Detection of AR-V7 using cell-free RNA (cfRNA) in prostate cancer.

e16613 Background: In prostate cancer (PC), the androgen-receptor isoform that is encoded by the splice variant 7 (AR-V7) is characterized by an absence of the ligand-binding domain that is the target of abiraterone and enzalutamide. Consequently, presence of AR-V7 in prostate cancer has been found to be associated with resistance to these two drug agents. In this study, we analyzed cell-free RNA (cfRNA) released into the bloodstream in order to quantitate AR gene expression and AR-V7 in PC patients. Methods: Blood samples were collected from 13 prostate cancer patients, 6 of whom were treated with androgen ablation agents. Plasma was fractionated from blood samples and nucleic acids were extracted. RNA was reverse-transcribed into cDNA using random primers, and then analyzed by quantitative RT-PCR using appropriate gene-specific primers. AR and AR-V7 were assayed in cDNA samples. Relative AR expression was calculated using beta-actin as a denominator reference gene, and AR expression status was tested for associations with disease status and patients' current therapies. Results: Of the 13 samples tested, a total of 7 had detectable levels of AR expression (53.8%). Of the 7 samples testing positive for AR expression, one also tested positive for AR-V7 (14.3%). Among those with detected levels of AR expression, 5 (71.4%) were metastatic castrate resistant, while the other 2 were hormone naïve. Among the 6 samples with no detectable levels of AR expression, 3 (50%) were metastatic castrate resistant, while the other 3 were hormone naïve. Among those with AR expression detected, 2 (28.6%) were on enzalutamide, 3 (42.9%) were on abiraterone, and the remaining 2 were on no therapy and Lupron injections, respectively. Neither disease status nor current therapies were associated with AR expression (p = 0.1839, Chi-Square). Conclusions: We found a surprisingly high frequency of lack of AR expression among these samples (approaching 50% overall), while AR-V7 frequency was consistent with previous findings. While loss of AR expression might be expected to arise as a means of resistance drugs such as enzalutamide and abiraterone, cancers from hormone-therapy naive patients also lacked AR expression. The mechanism and timing of the loss of AR remains to be investigated.