Sadar Lab

Hormonal therapies for prostate cancer target the androgen receptor (AR) ligand-binding domain (LBD). Clinical development for inhibitors that bind to the N-terminal domain (NTD) of AR has yielded ralaniten and its analogues. Ralaniten acetate is well tolerated in patients at 3600 mgs/day. Clinical trials are ongoing with a second-generation analogue of ralaniten. Binding sites on different AR domains could result in differential effects on AR-regulated gene expression. Here, we provide the first comparison between AR-NTD inhibitors and AR-LBD inhibitors on androgen-regulated gene expression in prostate cancer cells using cDNA arrays, GSEA, and RT-PCR. LBD inhibitors and NTD inhibitors largely overlapped in the profile of androgen-induced genes that they each inhibited. However, androgen also represses gene expression by various mechanisms, many of which involve protein-protein interactions. De-repression of the transcriptome of androgen-repressed genes showed profound variance between these two classes of inhibitors. In addition, these studies revealed a unique and strong induction of expression of the metallothionein family of genes by ralaniten by a mechanism independent of AR and dependent on MTF1, thereby suggesting this may be an off-target. Due to the relatively high doses that may be encountered clinically with AR-NTD inhibitors, identification of off-targets may provide insight into potential adverse events, contraindications, or poor efficacy.

Androgen receptor (AR) belongs to the steroid hormone receptor group of ligand-activated transcription factors in the nuclear receptor superfamily. AR mediates the action of physiological and exogenous androgens to regulate the expression of a network of genes in target tissues that are essential for the development and maintenance of the male phenotype and reproductive function as well as the function of numerous other tissues in both males and females. AR is ubiquitously expressed throughout the body. AR is a modular protein that comprises an N-terminal domain (NTD) that contains all of its transcriptional activity, a DNA-binding domain, a flexible hinge region, and a C-terminal ligand-binding domain (LBD). All clinically approved hormonal therapies target the AR LBD, either directly with antiandrogens and selective AR modulators or indirectly by reducing levels of androgens. Pathological conditions related to AR dysfunction involve altered levels of androgens and structural alterations in the AR. These include mutations, polymorphisms in the polyglutamine tract of the NTD, and alternative splicing of AR to yield constitutively active receptors. From the extensive list of AR-related diseases, herein we describe prostate cancer, androgen-insensitivity syndrome, polycystic ovary syndrome, breast cancer, and a few more pathological conditions in more detail.

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Steroid hormone receptors are ligand-activated transcription factors, acting as master regulators of gene expression. Steroid receptors mediate formation of large protein complexes by recruiting coregulatory proteins and transcriptional machinery to specific genomic regions. Unlike the structured ligand-binding or DNA-binding domains, the N-terminal domain (NTD), where many of these protein-protein interactions occur, contains extended regions of intrinsic disorder. Interactions in the NTD and allosteric binding elsewhere induce temporary and reversible changes in the NTD structure, substantially influencing the repertoire of potential binding partners. This structural plasticity is key for the steroid receptors to coordinate intra- and intercellular signals into a tissue specific response. Designing small molecule inhibitors against intrinsically disordered proteins (IDP) in general has proven difficult as structural information is limited. While some progress has been made in this area, only recently has any molecule targeting IDPs progressed beyond the preclinical stage. Here we summarize the discovery and development of sintokamides, niphatenones, and EPI compounds which target the intrinsically disordered NTD of the androgen receptor. These are the first drugs to target the NTD of any steroid receptor, and EPI-506 and EPI-7386 remain the only compounds that bind to an IDP to have been tested in clinical trials.

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Intrinsically disordered proteins (IDPs) and regions (IDRs) lack stable three-dimensional structure making drug discovery challenging. A validated therapeutic target for diseases such as prostate cancer is the androgen receptor (AR) which has a disordered amino-terminal domain (NTD) that contains all of its transcriptional activity. Drug discovery against the AR-NTD is of intense interest as a potential treatment for disease such as advanced prostate cancer that is driven by truncated constitutively active splice variants of AR that lack the C-terminal ligand-binding domain (LBD).: This article presents an overview of the relevance of AR and its intrinsically disordered NTD as a drug target. AR structure and approaches to blocking AR transcriptional activity are discussed. The discovery of small molecules, including the libraries used, proven binders to the AR-NTD, and site of interaction of these small molecules in the AR-NTD are presented along with discussion of the Phase I clinical trial.: The lack of drugs in the clinic that directly bind IDPs/IDRs reflects the difficulty of targeting these proteins and obtaining specificity. However, it may also point to an inappropriateness of too closely borrowing concepts and resources from drug discovery to folded proteins.