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Current Pharmaceutical Design Kamlesh K. Yadav^ 1 , Khader Shameer^ 2,3 Ben Readhead 2,3 , Shalini S. Yadav 1 , Li Li 2,3 , Andrew Kasarskis 3 , Ashutosh K. Tewari 1, * and Joel T. Dudley 2,3,4, * 1 Department of Urology, Icahn School of Medicine at Mount Sinai, New York, New York, USA; 2 Harris Center for Precision Well- ness, Icahn School of Medicine at Mount Sinai, New York, New York, USA; 3 Department of Genetics and Genomic Sciences, Icahn School of Medicine at Mount Sinai, New York, New York, USA; 4 Department of Health Evidence and Policy, Icahn School of Medi- cine at Mount Sinai, New York, New York, USA A R T I C L E H I S T O R Y Received: March 10, 2016 Accepted: May 25, 2016 DOI: 10.2174/1381612822666160513145924 Abstract: Background: Prostate cancer is the most commonly diagnosed cancer in men. More than 200,000 new cases are added each year in the US, translating to a lifetime risk of 1 in 7 men. Neuroendocrine prostate cancer (NEPC) is an aggressive and treatment- resistant form of prostate cancer. A subset of patients treated with aggressive androgen deprivation therapy (ADT) present with NEPC. Patients with NEPC have a reduced 5-year overall survival rate of 12.6%. Knowledge integration from genetic, epigenetic, biochemi- cal and therapeutic studies suggests NEPC as an indicative mechanism of resistance devel- opment to various forms of therapy. Methods: In this perspective, we discuss various experimental, computational and risk prediction methodologies that can be utilized to identify novel therapies against NEPC. We reviewed literature from PubMed and computationally analyzed publicly available genomics data to present dif- ferent possibilities for developing systems medicine based therapeutic and curative models to understand and target prostate cancer and NEPC. Results: We discuss strategies including gene-set analyses, network analyses, genomics and phenomics aided drug development, microRNA and peptide-based therapeutics, pathway modeling, drug repositioning and can- cer immunotherapies. We also discuss the application of cancer risk estimations and mining of electronic medi- cal records to develop personalized risk predictions models for NEPC. Preemptive stratification of patients who are at risk of evolving NEPC phenotypes using predictive models could also help to design and deliver better therapies. Conclusion: Collectively, understanding the mechanism of NEPC evolution from prostate cancer using systems biology approaches would help in devising better treatment strategies and is critical and unmet clinical need. Keywords: Prostate cacner, precision medicine, systems biology, neuroendocrine prostate cancer. INTRODUCTION An estimated 2.9 million people are living with prostate cancer (PCa) in the US. Nearly 27,500 patients will die this year due to complications arising from the disease, making it the second lead- ing cause of cancer-related deaths in American men [1]. The SEER database also suggests PCa to be the most commonly diagnosed cancer in American men; more than 220,000 new cases diagnosed each year with a lifetime risk of 1 in 7. Localized prostate cancer is usually treated by either surgical excision (Radical Prostatectomy) and/or radiotherapy, both of which cure a majority of the patients. However, in a small proportion, recurrence and metastasis devel- ops. The metastatic disease is routinely treated using androgen dep- rivation therapy (ADT) but invariably progresses to a castration- resistant prostate cancer (CRPC) phase with poor prognosis. Re- cently approved AR-signaling inhibitors Enzalutamide and Abi- raterone have been shown to be effective in more than 50% of the CRPCs [2-7]. However, 50% of the responders develop resistance *Address correspondence to these authors at the Department of Urology (AKT, Email: [email protected]) and Department of Genetics and Genomic Sciences (JTD: E-mail: [email protected]), Mount Sinai Health System, New York, NY, USA ^ Equally contributed to this work. to the new treatments within two years [8-11]. Analysis of tumors from these patients suggests that nearly 25% of them present with an aggressive and treatment resistant form of the disease called neuroendocrine prostate cancer (NEPC) [12, 13]. There is a recent interest in understanding the mechanism of differentiation of CRPC to a neuroendocrine form and developing better diagnostic and precision therapies to manage these lethal and drug resistant tu- mors. In this comprehensive perspective, we discuss the cancer biology of NEPC, various strategies to identify potential biomarkers of NEPC, opportunities to characterize potential drug targets and develop precision therapy using systems medicine-based ap- proaches. NEUROENDOCRINE PROSTATE CANCER The prostate parenchyma is made up of prostatic acini sur- rounded by stromal, fibroblast, nerve and endothelial tissues. The acini are comprised of basal and luminal cells and few interspersed neuroendocrine cells around the basal cells that are AR-negative (Fig. 1). These cells secrete neuropeptides such as bombesin, neuro- tensin, serotonin and calcitonin that have been postulated to support the growth of both normal and malignant prostate tissues [13, 14]. The diagnosis and sub-classification of NEPC is principally carried out using the evidence gathered from cellular morphology, distribu- 1873-4286/16 $58.00+.00 © 2016 Bentham Science Publishers Send Orders for Reprints to [email protected] 5234 Current Pharmaceutical Design, 2016, 22, 5234-5248 REVIEW ARTICLE Systems Medicine Approaches to Improving Understanding, Treatment, and Clinical Management of Neuroendocrine Prostate Cancer

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Kamlesh K. Yadav^1, Khader Shameer^

2,3 Ben Readhead

2,3, Shalini S. Yadav

1, Li Li

2,3, Andrew Kasarskis

3,

Ashutosh K. Tewari1,* and Joel T. Dudley

2,3,4,*

1Department of Urology, Icahn School of Medicine at Mount Sinai, New York, New York, USA; 2Harris Center for Precision Well-ness, Icahn School of Medicine at Mount Sinai, New York, New York, USA; 3Department of Genetics and Genomic Sciences, Icahn School of Medicine at Mount Sinai, New York, New York, USA; 4Department of Health Evidence and Policy, Icahn School of Medi-cine at Mount Sinai, New York, New York, USA

A R T I C L E H I S T O R Y

Received: March 10, 2016

Accepted: May 25, 2016

DOI: 10.2174/1381612822666160513145924

Abstract: Background: Prostate cancer is the most commonly diagnosed cancer in men.

