Discovering potential drug-targets for personalized treatment of autoimmune disorders - what we learn from epidermolysis bullosa acquisita

Introduction: Epidermolysis bullosa acquisita (EBA) is a chronic autoimmune bullous dermatosis (AIBD). Treatment of EBA is challenging and mostly relies on systemic immunosuppression. During the last decade, intensive research led to the identification of new potential therapeutic targets that interfere in different phases of disease progression. Therapeutic interventions acting upon these candidate drug targets in animal models of EBA, such as cytokine-modulating biologics and small molecules, have validated them as potential new therapeutic strategies for EBA patients.

Areas covered: In this paper, we review the current treatments for EBA, describe the pathogenesis of the disease, and finally specify new drug candidates for the development of a more specific therapy with minimized side-effects for EBA and potentially other autoimmune diseases.

Expert opinion: We currently understand EBA as a disease that evolves from the interplay of many different signaling pathways. These signaling pathways, which are described in this review, provide new targets for EBA treatment. The ultimate goal of this research field is the development of specific, pathogenesis-based therapeutic strategies. Through identification of up- or downregulated pathways that dominate disease progression in individual patients, we expect therapy in EBA to become more and more precise and move towards a patient-based therapy.     

Promising therapies for the treatment of frontotemporal dementia clinical phenotypes: from symptomatic to disease-modifying drugs

Introduction: Frontotemporal dementia (FTD) is a heterogeneous clinical entity that includes several disorders characterized by different cellular mechanisms. Distinctive clinical features in FTD include behavioral, affective, and cognitive symptoms. Unfortunately, little progress has been made over the past 20 years in terms of the development of effective disease-modifying drugs with the currently available symptomatic treatments having limited clinical utility.

Areas covered: This article reviews the principal pharmacological intervention studies for FTD. These are predominantly randomized clinical trials and include symptomatic treatments and potential disease-modifying drugs.

Expert opinion: There is insufficient evidence on effective treatments for FTD and studies with better methodological backgrounds are needed. Most studies reporting therapeutic benefits were conducted with selective serotonin reuptake inhibitors, while anti-dementia drugs have been ineffective in FTD. Since the underlying pathology of FTD mostly consists of abnormal tau protein or TDP-43 aggregates, treatments are being developed to interfere with their aggregation process or with the clearance of these proteins. Furthermore, disease-modifying treatments remain years away as demonstrated by the recent negative Phase III findings of a tau aggregation inhibitor (LMTM) for treating the behavioral variant of FTD. The results from current ongoing Phase I/II trials will hopefully give light to future treatment options.     

Targeting leucine-rich repeat kinase 2 in Parkinson's disease

Introduction: Parkinson's disease (PD), is a common progressive neurodegenerative disorder, and missense mutations in the LRRK2 gene are the most common single genetic cause of autosomal dominant PD and polymorphic variants modulate risk in sporadic PD. Earlier research focused on LRRK2 genetics, but with the recent discoveries of LRRK2 substrates/interactors, LRRK2-specific mechanisms are being unveiled.

Areas covered: As a multi-domain protein, LRRK2 possess diverse functions that range from housekeeping, signaling to clearance of proteins. Proteins that interact with LRRK2 have drawn attention to several pathophysiologic pathways that could potentially be targeted in the development of new therapies. This review will discuss the possible physiological roles of LRRK2 based on recently reported interactors as well as place LRRK2 in the disease context. Current LRRK2 inhibition studies and reports about LRRK2 biomarkers will also be discussed as they are important considerations for LRRK2 translational treatment options.

Expert opinion: The discovery of LRRK2 as a pathogenic gene in PD has contributed enormously to our understanding of clinical genetics. One of the challenges is to understand the physiologic role of LRRK2 and its specific function that gets disrupted when it is mutated. In vivo LRRK2 models have provided insights but they do not full recapitulate human PD. The identification of LRRK2 interactors opens the opportunities for identification of new therapeutic targets. Ways of quantification of kinase activity in vivo and determining what constitutes physiologic inhibition will need to be further investigated before specific pharmacologic agents can be meaningfully utilized.     

