Evaluation of circulating tumor cells (CTCs) enumeration and 18F-Choline positron emission tomography/computed tomography (FCH PET/CT) as early efficacy response biomarkers in metastatic castration-resistant prostate cancer (CRPC) patients (pts) treated with abiraterone acetate

63 Background: Circulating tumor cells (CTCs) and FCH PET/CT are both promising tools for treatment monitoring in CRPC pts. In this study, we assessed prostate specific antigen (PSA), CTC and FCH PET/CT for monitoring treatment response in metastatic CRPC pts receiving recently approved abiraterone acetate therapy after chemotherapy. Methods: Twenty nine metastatic CRPC pts progressing after docetaxel chemotherapy received abiraterone acetate 1,000 mg daily with prednisone 5 mg twice daily in continuous 28-day cycles. Pts were evaluated monthly for efficacy and safety. CTCs counts (using the CellSearch assay) and FCH PET/CT were performed at baseline and at 4 weeks (PERCIST criteria and SUV Peak). Results: After one month of treatment, 12 pts (41%) had a PSA rise and 17 pts (59%) had a PSA decline. At baseline, 18 pts had detectable CTCs (at least 5 CTCs), and 6 of them (33%) had a decline from at least 5 to less than 5 CTCs after 4 weeks of treatment. 10 of 29 pts (34%) had a > or = 30% decline in CTCs. Amongst 22 pts who were evaluable by FCH PET/CT, response assessment was as follows: response (n=8), stable disease (n=12), progression (n=1). PSA decline and CTCs levels correlated in 16 of 29 pts (55%), and were discordant in 13 of 29 pts (45%). In all 12/29 pts (41%) with a 50% or more decline in PSA both a CTCs response and a FCH PET/CT response was observed. In patients with a PSA rise while on abiraterone acetate, 8 pts did not have a CTCs increase or a CTC conversion from unfavourable to favorable, and 7 of 8 pts had a PSA decline at 3 months or a FCH PET/CT response. No flare phenomenon on FCH PET/CT was observed. Conclusions: Monitoring pts with CRPC who receive abiraterone using both CTCs and metabolic imaging indicate discrepancy in 45% with response assessment using PSA. Association between CTC drop, early FCH PET/CT response and clinical benefit is currently under study.