More than 200,000 new cases are added each year in the US, translating to a lifetime risk

of 1 in 7 men. Neuroendocrine prostate cancer (NEPC) is an aggressive and treatment-

resistant form of prostate cancer. A subset of patients treated with aggressive androgen

deprivation therapy (ADT) present with NEPC. Patients with NEPC have a reduced 5-year

overall survival rate of 12.6%. Knowledge integration from genetic, epigenetic, biochemi-

cal and therapeutic studies suggests NEPC as an indicative mechanism of resistance devel-

opment to various forms of therapy.

Methods: In this perspective, we discuss various experimental, computational and risk

prediction methodologies that can be utilized to identify novel therapies against NEPC. We

reviewed literature from PubMed and computationally analyzed publicly available genomics data to present dif-

ferent possibilities for developing systems medicine based therapeutic and curative models to understand and

target prostate cancer and NEPC.

Results: We discuss strategies including gene-set analyses, network analyses, genomics and phenomics aided

drug development, microRNA and peptide-based therapeutics, pathway modeling, drug repositioning and can-

cer immunotherapies. We also discuss the application of cancer risk estimations and mining of electronic medi-

cal records to develop personalized risk predictions models for NEPC. Preemptive stratification of patients who

are at risk of evolving NEPC phenotypes using predictive models could also help to design and deliver better

therapies.

Conclusion: Collectively, understanding the mechanism of NEPC evolution from prostate cancer using systems

biology approaches would help in devising better treatment strategies and is critical and unmet clinical need.

Keywords: Prostate cacner, precision medicine, systems biology, neuroendocrine prostate cancer.

INTRODUCTION

An estimated 2.9 million people are living with prostate cancer (PCa) in the US. Nearly 27,500 patients will die this year due to complications arising from the disease, making it the second lead-ing cause of cancer-related deaths in American men [1]. The SEER database also suggests PCa to be the most commonly diagnosed cancer in American men; more than 220,000 new cases diagnosed each year with a lifetime risk of 1 in 7. Localized prostate cancer is usually treated by either surgical excision (Radical Prostatectomy) and/or radiotherapy, both of which cure a majority of the patients. However, in a small proportion, recurrence and metastasis devel-ops. The metastatic disease is routinely treated using androgen dep-rivation therapy (ADT) but invariably progresses to a castration-resistant prostate cancer (CRPC) phase with poor prognosis. Re-cently approved AR-signaling inhibitors Enzalutamide and Abi-raterone have been shown to be effective in more than 50% of the CRPCs [2-7]. However, 50% of the responders develop resistance

*Address correspondence to these authors at the Department of Urology

(AKT, Email: [email protected]) and Department of Genetics and Genomic Sciences (JTD: E-mail: [email protected]), Mount Sinai

Health System, New York, NY, USA

^ Equally contributed to this work.

to the new treatments within two years [8-11]. Analysis of tumors from these patients suggests that nearly 25% of them present with an aggressive and treatment resistant form of the disease called neuroendocrine prostate cancer (NEPC) [12, 13]. There is a recent interest in understanding the mechanism of differentiation of CRPC to a neuroendocrine form and developing better diagnostic and precision therapies to manage these lethal and drug resistant tu-mors. In this comprehensive perspective, we discuss the cancer biology of NEPC, various strategies to identify potential biomarkers of NEPC, opportunities to characterize potential drug targets and develop precision therapy using systems medicine-based ap-proaches.

NEUROENDOCRINE PROSTATE CANCER

The prostate parenchyma is made up of prostatic acini sur-rounded by stromal, fibroblast, nerve and endothelial tissues. The acini are comprised of basal and luminal cells and few interspersed neuroendocrine cells around the basal cells that are AR-negative (Fig. 1). These cells secrete neuropeptides such as bombesin, neuro-tensin, serotonin and calcitonin that have been postulated to support the growth of both normal and malignant prostate tissues [13, 14]. The diagnosis and sub-classification of NEPC is principally carried out using the evidence gathered from cellular morphology, distribu-

1873-4286/16 $58.00+.00 © 2016 Bentham Science Publishers

Send Orders for Reprints to [email protected] 5234

Current Pharmaceutical Design, 2016, 22, 5234-5248

REVIEW ARTICLE

Systems Medicine Approaches to Improving Understanding, Treatment,and Clinical Management of Neuroendocrine Prostate Cancer

Systems Medicine Approaches for Targeting Neuroendocrine Prostate Cancer Current Pharmaceutical Design, 2016, Vol. 22, No. 34 5235

tion and expression of well-known neuronal markers, such as neu-ron-specific enolase (NSE, expressed from ENO2), chromogranins (CHGA/CHGB), synaptophysin (SYP) and CD56 [15, 16]. NEPC can be divided into five subclasses: a) adenocarcinomas with focal NE differentiation, b) adenocarcinomas with Paneth-cell-like NE differentiation, c) carcinoid tumors d) large cell carcinoma and e) small carcinoma. Of these, small cell carcinomas are the most common representative of the cellular phenotype [17].

The percentage of neuroendocrine (NE) cells has been shown to increase with disease aggressiveness, suggesting their role in either supporting the growth of cancer cells or bringing about resistance to therapy. Whereas PCa primarily arises primarily from the acinar cells [18], newly diagnosed NEPC, which comprises less than 1-2% of all prostate cancers, arise from the NE cells [17]. The concomi-tant increase in the NE cells with aggressive disease progression has been purported as a mechanism of resistance development, and termed as treatment induced neuroendocrine prostate cancer (tNEPC) [14, 19]. Indeed in vitro studies with PCa cell lines sug-gest NEPC development as a mode of resistance to ADT and radia-tion therapy [20-22].

