Reduced expression of the androgen receptor by third generation of antisense shows antitumor activity in models of prostate cancer

Landscape of small nucleic acid therapeutics: moving from the bench to the clinic as next-generation medicines

The ability of small nucleic acids to modulate gene expression via a range of processes has been widely explored. Compared with conventional treatments, small nucleic acid therapeutics have the potential to achieve long-lasting or even curative effects via gene editing. As a result of recent technological advances, efficient small nucleic acid delivery for therapeutic and biomedical applications has been achieved, accelerating their clinical translation. Here, we review the increasing number of small nucleic acid therapeutic classes and the most common chemical modifications and delivery platforms. We also discuss the key advances in the design, development and therapeutic application of each delivery platform. Furthermore, this review presents comprehensive profiles of currently approved small nucleic acid drugs, including 11 antisense oligonucleotides (ASOs), 2 aptamers and 6 siRNA drugs, summarizing their modifications, disease-specific mechanisms of action and delivery strategies. Other candidates whose clinical trial status has been recorded and updated are also discussed. We also consider strategic issues such as important safety considerations, novel vectors and hurdles for translating academic breakthroughs to the clinic. Small nucleic acid therapeutics have produced favorable results in clinical trials and have the potential to address previously "undruggable" targets, suggesting that they could be useful for guiding the development of additional clinical candidates.

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.

Polycation Architecture Affects Complexation and Delivery of Short Antisense Oligonucleotides: Micelleplexes Outperform Polyplexes

Herein, we examine the complexation and biological delivery of a short single-stranded antisense oligonucleotide (ASO) payload with four polymer derivatives that form two architectural variants (polyplexes and micelleplexes): a homopolymer poly(2-dimethylaminoethyl methacrylate) (D), a diblock polymer poly(ethylene glycol)methylether methacrylate-block-poly(2-dimethylaminoethyl methacrylate) (O_bD), and two micelle-forming variants, poly(2-dimethylaminoethyl methacrylate)-block-poly(n-butyl methacrylate) (DB) and poly(ethylene glycol)methylether methacrylate-block-poly(2-dimethylaminoethyl methacrylate)-block-poly(n-butyl methacrylate) (O_bDB). Both polyplexes and micelleplexes complexed ASOs, and the incorporation of an O_b brush enhances colloidal stability. Micellplexes are templated by the size and shape of the unloaded micelle and that micelle-ASO complexation is not sensitive to formulation/mixing order, allowing ease, versatility, and reproducibility in packaging short oligonucleotides. The DB micelleplexes promoted the largest gene silencing, internalization, and tolerable toxicity while the O_bDB micelleplexes displayed enhanced colloidal stability and highly efficient payload trafficking despite having lower cellular uptake. Overall, this work demonstrates that cationic micelles are superior delivery vehicles for ASOs denoting the importance of vehicle architecture in biological performance.

Selective targeting of MYC mRNA by stabilized antisense oligonucleotides

MYC is a prolific proto-oncogene driving the malignant behaviors of numerous common cancers, yet potent and selective cell-permeable inhibitors of MYC remain elusive. In order to ultimately realize the goal of therapeutic MYC inhibition in cancer, we have initiated discovery chemistry efforts aimed at inhibiting MYC translation. Here we describe a series of conformationally stabilized synthetic antisense oligonucleotides designed to target MYC mRNA (MYCASOs). To support bioactivity, we designed and synthesized this focused library of MYCASOs incorporating locked nucleic acid (LNA) bases at the 5'- and 3'-ends, a phosphorothioate backbone, and internal DNA bases. Treatment of MYC-expressing cancer cells with MYCASOs leads to a potent decrease in MYC mRNA and protein levels. Cleaved MYC mRNA in MYCASO-treated cells is detected with a sensitive 5' Rapid Amplification of cDNA Ends (RACE) assay. MYCASO treatment of cancer cell lines leads to significant inhibition of cellular proliferation while specifically perturbing MYC-driven gene expression signatures. In a MYC-induced model of hepatocellular carcinoma, MYCASO treatment decreases MYC protein levels within tumors, decreases tumor burden, and improves overall survival. MYCASOs represent a new chemical tool for in vitro and in vivo modulation of MYC activity, and promising therapeutic agents for MYC-addicted tumors.

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.

Targeting prostate cancer cells with enzalutamide-HDAC inhibitor hybrid drug 2-75

BACKGROUND

The progression of castration-resistant prostate cancer (CRPC) still relies on the function of androgen receptor (AR), achieved by evolving mechanisms to reactivate AR signaling under hormonal therapy. Histone deacetylase inhibitors (HDACis) disrupt cytoplasmic AR chaperone heat shock protein 90 (Hsp90) via HDAC6 inhibition, leading to AR degradation and growth suppression of prostate cancer (PCa) cells. However, current HDACis are not effective in clinical trials treating CRPC.

METHODS

We designed hybrid molecules containing partial chemical scaffolds of AR antagonist enzalutamide (Enz) and HDACi suberoylanilide hydroxamic acid (SAHA) as new anti-PCa agents. We previously demonstrated that Enz-HDACi hybrid drug 2-75 targets both AR and Hsp90, which inhibits the growth of Enz-resistant C4-2 cells. In the current study, we further investigate the molecular and cellular actions of 2-75 and test its anti-PCa effects in vivo.

