Prospective Multicenter Validation of Androgen Receptor Splice Variant 7 and Hormone Therapy Resistance in High-Risk Castration-Resistant Prostate Cancer: The PROPHECY Study.

PURPOSE: Androgen receptor splice variant 7 (AR-V7) results in a truncated receptor, which leads to ligand-independent constitutive activation that is not inhibited by anti-androgen therapies, including abiraterone or enzalutamide. Given that previous reports suggested that circulating tumor cell (CTC) AR-V7 detection is a poor prognostic indicator for the clinical efficacy of secondary hormone therapies, we conducted a prospective multicenter validation study. PATIENTS AND METHODS: PROPHECY ( ClinicalTrials.gov identifier: NCT02269982) is a multicenter, prospective-blinded study of men with high-risk mCRPC starting abiraterone acetate or enzalutamide treatment. The primary objective was to validate the prognostic significance of baseline CTC AR-V7 on the basis of radiographic or clinical progression free-survival (PFS) by using the Johns Hopkins University modified-AdnaTest CTC AR-V7 mRNA assay and the Epic Sciences CTC nuclear-specific AR-V7 protein assay. Overall survival (OS) and prostate-specific antigen responses were secondary end points. RESULTS: We enrolled 118 men with mCRPC who were starting abiraterone or enzalutamide treatment. AR-V7 detection by both the Johns Hopkins and Epic AR-V7 assays was independently associated with shorter PFS (hazard ratio, 1.9 [95% CI, 1.1 to 3.3; P = .032] and 2.4 [95% CI, 1.1 to 5.1; P = .020], respectively) and OS (hazard ratio, 4.2 [95% CI, 2.1 to 8.5] and 3.5 [95% CI, 1.6 to 8.1], respectively) after adjusting for CTC number and clinical prognostic factors. Men with AR-V7-positive mCRPC had fewer confirmed prostate-specific antigen responses (0% to 11%) or soft tissue responses (0% to 6%). The observed percentage agreement between the two AR-V7 assays was 82%. CONCLUSION: Detection of AR-V7 in CTCs by two blood-based assays is independently associated with shorter PFS and OS with abiraterone or enzalutamide, and such men with mCRPC should be offered alternative treatments.

Requirement for C-terminal sequences in regulation of Ect2 guanine nucleotide exchange specificity and transformation.

Ect2 was identified originally as a transforming protein and a member of the Dbl family of Rho guanine nucleotide exchange factors (GEFs). Like all Dbl family proteins, Ect2 contains a tandem Dbl homology (DH) and pleckstrin homology (PH) domain structure. Previous studies demonstrated that N-terminal deletion of sequences upstream of the DH domain created a constitutively activated, transforming variant of Ect2 (designated DeltaN-Ect2 DH/PH/C), indicating that the N terminus served as a negative regulator of DH domain function in vivo. The role of sequences C-terminal to the DH domain has not been established. Therefore, we assessed the consequences of mutation of C-terminal sequences on Ect2-transforming activity. Surprisingly, in contrast to observations with other Dbl family proteins, we found that mutation of the invariant tryptophan residue in the PH domain did not impair DeltaN-Ect2 DH/PH/C transforming activity. Furthermore, although the sequences C-terminal to the PH domain lack any known functional domains or motifs, deletion of these sequences (DeltaN-Ect2 DH/PH) resulted in a dramatic reduction in transforming activity. Whereas DeltaN-Ect2 caused formation of lamellipodia, DeltaN-Ect2 DH/PH enhanced actin stress fiber formation, suggesting that C-terminal sequences influenced Ect2 Rho GTPase specificity. Consistent with this possibility, we determined that DeltaN-Ect2 DH/PH activated RhoA, but not Rac1 or Cdc42, whereas DeltaN-Ect2 DH/PH/C activated all three Rho GTPases in vivo. Taken together, these observations suggest that regions of Ect2 C-terminal to the DH domain alter the profile of Rho GTPases activated in vivo and consequently may contribute to the enhanced transforming activity of DeltaN-Ect2 DH/PH/C.