Amy H. Tien

An androgen receptor N-terminal domain antagonist for treating prostate cancer

Hormone therapies for advanced prostate cancer target the androgen receptor (AR) ligand-binding domain (LBD), but these ultimately fail and the disease progresses to lethal castration-resistant prostate cancer (CRPC). The mechanisms that drive CRPC are incompletely understood, but may involve constitutively active AR splice variants that lack the LBD. The AR N-terminal domain (NTD) is essential for AR activity, but targeting this domain with small-molecule inhibitors is complicated by its intrinsic disorder. Here we investigated EPI-001, a small-molecule antagonist of AR NTD that inhibits protein-protein interactions necessary for AR transcriptional activity. We found that EPI analogs covalently bound the NTD to block transcriptional activity of AR and its splice variants and reduced the growth of CRPC xenografts. These findings suggest that the development of small-molecule inhibitors that bind covalently to intrinsically disordered proteins is a promising strategy for development of specific and effective anticancer agents.

Altered Immunity Accompanies Disease Progression in a Mouse Model of Prostate Dysplasia

Increasing evidence suggests that altered immune function accompanies, and indeed may facilitate, cancer progression. In this study, we sought to determine the nature of, and cellular mechanisms underlying, changes in immune status during disease progression in a transgenic mouse model of prostate dysplasia. Immune cells in the tumor microenvironment, as well as in the secondary lymphoid tissues, displayed altered phenotypes. Although evidence of antitumor immunity was detected, there was a paradoxical decrease in the ability of T cells to proliferate in vitro at later stages of disease progression. Detailed analysis of the draining lumbar lymph nodes revealed an increased frequency and number of CD4(+)CD25(+) T cells and an enhanced production of inhibitory cytokines, which correlated with impaired T-cell function. Functional studies confirmed a role for CD4(+)CD25(+) regulatory T cells in suppressing T-cell proliferation as well as regulating the growth of transplanted prostate tumor cells. In addition, our studies show for the first time that anti-CD25 antibody treatment reduces, but does not prevent, tumor growth in a transgenic mouse model of prostate dysplasia. Taken together, this work provides compelling evidence that prostate tumor progression is accompanied by altered immune function and, moreover, that regulatory T cells play an important role in this process. These studies thus provide the impetus for development of specific and effective strategies to deplete regulatory T cells, or suppress their function, as an alternative or adjunct strategy for reducing tumor growth.

Niphatenones, glycerol ethers from the sponge Niphates digitalis block androgen receptor transcriptional activity in prostate cancer cells: structure elucidation, synthesis, and biological activity.

Extracts of the marine sponge Niphates digitalis collected in Dominica showed strong activity in a cell-based assay designed to detect antagonists of the androgen receptor (AR) that could act as lead compounds for the development of a new class of drugs to treat castration recurrent prostate cancer (CRPC). Assay-guided fractionation showed that niphatenones A (3) and B (4), two new glycerol ether lipids, were the active components of the extracts. The structures of 3 and 4 were elucidated by analysis of NMR and MS data and confimed via total synthesis. Biological evaluation of synthetic analogues of the niphatenones has shown that the enantiomers 7 and 8 are more potent than the natural products in the screening assay and defined preliminary SAR for the new AR antagonist pharmacophore, including the finding that the Michael acceptor enone functionality is not required for activity. Niphatenone B (4) and its enantiomer 8 blocked androgen-induced proliferation of LNCaP prostate cancer cells but had no effect on the proliferation of PC3 prostate cancer cells that do not express functional AR, consistent with activity as AR antagonists. Use of the propargyl ether 44 and Click chemistry showed that niphatenone B binds covalently to the activation function-1 (AF1) region of the AR N-terminus domain (NTD).

