Synergistic effect of a novel antiandrogen, VN/124-1, and signal transduction inhibitors in prostate cancer progression to hormone independence in vitro

Updates on Overcoming Bicalutamide Resistance: A Glimpse into Resistance to a Novel Antiandrogen

The standard androgen deprivation therapy for advanced prostate cancer includes the use of bicalutamide, which is a well-known antagonist of androgen receptors. Despite numerous benefits of the drugs in prostate cancer treatment, there is always a risk of developing a resistant phenotype, which paves the way for a more aggressive and low-survival type of prostate cancer. Over the years, many studies have investigated the candidate mechanisms of such resistance and have managed to find possible therapeutic solutions. In this Review, we shed light on the heterogeneous dynamics of progression to resistance against bicalutamide treatment, referring to the most recent studies and the approaches that have been so far discussed. This Review tries to offer a deep and comprehensive understanding about how the resistant cells become sensitive to the drug and what corresponding pathways lead to an appropriate solution for the antiandrogen resistance challenge.

Benzimidazole and its derivatives as cancer therapeutics: The potential role from traditional to precision medicine

Cancer is the second leading cause of mortality globally which remains a continuing threat to human health today. Drug insensitivity and resistance are critical hurdles in cancer treatment; therefore, the development of new entities targeting malignant cells is considered a high priority. Targeted therapy is the cornerstone of precision medicine. The synthesis of benzimidazole has garnered the attention of medicinal chemists and biologists due to its remarkable medicinal and pharmacological properties. Benzimidazole has a heterocyclic pharmacophore, which is an essential scaffold in drug and pharmaceutical development. Multiple studies have demonstrated the bioactivities of benzimidazole and its derivatives as potential anticancer therapeutics, either through targeting specific molecules or non-gene-specific strategies. This review provides an update on the mechanism of actions of various benzimidazole derivatives and the structure‒activity relationship from conventional anticancer to precision healthcare and from bench to clinics.

Signalling transduction of O-GlcNAcylation and PI3K/AKT/mTOR-axis in prostate cancer

Hexosamine biosynthetic (HBP) and PI3K/AKT/mTOR pathways are found to predominate the proliferation and survival of prostate cancer cells. Both these pathways have their own specific intermediates to propagate the secondary signals in down-stream cascades and besides having their own structured network, also have shared interconnecting branches. These interconnections are either competitive or co-operative in nature depending on the microenvironmental conditions. Specifically, in prostate cancer HBP and mTOR pathways increases the expression and protein level of androgen receptor in order to support cancer cell proliferation, advancement and metastasis. Pharmacological inhibition of a single pathway is therefore insufficient to stop disease progression as the cancer cells manage to alter the signalling channel. This is one of the primary reasons for the therapeutic failure in prostate cancer and emergence of chemoresistance. Inhibition of these multiple pathways at their common junctures might prove to be of benefit in men suffering from an advanced disease state. Hence, a thorough understanding of these cellular intersecting points and their significance with respect to signal transduction mechanisms might assist in the rational designing of combinations for effective management of prostate cancer.

Impact of Addition of Metformin to Abiraterone in Metastatic Castration-Resistant Prostate Cancer Patients With Disease Progressing While Receiving Abiraterone Treatment (MetAb-Pro): Phase 2 Pilot Study

BACKGROUND

There is evidence linking metformin to improved prostate cancer-related outcomes.

PATIENTS AND METHODS

Twenty-five men with metastatic castration-resistant prostate cancer and prostate-specific antigen (PSA) progression while receiving treatment with abiraterone from 3 Swiss centers were included in this single-arm phase 2 trial between November 2013 and September 2016. Metformin was added to abiraterone continuously at 1000 mg twice daily in uninterrupted 4-week cycles. The primary end point was the absence of disease progression at 12 weeks (PFS12). The Fleming single-stage design was applied. With a 5% significance level and 80% power, 25 patients were required to test PFS12 ≤ 15% (H0) compared to ≥ 35% (H1). Secondary end points included toxicity and safety issues. The study was registered at ClinicalTrials.gov (NCT01677897).

RESULTS

The primary end point PFS12 was 12% (3 of 25 patients) (95% confidence interval, 3-31). Most patients had PSA progression, almost half had radiographic progression, but only 1 patient had symptomatic progression. Eleven (44%) of 25 patients had grade 1 and 2 patients each grade 2 (8%) or grade 3 (8%) gastrointestinal toxicity (nausea, diarrhea, loss of appetite). One patient discontinued treatment at week 5 because of intolerable grade 3 diarrhea.

CONCLUSION

The addition of metformin to abiraterone for patients with metastatic castration-resistant prostate cancer and PSA progression while receiving abiraterone therapy does not affect further progression and has no meaningful clinical benefit. A higher-than-expected gastrointestinal toxicity attributed to metformin was observed.

