PURPOSE: To evaluate outcomes of disease progression based on the sequence of abiraterone and enzalutamide in veterans diagnosed with metastatic castration-resistant prostate cancer (mCRPC).
BACKGROUND: Two of the current options for mCRPC treatment are the novel oral hormonal agents abiraterone and enzalutamide. After progression on one of these agents, one option is to switch to the other agent not previously used. Previously published retrospective studies and one prospective study have shown a difference in outcomes favoring abiraterone followed by enzalutamide, while others have shown no difference based on sequence. The optimal sequence of abiraterone and enzalutamide is still unclear.
METHODS: This was a retrospective chart review of patients who received abiraterone and enzalutamide in sequence for the treatment of mCRPC within our healthcare system from April 28, 2011 through October 31, 2019. Baseline demographic information such as age, race, Gleason score, and prior treatments were collected. The primary outcome was combined prostate-specific antigen progression-free survival (cPSA-PFS). Secondary outcomes included radiographic PFS (rPFS), overall survival (OS), adverse events causing treatment discontinuation, and medication adherence. Between-group survival differences were estimated by the Kaplan-Meier method and an unadjusted Cox regression model.
RESULTS: A total of 77 patients met criteria for study inclusion, with 51 in the abiraterone-to-enzalutamide group (ABI-ENZ) and 26 in the enzalutamide-to-abiraterone group (ENZ-ABI). For the primary outcome of cPSA-PFS, the median survival of the ABI-ENZ and ENZ-ABI groups was 17.3 months (95% CI, 10.3-24.3 months) and 10.2 months (95% CI, 8.5-11.8 months), respectively, which was significantly different (log-rank P=0.009) in favor of the ABI-ENZ sequence (HR 0.46; 95% CI, 0.26-0.83). Secondary outcomes of rPFS and OS were not significantly different between groups.
CONCLUSION: This study adds to the evidence supporting the sequence of abiraterone before enzalutamide for improving PSA-PFS. It is thought this might be related to differences in mechanisms of resistance between the two drugs. This benefit has not yet translated to an improvement in rPFS and OS. Based on the results of this study in conjunction with previously published studies, use of abiraterone before enzalutamide should be considered over the alternate sequence.