1043 Background: Tucatinib (TUC) is an investigational TKI, highly selective for HER2 without significant inhibition of EGFR. HER2CLIMB is a randomized trial of TUC vs placebo in combination with trastuzumab and capecitabine in patients (pts) with HER2+ breast cancer (NCT02614794, Murthy NEJM 2019). The most common G ≥3 adverse events (AEs) with higher incidence on the TUC arm (diarrhea, palmar-plantar erythrodysesthesia syndrome [PPE], and elevated liver enzymes) are described herein. Methods: Given that pts on the TUC arm had a longer duration of tx than those on the control arm, time-at-risk exposure-adjusted incidence rates of diarrhea, AST, ALT, and PPE were calculated as the number of pts with an event divided by the total exposure time-at-risk of an initial occurrence of the event among pts in the tx group. Time-to-event analyses were conducted for AST/ALT/bilirubin (in aggregate), diarrhea, and PPE. Results: Diarrhea and elevated AST/ALT/bilirubin on both the TUC and control arms were primarily G1/2 and manageable with dose modifications, and in some cases of diarrhea, with antidiarrheal tx. Median time to diarrhea onset was shorter on the TUC arm compared to control. For AST/ALT/bilirubin and PPE, median time to first onset was Cycles 1 and 2. On the TUC arm, antidiarrheals were used in 49.7% of cycles in which diarrhea was reported (39.8% on the control arm), and when used, the median duration of use on each arm was 3 days per cycle. Prophylactic antidiarrheals were not required per protocol. When adjusted for exposure (time-at-risk exposure-adjusted incidence rate per 100 person-years), the difference in G ≥3 events between tx arms becomes similar for diarrhea and PPE (21 vs 17 and 21 vs 19). The difference in G ≥3 events between arms is reduced for AST and ALT (7 vs 1 and 8 vs 1). Conclusions: TUC with trastuzumab and capecitabine was well-tolerated. Rates of G ≥3 diarrhea and PPE were similar between tx arms. Elevated liver enzymes were higher on the TUC arm, but were transient and reversible. Discontinuation of TUC due to AEs was rare. Clinical trial information: NCT02614794 . [Table: see text]