In the human phenotype ontology, prostate neoplasm and endo-crine neoplasm are described using separate term hierarchies (Fig. 2) and the transition of PCa phenotype to NEPC as part of the che-motherapy resistance mechanism combines the pathological fea-tures of both phenotypes. In the tumor evolution setting (where ADENOCARCINOMA � CRPC � NEPC; Figs. 1 & 3) a series of changes are brought about not only to promote the tumor prolif-eration and migration to visceral organs, but also to initiate signal-ing alterations that drive neuronal gene expression [23, 24].

NEURONAL CORRELATIONS OF PROSTATE CANCER

Several lines of evidence indicate a high correlation between neuronal structures and prostate cancer. PCa involved with perineu-ral invasion show more aggressive and proliferative phenotype (Fig. 3) [25]. Studies in mice models suggest the involvement of the autonomic nervous system in the initiation (sympathetic nervous system) and progression (parasympathetic system) of PCa [26]. Furthermore, epidemiological studies have identified an inverse correlation between PCa and nervous system disorders such as pa-tients with spinal cord disorders have small prostate sizes and lower chances of developing PCa [27], and patients with central nervous system (CNS) diseases including schizophrenia, Parkinson’s and Alzheimer’s disease have a lower likelihood of developing PCa [28]. Finally, interleukin-6 (IL6) not only plays a significant role in PCa progression and NE differentiation [29-32] but also has a

pathological role in CNS disorder schizophrenia [33-35]. Thus, emerging evidence suggests that PCa has a predisposition towards developing into a neuronal phenotype due to the shared signaling and genetic cues and may explain the observation of anti-psychosis drugs having a suppressive effect on PCa development [36, 37].

EXPERIMENTAL MODEL FOR NEPC

To understand the processes underlying the differentiation of ADENOCARCINOMA � CRPC � NEPC, in vitro models using prostate cell-lines have been developed. Upon transformation, the cells display dramatic morphological changes associated with neu-rons such as branched neurite formation and expression of neuronal markers (Fig. 2). One of the first such studies identified cAMP as a ligand that brings about differentiation of LNCaP (an AR respon-sive adenocarcinoma cell line model derived from lymph node me-tastasized PCa) cells in vitro [38]. Another study identified andro-gen deprivation leads to neuronal differentiation of LNCaP cells as well [39, 40]. Thereafter, several other studies demonstrated the use of other factors such as IL6 [41], neuropeptide bombesin [42], WNT-11 [43], radiation [22], valporic acid [44], genistein [45], vasoactive intestinal peptides [46, 47], Jolkilonide B, [48], and COX-2 inhibitor [49] to bring about NE differentiation. However, most of these studies have been limited to using the LNCaP adeno-carcinoma cell line model, which does not reflect clinical observa-tion of ADENOCARCINOMA � CRPC � NEPC. A ligand capa-ble of converting prostate cancer cell lines in to the NE form, irre-spective of AR status, would be crucial in understanding the pro-gressive mechanism of prostate tumor evolving towards a neuroen-docrine phenotype. Unfortunately, the in vitro transformed cells undergo post-mitotic arrest preventing their use as a model system to test drug efficacy studies.

The poor availability of a purely NEPC cell line has limited drug screening and testing efforts. The PC3 cells (an AR negative CRPC line derived from a bone metastasized PCa) have been shown to share characteristics of NEPC and express markers such as NSE and Chromogranin [50]. However, the NCI-H660 cell-line, which was initially attributed as a small cell lung cancer cell line [51], and later confirmed to be of prostate origin using cytogenetic studies [52], exhibits all characteristics of NEPC. It also harbors the prominent TMPRSS2-ERG fusion that is present in more than 50% of all prostate cancers as well as a mutated form of TP53 [53, 54]. Importantly, genomic and biochemical studies have identified it to be a suitable model of NEPC, expressing all the neuronal markers, and has been successfully utilized to test small-molecule drug against NEPC [23].

Fig. (1). Evolution of Neuroendocrine prostate cancer (NEPC).

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5236 Current Pharmaceutical Design, 2016, Vol. 22, No. 34 Yadav et al.

SYSTEMS MEDICINE APPROACHES FOR TARGETING NEPC

Systems medicine is defined as an interdisciplinary approach to study a disease at the systems level of the human body by consider-ing it as part of an integrated complex system with biochemical, physiological, and environmental interactions [55]. NEPC is treated primarily with chemotherapy, especially a combination of docetaxel with etoposide and cisplatin. However, resistance development is very rapid, and survival is less than 1.5 years [56, 57]. Pre-clinical success with zoledronic acid [58], plumbagin [59] and danusertib [23] has also been reported. The prevalence of high-throughput sequencing and phenotyping is revolutionizing design, development and delivery of precision medicine for various cancers including prostate [60]. Kidd et al. illustrated an example of a case-study on how data-driven precision therapy could guide in designing patient-specific therapy [61]. While personalized molecular profiling and individualized treatment strategies have conflicting outcome reports [62] – personalized, data-driven recommendation and its integration with predictive models could provide a better outcome for patients with NEPC.

THERAPEUTIC STRATEGIES TO TARGET NEPC

Designing experimental and computational strategies for the systematic targeting of pathways, genes or genetic variations in-volved in PCa or NEPC could help to find better therapies for NEPC. Such methods include gene-set-based target discovery, net-work medicine approaches, genotype-phenotype aided drug discov-ery, development of protein or peptide drugs, miRNA-informed therapy, targeting gene or transcript-fusion events, pathway-based drug development, high-throughput screening of small molecules and drug repositioning approaches.

