Cellular variant focal segmental glomerulosclerosis during trastuzumab emtansine treatment in HER2-positive breast cancer: a case report.

Onconephrology is an emerging subspecialty that encompasses comprehensive kidney management in patients with cancer. Trastuzumab emtansine (T-DM1) is an antibody-drug conjugate composed of trastuzumab linked to the microtubule inhibitor. T-DM1 is widely used in the treatment of human epidermal growth factor receptor 2-positive breast cancer, and glomerular disease arising during T-DM1 therapy remains rarely reported. The present report describes the case of a 50-year-old Japanese woman who developed nephrotic-range proteinuria approximately five months after initiating T-DM1 adjuvant therapy. Kidney biopsy confirmed the diagnosis of cellular variant focal segmental glomerulosclerosis. Other secondary causes, including autoimmune disease and infection, were excluded, and the temporal relationship between symptom onset and T-DM1 administration suggested a possible drug-related etiology. Given the absence of alternative treatment regimens for her breast cancer, we decided to continue T-DM1 without immunosuppressive therapy, with careful monitoring of kidney function and proteinuria. The scheduled anticancer treatment course was successfully completed, and proteinuria improved significantly within 7 weeks of treatment completion. This case highlights the importance of recognizing rare glomerular adverse effects of T-DM1 and underscores the need to carefully balance oncological treatment benefits against renal toxicity, emphasizing the value of multidisciplinary collaboration between oncologists and nephrologists.