Inhibition of Androgen Receptor Nuclear Localization and Castration-Resistant Prostate Tumor Growth by Pyrroloimidazole-based Small Molecules

Deciphering the roles of aryl hydrocarbon receptor (AHR) in regulating carcinogenesis

Aryl hydrocarbon receptor (AHR) is a ligand-dependent receptor that belongs to the superfamily of basic helix-loop-helix (bHLH) transcription factors. The activation of the canonical AHR signaling pathway is known to induce the expression of cytochrome P450 enzymes, facilitating the detoxification metabolism in the human body. Additionally, AHR could interact with various signaling pathways such as epidermal growth factor receptor (EGFR), signal transducer and activator of transcription 3 (STAT3), hypoxia-inducible factor-1α (HIF-1α), nuclear factor ekappa B (NF-κβ), estrogen receptor (ER), and androgen receptor (AR) signaling pathways. Over the past 30 years, several studies have reported that various chemical, physical, or biological agents, such as tobacco, hydrocarbon compounds, industrial and agricultural chemical wastes, drugs, UV, viruses, and other toxins, could affect AHR expression or activity, promoting cancer development. Thus, it is valuable to overview how these factors regulate AHR-mediated carcinogenesis. Current findings have reported that many compounds could act as AHR ligands to drive the expressions of AHR-target genes, such as CYP1A1, CYP1B1, MMPs, and AXL, and other targets that exert a pro-proliferation or anti-apoptotic effect, like XIAP. Furthermore, some other physical and chemical agents, such as UV and 3-methylcholanthrene, could promote AHR signaling activities, increasing the signaling activities of a few oncogenic pathways, such as the phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathways. Understanding how various factors regulate AHR-mediated carcinogenesis processes helps clinicians and scientists plan personalized therapeutic strategies to improve anti-cancer treatment efficacy. As many studies that have reported the roles of AHR in regulating carcinogenesis are preclinical or observational clinical studies that did not explore the detailed mechanisms of how different chemical, physical, or biological agents promote AHR-mediated carcinogenesis processes, future studies should focus on conducting large-scale and functional studies to unravel the underlying mechanism of how AHR interacts with different factors in regulating carcinogenesis processes.

An integrated in vitro/in silico approach to assess the anti-androgenic potency of isobavachin

Isobavachin is a dietary flavanone with multiple biological activities. Our previous research has confirmed the estrogenicity of isobavachin, and this work aims to assess the anti-androgenic potency of isobavachin by an integrated in vitro and in silico approach. Isobavachin can limit the proliferation of prostate cancer cells by inducing a distinct G1 cell-cycle arrest. In addition, isobavachin also significantly represses the transcription of androgen receptor (AR)-downstream targets such as prostate specific antigen. Mechanistically, we demonstrated that isobavachin can disrupt the nuclear translocation of AR and promote its proteasomal degradation. The results of computer simulations showed that isobavachin can stably bind to AR, and the amino acid residue Gln711 may play a critical role in AR binding of both AR agonists and antagonists. In conclusion, this work has identified isobavachin as a novel AR antagonist.

Antiandrogen-Equipped Histone Deacetylase Inhibitors Selectively Inhibit Androgen Receptor (AR) and AR-Splice Variant (AR-SV) in Castration-Resistant Prostate Cancer (CRPC)

Background: Epigenetic modification influences androgen receptor (AR) activation, often resulting in prostate cancer (PCa) development and progression. Silencing histone-modifying enzymes (histone deacetylases-HDACs) either genetically or pharmacologically suppresses PCa proliferation in preclinical models of PCa; however, results from clinical studies were not encouraging. Similarly, PCa patients eventually become resistant to androgen ablation therapy (ADT). Our goal is to develop dual-acting small molecules comprising antiandrogen and HDAC-inhibiting moieties that may overcome the resistance of ADT and effectively suppress the growth of castration-resistant prostate cancer (CRPC). Methods: Several rationally designed antiandrogen-equipped HDAC inhibitors (HDACi) were synthesized, and their efficacy on CRPC growth was examined both in vitro and in vivo. Results: While screening our newly developed small molecules, we observed that SBI-46 significantly inhibited the proliferation of AR+ CRPC cells but not AR- CRPC and normal immortalized prostate epithelial cells (RWPE1) or normal kidney cells (HEK-293 and VERO). Molecular analysis confirmed that SBI-46 downregulated the expressions of both AR+ and AR-splice variants (AR-SVs) in CRPC cells. Further studies revealed the downregulation of AR downstream (PSA) events in CRPC cells. The oral administration of SBI-46 abrogated the growth of C4-2B and 22Rv1 CRPC xenograft tumors that express AR or both AR and AR-SV in xenotransplanted nude mice models. Further, immunohistochemical analysis confirmed that SBI-46 inhibits AR signaling in xenografted tumor tissues. Conclusion: These results demonstrate that SBI-46 is a potent agent that inhibits preclinical models of CRPC by downregulating the expressions of both AR and AR-SV. Furthermore, these results suggest that SBI-46 may be a potent compound for treating CRPC.