MOLECULAR ARCHITECTURE OF PCA AND NEPC

Multiple gene sets are currently available to perform systematic analyses of PCa and NEPC. From the first report in the late ‘90s on the role of BMX (Etk/BMx) in NEPC [41] till 2015 more than 30 additional genes, such as (DEK [63], PEG10 [64], CREB, IL-6 [32, 65-67], EGF [68], ASH-1 [69] Midkine [70], Mash1 expression [71], AKT/hnRNPK/AR/beta-catenin [72], MnSOD [73], Wnt-11 [43], SNAI1 transcription factor [74], SOD2 [75], CD44 [76], Neu-roD1 [77], Cystatin C [78], IL8 [79], Granin A and Calcitonin [80], Ps2 protein [80], BCL2 [81]), and their expression pattern have been studied using in vitro and in vivo model systems. Next genera-tion studies (NGS) over the last few years, including the recent NGS-report from the TCGA, have provided extensive catalog of genes and subtypes associated with PCa and its subtypes including NEPC [20, 23, 82, 83]. Observation of factors previously identified in adenocarcinomas (such as TMPRSS2-ERG translocation [84]), strongly suggests the progressive development of NEPC from ade-nocarcinomas through the CRPC phase [85] . Specific oncogenic drivers (such as MYCN [86, 87], MYCC [88], PLK1 [89], and AURKA [23] are activated, and tumor suppressors such as TP53 [90], RB1 [91], along with transcription factor, RE1-Silencing Transcription Factor (REST1, alias Neural-Restrictive Silencing Factor [24]), are lost or downregulated. Upregulation of epigenetic regulators such as EZH2, H2S/Cav3.2 [92], Secretogranin II [93], Orexin type 1 [94], AGR2 [95] have also been identified as key players of NEPC pathology. Selective inhibition of driver genes, such as AURKA, has been shown to alleviate the NEPC phenotype [23]. Several canonical gene sets are reported in the literature indi-cating involvment in NEPC and pleiotropic mechanisms where one gene influences multiple traits have been implicated in PCa [96]. Cancer sub-type specific gene-sets from whole-exome, genome and transcriptome studies are reported as part of TCGA studies. A re-

Fig. (2). Taxonomy of prostate neoplasm and neuroendocrine neoplasm using human phenotype ontology.

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Systems Medicine Approaches for Targeting Neuroendocrine Prostate Cancer Current Pharmaceutical Design, 2016, Vol. 22, No. 34 5237

cent study also reported the expression quantitative trait loci (eQTLs) associated with genetic variants involved in PCa [97]. Such lists of genes, transcripts and proteins associated with PCa and NEPC are continuously expanding. Thus, designing drugs that can reverse the validated expression signature could aid in developing new therapies for NEPC.

NETWORK ANALYSES OF GENE-SETS TO FIND KEY HUBS DRIVING CANCER PATHOLOGIES

Network analyses of the candidate, or curated genes discovered from NGS profiling can be used to not only find driver and passen-ger genes but also genes that are vital to the disease pathways [98]. Deriving network analyses based connectivity metrics of PCa or NEPC genes can be useful to pinpoint key players of pathophysi-ological mechanisms. Understanding these fundamental mecha-nisms could help to identify new targets and new therapies. A re-cent study explored the key proteins associated with ovarian cancer by converging nanobiotechnology and network analyses [99]. Briefly, nanoparticles were used as therapeutic agents to quench the growth of ovarian cancer cell lines. Using gold-nanoparticle tagging and mass spectrometry, the proteins involved at various progressive phases were identified. The proteins were then used to derive puta-tive functional networks. These networks were then prioritized using connectivity indices like degree, radiality and eccentricity. Developing similar network driven approaches using canonical gene-lists or patient-derived gene sets and combining clinical and biological relevance would help to find key hubs that drive aggres-sive cancer pathologies of the prostate.

Using a similar computation method, we used the NEPC gene sets from a recent study [23] and compiled a canonical network (Fig. 4). This network can be further perturbed using network met-rics and synthetic lethality [100] to find key hubs.

GWAS (GENOME-WIDE ASSOCIATION STUDIES) AND PheWAS (PHENOME-WIDE ASSOCIATION STUDIES)-

DRIVEN DRUG DISCOVERY

Whereas GWAS studies aim to find genetic variants associated with a given phenotype using large case and control cohorts, Phe-WAS is a unique analysis that aims to find the enriched phenotypes associated with genetic variants of a primary phenotype such as phenotypes associated with PCa. PheWAS is generally possible when GWAS patient cohorts are derived from electronic medical record (EMR)-linked biorepositories. GWAS studies have identi-fied several genetic variants and genes associated with PCa, albeit with low risk penetration. Analyzing the compendium of genetic variants, genes and the phenotypes enriched across patients harbor-ing risk variants could help to develop better therapies. The high-quality data captured during the clinical trial or clinical operations are in the silos of clinical registries or internal databases of

healthcare or pharma organizations and not readily available for data mining. Current PheWAS are leveraging EMR data as the primary data source that provides the breadth of the phenotypes suitable for high-throughput human phenomics studies.

Genotypes and Genes Associated with PCa

GWAS data is currently being explored to find new targets for a variety of diseases including cancers and autoimmune disease [101]. While there is no phenotype specific GWAS on NEPC, a total of 73 genetic variants have been identified from GWAS in case-control populations of different ancestries with PCa (Fig. 5). These genetic variants were reported or mapped to 511 genes. While a majority of these genes have known biochemical or com-putationally derived functional roles and 7.4% of these genes are orphan genes with no validated functional or biochemical roles. Exploring the functional or regulatory role of these genetic variants and genes [102] in NEPC could aid in developing GWAS-aided therapies for NEPC patient population.

