N-terminal targeting of androgen receptor variant enhances response of castration resistant prostate cancer to taxane chemotherapy

Unveiling the mechanisms and challenges of cancer drug resistance

Cancer treatment faces many hurdles and resistance is one among them. Anti-cancer treatment strategies are evolving due to innate and acquired resistance capacity, governed by genetic, epigenetic, proteomic, metabolic, or microenvironmental cues that ultimately enable selected cancer cells to survive and progress under unfavorable conditions. Although the mechanism of drug resistance is being widely studied to generate new target-based drugs with better potency than existing ones. However, due to the broader flexibility in acquired drug resistance, advanced therapeutic options with better efficacy need to be explored. Combination therapy is an alternative with a better success rate though the risk of amplified side effects is commonplace. Moreover, recent groundbreaking precision immune therapy is one of the ways to overcome drug resistance and has revolutionized anticancer therapy to a greater extent with the only limitation of being individual-specific and needs further attention. This review will focus on the challenges and strategies opted by cancer cells to withstand the current therapies at the molecular level and also highlights the emerging therapeutic options -like immunological, and stem cell-based options that may prove to have better potential to challenge the existing problem of therapy resistance. Video Abstract.

Treatments Targeting the Androgen Receptor and Its Splice Variants in Breast Cancer

Breast cancer is a major cause of death worldwide. The complexity of endocrine regulation in breast cancer may allow the cancer cells to escape from a particular treatment and result in resistant and aggressive disease. These breast cancers usually have fewer treatment options. Targeted therapies for cancer patients may offer fewer adverse side effects because of specificity compared to conventional chemotherapy. Signaling pathways of nuclear receptors, such as the estrogen receptor (ER), have been intensively studied and used as therapeutic targets. Recently, the role of the androgen receptor (AR) in breast cancer is gaining greater attention as a therapeutic target and as a prognostic biomarker. The expression of constitutively active truncated AR splice variants in breast cancer is a possible mechanism contributing to treatment resistance. Therefore, targeting both the full-length AR and AR variants, either through the activation or suppression of AR function, depending on the status of the ER, progesterone receptor, or human epidermal growth factor receptor 2, may provide additional treatment options. Studies targeting AR in combination with other treatment strategies are ongoing in clinical trials. The determination of the status of nuclear receptors to classify and identify patient subgroups will facilitate optimized and targeted combination therapies.

Current and emerging approaches to noncompetitive AR inhibition

The androgen receptor (AR) has been shown to be a key determinant in the pathogenesis of castration-resistant prostate cancer (CRPC). The current standard of care therapies targets the ligand-binding domain of the receptor and can afford improvements to life expectancy often only in the order of months before resistance occurs. Emerging preclinical and clinical compounds that inhibit receptor activity via differentiated mechanisms of action which are orthogonal to current antiandrogens show promise for overcoming treatment resistance. In this review, we present an authoritative summary of molecules that noncompetitively target the AR. Emerging small molecule strategies for targeting alternative domains of the AR represent a promising area of research that shows significant potential for future therapies. The overall quality of lead candidates in the area of noncompetitive AR inhibition is discussed, and it identifies the key chemotypes and associated properties which are likely to be, or are currently, positioned to be first in human applications.

Targeting the N-terminal domain of the androgen receptor: The effective approach in therapy of CRPC

The androgen receptor (AR) is dominant in prostate cancer (PCa) pathology. Current therapeutic agents for advanced PCa include androgen synthesis inhibitors and AR antagonists that bind to the hormone binding pocket (HBP) at the ligand binding domain (LBD). However, AR amplification, AR splice variants (AR-Vs) expression, and intra-tumoral de novo synthesis of androgens result in the reactivation of AR signalling. The AR N-terminal domain (NTD) plays an essential role in AR transcriptional activity. The AR inhibitor targeting NTD could potentially block the activation of both full-length AR and AR-Vs, thus overcoming major resistance mechanisms to current treatments. This review discusses the progress of research in various NTD inhibitors and provides new insight into the development of AR-NTD inhibitors.

Prostate cancer detection: a systematic review of urinary biosensors

BACKGROUND

Current diagnostic methods for prostate cancer are invasive and lack specificity towards aggressive forms of the disease, which can lead to overtreatment. A new class of non-invasive alternatives is under development, in which urinary biomarkers are detected using biosensing devices to offer rapid and accurate prostate cancer diagnosis. These different approaches are systematically reviewed and their potential for translation to clinical practice is evaluated.

METHODS

A systematic review of the literature was performed in May 2021 using PubMed Medline database, Embase, and Web of Science. The objective was to review the structural designs and performance of biosensors tested on urine samples from patients with prostate cancer.

RESULTS

A total of 76 records were identified. After screening and eligibility, 14 articles were included and are discussed in this paper. The biosensors were discussed based on the target biomarkers and detection technologies used, as well as the results of the clinical studies. Most of the works reported good discrimination between patients with prostate cancer and controls.

CONCLUSIONS

This review highlights the potential of urinary biosensors for non-invasive prostate cancer detection. However, clinical studies have so far only been conducted on small cohorts of patient, with large scale trials still needed to validate the proposed approaches. Overall, the consensus arising from the proof of concepts studies reviewed here, is that an adequate combination of biomarkers into multiplex biosensor platforms is required to achieve accurate diagnostic tests. Furthermore, whether such devices can discriminate between aggressive and indolent cancer has not yet been addressed, because it entails optimized biomarkers panels and long-term clinical trials.

Drugging the Undruggable: Targeting the N-Terminal Domain of Nuclear Hormone Receptors

This chapter focuses on the development of drugs targeting the N-terminal domain of nuclear hormone receptors, using progress with the androgen receptor as an example. Historically, development of therapies targeting nuclear hormone receptors has focused on the folded C-terminal ligand-binding domain. Therapies were traditionally not developed to target the intrinsically disordered N-terminal domain as it was considered "undruggable". Recent developments have now shown it is possible to direct therapies to the N-terminal domain. This chapter will provide an introduction of the structure and function of the domains of nuclear hormone receptors, followed by a discussion of the rationale supporting the development of N-terminal domain inhibitors. Chemistry and mechanisms of action of small molecule inhibitors will be described with emphasis on N-terminal domain inhibitors developed to the androgen receptor including those in clinical trials.

Eighty Years of Targeting Androgen Receptor Activity in Prostate Cancer: The Fight Goes on

Simple Summary

Prostate cancer is the second most common cancer in men world-wide, with nearly 1.3 million new cases each year, and over the next twenty years the incidence and death rate are predicted to nearly double. For decades, this lethal disease has been more or less successfully treated using hormonal therapy, which has the ultimate aim of inhibiting androgen signalling. However, prostate tumours can evade such hormonal therapies in a number of different ways and therapy resistant disease, so-called castration-resistant prostate cancer (CRPC) is the major clinical problem. Somewhat counterintuitively, the androgen receptor remains a key therapy target in CRPC. Here, we explain why this is the case and summarise both new hormone therapy strategies and the recent advances in knowledge of androgen receptor structure and function that underpin them.

