Abstract Purpose: To evaluate drug–drug interactions between the human epidermal growth factor receptor 2 (HER2)–targeted antibody-drug conjugate trastuzumab deruxtecan (T-DXd; DS-8201a) and the OATP1B/CYP3A inhibitor ritonavir or the strong CYP3A inhibitor itraconazole. Patients and Methods: Patien...