PI3K pathway and Bcl-2 family. Clinicopathological features in prostate cancer

The phosphatidylinositol 3-kinase (PI3K)/AKT/mTOR pathways and Bcl-2 family play a central role in prostate cancer (PC). The aim was to determine influence in the biochemical progression in PC. To evaluate the association between clinic pathological and immunohistochemical variables, Spearman's test was performed. Log-rank test and Kaplan-Meier curves were used for survival comparisons. To explore the correlation of the studied immunohistochemical parameters and the established prognostic variables with biochemical progression, univariate and multivariate Cox proportional Hazard regression analyses were performed. Spearman analysis showed correlation between stroma expression and tumor expression of PI3K with biochemical progression (p = .009, p = .004), respectively, and tumor immunohistochemical score with biochemical progression (p = .051). In the multivariate Cox regression model, only PI3K was retained as independent predictors of biochemical progression. In stroma expression, PI3K is (HR 0.172, 95% CI 0.065-0.452, p = .000); tumor expression, PI3K is (HR 0.087, 95% CI 0.026-0.293, p = .000), and tumor immunohistochemical score (HR 0.382, 95% CI 0.209-0.697 p = .002). Our results suggest a role for prostatic expression of PI3K was prognostic markers for PC. PI3K/AKT/mTOR and Bcl-2 family are becoming an important therapeutic target and predictive biomarkers of onset and progression of PC.

Novel non-AR therapeutic targets in castrate resistant prostate cancer

Castrate resistant prostate cancer (CRPC) remains a disease with significant morbidity and mortality. The recent approval of abiraterone and enzalutamide highlight the improvements which can be made targeting the androgen receptor (AR) axis. Nonetheless, resistance inevitably develops and there is continued interest in targeting alternate pathways which cause disease resistance and progression. Here, we review non-AR targets in CRPC, with an emphasis on novel agents now in development. This includes therapeutics which target the tumour microenvironment, the bone metastatic environment, microtubules, cellular energetics, angiogenesis, the stress response, survival proteins, intracellular signal transduction, DNA damage repair and dendritic cells. Understanding the hallmarks of prostate cancer resistance in CRPC has led to the identification and development of these new targets. We review the molecular rationale, as well at the clinical experience for each of these different classes of agents which are in clinical development.

Androgen Receptor Modulation Optimized for Response (ARMOR) Phase I and II Studies: Galeterone for the Treatment of Castration-Resistant Prostate Cancer

PURPOSE

Galeterone is a selective, multitargeted agent that inhibits CYP17, antagonizes the androgen receptor (AR), and reduces AR expression in prostate cancer cells by causing an increase in AR protein degradation. These open-label phase I and II studies [Androgen Receptor Modulation Optimized for Response-1 (ARMOR1) and ARMOR2 part 1] evaluated the efficacy and safety of galeterone in patients with treatment-naive nonmetastatic or metastatic castration-resistant prostate cancer (CRPC) and established a dose for further study.

EXPERIMENTAL DESIGN

In ARMOR1, 49 patients received increasing doses (650-2,600 mg) of galeterone in capsule formulation; 28 patients in ARMOR2 part 1 received increasing doses (1,700-3,400 mg) of galeterone in tablet formulation for 12 weeks. Patients were evaluated biweekly for safety and efficacy, and pharmacokinetic parameters were assessed.

RESULTS

In ARMOR1, across all doses, 49.0% (24/49) achieved a ≥30% decline in prostate-specific antigen (PSA; PSA30) and 22.4% (11/49) demonstrated a ≥50% PSA decline (PSA50). In ARMOR2 part 1, across all doses, PSA30 was 64.0% (16/25) and PSA50 was 48.0% (12/25). In the 2,550-mg dose cohort, PSA30 was 72.7% (8/11) and PSA50 was 54.5% (6/11). Galeterone was well tolerated; the most common adverse events were fatigue, increased liver enzymes, gastrointestinal events, and pruritus. Most were mild or moderate in severity and required no action and there were no apparent mineralocorticoid excess (AME) events.

CONCLUSIONS

The efficacy and safety from ARMOR1 and ARMOR2 part 1 and the pharmacokinetic results support the galeterone tablet dose of 2,550 mg/d for further study. Galeterone was well tolerated and demonstrated pharmacodynamic changes consistent with its selective, multifunctional AR signaling inhibition.

Galeterone and VNPT55 induce proteasomal degradation of AR/AR-V7, induce significant apoptosis via cytochrome c release and suppress growth of castration resistant prostate cancer xenografts in vivo

Galeterone (Gal) is a first-in-class multi-target oral small molecule that will soon enter pivotal phase III clinical trials in castration resistant prostate cancer (CRPC) patients. Gal disrupts androgen receptor (AR) signaling via inhibition of CYP17, AR antagonism and AR degradation. Resistance to current therapy is attributed to up-regulation of full-length AR (fAR), splice variants AR (AR-Vs) and AR mutations. The effects of gal and VNPT55 were analyzed on f-AR and AR-Vs (AR-V7/ARv567es) in LNCaP, CWR22Rv1 and DU145 (transfected with AR-Vs) human PC cells in vitro and CRPC tumor xenografts. Galeterone/VNPT55 decreased fAR/AR-V7 mRNA levels and implicates Mdm2/CHIP enhanced ubiquitination of posttranslational modified receptors, targeting them for proteasomal degradation. Gal and VNPT55 also induced significant apoptosis in PC cells via increased Bax/Bcl2 ratio, cytochrome-c release with concomitant cleavage of caspase 3 and PARP. More importantly, gal and VNPT55 exhibited strong in vivo anti-CRPC activities, with no apparent host toxicities. This study demonstrate that gal and VNPT55 utilize cell-based mechanisms to deplete both fAR and AR-Vs. Importantly, the preclinical activity profiles, including profound apoptotic induction and inhibition of CRPC xenografts suggest that these agents offer considerable promise as new therapeutics for patients with CRPC and those resistant to current therapy.

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.

