PURPOSEPyrotinib, an irreversible pan-ErbB inhibitor, showed promising antitumor activity and acceptable tolerability in a phase I trial. We assessed the efficacy and tolerability of pyrotinib versus lapatinib, both in combination with capecitabine, in women with human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer in an open-label, multicenter, randomized phase II study.PATIENTS AND METHODSChinese patients with HER2-positive relapsed or metastatic breast cancer previously treated with taxanes, anthracyclines, and/or trastuzumab were assigned (1:1) to receive 400 mg pyrotinib or lapatinib 1,250 mg orally once per day for 21-day cycles in combination with capecitabine (1,000 mg/m(2) orally twice per day on days 1 to 14). The primary end point was investigator-assessed overall response rate per Response Evaluation Criteria in Solid Tumors (RECIST), version 1.1.RESULTSBetween May 29, 2015, and March 15, 2016, 128 eligible patients were randomly assigned to the pyrotinib (n = 65) or lapatinib (n = 63) treatment groups. The overall response rate was 78.5% (95% CI, 68.5% to 88.5%) with pyrotinib and 57.1% (95% CI, 44.9% to 69.4%) with lapatinib (treatment difference, 21.3%; 95% CI, 4.0% to 38.7%; P = .01). The median progression-free survival was 18.1 months (95% CI, 13.9 months to not reached) with pyrotinib and 7.0 months (95% CI, 5.6 to 9.8 months) with lapatinib (adjusted hazard ratio, 0.36; 95% CI, 0.23 to 0.58; P < .001). The most frequent grade 3 to 4 adverse events were hand-foot syndrome in 16 of 65 patients (24.6%) in the pyrotinib group versus 13 of 63 (20.6%) in the lapatinib group; diarrhea in 10 patients (15.4%) versus three patients (4.8%), respectively; and decreased neutrophil count in six patients (9.2%) versus two patients (3.2%), respectively.CONCLUSIONIn women with HER2-positive metastatic breast cancer previously treated with taxanes, anthracyclines, and/or trastuzumab, pyrotinib plus capecitabine yielded statistically significant better overall response rate and progression-free survival than lapatinib plus capecitabine in this randomized phase II trial.