Background: Sabizabulin is a novel oral cytoskeleton disruptor being developed for use in metastatic castration resistant prostate cancer (mCRPC). A Phase 1b/2 clinical study was conducted to establish the maximum tolerated dose (MTD) and evaluate the preliminary efficacy in men with mCRPC resistant to androgen receptor targeting agents. Methods: The Phase 1b portion of the study in 39 men utilized escalating and expanding dose and duration. The Phase 2 portion studied 41 men with mCRPC at the recommended Phase 2 dose of 63 mg daily. Efficacy was assessed by bone/CT scans. A final analysis of the safety and efficacy data including the primary endpoint, the median progression-free survival was conducted. Results: Although the MTD was not reached in the Phase 1b, the recommended Phase 2 dose was set at 63 mg/day to maximize GI tolerability. The most common adverse events (> 10% frequency) at the 63 mg oral daily dosing (combined Phase 1b/2 data) were predominantly Grade 1-2. Grade ³3 events included diarrhea (7.4%), fatigue (5.6%) and ALT/AST elevations (5.6% and 3.7%, respectively). Neurotoxicity and neutropenia were not observed. Preliminary efficacy data in patients treated with ≥1 continuous cycle (21 days) of 63 mg or higher (n = 55) included an objective response rate of 6/29 (20.7%) in patients with measurable disease (1 complete, 5 partial). 14/48 (29.2%) of the patients had PSA declines. The Kaplan-Meier median radiographic progression-free survival was estimated to be 11.4 months (95% C.I. 29.63-65.79) (n = 55). Durable responses lasting > 2.75 years were observed with 14.5% (8/55) demonstrating a response greater than 12 months. Conclusions: This clinical trial demonstrated that chronic oral daily dosing of sabizabulin has a favorable safety profile with significant preliminary cytotoxic and cytostatic antitumor activity. These data support the ongoing Phase 3 VERACITY trial of sabizabulin in men with mCRPC who have progressed on an androgen receptor targeting agent. Clinical trial information: NCT03752099.