Identification of ML-9 as a lysosomotropic agent targeting autophagy and cell death

The growing number of studies suggested that inhibition of autophagy enhances the efficacy of Akt kinase inhibitors in cancer therapy. Here, we provide evidence that ML-9, a widely used inhibitor of Akt kinase, myosin light-chain kinase (MLCK) and stromal interaction molecule 1 (STIM1), represents the ‘two-in-one' compound that stimulates autophagosome formation (by downregulating Akt/mammalian target of rapamycin (mTOR) pathway) and inhibits their degradation (by acting like a lysosomotropic agent and increasing lysosomal pH). We show that ML-9 as a monotherapy effectively induces prostate cancer cell death associated with the accumulation of autophagic vacuoles. Further, ML-9 enhances the anticancer activity of docetaxel, suggesting its potential application as an adjuvant to existing anticancer chemotherapy. Altogether, our results revealed the complex effect of ML-9 on autophagy and indentified ML-9 as an attractive tool for targeting autophagy in cancer therapy through dual inhibition of both the Akt pathway and the autophagy.

Steroidal 5α-reductase and 17α-hydroxylase/17,20-lyase (CYP17) inhibitors useful in the treatment of prostatic diseases

The role of steroidal inhibitors of androgen biosynthesis as potential weapons in the treatment of prostatic diseases, such as benign prostatic hyperplasia and prostatic cancer will be reviewed. Two enzymes have been targeted in the development of inhibitors that potentially could be useful in the management of such conditions. 5α-Reductase is primarily of interest in benign prostatic disease, though some role in the chemoprevention of prostatic carcinoma have been considered, whereas the 17α-hydroxylase/17,20-lyase (CYP17) enzyme is of interest in the treatment of malignant disease. An overview of the main achievements obtained during the past years will be presented, however special focus will be made on steroidal molecules that reached clinical trials or have been commercially launched. Relevant examples of such drugs are finasteride, dutasteride, abiraterone acetate and galeterone (TOK-001, formerly known as VN/124-1). This article is part of a Special Issue entitled "Synthesis and biological testing of steroid derivatives as inhibitors".

Tolerability, safety and pharmacokinetics of ridaforolimus in combination with bicalutamide in patients with asymptomatic, metastatic castration-resistant prostate cancer (CRPC)

PURPOSE

Recent data indicate that there is a significant cross-talk between the PI3K/Akt/mTOR and androgen receptor signaling pathways. We evaluated safety and tolerability as well as potential drug-drug interaction of ridaforolimus, a mammalian target of rapamycin (mTOR) inhibitor, when combined with the androgen receptor inhibitor bicalutamide in patients with asymptomatic, metastatic castration-resistant prostate cancer.

PATIENTS AND METHODS

Patients were treated with the combination of ridaforolimus 30 mg/day for 5 consecutive days each week and bicalutamide 50 mg/day. Ridaforolimus pharmacokinetics was assessed with and without bicalutamide.

RESULTS

Twelve patients were enrolled including 1 screen failure. Dose reductions were required in 7 patients. Three of the 11 patients experienced a dose-limited toxicity, 1 with Grade 3 hyperglycemia and 2 with Grade 2 stomatitis leading to <75 % of planned ridaforolimus dose during the first 35 days of study treatment. The pharmacokinetic results showed no differences in exposures to ridaforolimus with and without concomitant bicalutamide administration.

CONCLUSIONS

Although there was no evidence of a clinically relevant pharmacological drug-drug interaction, the occurrence of dose-limiting toxicities in 3 of 11 evaluable patients at a reduced dose of ridaforolimus of 30 mg/day suggests that this combination may not be well suited for asymptomatic or minimally symptomatic prostate cancer patients.

Surface α-enolase promotes extracellular matrix degradation and tumor metastasis and represents a new therapeutic target

In previous research, we found α-enolase to be inversely correlated with progression-free and overall survival in lung cancer patients and detected α-enolase on the surface of lung cancer cells. Based on these findings, we hypothesized that surface α-enolase has a significant role in cancer metastasis and tested this hypothesis in the current study. We found that α-enolase was co-immunoprecipitated with urokinase-type plasminogen activator, urokinase-type plasminogen activator receptor, and plasminogen in lung cancer cells and interacted with these proteins in a cell-free dot blotting assay, which can be interrupted by α-enolase-specific antibody. α-Enolase in lung cancer cells co-localized with these proteins and was present at the site of pericellular degradation of extracellular matrix components. Treatment with antibody against α-enolase in vitro suppressed cell-associated plasminogen and matrix metalloproteinase activation, collagen and gelatin degradation, and cell invasion. Examination of the effect of treatment with shRNA plasmids revealed that down regulation of α-enolase decreases extracellular matrix degradation by and the invasion capacity of lung cancer cells. Adoptive transfer of α-enolase-specific antibody to mice resulted in accumulation of antibody in subcutaneous tumor and inhibited the formation of tumor metastasis in lung and bone. This study demonstrated that surface α-enolase promotes extracellular matrix degradation and invasion of cancer cells and that targeting surface α-enolase is a promising approach to suppress tumor metastasis.

Agents that target androgen synthesis in castration-resistant prostate cancer

In April 2011, abiraterone acetate (in combination with low-dose steroids) was approved by the US Food and Drug Administration for the treatment of men with metastatic, castration-resistant prostate cancer who have previously been treated with docetaxel-based chemotherapy. The development of abiraterone was the successful result of an improved understanding of the role of the androgen receptor signaling pathway in castration-resistant prostate cancer. Abiraterone is a rationally designed potent inhibitor of cytochrome P450, family 17, subfamily A, polypeptide 1, which is essential for synthesis of testosterone from nongonadal precursors. More recently, other drugs that act along the androgen0synthesis pathway, such as orteronel (TAK-700) and galeterone (TOK-001), have shown promise in early clinical trials. Here, we review the discovery and clinical development of abiraterone and other novel androgen-synthesis inhibitors for the management of advanced prostate cancer.