Abstract

Prostate cancer (PCa) is the most common cancer in men in the West, other than skin cancer, accounting for over a quarter of cancer diagnoses in US men. In a seminal paper from 1941, Huggins and Hodges demonstrated that prostate tumours and metastatic disease were sensitive to the presence or absence of androgenic hormones. The first hormonal therapy for PCa was thus castration. In the subsequent eighty years, targeting the androgen signalling axis, where possible using drugs rather than surgery, has been a mainstay in the treatment of advanced and metastatic disease. Androgens signal via the androgen receptor, a ligand-activated transcription factor, which is the direct target of many such drugs. In this review we discuss the role of the androgen receptor in PCa and how the combination of structural information and functional screenings is continuing to be used for the discovery of new drug to switch off the receptor or modify its function in cancer cells.

The regulatory effect of cabazitaxel on epithelial-mesenchymal transition in metastatic prostate cancer

Introduction

: Epithelial-mesenchymal transition (EMT) is a critical mechanism that promotes cancer cells to metastasis. Therefore, EMT regulation has become an important target in anticancer therapy approaches in recent years. However, in metastatic prostate cancer (PC), the EMT regulatory effect has not fully understood for cabazitaxel (Cbx), a third line taxane-based chemotherapeutic for metastatic castration-resistant PC.

Aim

In this study, we investigated the antimetastatic and EMT-regulatory effects of Cbx on hormone-sensitive metastatic PC cells.

Materials and Methods

The anticancer effects of Cbx were assessed by WST-1 and Annexin V analysis. The antimetastatic effect of Cbx was evaluated by wound healing and quantitative reverse transcription polymerase chain reaction through EMT-mesenchymal-to-epithelial transition (MET) markers as well as EMT-repressor microRNAs (miRNAs) in Cbx-treated LNCaP cells.

Results

Our results showed that, in addition to its apoptotic and anti-migratory activities, Cbx exhibited the EMT-repressor effects through the prominent downregulation of matrix metalloproteinase-9 and Snail levels as EMT-promoting factors, and the significant upregulation of the certain miRNAs, including miR-205, miR-524, and miR-124, which play a role in EMT-repressing by targeting regulators of the EMT-associated genes.

Conclusion

Although further evaluations are needed to improve the findings, we showed that, in addition to its classical taxane function, Cbx has a regulatory effect on EMT-MET cycling in hormone-sensitive metastatic PC.

Recent Discoveries in the Androgen Receptor Pathway in Castration-Resistant Prostate Cancer

The androgen receptor (AR) is the main therapeutic target in advanced prostate cancer, because it regulates the growth and progression of prostate cancer cells. Patients may undergo multiple lines of AR-directed treatments, including androgen-deprivation therapy, AR signaling inhibitors (abiraterone acetate, enzalutamide, apalutamide, or darolutamide), or combinations of these therapies. Yet, tumors inevitably develop resistance to the successive lines of treatment. The diverse mechanisms of resistance include reactivation of the AR and dysregulation of AR cofactors and collaborative transcription factors (TFs). Further elucidating the nexus between the AR and collaborative TFs may reveal new strategies targeting the AR directly or indirectly, such as targeting BET proteins or OCT1. However, appropriate preclinical models will be required to test the efficacy of these approaches. Fortunately, an increasing variety of patient-derived models, such as xenografts and organoids, are being developed for discovery-based research and preclinical drug screening. Here we review the mechanisms of drug resistance in the AR signaling pathway, the intersection with collaborative TFs, and the use of patient-derived models for novel drug discovery.

LncRNA HORAS5 promotes taxane resistance in castration-resistant prostate cancer via a BCL2A1-dependent mechanism

Background: Castration-resistant prostate cancer (CRPC) is an incurable malignancy. Long noncoding RNAs (lncRNAs) play key roles in drug resistance. Materials & methods: LncRNA HORAS5 role in cabazitaxel resistance (i.e., cell-count, IC50 and caspase activity) was studied via lentiviral-mediated overexpression and siRNA-based knockdown. Genes expression was analyzed with RNA-sequencing, reverse transcription quantitative PCR (RT-qPCR) and western blot. HORAS5 expression was queried in clinical database. Results: Cabazitaxel increased HORAS5 expression that upregulated BCL2A1, thereby protecting CRPC cells from cabazitaxel-induced apoptosis. BCL2A1 knockdown decreased cell-count and increased apoptosis in CRPC cells. HORAS5-targeting antisense oligonucleotide decreased cabazitaxel IC50. In CRPC clinical samples, HORAS5 expression increased upon taxane treatment. Conclusion: HORAS5 stimulates the expression of BCL2A1 thereby decreasing apoptosis and enhancing cabazitaxel resistance in CRPC cells.

Impact of taxanes on androgen receptor signaling

The development and progression of metastatic castration-resistant prostate cancer is the major challenge in the treatment of advanced prostate cancer. The androgen receptor signaling pathway remains active in metastatic castration-resistant prostate cancer. Docetaxel and cabazitaxel are the first- and second-line chemotherapy, respectively, for patients with metastatic castration-resistant prostate cancer. These two taxanes, in general, function by (i) inhibiting mitosis and inducing apoptosis and (ii) preventing microtubule-dependent cargo trafficking. In prostate cancer, taxanes have been reported to inhibit the nuclear translocation and activity of the androgen receptor. However, whether this is attainable or not clinically remains controversial. In this review, we will provide a comprehensive view of the effects of taxanes on androgen receptor signaling in prostate cancer.

p300 is upregulated by docetaxel and is a target in chemoresistant prostate cancer

Administration of the microtubule inhibitor docetaxel is a common treatment for metastatic castration-resistant prostate cancer (mCRPC) and results in prolonged patient overall survival. Usually, after a short period of time chemotherapy resistance emerges and there is urgent need to find new therapeutic targets to overcome therapy resistance. The lysine-acetyltransferase p300 has been correlated to prostate cancer (PCa) progression. Here, we aimed to clarify a possible function of p300 in chemotherapy resistance and verify p300 as a target in chemoresistant PCa. Immunohistochemistry staining of tissue samples revealed significantly higher p300 protein expression in patients who received docetaxel as a neoadjuvant therapy compared to control patients. Elevated p300 expression was confirmed by analysis of publicly available patient data, where significantly higher p300 mRNA expression was found in tissue of mCRPC tumors of docetaxel-treated patients. Consistently, docetaxel-resistant PCa cells showed increased p300 protein expression compared to docetaxel-sensitive counterparts. Docetaxel treatment of PCa cells for 72 h resulted in elevated p300 expression. shRNA-mediated p300 knockdown did not alter colony formation efficiency in docetaxel-sensitive cells, but significantly reduced clonogenic potential of docetaxel-resistant cells. Downregulation of p300 in docetaxel-resistant cells also impaired cell migration and invasion. Taken together, we showed that p300 is upregulated by docetaxel, and our findings suggest that p300 is a possible co-target in treatment of chemoresistant PCa.

