Trastuzumab Deruxtecan in Metastatic Urothelial Carcinoma with NGS-Detected ERBB2 Amplification: A Four-Patient Real-World Case Series

Background: Next-generation sequencing (NGS)-detected ERBB2 amplification occurs in a subset of urothelial carcinomas, but its role as a treatment-selection marker for trastuzumab deruxtecan (T-DXd) remains uncertain. Methods: We retrospectively reviewed four patients with metastatic urothelial carcinoma treated with T-DXd in routine practice from 2024. Treatment selection was based on NGS-detected ERBB2 amplification because HER2 immunohistochemistry and in situ hybridization were unavailable. Results: Four men aged 65–76 years received T-DXd: one in the second line and three in the fourth or fifth line. The best radiological responses, abstracted from contemporaneous radiology reports and oncology medical records, were complete response in one patient, partial response in one, and stable disease in two. Three patients had previously received enfortumab vedotin. Documented adverse events included fatigue, anemia, diarrhea, rash, and leukopenia. No interstitial lung disease or pneumonitis was documented in the available records. Conclusions: These observations are descriptive and hypothesis-generating. They do not establish the efficacy or safety of T-DXd or validate ERBB2 amplification as a predictive biomarker, but they support prospective evaluation of genomic ERBB2 amplification when standard HER2 testing is unavailable.