Systematic structure modifications of multitarget prostate cancer drug candidate galeterone to produce novel androgen receptor down-regulating agents as an approach to treatment of advanced prostate cancer

As part of our program to explore the influence of small structural modifications of our drug candidate 3β-(hydroxy)-17-(1H-benzimidazol-1-yl)androsta-5,16-diene (galeterone, 5) on the modulation of the androgen receptor (AR), we have prepared and evaluated a series of novel C-3, C-16, and C-17 analogues. Using structure activity analysis, we established that the benzimidazole moiety at C-17 is essential and optimal and also that hydrophilic and heteroaromatic groups at C-3 enhance both antiproliferative (AP) and AR degrading (ARD) activities. The most potent antiproliferative compounds were 3β-(1H-imidazole-1-carboxylate)-17-(1H-benzimidazol-1-yl)androsta-5,16-diene (47), 3-((EZ)-hydroximino)-17-(1H-benzimidazol-1-yl)androsta-4,16-diene (36), and 3β-(pyridine-4-carboxylate)-17-(1H-benzimidazol-1-yl)androsta-5,16-diene (43), with GI50 values of 0.87, 1.91, and 2.57 μM, respectively. Compared to 5, compound 47 was 4- and 8-fold more potent with respect to AP and ARD activities, respectively. Importantly, we also discovered that our compounds, including 5, 36, 43, and 47, could degrade both full-length and truncated ARs in CWR22rv1 human prostate cancer cells. With these activities, they have potential for development as new drugs for the treatment of all forms of prostate cancer.

Evolving landscape and novel treatments in metastatic castrate-resistant prostate cancer

Treatment options for castrate-resistant prostate cancer (CRPC) have advanced in recent years and significantly improved the outlook for patients with this aggressive and lethal disease. Further understanding of the biology of CRPC has led to several new targeted therapies and continues to emphasize the importance of androgen receptor (AR) directed therapy. The treatment landscape is rapidly changing and further biologically rationale, biomarker-based ongoing clinical trials are needed. We review the recent results of major clinical trials in CRPC. New and investigational agents now in clinical evaluation are reviewed including inhibitors of angiogenesis, microtubules, chaperones, AR and intracellular kinases, as well as immunotherapy, radiopharmaceuticals and bone-targeted agents. The recent improvement in prognosis for CRPC brings continued optimism for further improvements. Thoughtful planning of clinical trials and further understanding of the mechanisms of resistance to therapies will allow for continued progress in patient care.

Targeting the androgen receptor

Androgen receptor (AR)-mediated signaling is critical to the growth and survival of prostate cancer. Although medical castration and antiandrogen therapy can decrease AR activity and lower PSA, castration resistance eventually develops. Recent work exploring the molecular structure and evolution of AR in response to hormonal therapies has revealed novel mechanisms of progression of castration-resistant prostate cancer and yielded new targets for drug development. This review focuses on understanding the mechanisms of persistent AR signaling in the castrate environment, and highlights new therapies either currently available or in clinical trials, including androgen synthesis inhibitors and novel direct AR inhibitors.

Modeling androgen receptor flexibility: a binding mode hypothesis of CYP17 inhibitors/antiandrogens for prostate cancer therapy

Prostate Cancer (PCa), a leading cause of cancer death worldwide (www.cancer.gov), is a complex malignancy where a spectrum of targets leads to a diversity of PCa forms. A widely pursued therapeutic target is the Androgen Receptor (AR). As a Steroid Hormone Receptor, AR serves as activator of transcription upon binding to androgens and plays a central role in the development of PCa. AR is a structurally flexible protein, and conformational plasticity of residues in the binding-pocket is a key to its ability to accommodate ligands from various chemical classes. Besides direct modulation of AR activity by antagonists, inhibition of cytochrome CYP17 (17α-hydroxylase/17,20-lyase), essential in androgen biosynthesis, has widely been considered an effective strategy against PCa. Interestingly, Handratta et al. (2005) discovered new, potent inhibitors of CYP17 (C-17 steroid derivatives) with pure AR antagonistic properties. Although the antiandrogenic activity of their lead compound (VN/124-1) has been experimentally proven both in vitro and in vivo, no structural data are currently available to elucidate the molecular determinants responsible for these desirable dual inhibitory properties. We implemented a Structure-based Drug Design (SBDD) approach to generate a valuable hypothesis as to the binding modes of steroidal CYP17 inhibitors/antiandrogens against the AR. To deal with the plasticity of residues buried in the Ligand Binding Domain (LBD), we developed a flexible-receptor Docking protocol based on Induced-Fit (IFD) methodology (www.schrodinger.com/). Our results constitute an ideal starting point for the rational design of next-generation analogues of CYP17 inhibitors/antiandrogens as well as an attractive tool to suggest novel chemical classes of AR antagonists.