Osteoblast-Derived Factors Induce an Expression Signature that Identifies Prostate Cancer Metastasis and Hormonal Progression

Identification of gene expression signatures associated with metastases provides a tool to discern mechanisms and potential therapeutic targets and may lead toward a molecular classification system in pathology. Prostate cancer (CaP) frequently metastasizes to the bone to form osteoblastic lesions. Correlative clinical data and in vitro evidence have led to the hypothesis that osteoblast-derived factors promote hormonal progression of CaP cells. Here, the gene expression signature of CaP exposed to osteoblast-derived factors was identified. This signature included known androgen-regulated genes, oncogenes, tumor suppressors, and genes whose products are involved in apoptosis and cell cycle. A comparative functional genomic approach involved the application of this responsive gene expression signature to clinical samples of human CaP, melanomas, and oral cancers. Cluster analysis revealed that this gene expression signature had specificity for CaP and could resolve clinical specimens according to stage (benign, localized, and metastatic) and androgen sensitivity with an accuracy of 100% and 80%, respectively. Together, these results suggest that factors derived from osteoblasts induce a more advanced phenotype of CaP and promotes hormonal progression.

Characterization of Niphatenones that Inhibit Androgen Receptor N-Terminal Domain

Androgen ablation therapy causes a temporary reduction in tumor burden in patients with advanced prostate cancer. Unfortunately the malignancy will return to form lethal castration-recurrent prostate cancer (CRPC). The androgen receptor (AR) remains transcriptionally active in CRPC in spite of castrate levels of androgens in the blood. AR transcriptional activity resides in its N-terminal domain (NTD). Possible mechanisms of continued AR transcriptional activity may include, at least in part, expression of constitutively active splice variants of AR that lack the C-terminal ligand-binding domain (LBD). Current therapies that target the AR LBD, would not be effective against these AR variants. Currently no drugs are clinically available that target the AR NTD which should be effective against these AR variants as well as full-length AR. Niphatenones were originally isolated and identified in active extracts from Niphates digitalis marine sponge. Here we begin to characterize the mechanism of niphatenones in blocking AR transcriptional activity. Both enantiomers had similar IC50 values of 6 µM for inhibiting the full-length AR in a functional transcriptional assay. However, (S)-niphatenone had significantly better activity against the AR NTD compared to (R)-niphatenone. Consistent with niphatenones binding to and inhibiting transactivation of AR NTD, niphatenones inhibited AR splice variant. Niphatenone did not affect the transcriptional activity of the related progesterone receptor, but slightly decreased glucocorticoid receptor (GR) activity and covalently bound to GR activation function-1 (AF-1) region. Niphatenone blocked N/C interactions of AR without altering either AR protein levels or its intracellular localization in response to androgen. Alkylation with glutathione suggests that niphatenones are not a feasible scaffold for further drug development.

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

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.

Spongian Diterpenoids Inhibit Androgen Receptor Activity

Androgen receptor is a ligand-activated transcription factor and a validated drug target for all stages of prostate cancer. Antiandrogens compete with physiologic ligands for androgen receptor ligand-binding domain (LBD). High-throughput screening of a marine natural product library for small molecules that inhibit androgen receptor transcriptional activity yielded the furanoditerpenoid spongia-13(16),-14-dien-19-oic acid, designated terpene 1 (T1). Characterization of T1 and the structurally related semisynthetic analogues (T2 and T3) revealed that these diterpenoids have antiandrogen properties that include inhibition of both androgen-dependent proliferation and androgen receptor transcriptional activity by a mechanism that involved competing with androgen for androgen receptor LBD and blocking essential N/C interactions required for androgen-induced androgen receptor transcriptional activity. Structure–activity relationship analyses revealed some chemical features of T1 that are associated with activity and yielded T3 as the most potent analogue. In vivo, T3 significantly reduced the weight of seminal vesicles, which are an androgen-dependent tissue, thereby confirming the on-target activity of T3. The ability to create analogues of diterpenoids that have varying antiandrogen activity represents a novel class of chemical compounds for the analysis of androgen receptor ligand-binding properties and therapeutic development. Mol Cancer Ther; 12(5); 621–31. ©2013 AACR.

