Acute toxicity testing of chemicals—Opportunities to avoid redundant testing and use alternative approaches

Microbial pesticides - challenges and future perspectives for testing and safety assessment with respect to human health

Plant protection measures are necessary to prevent pests and diseases from attacking and destroying crop plants and to meet consumer demands for agricultural produce. In the last decades the use of chemical pesticides has largely increased. Farmers are looking for alternatives. Biopesticides should be considered a sustainable solution. They may be less toxic than chemical pesticides, be very specific to the target pest, decompose quickly, and be less likely to cause resistance. On the other hand, lower efficacy and higher costs are two disadvantages of many biopesticides. Biopesticides include macroorganisms, natural compounds and microorganisms. Microbial pesticides are the most widely used and studied class of biopesticides. The greatest difference between microbial and chemical pesticides is the ability of the former to potentially multiply in the environment and on the crop plant after application. The data requirements for the European Union and the United States Environmental Protection Agency are highlighted, as these regulatory processes are the most followed in regions where local regulations for biopesticide products are not available or vague. New Approach Methods already proposed or harmonized for chemical pesticides are presented and discussed with respect to their use in evaluating microbial pesticide formulations. Evaluating the microbials themselves is not as simple as using the same validated New Approach Methods as for synthetic pesticides. Therefore, the authors suggest considering New Approach Method strategies specifically for microbials and global harmonization with acceptability with the advancements of such approaches. Further discussion is needed and greatly appreciated by the experts.

The application of acute oral toxicity computational models in dangerous goods classification

Acute oral toxicity (AOT) data inform the acute toxicity potential of a compound and guides occupational safety and transportation practices. AOT data enable the categorization of a chemical into the appropriate AOT Globally Harmonized System (GHS) category based on the severity of the hazard. AOT data are also utilized to identify compounds that are Dangerous Goods (DGs) and subsequent transportation guidance for shipping of these hazardous materials. Proper identification of DGs is challenging for novel compounds that lack data. It is not feasible to err on the side of caution for all compounds lacking AOT data and to designate them as DGs, as shipping a compound as a DG has cost, resource, and time implications. With the wealth of available historical AOT data, AOT testing approaches are evolving, and in silico AOT models are emerging as tools that can be utilized with confidence to assess the acute toxicity potential of de novo molecules. Such approaches align with the 3R principles, offering a reduction or even replacement of traditional in vivo testing methods and can also be leveraged for product stewardship purposes. Utilizing proprietary historical in vivo AOT data for 210 pharmaceutical compounds (PCs), we evaluated the performance of two established in silico AOT programs: the Leadscope AOT Model Suite and the Collaborative Acute Toxicity Modeling Suite. These models accurately identified 94% and 97% compounds that were not DGs (GHS categories 4, 5, and not classified (NC)) suggesting that the models are fit-for-purpose in identifying PCs with low acute oral toxicity potential (LD50 >300 mg/kg). Utilization of these models to identify compounds that are not DGs can enable them to be de-prioritized for in vivo testing. This manuscript provides a detailed evaluation and assessment of the two models and recommends the most suitable applications of such models.

Principles and Procedures for Assessment of Acute Toxicity Incorporating In Silico Methods

Acute toxicity in silico models are being used to support an increasing number of application areas including (1) product research and development, (2) product approval and registration as well as (3) the transport, storage and handling of chemicals. The adoption of such models is being hindered, in part, because of a lack of guidance describing how to perform and document an in silico analysis. To address this issue, a framework for an acute toxicity hazard assessment is proposed. This framework combines results from different sources including in silico methods and in vitro or in vivo experiments. In silico methods that can assist the prediction of in vivo outcomes (i.e., LD50) are analyzed concluding that predictions obtained using in silico approaches are now well-suited for reliably supporting assessment of LD50-based acute toxicity for the purpose of GHS classification. A general overview is provided of the endpoints from in vitro studies commonly evaluated for predicting acute toxicity (e.g., cytotoxicity/cytolethality as well as assays targeting specific mechanisms). The increased understanding of pathways and key triggering mechanisms underlying toxicity and the increased availability of in vitro data allow for a shift away from assessments solely based on endpoints such as LD50, to mechanism-based endpoints that can be accurately assessed in vitro or by using in silico prediction models. This paper also highlights the importance of an expert review of all available information using weight-of-evidence considerations and illustrates, using a series of diverse practical use cases, how in silico approaches support the assessment of acute toxicity.