RESULTS

Compared with Enz, 2-75 had greater AR antagonistic effects by decreasing the stability, transcriptional activity, and nuclear translocation of intracellular AR. In addition to inhibition of full-length AR (FL AR), 2-75 downregulated the AR-V7 variant in multiple PCa cell lines. Mechanistic studies indicated that the AR affinity of 2-75 retains the drug in the cytoplasm of AR + PCa cells and further directs 2-75 to the AR-associated protein complex, which permits localized effects on AR-associated Hsp90. Further, unlike pan-HDACi SAHA, the cytoplasm-retaining property allows 2-75 to significantly inhibit cytoplasmic HDAC6 with limited impact on nuclear HDACs. These selective cytoplasmic actions of 2-75 overcome the unfavorable resistance and toxicity properties associated with classical AR antagonists, HDACis, and Hsp90 inhibitors. Finally, 2-75 showed greater antitumor activities than Enz in vivo on SQ xenografts derived from LNCaP cells.

CONCLUSIONS

Novel therapeutic strategy using newly designed 2-75 and related AR antagonist-HDACi hybrid drugs has great potential for effective treatment of CRPC.

A magic drug target: Androgen receptor

Androgen receptor (AR) is closely associated with a group of hormone-related diseases including the cancers of prostate, breast, ovary, pancreas, etc and anabolic deficiencies such as muscle atrophy and osteoporosis. Depending on the specific type and stage of the diseases, AR ligands including not only antagonists but also agonists and modulators are considered as potential therapeutics, which makes AR an extremely interesting drug target. Here, we at first review the current understandings on the structural characteristics of AR, and then address why and how AR is investigated as a drug target for the relevant diseases and summarize the representative antagonists and agonists targeting five prospective small molecule binding sites at AR, including ligand-binding pocket, activation function-2 site, binding function-3 site, DNA-binding domain, and N-terminal domain, providing recent insights from a target and drug development view. Further comprehensive studies on AR and AR ligands would bring fruitful information and push the therapy of AR relevant diseases forward.

Perspectives on the current and emerging chemical androgen receptor antagonists for the treatment of prostate cancer

INTRODUCTION

 Prostate cancer is the most common cancer in men. Regardless of the initial treatment of localized disease, almost all patients develop castration resistant prostate cancer (CRPC). A better understanding of the molecular mechanisms behind castration resistance has led to the approval of novel oral androgen receptor (AR) antagonists, such as enzalutamide and apalutamide. Indeed, research has accelerated with numerous agents being studied for the management of CRPC. Areas covered: Herein, the authors present currently used and emerging AR antagonists for the treatment of CRPC. Emerging agents include darolutamide, EZN-4176, AZD-3514, and AZD-5312, apatorsen, galeterone, ODM-2014, TRC-253, BMS-641988, and proxalutamide. Expert opinion: Further understanding of the mechanisms leading to castration resistance in prostate cancer can reveal potential targets for the development of novel AR antagonists. Current novel agents are associated with modest clinical and survival benefit, while acquired resistance and safety issues are under continuous evaluation. The combination of AR antagonists used and ideal sequencing strategies are key tasks ahead, along with the investigation of molecular biomarkers for future personalized targeted therapies. In the future, the challenge will be to determine an AR antagonist with the best combination of outcome and tolerability.

PEG-modification on the endo-position of an antisense oligonucleotide increases tumor accumulation via the EPR effect

Nucleic acid medicine is the next-generation therapeutic modality for refractory diseases with its unique mode of action as an alternative to traditional therapies. A nucleic acid delivery system targeted to liver was validated clinically; however, the delivery system of nucleic acids targeting solid tumors following systemic administration is not efficient enough for clinical use. In this study, we first utilized an antisense oligonucleotide (ASO) and polyethylene glycol (PEG) in one-to-one conjugation (PEG-ASO) at the endo-position of the ASO (endo-PEG-ASO). The effects of ASO modification position, PEG structure and molecular weight, and PEG-ASO tumor accumulation were evaluated in vivo. The endo-PEG-ASO showed prolonged pharmacokinetics and enhanced tumor accumulation compared with the conventional ASO and the PEG-ASO modified at the ASO exo-position (exo-PEG-ASO), indicating that the modification position of PEG is crucial for targeting tumors. We also observed that the endo-PEG-ASO indicated possibility of enhanced permeability inside the tumor. Further research is needed to optimize the linker in the endo-PEG-ASO for clinical application as a novel and promising therapeutic format for targeting solid tumors.

Clinical relevance of androgen receptor alterations in prostate cancer

Prostate cancer (PC) remains a leading cause of cancer-related deaths among men worldwide, despite continuously improved treatment strategies. Patients with metastatic disease are treated by androgen deprivation therapy (ADT) that with time results in the development of castration-resistant prostate cancer (CRPC) usually established as metastases within bone tissue. The androgen receptor (AR) transcription factor is the main driver of CRPC development and of acquired resistance to drugs given for treatment of CRPC, while a minority of patients have CRPC that is non-AR driven. Molecular mechanisms behind epithelial AR reactivation in CRPC include AR gene amplification and overexpression, AR mutations, expression of constitutively active AR variants, intra-tumoural and adrenal androgen synthesis and promiscuous AR activation by other factors. This review will summarize AR alterations of clinical relevance for patients with CRPC, with focus on constitutively active AR variants, their possible association with AR amplification and structural rearrangements as well as their ability to predict patient resistance to AR targeting drugs. The review will also discuss AR signalling in the tumour microenvironment and its possible relevance for metastatic growth and therapy.