Targeted Degradation of Androgen Receptor by VNPP433-3β in Castration-Resistant Prostate Cancer Cells Implicates Interaction with E3 Ligase MDM2 Resulting in Ubiquitin-Proteasomal Degradation

Targeted protein degradation is a fast-evolving therapeutic strategy to target even the traditionally undruggable target proteins. Contrary to the traditional small-molecule inhibitors of enzyme or receptor antagonists that bind the active site pockets in the target protein, molecular glue degraders facilitate interaction of target proteins with E3 ubiquitin ligases by stabilizing the ternary complex and induce physical proximity, thereby triggering ubiquitination and subsequent proteasomal degradation. AR plays a key role in all stages of prostate cancer. It is activated by the binding of androgenic hormones and transcriptionally regulates multiple genes including the ones that regulate cell cycle. Using HiBiT CRISPR cell line, biochemical methods, and RNA sequencing, we report the potential role of VNPP433-3β, the next generation galeterone analog as molecular glue that brings together AR, the key driver of prostate cancer and MDM2, an E3 ubiquitin ligase leading to ubiquitination and subsequent degradation of f-AR and AR-V7 in prostate cancer cells.

Di-, tetra-, and perhydropyrrolo[1,2-a]imidazoles: The Methods of Synthesis and Some Aspects of Application

The review summarizes and systematizes the literature data on the synthesis and some aspects of application of pyrrolo[1,2-a]imidazoles. Synthetic approaches are grouped according to the degree of saturation of the product pyrroloimidazole ring. The bibliography of the review includes 110 sources over the last 15 years.

Modulating the Activity of Androgen Receptor for Treating Breast Cancer

The androgen receptor (AR) is a steroid hormone receptor widely detected in breast cancer. Evidence suggests that the AR might be a tumor suppressor in estrogen receptor alpha-positive (ERα+ve) breast cancer but a tumor promoter in estrogen receptor alpha-negative (ERα-ve) breast cancer. Modulating AR activity could be a potential strategy for treating breast cancer. For ERα+ve breast cancer, activation of the AR had been demonstrated to suppress the disease. In contrast, for ERα-ve breast cancer, blocking the AR could confer better prognosis to patients. These studies support the feasibility of utilizing AR modulators as anti-cancer drugs for different subtypes of breast cancer patients. Nevertheless, several issues still need to be addressed, such as the lack of standardization in the determination of AR positivity and the presence of AR splice variants. In future, the inclusion of the AR status in the breast cancer report at the time of diagnosis might help improve disease classification and treatment decision, thereby providing additional treatment strategies for breast cancer.

MYB interacts with androgen receptor, sustains its ligand-independent activation and promotes castration resistance in prostate cancer

BACKGROUND

Aberrant activation of androgen receptor signalling following castration therapy is a common clinical observation in prostate cancer (PCa). Earlier, we demonstrated the role of MYB overexpression in androgen-depletion resistance and PCa aggressiveness. Here, we investigated MYB-androgen receptor (AR) crosstalk and its functional significance.

METHODS

Interaction and co-localization of MYB and AR were examined by co-immunoprecipitation and immunofluorescence analyses, respectively. Protein levels were measured by immunoblot analysis and enzyme-linked immunosorbent assay. The role of MYB in ligand-independent AR transcriptional activity and combinatorial gene regulation was studied by promoter-reporter and chromatin immunoprecipitation assays. The functional significance of MYB in castration resistance was determined using an orthotopic mouse model.

RESULTS

MYB and AR interact and co-localize in the PCa cells. MYB-overexpressing PCa cells retain AR in the nucleus even when cultured under androgen-deprived conditions. AR transcriptional activity is also sustained in MYB-overexpressing cells in the absence of androgens. MYB binds and promotes AR occupancy to the KLK3 promoter. MYB-overexpressing PCa cells exhibit greater tumorigenicity when implanted orthotopically and quickly regain growth following castration leading to shorter mice survival, compared to those carrying low-MYB-expressing prostate tumours.

