A Case of Reversible Acute Kidney Injury and Life-Threatening Gastrointestinal Bleed with Colitis in a Patient Receiving Tucatinib for HER2-Positive Breast Cancer

This case report describes the complex clinical course of a 73-year-old female with HER2-positive breast cancer enrolled in the CompassHER2 RD trial who developed acute kidney injury (AKI) requiring emergent hemodialysis shortly after initiation of tucatinib. In addition to severe renal dysfunction, the patient experienced a life-threatening lower gastrointestinal (GI) bleed in the setting of suspected drug-induced colitis and profound thrombocytopenia while receiving combination therapy with trastuzumab emtansine (T-DM1). Although tucatinib has demonstrated promising efficacy in improving outcomes for patients with high-risk HER2-positive disease, serious renal and gastrointestinal complications remain uncommon and underreported. This case underscores the importance of early recognition of severe adverse effects, careful laboratory surveillance, and multidisciplinary management in patients undergoing targeted HER2-directed therapy.