Can in vitro assessment provide relevant end points for cognitive drug programs?

Several start-up biotechnology companies have been created with the primary intent of developing cognitive enhancers. In addition, established pharmaceutical companies also frequently focus their efforts on cognitive drug discovery. In many instances, the rationale and evidence for these endeavors are based largely on in vitro assessments. In particular, the experimental paradigm, know as long-term potentiation (LTP), a cellular model of synaptic plasticity and memory encoding, is being increasing used preclinically for assessing potential nootropic drugs in vitro. Central to this thinking is the idea that the modulation of LTP and/or glutamate receptors are the key criteria that must be met for the development of cognitive enhancers. However, programs targeting the NMDA receptor, a glutamate receptor subtype, over the years have been less than fruitful. In addition, skeptics criticize the relevance of some in vitro tests such as LTP for simulating human cognitive function. Given these considerations, one may wonder if in vitro assessments in general, and the LTP paradigm in particular, provide relevant end points for cognitive drug discovery and development programs. The focus of this article is to address this question and to present evidence as to why in vitro assessment is still critical to the success of any cognitive drug program.     

PDE4 as a target for cognition enhancement

Introduction: The second messengers cAMP and cGMP mediate fundamental aspects of brain function relevant to memory, learning, and cognitive functions. Consequently, cyclic nucleotide phosphodiesterases (PDEs), the enzymes that inactivate the cyclic nucleotides, are promising targets for the development of cognition-enhancing drugs.

Areas covered: PDE4 is the largest of the 11 mammalian PDE families. This review covers the properties and functions of the PDE4 family, highlighting procognitive and memory-enhancing effects associated with their inactivation.

Expert opinion: PAN-selective PDE4 inhibitors exert a number of memory- and cognition-enhancing effects and have neuroprotective and neuroregenerative properties in preclinical models. The major hurdle for their clinical application is to target inhibitors to specific PDE4 isoforms relevant to particular cognitive disorders to realize the therapeutic potential while avoiding side effects, in particular emesis and nausea. The PDE4 family comprises four genes, PDE4A-D, each expressed as multiple variants. Progress to date stems from characterization of rodent models with selective ablation of individual PDE4 subtypes, revealing that individual subtypes exert unique and non-redundant functions in the brain. Thus, targeting specific PDE4 subtypes, as well as splicing variants or conformational states, represents a promising strategy to separate the therapeutic benefits from the side effects of PAN-PDE4 inhibitors.     

Pre-encoding administration of amphetamine or THC preferentially modulates emotional memory in humans

Rationale

Many addictive drugs are known to have effects on learning and memory, and these effects could motivate future drug use. Specifically, addictive drugs may affect memory of emotional events and experiences in ways that are attractive to some users. However, few studies have investigated the effects of addictive drugs on emotional memory in humans.

Objectives

This study examined the effects of the memory-enhancing drug dextroamphetamine (AMP) and the memory-impairing drug Δ⁹-tetrahydrocannabinol (THC) on emotional memory in healthy volunteers.

Methods

Participants completed three experimental sessions across which they received capsules containing placebo and two doses of either AMP (10 and 20 mg; N?=?25) or THC (7.5 and 15 mg; N?=?25) before viewing pictures of positive (pleasant), neutral, and negative (unpleasant) scenes. Memory for the pictures was assessed 2 days later, under drug-free conditions.

Results

Relative to placebo, memory for emotional pictures was improved by AMP and impaired by THC, but neither drug significantly affected memory for unemotional pictures. Positive memory biases were not observed with either drug, and there was no indication that the drugs’ memory effects were directly related to their subjective or physiological effects alone.

Conclusions

This study provides the first clear evidence that stimulant drugs can preferentially strengthen, and cannabinoids can preferentially impair, memory for emotional events in humans. Although addictive drugs do not appear to positively bias memory, the possibility remains that these drugs’ effects on emotional memory could influence drug use among certain individuals.     

Peptidomics in drug research

Background: Because of their wide range of functions, endogenous peptides have great potential either as drugs themselves or as drug targets. Objective: To provide an overview of the current use of peptides as drugs (targets) and describe how improvements in peptide biochemistry and the application of peptidomics studies can lead to the discovery of new diagnostic and therapeutic targets. Methods: We discuss the different peptidomics technologies and their application in the study of human and animal disease models, animal venoms, antimicrobial peptides, G-protein-coupled receptor ligands and biomarkers. Results/conclusions: At present, peptide drugs represent a small but growing number of pharmaceutical molecules. The peptidomics methodology, which was introduced 7 years ago to study naturally occurring peptides, will lead to a plethora of new peptide drug leads.     