CONCLUSIONS

Our findings reveal a novel MYB-AR crosstalk in PCa and establish its role in castration resistance.

Anti-Androgen Receptor Therapies in Prostate Cancer: A Brief Update and Perspective

Prostate cancer is a major health issue in western countries and is the second leading cause of cancer death in American men. Prostate cancer depends on the androgen receptor (AR), a transcriptional factor critical for prostate cancer growth and progression. Castration by surgery or medical treatment reduces androgen levels, resulting in prostatic atrophy and prostate cancer regression. Thus, metastatic prostate cancers are initially managed with androgen deprivation therapy. Unfortunately, prostate cancers rapidly relapse after castration therapy and progress to a disease stage called castration-resistant prostate cancer (CRPC). Currently, clinical treatment for CRPCs is focused on suppressing AR activity with antagonists like Enzalutamide or by reducing androgen production with Abiraterone. In clinical practice, these treatments fail to yield a curative benefit in CRPC patients in part due to AR gene mutations or splicing variations, resulting in AR reactivation. It is conceivable that eliminating the AR protein in prostate cancer cells is a promising solution to provide a potential curative outcome. Multiple strategies have emerged, and several potent agents that reduce AR protein levels were reported to eliminate xenograft tumor growth in preclinical models via distinct mechanisms, including proteasome-mediated degradation, heat-shock protein inhibition, AR splicing suppression, blockage of AR nuclear localization, AR N-terminal suppression. A few small chemical compounds are undergoing clinical trials combined with existing AR antagonists. AR protein elimination by enhanced protein or mRNA degradation is a realistic solution for avoiding AR reactivation during androgen deprivation therapy in prostate cancers.

Regulation and targeting of androgen receptor nuclear localization in castration-resistant prostate cancer

Nuclear localization of the androgen receptor (AR) is necessary for its activation as a transcription factor. Defining the mechanisms regulating AR nuclear localization in androgen-sensitive cells and how these mechanisms are dysregulated in castration-resistant prostate cancer (CRPC) cells is fundamentally important and clinically relevant. According to the classical model of AR intracellular trafficking, androgens induce AR nuclear import and androgen withdrawal causes AR nuclear export. The present study has led to an updated model that AR could be imported in the absence of androgens, ubiquitinated, and degraded in the nucleus. Androgen withdrawal caused nuclear AR degradation, but not export. In comparison with their parental androgen-sensitive LNCaP prostate cancer cells, castration-resistant C4-2 cells exhibited reduced nuclear AR polyubiquitination and increased nuclear AR level. We previously identified 3-(4-chlorophenyl)-6,7-dihydro-5H-pyrrolo[1,2-a]imidazole (CPPI) in a high-throughput screen for its inhibition of androgen-independent AR nuclear localization in CRPC cells. The current study shows that CPPI is a competitive AR antagonist capable of enhancing AR interaction with its E3 ligase MDM2 and degradation of AR in the nuclei of CRPC cells. Also, CPPI blocked androgen-independent AR nuclear import. Overall, these findings suggest the feasibility of targeting androgen-independent AR nuclear import and stabilization, two necessary steps leading to AR nuclear localization and activation in CRPC cells, with small molecule inhibitors.

Use of computational toxicology (CompTox) tools to predict in vivo toxicity for risk assessment

Computational Toxicology tools were used to predict toxicity for three pesticides: propyzamide (PZ), carbaryl (CB) and chlorpyrifos (CPF). The tools used included: a) ToxCast/Tox21 assays (AC₅₀ s μM: concentration 50% maximum activity); b) in vitro-to-in vivo extrapolation (IVIVE) using ToxCast/Tox21 AC₅₀s to predict administered equivalent doses (AED: mg/kg/d) to compare to known in vivo Lowest-Observed-Effect-Level (LOEL)/Benchmark Dose (BMD); c) high throughput toxicokinetics population based (HTTK-Pop) using AC₅₀s for endpoints associated with the mode of action (MOA) to predict age-adjusted AED for comparison with in vivo LOEL/BMDs. ToxCast/Tox21 active-hit-calls for each chemical were predictive of targets associated with each MOA, however, assays directly relevant to the MOAs for each chemical were limited. IVIVE AEDs were predictive of in vivo LOEL/BMD₁₀s for all three pesticides. HTTK-Pop was predictive of in vivo LOEL/BMD₁₀s for PZ and CPF but not for CB after human age adjustments 11-15 (PZ) and 6-10 (CB) or 6-10 and 11-20 (CPF) corresponding to treated rat ages (in vivo endpoints). The predictions of computational tools are useful for risk assessment to identify targets in chemical MOAs and to support in vivo endpoints. Data can also aid is decisions about the need for further studies.

