Novel Therapeutic Approaches to Overcome Acquired Resistance to Enzalutamide in Patients with Advanced Prostate Cancer

Abstract

This thesis addresses the development of novel therapeutic approaches to combat acquired resistance to enzalutamide, a second-generation androgen receptor antagonist (SG-ARA), in patients with advanced prostate cancer. Numerous mechanisms of acquired resistance to enzalutamide have been identified via extensive research efforts, including alterations to the androgen receptor (AR), DNA repair, PI3K/AKT, and WNT/β-catenin pathways. An open-label phase 2 clinical trial evaluating the safety and efficacy of seviteronel, a selective CYP17 lyase inhibitor with activity against AR point mutations associated with enzalutamide resistance (e.g., F877L, T877A), was initiated in patients with metastatic castration-resistant prostate cancer (mCRPC) previously treated with enzalutamide. Overall, seviteronel was not generally well tolerated and did not demonstrate clinically meaningful anti-tumor activity based on prostate-specific antigen (PSA) response, suggesting limited benefit of solely targeting of the AR pathway in patients with advanced prostate cancer following enzalutamide monotherapy. To alternatively overcome enzalutamide resistance, it was proposed to target of hypoxia-inducible factor (HIF)-1α, a transcription factor upregulated in hypoxic tumors and previously shown to interact with the AR and β-catenin, in combination with enzalutamide. NLG207, a nanoparticle-drug conjugate of the potent topoisomerase I inhibitor with HIF-1α modulating properties, camptothecin (CPT), was evaluated in combination with enzalutamide for anti-tumor activity in two preclinical prostate cancer xenograft models of enzalutamide resistance mechanisms, including AR splice variant expression (e.g. AR-V7) and AR overexpression. The addition of NLG207 enhanced the in vivo anti-tumor activity of enzalutamide, significantly reducing tumor volume after three weeks of treatment. Correlative in vitro analyses supported the effectiveness of NLG207 to drive down enzalutamide resistance via the downregulation of AR-V7 mRNA expression. These data provided scientific rationale to investigate dual targeting of HIF-1α and the AR via combination treatment with NLG207 and enzalutamide in a clinical trial setting. To support the clinical investigation of NLG207, a novel, robust, selective, accurate, and precise bioanalytical method to quantify CPT concentrations, both bound and unbound to NLG207, was developed and validated. Clinical samples from 27 patients receiving NLG207 on two National Cancer Institute sponsored clinical trials (NCT02769962 and NCT03531827) were quantitated via the validated bioanalytical method and used to build a population pharmacokinetic (popPK) model via non-linear mixed-effects modelling. The popPK model confirmed the nanoparticle behavior critical to drug delivery facilitated by NLG207 and mechanistically characterized the release of CPT. NCT03531827 is an ongoing open-label phase 2 study with a lead-in dose escalation evaluating the efficacy of NLG207 in combination with enzalutamide in patients with mCRPC previously treated with enzalutamide, with only three patients enrolled to date. The primary objective of the study seeks to improve the overall response rate in this patient population, with response defined as either a PSA decline greater than 50% or stable disease on imaging after 5 months. The study will also evaluate numerous biomarkers of enzalutamide resistance, including those associated with the AR, HIF-1α, and DNA repair pathways. Through increased understanding of acquired resistance to enzalutamide, the collective work aims to facilitate further optimization of treatment for patients with advanced prostate cancer.