Background: A significant unmet need exists for drugs targeting allosteric ErbB mutations (non-canonical mutations outside the ATP binding site). Current EGFR and HER2 tyrosine kinase inhibitors or mAbs have limited antitumor activity against allosteric mutations, resulting in toxicity before adequate drug exposure (Connell and Doherty, 2017). BDTX-189 is a potent and selective orally available irreversible inhibitor targeting unique oncogenic driver mutations of ErbB kinases in EGFR and HER2, while sparing WT EGFR. Preclinical studies demonstrated antitumor activity across a range of allosteric ErbB mutants, including extracellular domain allosteric mutations of HER2 as well as EGFR and HER2 kinase domain exon 20 insertions (Buck, 2019). This first-in-human trial (NCT04209465) is aimed to determine the recommended phase 2 dose (RP2) and schedule (Phase 1, P1), and evaluate the efficacy (Phase 2, P2) of BDTX-189. P1 primary objective is to determine the RP2 dose and schedule of monotherapy BDTX-189. Secondary objectives include assessment of safety, tolerability, pharmacokinetics (PK), pharmacodynamic (PD) effects in tumor, and preliminary efficacy. The P2 primary objective is to assess antitumor activity of monotherapy BDTX-189. Methods: The study will enroll patients (pts) ≥18 yrs with histologically or cytologically confirmed locally advanced or metastatic solid tumors with no standard therapy available or for whom standard therapy is unsuitable or intolerable. P1 dose-escalation will use a BOIN design (Yuan, 2016) and will enroll ≤ 88 pts with allosteric HER2 or HER3 mutation; EGFR or HER2 exon 20 insertion mutation; HER2 amplified or overexpressing tumor; or EGFR exon 19 deletion or L858R mutation. BDTX-189 will be dosed orally (PO) initially QD in 3 wk cycles. Regimen optimization will use PK, PD and safety data and may explore a BID schedule. An expansion cohort of ≤12 pts will further evaluate safety and preliminary efficacy of BDTX-189 prior to P2. P2, utilizing a Simon 2-stage design, will enroll ≤100 pts with NSCLC with EGFR or HER2 exon 20 insertion mutations (cohort 1); breast cancer with an allosteric ErbB mutation (cohort 2); tumors (except breast) with S310F/Y mutation (cohort 3); and other allosteric ErbB mutations not defined in cohorts 1-3 (cohort 4). Assessments include safety, tolerability, DLTs, evaluation of MTD, PK, PD, and preliminary antitumor activity. Enrollment began 1/2020. Clinical trial information: NCT04209465.