235Background: TMPRSS2-ERG expression at peripheral blood has been correlated with lower docetaxel benefit. This multicenter study prospectively assessed the role of TMPRSS2-ERG mRNA as a taxane-resistance biomarker in blood and retrospectively in tumors, and explored the impact of prior abiraterone/enzalutamide (A/E) in castration-resistant prostate cancer (CRPC) patients and in vitro. Methods: TMPRSS2-ERG was tested by quantitative reverse-transcription PCR. We included 204 patients (137 blood and 124 tumor samples) treated with taxanes. TMPRSS2-ERG expression was correlated with PSA-progression-free survival (PFS), radiological-PFS (RX-PFS), and overall survival (OS). Independent association with survival was evaluated by multivariate Cox modeling. ERG knockdown, combinatorial and sequential experiments with enzalutamide and docetaxel were performed in VCaP cells. Results: In the multivariate analysis prior A/E (HR: 1.833; P = 0.005) and blood TMPRSS2-ERG (HR: 2, 95%CI; 1.1-3.67; P = 0.018) , were independently associated to lower PSA-PFS. In patients without prior A/E, both blood and tumor TMPRSS2-ERG independently predicted lower PSA-PFS (HR 2.92; P = 0.014 and HR 1.82; P = 0.045, respectively) to taxanes. This was not observed in patients with prior A/E. There was a significant interaction between blood TMPRSS2-ERG detection and prior A/E related to PSA-PFS (P = 0.032) and RX-PFS (P = 0.009). In vitro stable ERG inhibition did not sensitize VCaP cells to docetaxel. Concomitant enzalutamide and taxanes were synergistic, but prior enzalutamide reduced docetaxel cytotoxicity in VCaP cells. Enzalutamide induced neuroendocrine markers and reduced E-cadherin expression. Conclusions: Prior hormone-therapy may influence taxanes response and TMPRSS2-ERG prognostic value. Thus, multiple and sequential biomarkers are needed in CRPC follow-up evaluation.