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Interest in developing androgen receptor (AR) inhibitors with novel mechanism of action is on the rise since the commercial anti-androgens (including recently approved drug, Enzalutamide) face clinical limitations. The current therapies fail over a period of time because they all target mutation-prone hormone binding pocket on AR to which the receptor has already developed effective resistance mechanisms. Hence, there is a pressing need for novel therapeutics that inhibit the AR through alternative modes of action. Recent studies have identified a novel binding pocket on the surface of AR called binding function-3 (BF3) that is critical for the AR transcriptional activity. In order to develop promising drug-like candidates, we performed structural optimization of our lead AR BF3 inhibitor, followed by a series of experimental validation of the synthetic analogues. As a result, we have discovered a chemical series of quinoline as new lead BF3 inhibitors. One of the most potent inhibitors identified, VPC-13566, demonstrated an IC50 of 0.071μM in AR eGFP transcriptional assay. Confirming it as a true BF3 binder, VPC-13566 displaced Bag-1L peptide, a co-regulator protein that binds to AR BF3 pocket. Additionally, the Biolayer Interferometry assay detected direct reversible interactions between the AR ligand binding domain and the inhibitor. To confirm VPC-13566 binding to the BF3 pocket, a mutagenesis study was performed. Results with mutants F673E, E837A and N833W confirmed that compound did not show any binding to the protein compared with the wild type in a BLI assay. VPC-13566 demonstrated strong anti-proliferative activity against LNCaP and Enzalutamide-resistant prostate cancer cell lines (MR49F) whereas it did not affect the growth of AR independent PC3 cell line. It also reduced prostate specific antigen (PSA) in both LNCaP and MR49F and reduced expression of AR target genes, PSA and TMPRSS2. These findings suggest that VPC-13566 exhibits AR BF3 specific mechanism of action. Furthermore, VPC-13566 significantly reduced AR-dependent growth of tumors and PSA levels in LNCaP and MR49F xenograft models. Based on these outcomes, it can be anticipated that such drug prototypes will lay a foundation for the development of alternative or supplementary small-molecule therapies capable of combating PCa even in its drug resistant forms. Because the emergence of castration resistance is the lethal end stage of the disease, we anticipate that the proposed research will eventually have a substantial impact on patient survival. Citation Format: Ravi Shashi Nayana Munuganti, Mohamed DH Hassona, Eric Leblanc, Emma T. Guns, Paul S. Rennie, Artem Cherkasov. Targeting binding function-3 site on the androgen receptor to treat Enzalutamide-resistant prostate cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3654. doi:10.1158/1538-7445.AM2015-3654

The androgen receptor (AR) is a hormone-activated transcription factor implicated in the development and progression of prostate cancer. The AR contains an N-terminal domain (NTD), followed by DNA binding (DBD) and ligand-binding (LBD) domains. Upon binding of androgens to the LBD, the AR moves into the nucleus where it interacts with target genes via the conserved AR-DBD. Prostate cancer treatment involves the use of small-molecules to block androgen production or to compete with androgens for binding to the AR-LBD. Drug-resistance occurs when LBD mutations render small-molecule treatment ineffective or when constitutively active AR splice variants, lacking the LBD, become overexpressed. We recently identified a surface pocket on the AR-DBD that may serve as an alternative drug-target site. The DBD-specific, small molecule compounds presented here block transcriptional activity of full-length and splice variant AR forms across multiple cell lines and show no cross-reactivity with related steroid nuclear receptors. The inhibition is lost when residues in the binding pocket are mutated or when the AR-DBD is replaced with a different DNA binding domain. Biophysical techniques were used to explore the interaction of these compounds with the purified, recombinant DBD protein. Gene expression analysis demonstrated the down-regulation of AR target genes following compound treatment, without any obvious expression signature associated with toxicity. The compounds affected DNA binding in vitro and also blocked interactions of the AR with chromatin inside the nucleus. Finally, the lead compound suppressed the growth of LNCaP tumor xenografts and was accompanied by down-regulation of AR target genes, such as PSA. These results suggest a mechanism whereby our developed inhibitory compounds interfere with DNA binding by the AR, potentially leading to effective treatment of advanced, castration-resistant prostate cancers. Citation Format: Kush Dalal, Mani Roshan-Moniri, Aishwariya Sharma, Huifang Li, Fuqiang Ban, Mohamed D. Hassona, Michael Hsing, Kriti Singh, Eric LeBlanc, Scott Dehm, Emma Tomlinson Guns, Artem Cherkasov, Paul S. Rennie. Selectively targeting the DNA-binding domain of the androgen receptor as a prospective therapy for prostate cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1778. doi:10.1158/1538-7445.AM2015-1778