Molecules targeting the androgen receptor (AR) signaling axis beyond the AR-Ligand binding domain

Prostate cancer (PCa) is the second most common cause of cancer-related mortality in men in the United States. The androgen receptor (AR) and the physiological pathways it regulates are central to the initiation and progression of PCa. As a member of the nuclear steroid receptor family, it is a transcription factor with three distinct functional domains (ligand-binding domain [LBD], DNA-binding domain [DBD], and transactivation domain [TAD]) in its structure. All clinically approved drugs for PCa ultimately target the AR-LBD. Clinically active drugs that target the DBD and TAD have not yet been developed due to multiple factors. Despite these limitations, the last several years have seen a rise in the discovery of molecules that could successfully target these domains. This review aims to present and comprehensively discuss such molecules that affect AR signaling through direct or indirect interactions with the AR-TAD or the DBD. The compounds discussed here include hairpin polyamides, niclosamide, marine sponge-derived small molecules (eg, EPI compounds), mahanine, VPC compounds, JN compounds, and bromodomain and extraterminal domain inhibitors. We highlight the significant in vitro and in vivo data found for each compound and the apparent limitations and/or potential for further development of these agents as PCa therapies.

Prostate tumor neuroendocrine differentiation via EMT: The road less traveled

The long-standing challenge in the treatment of prostate cancer is to overcome therapeutic resistance during progression to lethal disease. Aberrant transforming-growth factor-β (TGF-β) signaling accelerates prostate tumor progression in a transgenic mouse model via effects on epithelial-mesenchymal transition (EMT), and neuroendocrine differentiation driving tumor progression to castration-resistant prostate cancer (CRPC). Neuroendocrine prostate cancer (NEPC) is highly aggressive exhibiting reactivation of developmental programs associated with EMT induction and stem cell-like characteristics. The androgen receptor (AR) is a critical driver of tumor progression as well as therapeutic response in patients with metastatic CRPC. The signaling interactions between the TGF-β mechanistic network and AR axis impact the EMT phenotypic conversions, and perturbation of epithelial homeostasis via EMT renders a critical venue for epithelial derived tumors to become invasive, acquire the neuroendocrine phenotype, and rapidly metastasize. Combinations of microtubule targeting taxane chemotherapy and androgen/AR targeting therapies have survival benefits in CRPC patients, but therapeutic resistance invariability develops, leading to mortality. Compelling evidence from our group recently demonstrated that chemotherapy (cabazitaxel, second line taxane chemotherapy), or TGF-β receptor signaling targeted therapy, caused reversion of EMT to mesenchymal-epithelial transition and tumor re-differentiation, in in vitro and in vivo prostate cancer models. In this review, we discuss the functional contribution of EMT dynamic changes to the development of the neuroendocrine phenotype-the newly characterized pathological feature of prostate tumors in the context of the tumor microenvironment-navigated cell lineage changes and the role of this neuroendocrine phenotype in metastatic progression and therapeutic resistance.

Urine-based liquid biopsy: non-invasive and sensitive AR-V7 detection in urinary EVs from patients with prostate cancer

Androgen-receptor splice variant 7 (AR-V7) is associated with castration-resistant prostate cancer (CRPC) and resistance to anti-androgen therapy. Despite its clinical importance, the lack of efficient methods for AR-V7 analysis remains a challenge for broader use of this biomarker in routine clinical practice. Herein, we suggest a practical and non-invasive liquid biopsy method for analysis of AR-V7 in the RNA of urine-derived extracellular vesicles (EVs) without the need for blood withdrawal. Urine-derived EVs were isolated by a lab-on-a-disc integrated with six independent nanofiltration units (Exo-Hexa) allowing simultaneous processing of six individual samples. Rapid enrichment of EVs (<30 min) from each 4 mL urine sample was followed by mRNA extraction, and AR-V7 and androgen receptor full-length (AR-FL) mRNA levels in the urinary EVs were quantified by droplet digital polymerase chain reaction (ddPCR) as absolute concentrations (copies per mL). Higher AR-V7 and lower AR-FL expressions were detected in urine-derived EVs from 14 patients with CRPC than in those from 22 patients with hormone-sensitive prostate cancer. Additionally, we found that AR-V7 transcript levels and the AR-V7/AR-FL ratio in urinary EVs were higher in patients with advanced prostate cancer. This study is the first to report that RNA of urine-derived EVs is a reliable source for AR-V7 expression analysis. The proposed method for quantifying AR-V7 in urinary EVs prepared by a lab-on-a-disc is therefore a simple and promising approach to liquid biopsy with great potential for therapeutic impact on prostate cancer.

Androgen Receptor Splice Variant 7 Drives the Growth of Castration Resistant Prostate Cancer without Being Involved in the Efficacy of Taxane Chemotherapy

Expression of androgen receptor (AR) splice variant 7 (AR-V7) has been identified as the mechanism associated with the development of castration-resistant prostate cancer (CRPC). However, a potential link between AR-V7 expression and resistance to taxanes, such as docetaxel or cabazitaxel, has not been unequivocally demonstrated. To address this, we used LNCaP95-DR cells, which express AR-V7 and exhibit resistance to enzalutamide and docetaxel. Interestingly, LNCaP95-DR cells showed cross-resistance to cabazitaxel. Furthermore, these cells had increased levels of P-glycoprotein (P-gp) and their sensitivity to both docetaxel and cabazitaxel was restored through treatment with tariquidar, a P-gp antagonist. Results generated demonstrated that P-gp mediated cross-resistance between docetaxel and cabazitaxel. Although the LNCaP95-DR cells had increased expression of AR-V7 and its target genes (UBE2C, CDC20), the knockdown of AR-V7 did not restore sensitivity to docetaxel or cabazitaxel. However, despite resistance to docetaxel and carbazitaxel, EPI-002, an antagonist of the AR amino-terminal domain (NTD), had an inhibitory effect on the proliferation of LNCaP95-DR cells, which was similar to that achieved with the parental LNCaP95 cells. On the other hand, enzalutamide had no effect on the proliferation of either cell line. In conclusion, our results suggested that EPI-002 may be an option for the treatment of AR-V7-driven CRPC, which is resistant to taxanes.

