Discovery of drugs that directly target the intrinsically disordered region of the androgen receptor

Abstract

Introduction: 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).Areas covered: 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.Expert opinion: 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.

Keywords: Androgen receptor; EPI-002; N-terminal domain inhibitor; clinical trial; drug discovery; intrinsically disordered protein; mechanism of action; prostate cancer; ralaniten; sintokamides.

Fig 1.

Structure of the androgen receptor. N-terminal domain (NTD) is intrinsically disordered. DNA-binding domain (DBD) is an ordered and folded domain. Hinge region is disordered. Ligand-binding domain (LBD) is an ordered and folded domain.

Fig 2.

Ronn plot of the androgen receptor with the EPI-002 binding site (EBS1–3). A. The probability of protein disorder across the amino acid residues of androgen receptor. A probability score below 0.5 is considered ordered and above 0.5 as disordered. Transactivation units (Tau) 1 and 5 are shown within the N-terminal domain. The EPI-002 binding site is depicted with the regions of interaction shown in light red. DBD, DNA-binding domain; Hinge region in purple; LBD, ligand-binding domain. B. Regions of posttranslational modification and interactions with some other proteins on the N-terminal domain in context to EPI-002 binding site. Within the EPI-binding site are regions for phosphorylation (P) and sumolyation (S). Protein interactions shown include chaperones hsp40 and hsp90; AF-2, activation function-2 in the AR LBD for N/C interaction; RAP74 of the basal transcriptional machinery; p300/CBP, BRD4, TAB2, CHIP, MAGE-11, BAG1L, Gli, and STAT3. C. Residues in the three EPI-002 binding sites (EBS) and the flanking residues. EBS1, EPI-002 binding site 1; EBS2, EPI-002 binding site 2; EBS3, EPI-002 binding site 3.

Fig 3.

A model proposing that direct binding of EPI to Tau-5 in the AR-NTD creates a conformation that prevents interactions with CBP and the basal transcriptional machinery (BTM).