Emerging cognitive enhancing drugs

Background: Advances in health sciences during the last century have increased the average age in industrialized nations. Despite this progress, neurodegenerative diseases that affect higher order thinking and memory continue to increase in prevalence as they take a devastating toll on human productivity in the later years. There is an acute need for new drugs and therapeutic approaches for treating these severe diseases, and also for improving the quality of cognitive function associated with normal aging and in many other disorders and syndromes that present with cognitive dysfunction. Objective: The purpose of this review is to ascertain the pharmacological approaches being exploited to improve cognition and memory and to determine the most relevant and effective directions taken for new drug discovery. Limitations and difficulties encountered in this effort also are discussed. Methods: This review focuses primarily on compounds already undergoing clinical trials for improving cognition and memory with some discussion of rising new drug targets. Results/conclusion: Compounds that act on allosteric sites on neurotransmitter receptors are expected to lead the field with new levels of specificity and reduced side effects. New multi-functional compounds can be designed that can both improve cognition and slow the process of disease.     

Development of transplant immunosuppressive agents – considerations in the use of animal models

ABSTRACT

Introduction: The development of all immunosuppressant agents to date has involved the experimental use of large and small animal models. Over the last half-century, immunosuppressive drugs have extended the lives of transplant patients worldwide. However, the use of animal models in the development of these drugs is not perfect, and this has brought to light a number of issues including idiosyncratic reactions that are found in animal models but not in humans. The 2006 highly publicized case of the ‘elephant man’ TGN 1412 drug trial highlights the importance of being cogent of the limitations of animal models.

Areas covered: This review covers the utility and limitations of the use of animal models for the development of immunosuppressant agents. This includes both large and small animal models, particularly rodent models in the transplant setting.

Expert opinion: The use of animal models represents a critical stage in the development of immunosuppressive drugs. Limitations include physiological differences to humans; this is especially true of immunologically naïve lab rodents with small memory cell populations. Toxic drug levels may differ widely between species. Animal models are also costly and raise ethical concerns. However, there is currently no way to recreate the complex environment of the human immune system purely in vitro.     

MPTP-induced changes in hippocampal synaptic plasticity and memory are prevented by memantine through the BDNF-TrkB pathway

Background and Purpose

Mild cognitive deficit in early Parkinson''s disease (PD) has been widely studied. Here we have examined the effects of memantine in preventing memory deficit in experimental PD models and elucidated some of the underlying mechanisms.

Experimental Approaches

I.p. injection of 1-methyl-4- phenyl-1,2,3,6-tetrahydro pyridine (MPTP) in C57BL/6 mice was used to produce models of PD. We used behavioural tasks to test memory. In vitro, we used slices of hippocampus, with electrophysiological, Western blotting, real time PCR, elisa and immunochemical techniques.

Key Results

Following MPTP injection, long-term memory was impaired and these changes were prevented by pre-treatment with memantine. In hippocampal slices from MPTP treated mice, long-term potentiation (LTP) –induced by θ burst stimulation (10 bursts, 4 pulses) was decreased, while long-term depression (LTD) induced by low-frequency stimulation (1 Hz, 900 pulses) was enhanced, compared with control values. A single dose of memantine (i.p., 10 mg·kg⁻¹) reversed the decreased LTP and the increased LTD in this PD model. Activity-dependent changes in tyrosine kinase receptor B (TrkB), ERK and brain-derived neurotrophic factor (BDNF) expression were decreased in slices from mice after MPTP treatment. These effects were reversed by pretreatment with memantine. Incubation of slices in vitro with 1-methyl-4-phenylpyridinium (MPP⁺) decreased depolarization-induced expression of BDNF. This effect was prevented by pretreatment of slices with memantine or with calpain inhibitor III, suggesting the involvement of an overactivated calcium signalling pathway.

Conclusions and Implications

Memantine should be useful in preventing loss of memory and hippocampal synaptic plasticity in PD models.     