Androgen Receptor and Beyond, Targeting Androgen Signaling in Castration-Resistant Prostate Cancer

The development of metastatic castration-resistant prostate cancer (mCRPC) signals the terminal disease phase. The preceding hormone-dependent disease setting is effectively managed with androgen deprivation therapy. This foundation of treatment has a high rate of biochemical and clinical response and meaningful clinical benefit but is finite in duration as most cancers will progress to castration resistance. Historically, treatment for mCRPC entailed androgen receptor (AR) inhibitors (nilutamide, flutamide, bicalutamide), nonspecific steroidal biosynthesis inhibitors (ketoconazole, itraconazole), steroids (prednisone, diethylstilbesterol, dexamethasone), or palliative chemotherapy (mitoxantrone, estramustine), but none of these strategies impacted survival. Docetaxel was the first agent to demonstrate a survival improvement in this population, and other therapies followed (cabazitaxel, sipuleucel-T and radium-223). Understanding how prostate cancer cells grow in a systemic androgen-deprived environment further changed this clinical landscape. Deciphering what steroidogenic enzymes are overactive and required for testosterone/dihydrotestosterone synthesis has yielded therapies directed toward both adrenal and tumor-derived androgens. All androgens normally act through AR, and this fact remains true in mCRPC. The cancer accomplishes this by overexpressing the receptor (by genomic copy-number gains or RNA amplification), mutating it directly to lose its selectivity for testosterone/dihydrotestosterone, or selecting for splice variants that do not require ligand at all. These resistance mechanisms result in persistent AR-mediated signaling. Through this understanding, drugs targeting non-ligand-binding aspects of AR functioning (e.g., nuclear translocation, cofactor recruitment) have been developed. Finally, how AR interacts with other signaling pathway is being explored, and new combinations of targets to test are being proposed. Multiple compounds remain in various stages of clinical development based on targeting these resistance pathways, and hopefully, they will further the armamentarium for mCRPC. This review visits these mechanisms of resistance, how they are targeted, and remaining challenges in implementing these therapies into clinical practice among the other approved treatments.

6BIO Enhances Oligonucleotide Activity in Cells: A Potential Combinatorial Anti-androgen Receptor Therapy in Prostate Cancer Cells

Approximately 15%-25% of men diagnosed with prostate cancer do not survive their disease. The American Cancer Society estimated that for the year 2016 the number of prostate cancer deaths will be 26,120. Thus, there is a critical need for novel approaches to treat this deadly disease. Using high-throughput small-molecule screening, we found that the small molecule 6-bromo-indirubin-3'-oxime (6BIO) significantly improves the targeting of antisense oligonucleotides (ASOs) delivered by gymnosis (i.e., in the absence of any transfection reagents) in both the cell cytoplasm and the nucleus. Furthermore, as a single agent, 6BIO had the unexpected ability to simultaneously downregulate androgen receptor (AR) expression and AR signaling in prostate cancer cells. This includes downregulating levels of the AR-V7, a drug-resistance-related AR splice variant that is important in the progression of prostate cancer. Combining 6BIO and an anti-AR oligonucleotide (AR-ASO) can augment the downregulation of AR expression. We also demonstrated that 6BIO enhances ASO function and represses AR expression through the inhibition of the two main glycogen synthase kinase 3 (GSK-3) isoforms: GSK-3α and GSK-3β activity. Our findings provide a rationale for the use of 6BIO as a single agent or as part of a combinatorial ASO-based therapy in the treatment of human prostate cancer.

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.

Novel mechanism-based therapeutics for androgen axis blockade in castration-resistant prostate cancer

PURPOSE OF REVIEW

Understanding the mechanisms by which castration-resistant prostate cancer (CRPC) progresses provides an opportunity to identify novel therapeutic strategies to treat this disease. This understanding has led to approaches to attack prostate cancer's androgen axis in unique ways. This review will examine the classes of novel therapies for androgen axis blockade in CRPC, with a particular focus on the unique characteristics of drugs in various stages of clinical development.

RECENT FINDINGS

The success of abiraterone and enzalutamide has stimulated multiple investigations into novel approaches to attack the androgen-signaling pathway. Drugs under development include cytochrome P17 inhibitors with 17,20-lyase specificity, androgen receptor antagonists that are active against mutated and constitutively active splice variant forms of the protein, androgen receptor degraders, and bromodomain/bromodomain extra-terminal inhibitors that prevent chromatin binding of activated receptors. The clinical development of several of these experimental agents is reviewed.

SUMMARY

Given the unique mechanisms of action for drugs in development, and the possibility that the novel agents may be active in the setting of common resistance mechanisms, treatment options for patients are likely to expand greatly in the coming years. Future studies should prioritize combinations of agents with unique mechanisms of action to optimize outcomes for patients, and should rely on precision-medicine approaches to target known molecular alterations.

Phase I and II therapies targeting the androgen receptor for the treatment of castration resistant prostate cancer

INTRODUCTION

Prostate cancer is the most common cancer in elderly males. Regardless of the initial hormonal treatment in metastatic disease, a significant proportion of patients develop castration resistant prostate cancer (CRPC). A better understanding of the molecular mechanisms behind castration resistance has led to the approval of oral medications such as abiraterone acetate and enzalutamide. Relevant research is accelerated with numerous agents being tested for the management of CRPC.

AREAS COVERED

The authors present Phase I and II studies targeting the androgen receptor for the treatment of CRPC. Three groups of agents are identified according to the mechanism of action. These include the CYP-17 modulators (Orteronel, Galeterone, VT-464 and CFG-920), novel antiandrogens (Apatorsen, ARN-509, ODM-201, EZN-4176, AZD-3514) and bipolar androgen therapy.

EXPERT OPINION

Further understanding of the mechanisms leading to castration resistance in prostate cancer can reveal potential targets for the development of novel anti-cancer agents. Except for the development of novel antiandrogens and CYP-17 modulators, bipolar androgen therapy is an interesting therapeutic approach. The combinations of the novel agents tested in Phase I and II studies with established agents is another field of interest. The real challenge is to distinguish a novel anti-cancer agent with acceptable tolerability and the best outcome.

