The value and limitations of transgenic mouse models used in drug discovery for Alzheimer's disease: an update

INTRODUCTION: The exponential growth in the world's aged population has increased pressure on drug discovery efforts to identify innovative therapies for Alzheimer's disease (AD). The long and uncertain clinical trial path utilized to test the potential efficacy of these novel agents is challenging. For these and other reasons, there has been an explosion in the generation and availability of transgenic mouse models that mimic some, but not all aspects of AD. The largely overwhelmingly positive results obtained when testing potential clinical agents in these same animal models have failed to translate into similar positive clinical outcomes. AREAS COVERED: This review discusses the value and limitations associated with currently available transgenic mouse models of AD. Furthermore, the article proposes ways in which researchers can better characterize pharmacodynamic and pharmacokinetic endpoints to increase the success rate for novel therapies advancing into clinical development. Lastly, the author discusses ways in which researchers can supplement, expand and improve transgenic mouse models used in AD drug discovery. EXPERT OPINION: The use of transgenic mouse models that recapitulate various aspects of AD has expanded our knowledge and understanding of disease pathogenesis immensely. Further success in testing and translating novel therapies from animal models into bona fide medicines would be enhanced by i) the availability of better models that more fully recapitulate the disease spectrum, ii) defining and measuring standardized endpoints that display a pharmacodynamic range, iii) building and including translatable biomarkers and iv) including novel endpoints that would be expected to translate into clinically beneficial outcomes.     

Rodent models for Alzheimer’s disease drug discovery

Introduction: Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by memory loss and personality changes, leading to dementia. Histopathological hallmarks are represented by aggregates of beta-amyloid peptide (Aβ) in senile plaques and deposition of hyperphosphorylated tau protein in neurofibrillary tangles in the brain. Rare forms of early onset familial Alzheimer’s disease are due to gene mutations. This has prompted researchers to develop genetically modified animals that could recapitulate the main features of the disease. The use of these models is complemented by non-genetically modified animals.

Areas covered: This review summarizes the characteristics of the most used transgenic (Tg) and non-Tg models of AD. The authors have focused on models mainly used in their laboratories including amyloid precursor protein (APP) Tg2576, APP/presenilin 1, 3xAD, single h-Tau, non-Tg mice treated with acute injections of Aβ or tau, and models of physiological aging.

Expert opinion: Animal models of disease might be very useful for studying the pathophysiology of the disease and for testing new therapeutics in preclinical studies but they do not reproduce the entire clinical features of human AD. When selecting a model, researchers should consider the various factors that might influence the phenotype. They should also consider the timing of testing/treating animals since the age at which each model develops certain aspects of the AD pathology varies.     

Colorectal cancer models for novel drug discovery

Introduction: Despite increased screening rates and advances in targeted therapy, colorectal cancer (CRC) remains the third leading cause of cancer-related mortality. CRC models that recapitulate key features of human disease are essential to the development of novel and effective therapeutics. Classic methods of modeling CRC such as human cell lines and xenograft mice, while useful for many applications, carry significant limitations. Recently developed in vitro and in vivo models overcome some of these deficiencies and thus can be utilized to better model CRC for mechanistic and translational research.

Areas covered: The authors review established models of in vitro cell culture and describe advances in organoid culture for studying normal and malignant intestine. They also discuss key features of classic xenograft models and describe other approaches for in vivo CRC research, including patient-derived xenograft, carcinogen-induced, orthotopic transplantation and transgenic mouse models. We also describe mouse models of metastatic CRC.

Expert opinion: No single model is optimal for drug discovery in CRC. Genetically engineered models overcome many limitations of xenograft models. Three-dimensional organoids can be efficiently derived from both normal and malignant tissue for large-scale in vitro and in vivo (transplantation) studies and are thus a significant advance in CRC drug discovery.     

