Abstract

TPS5111 Background: Current therapeutic strategies for metastatic castration-resistant prostate cancer (mCRPC) include treatment with next-generation androgen receptor pathway inhibitors (ARPIs), which target the ligand-binding domain (LBD) of the AR. Approximately 5% to 10% of patients will have primary resistance to ARPIs and most initial responders will develop resistance within 1 to 3 years. Eventually, nearly all men with prostate cancer treated with ARPIs develop resistance via several mechanisms including AR genomic alterations, epigenetic alterations, and expression of truncated constitutively active AR splice variants. Hence, there is substantial unmet need for treatment of men with mCRPC resistant to at least one next-generation ARPI. ONCT-534 is a DAARI with a novel mechanism of action that combines inhibition of AR function with degradation of the AR protein. Importantly, this activity includes interaction with both LBD and the N-terminal domain (NTD) of the AR, rendering it effective against splice variants and LBD mutants, unlike existing ARPIs. ONCT-534 has demonstrated preclinical activity in prostate cancer models against unmutated AR and multiple forms of AR alteration, including amplification, mutations in the LBD, and splice variants with loss of LBD. Methods: ONCT-534-101 is a phase 1/2, multi-center study to evaluate the safety, tolerability, antitumor activity, and pharmacokinetics of ONCT-534 in subjects with histologically confirmed mCRPC without neuroendocrine differentiation or small cell features who have relapsed or are refractory (R/R) to at least one next-generation ARPI. The study is separated into Phase 1 Dose Escalation and a Phase 2 Dose Expansion portions. The Phase 1 portion will evaluate approximately 27 subjects in 5 dose levels using an adaptive Bayesian Optimal Interval (BOIN) design to assess safety, tolerability and dose limiting toxicities (DLTs) at escalating doses and to determine the maximum tolerated dose and inform the 2 dose levels or schedules to be tested in Phase 2. DLTs will be assessed during the first 28 days of treatment. AR phenotype and AR levels will be evaluated for each subject pre- and post-treatment. The Phase 2 portion will evaluate approximately 32 subjects in 2 randomized cohorts to assess safety and tolerability of ONCT-534, compare the 2 different dose levels or schedules to select the optimal dose for further study, and assess the preliminary antitumor activity of ONCT-534. In both phases, after a screening period, eligible subjects with mCRPC will receive their assigned dose regimen of ONCT-534, which will be administered orally daily for 2 years or until disease progression and no longer clinically benefiting from treatment or development of unacceptable toxicity. Cohorts 1 and 2 have been completed without DLT. Enrollment into Cohort 3 began in January 2024 (NCT 05917470). Clinical trial information: NCT05917470 .