Selective targeting of RANK signaling pathways as new therapeutic strategies for osteoporosis

Importance of the field: Osteoporosis has become a worldwide health and social issue due to an aging population. Four major antiresorptive drugs (agents capable of inhibiting osteoclast formation and/or function) are currently available on the market: estrogen, selective estrogen receptor modulators (SERMs), bisphosphonates and calcitonin. These drugs either lack satisfactory efficacy or have potential to cause serious side effects. Thus, development of more efficacious and safer drugs is warranted.

Areas covered in this review: The discovery of the receptor activator of NF-κB ligand (RANKL) and its two receptors, RANK and osteoprotegerin (OPG), has not only established a crucial role for the RANKL/RANK/OPG axis in osteoclast biology but also created a great opportunity to develop new drugs targeting this system for osteoporosis therapy. This review focuses on discussion of therapeutic targeting of RANK signaling.

What the reader will gain: An update on the functions of RANKL and an overview of the known RANK signaling pathways in osteoclasts. A discussion of rationales for exploring RANK signaling pathways as potent and specific therapeutic targets to promote future development of better drugs for osteoporosis.

Take home message: Several RANK signaling components have the potential to serve as potent and specific therapeutic targets for osteoporosis.     

Targeting Pyk2 for therapeutic intervention

Importance of the field: The focal adhesion tyrosine kinases FAK and Pyk2 are uniquely situated to act as critical mediators for the activation of signaling pathways that regulate cell migration, proliferation and survival. By coordinating adhesion and cytoskeletal dynamics with survival and growth signaling, FAK and Pyk2 represent molecular therapeutic targets in cancer as malignant cells often exhibit defects in these processes.

Areas covered in this review: This review examines the structure and function of the focal adhesion kinase Pyk2 and intends to provide a rationale for the employment of modulating strategies that include both catalytic and extra-catalytic approaches that have been developed in the last 3 – 5 years.

What the reader will gain: Targeting tyrosine kinases in oncology has focused on the ATP binding pocket as means to inhibit catalytic activity and downregulate pathways involved in tumor invasion. This review discusses the available catalytic inhibitors and compares them to the alternative approach of targeting protein–protein interactions that regulate kinase activity.

Take home message: Development of specific catalytic inhibitors of the focal adhesion kinases has improved but significant challenges remain. Thus, approaches that inhibit the effector function of Pyk2 by targeting regulatory modules can increase specificity and will be a welcome asset to the therapeutic arena.     

Targeting α-synuclein as a therapeutic strategy for Parkinson’s disease

Introduction: α-Synuclein, a neuronal protein, plays a central role in the pathophysiology of Parkinson’s disease (PD), the second most prevalent neurodegenerative disorder. Cases of PD have increased tremendously over the past decade necessitating the identification of new therapeutic targets to reduce patient morbidity and to improve PD patients’ quality of life.

Areas covered: The purpose of this article is to provide an update on the role of α-synuclein in fibrils formation and review its role as an effective immunotherapeutic target for PD. The rapidly expanding evidence for the contribution of α-synuclein to the pathogenesis of PD led to the development of antibodies against the C terminus of α-synuclein and other molecules involved in the inflammatory signaling pathways that were found to contribute significantly to initiation and progression of the disease.

Expert opinion: The readers will obtain new insights on the mechanisms by which α-synuclein can trigger the development of PD and other related degenerative disorders along with the potential role of active and passive antibodies targeted against specific form of α-synuclein aggregates to clear neurotoxicity, stop the propagation of the prion-like behavior of these oligomers and reverse neuronal degeneration associated with PD.     

Targeting Fas in osteoresorptive disorders

Importance of the field: Fas receptor is a mediator of the external apoptotic pathway in many cells and tissues. It is proposed that Fas receptor mediates osteoresorptive effects of estrogen deficiency and local/systemic inflammation.

Areas covered in this review: This review covers the past two decades of research on the expression and function of the Fas–Fas ligand system on bone cells, involvement in the pathogenesis of osteoresorption and potential therapeutic modulation.