New approaches for pediatric rhabdomyosarcoma drug discovery: targeting combinatorial signaling

Introduction: Rhabdomyosarcomas (RMS) are rare heterogeneous pediatric tumors that are treated by surgery, chemotherapy and irradiation. New therapeutic approaches are needed, especially in the advanced stages to target the pro-oncogenic signals. Exploring the molecular interactions of the regulatory signals and their roles in the developmental aspects of different subtypes of RMS is essential to identify potential targets and develop new therapeutic drugs.

Areas covered: Insights into different drug discovery approaches are discussed with specific emphasis on gene expression profiling, fusion protein, role of small interfering RNA (siRNA)- and microRNA (miRNA)-based discovery approaches, targeting cancer stem cells, and in vitro and in vivo model systems. Targeting some overexpressed signals along with the possibilities of combination therapy of validated drug targets is discussed. Additionally, methods to overcome the limitations of discovery-based research are briefly discussed.

Expert opinion: Due to drug resistance, ineffective therapy in advanced stages and relapse, there is a demand to explore new drug targets and discovery approaches. Implementing miRNA-based profiling would reveal the extent of miR-based regulation, various biomarkers and potential targets in RMS. A suitable combination of innovative techniques and the use of model systems might assist the identification and validation of novel targets and drug discovery methods. Combining specific drugs along with type-specific target inhibition of overexpressed mRNAs through siRNA approaches would enable the development of personalized therapy.     

Effects of pharmaceuticals and other active chemicals at biological targets: mechanisms,interactions, and integration into PB-PK/PD models

Biologically active molecules, that is pharmaceuticals and other chemical substances, exert their therapeutic and/or toxic effects by complex interactions with their biological targets/active sites. This review discusses the factors and processes governing the kinetics and effects of active molecules at their cellular targets, the chain of events leading to clinical effects, and the crosstalk between regulatory pathways controlling these processes. Special attention is given to the discussion of effects of a single drug or other chemical on multiple targets, to the interaction of multiple ligands with a single target/receptor and the effects of single ligand–target complexes on multiple signal transduction pathways, and to the control of physiological functions, such as regulation of blood glucose levels, by numerous primary mechanisms acting on different cellular targets. Physiologically based-pharmacokinetic/pharmacodynamic (PB-PK/PD) models are of great value for the design of active principles by the pharmaceutical industry and for the optimization and individualization of patient therapy. Experimental results from in vitro and in vivo studies can be used for building such models. On the other hand, properly designed models and simulation can contribute to a better design of experiments. Much of what is presented in this article applies equally well to drugs and other chemicals. Unless specified otherwise, reference to drugs applies also to other chemicals. This review is based on an expert meeting organized by COST Action B25 held in Eilat, Israel, on 14 – 15 February 2008. The authors have prepared this article to reflect the presentations and discussions at that meeting.     

Animal models of dementia

The major use of animal models for dementia is in the development of new therapeutic drugs. In contrast to other areas in psychopharmacology, where the predictive validity of animal models can be established by examining their response to known therapeutic agents, the search for antidementia drugs is hampered by the absence of recognized clinically effective reference compounds. Research scientists are therefore obliged to validate their models in terms of their similarity to the clinical condition. A second problem is the therapeutic target. Although antidementia drugs are intended to improve cognition in a general sense, the foremost therapeutic targets are the deficits in memory and attention associated with aging. The aging animal would appear to represent the best available model providing it can be shown that the cognitive deficits observed are similar to those observed in old age in humans. A logical consequence of this approach is to validate other experimental models in terms of their resemblance to naturally occurring age-related impairments. To illustrate these points, this paper reviews data obtained in aging animals in various behavioral models and the effects of drugs thereon. Comparisons are made between drug effects in aged animals with those observed in experimental models in young animals in which cognitive deficits are induced by anticholinergic drug treatment or lesions to cholinergic-rich brain regions. It is concluded that aging induces cognitive deficits mainly associated with short-term memory but that other aspects of cognition, namely attention and information processing speed, also decline with age. Putative antidementia drugs, particularly those facilitating cholinergic transmission, appear frequently to have more marked effects in experimental deficit models than in aged animals themselves. © 1995 Wiley-Liss, Inc.     