Androgen-Responsive Gene Expression in Prostate Cancer Progression

Benign and cancerous prostate tissue is dependent upon androgens. Androgen ablation causes prostate tissue to undergo apoptosis which thereby provides the rationale of castration as a systemic therapy for advanced prostate cancer. The full-length androgen receptor is a ligand-activated transcription factor that regulates the expression of genes required for growth, function, and survival of prostate cells in response to androgen. Androgen binds to the androgen receptor which then translocates to the nucleus to bind to androgen response elements on target genes termed “androgen-responsive genes” (ARGs) to regulate their transcription and levels of expression. Identification and characterization of ARGs may provide an understanding of androgen receptor signaling, resistance mechanisms to current hormonal therapies, and reveal biomarkers for patient selection and sequential application of current and new therapies targeting the androgen axis. This review addresses differential expression of ARGs following androgen ablation treatment during progression of advanced prostate cancer.

Abstract B14: Developing small-molecule inhibitors to the androgen receptor N-terminus domain for the treatment of advanced prostate cancer

Abstract Androgen ablation therapy remains the gold standard for the treatment of advanced prostate cancer, but unfortunately, it is not curative and eventually the disease will return as lethal castration-resistant prostate cancer (CRPC). There is evidence supporting the concept that development of CRPC is causally related to continued transactivation of androgen receptor (AR). Suspected mechanisms for continued AR activity in spite of castrate levels of androgen include: amplification or overexpression of AR; gain-of-function mutations allowing AR to be activated by steroids or antiandrogens; ligand-independent activation by growth factors, cytokines, or kinases; overexpression of AR coactivators; intracrine signaling by increased intratumoral androgens; and/or expression of constitutively active splice variants of AR that lack the C-terminal ligandbinding domain (LBD). All current therapies that target the AR are dependent on the presence of its C-terminal LBD. However, it is the N-terminal domain (NTD) of the AR that is the “Achilles Heel” of AR activity, with activation function-1 (AF-1) being essential for AR activity regardless of androgen. Our efforts have been focused upon developing drugs to the AR NTD and have yielded EPI-001 a small molecule, sintokamide peptides, and decoys to the AR NTD. Of these, EPI-001 is the best characterized as previously shown to inhibit essential protein-protein interactions that are required for AR transcriptional activity. EPI-001 and its analogues (generally referred to as “EPI”) have great promise for clinical development based upon its unique mechanism of action, specificity, low toxicity, and cytoreductive antitumor activity. EPI blocked transcriptional activity of full-length and AR variants as well as specifically inhibited AR-dependent cell proliferation. EPI directly and specifically interacted with AF1 and did not interact with denatured AF1 as shown using in vitro binding assays. Specific and direct interaction of EPI with the endogenous AR occurred in living cells as shown using click chemistry. EPI-001 had 86% oral bioavailability, a half-life of 3.4 hours, and plasma levels at the effective concentration of 10 ug/ml were achieved with oral dosing. Consistent with excellent oral bioavailability, oral dosing of EPI inhibited VCaP tumor growth in castrated animals. VCaP human prostate cancer cells express an abundance of full-length AR as well as constitutively active AR splice variant lacking LBD. Evidence for EPI targeting the AR transcriptional program in vivo, was provided by reduced transcripts of UBE2C, CDC20, cyclinA2, and AKT1 in harvested VCaP xenografts from animals treated orally with EPI. In conclusion, EPI is an antagonist of AR NTD that blocks the activity of AR, including constitutively active AR splice variants, by a mechanism that involves direct interaction with the NTD. Oral dosing of EPI has antitumor activity in prostate cancer xenografts that express AR variant. Together these data support the clinical development of EPI for the treatment of CRPC. Funding: NIH (2R01 CA105304) and US Army Medical Research and Materiel Command Prostate Cancer Research Program (PC100761). Note: This abstract was not presented at the conference because the presenter was unable to attend. Citation Format: Marianne D. Sadar, Jae-Kyung Myung, Iain McEwan, Stephen Plymate, Raymond J. Andersen, Carmen A. Banuelos, Nasrin R. Mawji, Jun Wang, Javier Garcia Fernandez, Amy Tien, Iran Tavakoli, Yu Chi Yang, Simon Haile. Developing small-molecule inhibitors to the androgen receptor N-terminus domain for the treatment of advanced prostate cancer [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr B14.