Prognostic association of plasma cell-free DNA-based androgen receptor amplification and circulating tumor cells in pre-chemotherapy metastatic castration-resistant prostate cancer patients

Background The prognostic significance of plasma cell-free DNA (cfDNA) androgen receptor amplification (ARamp) in metastatic castration-resistant prostate cancer (mCRPC) compared with circulating tumor cell (CTC) counts is not known. Methods As part of correlative aims of a prospective study in mCRPC, concurrent and serial collections of plasma and CTCs were performed. Specimen collections were performed at baseline after progression on androgen deprivation therapy and then 12 weeks later. QuantStudio3D digital PCR system was used to determine plasma cfDNA AR copy number variations and Cell search assay for enumerating CTC counts. Association of baseline cfDNA ARamp status/CTC counts with overall survival (OS) (primary goal) was evaluated using Kaplan-Meier method and log-rank test (p <= 0.05 for significance) and receiver operator curves (ROCs) for ARamp status and CTC counts >= 5. A multivariate analysis was performed using Cox regression models that included ARamp, CTC counts, and other clinical factors. Results ARamp was detected in 19/70 patients at baseline. At the time of analysis, 28/70 patients had died (median followup 806 days; interquartile range: 535-966). ARamp was associated with poor OS (2-year OS of 35% in ARamp vs. 71% in non-ARamp; log-rank p value 0.0001). Baseline CTC counts >= 5 (vs. <5) was also associated with poor survival (2-year OS of 44 vs. 74%; log-rank p = 0.001). ROC analysis demonstrated area under the curve of 0.66 for ARamp-based prognosis and 0.68 for CTC count-based prognosis (p = 0.84 for difference). The best two variables included for multivariable analysis were ARamp and CTC counts >= 5; however, the two-factor model was not significantly better than using ARamp alone for predicting survival (hazard ratio = 5.25; p = 0.0002). Conclusions CTCs and plasma cfDNA ARamp were observed to have equal prognostic value in mCRPC. Larger cohorts that incorporate molecular and clinical factors are needed to further refine prognosis in CRPC.