Xenograft and genetically engineered mouse model systems of osteosarcoma and Ewing's sarcoma: tumor models for cancer drug discovery

Introduction: There are > 75 histological types of solid tumors that are classified into two major groups: bone and soft-tissue sarcomas. These diseases are more prevalent in children, and pediatric sarcomas tend to be highly aggressive and rapidly progressive. Sarcomas in adults may follow a more indolent course, but aggressive tumors are also common. Sarcomas that are metastatic at diagnosis, or recurrent following therapy, remain refractory to current treatment options with dismal overall survival rates. A major focus of clinical trials, for patients with sarcoma, is to identify novel and more effective therapeutic strategies targeted to genomic or proteomic aberrations specific to the malignant cells. Critical to the understanding of the potential for targeted therapies are models of disease that are representative of clinical disease and predictive of relevant clinical responses.

Areas covered: In this article, the authors discuss the use of mouse xenograft models and genetically engineered mice in cancer drug discovery. The authors provide a special focus on models for the two most common bone sarcomas: osteosarcoma (OS) and Ewing's sarcoma (ES).

Expert opinion: Predicting whether a new anticancer agent will have a positive therapeutic index in patients with OS and ES remains a challenge. The use of mouse sarcoma models for understanding the mechanisms involved in the response of tumors to new treatments is an important step in the process of drug discovery and the development of clinically relevant therapeutic strategies for these diseases.     

Animal models of skin disease for drug discovery

Introduction: Discovery of novel drugs, treatments, and testing of consumer products in the field of dermatology is a multi-billion dollar business. Due to the distressing nature of many dermatological diseases, and the enormous consumer demand for products to reverse the effects of skin photodamage, aging, and hair loss, this is a very active field.

Areas covered: In this paper, we will cover the use of animal models that have been reported to recapitulate to a greater or lesser extent the features of human dermatological disease. There has been a remarkable increase in the number and variety of transgenic mouse models in recent years, and the basic strategy for constructing them is outlined.

Expert opinion: Inflammatory and autoimmune skin diseases are all represented by a range of mouse models both transgenic and normal. Skin cancer is mainly studied in mice and fish. Wound healing is studied in a wider range of animal species, and skin infections such as acne and leprosy also have been studied in animal models. Moving to the more consumer-oriented area of dermatology, there are models for studying the harmful effect of sunlight on the skin, and testing of sunscreens, and several different animal models of hair loss or alopecia.     

Drug discovery in prostate cancer mouse models

Introduction: The mouse is an important, though imperfect, organism with which to model human disease and to discover and test novel drugs in a preclinical setting. Many experimental strategies have been used to discover new biological and molecular targets in the mouse, with the hopes of translating these discoveries into novel drugs to treat prostate cancer in humans. Modeling prostate cancer in the mouse, however, has been challenging, and often drugs that work in mice have failed in human trials.

Areas covered: The authors discuss the similarities and differences between mice and men; the types of mouse models that exist to model prostate cancer; practical questions one must ask when using a mouse as a model; and potential reasons that drugs do not often translate to humans. They also discuss the current value in using mouse models for drug discovery to treat prostate cancer and what needs are still unmet in field.

Expert opinion: With proper planning and following practical guidelines by the researcher, the mouse is a powerful experimental tool. The field lacks genetically engineered metastatic models, and xenograft models do not allow for the study of the immune system during the metastatic process. There remain several important limitations to discovering and testing novel drugs in mice for eventual human use, but these can often be overcome. Overall, mouse modeling is an essential part of prostate cancer research and drug discovery. Emerging technologies and better and ever-increasing forms of communication are moving the field in a hopeful direction.     

Developing models for cachexia and their implications in drug discovery

Introduction: Cachexia is a complex metabolic syndrome associated with underlying illness and characterized by loss of muscle with or without loss of fat mass. Systemic inflammation plays a central role in its pathophysiology. As millions of patients are in a cachectic state of chronic disease, cachexia is one of the major causes of death worldwide. Difficulties in the recruitment and follow-up of clinical trials mean that well-characterized animal models are of great importance in developing cachexia therapies. However, some of the widely used animal models have limitations in procedural reproducibility or in recapitulating in the cachectic phenotype, which has warranted the development of novel models for cachexia.

