T-DM1 permeates cultured mouse megakaryocytes and mouse/human platelets. Trastuzumab incorporation was examined in megakaryocytes from at least 3 different cultures and washed platelets from at least 3 different mouse/human donors after incubation with 100 μg/mL of T-DM1, 5B6-DM1 (trastuzumab specificity control), and trastuzumab Ab control. (A) Representative differential interference contrast image. T-DM1 and 5B6-DM1 treatment resulted in inhibition of proplatelet production 24 hours after treatment. Immunofluorescence microscopy reveals that both T-DM1 and trastuzumab (red) permeate cultured mouse megakaryocytes (B) and mouse/human platelets (C-D) and localize cytoplasmically. T-DM1 disrupts microtubule organization (green).