Phenomic Correlations of Patients with PCa Genotypes

To understand the phenotype-genotype correlation of PCa and other clinical phenotypes, we have compiled the phenome-wide association datasets from PheWAS database (https: //phewas.mc.vanderbilt.edu/). We compiled 73 variants associated with PCa and their associations with 1218 phenotypes. Peyronie's disease was the most significant association but with a small num-ber of sample size (n=44, rs2735839, P= 4.18E-06; See Fig. 6). This association would need further investigation as Peyronie’s disease is an outcome associated with men that have undergone radical prostatectomy. The genetic variant rs2735839 is an intronic variant of KLK3, a crucial prostate cancer gene and may play a mechanistic role in NEPC. The phenotype “adverse effects of antir-heumatics” was the phenotype with the lowest odds ratio (n=56, rs10187424, P= 4.16E-05, OR= 0.3765). The phenotype “cardiac conduction disorders” had a lower risk in patients harboring vari-ants for PCa (n=2685, rs1983891, P= 0.0008882, OR=0.8627). The variant rs1983891 is localized to the intronic region of FOXP1, which negatively regulate PCa [103]. Hepatic cancer was the phe-notype with highest odds ratio among the 1645 tested phenotypes (n= 30, rs7837688, P= 0.0001333). The variant rs7837688 is local-ized to intergenic regions with CASC8 and LOC105375754; and proximal to MYC, a known PCa and NEPC gene. Intronic variants identified from GWAS have been implicated in driving pathologi-cal mechanism in various diseases including myocardial infarction [104, 105], and exploring the expression, function and regulatory mechanisms of these variants or their eQTLs may lead to discovery of new mechanisms underlying PCa and NEPC.

PROTEIN AND PEPTIDE DRUGS FOR NEPC

Protein drugs are evolving as a competing therapeutic domain to improve or augment small-molecule based therapies. Peptides

Fig. (3). Morphological characteristics of adenocarcinoma and transformed neuroendorcine cells. The androgen positive adenocarcinoma cell line LNCaP

(left) was serum starved for 3 weeks to mimic androgen depravation.

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5238 Current Pharmaceutical Design, 2016, Vol. 22, No. 34 Yadav et al.

have been shown to be effective in inhibiting previously ‘un-druggable’ target such as Ras [106] and transcription factors such as NOTCH [107]. NEPC has phenotypes regulated by peptides and developing specific protein or peptide drugs to modulate these pep-tides could be of high therapeutic value. For example, NEPC re-lated peptides promote IGF-1R signaling and promote NEPC [108]. Neuropeptides 26RFa [109] and Manserin [110] were implicated in NEPC. Midkine a heparin binding growth factor [70], recombinant human IFN-beta [111] and vasoactive intestinal peptide (VIP) [112] have been shown to have a role in NEPC phenotype development and growth. Inhibiting these peptides using stable peptidomimetics could aid in finding therapies for NEPC.

TARGETING FUSION TRANSCRIPTS ASSOCIATED WITH

NEPC Chromosomal translocations are regarded as a cytogenetic fea-ture of various tumors including PCa [113]. NGS technologies have revealed a large number of fusion genes or fusion transcripts and their putative role in diseases including various cancers, cancer resistance and relapse [114]. For example TMPRSS2-ERG, SLC45A3-BRAF, SLC45A3-ELK4 and ESRP1-RAF1 fusion tran-scripts have been implicated in PCa [115-117]. Targeting fusion transcripts could pave way towards development of better therapy, similar to strategies utilized to target BCR-Abl with imatininb for CML (chronic myelogenous leukemia) [118].

Fig. (4). Functional biochemical network of NEPC gene list compiled from TCGA

Network reconstructed using data from STRING database. Nodes are proteins and edges are biochemical actions represented as an interaction. Edge color

legend: activation=green; inhibition=red, protein-protein binding=blue; phenotype=cyan, enzyme catalysis=purple, post-translational modification=violet,

reaction=light blue; coexpression=yellow.

Systems Medicine Approaches for Targeting Neuroendocrine Prostate Cancer Current Pharmaceutical Design, 2016, Vol. 22, No. 34 5239

MicroRNA-BASED THERAPY FOR NEPC

Micro RNAs (miRNAs) are non-coding RNAs that regulate expression of genes post-transcriptionally. Regulation of several androgen-induced genes has been observed by repressing three miRNAs: hsa-mir-125b-1, hsa-mir-125b-2 and hsa-mir-99a [119]. In another study, dysregulation of hsa-mir-145, which targets and regulates expression of ERG [120], and, hsa-mir-183, which targets DKK3 and SMAD4, have been implicated in PCa [121]. Collec-tively these microRNAs inhibit a set of genes (PRNP, S1PR1, TACSTD2, NR3C1, SOD2, ZNF682, WDR12, CDNF, BMPR2, COX20, IGF1R, ART4, PLEKHA1, AMOTL1, DAZAP2, BCL2L11, GMFB, BTG2, RPL9, UBN2, TNRC6C, LHFPL2, VAV3, HNRNPA1, ZNF529, WEE1, PTMA, SLC38A1). While a specific role or function of miRNAs in NEPC has not been elucidated, per-forming miRNA sequencing and analyses of downstream targets could help in the identification of new therapeutic targets for NEPC.

PATHWAY-DRIVEN DRUG DISCOVERY TO TARGET PATHWAY CROSS-TALKS IN NEPC

Biological pathways act in concert to perform a variety of functions in the “normal” and “disease” states [122]. Emerging evidence from systems-biology studies suggests that pathway cross-talks plays an important role in imparting pathophysiology of mulitple diseases and the switch between normal and disease states can be modulated through pathways with regulatory activities [123, 124]. Such approaches can use biological pathway evidences, including pathway cross-talk, and can help in the identification of new indications for existing compounds [125]. There are multiple computational experimental strategies to find new pathway modulators or targets from pathways. For example, vascular endothelial growth factor signaling pathway is targeted by a diverse set of drugs (n=67; see: http: //www.genome.jp/dbget-bin/www_bget?pathway+hsa04370) (VEGF, See Fig. 7). VEGF is a critical pathway composed of 61 genes and six organic compounds [126]. VEGF has also been implicated to play a key role in NEPC