Collateral resistance to taxanes in enzalutamide-resistant prostate cancer through aberrant androgen receptor and its variants

Currently, the optimal sequential use of androgen receptor (AR) axis-targeted agents and taxane chemotherapies remains undetermined. We aimed to elucidate the resistance status between taxanes and enzalutamide, and the functional role of the AR axis. Enzalutamide-resistant 22Rv1 cells showed collateral resistance to taxanes, including docetaxel and cabazitaxel. However, taxane-resistant cells showed no collateral resistance to enzalutamide; taxane-resistant cells expressed comparable protein levels of full-length AR and AR variants. Knockdown of both full-length AR and AR variants rendered cells sensitive to taxanes, whereas knockdown of AR variants sensitized cells to enzalutamide, but not to taxanes. In contrast, overexpression of full-length AR rendered cells resistant to taxanes. Consistently, the prostate-specific antigen response and progression-free survival in docetaxel chemotherapy were worse in cases with prior use of ARAT agents compared with cases without. Collateral resistance to taxanes was evident after obtaining enzalutamide resistance, and aberrant AR signaling might be involved in taxane resistance.

Profiling Prostate Cancer Therapeutic Resistance

The major challenge in the treatment of patients with advanced lethal prostate cancer is therapeutic resistance to androgen-deprivation therapy (ADT) and chemotherapy. Overriding this resistance requires understanding of the driving mechanisms of the tumor microenvironment, not just the androgen receptor (AR)-signaling cascade, that facilitate therapeutic resistance in order to identify new drug targets. The tumor microenvironment enables key signaling pathways promoting cancer cell survival and invasion via resistance to anoikis. In particular, the process of epithelial-mesenchymal-transition (EMT), directed by transforming growth factor-β (TGF-β), confers stem cell properties and acquisition of a migratory and invasive phenotype via resistance to anoikis. Our lead agent DZ-50 may have a potentially high efficacy in advanced metastatic castration resistant prostate cancer (mCRPC) by eliciting an anoikis-driven therapeutic response. The plasticity of differentiated prostate tumor gland epithelium allows cells to de-differentiate into mesenchymal cells via EMT and re-differentiate via reversal to mesenchymal epithelial transition (MET) during tumor progression. A characteristic feature of EMT landscape is loss of E-cadherin, causing adherens junction breakdown, which circumvents anoikis, promoting metastasis and chemoresistance. The targetable interactions between androgens/AR and TGF-β signaling are being pursued towards optimized therapeutic regimens for the treatment of mCRPC. In this review, we discuss the recent evidence on targeting the EMT-MET dynamic interconversions to overcome therapeutic resistance in patients with recurrent therapeutically resistant prostate cancer. Exploitation of the phenotypic landscape and metabolic changes that characterize the prostate tumor microenvironment in advanced prostate cancer and consequential impact in conferring treatment resistance are also considered in the context of biomarker discovery.

Development and Implementation of a High-Throughput High-Content Screening Assay to Identify Inhibitors of Androgen Receptor Nuclear Localization in Castration-Resistant Prostate Cancer Cells

Patients with castration-resistant prostate cancer (CRPC) can be treated with abiraterone, a potent inhibitor of androgen synthesis, or enzalutamide, a second-generation androgen receptor (AR) antagonist, both targeting AR signaling. However, most patients relapse after several months of therapy and a majority of patients with relapsed CRPC tumors express the AR target gene prostate-specific antigen (PSA), suggesting that AR signaling is reactivated and can be targeted again to inhibit the relapsed tumors. Novel small molecules capable of inhibiting AR function may lead to urgently needed therapies for patients resistant to abiraterone, enzalutamide, and/or other previously approved antiandrogen therapies. Here, we describe a high-throughput high-content screening (HCS) campaign to identify small-molecule inhibitors of AR nuclear localization in the C4-2 CRPC cell line stably transfected with GFP-AR-GFP (2GFP-AR). The implementation of this HCS assay to screen a National Institutes of Health library of 219,055 compounds led to the discovery of 3 small molecules capable of inhibiting AR nuclear localization and function in C4-2 cells, demonstrating the feasibility of using this cell-based phenotypic assay to identify small molecules targeting the subcellular localization of AR. Furthermore, the three hit compounds provide opportunities to develop novel AR drugs with potential for therapeutic intervention in CRPC patients who have relapsed after treatment with antiandrogens, such as abiraterone and/or enzalutamide.

Anoikis and EMT: Lethal "Liaisons" during Cancer Progression

Anoikis is a unique mode of apoptotic cell death that occurs consequentially to insufficient cell-matrix interactions. Resistance to anoikis is a critical contributor to tumor invasion and metastasis. The phenomenon is regulated by integrins, which upon engagement with components of the extracellular matrix (ECM) form adhesion complexes and the actin cytoskeleton drives the formation of cell protrusions used to adhere to ECM, directing cell migration. The epithelial-mesenchymal transition (EMT) confers stem cell properties and leads to acquisition of a migratory and invasive phenotype by causing adherens junction breakdown and circumventing anoikis in the tumor microenvironment. The investigation of drug discovery platforms for apoptosis-driven therapeutics identified several novel agents with antitumor action via reversing resistance to anoikis, inhibiting survival pathways and impacting the EMT landscape in human cancer. In this review, we discuss current evidence on the contribution of the anoikis phenomenon functionally linked to EMT to cancer metastasis and the therapeutic value of antitumor drugs that selectively reverse anoikis resistance and/or EMT to impair tumor progression toward the development/optimization of apoptosis-driven therapeutic targeting of metastatic disease.

A sting in the tail: the N-terminal domain of the androgen receptor as a drug target

The role of androgen receptor (AR) in the initiation and progression of prostate cancer (PCa) is well established. Competitive inhibition of the AR ligand-binding domain (LBD) has been the staple of antiandrogen therapies employed to combat the disease in recent years. However, their efficacy has often been limited by the emergence of resistance, mediated through point mutations, and receptor truncations. As a result, the prognosis for patients with malignant castrate resistant disease remains poor. The amino-terminal domain (NTD) of the AR has been shown to be critical for AR function. Its modular activation function (AF-1) is important for both gene regulation and participation in protein-protein interactions. However, due to the intrinsically disordered structure of the domain, its potential as a candidate for therapeutic intervention has been dismissed in the past. The recent emergence of the small molecule EPI-001 has provided evidence that AR-NTD can be targeted therapeutically, independent of the LBD. Targeting of AR-NTD has the potential to disrupt multiple intermolecular interactions between AR and its coregulatory binding partners, in addition to intramolecular cross-talk between the domains of the AR. Therapeutics targeting these protein-protein interactions or NTD directly should also have efficacy against emerging AR splice variants which may play a role in PCa progression. This review will discuss the role of intrinsic disorder in AR function and illustrate how emerging therapies might target NTD in PCa.