Putting the brakes on continued androgen receptor signaling in castration-resistant prostate cancer

Patients with advanced prostate cancer initially respond very well to medical or surgical castration. Despite a good initial response, the disease progresses to a castration-resistant state. Castration-resistant prostate cancer (CRPC) remains addicted to androgen receptor signaling. The addition of conventional anti-androgen agents, such as bicalutamide, only provides a transient benefit. This has led to a search for further drug targets. Cytochrome P450 17 (CYP17) is an enzyme that is vital for the adrenal biosynthesis of androgens. The CYP17 inhibitor abiraterone acetate has a proven benefit in a phase III randomized trial and other CYP17 inhibitors are currently being evaluated. The novel antiandrogen MDV3100 is a small molecule androgen receptor antagonist with promising activity. Heat shock proteins (HSPs) bind to the androgen receptor and modify its activity. Several HSP inhibitors are under evaluation in clinical trials. This review explores the role of CYP17 inhibitors, MDV3100, and HSP inhibitors.

Synergistic activity of the mTOR inhibitor ridaforolimus and the antiandrogen bicalutamide in prostate cancer models

Although androgen ablation therapy is the foundation of current prostate cancer treatment, most patients ultimately develop castration-resistant disease. One proposed mechanism to account for androgen receptor (AR) activity in the castrate environment is via crosstalk with other signaling pathways. Specifically, reciprocal interactions between the AKT/mTOR and AR pathways have been implicated in prostate cancer progression. Here, we used the potent inhibitor ridaforolimus to target mTOR signaling alone and in combination with AR blockade by bicalutamide to examine the effect of abrogating these signaling pathways. Ridaforolimus treatment inhibited the proliferation of all six prostate cancer cell lines examined with the greatest sensitivity associated with loss of PTEN and elevated AKT/mTOR pathway activity. Dual inhibition of the AR and mTOR signaling pathways provided further benefit with the ridaforolimus-bicalutamide combination producing synergistic antiproliferative effects in prostate cancer cells in vitro when compared with each agent alone. Pharmacodynamic analysis confirmed that combination treatment resulted in full inhibition of each of the respective pathways. Importantly, the ridaforolimus-bicalutamide combination exhibited potent antitumor activity with parallel reductions in plasma PSA levels in vivo. Taken together, ridaforolimus exhibited potent antiproliferative and antitumor activity in prostate cancer models and the addition of bicalutamide represents a potentially effective combination strategy for patient therapy.

Antitumor agents 290. Design, synthesis, and biological evaluation of new LNCaP and PC-3 cytotoxic curcumin analogs conjugated with anti-androgens

In our continuing study of curcumin analogs as potential anti-prostate cancer drug candidates, 15 new curcumin analogs were designed, synthesized and evaluated for cytotoxicity against two human prostate cancer cell lines, androgen-dependent LNCaP and androgen-independent PC-3. Twelve analogs (5-12, 15, 16, 19, and 20) are conjugates of curcumin (1) or methyl curcumin (2) with a flutamide- or bicalutamide-like moiety. Two compounds (22 and 23) are C4-mono- and difluoro-substituted analogs of dimethyl curcumin (DMC, 21). Among the newly synthesized conjugates compound 15, a conjugate of 2 with a partial bicalutamide moiety, was more potent than bicalutamide alone and essentially equipotent with 1 and 2 against both prostate tumor cell lines with IC(50) values of 41.8 μM (for LNCaP) and 39.1 μM (for PC-3). A cell morphology study revealed that the cytotoxicity of curcumin analogs or curcumin-anti-androgen conjugates detected from both prostate cancer cell lines might be due to the suppression of pseudopodia formation. A molecular intrinsic fluorescence experiment showed that 1 accumulated mainly in the nuclei, while conjugate 6 was distributed in the cytosol. At the tested conditions, anti-androgens suppressed pseudopodia formation in PC-3 cells, but not in LNCaP cells. The evidence suggests that distinguishable target proteins are involved, resulting in the different outcomes toward pseudopodia suppression.

Emerging targeted therapies for castration-resistant prostate cancer

Until recently, few therapeutic options were available for patients with castration-resistant prostate cancer (CRPC). Since 2010, four new molecules with a demonstrated benefit (sipuleucel-T, cabazitaxel, abiraterone, and denosumab) have been approved in this setting, and to-date several other agents are under investigation in clinical trials. The purpose of this review is to present an update of targeted therapies for CRPC. Presented data are obtained from literature and congress reports updated until December 2011. Targeted therapies in advanced phases of clinical development include novel androgen signaling inhibitors, inhibitors of alternative signaling pathways, anti-angiogenic agents, inhibitors that target the bone microenvironment, and immunotherapeutic agents. Radium-223 and MDV3100 demonstrated a survival advantage in phase III trials and the road for their introduction in clinical practice is rapidly ongoing. Results are also awaited for phase III studies currently underway or planned with new drugs given as monotherapy (TAK-700, cabozantinib, tasquinimod, PROSTVAC-VF, ipilimumab) or in combination with docetaxel (custirsen, aflibercept, dasatinib, zibotentan). The optimal timing, combination, and sequencing of emerging therapies remain unknown and require further investigation. Additionally, the identification of novel markers of response and resistance to these therapies may better individualize treatment for patients with CRPC.

