Objectives: Endometrial cancer is the most common gynaecologic malignancy, and its mortality rate is rising. Advanced or recurrent disease remains challenging because historically there have been limited therapeutic options. We aim to describe a complete and durable response to the HER2-directed antibody–drug conjugate trastuzumab deruxtecan (T-DXd) in a heavily pretreated patient with HER2-positive, mismatch-repair-deficient metastatic serous endometrial cancer. Methods: A 72-year-old woman underwent hysterectomy, bilateral salpingo-oophorectomy, and staging procedures for FIGO stage IIIA, high-grade serous papillary endometrial carcinoma. Tumour profiling revealed dMMR, a p53 abnormal pattern, and HER2 overexpression (IHC 3+). She received carboplatin/paclitaxel plus avelumab, followed by pegylated liposomal doxorubicin and weekly paclitaxel. After progression on paclitaxel, off-label T-DXd was initiated. Molecular data (FoundationOne CDx) were collected, along with and serial imaging and CA125 assessments. Results: The patient developed cough after two cycles of T-DXd; interstitial lung disease was excluded, and treatment resumed with steroid cover. By December 2024, PET/CT demonstrated complete metabolic response, with resolution of vaginal-vault and para-aortic lesions and normalisation of CA125. Real-world progression-free survival exceeded eight months, with ongoing symptom improvement. Treatment was generally well tolerated; the principal adverse event was grade 3 neutropenia requiring dose reduction. No cardiotoxicity or interstitial lung disease occurred. Conclusions: This case illustrates that T-DXd can induce deep and durable remission in HER2-positive, dMMR metastatic serous endometrial cancer after multiple lines of therapy. It adds real-world evidence supporting further investigation of HER2-directed antibody–drug conjugates in gynaecologic malignancies, and underscores the need for confirmatory trials and refined biomarker-driven patient selection.

Trastuzumab deruxtecan (T-DXd) has demonstrated efficacy in HER2-positive and HER2-low breast cancer. Its main safety concern is interstitial lung disease, while clinically relevant hepatotoxicity is rarely reported. In our case report we describe a 49-year-old woman with HER2-positive advanced breast cancer that developed persistent grade 3 transaminase elevations after 2 cycles of T-DXd, refractory to corticosteroid treatment and requiring treatment discontinuation. This case underlines the unpredictable and idiosyncratic nature of T-DXd associated hepatotoxicity and the importance of liver function monitoring. Clinicians should consider DILI in patients with unexplained liver enzyme elevations during therapy. Further studies are needed to clarify mechanisms and risk factors.