Clinical and molecular landscape of metastatic extramammary Paget's disease.
BACKGROUND: Extramammary Paget's disease (EMPD) is a rare malignancy without established systemic therapy. EMPD shares molecular features with breast cancer, such as human epidermal growth factor receptor 2 (HER2) and hormone receptor (HR) expression, but their clinical relevance remains unclear. MATERIALS AND METHODS: Tumors from 20 metastatic invasive EMPD cases were analyzed for molecular and biological features. Genomic features, transcriptomic profiles, and HER2 and HR expression status were investigated using immunohistochemistry, fluorescence in situ hybridization, and targeted-genome next-generation sequencing and nCounter BC360 panels. Metastatic breast cancer samples were used as a comparison to clarify metastatic EMPD's clinical relevance. RESULTS: Estrogen receptor expression was observed in 45% of EMPD tumors, while only 10% expressed progesterone receptor. HER2 was overexpressed in 30% of cases, and HER2-directed therapies were durably effective. Among 8 patients with NGS data, 63% (5/8) harbored oncogenic ERBB2 alterations independent of HER2 expression. BC360 profiling revealed biological differences between EMPD and breast cancer, particularly poor biological compatibility for HR-positive tumors. Immune profiling showed that a subset of EMPD tumors exhibited CD8+ T-cell signatures and PD-1/PD-L1 gene expression comparable to triple-negative breast cancer. The median overall survival was 22.1 months (95% CI, 12.0-42.2), with 16 patients (80%) treated with systemic therapy, including anti-HER2 therapy, hormonal therapy, or cytotoxic therapies based on their molecular features. CONCLUSIONS: This study highlights the unique molecular and biological features of metastatic EMPD, emphasizing the need for tailored treatment approaches. This information should be used to guide future clinical strategies for metastatic EMPD.