Areas covered: This review focuses on some of the currently developing rodent models designed to mimic each co-morbidity in cachexia.

Expert opinion: Through developing cancer models, researchers have been seeking more targets for intervention. In cardiac cachexia, technical issues have been overcome by transgenic models. Furthermore, the development of new animal models has enabled the elucidation of the roles of inflammation, anabolism/catabolism in muscle/fat tissue and anorexia on cachexia. As metabolic and inflammatory pathways in cachexia may compromise cardiac muscle, the analysis of cardiac function/tissue in non-cardiac cachexia may be a useful component of cachexia assessment common to different underlying diseases and pave the way for novel drug discovery.     

Progress in the development of animal models of acute kidney injury and its impact on drug discovery

Introduction: Acute kidney injury (AKI) is a clinical syndrome characterized by the acute loss of kidney function. AKI is increasingly frequent and is associated with impaired survival and chronic kidney disease progression. Experimental AKI models have contributed to a better understanding of pathophysiological mechanisms but they have not yet resulted in routine clinical application of novel therapeutic approaches.

Areas covered: The authors present the advances in experimental AKI models over the last decade. Furthermore, the authors review their current and expected impact on novel drug discovery.

Expert opinion: New AKI models have been developed in rodents and non-rodents. Non-rodents allow the evaluation of specific aspects of AKI in both bigger animals and simpler organisms such as drosophila and zebrafish. New rodent models have recently reproduced described clinical entities, such as aristolochic and warfarin nephropathies, and have also provided better models for old entities such as thrombotic microangiopathy-induced AKI. Several therapies identified in animal models are now undergoing clinical trials in human AKI, including p53 RNAi and bone-marrow derived mesenchymal stem cells. It is conceivable that further refinement of animal models in combination with ongoing trials and novel trials based on already identified potential targets will eventually yield effective therapies for clinical AKI.     

Orthotopic mouse models expressing fluorescent proteins for cancer drug discovery

Importance of the field: Currently used rodent tumor models, including transgenic tumor models, or subcutaneously growing human tumors in immunodeficient mice, do not sufficiently represent clinical cancer, especially with regard to metastasis and drug sensitivity.

Areas covered in this review: To obtain clinically accurate models, we have developed the technique of surgical orthotopic implantation (SOI) to transplant histologically intact fragments of human cancer, including tumors taken directly from the patient, to the corresponding organ of immunodeficient rodents. SOI allows the growth and metastatic potential of the transplanted tumors to be expressed and reflects clinical cancer of all types. Effective drugs can be discovered and evaluated in the SOI models utilizing human tumor cell lines and patient tumors. Visualization of many aspects of cancer initiation and progression in vivo has been achieved with fluorescent proteins. Tumors and metastases in the SOI models that express fluorescent proteins can be visualized noninvasively in intact animals, greatly facilitating drug discovery.

What the reader will gain: This review will provide information on the imageable mouse models of cancer that are clinically relevant, especially regarding metastasis and their use for drug discovery and evaluation.

Take home message: SOI mouse models of cancer reproduce the features of clinical cancer.     

Curcumin and neurodegenerative diseases: a perspective

Introduction: Curcumin, a dietary polyphenol found in the curry spice turmeric, possesses potent antioxidant and anti-inflammatory properties and an ability to modulate multiple targets implicated in the pathogenesis of chronic illness. Curcumin has shown therapeutic potential for neurodegenerative diseases including Alzheimer's disease (AD) and Parkinson's disease (PD).

Areas covered: This article highlights the background and epidemiological evidence of curcumin's health benefits and its pharmacodynamic and pharmacokinetic profile. Curcumin's ability to counteract oxidative stress and inflammation and its capacity to modulate several molecular targets is reviewed. We highlight the neuroprotective properties of curcumin including pre-clinical evidence for its pharmacological effects in experimental models of AD and PD. The bioavailability and safety of curcumin, the development of semi-synthetic curcuminoids as well as novel formulations of curcumin are addressed.

