AR-V7 Transcripts in Whole Blood RNA of Patients with Metastatic Castration Resistant Prostate Cancer Correlate with Response to Abiraterone Acetate

Purpose: The expression of AR-V7 (androgen receptor splice variant) 7 in circulating tumor cells has been associated with resistance to abiraterone and enzalutamide in patients with metastatic castration resistant prostate cancer. We used a sensitive, whole blood reverse transcriptase-polymerase chain reaction assay that does not require circulating tumor cell enrichment to correlate outcomes of abiraterone with whole blood expression of AR-V7 and other prostate cancer associated transcripts. Materials and Methods: We assessed the expression of AR-V7, FOXA1, GRHL2, HOXB13, KLK2, KLK3 and TMPRSS2: ERG mRNA in 2.5 ml whole blood from each of 27 patients with metastatic castration resistant prostate cancer and 33 controls without cancer as the discovery cohort. Cycle threshold values of controls with the highest gene expression were set as the threshold for a positive test. Thresholds were then applied to a validation cohort of 37 patients with metastatic castration resistant prostate cancer who were commencing abiraterone. Gene expression was correlated with the prostate specific antigen response rate using the chi-square test, and with time to prostate specific antigen progression and overall survival using the log rank test. Results: In the discovery cohort 3 of 27 patients (11.1%) with metastatic castration resistant prostate cancer were AR-V7 positive vs 4 of 37 (10.8%) in the validation cohort. In the validation cohort patients with a positive AR-V7 test had a lower prostate specific antigen response rate (0% vs 42%, p = 0.27) together with shorter median prostate specific antigen progression (0.7 vs 4.0 months, p < 0.001) and median overall survival (5.5 vs 22.1 months, p < 0.001). Conclusions: Reverse transcriptase-polymerase chain reaction detection of AR-V7 transcripts in whole blood was associated with inferior outcomes in patients treated with abiraterone. These results reinforce the potential usefulness of AR-V7 as a prognostic and predictive biomarker for metastatic castration resistant prostate cancer.