Epithelial-mesenchymal-transition regulators in prostate cancer: Androgens and beyond

Castration resistant prostate cancer (CRPC) remains one of the leading causes of cancer deaths among men. Conventional therapies targeting androgen signaling driven tumor growth have provided limited survival benefit in patients. Recent identification of the critical molecular and cellular events surrounding tumor progression, invasion, and metastasis to the bone as well as other sites provide new insights in targeting advanced disease. Epithelial mesenchymal transition (EMT) is a process via which epithelial cells undergo morphological changes to a motile mesenchymal phenotype, a phenomenon implicated in cancer metastasis but also therapeutic resistance. Therapeutic targeting of EMT has the potential to open a new avenue in the treatment paradigm of CRPC through the reversion of the invasive mesenchymal phenotype to the well differentiated tumor epithelial tumor phenotype. Overcoming therapeutic resistance in metastatic prostate cancer is an unmet need in today's clinical management of advanced disease. This review outlines our current understanding of the contribution of EMT and its reversal to MET in prostate cancer progression and therapeutic resistance, and the impact of selected targeting of mechanisms of resistance via EMT towards a therapeutic benefit in patients with CRPC.

Mechanisms of Therapeutic Resistance in Prostate Cancer

Prostate cancer is the second leading cause of cancer deaths in the USA. The challenge in managing castration-resistant prostate cancer (CRPC) stems not from the lack of therapeutic options but from the limited duration of clinical and survival benefit offered by treatments in this setting due to primary and acquired resistance. The remarkable molecular heterogeneity and tumor adaptability in advanced prostate cancer necessitate optimization of such treatment strategies. While the future of CRPC management will involve newer targeted therapies in deliberately biomarker-selected patients, interventions using current approaches may exhibit improved clinical benefit if employed in the context of optimal sequencing and combinations. This review outlines our current understanding of mechanisms of therapeutic resistance in progression to and after the development of castration resistance, highlighting targetable and reversible mechanisms of resistance.

Androgen receptor signaling in castration-resistant prostate cancer: a lesson in persistence

The androgen receptor (AR) signaling axis drives all stages of prostate cancer, including the lethal, drug-resistant form of the disease termed castration-resistant prostate cancer (CRPC), which arises after failure of androgen deprivation therapy (ADT). Persistent AR activity in spite of ADT and the second-generation AR-targeting agents enzalutamide and abiraterone is achieved in many cases by direct alterations to the AR signaling axis. Herein, we provide a detailed description of how such alterations contribute to the development and progression of CRPC. Aspects of this broad and ever-evolving field specifically addressed in this review include: the etiology and significance of increased AR expression; the frequency and role of gain-of-function mutations in the AR gene; the function of constitutively active, truncated forms of the AR termed AR variants and the clinical relevance of alterations to the activity and expression of AR coregulators. Additionally, we examine the novel therapeutic strategies to inhibit these classes of therapy resistance mechanisms, with an emphasis on emerging agents that act in a manner distinct from the current ligand-centric approaches. Throughout, we discuss how the central role of AR in prostate cancer and the constant evolution of the AR signaling axis during disease progression represent archetypes of two key concepts in oncology, oncogene addiction and therapy-mediated selection pressure.

Androgen receptor variation affects prostate cancer progression and drug resistance

Significant therapeutic progress has been made in treating prostate cancer in recent years. Drugs such as enzalutamide, abiraterone, and cabazitaxel have expanded the treatment armamentarium, although it is not completely clear which of these drugs are the most-effective option for individual patients. Moreover, such advances have been tempered by the development of therapeutic resistance. The purpose of this review is to summarize the current literature pertaining to the biochemical effects of AR variants and their consequences on prostate cancer therapies at both the molecular level and in clinical treatment. We address how these AR splice variants and mutations affect tumor progression and therapeutic resistance and discuss potential novel therapeutic strategies under development. It is hoped that these therapies can be administered with increasing precision as tumor genotyping methods become more sophisticated, thereby lending clinicians a better understanding of the underlying biology of prostate tumors in individual patients.

Emerging data on androgen receptor splice variants in prostate cancer

Androgen receptor splice variants are alternatively spliced variants of androgen receptor, which are C-terminally truncated and lack the canonical ligand-binding domain. Accumulating evidence has indicated a significant role of androgen receptor splice variants in mediating resistance of castration-resistant prostate cancer to current therapies and in predicting therapeutic responses. As such, there is an urgent need to target androgen receptor splicing variants for more effective treatment of castration-resistant prostate cancer. Identification of precise and critical targeting points to deactivate androgen receptor splicing variants relies on a deep understanding of how they are generated and the mechanisms of their action. In this review, we will focus on the emerging data on their generation, clinical significance and mechanisms of action as well as the therapeutic influence of these findings.

Adaptive pathways and emerging strategies overcoming treatment resistance in castration resistant prostate cancer

The therapies available for prostate cancer patients whom progress from hormone-sensitive to castration resistant prostate cancer include both systemic drugs, including docetaxel and cabazitaxel, and drugs that inhibit androgen signaling such as enzalutamide and abiraterone. Unfortunately, it is estimated that up to 30% of patients have primary resistance to these treatments and over time even those who initially respond to therapy will eventually develop resistance and their disease will continue to progress regardless of the presence of the drug. Determining the mechanisms involved in the development of resistance to these therapies has been the area of intense study and several adaptive pathways have been uncovered. Androgen receptor (AR) mutations, expression of AR-V7 (or other constitutively active androgen receptor variants), intracrine androgen production and overexpression of androgen synthesis enzymes such as Aldo-Keto Reductase Family 1, Member C3 (AKR1C3) are among the many mechanisms associated with resistance to anti-androgens. In regards to the taxanes, one of the key contributors to drug resistance is increased drug efflux through ATP Binding Cassette Subfamily B Member 1 (ABCB1). Targeting these resistance mechanisms using different strategies has led to various levels of success in overcoming resistance to current therapies. For instance, targeting AR-V7 with niclosamide or AKR1C3 with indomethacin can improve enzalutamide and abiraterone treatment. ABCB1 transport activity can be inhibited by the dietary constituent apigenin and antiandrogens such as bicalutamide which in turn improves response to docetaxel. A more thorough understanding of how drug resistance develops will lead to improved treatment strategies. This review will cover the current knowledge of resistance mechanisms to castration resistant prostate cancer therapies and methods that have been identified which may improve treatment response.

