Efficacy, safety, tolerability, and pharmacokinetics of EPI-506 (ralaniten acetate), a novel androgen receptor (AR) N-terminal domain (NTD) inhibitor, in men with metastatic castration-resistant prostate cancer (mCRPC) progressing after enzalutamide and/or abiraterone.
5032 Background: EPI-506 (ralaniten acetate) is a first-in-class small molecule transcription inhibitor of the AR NTD. Nonclinical studies demonstrated activity against both full length and resistance-related AR species, including AR-v7. Methods: Open-label, single-arm, Phase 1/2 study evaluating EPI-506 administered orally once-daily. The Phase 1 is a 3+3 design to establish the safety, pharmacokinetic (PK) profile, and recommended phase 2 dose of EPI-506. Anti-tumor activity is also evaluated. Inclusion criteria included: mCRPC with progression after ≥1 line of hormonal therapy or chemotherapy, and progression on enzalutamide and/or abiraterone. Results: Eighteen patients (pts) have been enrolled in the dose escalation phase over 5 dose levels (80, 160, 320, 640, 1280 mg). Median age was 71 (range 58-87). Prior treatments included enzalutamide only (N = 7), abiraterone only (N = 2) or both (N = 9). Six pts have had prior chemotherapy. Seven pts have discontinued due to disease progression and 2 pts due to adverse events (AEs): Grade 4 elevated amylase (related) and Grade 4 gastrointestinal bleeding (unrelated). Median exposure was 98.5 days (range 26-338). Most frequently reported treatment emergent AEs were diarrhea (N = 7), nausea (N = 5) and fatigue (N = 3). One Grade 3 AE (AST elevation) at 1280 mg and one Grade 4 AE (increased amylase) at 640 mg were reported. PK data demonstrate a dose-proportional profile for Cmax and AUC coupled with an initial negative food effect up to 640 mg. At week 4 of continuous dosing, 3 of 18 evaluable pts demonstrated PSA declines ranging from 9 to 18% receiving doses ≥640 mg. Conclusions: EPI-506 is well-tolerated with a favorable safety profile. PK indicates dose-proportionality. PSA declines have been observed at doses associated with sub-therapeutic exposure in preclinical studies. This study is the first to evaluate targeting the AR NTD, a region critical for transcriptional function of all known AR species. Clinical trial information: NCT02606123.