Although landmark trials in the metastatic (CLEOPATRA) and neo-adjuvant (NeoSphere; TRYPHAENA) settings identified all-grade diarrhea as a pertuzumab-associated adverse event, it was not classified as dose-limiting. In actual practice, diarrhea is often a reason for treatment modifications.To compare the risk of pertuzumab-associated diarrhea in actual practice to the risks in randomized controlled trials.We conducted a retrospective cohort study of HER2/neu-positive breast cancer patients who received a pertuzumab-containing regimen between January 2012 and August 2015. We calculated the risk of diarrhea with 95% confidence limits (CLs), and then used two-sample t-tests to compare the risk between trials and actual practice.A total of 27 patients in the study cohort received a pertuzumab-containing treatment regimen for HER2/neu-positive breast cancer. The overall risk of all-grade and severe diarrhea in actual practice was 70% (95% CLs 55-90%) and 37% (95% CLs 20-66%), respectively. No severe diarrhea was observed in the metastatic setting, and the risk of all-grade diarrhea (44%, 95% CLs 21-92%) was similar to the CLEOPATRA study (67%). The risk of all-grade diarrhea in the neo-adjuvant setting was 83% (95% CLs 68-100%), compared to 46% in the NeoSphere trial (p = 0.03). The risk of severe diarrhea (Grade 3-4) in the neo-adjuvant setting was 47% (95% CLs 27-80%) versus 6% in the NeoSphere (p < 0.0001) and 12% in the TRYPHAENA (p < 0.01) trials.The risk of all-grade and severe diarrhea associated with neoadjuvant pertuzumab use for HER2/neu-positive breast cancer was greater in actual practice than in trials.