Biodegradable brush-type copolymer modified with targeting peptide as a nanoscopic platform for targeting drug delivery to treat castration-resistant prostate cancer

Well-defined amphiphilic tumor-targeting brush-type copolymers, poly(oligo(ethylene glycol) monomethyl ether methacrylate-co-G3-C12)-g-poly(ε-caprolactone) (P(OEGMA-co-G3-C12)-g- PCL), were synthesized by the combination of ring-opening polymerization (ROP), reversible addition-fragmentation transfer (RAFT) polymerization and polymer post-functionalization, in which G3-C12 was castration-resistant prostate cancer (CRPC) targeting peptide. The obtained polymers were then employed for the targeted treatment of CRPC by delivering a hydrophobic anticancer drug (bufalin, BUF). Polymerizable monomer, 3-((2-(methacryloyloxy)ethyl)thio)propanoic acid (BSMA) and PCL-based macromolecular monomer (PCLMA) were synthesized at first. RAFT polymerization of OEGMA, BSMA, and PCLMA afforded amphiphilic brush-type copolymers, P(OEGMA-co-BSMA)-g-PCL. Post-functionalization of the obtained polymers with G3-C12 led to the formation of the final amphiphilic targeting brush-type copolymers, P(OEGMA-co-G3-C12)-g- PCL. In aqueous media, P(OEGMA-co-G3-C12)-g-PCL self-assembles into micelles with a hydrodynamic diameter (Dh) of ∼66.1±0.44nm. It was demonstrated that the obtained micellar nanoparticles exhibited good biocompatibility and biodegradability. Besides, BUF-loaded micellar nanoparticles assembled from P(OEGMA-co-G3-C12)-g-PCL, BUF-NP-(G3-C12), showed a controlled drug release in vitro and improved anticancer efficacy both in vitro and in vivo.

Clinical Relevance of Androgen Receptor Splice Variants in Castration-Resistant Prostate Cancer

OPINION STATEMENT

Metastatic castration-resistant prostate cancer (mCRPC) currently benefits from a wealth of treatment options, yet still remains lethal in the vast majority of patients. It is becoming increasingly understood that this disease entity continues to evolve over time, acquiring additional and diverse resistance mechanisms with each subsequent therapy used. This dynamic relationship between treatment pressure and disease resistance can be challenging for the managing clinician. The recent discovery of alternate splice variants of the androgen receptor (AR) is one potential mechanism of escape in mCRPC, and recognizing this resistance mechanism might be important for optimal treatment selection for our patients. AR-V7 appears to be the most relevant AR splice variant, and early clinical data suggest that it is a negative prognostic marker in mCRPC. Emerging evidence also suggests that detection of AR-V7 may be associated with resistance to novel hormonal therapy (abiraterone and enzalutamide) but may be compatible with sensitivity to taxane chemotherapy (docetaxel and cabazitaxel). Adding to this complexity is the observation that AR-V7 is a dynamic marker whose status may change across time and depending on selective pressures induced by different therapies. Finally, it is possible that AR-V7 may represent a therapeutic target in mCRPC if drugs can be designed that degrade or inhibit AR splice variants or block their transcriptional activity. Several such agents (including galeterone, EPI-506, and bromodomain/BET inhibitors) are now in clinical development.

Rh2E2, a novel metabolic suppressor, specifically inhibits energy-based metabolism of tumor cells

Energy metabolism in cancer cells is often increased to meet their higher proliferative rate and biosynthesis demands. Suppressing cancer cell metabolism using agents like metformin has become an attractive strategy for treating cancer patients. We showed that a novel ginsenoside derivative, Rh2E2, is as effective as aspirin in preventing the development of AOM/DSS-induced colorectal cancer and suppresses tumor growth and metastasis in a LLC-1 xenograft. A sub-chronic and acute toxicity LD50 test of Rh2E2 showed no harmful reactions at the maximum oral dosage of 5000 mg/kg body weight in mice. Proteomic profiling revealed that Rh2E2 specifically inhibited ATP production in cancer cells via down-regulation of metabolic enzymes involving glycolysis, fatty acid β-oxidation and the tricarboxylic acid cycle, leading to specific cytotoxicity and S-phase cell cycle arrest in cancer cells. Those findings suggest that Rh2E2 possesses a novel and safe anti-metabolic agent for cancer patients by specific reduction of energy-based metabolism in cancer cells.

Multinucleation and Mesenchymal-to-Epithelial Transition Alleviate Resistance to Combined Cabazitaxel and Antiandrogen Therapy in Advanced Prostate Cancer

Patients with metastatic castration-resistant prostate cancer (CRPC) frequently develop therapeutic resistance to taxane chemotherapy and antiandrogens. Cabazitaxel is a second-line taxane chemotherapeutic agent that provides additional survival benefits to patients with advanced disease. In this study, we sought to identify the mechanism of action of combined cabazitaxel and androgen receptor (AR) targeting in preclinical models of advanced prostate cancer. We found that cabazitaxel induced mitotic spindle collapse and multinucleation by targeting the microtubule depolymerizing kinesins and inhibiting AR. In androgen-responsive tumors, treatment with the AR inhibitor, enzalutamide, overcame resistance to cabazitaxel. Combination treatment of human CRPC xenografts with cabazitaxel and enzalutamide reversed epithelial-mesenchymal transition (EMT) to mesenchymal-epithelial transition (MET) and led to multinucleation, while retaining nuclear AR. In a transgenic mouse model of androgen-responsive prostate cancer, cabazitaxel treatment induced MET, glandular redifferentiation, and AR nuclear localization that was inhibited by androgen deprivation. Collectively, our preclinical studies demonstrate that prostate tumor resistance to cabazitaxel can be overcome by antiandrogen-mediated EMT-MET cycling in androgen-sensitive tumors but not in CRPC. Moreover, AR splice variants may preclude patients with advanced disease from responding to cabazitaxel chemotherapy and antiandrogen combination therapy. This evidence enables a significant insight into therapeutic cross-resistance to taxane chemotherapy and androgen deprivation therapy in advanced prostate cancer.

Moving Beyond the Androgen Receptor (AR): Targeting AR-Interacting Proteins to Treat Prostate Cancer

Medical or surgical castration serves as the backbone of systemic therapy for advanced and metastatic prostate cancer, taking advantage of the importance of androgen signaling in this disease. Unfortunately, resistance to castration emerges almost universally. Despite the development and approval of new and more potent androgen synthesis inhibitors and androgen receptor (AR) antagonists, prostate cancers continue to develop resistance to these therapeutics, while often maintaining their dependence on the AR signaling axis. This highlights the need for innovative therapeutic approaches that aim to continue disrupting AR downstream signaling but are orthogonal to directly targeting the AR itself. In this review, we discuss the preclinical research that has been done, as well as clinical trials for prostate cancer, on inhibiting several important families of AR-interacting proteins, including chaperones (such as heat shock protein 90 (HSP90) and FKBP52), pioneer factors (including forkhead box protein A1 (FOXA1) and GATA-2), and AR transcriptional coregulators such as the p160 steroid receptor coactivators (SRCs) SRC-1, SRC-2, SRC-3, as well as lysine deacetylases (KDACs) and lysine acetyltransferases (KATs). Researching the effect of-and developing new therapeutic agents that target-the AR signaling axis is critical to advancing our understanding of prostate cancer biology, to continue to improve treatments for prostate cancer and for overcoming castration resistance.