Preclinical safety of tetrahydrocurcumin loaded lipidic nanoparticles incorporated into tacrolimus ointment: In vitro and in vivo evaluation

Preclinical safety and proof of concept studies for a topical ointment comprising of concentrated tetrahydrocurcumin loaded lipidic nanoparticles (THC-LNs) and tacrolimus ointment (TTO) is proposed in the present investigation. The skin irritation potential and acute dermal toxicity were performed in rats in compliance with the Organization for Economic Cooperation and Development (OECD) guidelines (402, 404 and 410) while the cytotoxic potential was performed in HaCaT cells. Finally, in vivo evaluation was performed in Imiquimod mice model of psoriasis. In primary skin irritation assessment, TTO formulation, marketed formulation (Tacroz® Forte), THC-LNs, and blank LNs were topically applied on intact skin sites in rats while another group served as a negative control group for 72 h. TTO did not induce any adverse reactions. Repeated 28 days dermal toxicity followed by biochemical and histopathological assessment showed negligible alternations and skin lesions. THC-LNs revealed negligible cytotoxic potential in HaCaT cells. TTO showed significantly high anti-psoriatic activity in comparison to marketed ointment. This was also confirmed via histopathological evaluation. Based on these findings, it can be ascertained that TTO showed minimal toxicity and has ample potential for further clinical analysis. The above studies affirm the potential of TTO as an alternative for psoriasis.

STopTox: An in Silico Alternative to Animal Testing for Acute Systemic and Topical Toxicity

BACKGROUND

Modern chemical toxicology is facing a growing need to Reduce, Refine, and Replace animal tests (Russell 1959) for hazard identification. The most common type of animal assays for acute toxicity assessment of chemicals used as pesticides, pharmaceuticals, or in cosmetic products is known as a "6-pack" battery of tests, including three topical (skin sensitization, skin irritation and corrosion, and eye irritation and corrosion) and three systemic (acute oral toxicity, acute inhalation toxicity, and acute dermal toxicity) end points.

METHODS

We compiled, curated, and integrated, to the best of our knowledge, the largest publicly available data sets and developed an ensemble of quantitative structure-activity relationship (QSAR) models for all six end points. All models were validated according to the Organisation for Economic Co-operation and Development (OECD) QSAR principles, using data on compounds not included in the training sets.

RESULTS

In addition to high internal accuracy assessed by cross-validation, all models demonstrated an external correct classification rate ranging from 70% to 77%. We established a publicly accessible Systemic and Topical chemical Toxicity (STopTox) web portal (https://stoptox.mml.unc.edu/) integrating all developed models for 6-pack assays.

CONCLUSIONS

We developed STopTox, a comprehensive collection of computational models that can be used as an alternative to in vivo 6-pack tests for predicting the toxicity hazard of small organic molecules. Models were established following the best practices for the development and validation of QSAR models. Scientists and regulators can use the STopTox portal to identify putative toxicants or nontoxicants in chemical libraries of interest. https://doi.org/10.1289/EHP9341.

Emerging Technologies for In Vitro Inhalation Toxicology

Respiratory toxicology remains a major research area in the 21st century since current scenario of airborne viral infection transmission and pollutant inhalation is expected to raise the annual morbidity beyond 2 million. Clinical and epidemiological research connecting human exposure to air contaminants to understand adverse pulmonary health outcomes is, therefore, an immediate subject of human health assessment. Important observations in defining systemic effects of environmental contaminants on inhalation metabolic dysfunction, liver health, and gastrointestinal tract have been well explored with in vivo models. In this review, a framework is provided, a paradigm is established about inhalation toxicity testing in vitro, and a brief overview of breathing Lungs-on-Chip (LoC) as design concepts is given. The optimized bioengineering approaches and microfluidics with their fundamental pros, and cons are presented. There are different strategies that researchers apply to inhalation toxicity studies to assess a variety of inhalable substances and relevant LoC approaches. A case study from published literature and frame arguments about reproducibility as well as in vitro/in vivo correlations are discussed. Finally, the opportunities and challenges in soft robotics, systems inhalation toxicology approach integrating bioengineering, machine learning, and artificial intelligence to address a multitude model for future toxicology are discussed.