Protein Kinase C-α is a Critical Protein for Antisense Oligonucleotide-mediated Silencing in Mammalian Cells

We have identified the existence of a productive, PKC-α-dependent endocytotic silencing pathway that leads gymnotically-delivered locked nucleic acid (LNA)-gapmer phosphorothioate antisense oligonucleotides (ASOs) into late endosomes. By blocking the maturation of early endosomes to late endosomes, silencing the expression of PKC-α results in the potent reduction of ASO silencing ability in the cell. We have also demonstrated that silencing of gene expression in the cytoplasm is vitiated when PKC-α expression is reduced. Restoring PKC-α expression via a reconstitution experiment reinstates the ability of ASOs to silence. These results advance our understanding of intracellular ASO trafficking and activity following gymnotic delivery, and further demonstrate the existence of two distinct silencing pathways in mammalian cells, one in the cytoplasmic and the other in the nuclear compartment.

Therapeutic Rationales, Progresses, Failures, and Future Directions for Advanced Prostate Cancer

Patients with localized prostate cancer (PCa) have several therapeutic options with good prognosis. However, survival of patients with high-risk, advanced PCa is significantly less than patients with early-stage, organ-confined disease. Testosterone and other androgens have been directly linked to PCa progression since 1941. In this review, we chronicle the discoveries that led to modern therapeutic strategies for PCa. Specifically highlighted is the biology of androgen receptor (AR), the nuclear receptor transcription factor largely responsible for androgen-stimulated and castrate-recurrent (CR) PCa. Current PCa treatment paradigms can be classified into three distinct but interrelated categories: targeting AR at pre-receptor, receptor, or post-receptor signaling. The continuing challenge of disease relapse as CR and/or metastatic tumors, destined to occur within three years of the initial treatment, is also discussed. We conclude that the success of PCa therapies in the future depends on targeting molecular mechanisms underlying tumor recurrence that still may affect AR at pre-receptor, receptor, and post-receptor levels.

siRNA Lipid Nanoparticle Potently Silences Clusterin and Delays Progression When Combined with Androgen Receptor Cotargeting in Enzalutamide-Resistant Prostate Cancer

PURPOSE

Lipid nanoparticle (LNP) formulations facilitate tumor uptake and intracellular processing through an enhanced permeation and retention effect (EPR), and currently multiple products are undergoing clinical evaluation. Clusterin (CLU) is a cytoprotective chaperone induced by androgen receptor (AR) pathway inhibition to facilitate adaptive survival pathway signaling and treatment resistance. In our study, we investigated the efficacy of siRNA tumor delivery using LNP systems in an enzalutamide-resistant (ENZ-R) castration-resistant prostate cancer (CRPC) model.

EXPERIMENTAL DESIGN

Gene silencing of a luciferase reporter gene in the PC-3M-luc stable cell line was first assessed in subcutaneous and metastatic PC-3 xenograft tumors. Upon validation, the effect of LNP siRNA targeting CLU in combination with AR antisense oligonucleotides (ASO) was assessed in ENZ-R CRPC LNCaP in vitro and in vivo models.

RESULTS

LNP LUC-siRNA silenced luciferase expression in PC-3M-luc subcutaneous xenograft and metastatic models. LNP CLU-siRNA potently suppressed CLU and AR ASO-induced CLU and AKT and ERK phosphorylation in ENZ-R LNCaP cells in vitro, more potently inhibiting ENZ-R cell growth rates and increased apoptosis when compared with AR-ASO monotherapy. In subcutaneous ENZ-R LNCaP xenografts, combinatory treatment of LNP CLU-siRNA plus AR-ASO significantly suppressed tumor growth and serum PSA levels compared with LNP LUC-siRNA (control) and AR-ASO.

CONCLUSIONS

LNP siRNA can silence target genes in vivo and enable inhibition of traditionally non-druggable genes like CLU and other promising cotargeting approaches in ENZ-R CRPC therapeutics.

Targeting the androgen receptor pathway in castration-resistant prostate cancer: progresses and prospects

Androgen receptor (AR) signaling is a critical pathway for prostate cancer cells, and androgen-deprivation therapy (ADT) remains the principal treatment for patients with locally advanced and metastatic disease. However, over time, most tumors become resistant to ADT. The view of castration-resistant prostate cancer (CRPC) has changed dramatically in the last several years. Progress in understanding the disease biology and mechanisms of castration resistance led to significant advancements and to paradigm shift in the treatment. Accumulating evidence showed that prostate cancers develop adaptive mechanisms for maintaining AR signaling to allow for survival and further evolution. The aim of this review is to summarize molecular mechanisms of castration resistance and provide an update in the development of novel agents and strategies to more effectively target the AR signaling pathway.

Generation 2.5 antisense oligonucleotides targeting the androgen receptor and its splice variants suppress enzalutamide-resistant prostate cancer cell growth

PURPOSE

Enzalutamide (ENZ) is a potent androgen receptor (AR) antagonist with activity in castration-resistant prostate cancer (CRPC); however, progression to ENZ-resistant (ENZ-R) CRPC frequently occurs with rising serum PSA levels, implicating AR full-length (ARFL) or variants (AR-Vs) in disease progression.

EXPERIMENTAL DESIGN

To define functional roles of ARFL and AR-Vs in ENZ-R CRPC, we designed 3 antisense oligonucleotides (ASO) targeting exon-1, intron-1, and exon-8 in AR pre-mRNA to knockdown ARFL alone or with AR-Vs, and examined their effects in three CRPC cell lines and patient-derived xenografts.