Advancements in the delivery of epigenetic drugs

Introduction: Advancements in epigenetic treatments are not only coming from new drugs, but also from modifications or encapsulation of the existing drugs into different formulations leading to greater stability and enhanced delivery to the target site. The epigenome is highly regulated and complex; therefore, it is important that off-target effects of epigenetic drugs be minimized. The step from in vitro to in vivo treatment of these drugs often requires development of a method of effective delivery for clinical translation.

Areas covered: This review covers epigenetic mechanisms such as DNA methylation, chromatin remodeling and small-RNA-mediated gene regulation. There is a section in the review with examples of diseases where epigenetic alterations lead to impaired pathways, with an emphasis on cancer. Epigenetic drugs, their targets and clinical status are presented. Advantages of using a delivery method for epigenetic drugs as well as examples of current advancements and challenges are also discussed.

Expert opinion: Epigenetic drugs have the potential to be very effective therapy against a number of diseases, especially cancers and neurological disorders. As with many chemotherapeutics, undesired side effects need to be minimized. Finding a suitable delivery method means reducing side effects and achieving a higher therapeutic index. Each drug may require a unique delivery method exploiting the drug’s chemistry or other physical characteristic requiring interdisciplinary participation and would benefit from a better understanding of the mechanisms of action.     

Isoflurane anaesthesia reversibly improves cognitive function and long-term potentiation (LTP) via an up-regulation in NMDA receptor 2B subunit expression

Postoperative cognitive dysfunction (POCD) is a decline in cognitive performance after a surgery performed under anaesthesia. The exact roles of surgery and/or anaesthesia for facilitating POCD are unclear. This study investigates the effects of isoflurane anaesthesia on cognitive performance and cellular mechanisms involved in learning and memory function. Male C57BL6/J mice (age: 4-5 months) were anaesthetized with isoflurane in oxygen/air (FiO₂ = 0.5) for 2 h, non-anaesthetized mice served as controls. After 24 h, neurocognitive function, in vitro long-term potentiation (LTP), or protein expression were evaluated. In a visuospatial test, anaesthetized mice showed better cognitive performance as they learned faster compared to controls. In hippocampal slices of anaesthetized mice, in vitro LTP was enhanced as reflected in an increased extracellular field potential (fEPSP) slope after 1 h to 210.2 ± 17% (control: 156.8 ± 7.2%; n = 14; p < 0.05). NR2B subunits of the NMDA receptors were selectively up-regulated in hippocampal neurones after anaesthesia. Blocking these receptors either with the NR2B selective antagonists ifenprodil or RO25-6981 (R-(R,S)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propranol), prevents the anaesthesia-induced improvement in cognitive function as well as enhancement of in vitro LTP. The anaesthesia-mediated effects on NR2B subunits were fully reversed to control levels seven days after anaesthesia. The present data suggests that isoflurane anaesthesia induces a hippocampus-specific elevation of NR2B subunit composition, enhances LTP in CA1 neurones, and produces hippocampal-dependent cognitive improvement.     

Nootropic,anxiolytic and CNS-depressant studies on different plant sources of shankhpushpi

Context: Shankhpushpi, a well-known drug in Ayurveda, is extensively used for different central nervous system (CNS) effects especially memory enhancement. Different plants are used under the name shankhpushpi in different regions of India, leading to an uncertainty regarding its true source. Plants commonly used under the name shankhpushpi are: Convolvulus pluricaulis Chois., Evolvulus alsinoides Linn., both from Convolvulaceae, and Clitoria ternatea Linn. (Leguminosae).

Objective: To find out the true source of shankhpushpi by evaluating and comparing memory-enhancing activity of the three above mentioned plants. Anxiolytic, antidepressant and CNS-depressant activities of these three plants were also compared and evaluated.

Materials and methods: The nootropic activity of the aqueous methanol extract of each plant was tested using elevated plus-maze (EPM) and step-down models. Anxiolytic, antidepressant and CNS-depressant studies were evaluated using EPM, Porsolt’s swim despair and actophotometer models, respectively.

Results: C. pluricaulis extract (CPE) at a dose of 100?mg/kg, p.o. showed maximum nootropic and anxiolytic activity (p < 0.001). E. alsinoides extract (EAE) and C. ternatea extract (CTE) showed maximum memory-enhancing and anxiolytic activity (p < 0.001) at 200 and 100?mg/kg, respectively. Amongst the three plants, EAE and CTE showed significant (p < 0.05), while CPE did not exhibit any antidepressant activity. All the three plants showed CNS-depressant action at higher dose levels.

