Baseline gene expression and dynamic endocrine therapy use in ER+ breast cancer: associations with recurrence in a real-world cohort.

Endocrine therapy (ET) selection in estrogen receptor-positive breast cancer (ER + BC) is guided by menopausal status and tolerability rather than tumor biology, despite substantial heterogeneity in recurrence. We hypothesized that baseline gene expression differentially associates with recurrence depending on initial ET class. In a retrospective cohort of 74 ER+/HER2- patients treated with adjuvant ET, we profiled baseline tumor RNA and fitted adjusted Cox models for gene and ET interactions on time-to-recurrence and time-to-switch. Among selective estrogen receptor modulators-initiated patients, higher expression of TP53, WNT7B, UBE2T, BAG1, and ACTR3B was associated with markedly increased recurrence, while no comparable association was observed among aromatase inhibitors-initiated patients. Forty-one percent of patients switched ET at least once, predominantly due to intolerance (joint pain, unspecified side effects); switching was not consistently associated with tumor biology. These hypothesis-generating findings identify candidate gene and ET interactions and motivate validation in larger, independent cohorts.