Metastatic extramammary Paget’s disease successfully treated by trastuzumab deruxtecan: a rare case report

Metastatic extramammary Paget's disease is a rare type of cancer with poor prognosis. Approximately 40 percent of cases overexpress the Her2/neu protein, making it an important target for precision-based treatment. The article describes a case of a 73-year-old man with a chronic red patch on the supra-pubic and scrotal area that slowly enlarged over time. Genetic testing revealed a high tumor mutation burden (30 mutations per megabase) and mutations in ERBB2, PIK3CA, and PTEN genes. The patient had low Her2/neu expression (++/+++). Initial treatment with trastuzumab and pembrolizumab was ineffective. After switching to trastuzumab deruxtecan (an anti-Her2/neu antibody-drug conjugate), a partial response was achieved with regression of enlarged lymph nodes and bladder lesions. The results suggest that extramammary Paget's disease with low Her2/neu expression and high tumor mutation burden may respond better to anti-Her2/neu antibody-drug conjugates than to combined immunotherapy.