RESULTS

ENZ-R-LNCaP cells express high levels of both ARFL and AR-V7 compared with CRPC-LNCaP; in particular, ARFL levels were approximately 12-fold higher than AR-V7. Both ARFL and AR-V7 are highly expressed in the nuclear fractions of ENZ-R-LNCaP cells even in the absence of exogenous androgens. In ENZ-R-LNCaP cells, knockdown of ARFL alone, or ARFL plus AR-Vs, similarly induced apoptosis, suppressed cell growth and AR-regulated gene expression, and delayed tumor growth in vivo. In 22Rv1 cells that are inherently ENZ-resistant, knockdown of both ARFL and AR-Vs more potently suppressed cell growth, AR transcriptional activity, and AR-regulated gene expression than knockdown of ARFL alone. Exon-1 AR-ASO also inhibited tumor growth of LTL-313BR patient-derived CRPC xenografts.

CONCLUSIONS

These data identify the AR as an important driver of ENZ resistance, and while the contributions of ARFL and AR-Vs can vary across cell systems, ARFL is the key driver in the ENZ-R LNCaP model. AR targeting strategies against both ARFL and AR-Vs is a rational approach for AR-dependent CRPC.

Acute targeting of general transcription factor IIB restricts cardiac hypertrophy via selective inhibition of gene transcription

BACKGROUND

We previously reported that specialized and housekeeping genes are differentially regulated via de novo recruitment and pause-release of RNA polymerase II, respectively, during cardiac hypertrophy. However, the significance of this finding remains to be examined. Therefore, the purpose of this study was to determine the mechanisms that differentially regulate these gene groups and exploit them for therapeutic targeting.

METHODS AND RESULTS

Here, we show that general transcription factor IIB (TFIIB) and cyclin-dependent kinase 9 are upregulated during hypertrophy, both targeted by microRNA-1, and play preferential roles in regulating those 2 groups of genes. Chromatin immunoprecipitation-sequencing reveals that TFIIB is constitutively bound to all paused, housekeeping, promoters, whereas de novo recruitment of TFIIB and polymerase II is required for specialized genes that are induced during hypertrophy. We exploited this dichotomy to acutely inhibit induction of the latter set, which encompasses cardiomyopathy, immune reaction, and extracellular matrix genes, using locked nucleic acid-modified antisense TFIIB oligonucleotide treatment. This resulted in suppression of all specialized genes, while sparing the housekeeping ones, and, thus, attenuated pathological hypertrophy.

CONCLUSIONS

The data for the first time reveal distinct general TFIIB dynamics that regulate specialized versus housekeeping genes during cardiac hypertrophy. Thus, by acutely targeting TFIIB, we were able to inhibit selectively the former set of genes and ameliorate pressure overload hypertrophy. We also demonstrate the feasibility of acutely and reversibly targeting cardiac mRNA for therapeutic purposes using locked nucleic acid-modified antisense oligonucleotides.

Novel anti-androgen receptor signaling agents: Understanding the mechanisms of resistance

Prostate cancer remains an intractable threat to the lives of men worldwide. Although deaths from prostate cancer (PCa) in the United States have declined in recent years, in other parts of the world Pca mortality is increasing. The introduction of 2nd generation anti-androgen receptor agents into the therapeutic armamentarium for metastatic castration-resistant prostate cancer (mCRPC) has resulted in modestly increased survival advantages as demonstrated by initial clinical trials. However, analysis of the molecular pathways affected by these agents may lead to new insight into mechanisms of resistance that drive mCRPC, including proliferation and survival signaling pathways that are derepressed by maximum repression of androgen signaling. Combination therapies that involve anti-AR signaling agents together with agents that target these pathways establish a paradigm for the development of more effective treatment of mCRPC. In this review, we briefly summarize the current clinical trial literature with regard to novel anti-AR signaling agents such as abiraterone acetate and enzalutamide. We discuss observational data that point to mechanisms of resistance that emerged from these studies. We further present and discuss recent experimental studies that address the mechanisms of resistance to these treatments. Finally, we discuss novel and rational therapeutic approaches, including combination therapy, for patients with mCRPC.

Androgen receptor antagonists for prostate cancer therapy

Androgen deprivation is the mainstay therapy for metastatic prostate cancer (PCa). Another way of suppressing androgen receptor (AR) signaling is via AR antagonists or antiandrogens. Despite being frequently prescribed in clinical practice, there is conflicting evidence concerning the role of AR antagonists in the management of PCa. In the castration-resistant settings of PCa, docetaxel has been the only treatment option for decades. With recent evidence that castration-resistant PCa is far from AR-independent, there has been an increasing interest in developing new AR antagonists. This review gives a concise overview of the clinically available antiandrogens and the experimental AR antagonists that tackle androgen action with a different approach.

Targeting the androgen receptor in prostate cancer

INTRODUCTION

The androgen receptor (AR) is a ligand-activated transcription factor that is expressed in primary and metastatic prostate cancers. There are advances in endocrine therapy for prostate cancer that are based on improved understanding of AR function.

AREAS COVERED

PubMed has been used to include most important publications on targeting the AR in prostate cancer. AR expression may be downregulated by agents used for chemoprevention of prostate cancer or, in models of advanced prostate cancer, by antisense oligonucleotides. New drugs that inhibit the steroidogenic enzyme CYP17A1 (abiraterone acetate) or diminish nuclear translocation of the AR (enzalutamide) have been shown to improve patients' survival in prostate cancer. However, it is clear that there is a development of resistance to these novel therapies. They may include increased expression of truncated, constitutively active AR or activation of the signaling pathway of signal transducers and activators of transcription.