[127]. Drugs targeting VEGF have a variety of activities including analgesic, anti-inflammatory, anti-neoplastic and anti-rheumatic. Specific disease phenotypes associated with drugs targeting VEGF pathway includes solid tumors, atherosclerosis, neuropathic pain, Acute respiratory distress syndrome (ARDS), major depressive disorder, acute coronary syndrome, Crohn’s disease, Psoriasis and age-related macular degeneration. VEGF is an example of a pathway that can be activated or inhibited by multiple compounds for therapeutic interventions. Identification of similar pathways and their cross-talks using systems biology approaches including quantitative modeling and pathway analtyics could help to reposition drugs for new indications [128-136]. Pathways such as Notch signaling [137] and cAMP signaling [138] have established role in NEPC and pathway-based drug development can be used to improve available therapies that can potentially target NEPC.

Targeting Signal Transduction Events of NEPC

Biochemical modulation of pathways or disease mechanisms by inhibition or activation of post-translational modifications including phosphorylation events can act as therapeutic strategies for NEPC. Phosphorylation of protein kinase A by RhoA is reported as a sig-naling event associated with NEPC [139]. Hormone signaling by paracrine factors [140] has been implicated in NEPC, although the precise roles of these modulations are yet to be ascertained. In-flammation, immune response and activation of macrophages [141] have also been indicated as a mechanistic basis of NEPC. Under-standing key modifiers of proteins and pathways of PCa and their role in recurrence is key to understand signaling pathways driving the progression of PCa to NEPC.

Metabolic Pathway Modeling to Identify Drug Targets for NEPC

Targeting metabolic pathways by leveraging computational modeling and simulation offers an effective method to identify novel or better drug targets [142-144]. Computational methods, that can model metabolic pathways, simulate enzyme kinetic activities and perform in-silico gene knockouts, could help in identification and prioritization of new drug targets [145]. Modeling techniques

Fig. (5). Genetic variants associated with prostate cancer mapped to human chromosomes. (Highlighted a blacke dot).

5240 Current Pharmaceutical Design, 2016, Vol. 22, No. 34 Yadav et al.

like flux balance analysis [146] and its derivatives use genome scale reconstructions of metabolic models to predict and prioritize the role of single, pair or multiple genes using gene or metabolic reaction perturbations. Briefly, the method involves five steps: 1) construction of a draft model using automated genome annotation pipelines, 2) refinement of draft models by leveraging high quality annotations using bio-curation, 3) generation of a computable model and definition of model constrains and parameters for simulations, 4) evaluation of model and simulation of gene or reaction knockout experiments, 5) interpretation of results and experimental validation.

Genome-scale metabolic models for target prioritization can be enhanced by integrating data from transcriptomics, metabolomics, proteomics, biochemical studies and predictive algorithms [147-152]. A list of software packages (See: http: //sbml.org/ SBML_Software_Guide) and detailed protocols for performing genome-scale reconstruction and modeling is available here [153, 154]. Metabolic modeling has been used to identify drug targets for infectious diseases ( tuberculosis [155] and multidrug-resistant Staphylococcus aureus [156]), cardiovascular diseases [157-159], neurodegenerative disease [160], cancer [161], diabetes, and obe-sity [162]. Metabolic models have also been employed to predict biochemical or physiological response including drug toxicity evaluations as part of drug discovery investigations [163].

HIGH-THROUGPUT SCREENING TO FIND COMPOUNDS TARGETING NEPC

Target based drug discovery relies on experimental screening of the compounds using a large number of compounds from custom or commercial ligand libraries. The compounds are tested using high-throughput screening (HTS) [164] platforms that enable rapid

testing of a large collection of compounds and their agonistic or antagonistic perturbations using cell line based screening assays or specific protein-ligand binding assays (LBA) [165]. Such experi-mental approaches can be optimized using computational screening method that simulate the screening using virtual-ligand screening method [166]. Virtual ligand screening methods help to reduce the number of compounds that need experimental evaluation and thus help in optimizing the cost of drug discovery investigations. Briefly, the virtual ligand screening method comprise of protein modeling, ligand modeling and structure optimization, protein-ligand docking and assessment of docked complexes using a variety of chemoinformatic parameters [167-170]. Compounds can be assessed using Quantitative Structure-Activity Relationships (QSAR) [171] that provides a quantitative inference of how well the ligand can perturb the target, pharmacophore properties, adsorption, distribution, metabolism and excretion and toxicity (ADMET) profiling and other chemoinformatic parameters that are relevant to identifying the optimal ligand for a given target based on disease biology, physicochemical properties and binding kinetics. Computational evaluation of drug-target interactions would help in focusing downstream functional studies and clinical trials on a subset of compounds and thus help in improving the process of drug discovery by reducing the cost and time to translate therapies from bench to the clinical setting [172, 173].

DRUG REPOSITIONING FOR NEPC

Drug repositioning comprises the use of computational and experimental analyses to identify the suitability of an existing, ap-proved or investigational compound for a new indication. This ap-proach has been used as a therapeutic strategy for more than 250 drugs and spans multiple indications from all classes of diseases

Fig. (6). Phenome-wide associations of a prostate cancer genetic variant (rs2735839, mapped to KLK3).

