Interplay between HER2-Low status, hormone receptor expression, and therapeutic response to cyclin-dependent kinase 4/6 inhibitors as first-line treatment in luminal-like metastatic breast cancer: The CYCLHER study

BACKGROUND: In hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2-) metastatic breast cancer (mBC), the prognostic significance of estrogen receptor (ER), progesterone receptor (PgR), and HER2 immunohistochemical expression levels in patients treated with cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) remains unclear. The CYCLHER study was designed to evaluate treatment outcomes and identify biologically driven prognostic factors. METHODS: CYCLHER (NCT06243432) is a retrospective, multicenter study conducted across 16 Italian oncology centers. Patients with HR+/HER2- mBC who received first-line endocrine therapy (ET) plus CDK4/6i between November 2016 and May 2023 were included. ER, PgR and HER2 status were assessed on metastatic or primary tumor samples. The primary endpoint was real-world progression-free survival (rwPFS). Secondary endpoints included overall survival (OS), objective response rate (ORR), disease control rate (DCR), duration of response (DoR), and attrition rate. Optimal cut-off values for ER and PgR expression were determined using Cut-off Finder v1.0. RESULTS: Among 597 eligible patients, median rwPFS was 28.1 months at a median follow-up of 41 months. HER2-low status emerged as a negative prognostic factor, being independently associated with shorter rwPFS compared to HER2-0 (p = 0.02). ER ≥ 87% and PgR ≥60% were associated with improved rwPFS, DCR, and 5-year OS rates. A prognostic score derived from the three biomarkers identified four groups with significantly different rwPFS outcomes (global log-rank p < 0.0001). The first-to-second line attrition rate was 16.2% (95% CI 12.4-20.0). The identified biomarkers did not significantly impact treatment outcomes in the second-line setting. CONCLUSIONS: CYCLHER confirms the prognostic relevance of ER, PgR, and HER2 expression in patients with HR+/HER2-mBC treated with CDK4/6i. HER2-low status was associated with poorer outcomes, supporting its potential role as a negative prognostic marker. The proposed score may facilitate personalized treatment strategies and enhance risk stratification in clinical practice.