EXPERT OPINION

Although introduction of novel drugs have improved patients' survival, there is a need to investigate the mechanisms of resistance further. The role of truncated AR and compensatory activation of signaling pathways as well as the development of scientifically justified combination therapies seems to be issues of a high priority.

Relaxin receptor antagonist AT-001 synergizes with docetaxel in androgen-independent prostate xenografts

Androgen hormones and the androgen receptor (AR) pathway are the main targets of anti-hormonal therapies for prostate cancer. However, resistance inevitably develops to treatments aimed at the AR pathway resulting in androgen-independent or hormone-refractory prostate cancer (HRPC). Therefore, there is a significant unmet need for new, non-androgen anti-hormonal strategies for the management of prostate cancer. We demonstrate that a relaxin hormone receptor antagonist, AT-001, an analog of human H2 relaxin, represents a first-in-class anti-hormonal candidate treatment designed to significantly curtail the growth of androgen-independent human prostate tumor xenografts. Chemically synthesized AT-001, administered subcutaneously, suppressed PC3 xenograft growth by up to 60%. AT-001 also synergized with docetaxel, standard first-line chemotherapy for HRPC, to suppress tumor growth by more than 98% in PC3 xenografts via a mechanism involving the downregulation of hypoxia-inducible factor 1 alpha and the hypoxia-induced response. Our data support developing AT-001 for clinical use as an anti-relaxin hormonal therapy for advanced prostate cancer.

Evolution of androgen receptor targeted therapy for advanced prostate cancer

The discovery of androgen dependence in prostate cancer in 1941 by Huggins and colleagues has remained the backbone for the treatment of this disease. However, although many patients initially respond to androgen depletion therapy, they almost invariably relapse and develop resistance with transition of the disease to a castration-resistant state. Over the past decade, the better understanding of the mechanisms that drive resistance to castration has led to the development of next-generation androgen receptor targeting agents such as abiraterone acetate and enzalutamide. This Review aims to revisit the discovery and evolution of androgen receptor targeting therapeutics for the treatment of advanced-stage prostate cancer over the years and to discuss the upcoming future and challenges in the treatment of this common cancer.

Targeting metastatic castration-resistant prostate cancer: mechanisms of progression and novel early therapeutic approaches

INTRODUCTION

Advances in clinical research have led to official approval of several new treatments for metastatic prostate cancer in the last three years: sipuleucel-T, cabazitaxel, abiraterone acetate, radium-223 and enzalutamide. Although these agents have all been shown to improve overall survival in randomized Phase III trials, metastatic castration-resistant prostate cancer (mCRPC) remains incurable.

AREAS COVERED

First, the review summarizes the current literature on the biology of mCRPC. The emerging data are increasing our understanding of the mechanisms that underlie the pathogenesis of castrate resistance and where future treatment might be headed. In the second part of the review, the authors assess the future directions in disease therapy. Indeed, novel selected therapeutic approaches, including novel agents and combinatorial therapies, are showing promising early results.

EXPERT OPINION

Targeting different molecular pathways in combination with immunotherapy can be a promising direction in metastatic castration prostate cancer treatment. However, several challenges still exist including elucidating the optimal use and sequencing of these new agents. There are also challenges in both the design and the interpretation of the results from clinical trials.

First-in-human Phase I study of EZN-4176, a locked nucleic acid antisense oligonucleotide to exon 4 of the androgen receptor mRNA in patients with castration-resistant prostate cancer

BACKGROUND

Prostate cancer remains dependent of androgen receptor (AR) signalling, even after emergence of castration resistance. EZN-4176 is a third-generation antisense oligonucleotide that binds to the hinge region (exon 4) of AR mRNA resulting in full-length AR mRNA degradation and decreased AR protein expression. This Phase I study aimed to evaluate EZN-4176 in men with castration-resistant prostate cancer (CRPC).

METHODS

Patients with progressing CRPC were eligible; prior abiraterone and enzalutamide treatment were allowed. EZN-4176 was administered as a weekly (QW) 1-h intravenous infusion. The starting dose was 0.5 mg kg(-1) with a 4-week dose-limiting toxicity (DLT) period and a 3+3 modified Fibonacci dose escalation design. After determination of the DLT for weekly administration, an every 2 weeks schedule was initiated.

RESULTS

A total of 22 patients were treated with EZN-4176. At 10 mg kg(-1) QW, two DLTs were observed due to grade 3-4 ALT or AST elevation. No confirmed biochemical or soft tissue responses were observed. Of eight patients with <5 circulating tumour cells at baseline, a conversion to <5 was observed in three (38%) patients. The most common EZN-4176-related toxicities (all grades) were fatigue (59%), reversible abnormalities in liver function tests ALT (41%) and AST (41%) and infusion-related reactions including chills (36%) and pyrexia (14%).

CONCLUSION

Activity of EZN-4176 at the doses and schedules explored was minimal. The highest dose of 10 mg kg(-1) QW was associated with significant but reversible transaminase elevation.

Molecular pathogenesis and progression of prostate cancer

Prostate cancer (PCa) is the most commonly diagnosed noncutaneous malignancy and second leading cause of cancer-related deaths in US males. Clinically, locally confined disease is treated surgically and/or with radiation therapy. Invasive disease, however, must be treated with pharmacological inhibitors of androgen receptor (AR) activity, since disease progression is fundamentally reliant on AR activation. However, despite initially effective treatment options, recurrent castration-resistant PCa (CRPC) often occurs due to aberrant reactivation of AR. Additionally, it is appreciated that many other signaling molecules, such as transcription factors, oncogenes, and tumor suppressors, are often perturbed and significantly contribute to PCa initiation and progression to incurable disease. Understanding the interplay between AR signaling and other signaling networks altered in PCa will advance therapeutic approaches. Overall, comprehension of the molecular composition promoting neoplastic growth and formation of CRPC is paramount for developing durable treatment options.