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(RepurposeDB, Manuscript in press). Since drug repositioning can help identify therapeutic compounds for complex, chronic or or-phan diseases using molecular information derived from a single patient, family members with rare clinical manifestation or case-control cohorts, it can prove to be useful in increasing the repertoire of currently limited therapeutic options available to treat NEPC, Fig. (8). Successful indications have been reported using drug repo-sitioning across different therapeutic domains including inflamma-tory bowel disease and non-small cell lung cancer ([174, 175].). Publicly available resources such as mRNA transcriptional profiles in databases like The Cancer Genome Atlas (TCGA) [176-179], cBioPortal [180], ArrayExpress (https: //www.ebi.ac.uk/ arrayex-press/) and Gene Expression Omnibus (GEO, http: //www.ncbi.nlm.nih.gov/geo/) can be integrated to design a global expression map of NEPC. This map can be matched in-silico using bioinformatics resource like Connectivity Map (CMap, http: //www.broadinstitute.org/cmap/) and LINCS (http: //www.lincsproject.org/) to discover drugs that can potentially re-verse the direction of gene expression phenotypes. A detailed over-view of computational and experimental strategies and translational bioinformatics resources available for drug repositioning is dis-cussed elsewhere (See Shameer et al. [181] and Readhead et.al [182]). Drug repositioning methods enables rapid testing of existing compound libraries against molecular profiles to modulate clinical phenotypes of a patient with the rare disease [183, 184].

CANCER IMMUNOTHERAPY FOR NEPC

Cancer immunotherapy is the emerging field of research to design and develop therapeutic interventions, which can activate the immune system for recognition and killing of cancer cells. Immu-notherapy was shown to be effective for complex diseases including asthma and melanoma [185-188]. Immunotherapy for cancer com-prises of the various strategies employed to either activate the im-

mune effector functions (both innate as well as adaptive) or antago-nizing immune inhibitory functions, or both [189-192]. Examples of strategies for activating immune effector functions include adop-tive cell therapy (ACT), which introduces immune cells directly into the patient [193, 194], vaccination with tumor antigens [195], cytokine treatment (such as IL-2) [196], induction of antigen pres-entation (dendritic cell pulsing and administration) [197] and anti-bodies targeting co-stimulatory molecules for T-cell activation (OX40) [198]. Examples of inhibitory strategies include antibodies that deplete regulatory T cells (anti-CD25) [199] and antibodies targeting immune checkpoints (cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed cell death- 1(PD-1) [200]. Several of the above strategies that are in various phases of clinical trails for PCa include; Sipuleucel-T which is a dendritic cell-based vaccine [201, 202], Ipilimumab a fully humanized anti-CTLA-4 monoclonal antibody based immunotherapy [201, 203], Prostavac-VF (also known as PSA-TRICOM) an attenuated recombinant Vac-cinica/Fowlpox viral-vector based vaccine [202] and GVAX, which is a PCa cell-line based vaccine [204, 205].

PCa and NEPC is ideally suited for immunotherapy as it has a long natural history, thus providing ample time for immune effector functions to develop and act against the tumor. It is also rich in several neo-antigens such as PSA and PSMA, thereby providing an excellent repertoire of antigens for targeted immunotherapy. Impor-tantly, the above ongoing-clinical studies suggest immunotherapies to be more efficacious when carried out at an earlier stage where tumor burden is low. Thus, vaccination either alone or in combina-tion with androgen deprivation therapy (ADT) or radiotherapy in pre-surgical disease settings is essential as it will serve three impor-tant purposes; 1) define true response criteria to immunotherapies which generally elicit slower responses, 2) help identify markers of disease progression by providing an opportunity to sample tumor and blood both before and after treatment, and 3) provide an oppor-

Fig. (7). Pathway based drug targeting: an example using VEGF signaling pathway.

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tunity to identify neo-antigens which could be harnessed for per-sonalized immunotherapy regimes. Although no immunotherapy-based clinical trials are currently focused on NEPC, the advent of new genomics based-computational prowess that can predict, map and reposition drug that can modulate the immune system holds immense promise in the field [206]. In the future, such systems-based approaches and clinical trials of vaccines in pre-surgical set-tings would pave the ways towards designing immunotherapies targeting NEPC.

PREDICTIVE STRATEGIES FOR NEPC PATIENT STRATIFICATION

Therapeutic interventions are important in cancer management to provide positive patient outcomes including improvement in the quality of life of patient and reduction of symptom burden. NEPC is primarily a treatment induced, aggressive form of cancer that occur in a subset of patients with poor response to treatment. In this sce-nario, developing predictive models by combining deep profiling data from tumors and pharamacogenes with phenomic data could aid in developing risk models to see which patients are at risk for NEPC. Identifying the variable to develop such predictive models often need large-scale clinical trials. With the advent of automated data mining techniques and phenotyping algorithms, this is chang-ing. Algorithms can be designed to precisely phenotype PCa or NEPC and these algorithms can be used to compile a large patient population retrospectively. Data can also be assembled from cancer registries and public biomedical and healthcare databases. There are multiple ways to develop such predictive models; here we highlight two cases using EMR mining and risk model development.

MINING OF EHR AND CLINICAL TRIAL DATABASES TO

FIND PCa AND NEPC TRAJECTORIES Systematic analyses of data generated as part of the care-delivery of PCa or NEPC patient populations using data compiled in electronic health records (EHR) would also help to understand clinical implications of NEPC and the evolution of patients from PCa to NEPC or how the patients responds to NEPC treatments.

EHR based data mining and digital epidemiology studies are cost-effective, efficient and provide clinical insights more quickly than clinical trials. EHR-linked biorepositories have been used to do phenomics, phenome-wide enrichment analyses and discovering genetic variants associated red blood cell traits, platelet traits and diseases including peripheral arterial diseases [207-210]. Mining multiple data types from EHR has shown to be an important way to find patient clusters that can be treated with an intervention or ther-apy. For example, Li et al. (See: http://stm.sciencemag.org/content/7/311/311ra174) have shown that utilizing clinical characteristics including patient demographics, lab measures, and drug/s taken could help to segregate Type-2 diabetes population as three subtypes. Individualized or population scale therapies can now be designed to target these subtypes and improve outcomes. Similar approaches can be adapted for PCa and NEPC cohort and could help to determine patient clusters based on genetic variation, phenotypes or treatment response rates.

