Approaches to Ras signaling modulation and treatment of Ras-dependent disorders: a patent review (2007 – present)

Introduction: Ras proteins are small GTPases molecular switches that cycle through two alternative conformational states, a GDP-bound inactive state and a GTP-bound active state. In the active state, Ras proteins interact with and modulate the activity of several downstream effectors regulating key cellular processes including proliferation, differentiation, survival, senescence, migration and metabolism. Activating mutations of RAS genes and of genes encoding Ras signaling members have a great incidence in proliferative disorders, such as cancer, immune and inflammatory diseases and developmental syndromes. Therefore, Ras and Ras signaling represent important clinical targets for the design and development of pharmaceutically active agents, including anticancer agents.

Areas covered: The authors summarize methods available to down-regulate the Ras pathway and review recent patents covering Ras signaling modulators, as well as methods designed to kill specifically cancer cells bearing activated RAS oncogene.

Expert opinion: Targeted therapy approach based on direct targeting of molecules specifically altered in Ras-dependent diseases is pursued with molecules that down-regulate expression or inhibit the biological function of mutant Ras or Ras signaling members. The low success rate in a clinical setting of molecules targeting activated members of the Ras pathway may require development of novel approaches, including combined and synthetic lethal therapies.     

Emerging pathways and future targets for the molecular therapy of pancreatic cancer

Introduction: Pancreatic cancer treatment remains a challenge for clinicians and researchers. Despite undisputable advances in the comprehension of the molecular mechanisms underlying cancer development and progression, early disease detection and clinical management of patients has made little, if any, progress in the past 20 years. Clinical development of targeted agents directed against validated pathways, such as the EGF/EGF receptor axis, the mutant KRAS protein, MMPs, and VEGF-mediated angiogenesis, alone or in combination with gemcitabine-based standard chemotherapy, has been disappointing.

Areas covered: This review explores the preclinical rationale for clinical approaches aimed at targeting the TGF-β, IGF, Hedgehog, Notch and NF-κB signaling pathways, to develop innovative therapeutic strategies for pancreatic cancer.

Expert opinion: Although some of the already clinically explored approaches (particularly EGFR and KRAS targeting) deserve further clinical consideration, by employing more innovative and creative clinical trial designs than the gemcitabine–targeted agent paradigm that has thus far invariably failed, the targeting of emerging and relatively unexplored signaling pathways holds great promise to increase our understanding of the complex molecular biology and to advance the clinical management of pancreatic cancer.     

Potential therapeutic approaches for Angelman syndrome

Introduction: Angelman syndrome (AS) is a neurodevelopmental disorder caused by deficiency of maternally inherited UBE3A, an ubiquitin E3 ligase. Despite recent progress in understanding the mechanism underlying UBE3A imprinting, there is no effective treatment. Further investigation of the roles played by UBE3A in the central nervous system (CNS) is needed for developing effective therapies.

Area covered: This review covers the literature related to genetic classifications of AS, recent discoveries regarding the regulation of UBE3A imprinting, alterations in cell signaling in various brain regions and potential therapeutic approaches. Since a large proportion of AS patients exhibit comorbid autism spectrum disorder (ASD), potential common molecular bases are discussed.

Expert opinion: Advances in understanding UBE3A imprinting provide a unique opportunity to induce paternal UBE3A expression, thus targeting the syndrome at its ‘root.’ However, such efforts have yielded less-than-expected rescue effects in AS mouse models, raising the concern that activation of paternal UBE3A after a critical period cannot correct all the CNS defects that developed in a UBE3A-deficient environment. On the other hand, targeting abnormal downstream cell signaling pathways has provided promising rescue effects in preclinical research. Thus, combined reinstatement of paternal UBE3A expression with targeting abnormal signaling pathways should provide better therapeutic effects.     