Expert opinion: Curcumin possesses therapeutic potential in the amelioration of a host of neurodegenerative ailments as evidenced by its antioxidant, anti-inflammatory and anti-protein aggregation effects. However, issues such as limited bioavailability and a paucity of clinical studies examining its therapeutic effectiveness in illnesses such as AD and PD currently limit its therapeutic outreach. Considerable effort will be required to adapt curcumin as a neuroprotective agent to be used in the treatment of AD, PD and other neurodegenerative diseases.     

Update on the pharmacotherapy for myelodysplastic syndromes

Introduction: For many decades, myelodysplastic syndromes (MDS) were a poorly understood disease group with no approved therapies, and patient management largely relied upon supportive care and intensive chemotherapy. The last decade has seen many scientific and therapeutic advances culminating in the US FDA approval of three drugs for the treatment of these complex malignancies: lenalidomide, azacitidine and decitabine.

Areas covered: This review summarizes the major prognostic risk models that guide treatment decisions and examines the available literature on the mechanism of action and efficacy of each of the approved agents. The authors also discuss evidence supporting the use of other therapies that have entered the standard of care including growth factors, immunosuppressive therapy and stem-cell transplantation.

Expert opinion: While significant progress has been made in understanding the molecular basis of MDS, much of this has yet to translate into therapeutic benefit. Each of the available treatment modalities has shortcomings, and both combination strategies and novel agents are under investigation in clinical trials to improve outcomes.     

Lost in translation? A critical look at the role that animal models of obsessive compulsive disorder play in current drug discovery strategies

Introduction: Obsessive-compulsive disorder (OCD) is a severe neuropsychiatric illness estimated to affect between 1–3% of the population. In today’s literature, there are a number well-validated and convincing animal models of OCD described.

Areas covered: Herein, the authors look at the role that animal models of OCD (including transgenic models, deer mouse stereotypy, quinpirole sensitization, post-training signal attenuation, and mouse marble burying) have played in determining the current directions of OCD drug discovery. Specifically, the article reviews new OCD drug therapies currently under investigation including drugs that target glutamate, dopamine, serotonin, and endocannabinoid systems. The authors review the published results of these clinical trials, and critically examine the contribution of animal models to the development of these novel therapies.

Expert opinion: Nitric oxide inhibitors, oxycarbazepine, and modulators of serotonin and metabotropic glutamate receptors should be further explored in animal models as well as in clinical trials. Pregabalin, topiramate, lamotrigine, sarcosine, minocycline, L-carnosine, celecoxib, and ondansetron, which have shown promise in clinical trials, should be explored in animal models with the goal of understanding the neurobiology of their effects. A multidisciplinary, interactive approach to OCD drug discovery, where animal models generate neurobiological hypotheses that can be tested in the clinic, and vice versa, should be cultivated.     

Computational modeling in melanoma for novel drug discovery

ABSTRACT

Introduction: There is a growing body of evidence highlighting the applications of computational modeling in the field of biomedicine. It has recently been applied to the in silico analysis of cancer dynamics. In the era of precision medicine, this analysis may allow the discovery of new molecular targets useful for the design of novel therapies and for overcoming resistance to anticancer drugs. According to its molecular behavior, melanoma represents an interesting tumor model in which computational modeling can be applied. Melanoma is an aggressive tumor of the skin with a poor prognosis for patients with advanced disease as it is resistant to current therapeutic approaches.

Areas covered: This review discusses the basics of computational modeling in melanoma drug discovery and development. Discussion includes the in silico discovery of novel molecular drug targets, the optimization of immunotherapies and personalized medicine trials.

