AbstractThe mechanism by which trastuzumab‐emtansine (T‐DM1) causes systemic toxicities apart from trastuzumab alone is currently unknown. We hypothesized that the systemic toxicities from T‐DM1 may have been caused by the free and active maytansine released from the lysed HER2+ tumor cells, and if so, they may correlate with the response to treatment and eventually disease‐free survival or patient outcome. In a retrospective, observational study, we evaluated 73 patients from three centers in the United States and Canada with advanced HER2+ breast cancer that received at least one dose of T‐DM1. Toxicity grades were summed to create a corresponding toxicity sum score (TSS), and its association with clinical outcomes was analyzed. A higher TSS was significantly associated with longer progression‐free survival with an HR = 0.66 [95% confidence interval [CI]: 0.47‐0.92], P = .014, for each 1‐point increase in the TSS score. Adjusted for baseline platelet count, aspartate transaminase and alanine transaminase, higher TSS remains significantly associated with longer progression‐free survival with adjusted HR = 0.67 [95% CI: 0.47‐0.93], P = .020. The analysis suggests that the systemic toxicities of T‐DM1 were significantly correlated with its clinical efficacy. This is the first report to correlate the systemic toxicities of T‐DM1 with clinical outcome. Further, this suggests that systemic toxicities of antibody‐drug conjugates (ADCs) may serve as a predictive biomarker, particularly if noncleavable linkers are used. If confirmed in larger prospective studies, the present finding is significant because most ADCs do not have a biomarker predictive of clinical outcome other than the presence or absence of the antibody target.