What the reader will gain: We review the structure, biological function and intracellular signaling pathways of the Fas–Fas ligand system emphasizing the role of the non-apoptotic signaling pathways in bone cells, particularly osteoblast differentiation. We also present data on the in vitro expression and function of the Fas–Fas ligand system on osteoblast/osteoclast lineage cells, animal and human studies confirming its involvement in osteoresorptive disorders and potential therapeutic approaches to modulate its function.

Take home message: Tissue specific therapeutic approaches need to be established to modify the Fas–Fas ligand system in osteoresorptive disorders as systemic targeting has many side effects. The most promising approach would be to target Fas signaling molecules coupled with osteoblast/osteoclast differentiation pathways, but a precise definition of these targets is still needed.     

Progranulin as a therapeutic target for dementia

Introduction: Progranulin (PGRN) is an acrosomal glycoprotein that is synthesized during spermatogenesis. It is overexpressed in tumors and has anti-inflammatory properties. The protein may be cleaved into granulins which display pro-inflammatory properties. In 2006, mutations in progranulin gene (GRN) that cause haploinsufficiency were found in familial cases of frontotemporal dementia (FTD). Patients with null mutations in GRN display very low-plasma PGRN levels; this analysis is useful for identifying mutation carriers, independent of the clinical presentation, and in those before the appearance of symptoms.

Areas covered: Here, we review the current knowledge of PGRN physiological functions and GRN mutations associated with FTD; we also summarize state of the art clinical trials and those compounds able to replace PGRN loss in preclinical models.

Expert opinion: PGRN represents a promising therapeutic target for FTD. Cohorts suitable for treatment, ideally at the preclinical stage, where pathogenic mechanisms ongoing in the brain are targeted, are available. However, PGRN may have side effects, such as the risk of tumorigenesis, and the risk/benefit ratio of any intervention cannot be predicted. Furthermore, at present, the situation is complicated by the absence of adequate outcome measures.     

Glycobiology of the Leishmania parasite and emerging targets for antileishmanial drug discovery

Importance of the field: Parasitic diseases that pose a threat to human life include leishmaniasis – caused by protozoa of Leishmania species. Existing drugs have limitations due to deleterious side effects like teratogenicity and factors like cost and drug resistance, thus furthering the need to develop this area of research.

Areas covered in this review: We came across drug targets, very recently characterised, cloned and validated by genomics and bioinformatics. We bring these promising drug targets into focus so that they can be explored to their fullest.

What the reader will gain: In an effort to bridge the gaps between existing knowledge and future prospects of drug discovery, we found interesting studies validating drug targets and paving the way for better experiments to be designed. In a few cases, novel pathways have been characterized, while in others, well established pathways when probed further, led to the discovery of new drug targets.

Take home message: The review constitutes a comprehensive report on upcoming drug targets, with emphasis on glycosylphosphatidylinositol (GPI)-anchored glycoconjugates along with related biochemistry of enolase, glycosome and purine salvage pathways, as we strive to bring ourselves a step closer to being able to combat this deadly disease.     

Chaperone-mediated autophagy as a therapeutic target for Parkinson disease

ABSTRACT

Introduction: Parkinson disease (PD) is the most common neurodegenerative movement disorder. Currently only symptomatic treatments exist for PD, and so the search for potential neuroprotective drug targets is of great importance. Chaperone mediated autophagy (CMA) is one of the key cellular mechanisms in protein homeostasis. Many of the pathogenic pathways thought to be important in PD converge on CMA, thus rendering it an attractive therapeutic target.

Areas covered: In this review we discuss current up-to-date knowledge of the molecular mechanisms involved in CMA function and regulation. We go on to discuss the links between CMA and PD including CMA’s role in ɑ-synuclein processing, oxidative stress, and mitochondrial function. We finish by exploring the potential benefits of how upregulation of CMA may be beneficial in PD and strategies to achieve this.

Expert opinion: Upregulation of CMA is an attractive therapeutic target in PD due to its links with several pathogenic pathways . Currently more knowledge of the mechanisms that regulate CMA is required to allow for the development of specific CMA modulators. However, recent studies demonstrating the role of retinoic acid derivatives and miRNAs in regulating CMA are promising, and indirect upregulation of CMA by modulating other lysosomal pathways may be helpful.     

