To assess the predictive value of molecular breast cancer subtypes in premenopausal hormone receptor-positive early breast cancer patients who received adjuvant endocrine treatment or chemotherapy.

Molecular breast cancer subtypes were centrally assessed on whole tumor sections by immunohistochemistry (IHC) in patients of the Austrian Breast and Colorectal Cancer Study Group (ABCSG) Trial 5 who had received either 5 years of tamoxifen/3 years of goserelin or six cycles of cyclophosphamide, methotrexate, and fluorouracil (CMF). Luminal A disease was defined as Ki67 <20%, luminal B as Ki67 ≥20%. The luminal B/HER2-positive subtype displayed 3+ HER2-IHC or amplification by ISH. Recurrence-free survival (RFS) and overall survival (OS) were analyzed using Cox models adjusted for clinical and pathological factors.

185 (38%), 244 (50%) and 59 (12%) of 488 tumors were classified as luminal A, luminal B/HER2-negative and luminal B/HER2-positive, respectively. Luminal B subtypes were associated with poor outcome. Patients with Luminal B tumors had a significantly shorter RFS (adjusted hazard ratio [HR] for recurrence: 2.22, 95% confidence interval [CI] 1.41-3.49, P = 0.001) and OS (adjusted HR for death: 3.51, 95% CI 1.80-6.87, P < 0.001). No interaction between molecular subtypes and treatment was observed (test for interaction: P = 0.84 for RFS; P = 0.69 for OS).

Determination of molecular subtypes by IHC is an independent prognostic factor for recurrence and death in premenopausal women with early-stage, hormone receptor-positive breast cancer but is not predictive for outcome of adjuvant treatment with tamoxifen/goserelin or CMF.