Remarkable inhibition of mTOR signaling by the combination of rapamycin and 1,4-phenylenebis(methylene)selenocyanate in human prostate cancer cells

Preclinical studies and clinical analyses have implicated the mammalian target of rapamycin (mTOR) pathway in the progression of prostate cancer, suggesting mTOR as a potential target for new therapies. mTOR, a serine/threonine kinase, belongs to two distinct signaling complexes: mTORC1 and mTORC2. We previously showed that the synthetic organoselenium compound, p-XSC, effectively inhibits viability and critical signaling molecules (e.g., androgen receptor, Akt) in androgen responsive (AR) and androgen independent (AI) human prostate cancer cells. On the basis of its inhibition of Akt, we hypothesized that p-XSC modulates mTORC2, an upstream regulator of the kinase. We further hypothesized that combining p-XSC with rapamycin, an mTORC1 inhibitor, would be an effective combinatory strategy for the inhibition of prostate cancer. The effects of p-XSC and rapamycin, alone or in combination, on viability and mTOR signaling were examined in AR LNCaP prostate cancer cells and AI C4-2 and DU145 cells. Phosphorylation of downstream targets of mTORC1 and mTORC2 was analyzed by immunoblotting. The interaction of mTORC1- and mTORC2-specific proteins with mTOR was probed through immunoprecipitation and immunoblotting. p-XSC inhibited phosphorylation of mTORC2 downstream targets, Akt and PCKα, and decreased the levels of rictor, an mTORC2-specific protein, coimmunoprecipitated with mTOR in C4-2 cells. The combination of p-XSC and rapamycin more effectively inhibited viability and mTOR signaling in C4-2, LNCaP and DU145 cells than either agent individually.

1,2,4-Oxadiazoles: a new class of anti-prostate cancer agents

Sulfide and sulfonyl derivatives of 1,2,4-oxadiazoles were synthesized and screened by MTT assay on the prostate cancer cells, DU-145. Six compounds were identified as potential anti-prostate cancer agents with IC(50) values ranging from 0.5 to 5.1μM. These compounds exhibited good activity on the androgen independent cells PC-3, while the results were moderate on androgen dependent LNCaP cells, suggesting the possibility of a mechanism of action different from that of the bioisosteric bicalutamide. Also a very low cytotoxicity was observed on non-cancerous cells MCF-10A.

Direct regulation of androgen receptor activity by potent CYP17 inhibitors in prostate cancer cells

TOK-001 and abiraterone are potent 17-heteroarylsteroid (17-HAS) inhibitors of Cyp17, one of the rate-limiting enzymes in the biosynthesis of testosterone from cholesterol in prostate cancer cells. Nevertheless, the molecular mechanism underlying the prevention of prostate cell growth by 17-HASs still remains elusive. Here, we assess the effects of 17-HASs on androgen receptor (AR) activity in LNCaP and LAPC-4 cells. We demonstrate that both TOK-001 and abiraterone reduced AR protein and mRNA expression, and antagonized AR-dependent promoter activation induced by androgen. TOK-001, but not abiraterone, is an effective apparent competitor of the radioligand [(3)H]R1881 for binding to the wild type and various mutant AR (W741C, W741L) proteins. In agreement with these data, TOK-001 is a consistently superior inhibitor than abiraterone of R1881-induced transcriptional activity of both wild type and mutant AR. However, neither agent was able to trans-activate the AR in the absence of R1881. Our data demonstrate that phospho-4EBP1 levels are significantly reduced by TOK-001 and to a lesser extent by abiraterone alcohol, and suggest a mechanism by which cap-dependent translation is suppressed by blocking assembly of the eIF4F and eIF4G complex to the mRNA 5' cap. Thus, the effects of these 17-HASs on AR signaling are complex, ranging from a decrease in testosterone production through the inhibition of Cyp17 as previously described, to directly reducing both AR protein expression and R1881-induced AR trans-activation.

Synthesis and biological evaluations of putative metabolically stable analogs of VN/124-1 (TOK-001): head to head anti-tumor efficacy evaluation of VN/124-1 (TOK-001) and abiraterone in LAPC-4 human prostate cancer xenograft model

In a continuing study of our clinical candidate 5 VN/124-1 (TOK-001) and analogs as potential agents for prostate cancer therapy, putative metabolites (10, 15 and 18) of compound 5 were rationally designed and synthesized. However, none of these agents were as efficacious as 5 in several in vitro studies. Using western blot analysis, we have generated a preliminary structure-activity relationship (SAR) of 5 and related analogs as androgen receptor ablative agents (ARAAs). In vivo using the androgen-dependent LAPC-4 prostate cancer xenograft model, we demonstrated for the first time that 5 is more efficacious than the 17-lyase inhibitor 3 (abiraterone)/4 (abiraterone acetate) that is currently in phase III clinical trials. In our desire to optimize the potency of 5, compounds 6 (3ξ-fluoro-) and 9 (3β-sulfamate-) designed to increase the stability and oral bioavailability of 5, respectively were evaluated in vivo. We showed, that on equimolar basis, compound 6 was ∼2-fold more efficacious versus LAPC-4 xenografts than 5, but the toxicity observed with 6 is of concern. These studies further demonstrate the efficacy of 5 in a clinically relevant prostate cancer model and justify its current clinical development as a potential treatment of prostate cancer.

New insights into the androgen-targeted therapies and epigenetic therapies in prostate cancer

Prostate cancer is the most common cancer in men in the United States, and it is the second leading cause of cancer-related death in American men. The androgen receptor (AR), a receptor of nuclear family and a transcription factor, is the most important target in this disease. While most efforts in the clinic are currently directed at lowering levels of androgens that activate AR, resistance to androgen deprivation eventually develops. Most prostate cancer deaths are attributable to this castration-resistant form of prostate cancer (CRPC). Recent work has shed light on the importance of epigenetic events including facilitation of AR signaling by histone-modifying enzymes, posttranslational modifications of AR such as sumoylation. Herein, we provide an overview of the structure of human AR and its key structural domains that can be used as targets to develop novel antiandrogens. We also summarize recent findings about the antiandrogens and the epigenetic factors that modulate the action of AR.