The androgen receptor (AR) is one of the most validated therapeutic targets in prostate cancer (PCa). Conventional anti-androgens lose effectiveness as cancer therapeutics because anti-androgen resistance usually develops after long term treatment. The challenge is that the current therapeutics bind to the same site of the AR (hormone binding pocket) and act via the same mode, to which the receptor has already developed effective resistance mechanisms. Hence, there is a pressing need for novel therapeutics that inhibit the AR through novel, alternative modes of action. Recent studies have identified a novel binding pocket on the surface of AR called binding function 3 (BF3) that is important for the AR transcriptional activity. In order to identify compounds that specifically bind to BF3 site and inhibit the AR, we conducted a systematic in silico screen (that included large-scale docking, in-site rescoring, and consensus voting procedures) followed by experimental validation of the identified hit molecules. As a result, we have discovered a novel chemical series of indoles as lead BF3 inhibitors. One of the most potent inhibitors identified, VPC-13163, demonstrated an IC50 of 0.31µM in AR eGFP transcriptional assay. Confirming it as a true BF3 binder, VPC-13163 neither displaced the co-activator from an alternative coactivator binding site, activation function 2 site, nor androgen from the hormone binding pocket. Additionally, the Biolayer Interferometry assay detected direct reversible interactions between the AR ligand binding domain and the inhibitor. VPC-13163 demonstrated strong anti-proliferative activity against LNCaP and Enzalutamide-resistant prostate cancer cell lines (MR49F) whereas it did not affect the growth of AR independent PC3 cell line. It also inhibits prostate specific antigen (PSA) in both LNCaP and MR49F and reduces expression of AR target genes, PSA and TMPRSS2. These findings suggest that VPC-13163 exhibits AR BF3 specific mechanism of action. Furthermore, VPC-13163 reduces AR-dependent growth of xenograft tumors in vivo. Based on these outcomes, it can be anticipated that such drug prototypes will lay a foundation for the development of alternative or supplementary small-molecule therapies capable of combating PCa even in its drug resistant forms. Because the emergence of castration resistance is the lethal end stage of the disease, we anticipate that the proposed research will eventually have a substantial impact on patient survival. Citation Format: Ravi Shashi Nayana Munuganti, Mohamed DH Hassona, Eric Leblanc, Fuqiang Ban, Emma T. Guns, Paul S. Rennie, Artem Cherkasov. The development of anti-androgens with a novel mechanism of action for treatment of castration-resistant prostate cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2508. doi:10.1158/1538-7445.AM2014-2508

The human androgen receptor (AR) represents a well-established drug target for prostate cancer treatment. All clinically used antiandrogens, such as Bicalutamide and Enzalutamide, possess similar chemical structures and bind to the AR in the androgen binding site (ABS). These AR antagonists are initially effective, but resistance invariably developed in the castration-resistant prostate cancer (CRPC). Even in late-stage patients, the AR activity is still persistent in the progression of the disease. Thus, there is a continuing need for novel chemical classes of AR antagonists that could overcome the drug resistance. In this study, we performed a virtual screening against the AR ABS, and identified novel AR antagonists with chemical structures completely different from existing antiandrogens. A 10-(4-Hydroxybenzylidene)anthracen-9(10H)-one compound was discovered that not only effectively inhibits AR transcription and strongly displaces androgen in the ABS, but induces AR degradation in prostate cancer cells. Starting from the initial hit compound, a series of 10-benzylidene-10H-anthracen-9-ones were synthesized and evaluated by in vitro assays. A close analogue with enhanced potency was identified as a lead compound, which demonstrated strong androgen displacement potency, effective AR transcriptional inhibition, and a profound ability to cause degradation of AR. Notably, it exhibited significant activity against MDV3100-resistant prostate cancer cells. This lead compound was evaluated in both non-castrated and castration-resistant LNCaP xenograft models, and demonstrated significant effect on inhibiting tumor growth and reducing prostate specific antigen (PSA). This series of compounds were predicted to adopt a different binding mode from current antiandrogens, which may circumvent the resistance rendered by identified gain-of-function mutations. Further development of this series of AR antagonists may have great therapeutic potential in CRPC. Citation Format: Huifang Li, Mohamed DH Hassona, Nathan A. Lack, Peter Axerio-Cilies, Eric Leblanc, Emma T. Guns, Paul S. Rennie, Artem Cherkasov. Discovery of a novel series of androgen receptor antagonists with potential therapeutic applications in castration-resistant prostate cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1632. doi:10.1158/1538-7445.AM2014-1632

In the development and progression of prostate cancer, the androgen receptor (AR) plays a critical role and its hormone/ligand-binding site is the primary therapeutic target for all antiandrogens (eg Bicalutamide, MDV3100) currently used to treat advanced, metastatic forms of this disease. While treatment with these AR inhibitors initially suppresses prostate tumor growth, resistance to these drugs invariably emerges. Accordingly, there exists a clinical need for the development of new AR antagonists with different chemical structures and different mechanisms of action for targeting and inhibiting AR transactivation. To this end, we have focused on a surface pocket on the AR called Binding Function 3 (BF3). Applying iterative in silico screening to millions of chemicals in the ZINC database, coupled with biological screening for AR binding and activity, we identified several compounds that can bind directly to the BF3 pocket and inhibit AR transactivation. Cell-based screening assays for inhibition of a fluorescent ARE-reporter and PSA expression, together with Biolayer Interferometry analysis for measuring binding to the AR, revealed several chemically distinct BF-3 binders that showed significant IC50 inhibitions of the AR in the nano-molar to low micro-molar range. Validation for binding to the BF3 site was performed by x-ray crystallography. Furthermore, none of these compounds were able to displace DHT from the hormone binding site even at concentrations more than 10-fold higher than their IC50 values. In addition to wild type LNCaP prostate cancer cells, these BF3 binders effectively inhibited cell proliferation (MTS assay) and PSA expression in derived MDV3100-resistant prostate cancer cell lines, but had no effect on cell viability of AR-negative prostate cancer cell lines. In summary, we have identified an entirely new class of anti-androgens that bind to the BF3 surface site of the AR and inhibit its transactivation by a mechanism which does not involve binding to its hormone/ligand binding site and thereby bypasses the treatment resistance seen with conventional antiandrogens. (Supported by funds from the PCF, Canada Safeway, and CIHR). Citation Format: Ravi Munuganti, Fuqiang Ban, Huifang Li, Eric LeBlanc, Peter Axerio, Kate Frewin, Emma Tomlinson Guns, Artem Cherkasov, Paul S. Rennie. New potent inhibitors of the androgen receptor that target its BF3 surface binding site. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1070. doi:10.1158/1538-7445.AM2013-1070