PROJECT 1: ABSTRACT MYC oncoproteins (including c-MYC, L-MYC and N-MYC) have been implicated in up to 70% of all human cancers. In prostate cancer, elevated levels of MYC protein expression are observed across all grades. In castration-resistant prostate cancer (CRPC), there is evidence of further upregulation of c-MYC levels with gene amplification occurring in 45% of cases. In late-stage, therapy-resistant neuroendocrine prostate cancer (NEPC), N-MYC is overexpressed in 40% of cases. In preclinical studies, inhibition of MYC can effectively kill CRPC and NEPC cells. A viable therapeutic strategy to inhibit MYC is therefore likely to have a significant impact on this disease and to fulfill the ongoing need for novel impactful therapies spanning the spectrum of castration resistant prostate cancer. Despite its recognition as an attractive cancer target, MYC has proved difficult to target, and there are currently no clinically viable small molecule MYC inhibitors (MYCi) available. By employing a pharmacophore-based in silico screen of a large chemical library (32 million compounds) coupled to a rapid in vivo screen in mice, we identified a series of novel small molecule inhibitors. These MYC inhibitors are highly drug-like and have shown excellent pharmacokinetic, toxicological and anti-tumor activity profiles in MYC-driven models of prostate cancer and leukemia. The compounds engage MYC inside cells as shown by the cellular thermal shift assay (CETSA); disrupt MYC/MAX complex formation which is required for MYC activity; and inhibit MYC-driven target gene expression. Furthermore, the MYCi compounds enhance phosphorylation of MYC on threonine-58 (T58P) which promotes MYC degradation via a well-characterized ubiquitin-proteasome pathway. Consequently, treatment with MYCi impaired tumorigenicity in vitro and in vivo. The goals of this project are to develop the lead MYC inhibitor, MYCi975, for clinical application in the treatment of prostate cancer and to characterize the mechanisms of MYCi-induced degradation of c-MYC and N-MYC oncoproteins. We will implement the following Specific Aims: Aim 1 is to investigate the mechanisms of MYCi975 regulation of c- MYC and N-MYC phosphorylation and stability and the potential of MYC pT58 as a pharmacodynamic marker. Aim 2 will assess MYCi anti-tumor efficacy and impact on pharmacodynamic biomarkers in preclinical models of c-MYC and N-MYC driven prostate cancer. Aim 3 will seek to develop MYCi975 for use in patients by conducting formal IND-enabling toxicology studies and initiate a phase 1 trial in mCRPC patients. Impact: Successful completion of these studies could lead to first-in-class therapies for lethal prostate cancers dependent on c-MYC/N-MYC activity. This benefit can extend to other human cancers as well because of the pervasive role MYC proteins play in cancers of all types.