Sintokamide A Is a Novel Antagonist of Androgen Receptor That Uniquely Binds Activation Function-1 in Its Amino-terminal Domain

Abstract

Androgen receptor (AR) is a validated drug target for all stages of prostate cancer including metastatic castration-resistant prostate cancer (CRPC). All current hormone therapies for CRPC target the C-terminal ligand-binding domain of AR and ultimately all fail with resumed AR transcriptional activity. Within the AR N-terminal domain (NTD) is activation function-1 (AF-1) that is essential for AR transcriptional activity. Inhibitors of AR AF-1 would potentially block most AR mechanisms of resistance including constitutively active AR splice variants that lack the ligand-binding domain. Here we provide evidence that sintokamide A (SINT1) binds AR AF-1 region to specifically inhibit transactivation of AR NTD. Consistent with SINT1 targeting AR AF-1, it attenuated transcriptional activities of both full-length AR and constitutively active AR splice variants, which correlated with inhibition of growth of enzalutamide-resistant prostate cancer cells expressing AR splice variants. In vivo, SINT1 caused regression of CRPC xenografts and reduced expression of prostate-specific antigen, a gene transcriptionally regulated by AR. Inhibition of AR activity by SINT1 was additive to EPI-002, a known AR AF-1 inhibitor that is in clinical trials (NCT02606123). This implies that SINT1 binds to a site on AF-1 that is unique from EPI. Consistent with this suggestion, these two compounds showed differences in blocking AR interaction with STAT3. This work provides evidence that the intrinsically disordered NTD of AR is druggable and that SINT1 analogs may provide a novel scaffold for drug development for the treatment of prostate cancer or other diseases of the AR axis.

Combined N‐terminal androgen receptor and autophagy inhibition increases the antitumor effect in enzalutamide sensitive and enzalutamide resistant prostate cancer cells

“…Further research needs to focus on maximizing the anticancer effect of NTD‐ binding AR‐targeting compounds. Novel or modified compounds might enhance the promising effects of NTD AR inhibition . In a second step, optimal combinations of compounds or sequences need to be assessed.…”

“…Novel or modified compounds might enhance the promising effects of NTD AR inhibition. 35 In a second step, optimal combinations of compounds or sequences need to be assessed. A combination of AR inhibition with a co-targeting of alternative pathways has the potential to improve advanced prostate cancer treatment.…”

Non-Genomic Actions of the Androgen Receptor in Prostate Cancer

“…To date, two families of compounds have been discovered that bind to the AF1 region of the AR NTD and inhibit transactivation. These are sintokamide A and EPI compounds (69, 70). Both of these agents demonstrate promising results for inducing the regression of enzalutamide-resistant CRPC xenografts in mice (70, 71).…”

“…These are sintokamide A and EPI compounds (69, 70). Both of these agents demonstrate promising results for inducing the regression of enzalutamide-resistant CRPC xenografts in mice (70, 71). Sintokamide A and EPI have different mechanisms of action by binding unique sites on AF1.…”

“…Nuclear magnetic resonance spectroscopy analyses demonstrate that EPI-001 binds to a specific pocket formed by Tau5 of AR NTD with contact to three regions, residues 353–364, 397–407, and 433–466 (72), that overlaps with the residues 371–381 containing the polyproline sequence required to interact with Src. EPI blocks protein–protein interactions on the AR NTD, between AF1 and CREB-binding protein and TFIIF of the transcription preinitiation complex, and between full-length AR and STAT3, which binds within residues 234–558 of the AR NTD (7, 69, 70). EPI-506 is the first AR NTD antagonist to be tested in clinical trials for metastatic CRPC (NCT02606123).…”