Expert opinion: Mathematical and computational models are gradually being used to help understand biomedical data produced by high-throughput analysis. The use of advanced computer models allowing the simulation of complex biological processes provides hypotheses and supports experimental design. The research in fighting aggressive cancers, such as melanoma, is making great strides. Computational models represent the key component to complement these efforts. Due to the combinatorial complexity of new drug discovery, a systematic approach based only on experimentation is not possible. Computational and mathematical models are necessary for bringing cancer drug discovery into the era of omics, big data and personalized medicine.     

Emerging drugs for the treatment of bone metastasis

Introduction: Bone metastases are virtually incurable resulting in significant disease morbidity, reduced quality of life and mortality. Bone provides a unique microenvironment whose local interactions with tumor cells offer novel targets for therapeutic interventions. Increased understanding of the pathogenesis of bone disease has led to the discovery and clinical utility of bone-targeted agents other than bisphosphonates and denosumab, currently, the standard of care in this setting.

Areas covered: In this review, we present the recent advances in molecular targeted therapies focusing on therapies that inhibit bone resorption and/or stimulate bone formation and novel anti-tumoral agents that exerts significant effects on skeletal metastases, nowadays available in clinical practice or in phase of development.

Expert opinion: New emergent bone target therapies radium-223, mTOR inhibitors, anti-androgens have demonstrated the ability to increase overall survival in bone metastatic patients, other compounds, such as ET-1 and SRC inhibitors, up to now failed to clearly confirm in clinical trials their promising preclinical data.     

Therapeutic targets for Alzheimer's disease

Background: Alzheimer's disease (AD), the most common form of degenerative dementia, represents a tremendous unmet medical need. Although AD had already been described about 100 years ago and despite enormous research efforts, at present only few symptomatic treatment options exist for the more than 25 million patients worldwide. This situation might change as many targets for therapeutic intervention have been identified based on the in-depth study of the pathology of the disease in model systems and humans, and of its underlying genetics. Objective/methods: These targets are highlighted in the context of contemporary drug discovery for the identification of new therapies. Results/conclusions: ‘Translation’ of recent discoveries into disease-modifying therapies has not yet been accomplished. The future will show whether the current drug discovery and development ‘pipelines’ of pharmaceutical companies yield efficacious new medicines for AD.     

The role of murine models of prostate cancer in drug target discovery and validation

Background: Mice provide us with an excellent preclinical model of human prostate cancer. They have expanded our understanding of the molecular pathways involved in prostate carcinogenesis as well as allowing us to explore both novel and traditional treatment regimes based on the molecular profile of these lesions. Continuing refinement of the transgenic prostate models has proven challenging since no one model seems to represent the entire continuum of the disease, thus currently limiting its applicability to the human condition. This platform may potentially have major impact in validation of drug targeting specific biological process of prostate carcinogenesis, supplementing (or even replacing) many of the current in vitro and in vivo assays with the in vivo environment that transgenic prostate models provide. Objective/method: This review focuses primarily on the current state of murine model systems as a preclinical therapeutic platform for the treatment of prostate cancer, as well as hope for the future of the field. Conclusion: Much of the work in the drug discovery field has been done with the PTEN^-/- and TRAMP models of prostate cancer. Despite their limitations they have contributed much to our understanding of the pathophysiology of the disease. There is, however, a need for transgenic models that better reflect the stepwise progression found in the human condition. We feel that they will prove to be invaluable as a preclinical platform regarding efficacy and tolerability of various anticancer agents, which ultimately allows us to translate these findings to the clinical setting to prognosticate and ultimately render cancer patients disease-free.     

Transgenic mouse models of Alzheimer disease: developing a better model as a tool for therapeutic interventions

Alzheimer disease (AD) is the leading cause of dementia among elderly. Currently, no effective treatment is available for AD. Analysis of transgenic mouse models of AD has facilitated our understanding of disease mechanisms and provided valuable tools for evaluating potential therapeutic strategies. In this review, we will discuss the strengths and weaknesses of current mouse models of AD and the contribution towards understanding the pathological mechanisms and developing effective therapies.