AbstractAimPyrotinib (an irreversible pan‐ErbB small‐molecular tyrosine kinase inhibitor) was approved in human epidermal growth factor receptor 2 (HER2)‐positive breast cancer and showed great antitumor activity in preclinical studies of gastric cancer (GC). This study was first designed to prospectively assess pyrotinib in pretreated HER2‐positive GC.MethodsThis multicenter, phase I study followed a standard “3 + 3” design and included two parts. In the pyrotinib part, pyrotinib was administered orally, once per day at dose levels of 240, 320, 400, and 480 mg. In the pyrotinib plus docetaxel part, patients received pyrotinib (qd, d1‐21, q3W) combined with docetaxel (60 mg/m2, d1, q3W) at dose levels of 240, 320, and 400 mg. Primary endpoints were to determine the maximum tolerated dose (MTD) and recommended phase II dose (RP2D) of pyrotinib as monotherapy or coadministered with docetaxel.ResultsA total of 25 patients were enrolled and received pyrotinib (n= 15) or pyrotinib plus docetaxel (n= 10). One DLT was observed in pyrotinib monotherapy part (Grade 3 uncontrolled diarrhea after supportive care) and pyrotinib plus docetaxel part (Grade 4 neutropenia and leukopenia). In the pyrotinib monotherapy part, MTD was not reached. Diarrhea, anemia, neutropenia, and leukopenia were the most common treatment‐related adverse events (TRAEs). The RP2D for pyrotinib monotherapy was recommended as 400 mg. After combining with docetaxel, the risk of leukopenia and neutropenia was increased. Grade ≥3 TRAEs were reported for four patients in the monotherapy part and for eight patients in the combination part. Meant1/2was approximately 20 h. Pyrotinib exposure was dose‐dependent with a nonlinear relationship versus dose. There were five patients who had confirmed partial response (monotherapy: one each at 240, 400, and 480 mg dose cohort; combination therapy: two at 240 mg dose cohort), resulting in an objective response rate of 21% and 20%, respectively.ConclusionsPyrotinib alone and combined with docetaxel showed acceptable toxicities in patients with pretreated HER2‐positive GC.Trial RegistrationThis study was registered withClinicalTrials.gov, NCT02378389.