Background: Cabazitaxel demonstrated overall survival (OS) benefit for the treatment of CRPC progressing after docetaxel. CTC profiling could help to establish novel predictive biomarkers. In this prospective study (NCT03381326), we evaluated the prognostic role of CTC biomarkers expression and the association with plasma AR CN in pts treated with cabazitaxel. Methods: We enrolled pts receiving cabazitaxel from January 2015 to December 2018. Plasma DNA was isolated and digital PCR was performed to assess AR CN status. CTC enrichment was evaluated with AdnaTest EMT-2/StemCell kit. Expression analyses using real time PCR were performed for 17 genes and CTC positivity (CTC+) was defined as the expression of at least 1 of the following 7 relevant markers: AR-V7, AKT, AR, EPCAM, PSMA, PI3KCA, PSCA. Results: We enrolled 100 pts, 80 fully evaluable for this analysis. Median age was 72 years (range 49-82). All pts received prior docetaxel and 85% prior abiraterone and/or enzalutamide. Median OS and progression-free survival (PFS) were 16.4 months (mo) (95% CI 11.1-27.0) and 6.7 mo (95% CI 5.2-8.3), respectively. Fifty-eight (72.5%) showed CTC+ at baseline, whose 15 (26%) had >3 markers expressed in CTC. Significantly worse OS was observed in pts with >3 markers expressed in CTC compared to those with ≤3 markers and CTC negative pts [4.7 mo vs 15.2 vs 31.7 mo respectively, hazard ratio (HR) 6.05 (95% CI 2.07-17.73), p=0.004]. No significant difference was observed for PFS and PSA response. AR-V7 was expressed in 11 (19%) CTC+ pts, whose 10 (91%) had >3 markers expressed in CTC (p=0.0274), and 8 (73%) had plasma AR CN gain (p=0.048). A shorter OS was observed in AR-V7+ vs AR-V7- pts [10.6 vs 18.1 mo, HR 2.48 (95% CI 1.00-6.18), p=0.051]. Significantly worse OS and PFS were found in AR CN gain pts compared to AR normal [11.1 mo vs 27.0 mo, HR 2.28 (95% CI 1.19-4.38), p=0.013 and 5.9 mo vs 8.5 mo, HR 1.81 (95% CI 1.05-3.13), p=0.032], respectively. Conclusions: Liquid biopsy profiling may improve prognostication of CRPC patients treated with cabazitaxel. Further larger studies are warranted. Funding: Partially funded by Sanofi Genzyme. Clinical trial information: NCT03381326