A Novel Small Molecule Targets Androgen Receptor and Its Splice Variants in Castration-Resistant Prostate Cancer

Reactivation of androgen receptor (AR) appears to be the major mechanism driving the resistance of castration-resistant prostate cancer (CRPC) to second-generation antiandrogens and involves AR overexpression, AR mutation, and/or expression of AR splice variants lacking ligand-binding domain. There is a need for novel small molecules targeting AR, particularly those also targeting AR splice variants such as ARv7. A high-throughput/high-content screen was previously reported that led to the discovery of a novel lead compound, 2-(((3,5-dimethylisoxazol-4-yl)methyl)thio)-1-(4-(2,3-dimethylphenyl)piperazin-1-yl)ethan-1-one (IMTPPE), capable of inhibiting nuclear AR level and activity in CRPC cells, including those resistant to enzalutamide. A novel analogue of IMTPPE, JJ-450, has been investigated with evidence for its direct and specific inhibition of AR transcriptional activity via a pulldown assay and RNA-sequencing analysis, PSA-based luciferase, qPCR, and chromatin immunoprecipitation assays, and xenograft tumor model 22Rv1. JJ-450 blocks AR recruitment to androgen-responsive elements and suppresses AR target gene expression. JJ-450 also inhibits ARv7 transcriptional activity and its target gene expression. Importantly, JJ-450 suppresses the growth of CRPC tumor xenografts, including ARv7-expressing 22Rv1. Collectively, these findings suggest JJ-450 represents a new class of AR antagonists with therapeutic potential for CRPC, including those resistant to enzalutamide.

Assays to Interrogate the Ability of Compounds to Inhibit the AF-2 or AF-1 Transactivation Domains of the Androgen Receptor

Prostate cancer is the leading cause of cancer and second leading cause of cancer-related death in men in the United States. Twenty percent of patients receiving the standard of care androgen deprivation therapy (ADT) eventually progress to metastatic and incurable castration-resistant prostate cancer (CRPC). Current FDA-approved drugs for CRPC target androgen receptor (AR) binding or androgen production, but only provide a 2- to 5-month survival benefit due to the emergence of resistance. Overexpression of AR coactivators and the emergence of AR splice variants, both promote continued transcriptional activation under androgen-depleted conditions and represent drug resistance mechanisms that contribute to CRPC progression. The AR contains two transactivation domains, activation function 2 (AF-2) and activation function 1 (AF-1), which serve as binding surfaces for coactivators involved in the transcriptional activation of AR target genes. Full-length AR contains both AF-2 and AF-1 surfaces, whereas AR splice variants only have an AF-1 surface. We have recently prosecuted a high-content screening campaign to identify hit compounds that can inhibit or disrupt the protein-protein interactions (PPIs) between AR and transcriptional intermediary factor 2 (TIF2), one of the coactivators implicated in CRPC disease progression. Since an ideal inhibitor/disruptor of AR-coactivator PPIs would target both the AF-2 and AF-1 surfaces, we describe here the development and validation of five AF-2- and three AF-1-focused assays to interrogate and prioritize hits that disrupt both transactivation surfaces. The assays were validated using a test set of seven known AR modulator compounds, including three AR antagonists and one androgen synthesis inhibitor that are FDA-approved ADTs, two investigational molecules that target the N-terminal domain of AR, and an inhibitor of the Hsp90 (heat shock protein) molecular chaperone.

High-Content Screening Campaign to Identify Compounds That Inhibit or Disrupt Androgen Receptor-Transcriptional Intermediary Factor 2 Protein-Protein Interactions for the Treatment of Prostate Cancer