Targeting Mcl-1 for the therapy of cancer

Introduction: Human cancers are genetically and epigenetically heterogeneous and have the capacity to commandeer a variety of cellular processes to aid in their survival, growth and resistance to therapy. One strategy is to overexpress proteins that suppress apoptosis, such as the Bcl-2 family protein Mcl-1. The Mcl-1 protein plays a pivotal role in protecting cells from apoptosis and is overexpressed in a variety of human cancers.

Areas covered: Targeting Mcl-1 for extinction in these cancers, using genetic and pharmacological approaches, represents a potentially effectual means of developing new efficacious cancer therapeutics. Here we review the multiple strategies that have been employed in targeting this fundamental protein, as well as the significant potential these targeting agents provide in not only suppressing cancer growth, but also in reversing resistance to conventional cancer treatments.

Expert opinion: We discuss the potential issues that arise in targeting Mcl-1 and other Bcl-2 anti-apoptotic proteins, as well problems with acquired resistance. The application of combinatorial approaches that involve inhibiting Mcl-1 and manipulation of additional signaling pathways to enhance therapeutic outcomes is also highlighted. The ability to specifically inhibit key genetic/epigenetic elements and biochemical pathways that maintain the tumor state represent a viable approach for developing rationally based, effective cancer therapies.     

Genes Expressed in Cell Types after Their Exposure to UV Stresses or Oxidative Stresses,e.g., Keratinocytes,Fibroblasts in the Skin,or Endothelial Cells,Neurones or Astrocytes in Age-Related Pathologies Where Oxidative Stresses have been Shown to Occur,such as Atherosclerosis,Parkinson's Disease,Alzheimer's Disease

Introduction: The IGF system controls growth, differentiation, and development at the cellular, organ and organismal levels. IGF1 receptor (IGF1R) signaling is dysregulated in many cancers. Numerous clinical trials are currently assessing therapies that inhibit either growth factor binding or IGF1R itself. Therapeutic benefit, often in the form of stable disease, has been reported for many different cancer types.

Areas covered: Canonical IGF signaling and non-canonical pathways involved in carcinogenesis. Three recent insights into IGF1R signaling, namely hybrid receptor formation with insulin receptor (INSR), insulin receptor substrate 1 nuclear translocation, and evidence for IGF1R/INSR as dependence receptors. Different approaches to targeting IGF1R and mechanisms of acquired resistance. Possible mechanisms by which IGF1R signaling supports carcinogenesis and specific examples in different human tumors.

Expert opinion: Pre-clinical data justifies IGF1R as a target and early clinical trials have shown modest efficacy in selected tumor types. Future work will focus upon assessing the usefulness or disadvantages of simultaneously targeting the IGF1R and INSR, biomarker development to identify potentially responsive patients, and the use of IGF1R inhibitors in combination therapies or as an adjunct to conventional chemotherapy.     

Molecular targeted therapy in recurrent glioblastoma: current challenges and future directions

Introduction: The survival of patients with glioblastoma (GBM), which is the most common primary brain malignancy, remains poor with current treatment modalities. However, an enhanced understanding of gliomagenesis is supporting the development of targeted molecular therapies with the potential for improving clinical outcomes.

Areas covered: Epidermal growth factor receptor (EGFR) and vascular endothelial growth factor receptor (VEGFR) initiate key signaling pathways in GBM; however, trials with anti-EGFR agents have failed to show improved outcomes. Bevacizumab, a monoclonal antibody targeting VEGF, remains the only FDA-approved molecular drug in GBM; yet its use has only improved progression-free survival without any improvement in overall survival. We review the evidence supporting the continued evaluation of targeted molecular therapies in recurrent GBM. In addition, newer potential therapies targeting other signaling pathways, heat shock proteins and proteosomes, as well as the concept of targeting glioma stem cells are discussed.

Expert opinion: The complex genetic origin of GBM makes it challenging to identify molecular subsets that may benefit from specific targeted therapies. Pathway inhibition, via multisite kinase inhibitors or a carefully selected combination of molecular drugs with or without cytotoxic agents, is currently undergoing evaluation in clinical trials and may improve outcomes in these patients.     

