3510 Background: Pyrotinib is a potent, irreversible tyrosine kinase inhibitor (TKI) that blocks signal transduction through the erythroblastic leukemia viral oncogene homolog (erbB) receptors, which has previously demonstrated promising antitumor activity in patients (pts) with breast cancers. We studied the safety and efficacy of pyrotinib in pts with non-small cell lung cancer (NSCLC) and other solid tumors with activating human epidermal growth factor receptor 2 ( HER2, ERBB2) alterations. Methods: This is an open-label, multicenter phase 1 dose expansion basket trial of pyrotinib given 400mg oral daily at 28-day cycles. Expansion cohorts included pts with HER2-mutant NSCLC and advanced solid tumors with HER2 mutation or amplification. HER2 testing was conducted using next generation sequencing or fluorescence in situ hybridization. Primary endpoints included toxicities as evaluated by NCI CTCAE v5.0, and overall response rate (ORR) as evaluated by RECIST v1.1. Secondary objectives included progression-free survival (PFS). Results: A total of 62 pts were enrolled. The median age was 67 (range 40 – 86 years), 61% were female and the median lines of prior systemic therapy was 3 (range 1-11). There were no treatment related deaths. The most common adverse events were diarrhea (96.8%), nausea (82.3%) and vomiting (41.9%). The only ≥ grade 3 treatment related toxicity was diarrhea (24.2%). Prophylactic anti-diarrhea treatment was introduced to facilitate continuation of pyrotinib. At the Jan 13, 2020 cut-off, 24 pts with HER2-mutant NSCLC (20, i.e. 65% of which were the A775_G776insYVMA mutation) and 18 pts with HER2-mutant or amplified solid tumors completed end of Cycle 2 imaging scan and were evaluable for tumor responses. The ORR was 19% (8/42, 95% CI 7-31%); confirmed responses include a complete response (CR) and 3 partial responses (PRs) in HER2-mutant NSCLC, and 4 PRs in HER2-amplified cholangiocarcinoma, ovarian, endometrial and salivary gland carcinomas. There were 7 stable disease ≥ 6 months. Median progression-free survival was 5.4 months (95% CI 4.4-7.3). Conclusions: Pyrotinib demonstrated a manageable safety profile and encouraging efficacy in pts with heavily pre-treated HER2-mutant NSCLC. Furthermore, it is the first TKI to produce durable responses in pts with HER2-amplified biliary tract, ovarian, endometrial and salivary gland cancers. These results warrant further clinical development of pyrotinib. Clinical trial information: NCT02500199 .