LAUSR

BACKGROUND Glutathione S-transferase 1 (GSTP1) expression is inactivated in >90% of all prostate cancers in association with aberrant DNA methylation. Detection of serum free methylated GSTP1 (mGSTP1) DNA is associated with overall survival (OS) and response to docetaxel in metastatic castration-resistant prostate cancer (mCRPC) in test and internal validation cohorts. OBJECTIVE To assess the relationship between serum free mGSTP1 and treatment outcomes in SYNERGY, a phase 3 multicentre randomised trial testing the addition of custirsen to first-line chemotherapy with docetaxel in mCRPC. DESIGN, SETTING, AND PARTICIPANTS Serum free mGSTP1 DNA was measured by a sensitive methylation-specific polymerase chain reaction assay in paired samples (baseline and after two cycles of docetaxel) from 600 patients. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS Associations between serum free mGSTP1 at baseline, change in mGSTP1 after docetaxel, OS, and time to prostate-specific antigen (PSA) progression were examined using Cox proportional hazards models and Kaplan-Meier methods. RESULTS AND LIMITATIONS Serum free mGSTP1 was detectable at baseline in 458 (81%) patients. Of those with detectable mGSTP1 at baseline, mGSTP1 became undetectable after two cycles in 243 (53%). Undetectable mGSTP1 at baseline was associated with longer OS (hazard ratio [HR] 0.4, 95% confidence interval [CI] 0.29-0.55; p<0.00001). The event of mGSTP1 becoming undetectable after two cycles of chemotherapy was associated with longer OS (HR 0.36, 95% CI 0.29-0.46; p<0.00001) and longer time to PSA progression (HR 0.44, 95% CI 0.35-0.56; p<0.00001). Associations between mGSTP1 and clinical outcomes were independent of other established prognostic variables. Analysis was limited by the lack of radiographic progression-free survival data. CONCLUSIONS This is the first study to externally validate the prognostic role of a circulating epigenetic biomarker in mCRPC. Further studies are needed to validate serum free mGSTP1 as a surrogate endpoint for clinical trials and as a potential clinical decision tool. PATIENT SUMMARY In this study, we confirmed that a blood marker predicted outcomes after chemotherapy for metastatic prostate cancer. This marker may accelerate future clinical trials of new therapies and be useful in the clinic to guide treatment decisions.