Novel, potent anti-androgens of therapeutic potential: recent advances and promising developments

The beneficial effect of androgen ablation has been well established in prostate cancer therapy. Despite the initial response, patients typically relapse with a more aggressive form described as castration-resistant prostate cancer (CRCP), driven by continued androgen receptor (AR) signaling. This review details the current state of anti-androgen therapy, mainly for CRPC, with major emphasis on the most potent and promising compounds under development. Anti-androgen failure has been linked to elevated AR expression, increased expression of coactivator proteins, AR mutations, ligand-independent AR activation and persistent intraprostatic androgens. MDV3100, BMS-641988 and VN/124-1 were developed to overcome these mechanisms. In CRCP, prostate cancer cells still rely on intracellular androgens and, to a greater extent, on active AR for growth and survival. Therefore, potent anti-androgens that efficiently disrupt the functions (signaling) of AR are envisioned to be effective drugs for all types of prostate cancers.

CYP17 inhibitors for prostate cancer therapy

Prostate cancer (PC) is now the second most prevalent cause of death in men in the USA and Europe. At present, the major treatment options include surgical or medical castration. These strategies cause ablation of the production of testosterone (T), dihydrotestosterone (DHT) and related androgens by the testes. However, because these procedures do not affect adrenal, prostate and other tissues' androgen production, they are often combined with androgen receptor antagonists to block their action. Indeed, recent studies have unequivocally established that in castration-resistant prostate cancer (CRPC) many androgen-regulated genes become re-expressed and tissue androgen levels increase despite low serum levels. Clearly, inhibition of the key enzyme which catalyzes the biosynthesis of androgens from pregnane precursors, 17α-hydroxy/17,20-lyase (hereafter referred to as CYP17) could prevent androgen production from all sources. Thus, total ablation of androgen production by potent CYP17 inhibitors may provide effective treatment of prostate cancer patients. This review highlights the role of androgen biosynthesis in the progression of prostate cancer and the impact of CYP17 inhibitors, such as ketoconazole, abiraterone acetate, VN/124-1 (TOK-001) and TAK-700 in the clinic and in clinical development. Article from the special issue on Targeted Inhibitors.

Prolonging hormone sensitivity in prostate cancer xenografts through dual inhibition of AR and mTOR

BACKGROUND

To determine the mechanisms associated with loss of androgen dependency and disease progression in prostate cancer (PCa), we investigated the relationship between the androgen receptor (AR) and mTOR pathways and the impact of inhibiting both pathways in androgen-dependent and castration-resistant PCa models.

EXPERIMENTAL DESIGN

Androgen-dependent (LNCaP) and castration-resistant PCa (HP-LNCaP) cells were grown as tumours in SCID mice. Once tumours reached 500 mm(3), animals were grouped and injected subcutaneous with vehicle, our novel anti-androgen/androgen synthesis inhibitor, VN/124-1, bicalutamide, and everolimus. Tumour volumes were measured biweekly. The PSA and protein analyses were performed after completion of the treatment.

RESULTS

The addition of everolimus to bicalutamide treatment of resistant tumours significantly reduced tumour growth rates and tumour volumes. Anti-androgen treatment also increased protein expression of multiple signal transduction pathways earlier than vehicle-treated control xenografts. VN/124-1 plus everolimus acted in concert to reduce tumour growth rates in our castration-resistant xenograft model.

CONCLUSIONS

This study suggests that dual inhibition of AR and mTOR in castration-resistant xenograft models can restore sensitivity of tumours to anti-androgen therapy. Furthermore, after bicalutamide failure, dual inhibition with VN/124-1 and everolimus was the most effective treatment.

Abiraterone acetate: a promising drug for the treatment of castration-resistant prostate cancer

Abiraterone acetate (CB7630), a pregnenolone analog, is an orally administered small molecule that irreversibly inhibits a rate-limiting enzyme in androgen biosynthesis, CYP17, and blocks the synthesis of androgens in the testes, adrenal glands and prostate without causing adrenal insufficiency. In clinical studies, abiraterone acetate is well tolerated and shows promising clinical activity in castration-resistant prostate cancer. The recommended Phase II dose of abiraterone acetate is 1000 mg orally daily in combination with prednisone 5 mg twice daily. Side effects are minimal and mostly associated with secondary mineralocorticoid excess, owing to a compensatory increase in upstream steroids, such as deoxycorticosterone and corticosterone. These include hypertension, hypokalemia and edema and are easily manageable with a selective mineralocorticoid antagonist, such as eplerenone, or low-dose corticosteroids. Currently, abiraterone acetate is being tested in a Phase III trial for men with progressive castration-resistant prostate cancer who are chemotherapy naive. A Phase III trial for patients following prior chemotherapy has been completed and is awaiting analysis.

Treatment of castration-resistant prostate cancer: updates on therapeutics targeting the androgen receptor signaling pathway

Androgens play a critical role in the progression of castration-resistant prostate cancer through androgen receptor (AR)-regulated signaling pathways. Progress has been made in the development of potent agents designed to suppress androgen function by blocking the AR, inhibiting the synthesis of androgens, or targeting downstream AR signaling pathways. This review summarizes the development of novel therapies based on current insights into AR signaling pathways in castration-resistant prostate cancer.

