Post hoc analysis of a phase III study to test the association between circulating methylated glutathione s transferase (mGSTP1) DNA levels and response to docetaxel (DTX) in metastatic castration resistant prostate cancer (mCRPC).

Abstract

5014 Background: GSTP1 inactivation is associated with CpG island hypermethylation in > 99% prostate cancers. Detection of circulating mGSTP1 DNA predicts response to DTX and overall survival (OS) in phase I/II mCRPC cohorts. This post hoc analysis of a phase III study aims to test the association between circulating mGSTP1 DNA levels and outcomes. Methods: The phase III SYNERGY study tested DTX +/- custirsen as 1st line chemotherapy in mCRPC (n = 1022) with no OS benefit in the experimental arm. Serum samples were taken at baseline (BL) and preC3 of DTX +/- custirsen from 600 patients (pts) enrolled on the SYNERGY study. mGSTP1levels in free DNA were measured using a sensitive methylation specific PCR assay and correlated with PSA response, time to PSA progression (TTP) and OS. Results: On interim analysis of 300 pts, serum mGSTP1 was detectable at BL in 80% and preC3 in 44%. Undetectable preC3 mGSTP1 correlated with a ≥30% fall in PSA within 3m of starting DTX (p < 0.001). Detectable BL and preC3 mGSTP1 predicted shorter TTP after DTX (BL; HR 1.6 95%CI 1.1-2.3; p = 0.01 and preC3 HR 2.2 95%CI 1.6-2.9; p < 0.001). Detectable mGSTP1 at both time points predicted shorter OS (BL; median OS 18.4 vs 33.1m, HR 2.4 95%CI 1.6-3.7; p < 0.001 and preC3; median OS 13.9 vs 29m, HR 2.7 95%CI 2.0-3.6; p < 0.001). In those with detectable BL mGSTP1, 50% had undetectable preC3 mGSTP1 predicting > 30% fall in PSA within 3m (p < 0.001), improved TTP (HR 0.40 95%CI 0.29-0.57; p < 0.001) and improved OS (25.2 vs 13.9 m HR 0.38 95%CI 0.28-0.51; p < 0.001). On multivariable analysis including Hb, Karnofsky PS, LDH, PSA and visceral metastases, detectable preC3 mGSTP1 independently predicted shorter TTP (HR 1.9 95%CI 1.4-2.6; p < 0.001). Detectable mGSTP1at both time points independently predicted OS (BL; HR1.8 95%CI 1.2-2.8; p = 0.006 and preC3; HR 2.2 95%CI 1.6-3.0; p < 0.001). Results from the full cohort of 600 pts will be available for presentation at the meeting. Conclusions: This study should validate circulating mGSTP1 DNA as a marker of therapeutic benefit and prognosis in men with mCRPC receiving DTX and could be utilized for clinical management.