Severe thrombocytopaenia caused by trastuzumab emtansine in a patient with breast cancer: A case report.

We present the case of a 48-year-old woman with human epidermal growth factor receptor 2- and hormone receptor-positive left early breast cancer who developed severe thrombocytopaenia and moderate liver dysfunction after administration of trastuzumab emtansine as an adjuvant therapy. Briefly, she experienced grade 2 subcutaneous bleeding, decreased platelet count (18,000/µL), and elevated aspartate aminotransferase/alanine aminotransferase levels (254/193 IU), resulting in admission to the emergency room. Although thrombocytopaenia is a well-known adverse event associated with trastuzumab emtansine, we observed it immediately after trastuzumab emtansine administration in our patient. Based on the literature survey, we hypothesised that trastuzumab emtansine may have affected mature platelets in our patient. In addition, moderate hepatotoxicity may be partially explained based on the pharmacological mechanisms of trastuzumab emtansine action involving microtubule disorganisation in hepatocytes via cytoskeleton-associated protein 5 on the cell surface by emtansine. We discuss the mechanism of the development of thrombocytopaenia and liver dysfunction.