Cotargeting Androgen Receptor Splice Variants and mTOR Signaling Pathway for the Treatment of Castration-Resistant Prostate Cancer

PURPOSE

The PI3K/Akt/mTOR pathway is activated in most castration-resistant prostate cancers (CRPC). Transcriptionally active androgen receptor (AR) plays a role in the majority of CRPCs. Therefore, cotargeting full-length (FL) AR and PI3K/Akt/mTOR signaling has been proposed as a possible, more effective therapeutic approach for CRPC. However, truncated AR-splice variants (AR-V) that are constitutively active and dominant over FL-AR are associated with tumor progression and resistance mechanisms in CRPC. It is currently unknown how blocking the PI3K/Akt/mTOR pathway impacts prostate cancer driven by AR-Vs. Here, we evaluated the efficacy and mechanism of combination therapy to block mTOR activity together with EPI-002, an AR N-terminal domain (NTD) antagonist that blocks the transcriptional activities of FL-AR and AR-Vs in models of CRPC.

EXPERIMENTAL DESIGN

To determine the functional roles of FL-AR, AR-Vs, and PI3K/Akt/mTOR pathways, we employed EPI-002 or enzalutamide and BEZ235 (low dose) or everolimus in human prostate cancer cells that express FL-AR or FL-AR and AR-Vs (LNCaP95). Gene expression and efficacy were examined in vitro and in vivo

RESULTS

EPI-002 had antitumor activity in enzalutamide-resistant LNCaP95 cells that was associated with decreased expression of AR-V target genes (e.g., UBE2C). Inhibition of mTOR provided additional blockade of UBE2C expression. A combination of EPI-002 and BEZ235 decreased the growth of LNCaP95 cells in vitro and in vivo

CONCLUSIONS

Cotargeting mTOR and AR-NTD to block transcriptional activities of FL-AR and AR-Vs provided maximum antitumor efficacy in PTEN-null, enzalutamide-resistant CRPC. Clin Cancer Res; 22(11); 2744-54. ©2015 AACR.

Registered report: androgen receptor splice variants determine taxane sensitivity in prostate cancer

The Prostate Cancer Foundation-Movember Foundation Reproducibility Initiative seeks to address growing concerns about reproducibility in scientific research by conducting replications of recent papers in the field of prostate cancer. This Registered Report describes the proposed replication plan of key experiments from “Androgen Receptor Splice Variants Determine Taxane Sensitivity in Prostate Cancer” by Thadani-Mulero and colleagues (2014) published in Cancer Research in 2014. The experiment that will be replicated is reported in Fig. 6A. Thadani-Mulero and colleagues generated xenografts from two prostate cancer cell lines; LuCaP 86.2, which expresses predominantly the ARv567 splice variant of the androgen receptor (AR), and LuCaP 23.1, which expresses the full length AR as well as the ARv7 variant. Treatment of the tumors with the taxane docetaxel showed that the drug inhibited tumor growth of the LuCaP 86.2 cells but not of the LuCaP 23.1 cells, indicating that expression of splice variants of the AR can affect sensitivity to docetaxel. The Prostate Cancer Foundation-Movember Foundation Reproducibility Initiative is a collaboration between the Prostate Cancer Foundation, the Movember Foundation and Science Exchange, and the results of the replications will be published by PeerJ.

Androgen receptor splice variants circumvent AR blockade by microtubule-targeting agents

Docetaxel-based chemotherapy is established as a first-line treatment and standard of care for patients with metastatic castration-resistant prostate cancer. However, half of the patients do not respond to treatment and those do respond eventually become refractory. A better understanding of the resistance mechanisms to taxane chemotherapy is both urgent and clinical significant, as taxanes (docetaxel and cabazitaxel) are being used in various clinical settings. Sustained signaling through the androgen receptor (AR) has been established as a hallmark of CRPC. Recently, splicing variants of AR (AR-Vs) that lack the ligand-binding domain (LBD) have been identified. These variants are constitutively active and drive prostate cancer growth in a castration-resistant manner. In taxane-resistant cell lines, we found the expression of a major variant, AR-V7, was upregulated. Furthermore, ectopic expression of two clinically relevant AR-Vs (AR-V7 and ARV567es), but not the full-length AR (AR-FL), reduced the sensitivities to taxanes in LNCaP cells. Treatment with taxanes inhibited the transcriptional activity of AR-FL, but not those of AR-Vs. This could be explained, at least in part, due to the inability of taxanes to block the nuclear translocation of AR-Vs. Through a series of deletion constructs, the microtubule-binding activity was mapped to the LBD of AR. Finally, taxane-induced cytoplasm sequestration of AR-FL was alleviated when AR-Vs were present. These findings provide evidence that constitutively active AR-Vs maintain the AR signaling axis by evading the inhibitory effects of microtubule-targeting agents, suggesting that these AR-Vs play a role in resistance to taxane chemotherapy.

Recent advances in allosteric androgen receptor inhibitors for the potential treatment of castration-resistant prostate cancer

Prostate cancer (PC) is the second most frequent cause of male cancer death in the USA. As such, the androgen receptor (AR) plays a crucial role in PC, making AR the major therapeutic target for PC. Current antiandrogen chemotherapy prevents androgen binding to the ligand-binding pocket (LBP) of AR. However, PC frequently recurs despite treatment and it progresses to castration-resistant prostate cancer. Behind this regression is renewed AR signaling initiated via mutations in the LBP. Hence, there is a critical need to improve the therapeutic options to regulate AR activity in sites other than the LBP. Herein, recently disclosed (2010–2015) allosteric AR inhibitors are summarized and a perspective on the potential pharmaceutical intervention at these sites is provided.

Drug resistance in castration resistant prostate cancer: resistance mechanisms and emerging treatment strategies

Several mechanisms facilitate the progression of hormone-sensitive prostate cancer to castration-resistant prostate cancer (CRPC). At present, the approved chemotherapies for CRPC include systemic drugs (docetaxel and cabazitaxel) and agents that target androgen signaling, including enzalutamide and abiraterone. While up to 30% of patients have primary resistance to these treatments, each of these drugs confers a significant survival benefit for many. Over time, however, all patients inevitably develop resistance to treatment and their disease will continue to progress. Several key mechanisms have been identified that give rise to drug resistance. Expression of constitutively active variants of the androgen receptor, such as AR-V7, intracrine androgens and overexpression of androgen synthesis enzymes like AKR1C3, and increased drug efflux through ABCB1 are just some of the many discovered mechanisms of drug resistance. Treatment strategies are being developed to target these pathways and reintroduce drug sensitivity. Niclosamide has been discovered to reduce AR-V7 activity and synergized to enzalutamide. Indomethacin has been explored to inhibit AKR1C3 activity and showed to be able to reverse resistance to enzalutamide. ABCB1 transport activity can be mitigated by the phytochemical apigenin and by antiandrogens such as bicalutamide, with each improving cellular response to chemotherapeutics. By better understanding the mechanisms by which drug resistance develops improved treatment strategies will be made possible. Herein, we review the existing knowledge of CRPC therapies and resistance mechanisms as well as methods that have been identified which may improve drug sensitivity.