Twenty percent of prostate cancer (PCa) patients develop a noncurable drug-resistant form of the disease termed castration-resistant prostate cancer (CRPC). Overexpression of Androgen Receptor (AR) coactivators such as transcriptional intermediary factor 2 (TIF2) is associated with poor CRPC patient outcomes. We describe the implementation of the AR-TIF2 protein-protein interaction biosensor (PPIB) assay in a high-content screening (HCS) campaign of 143,535 compounds. The assay performed robustly and reproducibly and enabled us to identify compounds that inhibited dihydrotestosterone (DHT)-induced AR-TIF2 protein-protein interaction (PPI) formation or disrupted preexisting AR-TIF2 PPIs. We used multiparameter HCS data z-scores to identify and deprioritize cytotoxic or autofluorescent outliers and confirmed the resulting qualified actives in triplicate. None of the confirmed AR-TIF2 PPIB inhibitors/disruptors exhibited activity in a p53-hDM2 PPIB counter screen, indicating that they were unlikely to be either nonselective PPI inhibitors or to interfere with the biosensor assay format. However, eight confirmed AR-TIF2 PPIB actives also inhibited the glucocorticoid receptor (GR) nuclear translocation counter screen by >50%. These compounds were deprioritized because they either lacked AR specificity/selectivity, or they inhibited a shared component of the AR and GR signaling pathways. Twenty-nine confirmed AR-TIF2 PPIB actives also inhibited the AR nuclear localization counter screen, suggesting that they might indirectly inhibit the AR-TIF2 PPIB assay rather than directly blocking/disrupting PPIs. A total of 62.2% of the confirmed actives inhibited the DHT-induced AR-TIF2 PPI formation in a concentration-dependent manner with IC₅₀s < 40 μM, and 59.4% also disrupted preexisting AR-TIF2 PPI complexes. Overall, the hit rate for the AR-TIF2 PPIB HCS campaign was 0.12%, and most hits inhibited AR-TIF2 PPI formation and disrupted preexisting AR-TIF2 complexes with similar AR-red fluorescent protein distribution phenotypes. Further secondary and tertiary hit characterization assays are underway to select AR-TIF2 PPI inhibitor/disruptor hits suitable for medicinal chemistry lead optimization and development into novel PCa/CRPC therapeutics.

The potential of AR-V7 as a therapeutic target

INTRODUCTION

The androgen receptor variant AR-V7 is gaining attention as a potential predictive marker for as well as one of the resistance mechanisms to the most current anti-androgen receptor (AR) therapies in castration-resistant prostate cancer (CRPC). Accordingly, development of next-generation drugs that directly or indirectly target AR-V7 signaling is urgently needed. Areas covered: We review proposed mechanisms of drug resistance in relation to AR-V7 status, the mechanisms of generation of AR-V7, and its transcriptome, cistrome, and interactome. Pharmacological agents that interfere with these processes are being developed to counteract pan AR and AR-V7-specific signaling. Also, we address the current status of the preclinical and clinical studies targeting AR-V7 signaling. Expert opinion: AR-V7 is considered a true therapeutic target, however, it remains to be determined if AR-V7 is a principal driver or merely a bystander requiring heterodimerization with co-expressed full-length AR or other variants to drive CRPC progression. While untangling AR-V7 biology, multiple strategies are being developed to counteract drug resistance, including selective blockade of AR-V7 signaling as well as inhibition of pan-AR signaling. Ideally anti-AR therapies will be combined with agents preventing activation and enrichment of AR negative tumor cells that are otherwise depressed by AR activity axis.

Inhibition of Androgen Receptor Function and Level in Castration-Resistant Prostate Cancer Cells by 2-[(isoxazol-4-ylmethyl)thio]-1-(4-phenylpiperazin-1-yl)ethanone

The androgen receptor (AR) plays a critical role in the development of castration-resistant prostate cancer (CRPC) as well as in the resistance to the second-generation AR antagonist enzalutamide and the selective inhibitor of cytochrome P450 17A1 (CYP17A1) abiraterone. Novel agents targeting AR may inhibit the growth of prostate cancer cells resistant to enzalutamide and/or abiraterone. Through a high-throughput/high-content screening of a 220,000-member small molecule library, we have previously identified 2-[(isoxazol-4-ylmethyl)thio]-1-(4-phenylpiperazin-1-yl)ethanone (IMTPPE) (SID 3712502) as a novel small molecule capable of inhibiting AR transcriptional activity and protein level in C4-2 prostate cancer cells. In this study, we show that IMTPPE inhibits AR-target gene expression using real-time polymerase chain reaction, Western blot, and luciferase assays. IMTPPE inhibited proliferation of AR-positive, but not AR-negative, prostate cancer cells in culture. IMTPPE inhibited the transcriptional activity of a mutant AR lacking the ligand-binding domain (LBD), indicating that IMTPPE inhibition of AR is independent of the LBD. Furthermore, animal studies showed that IMTPPE inhibited the growth of 22Rv1 xenograft tumor, a model for enzalutamide-resistant prostate cancer. These findings suggest that IMTPPE is a potential lead compound for developing clinical candidates for the treatment of CRPC, including those resistant to enzalutamide.