Experimental evidence of persistent androgen-receptor-dependency in castration-resistant prostate cancer

In the majority of castration-resistant prostate cancer (CRPC), prostate-specific antigen (PSA), product of a gene that is almost exclusively regulated by the androgen receptor (AR), still acts as a serum marker reflecting disease burden, indicating that AR signaling is activated even under castrate level of serum androgen. Accumulated evidence shows that transcriptional ability of AR is activated both in ligand-dependent and -independent manners in CRPC cells. Some androgen-independent sublines derived from originally androgen-dependent LNCaP prostate cancer cells overexpress the AR and PSA, for which silencing the AR gene suppresses cellular proliferation. The overexpression of the AR confers androgen-independent growth ability on androgen-dependent prostate cancer cells. Some patient-derived prostate cancer xenograft lines also acquire castration-resistant growth ability secreting PSA. More recent publications have shown that the AR activated in CRPC cells regulates distinct gene sets from that in androgen-dependent status. This concept provides very important insights in the development of novel anti-prostate cancer drugs such as new generation anti-androgens and CYP17 inhibitors.

Recent progress in pharmaceutical therapies for castration-resistant prostate cancer

Since 2010, six drugs have been approved for the treatment of castration-resistant prostate cancer, i.e., CYP17 inhibitor Abiraterone, androgen receptor antagonist Enzalutamide, cytotoxic agent Cabazitaxel, vaccine Sipuleucel-T, antibody Denosumab against receptor activator of nuclear factor kappa B ligand and radiopharmaceutical Alpharadin. All these drugs demonstrate improvement on overall survival, expect for Denosumab, which increases the bone mineral density of patients under androgen deprivation therapy and prolongs bone-metastasis-free survival. Besides further CYP17 inhibitors (Orteronel, Galeterone, VT-464 and CFG920), androgen receptor antagonists (ARN-509, ODM-201, AZD-3514 and EZN-4176) and vaccine Prostvac, more drug candidates with various mechanisms or new indications of launched drugs are currently under evaluation in different stages of clinical trials, including various kinase inhibitors and platinum complexes. Some novel strategies have also been proposed aimed at further potentiation of antitumor effects or reduction of side effects and complications related to treatments. Under these flourishing circumstances, more investigations should be performed on the optimal combination or the sequence of treatments needed to delay or reverse possible resistance and thus maximize the clinical benefits for the patients.

Emerging therapeutics targeting castration-resistant prostate cancer: the AR-mageddon of tumor epithelial-mesenchymal transition

Advanced prostate cancer will claim nearly 30,000 lives among men in the USA in the year 2013. Most of these will be castration-resistant prostate cancers that are not responsive to traditional therapeutic modalities, and there is no available regimen that fully eradicates metastatic disease. This poses a significant clinical challenge for practitioners and has stimulated the development of novel agents that target these castration-resistant tumor cells. Development of metastatic prostate cancer is orchestrated by multiple signaling pathways that regulate cell survival, apoptosis, anoikis, epithelial-mesenchymal transition (EMT), invasion, the androgen signaling axis and angiogenesis. Disruption of the mechanisms underlying these processes is critical for development of agents that can target otherwise resistant tumor cells. Insights into the mechanisms by which rounds of EMT/mesenchymal-epithelial transition conversions facilitate the progression of localized prostate carcinomas to advanced metastatic and castration-resistant disease emerge as attractive targets for drug development. In this review, the authors discuss the current understanding of therapeutic resistance in castration-resistant prostate cancer with focus on the androgen receptor signaling axis and EMT. Novel therapeutic approaches targeting critical players of both pathways as well as the results from ongoing clinical trials will be discussed in this review.

Androgen receptor promotes ligand-independent prostate cancer progression through c-Myc upregulation

The androgen receptor (AR) is the principal therapeutic target in prostate cancer. For the past 70 years, androgen deprivation therapy (ADT) has been the major therapeutic focus. However, some patients do not benefit, and those tumors that do initially respond to ADT eventually progress. One recently described mechanism of such an effect is growth and survival-promoting effects of the AR that are exerted independently of the AR ligands, testosterone and dihydrotestosterone. However, specific ligand-independent AR target genes that account for this effect were not well characterized. We show here that c-Myc, which is a key mediator of ligand-independent prostate cancer growth, is a key ligand-independent AR target gene. Using microarray analysis, we found that c-Myc and AR expression levels strongly correlated with each other in tumors from patients with castration-resistant prostate cancer (CRPC) progressing despite ADT. We confirmed that AR directly regulates c-Myc transcription in a ligand-independent manner, that AR and c-Myc suppression reduces ligand-independent prostate cancer cell growth, and that ectopic expression of c-Myc attenuates the anti-growth effects of AR suppression. Importantly, treatment with the bromodomain inhibitor JQ1 suppressed c-Myc function and suppressed ligand-independent prostate cancer cell survival. Our results define a new link between two critical proteins in prostate cancer – AR and c-Myc – and demonstrate the potential of AR and c-Myc-directed therapies to improve prostate cancer control.