Recent advances in the design of Hedgehog pathway inhibitors for the treatment of malignancies

Introduction: The Hedgehog (Hh) signaling pathway is known to be dysregulated in several forms of cancer. Hence, specifically targeting this signaling cascade is a valid and promising strategy for successful therapeutic intervention. Several components within the Hh pathway have been proven to be druggable; however, challenges in the discovery and development process for small molecules targeting this pathway have been identified.

Areas covered: This review details both the current state and future potential of Hh pathway inhibitors as anticancer chemotherapeutics that target a variety of human malignancies.

Expert opinion: The initial development of Hh pathway inhibitors focused on small-molecule antagonists of Smoothened, a transmembrane protein that is a key regulator of pathway signaling. More recently, efforts to identify and develop inhibitors of pathway signaling that function through alternate mechanisms have been increasing. However, none of these have advanced into clinical trials. Further, early evidence suggesting the broad application of Hh pathway inhibitors as a monotherapy in a wide range of human cancers has not been validated. The potential for Hh pathway inhibitors as combination therapy has demonstrated promising preclinical results. However, more research to identify rational drug combinations to fully explore the potential of this anticancer drug class is warranted.     

KSR as a therapeutic target for Ras-dependent cancers

Introduction: Targeting downstream effectors required for oncogenic Ras signaling is a potential alternative or complement to the development of more direct approaches targeting Ras in the treatment of Ras-dependent cancers.

Areas covered: Here we review literature pertaining to the molecular scaffold Kinase Suppressor of Ras (KSR) and its role in promoting signals critical to tumor maintenance. We summarize the phenotypes in knockout models, describe the role of KSR in cancer, and outline the structure and function of the KSR1 and KSR2 proteins. We then focus on the most recent literature that describes the crystal structure of the kinase domain of KSR2 in complex with MEK1, KSR-RAF dimerization particularly in response to RAF inhibition, and novel attempts to target KSR proteins directly.

Expert opinion: KSR is a downstream effector of Ras-mediated tumorigenesis that is dispensable for normal growth and development, making it a desirable target for the development of novel therapeutics with a high therapeutic index. Recent advances have revealed that KSR can be functionally inhibited using a small molecule that stabilizes KSR in an inactive conformation. The efficacy and potential for this novel approach to be used clinically in the treatment of Ras-driven cancers is still being investigated.     

Targeting RhoA/ROCK pathway in pulmonary arterial hypertension

Introduction: Pulmonary arterial hypertension (PAH) is a rare disease with a complex pathogenesis. It is often associated with an increased vascular resistance, whilst in the more advanced stages there is a remodelling of the vascular walls. PAH has an intricate involvement of various signaling pathways, including the ras homolog family member A (RhoA)–Rho kinase (ROCK) axis. Currently, available therapies are not always able to significantly slow PAH progression. Therefore, newer approaches are needed.

Areas covered: In this review, areas covered include the role of the RhoA/ROCK in PAH pathogenesis and the plausibility of its therapeutic targeting. Furthermore, various inhibitory compounds are discussed, including Fasudil and SB-772077-B.

Expert opinion: Currently, specific RhoA/ROCK inhibition is the most promising therapeutic approach for PAH. Research has shown that it suppresses both the components of this axis and the upstream upregulating mediators. An inhaled RhoA/ROCK inhibitor may be a successful future therapy; however, further clinical trials are needed to support this approach.     

14-3-3 zeta as novel molecular target for cancer therapy

Introduction: 14-3-3ζ acts as a central hub in signaling networks, which promotes cell proliferation, adhesion and survival and inhibits apoptosis in multiple cancers. Development of inhibitors or agents that interfere with 14-3-3ζ-dependent signaling networks are likely to serve as novel molecular agents for targeted cancer therapy.