Targeted approaches to childhood cancer: progress in drug discovery and development

Introduction: Cancer is a leading cause of death in childhood. Encouraging progress has been made in the treatment of childhood malignancies, but there is an unmet need for new drugs to improve survival and reduce treatment-associated toxicities. Drug development in paediatric oncology has specific requirements with regard to the patient population and the regulatory background and presents several unique challenges that need addressing.

Areas covered: This review discusses the current framework of paediatric oncology drug development and some of the specific challenges in pre-clinical and clinical research. The authors discuss the recent developments in the targeting of various signalling pathways. These pathways represent a selection of targets that have been identified by pre-clinical and clinical investigators to be highly relevant in paediatric malignancies.

Expert opinion: The development of targeted agents in paediatric oncology must be driven by knowledge of tumour biology. Predictive and pharmacodynamic biomarkers should be incorporated within paediatric early clinical trials wherever possible. Faster dose-escalation, limited numbers of cohorts and novel adaptive designs can help to make paediatric early clinical trials more efficient. Close collaboration between academic/clinical researchers, the pharmaceutical industry, regulatory bodies and parent groups are crucial in overcoming the challenges associated with paediatric oncology drug development.     

Molecular targets on B-cells to prevent and treat antibody-mediated rejection in organ transplantation. Present and Future

Introduction: Waiting lists for transplant are increasing day after day since transplantation is still the best option for the treatment of end-stage organ failure. Likewise, our increasing knowledge about how immune system mounts the response against allograft is at the point that up to 95% of acute T-cell-mediated rejections are effectively controlled by current immunosuppressive therapy. Nevertheless, this is not the case for acute and chronic antibody-mediated rejections (ABMR), where treatments to reduce the level of donor specific antibodies (DSAs) remain suboptimal, causing the majority of acute and chronic graft losses.

Areas covered: T-cell immunosuppressant agents are usually ineffective to treat humoral rejection and current strategies to prevent ABMR include plasmapheresis; high doses of intravenous immunoglobulin; anti-CD20 monoclonal antibodies to treat B-cells; and in some cases Bortezomib, which mainly affects plasmoblast and plasma cells, or antibodies against components of complement cascade.

Expert Opinion: We reassess the B-cell ontogeny in order to propose new molecular targets that interrupt the antibody production pathways and their eventual pathological injury. We also take a look at the pharmaceutical industry to find agents that are currently being assayed for B-cell pathologies and that could be used in the future to prevent and treat ABMR.     

Novel approaches to the development of tyrosine kinase inhibitors and their role in the fight against cancer

Introduction: Protein tyrosine kinase inhibitors are currently one of the most important classes of cancer drugs and one of the most impressive approaches of targeted cancer therapy. Aberrant activation of tyrosine kinase pathways is among the most dysregulated molecular pathways in human cancers; therefore, a large number of tyrosine kinases may serve as valuable molecular targets. To date, several inhibitors of tyrosine kinases have been approved and there are hundreds more compounds that are in various stages of development. Because of the deregulation in human malignancies, the ABL1, SRC, the epidermal growth factor receptor and the vascular endothelial growth factor receptor kinases are among the protein kinases that are considered as prime molecular targets for selective inhibition.

Areas covered: This review focuses on most important small-molecule inhibitors that serve as a model for future development. They also provide a broad overview of some of the new approaches and challenges in the field.

Expert opinion: With the exception of a few malignancies seemingly driven by a limited number of genetic lesions, current targeted therapeutic approaches have shown only limited efficacy in advanced cancers. Consequently, more sophisticated strategies, such as identification of pathogenic ‘driver' mutations and optimization of personalized therapies are needed.     

Necroptosis inhibitors as therapeutic targets in inflammation mediated disorders - a review of the current literature and patents

Introduction: Recent studies have shown substantial interplay between the apoptosis and necroptosis pathways. Necroptosis, a form of programmed cell death, has been found to stimulate the immune system contributing to the pathophysiology of several inflammation-mediated disorders. Determining the contribution of necroptotic signaling pathways to inflammation may lead to the development of selective and specific molecular target implicated necroptosis inhibitors.

