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 þ , 42 (78%) were AR v567esþ , 29 (54%) were double positive, and 5 (9%) were double negative. PSA 50 response rates at any time were numerically higher for AR-V7 versus AR-V7 þ (78% vs. 58%; P ¼ 0.23) and for AR v567es versus AR v567esþ (92% vs. 57%; P ¼ 0.04) patients. When AR-V mRNA status was correlated with change in nuclear AR from cycle 1 day 1 to day 8 (n ¼ 24), AR-V7 þ patients (n ¼ 16) had a 0.4% decrease versus a 12.9% and 26.7% decrease in AR-V7/AR v567es (n ¼ 3) and AR-V7/AR v567esþ (n ¼ 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 þ (HR ¼ 0.38; P ¼ 0.01), and 12.71 versus 7.29 months for AR v567es versus AR v567esþ (HR ¼ 0.37; P ¼ 0.02). For AR-V7 þ , AR-V7/AR v567esþ , and AR-V7/ AR v567es patients, median PFS was 8.48, 11.17, and 16.62 months, respectively (P ¼ 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.
Bibliographical noteFunding Information:
This study was sponsored by Sanofi; editorial support was provided by Amber Wood and Olga Ucar of MediTech Media, funded by Sanofi. G. Galletti received support from the National Cancer Institute (NCI) under award number NIH T32 CA062948 and from the National Center for Advancing Translational Sciences of the NIH under award number UL1TR002384. S. Kim received support from the NCI-funded NIH T32 postdoctoral training grant (T32 CA203702) on Molecular and Translational Oncology Research. P. Giannakakou received support from the NCI under award number R21 CA216800.
© 2018 American Association for Cancer Research.