Background: Allosteric oncogenic mutations occur outside the canonical ATP-binding site of EGFR and HER2, and there are no approved therapies that target such mutations. BDTX-189 is a potent, selective, irreversible inhibitor of 48 allosteric EGFR and HER2 mutant variants under clinical evaluation in the ongoing MasterKey-01 trial (NCT04209465). BDTX-189 was designed to rapidly and irreversibly occupy the active site of targeted ErbB mutants, leading to sustained pharmacodynamic (PD) effects, and with selectivity over EGFR-WT in order to minimize EGFR-WT mediated toxicities. The pharmacokinetic (PK) profile was designed for rapid absorption and fast elimination to maintain target occupancy while minimizing prolonged drug exposure that could contribute to off-target associated toxicities. Methods: In MasterKey-01, BDTX-189 was administered orally once daily in continuous 21-day cycles, taken fasted. Dose escalation included cohorts of 1-2 patients receiving doses between 25 and 200 mg QD followed by 5-7 patients receiving 400 mg, 800 mg, or 1,200 mg QD fasted. The possible effects of a high fat meal on the PK of BDTX-189 were assessed in a subset of patients receiving single doses of 400 mg BDTX-189 fasted and immediately after a high-fat breakfast in a randomized crossover fashion with 3 days between doses. In addition, a dose escalation cohort investigating administration of BDTX-189 non-fasted was enrolled at 800 mg QD. Serial blood samples for analysis of plasma BDTX-189 concentrations were collected after each dose on C1D1 and C1D15. BDTX-189 levels were determined using LC-MS, and data analyzed using non-compartmental methods. Results: After single and multiple doses, BDTX-189 was rapidly absorbed (median tmax 1-2 h), with an elimination t_1/2 of 2-6 h. Dose-dependent increases in exposure from 200 to 800 mg QD fasted were observed, with no apparent accumulation or decline in exposures observed at steady-state. Administration of BDTX-189 with a high-fat meal increased AUC approximately 1.7-fold with minimal effect on Cmax, relative to administration in the fasted state. At 800 mg QD, mean AUC was similar in the non-fasting state relative to fasting and was within the target efficacious range defined by mouse models harboring allo-ErbB mutated tumors. Median tmax and t_1/2 values were similar after administration in the non-fasted and fasted states. Conclusions: BDTX-189 demonstrated rapid absorption and a short PK half-life consistent with the desired PK/PD profile, with exposures in the efficacious target range based on preclinical data. The pilot high fat food-effect data and non-fasting QD dosing regimen show similar or improved systemic exposure relative to dosing in the fasted state. The MasterKey-01 trial is ongoing, including refinement of the dosing regimen and identification of the recommended phase 2 dose. Clinical trial information: NCT04209465