Targeting the PI3K/AKT pathway for the treatment of prostate cancer

Despite recent advances in our understanding of the biological basis of prostate cancer, the management of the disease, especially in the castration-resistant phase, remains a significant challenge. Deregulation of the phosphatidylinositol 3-kinase pathway is increasingly implicated in prostate carcinogenesis. In this review, we detail the role of this pathway in the pathogenesis of prostate cancer and the rapidly evolving therapeutic implications of targeting it. In particular, we highlight the importance of the appropriate selection of agents and combinations, and the critical role of predictive and pharmocodynamic biomarkers.

Targeted therapy for advanced prostate cancer: inhibition of the PI3K/Akt/mTOR pathway

A large number of novel therapeutics is currently undergoing clinical evaluation for the treatment of prostate cancer, and small molecule signal transduction inhibitors are a promising class of agents. These inhibitors have recently become a standard therapy in renal cell carcinoma and offer significant promise in prostate cancer. Through an understanding of the key pathways involved in prostate cancer progression, a rational drug design can be aimed at the molecules critical to cellular signaling. This may enable administration of selective therapies based on the expression of molecular targets, more appropriately individualizing treatment for prostate cancer patients. One pathway with a prominent role in prostate cancer is the PI3K/Akt/mTOR pathway. Current estimates suggest that PI3K/Akt/mTOR signaling is upregulated in 30-50% of prostate cancers, often through loss of PTEN. Molecular changes in the PI3K/Akt/mTOR signaling pathway have been demonstrated to differentiate benign from malignant prostatic epithelium and are associated with increasing tumor stage, grade, and risk of biochemical recurrence. Multiple inhibitors of this pathway have been developed and are being assessed in the laboratory and in clinical trials, with much attention focusing on mTOR inhibition. Current clinical trials in prostate cancer are assessing efficacy of mTOR inhibitors in combination with multiple targeted or traditional chemotherapies, including bevacizumab, gefitinib, and docetaxel. Completion of these trials will provide substantial information regarding the importance of this pathway in prostate cancer and the clinical implications of its targeted inhibition. In this article we review the data surrounding PI3K/Akt/mTOR inhibition in prostate cancer and their clinical implications.

CYP17 inhibition as a hormonal strategy for prostate cancer

Androgen receptor (AR) signaling has a key role in the pathogenesis of prostate cancer. AR gene amplification, AR overexpression, and activating mutations in the AR occur more frequently as castration-resistant prostate cancer (CRPC) evolves, with intratumoral androgen levels remaining sufficient for AR activation despite castration. The source of these androgens might be either adrenal or intratumoral. AR signaling, therefore, remains a valid treatment target for patients with CRPC. CYP17 is a key enzyme for androgen biosynthesis. The imidazole antifungal agent ketoconazole weakly and nonspecifically inhibits CYP17, but remains unlicensed for this indication. Chemists at the Cancer Research UK Centre for Cancer Therapeutics have designed a novel, selective, irreversible inhibitor of CYP17 called abiraterone, which is more than 20 times more potent than ketoconazole. Abiraterone acetate, a prodrug, has undergone phase I assessment, and is rapidly progressing from phase II to phase III trials, in view of its high level of antitumor activity. This agent is safe and well tolerated, and activity profiles suggest that approximately 50% of CRPC remains AR-ligand driven. Other CYP17 inhibitors with alternative mechanisms of action, for example VN/124-1, are in preclinical development. The rationale for and implications of CYP17 inhibition and the CYP17-targeting agents in development are discussed in this Review.

CYP17 blockade by abiraterone: further evidence for frequent continued hormone-dependence in castration-resistant prostate cancer

The limited prognosis of patients with castration-resistant prostate cancer (CRPC) on existing hormonal manipulation therapies calls out for the urgent need for new management strategies. The novel, orally available, small-molecule compound, abiraterone acetate, is undergoing evaluation in early clinical trials and emerging data have shown that the selective, irreversible and continuous inhibition of CYP17 is safe with durable responses in CRPC. Importantly, these efficacy data along with strong preclinical evidence indicate that a significant proportion of CRPC remains dependant on ligand-activated androgen receptor (AR) signalling. Coupled with the use of innovative biological molecular techniques, including the characterisation of circulating tumour cells and ETS gene fusion analyses, we have gained insights into the molecular basis of CRPC. We envision that a better understanding of the mechanisms underlying resistance to abiraterone acetate, as well as the development of validated predictive and intermediate endpoint biomarkers to aid both patient selection and monitor response to treatment, will improve the outcome of CRPC patients.

Androgen receptor: role and novel therapeutic prospects in prostate cancer

Androgen receptor (AR) signaling is necessary for the development of prostate cancer. Androgen-deprivation therapy (ADT) for prostate cancer was described over 50 years ago and ADT remains the mainstay of systemic therapy. AR signaling remains intact as the disease evolves to castration-resistant prostate cancer (CRPC). Through cellular adaptations, CRPC continues to rely on androgens and AR growth signaling, and thus AR remains an important therapeutic target. CRPC cells upregulate enzymes used in androgen synthesis, thus providing an intracellular source of androgen despite systemic castration. Compounds in development, such as antiandrogens, lyase inhibitors, heat-shock protein-90 inhibitors, histone deacetylase inhibitors and others, will provide new tools to more effectively reduce ligand, inhibit AR and/or inhibit costimulatory pathways and result in improved clinical outcomes.