First-in-field small molecule inhibitors targeting BRN2 as a therapeutic strategy for small cell prostate cancer.

Abstract

260 Background: Resistance to newly developed androgen receptor pathway inhibitors (ARPIs), such as Enzalutamide (ENZ), rapidly emerges and patients generally die within two years. In particular, a subset of patients who relapse following ARPI therapy exhibit lineage switching whereby tumours shed their dependence on AR signaling and emerge with neuroendocrine features. These tumours, termed treatment induced neuroendocrine prostate cancer (t-NEPC), carry an extremely poor prognosis and, to date, treatment remains decades old cytotoxic chemotherapies; therefore, targeted therapies are desperately needed. Recently our group identified the neural transcription factor BRN2 as a major clinically relevant driver of NEPC and aggressive tumor growth, both in vitro and in vivo, suggesting targeting BRN2 is a promising strategy to prevent neuroendocrine differentiation or treat NEPC. Methods: Study the effects of BRN2 inhibition using siRNA, small molecule inhibitors (BRN2i) and CRISPR K/O models. The efficacy of the small molecules was examined using reporter assays, florescence polarization assays, Biolayer Interferometry, DARTS, chromatin fractionation, RNA-seq and ChIP-seq. Pharmacokinetic studies measured stability and bioavailability of the molecules and in-vivo efficacy is to be measured in NCI-H660 xenograft model. Results: Inhibition of BRN2 drastically reduced cell proliferation in NEPC cell lines 42DENZR and NCI-H660 cell lines. Targeting BRN2 with our first-in-field small molecule inhibitors lead to downregulation several known targets in NEPC like EZH2, ASCL1, SOX2 and PEG10. Treatment with BRN2i reduced recruitment of BRN2 to the chromatin by approximately 93% within 16 hours. Moreover, these BRN2 inhibitors displayed adequate pharmacokinetic properties and reduced NEPC proliferation in vivo. Conclusions: No therapies exist for highly lethal NEPC. Hence, the described work aims to lay the pre-clinical foundation for the integration of BRN2 targeted therapies into the treatment landscape to improve survival for patients suffering from small cell prostate cancer.