Background: Metastatic castration-resistant prostate cancer (mCRPC) remains a leading cause of cancer-related deaths worldwide. Although abiraterone (abi) in either the castration-sensitive or castration-resistant setting increases survival, resistance is universal and limits the efficacy of subsequent hormonal therapies. Polo-like kinase 1 (PLK1) is a serine/threonine protein kinase that regulates mitotic functions and promotes the progression of cells through mitosis, and it is highly upregulated in prostate cancer following castration. PLK1 inhibition enhances the efficacy of abi in cell line models as well as patient-derived tumor xenografts via several mechanisms. Onvansertib (PCM-075; Trovagene, Inc.) is the first orally available PLK1-specific inhibitor. In phase 1 testing, onvansertib demonstrated a manageable safety profile, with transient hematologic effects as its most prominent, yet reversible, toxicity. Methods: The goal of this phase 2 study (NCT03414034) is to observe the effects of onvansertib in combination with abi + prednisone on disease control, as assessed by prostate-specific antigen (PSA) decline or stabilization after 12 weeks of study treatment, in subjects with mCRPC and early resistance to abi. Patients will be enrolled at time of PSA progression while on standard abi. A completed 3-patient safety lead-in phase tested the safety of the combination of onvansertib with abi, and the accruing expansion phase will treat 29 more patients with onvansertib (24 mg/m² orally on days 1-5 of a 21-day cycle) plus abi (administered orally and continuously once daily with prednisone) until time of radiographic or symptomatic progression. With 32 patients, there will be 90% power to detect a change in disease-control rate from 10% (null) to 30% (alternative). Based on a Simon’s two-stage optimal design, the study will terminate early if < 2 of the first 13 patients achieve disease control. Exploratory analyses include evaluations of predictive genomic biomarkers in circulating tumor cells and circulating tumor DNA, including alterations of oncogenes and tumor suppressors implicated in PLK1 sensitivity within preclinical models. Clinical trial information: NCT03414034