RISK PREDICTION MODELS FOR RESPONSE TO PROS-

TATE CANCER TREATMENT AND NEPC

Developing predictive models to assess the treatment responses (no response, partial response or full response) will be a valuable clinical aid in managing PCa patients and understanding the demo-graphics of NEPC patient. GWAS enabled the availability of ge-netic variants and genes mediating complex disease, and risk esti-mation models are now being built using genetic data as part of their parameters [86, 211]. Pharmacogenomics risk models are currently explored in the setting of neurodegenerative diseases [212]. Candidate-gene-set-based risk estimation using genetic vari-ants in pro-inflammatory genes and PCa genes have also been sug-gested [213], but some of these recommendations were not repli-cated when tested using whole-genome or exome-wide approaches. Nevertheless, combining large patient population and mapping sub-population specific risk would help to delineate pharmacogenomic variants and genetic predisposition to chemotherapy responses.

Fig. (8). Drug repositioning of cancer drugs mapped across different disease groups across International Classification of Disease – v9 coding system. Dis-

eases categories are labeled using green font, name of medications are labeled in black.

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Systems Medicine Approaches for Targeting Neuroendocrine Prostate Cancer Current Pharmaceutical Design, 2016, Vol. 22, No. 34 5243

DISCUSSION

Delivering the right drug for the right patient in the right dose at the right time via the right route is a critical step in implementing precision therapy in a clinical setting. Recommending cancer treat-ments by leveraging variations in pharamacogenes (genes associ-ated with metabolism of drugs) and mutation landscape of tumor would help to develop precision therapy. NEPC is an aggressive cancer phenotype that evolves via complex and largely unknown mechanism and patient communities can benefit from such ap-proaches. Typically, NEPCs do not express AR, and subsequently prostate-specific antigen (PSA) used for PCa screening. Thus, early diagnosis of NEPC is challenging, under-reported, and. by the time diagnosis is made, therapeutic interventions and treatment stratifi-cation are limited. Furthermore, since PSA is also used to monitor biochemical recurrence during and after treatment, development of treatment-induced NEPC is under appreciated. Recent data suggests that upwards of 25% of patients treated with new generation anti-androgen signaling therapies targeting signaling modules progress towards NEPC [205, 214]. Thus, there is an unmet and urgent clini-cal need to not only develop better biomarkers for detecting NEPC early, but also for treatment strategies to overcome NEPC devel-opment. With an increasing number of diagnoses, limited treatment options, and high mortality rates, NEPC patient populations need better treatment modalities faster than the typical drug development processes deliver them. We discussed a variety to approaches to tackle NEPC, but other integrated approaches can also be used to understand and treat the NEPC. For example, designing RNA inter-ference based single molecule or gene-set based inhibition could help to find key players of NEPC. Drug screening using viability assays and chemical genomics of NEPC cell lines would then help to find primary candidates for NEPC. Developing better cell lines, patient-derived xenograft (PDX) models and other mouse models, and leveraging high-throughput sequencing could help to find new targets, processes and pathways of NEPC.

CONCLUSION

With the advent of precision phenotyping and personalized risk assessment, we see the rapid evolution of cancers and cancer sub-types aresistant to standard treatment methods. Better therapies that consider the dynamic characteristics of tumor evolution and tumor volatility to evolve, and emerge as metastatic cancer phenotypes needs to be addressed. Studies are needed on systems scale using deep profiling techniques to understand the disease trajectory of the evolution of NEPC from PCa. Computational approaches including systematic drug repurposing, drug-modeling, metabolic modeling, developing protein or peptide therapies, and the use of nanoparti-cles could play a significant role in discovering and developing effective therapies to target NEPC. In addition to the curative strategies, developing predictive models could aid to understand the individual risk of patients to develop chemotherapy resistance and NEPC. An integrated pharmacological risk assessment that uses pharmacogenomic profiling with tumor profiling would also be helpful to predict response to therapies. Developing personalized risk estimates using genetics, family history, diet, environmental exposure and behavioral end points would help to develop precision risk models. To conclude, developing precision risk models that use pharmacogenomic profiling, lifestyle data (including exercise, diet, and environmental exposure) coupled with deep molecular profiling using genomics, transcriptomics, metabolomics and proteomics will elaborate a more comprehensive, systems-wide understanding of NEPC and ultimately enable predictive and therapeutic strategies.

AUTHORS’ CONTRIBUTION

KKY and KS wrote the first draft manuscript. BR, SSY, LL and AK provided critical inputs. KS compiled the data and performed the empirical analyses. KKY and SSY performed in vitro assays and model of NEPC differentiation. AT and JTD contributed to the

overall planning of the analyses and the manuscript. All authors read and approved the final manuscript.

CONFLICT AND FINANCIAL INTEREST

KKY, KS, SSY: None declared;

AKT has received research grants from Intuitive Surgicals, Inc (Sunnyvale, CA), Prostate Cancer Foundation (PCF) and National Institutive of Bioimaging and Bioengineering. JTD has received consulting fees or honoraria from Janssen Pharmaceuticals, GSK, AstraZeneca, and Hoffman-La Roche. JTD holds equity in NuMedii Inc, Ayasdi, Inc. and Ontomics, Inc. No writing assistance was utilized in the production of this manuscript.

ACKNOWLEDGEMENTS

This work was funded by the following grants from National Institutes of Health: National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK, R01-DK098242-03); National Can-cer Institute (NCI, U54-CA189201-02) to JTD and National Center for Advancing Translational Sciences (NCATS, UL1TR000067) Clinical and Translational Science Award (CTSA) grant to JTD and KS. KKY and SSY thank the Prostate Cancer Foundation for Young Investigator Awards. Authors would like to thank Harris Center For Precision Wellness (http: //www.precisionwellness.org/), Icahn Institute for Genomics and Multiscale Biology (http: //icahn.mssm.edu/departments-and-institutes/genomics), and Deane Prostate Health of Mount Sinai Health System for infrastructural support.

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