Phase I/II safety and pharmacokinetic (PK) study of ARN-509 in patients with metastatic castration-resistant prostate cancer (mCRPC): Phase I results of a Prostate Cancer Clinical Trials Consortium study.

43 Background: In CRPC, androgen receptor (AR) overexpression is associated with resistance to first-generation anti-androgen therapy such as bicalutamide. ARN-509 is a novel small molecule AR antagonist that impairs AR nuclear translocation and binding to DNA, inhibiting tumor growth and promoting apoptosis, with no partial agonist activity. Preclinical data shows that ARN-509 binds AR with 5-fold greater affinity than bicalutamide, and induces tumor regression in hormone-sensitive and CRPC xenograft models. Methods: In this open-label, Phase 1/2 study, mCRPC patients received ARN-509 orally on a continuous daily dosing schedule. In Phase 1 , 7 doses (30, 60, 90, 120, 180, 240, 300 mg) were tested using standard 3x3 dose escalation criteria to assess safety, PK, and determine the recommended Phase 2 dose (RP2D). Preliminary anti-tumor activity was assessed by PSA kinetics, radiographic responses, circulating tumor cells (CTCs), and FDHT-PET imaging. Results: Twenty-four patients (median age 68 yrs, Gleason Score 8; prior docetaxel 13%) were enrolled. The most common Grade 1-2 treatment-related adverse events were fatigue (38%), nausea (29%), and pain (24%). There was only 1 treatment-related Grade 3 adverse event (abdominal pain) at 300 mg, possibly related to a higher pill burden, which led to an additional 3 patients being enrolled at the highest dose with no further dose limiting toxicities. PK was shown to be linear and dose-dependent. Twelve patients (55%) had ≥ 50% PSA declines. To date, 7 patients have discontinued the study due to progression, with the longest patient still on study for more than 1 year. FDHT-PET imaging demonstrated AR blockade at 4 weeks across multiple dose levels. Based on preclinical assessment of maximum efficacious dose, PK, and promising activity across all doses, 240 mg was selected as the RP2D. Conclusions: In this Phase 1 study, ARN-509 was shown to be safe and well tolerated, with promising preliminary activity based on PSA and pharmacodynamic evidence of AR antagonism. The Phase 2 portion of the study will enroll up to 90 patients with treatment-naïve non-metastatic and mCRPC.