Expression of AR-V7 and ARv in Circulating Tumor Cells Correlates with Outcomes to Taxane Therapy in Men with Metastatic Prostate Cancer Treated in TAXYNERGY.

Purpose: Biomarkers aiding treatment optimization in metastatic castration-resistant prostate cancer (mCRPC) are scarce. The presence or absence of androgen receptor (AR) splice variants, AR-V7 and AR, in mCRPC patient circulating tumor cells (CTC) may be associated with taxane treatment outcomes. A novel digital droplet PCR (ddPCR) assay assessed AR-splice variant expression in CTCs from patients receiving docetaxel or cabazitaxel in TAXYNERGY (NCT01718353). Patient outcomes were examined according to AR-splice variant expression, including prostate-specific antigen (PSA) response and progression-free survival (PFS).

Results: Of the 54 evaluable patients, 36 (67%) were AR-V7, 42 (78%) were AR, 29 (54%) were double positive, and 5 (9%) were double negative. PSA response rates at any time were numerically higher for AR-V7 versus AR-V7 (78% vs. 58%; = 0.23) and for AR versus AR (92% vs. 57%; = 0.04) patients. When AR-V mRNA status was correlated with change in nuclear AR from cycle 1 day 1 to day 8 ( = 24), AR-V7 patients ( = 16) had a 0.4% decrease versus a 12.9% and 26.7% decrease in AR-V7/AR ( = 3) and AR-V7/AR ( = 5) patients, respectively, suggesting a dominant role for AR-V7 over AR. Median PFS was 12.02 versus 8.48 months for AR-V7 versus AR-V7 (HR = 0.38; = 0.01), and 12.71 versus 7.29 months for AR versus AR (HR = 0.37; = 0.02). For AR-V7, AR-V7/AR, and AR-V7/AR patients, median PFS was 8.48, 11.17, and 16.62 months, respectively ( = 0.0013 for trend).

Conclusions: Although detection of both CTC-specific AR-V7 and AR by ddPCR influenced taxane outcomes, AR-V7 primarily mediated the prognostic impact. The absence of both variants was associated with the best response and PFS with taxane treatment.See related commentary by Dehm et al., p. 1696.