AR DNA binding domain inhibitors to treat AR splice variant-driven prostate cancer.

Abstract

293 Background: Nearly all prostate cancer (PC) cells depend on androgen receptor (AR) signaling for growth. Resistance to novel PC drugs like abiraterone and enzalutamide appears to occur through ligand-independent AR signaling, often by expression of AR splice variants (ARv’s) that lack the ligand binding domain (LBD). Thus, drugs that inhibit AR activity via domains other than the LBD may provide a way to inhibit the growth of drug-resistant PC. We discovered the first non-competitive, ligand-independent AR inhibitor, pyrvinium pamoate (PP), which functions by interacting with the DNA-binding domain (DBD) and preventing the recruitment of RNA polymerase II to transcription start sites. PP inhibits AR activity with low nanomolar potency and functions synergistically with competitive antagonists. Here, we undertook experiments to learn more about PP’s mechanism of action and developed an orally available derivative for translation to human studies. Methods: AR immunoprecipitation (IP) followed by mass spec sequencing was used to identify proteins that differentially bind to AR in the presence of PP. Additionally, RNA-seq was used to identify pathways affected by PP treatment. Functional studies of hits were conducted using over-expression and knock down in relevant PC cell lines. PP derivatives were developed through a medicinal chemistry approach. Lead compounds were tested in a battery of assays to determine potency in cultured cells and in vivo, as well as their solubility and pharmicokinetics. Results: RNA-seq results suggest that AR and hormone pathways are the primary target of PP in PC cells but that the splicing machinery is also be affected. Mass spec results also demonstrate that PP affects the interaction between AR and splicing machinery, as well as many transcription and translation-related proteins. Medicinal chemistry efforts led to a molecule with superior aqueous solubility, oral availability, and a half-life > 6hrs. This compound inhibited the activity of AR and ARv’s in cultured cells and in xenograft models. Conclusions: PP was the first identified AR DBD inhibitor and the development of an orally available derivative holds great promise for the treatment of ARv-driven PCs.