Effect of Low-dose 500 mg Abiraterone Acetate in Treatment of Metastatic Prostate Cancer Patients

This will be an open label, Phase I study to assess the efficacy of a reduced 500 mg dose of abiraterone acetate in patients with metastatic prostate cancer. Eligible metastatic hormone-sensitive prostate cancer (mHSPC) and metastatic castration-resistant prostate cancer (mCRPC) patients newly initiated on abiraterone acetate treatment will be recruited to receive a reduced 500 mg dose of abiraterone acetate plus prednisolone. The study treatment duration will span 12 weeks, after which patients being administered the reduced dose will be reverted to the standard 1000 mg dosing. Follow-up for mCRPC and mHSPC patients will last for 18 and 36 months respectively. The main question the study aims to answer is whether dose reduction of abiraterone acetate to 500 mg would achieve antitumor activity in mCRPC and mHPSC patients comparable to standard of care.

Primary Objectives: As a preliminary Phase I trial, the primary objective of the study would be to evaluate the percentage change in prostate specific antigen (PSA) from baseline to 12 weeks. Secondary Objectives: 1. determine the proportion of patients achieving PSA response (≥ 50% reduction in PSA after 12 weeks of therapy). 2. evaluate the pharmacokinetics associated with the 500 mg dose of abiraterone acetate. 3. investigate the correlation between plasma exposure of abiraterone and CP-I or CP-III in order to support their utility as a biomarker of OATP1B1/1B3 function. 4. assess the pharmacodynamic effects of the reduced 500 mg dose on the maximal percentage change in serum androgens (dehydroepiandrosterone (DHEA), dehydroepiandrosterone-sulfate (DHEA-S), testosterone, androstenedione) from baseline.