Abiraterone decanoate (AD): Potent and long-acting activity of a novel intramuscular (IM) abiraterone prodrug depot in a castrate monkey model.

Abstract

78 Background: Oral abiraterone acetate (AA) is a standard of care for castration-resistant (CRPC) and castration-sensitive prostate cancer (CSPC). Due to poor oral bioavailability, the recommended AA dose is 1,000 mg (4 x 250mg) once-daily on an empty stomach. The daily oral regimen produces high peak plasma concentrations that may be associated with safety issues (e.g., hepatotoxicity) and low trough concentrations that may be associated with inadequate CYP17 inhibition. AD is one of a series of novel abiraterone prodrugs that were designed to provide a controlled release of abiraterone and long-acting CYP17 inhibition with IM delivery. Following successful preclinical pilot studies, AD was further developed into a clinically acceptable formulation (PRL-02) and its PK/PD characteristics were evaluated and compared to oral AA in a castrate monkey pharmacology model in support of future clinical development. Methods: Sexually mature male cynomolgus monkeys underwent chemical castration using Lupron depot. Plasma samples were analyzed for prodrug, abiraterone and steroid concentrations following a single oral AA dose (5, 15 or 45 mg/kg) and a single IM AD injection (10, 30 or 100 mg/kg) (n=3/dose group). The combined activity of IM AD plus glucocorticoid replacement (single 0.5 mg/kg IM dexamethasone (DEX) dose or weekly doses of 1.29 mg/kg IM methylprednisolone acetate (MPA)) was also evaluated. AD, abiraterone and adrenal steroid levels were evaluated by LC-MS/MS. Results: Chemical castration resulted in a plasma testosterone (T) decrease of 72% from intact. T was further reduced to sub-castrate levels by both AA and AD (98.6% and 99.7% maximum decrease from castrate baseline, respectively). All dose levels of AD were highly effective in reducing plasma T yet led to abiraterone plasma concentrations (Cmax, AUC, Cmin) that were much less than those associated with steady-state clinical levels from oral AA. Sustained T suppression was observed following all single AD IM doses for 14 weeks, the last timepoint tested.The addition of a glucocorticoid replacement (DEX or MPA) starting at week 9 further reduced T in all AD groups. Conclusions: Single-dose IM AD suppressed T to the same or greater extent than single-dose oral AA in castrate monkeys, while providing much lower abiraterone exposures than the oral acetate prodrug. As such, IM AD may offer an improved risk-benefit profile due to the consistently lower abiraterone levels. The durable, profound T reductions provided by all AD doses are consistent with a 3-month clinical regimen that can be given in conjunction with Lupron. IM AD may thus offer patients with CSPC and CRPC a more convenient, safe and effective alternative than daily oral AA.