Discussion and conclusions: The above results showed all the three plants possess nootropic, anxiolytic and CNS-depressant activity. The results of memory-enhancing activity suggest C. pluricaulis to be used as true source of shankhpushpi.     

Three-dimensionally engineered biomimetic tissue models for in vitro drug evaluation: delivery,efficacy and toxicity

Introduction: Three-dimensionally (3D) engineered biomimetic tissue models are sought after due to their high fidelity in mimicking various native tissues of the human body, this quality of which gives them an important role at the forefront of drug discovery and development. A multitude of studies have consistently indicated that gene expression profiles, cellular phenotypes, differentiation capabilities and functionalities are all affected by tissue architecture. Thus, the drug evaluation process will stand to gain immense benefits from the fairly accurate predictions of cellular responses displayed by 3D-engineered tissue models when exposed to the drugs of interest in vitro. Stemming from this fact, many studies have set out to capitalize on developing tissue models that are tailored to specific aspects of drug evaluation including the tests of novel drug delivery systems, drug efficacy and toxicity.

Areas covered: The areas covered include fabrication methods and usage of 3D in vitro tumor models in cancer research, focusing on the evaluation of delivery and efficacy of various anticancer drugs or other therapeutic agents. Also covered are the use of 3D in vitro inflammatory tissue models in anti-inflammation research, centering on osteoarthritis (OA) and rheumatoid arthritis (RA) and the use of 3D in vitro tissue models designed for drug toxicity evaluation specifically with liver-mimetic tissues.

Expert opinion: Currently available 3D tissue models in various fields of research have already displayed their capabilities in predicting cellular responses to various therapeutic agents and delivery methods with better accuracy than their 2D counterparts, albeit being in need of much refinement before they can be successfully applied for reliable drug evaluation. Given further development and improvement, it is highly probable that the 3D-engineered tissue models may perform as living platforms for dynamic drug evaluation in vitro.     

Predicting and preventing antimicrobial resistance utilizing pharmacodynamics: Part I gram positive bacteria

Introduction: Antimicrobial resistance is a potentially inevitable consequence of widespread use of antibiotics in the healthcare system. An enhanced understanding of pharmacodynamic (PD) targets that prevent antimicrobial resistance development will improve currently availably therapies and help to guide future drug development strategies. Current in vitro methods to predict bacterial resistance to antimicrobials consist of serial dilution experiments, determination of the mutant prevention concentration (MPC), mutant selection window (MSW), and human simulated pharmacodynamics studies. Clinical trial data and real -world surveillance studies can help validate or disprove in vitro modeling.

Areas covered: This review will discuss methods of predicting development of resistance and how the use of pharmacodynamics can reduce or eliminate the emergence of resistance among Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus species.

Expert opinion: Pharmacodynamic targets can be used successfully to guide antimicrobial therapy to prevent resistance development. Currently, PD targets do not take into consideration horizontal resistance gene transfer and various factors may lead to different PD targets based on sites of infection. Further research is necessary to guide future drug development strategies and optimize new drug therapies.     

Cell based in vitro and ex vivo models in metabolic disease drug discovery: nice to have or critical path?

Background: The use of cellular models as tools in drug discovery is almost universal. However, in disease areas such as metabolic diseases, are they relevant to the process and do they add value? Objective: In this article, we explore the variety of cellular models now used in drug discovery in metabolic diseases as revealed by publication. We have tried to make some connections between drug phenotypes in these models with clinical parallels. We also ask the question as to whether such models add value in the drug discovery process. This overview is not about recombinant cell systems used in target-based screening; rather, we focus on in vitro, including ex vivo, models as physiological systems in drug discovery in obesity and diabetes. Conclusion: In terms of building target confidence, in vitro models are often the only mechanistic link to human systems early in a projects life. Many of the current targets in metabolic diseases in the early discovery phase are not yet clinically supported, let alone validated. In this respect, therefore, in vitro models warrant a place in the critical path in early discovery. In terms of any predictive role for decision-making today, this is much more difficult and is more likely pushed to a supporting role as part of a wider package. However, there is a rapid rate of advancement in this field and future developments hold much promise.     

