A Rare Case of Pulmonary Arterial Hypertension in a Patient With Breast Cancer Treated With Trastuzumab Emtansine

Breast cancer is the most common malignancy in women worldwide. A large proportion of breast cancer cases demonstrate human epidermal growth factor receptor 2 (HER2) overexpression. Trastuzumab emtansine (T-DM1) is an established therapy for HER2-positive metastatic breast cancer, though rare but serious adverse effects may occur. We report a 58-year-old woman with HER2-positive metastatic breast cancer who developed progressive exertional dyspnoea while receiving T-DM1. Echocardiography demonstrated right ventricular dilation and dysfunction with elevated pulmonary pressures. Right heart catheterisation confirmed pulmonary arterial hypertension (PAH) with increased pulmonary vascular resistance. Comprehensive evaluation, including computed tomography, pulmonary angiography, and ventilation-perfusion scanning, excluded thromboembolic disease. There was no significant parenchymal lung disease, and left ventricular function was preserved. In the absence of alternative causes and given the temporal relationship with T-DM1 exposure, drug-induced PAH was considered likely. The patient was treated with sildenafil and ambrisentan, resulting in symptomatic and haemodynamic improvement. Concurrent hepatotoxicity also resolved following discontinuation of T-DM1, further supporting a causal association. PAH is a rare complication of HER2-targeted therapies, with limited cases reported. This case highlights the importance of recognising PAH as a potential adverse effect of T-DM1. Clinicians should maintain a high index of suspicion in patients presenting with unexplained dyspnoea or right ventricular dysfunction to enable early diagnosis and appropriate management.