Areas covered: This review summarizes the recently published and patented necroptosis inhibitors as therapeutic targets in inflammation-mediated disorders. The role of several necroptosis inhibitors, focusing on specific signaling molecules, was discussed with particular attention to inflammation-mediated disorders. Data was obtained from Espacenet®, WIPO®, USPTO® patent websites, and other relevant sources (2006-2016).

Expert opinion: Necroptosis inhibitors hold promise for treatment of inflammation-mediated clinical conditions in which necroptotic cell death plays a major role. Although necroptosis inhibitors reviewed in this survey showed inhibitory effects against several inflammation-mediated disorders, only a few have passed to the stage of clinical testing and need extensive research for therapeutic practice. Revisiting the existing drugs and developing novel necroptosis inhibiting agents as well as understanding their mechanism are essential. A detailed study of necroptosis function in animal models of inflammation may provide us an alternative strategy for the development of drug-like necroptosis inhibitors.     

Small RNA drugs for prion disease: a new frontier

Introduction: Prion diseases, also known as transmissible spongiform encephalopathies, are a group of neurodegenerative diseases that are invariably incurable. In fact, intense laboratory and clinical research have failed to discover effective treatments, to date, which delay the onset or progression of any neurodegenerative conditions, including those caused by infectious prions. It has become clear that profound changes in the brains of patients are evident long before clinical signs and it is at this stage that the disease is reversible and presents ‘druggable' targets. However, research is beginning to uncover the molecular underpinnings involved in the early stages of disease pathogenesis. Targeting key genes and pathways using short non-coding RNA is a new avenue of exploratory research for the treatment of prion disease that holds much promise for the future.

Areas covered: This article reviews the novel approach of using RNA-based drugs as a therapeutic opportunity for prion disease. Furthermore, it discusses the challenges that currently exist in the development of these therapies and highlights the future opportunities in this area.

Expert opinion: Numerous challenges exist before this therapeutic option can be translated into effective treatments. First, the crucial genes and pathways targeted must be identified from the multitude of temporally and spatially altered genetic processes that occur during the disease. Second, patients must be before irreversible neuronal degeneration, that accompanies prion replication, has progressed. Finally, these small RNAs must be delivered to the affected region of the brain over long periods of time and without significant side effects.     

Androgenetic alopecia: combing the hair follicle signaling pathways for new therapeutic targets and more effective treatment options

ABSTRACT

Introduction: In the past 30 years, only two drugs have received FDA approval for the treatment of androgenetic alopecia reflecting a lack of success in unraveling novel targets for pharmacological intervention. However, as our knowledge of hair biology improves, new signaling pathways and organogenesis processes are being uncovered which have the potential to yield more effective therapeutic modalities.

Areas covered: This review focuses on potential targets for drug development to treat hair loss. The physiological processes underlying the promise of regenerative medicine to recreate new functional hair follicles in bald scalp are also examined.

Expert opinion: The discovery of promising new targets may soon enable treatment options that modulate the hair cycle to preserve or extend the growth phase of the hair follicle. These new targets could also be leveraged to stimulate progenitor cells and morphogenic pathways to reactivate miniaturized follicles in bald scalp or to harness the potential of wound healing and embryogenic development as an emerging paradigm to generate new hair follicles in barren skin.     

Targeting aneuploid cancer cells

Introduction: Most cancers are characterized by some degree of aneuploidy, although its relevance for tumor initiation or progression and the nature of the initial trigger are still not well understood. It was Theodor Boveri who first suggested a link between aneuploidy and cancer at the beginning of the last century, but it is only recently that the molecular mechanisms involved have started to be uncovered.

Areas covered: The molecular mechanisms that are at the origin of aneuploidy and their cellular consequences. Based on these new findings molecular targets have emerged which could lead to a specific treatment of at least some types of aneuploid tumors.

Expert opinion: Therapeutic intervention specifically for aneuploid cells is a very promising approach, however, although new promising targets have been spotted they still need to be tested for proof of concept. Targeting the spindle checkpoint could be an interesting approach for cancer therapy, however, as for other mitotic targets, the open question of the therapeutic window and sensitivity of normal hemopoietic cells has to be considered carefully. Future challenges will not only include identifying and validating druggable targets related to the relevant pathways, but also finding predictive biomarkers to define the responding patient population(s).