BACKGROUND: Abiraterone (ABI) and Enzalutamide (ENZA) are the most common medications used for metastatic castrate resistant prostate cancer (mCRPC). Post treatment prostate specific antigen (PSA) changes correlate with prognosis (1). Early PSA decline defined as PSA decline by >50 % at 4 weeks after starting treatment compared to baseline has been demonstrated to corresponds with improved PSA progression-free survival (PSA PFS) and overall survival (OS) (2). Based on our literature review, this relation has not been elucidated previously in the Veteran population.
METHODS: A retrospective chart review was conducted in subjects with mCRPC who had received ABI and ENZA from 2011 to 2019 at VA Fresno. Subjects must have received at least 3 months of each drug. The primary outcomes were to document PSAPFS and OS between patients with >50% and <50% decline in PSA at 4 weeks after starting treatment with either ABI or ENZA.
RESULTS: 50 patients were identified who were treated with Abiraterone. 30 (60%) patients had a PSA decrease of 50% or greater in the first 4 weeks. A 50% or greater decrease in PSA at 4 weeks after starting was associated with a better PSA PFS (15 vs 5 months) and better median OS (20.5 vs 12 months). 20 patients were identified treated with Enzalutamide. 13(65%) of patients had a PSA decrease of 50% or greater in the first 4 weeks. A 50% or greater decrease in PSA at 4 weeks was associated with a better PSA PFS (14.5 vs 5 months) and better median OS (20 vs 12 months). 50% of patients in the whole group were diagnosed with metastatic disease at initial diagnosis while the rest were previously treated for localized prostate cancer before being diagnosed as metastatic disease. Most baseline characteristics including, baseline PSA, the modality of initial localized treatment, Gleason score, previous treatment with chemotherapy were similar between the two comparison cohorts (>50% and <50% decrease in PSA after starting treatment with ABI or ENZA).
CONCLUSIONS: Early decrease in PSA is an independent marker for the efficacy of antiandrogen treatment with enzalutamide and abiraterone in metastatic hormone- resistant prostate cancer in the veteran population.