T-DM1 does not increase surface expression of P-selectin in platelets. P-selectin surface expression was examined in resting and thrombin (1 mU/μL)–activated washed platelets from at least 3 different mouse/human donors before and after incubation with a vehicle control or 100 μg/mL of T-DM1 or 5B6-DM1 (trastuzumab specificity control) for 4 hours. (A) Mouse/human platelets exhibit a change in morphology during incubation with T-DM1 and 5B6-DM1 as is evidenced by their forward- and side-scatter distribution relative to the vehicle control. (B) Mouse/human platelets treated with the vehicle control or T-DM1 show normal surface expression of P-selectin under resting and thrombin-activated conditions. Platelets treated with 5B6-DM1 show increased surface expression of P-selectin under resting conditions that is not increased further on treatment with thrombin.