Emerging molecularly targeted therapies in castration refractory prostate cancer

Androgen deprivation therapy (ADT) with medical or surgical castration is the mainstay of therapy in men with metastatic prostate cancer. However, despite initial responses, almost all men eventually develop castration refractory metastatic prostate cancer (CRPC) and die of their disease. Over the last decade, it has been recognized that despite the failure of ADT, most prostate cancers maintain some dependence on androgen and/or androgen receptor (AR) signaling for proliferation. Furthermore, androgen independent molecular pathways have been identified as drivers of continued progression of CRPC. Subsequently, drugs have been developed targeting these pathways, many of which have received regulatory approval. Agents such as abiraterone, enzalutamide, orteronel (TAK-700), and ARN-509 target androgen signaling. Sipuleucel-T, ipilimumab, and tasquinimod augment immune-mediated tumor killing. Agents targeting classic tumorogenesis pathways including vascular endothelial growth factor, hepatocyte growth factor, insulin like growth factor-1, tumor suppressor, and those which regulate apoptosis and cell cycles are currently being developed. This paper aims to focus on emerging molecular pathways underlying progression of CRPC, and the drugs targeting these pathways, which have recently been approved or have reached advanced stages of development in either phase II or phase III clinical trials.

Downregulation of HER3 by a novel antisense oligonucleotide, EZN-3920, improves the antitumor activity of EGFR and HER2 tyrosine kinase inhibitors in animal models

Among the four human EGF receptor (HER) family members (EGFR, HER2, HER3, HER4), HER3 is of particular interest as it interacts with HER2 and EGFR via heterodimerization and is a key link to the phosphoinositide 3-kinase (PI3K)/AKT signal transduction axis. Recent studies indicate that HER3 plays a critical role in mediating resistance to agents that target EGFR or HER2. As HER3 lacks significant kinase activity and cannot be inhibited by tyrosine kinase inhibitors, neutralizing antibodies and alternative inhibitors of HER3 have been sought as cancer therapeutics. We describe here a locked nucleic acid (LNA)-based HER3 antisense oligonucleotide, EZN-3920, that specifically downmodulated the expression of HER3, which was associated with growth inhibition. EZN-3920 effectively downmodulated HER3 expression, HER3-driven PI3K/AKT signaling pathway, and growth in tumors derived from BT474M1 breast and HCC827 lung carcinoma cell lines, which overexpress HER2 and EGFR, respectively. Furthermore, when EZN-3920 was coadministered with gefitinib or lapatinib in xenograft tumor models, enhanced antitumor activity compared with the effect of monotherapy was found. The effect was associated with a blockade of induced HER3 mRNA expression caused by lapatinib or gefitinib treatment. Finally, EZN-3920 sustained its antiproliferative effect in trastuzumab-resistant cells and three independently derived gefitinib-resistant cells. Our findings show that downmodulation of HER3 by EZN-3920 leads to the suppression of tumor growth in vitro and in vivo, suggesting that HER3 can be an effective target for the treatment of various cancers that have been activated by HER3 alone or where HER3 activation is associated with EGFR or HER2 expression.

Targeting the androgen receptor in the management of castration-resistant prostate cancer: rationale, progress, and future directions

Since the year 2000, tremendous progress has been made in the understanding of castration-resistant prostate cancer (crpc), a disease state now recognized to retain androgen receptor (ar)-dependency in most cases. That understanding led to the rational design of novel therapeutic agents targeting hormonal pathways in metastatic crpc. Two new drugs-the CYP17 inhibitor abiraterone acetate and the potent ar antagonist enzalutamide-were recently shown to prolong overall survival after chemotherapy treatment in patients with metastatic disease, with the former agent also demonstrating impressive activity in the pre-chemotherapy setting. Other new drugs targeting the ar-as well as drugs targeting heat shock proteins that protect cytoplasmic ar from degradation-are currently undergoing clinical development.This review briefly describes the molecular mechanisms underlying castration resistance and hormonal dependence in prostate tumours and summarizes the current ongoing and completed clinical trials that are targeting hormonal pathways in metastatic crpc. Potential mechanisms of resistance to these novel hormonal agents are reviewed. Finally, future research directions, including questions about drug sequencing and combination, are discussed.

Androgen receptor antagonists in castration-resistant prostate cancer

Persistent androgen receptor (AR) signaling despite low levels of serum androgens has been identified as a critical target for drug discovery in castration-resistant prostate cancer (CRPC). As proof of principle that the AR remains relevant in CRPC, 2 AR-targeted agents recently approved by the Food and Drug Administration-abiraterone and enzalutamide-have increased overall survival for patients with CRPC in the setting of prior chemotherapy. This review focuses on the AR and 2 direct antagonists, enzalutamide and ARN-509. These next-generation AR antagonists offer great promise for patients with advanced disease. Relative to conventional antiandrogens such as bicalutamide, they bind to the receptor with higher affinity, prevent nuclear translocation and DNA binding, and induce apoptosis without agonist activity in preclinical models. The success of these AR-targeted agents in the clinic has changed the landscape of therapy for patients with CRPC, and further therapeutic options building on this platform are currently in development.

Biological activity and biotechnological aspects of locked nucleic acids

Locked nucleic acid (LNA) is one of the most promising new nucleic acid analogues that has been produced under the past two decades. In this chapter, we have tried to cover many of the different areas, where this molecule has been used to improve the function of synthetic oligonucleotides (ONs). The use of LNA in antisense ONs, including gapmers, splice-switching ONs, and siLNA, as well as antigene ONs, is reviewed. Pharmacokinetics as well as pharmacodynamics of LNA ONs and a description of selected compounds in, or close to, clinical testing are described. In addition, new LNA modifications and the adaptation of enzymes for LNA incorporation are reviewed. Such enzymes may become important for the development of stabilized LNA-containing aptamers.