Spatial Mapping of Tamoxifen and its Metabolites in Breast Cancer Tissue via MALDI-MSI: Novel Insights into Endocrine Therapy Response

A fundamental question in tumor pharmacology is how systemically administered drugs reach and distribute within tumor tissues. In this study, we applied matrix-assisted laser desorption/ionization–based mass spectrometry imaging to surgical specimens from 16 patients with hormone receptor–positive, human epidermal growth factor receptor 2-negative breast cancer who received neoadjuvant tamoxifen therapy. Spatial distribution and heterogeneity of tamoxifen, together with its major metabolites N-desmethyl-tamoxifen, 4-hydroxy-tamoxifen, and endoxifen were directly visualized and correlated with histopathological features and therapeutic response. Tamoxifen was consistently detected in cancer regions regardless of histological subtype; however, its intratumoral intensity and spatial uniformity varied, reflecting stromal composition and tissue architecture. Cases with more homogeneous and higher intensity of tamoxifen distribution tended to show decreased Ki-67 expression and histopathological response. Among the metabolites, significant intratumoral accumulation of 4-hydroxy-tamoxifen, and endoxifen —but not N-desmethyl-tamoxifen—was associated with histopathological response. This study provides the first direct demonstration that intratumoral distribution of tamoxifen and its active metabolites in human breast cancer is spatially heterogeneous and linked to therapeutic efficacy. Mass spectrometry imaging-based visualization of drug pharmacokinetics at the molecular and spatial level offers a transformative approach for understanding and predicting treatment response, introducing drug distribution as a new dimension in personalized oncology with broad implications for tumor pharmacology.