Regulation of microtubule dynamics by DIAPH3 influences amoeboid tumor cell mechanics and sensitivity to taxanes

Taxanes are widely employed chemotherapies for patients with metastatic prostate and breast cancer. Here, we show that loss of Diaphanous-related formin-3 (DIAPH3), frequently associated with metastatic breast and prostate cancers, correlates with increased sensitivity to taxanes. DIAPH3 interacted with microtubules (MT), and its loss altered several parameters of MT dynamics as well as decreased polarized force generation, contractility, and response to substrate stiffness. Silencing of DIAPH3 increased the cytotoxic response to taxanes in prostate and breast cancer cell lines. Analysis of drug activity for tubulin-targeted agents in the NCI-60 cell line panel revealed a uniform positive correlation between reduced DIAPH3 expression and drug sensitivity. Low DIAPH3 expression correlated with improved relapse-free survival in breast cancer patients treated with chemotherapeutic regimens containing taxanes. Our results suggest that inhibition of MT stability arising from DIAPH3 downregulation enhances susceptibility to MT poisons, and that the DIAPH3 network potentially reports taxane sensitivity in human tumors.

Androgen pathway resistance in prostate cancer and therapeutic implications

INTRODUCTION

Metastatic prostate cancer is an incurable disease that is treated with a variety of hormonal therapies targeting various nodes of the androgen receptor (AR) pathway. Invariably patients develop resistance and become castration resistant. Common treatments for castration-resistant disease include novel hormonal therapies, such as abiraterone and enzalutamide, chemotherapy, immunotherapy and radiopharmaceuticals. As this disease generally remains incurable, understanding the molecular underpinnings of resistance pathways is critical in designing therapeutic strategies to delay or overcome such resistance.

AREAS COVERED

This review will explore the resistance mechanisms relevant to hormonal agents, such as AR-V7 expression and others, as well as discussing new approaches being developed to treat patients with castration-resistant prostate cancer that take advantage of these new insights. A literature search was performed to identify all published clinical trials related to androgen therapy mechanisms of drug resistance in metastatic castration-resistant prostate cancer.

EXPERT OPINION

Androgen therapy resistance mechanisms are varied, and include modification of all nodes in the androgen signaling pathway. The optimal treatment for men with relapsed metastatic castration-resistant prostate cancer is uncertain at this time. The authors recommend using available clinical data to guide treatment decision making until more specific biomarkers are clinically available.

Exploitation of the Androgen Receptor to Overcome Taxane Resistance in Advanced Prostate Cancer

Prostate cancer is a tumor addicted to androgen receptor (AR) signaling, even in its castration resistant state, and recently developed antiandrogen therapies including Abiraterone acetate and enzalutamide effectively target the androgen signaling axis, but there is ultimately recurrence to lethal disease. Development of advanced castration-resistant prostate cancer (CRPC) is a biological consequence of lack of an apoptotic response of prostate tumor cells to androgen ablation. Taxanes represent the major clinically relevant chemotherapy for the treatment of patients with metastatic CRPC; unfortunately, they do not deliver a cure but an extension of overall survival. First-generation taxane chemotherapies, Docetaxel (Taxotere), effectively target the cytoskeleton by stabilizing the interaction of β-tubulin subunits of microtubules preventing depolymerization, inducing G2M arrest and apoptosis. Shifting the current paradigm is a growing evidence to indicate that Docetaxel can effectively target the AR signaling axis by blocking its nuclear translocation and transcriptional activity in androgen-sensitive and castration-resistant prostate cancer cells, implicating a new mechanism of cross-resistance between microtubule-targeting chemotherapy and antiandrogen therapies. More recently, Cabazitaxel has emerged as a second-line taxane chemotherapy capable of conferring additional survival benefit to patients with CRPC previously treated with Docetaxel or in combination with antiandrogens. Similar to Docetaxel, Cabazitaxel induces apoptosis and G2M arrest; in contrast to Docetaxel, it sustains AR nuclear accumulation although it reduces the overall AR levels and FOXO1 expression. Cabazitaxel treatment also leads to downregulation of the microtubule-depolymerizing mitotic kinesins, MCAK, and HSET, preventing their ability to depolymerize microtubules and thus enhancing sensitivity to taxane treatment. The molecular mechanisms underlying taxane resistance involve mutational alterations in the tubulin subunits, microtubule dynamics, phenotyping programming of the epithelial-to-mesenchymal transition landscape, and the status of AR activity. This chapter discusses the mechanisms driving the therapeutic resistance of taxanes and antiandrogen therapies in CRPC, and the role of AR in potential interventions toward overcoming such resistance in patients with advanced metastatic disease.

N-terminal targeting of androgen receptor variant enhances response of castration resistant prostate cancer to taxane chemotherapy

Taxane-based chemotherapy is an effective treatment for castration-resistant-prostate cancer (CRPC) via stabilization of microtubules. Previous studies identified that the inhibitory effect of microtubule-targeting chemotherapy on androgen receptor (AR) activity was conferred by interfering with AR intracellular trafficking. The N-terminal domain (NTD) of AR was identified as a tubulin-interacting domain that can be effectively targeted by the novel small molecule inhibitor, EPI. Taken together this evidence provided the rationale that targeting AR nuclear translocation and activity via a combination of an antagonist of the AR NTD and taxane-based chemotherapy may enhance the therapeutic response in CRPC. The present study investigated the anti-tumor efficacy of a combination of EPI with Docetaxel chemotherapy, in cell models of CRPC, harboring the AR splice variants in addition to the full length AR. Our findings demonstrate that there was no significant effect on the androgen-mediated nuclear transport of AR variants and AR transcriptional activity by Docetaxel. The therapeutic response to Docetaxel was enhanced by inhibition of the NTD of AR (by EPI) through cycling of epithelial-mesenchymal-transition (EMT) to mesenchymal-epithelial-transition (MET) among prostate cancer epithelial cells. These results support that transient "programming" of EMT by the AR NTD inhibitor, potentially drives the sensitivity of prostate tumors with differential distribution of AR variants to microtubule-targeting chemotherapy. This study is of major significance in dissecting mechanisms to overcome taxane resistance in advanced CRPC.