Neurocognitive function in current and ex-users of ecstasy in comparison to both matched polydrug-using controls and drug-naïve controls

Rationale

Despite years of research and much controversy surrounding the effects of ecstasy use, findings are equivocal. Attempting to reduce methodological problems, such as concurrent use of other recreational drugs, could lead to a clearer picture of the effects of ecstasy use on cognitive function.

Objectives

The aim of this study is to investigate the effects of both prolonged abstention from ecstasy use and current use on cognitive function while controlling for the regular use of other recreational drugs used by ecstasy users (alcohol, cannabis, cocaine, amphetamines and lysergic acid diethylamide).

Materials and methods

A range of cognitive functions (including working memory, episodic memory, verbal learning and executive functions) was assessed in 109 participants: 25 current ecstasy users, 28 ex-ecstasy users (abstinent for at least 1 year), 29 polydrug-using controls (matched to both ecstasy-using groups for the use of other recreational drugs) and 27 drug-naïve controls.

Results

There was an overall tendency for impaired verbal learning and memory in both current ecstasy users and polydrug controls. There was also evidence of reduced response inhibition in the current ecstasy users and polydrug controls, which appeared to be related to recency of drug use. However, the majority of tests showed no group differences.

Conclusions

The results suggest that recreational drug use in general, rather than ecstasy use per se, can lead to subtle cognitive impairments and that recent drug use appears to impact strongest on cognitive performance. This study highlights the importance of controlling for the use of all recreational drugs and, in particular, recent drug use when investigating ‘effects of ecstasy’ on cognitive function.

     

Animal and cellular models for hypolipidemic drugs

Background: The development of effective and safe lipid-lowering agents should set out from and rely on robust preclinical investigation. Objective: To accomplish this aim, the selection of proper cellular and animal models is crucial. Results: Because lipid-lowering agents are ultimately supposed to reduce the atherosclerotic burden in the arterial wall, they need to tackle directly or indirectly the multifactorial nature of atherosclerotic disease. Hence, these drugs may essentially prevent triglyceride-rich lipoprotein assembly or enhance low-density lipoprotein (LDL) clearance through the LDL or related receptors in the liver. Established animal models such as the apolipoprotein E- and the LDL-receptor knockout mice are widely used to test drug actions on these pathways. A different approach is testing the ability of candidate drugs to increase plasma high-density lipoprotein (HDL) levels. More recently, the focus has shifted to drugs enhancing HDL function rather than just plasma HDL levels. This in turn requires in vitro and particularly in vivo models of reverse cholesterol transport, which have become available by now. Conclusion: A positive outcome of preclinical studies is necessary but not sufficient for an investigational new drug to be eventually approved for clinical use.     

Food additive carrageenan: Part I: A critical review of carrageenan in vitro studies,potential pitfalls,and implications for human health and safety

Carrageenan (CGN) has been used as a safe food additive for several decades. Confusion over nomenclature, basic CGN chemistry, type of CGN tested, interspecies biology, and misinterpretation of both in vivo and in vitro data has resulted in the dissemination of incorrect information regarding the human safety of CGN. The issue is exacerbated when mechanistic data obtained from in vitro experiments are directly translated to human hazard and used for risk assessment. This can lead to information that is taken out of experimental context and reported as a definitive effect in humans. In recent years, the use of cell-based models has increased and their ability to provide key information regarding chemical or drug safety is well established. In many instances, these new alternative approaches have started to replace the need to use animals altogether. In vitro systems can be extremely useful for understanding subcellular targets and mechanisms of adverse effects. However, care must be exercised when extrapolating the in vitro findings to in vivo effects. Often, issues such as chemical identity and purity, relevant dose, pharmacokinetic properties, solubility, protein binding, adsorption to plastics, and the use of cell models that are biologically and mechanistically relevant are overlooked or ignored. When this occurs, in vitro findings can provide misleading information that is not causally linked to in vivo events in animals or in humans. To date, there has not been a comprehensive review of the CGN in vitro literature, which has reported a wide range of biochemical effects related to this compound. An extensive effort has been made to evaluate as much of this literature as possible. This review focuses on the in vitro observation, the unique chemistry of CGN, and potential pitfalls of in vitro models used for hazard identification. The discussion of the in vitro studies discussed this review are supported by numerous in vivo studies. This provides a unique opportunity to have both the in vitro and in vivo studies reviewed together.