Research objectives and rationale Trastuzumab greatly enhances the efficacy of treatment in HER2+ metastatic breast cancer (MBC). Due to its potential to induce cardiomyopathy, however, trastuzumab is contraindicated in patients with baseline left ventricular ejection fraction (LVEF) 5% below baseline. Exploratory, we compared the incidence of severe cardiotoxicity in patients with and without cardioprotective medication at start trastuzumab. Results Of the 758 patients identified with HER2+ MBC, 41 patients were included with a LVEF Clinical characteristics of patients with and without cardioprotective medication from start trastuzAll patients (n=41)Patients with cardioprotective medication from start trastuzumab (n=12)Patients without cardioprotective medication fromStart trastuzumab (n=29)P-valueSevere cardiotoxicitya, n (%)16 (39)2 (17)14 (48)0.059Time to cardiotoxicity, months [IQR]7 [3 - 12]6 [not reached - 8]8 [4 - 11]0.515Reversibilityb, n (%)No4 (29)1 (8)3 (10)0.260Partial4 (29)0 (0)4 (14)Yes6 (43)0 (0)6 (21)Trastuzumab treatment , n (%)Continued27 (66)7 (58)20 (69)0.796Interrupted6 (15)2 (17)4 (14)Discontinued8 (20)3 (25)5 (17)LVEF, median % (IQR)Baseline46 [42 - 48]47 [44 - 49]46 [43 - 48]0.177Nadir42 [33 - 45]43 [40 - 47]40 [32 - 45]0.322Highest53 [50 - 57]52 [50 - 57]53 [49 - 58]0.761Difference nadir and highest13 [9 - 20]11 [7 - 19]13 [10 - 21]0.464Cardiac symptoms, n (%)16 (39)4 (33)12 (41)0.515 Citation Format: Nathalie I Bouwer, Tessa G Steenbruggen, Hanah N Rier, Jos JEM Kitzen, Carolien H Smorenburg, Marlies L Van Bekkum, Albert J Ten Tije, Paul C De Jong, Jan C Drooger, Cynthia Holterhues, Marcel JM Kofflard, Eric Boersma, Gabe S Sonke, Mark-David Levin, Agnes Jager. Cardiac function in patients receiving trastuzumab for HER2+ metastatic breast cancer with left ventricular ejection fraction