YB-1 expression and effectiveness of different dose-intensification strategies in high-risk breast cancer: Five-year follow-up results of prospective randomized WSG-AM-01 trial

563 Background: Y-box binding protein (YB-1), known as oncogenic transcription factor, is associated with up-regulation of MDR1, alters p53 function, and induces growth of an aggressive phenotype. In high-risk breast cancer, the prospective randomized WSG-AM-01 trial has reported significantly better event-free (EFS) and overall survival (OS) for tandem high-dose (HD) vs. dose-dense (DD) chemotherapy, especially in basal-like und HER2 subgroups. The present study examines the interaction of a drug resistant phenotype induced by YB-1 within the WSG-AM-01 collective at 5-year follow-up. Methods: 236 tumors (116 HD/120 DD) of 403 randomized patients (60%) were available for construction of tissue microarrays and determination of molecular classification by k-clustering of expression of 34 protein markers. Immunostaining of YB-1 by specific peptide antibody was scored semiquatitatively by intensity. Associations of YB-1 staining with other protein expression factors were studied by Pearson correlations. Univariate survival was estimated by Kaplan-Meier analysis and tested by log rank statistics. Multivariate survival modeling was performed by a generalized Cox model, with linear proportional hazards terms in the first block and time-varying interactions in the second block. Results: At a median follow-up of 61.7 months, the WSG-AM-01 confirms a significant EFS and OS benefit for HD in HRBC. In 60% of tumors, there was strong YB-1 expression. YB-1 was significantly associated with several proliferation and drug resistance markers, such as p53, EGFR, S6 and with basal-like/Her2 subtypes.YB-1 expression was highly predictive for response to HD: both EFS (HR=0.29, p=0.001) and OS (HR=0.16, p=0.0001) were significantly improved by HD compared to DD for YB-1 positive tumors; these favorable time-varying hazard ratios signify decreases in early (≤3 years) relapses and deaths, respectively. Conclusions: Among all investigated markers, only YB-1 expression was significantly associated by time- varying interaction analysis with efficacy of HD. The results suggest YB-1 as a potential stratification criterion for future trials as well as a target for treatment of drug resistant HRBC. No significant financial relationships to disclose.