Background: PRL-02 (abiraterone decanoate) is a novel, long-acting IM depot prodrug of abiraterone. In non-clinical models, PRL-02 provided high and durable concentrations of prodrug and abiraterone to target tissues including adrenal glands, lymph nodes and bone. Single doses of PRL-02 suppressed testosterone (T) through 14 weeks in a castrate monkey model to concentrations comparable to oral abiraterone acetate (AA) with lower and less variable plasma abiraterone exposures. Clinically, PRL-02 blocks androgens with minimal increases in mineralocorticoids or depletion of glucocorticoids via inhibition of CYP17 lyase and minimal inhibition of CYP17 hydroxylase. PRL-02 has the potential for a superior therapeutic index and safety profile compared to oral AA. Methods: Phase 1 is a standard 3+3 design intended to identify a recommended phase 2 dose (RP2D). Pts with metastatic castrate resistant or sensitive prostate cancer (mCRPC/mCSPC) and a screening T of 2 - 50 ng/dL were administered IM PRL-02 every 12 weeks with daily oral dexamethasone. Results: As of 8Sep22, 17 pts (6 mCRPC, 11 mCSPC) were treated across 5 dose cohorts (180, 360, 720, 1260, 1800mg). Generally, there was a dose-proportional increase in abiraterone concentrations following a single dose of PRL-02 with a Tmax of 14 - 28 days and a plasma half-life of 18.3 days. The median baseline T level was 7.45ng/dL. Among pts dosed at 720mg and above, 9 of 11 had a 90% reduction in T or values ≤ 1ng/dL at day 28, including 2 pts with T≤LLOQ of 0.1 ng/dL, and PSA50 responses were observed in 8 of 10 with post-baseline results. There were no treatment related serious adverse events (AEs) or dose limiting toxicities. G3 AEs related to PRL-02 included hip and shoulder pain. G2 related AEs included fatigue, decreased appetite, insomnia and hot flush; symptoms of mineralocorticoid excess were not reported.Minimal and transient changes in ‘up-stream’ steroids (e.g., progesterone (P) and corticosterone (C)) were observed through the 1800mg dose. Although serial radiology was not prospectively required, there was radiographic improvement in 6 pts with data available. Conclusions: PRL-02 was well tolerated. Dose-dependent T suppression was associated with clinical benefit including PSA responses and radiographic improvement. Based on a historical comparison, the levels of P and C are significantly lower than seen with AA + prednisone which appear to be due to greater CYP17 lyase selectivity. The 1260mg or 1800mg dose will be the RP2D. Clinical trial information: NCT04729114.