Prediction of Benefit from Docetaxel Chemotherapy for HR-positive Breast Cancer in the PACS-01 Trial

BACKGROUND:Adjuvant chemotherapy for hormone receptor-positive (HR+) breast cancer relies on taxanes, but existing tests do not identify which patients benefit from them. The SETER/PR index, a measure of endocrine-related transcriptional activity in HR + tumors, recently predicted benefit from paclitaxel when low (<0.75), but not for anthracycline-based therapy. We tested whether SETER/PR could predict benefit from docetaxel in node-positive, HR + patients enrolled in the PACS-01 trial (docetaxel after fluorouracil-epirubicin-cyclophosphamide (3FEC+3D) versus 6FEC). PATIENTS AND METHODS:SETER/PR index and Recurrence Score (RS) were obtained for 490 patients. SETER/PR was quantified using the QuantiGene Plex assay from RNA remaining after RS testing. Pre-specified analyses assessed SETER/PR as a continuous variable and using the predefined <0.75 cut point, as well as continuous RS and the >25 cut point. The primary endpoint was distant recurrence-free interval (DRFI). Predictive value was assessed using Cox models including biomarker, treatment arm, and interaction term. RESULTS:Continuous SETER/PR demonstrated significant interaction with treatment on DRFI (Pinteraction = 0.028), whereas predefined <0.75 cut point was not predictive (Pinteraction = 0.668). Exploratory analyses identified a cut point at 1.50 (Pinteraction = 0.013): patients with SETER/PR ≥ 1.50 had worse outcomes with 3FEC+3D versus 6FEC (HR 3.16, 95%CI 1.28-7.85), while outcomes were similar between arms when SETER/PR < 1.50 (HR 0.83, 95%CI 0.51-1.35). RS did not predict differential benefit, either continuously (Pinteraction = 0.670) or at the >25 threshold (Pinteraction = 0.534). CONCLUSION:Including sequential docetaxel (3FEC+3D) was less effective than continued anthracycline chemotherapy (6FEC) when breast cancer had high endocrine-related transcriptional activity (i.e., SETER/PR index ≥1.50). TRIAL REGISTRATION:PACS-01.