Low-dose abiraterone in metastatic castration-resistant prostate cancer.

164 Background: Abiraterone acetate (AA) is a selective and irreversible inhibitor of CYP17 approved for locally advanced and metastatic prostate cancer. The standard dose is 1000 mg orally while fasting. However, AA has a significant food effect, with serum levels increasing 5-7 times with a low-fat meal. Data suggest that a lower dose (250 mg with food, AALD) provides similar oncological outcomes to the standard dose, with a 75% cost reduction. In Brazil, 80% of patients are treated through the national public healthcare system, which does not afford AA in its label dose. Hospital de Base incorporated AALD since March 2021.Herein we present the data of patients with metastatic castration resistant prostate cancer treated with AALD. Methods: We retrospectively reviewed patients with mCRPC who received AALD at any treatment line, regardless of prior chemotherapy, from March 2021 to May 2023. The primary endpoint was PSA response rate (defined as a decrease of ≥ 50% in PSA concentration after 12 weeks of treatment). Secondary endpoints included time to treatment failure (TTF), overall survival (OS), and PSA response rate ≥ 50% at nadir. Data were summarized in medians, means and proportions. Chi-square was used for associations. Survival was calculated through Kaplan-Meier method. Cox-proportional hazards was used to compare groups. A p < 0.05 was considered statistically significant. Results: Ninety-six patients were included. The median age was 75 years (IQR 12.5), and most had an ECOG scoreof 1 (52.1%). Sixty-three patients (65.6%) received AALD in first-line, 26 (27.1%) in second-line, and 7 (7.3%) in third-line or beyond. The median PSA level before AALD was 20.9 (IQR 98.7). Twenty-seven patients (28.1%) had received docetaxel in mCRPC. After a median follow-up of 24.4 months, 53 patients (60.2%) had a ≥ 50% PSA response at 12 weeks, and 65 (69.9%) achieved a reduction of ≥ 50% at nadir. Patients treated with AALD pre-docetaxel had a higher PSA response at 12 weeks (68.3% vs. 40%, p = 0.015) and at nadir (76.1% vs. 53.8%, p = 0.036). First-line AALD was associated with better PSA responses. The median TTF was 7.9 months (5.5 – 10.3), and the median OS was 20.6 months (14.1 – 26). Achieving a ≥ 50% PSA response at 12 weeks was associated with improved TTF (13.6 vs. 4.6 months, HR = 0.37, p < 0.001) and OS (28.9 vs. 12.2 months, HR = 0.44, p = 0.004). Conclusions: AALD showed meaningful efficacy and survival outcomes. We advocate for AALD, particularly in settings where the standard dose is not affordable.