Areas covered: The role of 14-3-3ζ in cancer and its potential as a novel molecular target for therapy. The involvement of 14-3-3ζ in chemoresistance in multiple cancers provides a rationale for developing novel molecular therapies targeting this protein for more effective cancer management. The keywords used to conduct the literature search for this paper were ‘14-3-3/14-3-3zeta and cancer', ‘14-3-3 structure', ‘14-3-3 inhibitors', ‘14-3-3 cancer prognosis', ‘14-3-3 and cancer therapy', ‘role/ functions of 14-3-3'.

Expert opinion: 14-3-3ζ is a central cellular hub protein regulating multiple signaling pathways involved in cancer development, progression and therapeutic resistance. Thus, 14-3-3ζ may serve as a novel molecular target for cancer therapy. New approaches including synthetic and/or natural inhibitors that interfere with 14-3-3ζ–client interactions need to be developed for effective cancer therapy.     

Emerging MEK inhibitors

Importance of the field: The Ras/Raf/MEK/ERK pathway is often activated by genetic alterations in upstream signaling molecules. Integral components of this pathway such as Ras and B-Raf are also activated by mutation. The Ras/Raf/MEK/ERK pathway has profound effects on proliferative, apoptotic and differentiation pathways. This pathway can often be effectively silenced by MEK inhibitors.

Areas covered by this review: This review will discuss targeting of MEK which could lead to novel methods to control abnormal proliferation which arises in cancer and other proliferative diseases. This review will cover the scientific literature from 1980 to present and is a follow on from a review which focused on Emerging Raf Inhibitors published in this same review series.

What the reader will gain: By reading this review the reader will understand the important roles that genetics play in the response of patients to MEK inhibitors, the potential of combining MEK inhibitors with other types of therapy, the prevention of cellular aging and the development of cancer stem cells.

Take home message: Targeting MEK has been shown to be effective in suppressing many important pathways involved in cell growth and the prevention of apoptosis. MEK inhibitors have many potential therapeutic uses in the suppression of cancer, proliferative diseases and aging.     

Advances in the use of Xenopus for successful drug screening

Introduction: Understanding embryogenesis currently relies largely on the control of gene expression via several signaling pathways. Many of the embryonic signaling pathways guiding embryological events are implicated in diseases that lack effective cure or treatment. Because of the large number and size of the eggs, the rapid development of the embryos and the fact they are amenable to pharmacological, surgical and genetic techniques, Xenopus laevis has been successfully used in searching for compounds that target embryonic signaling pathways.

Areas covered: Here, the authors address the use of amphibian eggs/embryos in successful chemical screenings; egg extracts as well as embryo phenotypes have been assayed to reveal drug toxicology effects and novel compounds acting in the Wnt/β-catenin signaling pathway. They do not discuss the use of Xenopus oocyte two-electrode voltage clamps or genome editing tools as approaches for drug discovery because they have been discussed elsewhere.

Expert opinion: While high-throughput screening is commonly performed in egg extracts, the embryo axes perturbation system is more suited to the refinement and/or the validation of drug discovery targeting embryonic signaling (particularly the Wnt/β-catenin pathway). In addition, Xenopus has also been used in FETAX (frog embryo teratogenesis assay: Xenopus) to address chemical toxic/teratogenic effects. However, further studies are necessary.     

Targeting the Wnt signaling pathway in colorectal cancer

Introduction: The treatment of patients with advanced colorectal cancer still remains challenging, and identification of new target molecules and therapeutic avenues remains a priority. The great majority of colorectal cancers have mutations in one of two genes involved in the Wnt signaling pathway: the adenomatous polyposis coli (APC) and β-catenin (CTNNB1) genes. Up to now, however, no therapeutics for targeting this pathway have been established.

Areas covered: This review article begins with a brief summary of Wnt signaling from the viewpoints of genetics, cancer stem cell biology, and drug development. We then overview current attempts to develop drugs directed at various components of the Wnt signaling pathway.

