-time curve was 91300ng*h/mL. Pirtobrutinib disposition was significantly affected by body weight, renal function, and serum albumin, but not by formulation, cancer type, age, sex, race, ethnicity, or mild hepatic impairment. Conclusion The proposed starting dose of 200mg QD was associated with meaningful benefits. Most patients (96%) were expected to exceed 90% BTK inhibition, across the entire dosing interval, indicating extensive and durable engagement of the drug target. No dose adjustments based on body weight, serum albumin or renal function were recommended. Clinical trial registration NCT03740529