Background: There is an unmet need for effective TKIs against HER2 mutations in solid tumors, especially NSCLC. BI 1810631 is a highly potent and selective HER2 TKI that covalently binds to both wild-type and mutated HER2 receptors, including exon 20 insertions (ex20ins), while sparing EGFR. Beamion Lung 1 is an ongoing Phase Ia/Ib study to determine the safety, MTD, PK, PD, and preliminary efficacy of BI 1810631 in pts with HER2 aberration-positive solid tumors (NCT04886804). Preliminary results from Phase Ia are presented. Methods: In Phase Ia, pts with advanced/unresectable/metastatic solid tumors who were refractory or unsuitable for standard treatment (Tx) and had HER2 aberrations (e.g. overexpression, gene amplification, somatic mutation, or gene rearrangements) were enrolled. Pts received escalating doses of BI 1810631 BID starting from 15 mg or BI 1810631 QD starting from 60 mg via a Bayesian model. Phase Ib will initially include 30 pts with advanced HER2 tyrosine kinase domain mutation-positive, pre-treated NSCLC. Additional cohorts may be included. Primary endpoints: MTD based on DLTs; pts with DLTs (Phase Ia); ORR (Phase Ib). Secondary endpoints: number of pts with DLTs throughout entire Tx period and PK parameters (Phase Ia/Ib); DoR, DCR, duration of disease control, and PFS (Phase Ib). Results: As of 23 January 2023, 36 pts had been treated in the US, The Netherlands, Japan, and China. Pts had NSCLC (n = 22), colorectal cancer (n = 3), or other tumors (n = 11). Most pts (n = 26; ex20ins: n = 23) had a pathological HER2 mutation. Median (range) lines of prior systemic Tx: 2.5 (1–8). Prior HER2-targeted Tx: 12 (33%). Pts received BI 1810631 at 15, 30, 60, 100, 150 mg BID (n = 3/3/4/4/3) or 60, 120, 180, 240, 300 mg QD (n = 5/4/5/4/1). Median number of cycles was 4 (range 1–14). Tx is ongoing in 20 pts. To date, 3 DLTs have been observed (grade 2 edema [60 mg BID], grade 3 anemia [60 mg QD], grade 3 elevated ALT [180 mg QD]). The MTD has not been reached with either schedule. Tx-related adverse events (TRAEs) were reported in 25 pts (69%). The most common TRAEs were diarrhea (n = 10), anemia (n = 5), increased alkaline phosphatase, increased creatinine, increased ALT, hypoalbuminemia (all n = 4), hypocalcemia, increased AST, dry skin, increased GGT (all n = 3). Three pts had grade 3 TRAEs (anemia/increased GGT [n = 1], increased ALT [n = 2]). Strong preliminary efficacy signals were observed across all dose levels. Of 22 evaluable NSCLC patients, 10 responded (all PRs; all ex20ins) and 11 had SD. Two patients with other tumors (esophagus; cholangiocarcinoma) also achieved PR. Conclusions: These preliminary data indicate that BI 1810631 is well tolerated and shows encouraging anti-tumor activity in pts with HER2 aberration-positive solid tumors. Recruitment into Phase Ia is ongoing. Updated data, including durability endpoints, will be presented. Clinical trial information: NCT04886804.