Abstract 3083: Selective inhibition of transcription factor BRN2 as a treatment strategy for Small Cell Prostate Cancer

Introduction: Resistance to newly developed androgen receptor pathway inhibitors (ARPIs), such as Enzalutamide (ENZ), rapidly emerges. In particular, a subset of patients who relapse following ARPI therapy their dependence on AR signaling and emerge with neuroendocrine features. These tumors, termed treatment induced small-cell prostate cancer (t-SCPC) or neuroendocrine prostate cancer (t-NEPC), carry an extremely poor prognosis and, to date, treatment remains decades old cytotoxic chemotherapies. Previously, our group identified the neural transcription factor BRN2 as a major clinically relevant driver of SCPC and now report that targeting BRN2 is a promising strategy to prevent neuroendocrine differentiation or treat NEPC.Methods/Results: In silico screening of small molecules was conducted on a model of BRN2 which was later validated with the first-in-field crystal structure of BRN2 DNA binding domain. On the basis of the model, several small molecules were identified that showed direct binding to BRN2 and inhibited its activity. Pharmacokinetic studies measured stability and bioavailability of med-chem optimized lead compound (BRN2i) that significantly reduced tumor growth in multiple xenograft models with no measurable side-effects. In silico modeling predicted a 7Å “closing” in the DBD once it was bound to BRN2i, this shift translated to reduced interaction with DNA by chromatin fractionation and ChIP-seq, thus confirming the mode of action for BRN2i is through loss of DNA binding. Loss of BRN2 binding reduced cell proliferation in tSCPC cell line 42DENZR, de novo SCPC cell line NCI-H660 and NEPC organoids as well as downregulated several known targets like EZH2, ASCL1, SOX2 and PEG10. Down-regulation of these target genes was also measured in the xenograft tumors, confirming on target effect in vivo. Moreover, the specificity of BRN2i was validated with CRISPR/Cas9 mediated knockout of BRN2 with downstream mRNA expression and phenotypic changes. Conclusion: 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 neuroendocrine prostate cancer. Citation Format: Daksh Thaper, Ravi Munuganti, Adeleke Aguda, Soojin Kim, Shungyu Ku, Sahil Kumar, Sepideh Vahid, Shaghayegh Nouruzi, Olena Sivak, Dwaiyapan Ganguli, Colm Morrissey, Eva Corey, Himisha Beltran, Amina Zoubeidi. Selective inhibition of transcription factor BRN2 as a treatment strategy for Small Cell Prostate Cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3083.