Expert opinion: APC is a tumor suppressor, and therefore only downstream signal transducers of the APC protein can be considered as targets for pharmaceutical intervention. TRAF2 and NCK-interacting protein kinase (TNIK) was identified as the most downstream regulator of Wnt signaling by two independent research groups, and several classes of small-molecule inhibitors targeting this protein kinase have been developed. TNIK is a multifunctional protein with actions that extend beyond Wnt signaling regulation. Such TNIK inhibitors are expected to have a large variety of clinical applications.     

The prolactin receptor as a therapeutic target in human diseases: browsing new potential indications

Introduction: Prolactin (PRL) signaling has emerged as a relevant target in breast and prostate cancers. This has encouraged various laboratories to develop compounds targeting the PRL receptor (PRLR). As the latter is widely distributed, it is timely to address whether other conditions could also benefit from such inhibitors.

Areas covered: The authors briefly overview the two classes of PRLR blockers, which involve: i) PRL-core based analogs that have been validated as competitive antagonists in various preclinical models, and ii) anti-PRLR neutralizing antibodies that are currently in clinical Phase I for advanced breast and prostate cancers. The main purpose of this review is to discuss the multiple organs/diseases that may be considered as potential targets/indications for such inhibitors. This is done in light of reports suggesting that PRLR expression/signaling is increased in disease, and/or that systemic or locally elevated PRL levels correlate with (or promote) organ pathogenesis.

Expert opinion: The two immediate challenges in the field are i) to provide the scientific community with potent anti-prolactin receptor antibodies to map prolactin receptor expression in target organs, and ii) to take advantage of the availability of functionally validated PRLR blockers to establish the relevance of these potential indications in humans.     

Targeting the insulin-like growth factor signaling pathway: figitumumab and other novel anticancer strategies

Introduction: There are clear preclinical data that support the involvement of the insulin-like growth factor (IGF) signaling pathway in oncogenesis and cancer progression. Such evidence has led to the design and conduct of drug development programs targeting the IGF-I receptor (IGF-IR) over the past 10 years.

Areas covered: This review details the structure and function of different members of the IGF system and related pathways, describes the rationale for targeting IGF-IR in cancer and updates the current advances in drug development. The preclinical development of figitumumab, the furthest developed mAb against IGF-IR, is examined as well as the reported data from Phase I - III clinical trials. Future prospects for this target and pathway are also discussed.

Expert opinion: While there have been both successes and failures in the development of novel targeted therapeutics targeting the IGF pathway, the evaluation of such agents should continue, with greater emphasis placed on combinatorial strategies and the development of predictive biomarkers that enhance antitumor responses through appropriate patient selection.     

Molecular targeted therapy for patients with melanoma: the promise of MAPK pathway inhibition and beyond

Importance of the field: Recent discoveries have expanded the understanding of the molecular signaling events critical to melanomagenesis and led to the development of targeted therapeutic agents that are revolutionizing the treatment of patients with advanced melanoma.

Areas covered in this review: This article reviews current therapy and its limitations, describes the key pathogenic mechanisms in melanoma for which inhibitors have been tested, and summarizes the results of clinical trials involving molecularly targeted agents in this disease.

What the reader will gain: There has been an explosion of preclinical and clinical research aimed at targeting the key molecular alterations in melanoma for therapeutic benefit. These findings will be presented and placed in the proper clinical context, affording information regarding the current molecular targets in the melanoma and the activity and limitations of therapeutic agents directed against them.

Take home message: Greater understanding of the pathogenic mechanisms underlying melanoma development has prompted the development of new therapeutic approaches aimed at counteracting these processes. While progress made over the past few years has generated considerable excitement, the benefits of these new therapies are still limited by incomplete and transient tumor regressions. It is hoped that with further investigation, particularly into mechanisms of treatment de novo and acquired treatment resistance, these limitations can be overcome.