257 Background: Aniten compounds bind to the N-terminal domain (NTD) of the androgen receptor (AR) and inhibit AR dependent transcription. EPI-506, the pro-drug of EPI-002, was the first AR NTD inhibitor tested in a Phase 1 study in men with metastatic castration-resistant prostate cancer (mCRPC). The drug was well-tolerated but required high doses. At doses >1280 mg, EPI-506 treatment resulted in PSA declines; however, these were minor and of short duration, reflecting EPI-506’s low potency and short half-life. To understand EPI-506’s metabolic vulnerabilities, patient plasma samples were analyzed to identify metabolites. Methods: PSA serum levels were assessed after a month of dosing. Patient plasma samples were analyzed and pharmacokinetic (PK) parameters calculated. Three plasma samples from patients (one 80 and two 3,600 mg doses), were pooled across timepoints and metabolites were analyzed. EPI-506 metabolism was assessed in in vitro ADME assays and metabolite activity was measured. Results: EPI-002 patient plasma profiles exhibited dose-proportional Cmax and AUC following once or twice-daily EPI-506 administration. PSA declines (range of 8-29%) were observed, especially at higher doses (≥ 1,280 mg). A total of 19 metabolites were identified. Metabolite M19, a glycerol-moiety oxidant, was the major drug-related component. Other metabolic pathways included O-glucuronidation, sulfation, carboxylic acid formation, and oxidative chlorine loss. The major metabolites were tested in an AR driven reporter assay and were shown to be inactive. Interestingly, in vitro ADME assays predicted glucuronidation and sulfation but not cytochrome dependent metabolism. Conclusions: EPI-506 was tested in a phase 1 trial and showed minor PSA declines. The drug was well-tolerated but was highly metabolized. Patient plasma samples identified 19 metabolites. Newer molecules have been synthesized to address EPI-002’s metabolic liabilities and demonstrate > 20-fold improved potency and higher stability. These next generation Anitens are currently being characterized for IND filing. Clinical trial information: NCT02606123.
An open-label, phase 1 study of androgen receptor antagonist, apalutamide in Japanese patients with metastatic castration-resistant prostate cancer