Dose modification in enzalutamide and abiraterone plus prednisolone for castration-resistant prostate cancer: A subanalysis from the ENABLE study for PCa.

Background: A head-to-head comparison between enzalutamide (ENZ) and abiraterone plus prednisolone (ABI) revealed similar survival benefits for castration-resistant prostate cancer (CRPC) in the ENABLE study for PCa. Considering that a dose reduction of ENZ and ABI has demonstrated sufficient inhibitory ability of androgen receptor (AR) signaling, we analyzed the efficacy of modified doses of these agents in the ENABLE study for PCa.
Methods: This investigator-initiated, multicenter, randomized controlled trial that was conducted in Japan analyzed the prespecified survival endpoints, prostate-specific antigen (PSA) response rate ( ≥50% decline from baseline), and safety profile in patients treated with modified doses (ENZ ≤ 120 mg/day, ABI ≤ 750 mg/day) compared with those treated with a standard dose (ENZ 160 mg/day, ABI 1000 mg/day) as a starting dose.
Results: In total, 92 patients in each arm were treated and analyzed; 16 patients were treated with a modified dose in both the ENZ and ABI arms, respectively. Moreover, 32 patients treated with modified doses showed a significantly better time to PSA progression (TTPP) and overall survival (OS) compared with the 152 patients treated with a standard dose (HR 0.47, 95%CI 0.27-0.83, p = 0.0379, and HR 0.35, 95%CI 0.19-0.63, p = 0.0162). Despite a significantly longer TTPP in the modified ABI group than in the standard ABI group (HR 0.29, 95%CI 0.14-0.62, p = 0.0248), no significant difference was observed in the TTPP between the modified and standard ENZ groups (p = 0.5366). Furthermore, similar adverse event rates and grades were observed in each treatment dose group.
Conclusions: The modified doses of ABI showed better TTPP than the standard dose of ABI and may be a potential treatment option for CRPC patients; however, its mechanism is still unclear, although its ability to suppress AR signaling is equivalent to that of a standard dose.
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