Expression of AR-V7 and ARv567es 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(v567es), in mCRPC patient circulating tumor cells (CTC) may be associated with taxane treatment outcomes. Experimental Design: 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) 50 response and progression-free survival (PFS). Results: Of the 54 evaluable patients, 36 (67%) were AR-V7(+), 42 (78%) were AR(v567es+), 29 (54%) were double positive, and 5 (9%) were double negative. PSA50 response rates at any time were numerically higher for AR-V7(+) versus AR-V7(+) (78% vs. 58%; P = 0.23) and for AR(v567es+) versus AR(v567es+) (92% vs. 57%; P = 0.04) patients. When AR-V mRNA status was correlated with change in nuclear AR from cycle 1 day 1 to day 8 (n = 24), AR-V7(+) patients (n = 16) had a 0.4% decrease versus a 12.9% and 26.7% decrease in AR-V7(+)/AR(v567es+) (n = 3) and AR-V7(+)/AR(v567es+) (n = 5) patients, respectively, suggesting a dominant role for AR-V7 over AR(v567es). Median PFS was 12.02 versus 8.48 months for AR-V7(+) versus AR-V7(+) (HR = 0.38; P = 0.01), and 12.71 versus 7.29 months for AR(v567es+) versus AR(v567es+) (HR = 0.37; P = 0.02). For AR-V7(+), AR-V7(+)/AR(v567es+), and AR-V7(+)/AR(v567es+) patients, median PFS was 8.48, 11.17, and 16.62 months, respectively (P = 0.0013 for trend). Conclusions: Although detection of both CTC-specific AR-V7 and AR(v567es) 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