Curcumin-loaded nanocomplexes: Acute and chronic toxicity studies in mice and hamsters

We recently developed a modified solid dispersion of curcumin-loaded nanocomplexes (CNCs) in gums which promoted the prolonged and sustained release of curcumin. However, its safety assessment has not yet been investigated. Here, acute and chronic toxicities of CNCs were assayed using mice and hamsters. CNCs were orally administered to the animals. Doses of CNCs used for acute toxicity testing were 0.1, 1.1, 11.0 g/kg body weight for mice and 0.2, 2.1 and 21.4 g/kg body weight for hamsters. Doses of CNCs for chronic toxicity testing were 0.09, 0.27, 0.8 g/kg body weight/day for mice and 0.18, 0.54 and 1.61 g/kg body weight/day for hamsters. This regimen was followed daily for 6 months. Low and medium doses of CNCs did not induce any side effects in acute and chronic toxicity tests in either animal species. However, in acute toxicity testing, the organ-weight to body-weight ratio of spleen was significantly increased in mice treated with 11 g/kg body weight along with elevated levels of some biochemical parameters. There was a significant increase in organ-weight to body-weight ratios of stomach, liver and heart in hamsters treated with 21.4 g/kg body weight, but no elevated levels of biochemical parameters. Oral LD50 of CNCs in mice and hamsters were 8.9 and 16.8 g/kg body weight (equivalent to 2.5 and 4.7 g curcumin/kg body weight), respectively. Daily CNCs high-dose treatment for 6 months significantly increased organ-weight to body-weight ratios of stomach and intestine in mice and of lung and heart in hamsters. Elevated levels of glucose, total protein, ALT, AST and globulin in mice, and increased levels of AST, but decrease in cholesterol, in hamsters were concurrently observed with inflammation in liver and lung. These abnormalities were resolved within 28 days after cessation of treatment. The no-observed-adverse-effect level of CNCs was determined at 0.27 and 0.54 g/kg body weight/day in mice and hamsters. In conclusion, toxicity of high-dose CNCs treatment was graded as very low, possibly due to the components of the nanocomplex.

Bacteriocins: An Overview of Antimicrobial, Toxicity, and Biosafety Assessment by in vivo Models

The world is facing a significant increase in infections caused by drug-resistant infectious agents. In response, various strategies have been recently explored to treat them, including the development of bacteriocins. Bacteriocins are a group of antimicrobial peptides produced by bacteria, capable of controlling clinically relevant susceptible and drug-resistant bacteria. Bacteriocins have been studied to be able to modify and improve their physicochemical properties, pharmacological effects, and biosafety. This manuscript focuses on the research being developed on the biosafety of bacteriocins, which is a topic that has not been addressed extensively in previous reviews. This work discusses the studies that have tested the effect of bacteriocins against pathogens and assess their toxicity using in vivo models, including murine and other alternative animal models. Thus, this work concludes the urgency to increase and advance the in vivo models that both assess the efficacy of bacteriocins as antimicrobial agents and evaluate possible toxicity and side effects, which are key factors to determine their success as potential therapeutic agents in the fight against infections caused by multidrug-resistant microorganisms.

Preclinical safety evaluation of amphotericin A21: A novel antifungal

Safety studies are essential in drug development. This study evaluates the safety of Amphotericin A21 (AmB-A21), a derivative of amphotericin B with antifungal therapeutic potential. We performed a chronic toxicity study, a targeted organ study and a dermal irritation test. To evaluate chronic toxicity, 18 male adult rats were treated orally with AmB-21 (2 mg/kg) for 26 weeks. The effects on body-weight and animal health were measured, and haematological, clinical chemistry and histopathological tests were conducted on various organs. In the target organ toxicity study, male adult rats received a daily oral dose of AmB-21 (2 mg/kg) for 6 and 17 weeks; testicle histology and testosterone levels were then evaluated. For the dermal irritation study, AmB-21 (200 and 1000 mg/kg) was placed on the skin of adult male rabbits; macroscopic and microscopic studies, as well as haematological and clinical chemistry tests were then conducted. The chronic toxicity study revealed that AmB-21 caused testicle damage, and the testicle-targeted study showed structural alterations and changes in testosterone levels at 17 weeks. However, these alterations were no longer observed 8 weeks after discontinuation of treatment, and the testes showed very similar characteristics to those in the control group. The dermal irritation study showed skin thickening and reddening in rabbits treated with 2000 mg of AmB-A21 after 14 days of exposure. This same group also showed changes in liver enzymes, renal parameters and platelet levels. Based on our results, we consider AmB-21 to be a potential candidate for safe, long-term antifungal treatment given its reduced side effects.

Increasing confidence in waiving dermal toxicity studies: A comparison of oral and dermal acute data with alternative approaches for agrochemicals and products

The refinement of OECD 402 study design and criteria under which the study can be waived has been welcomed in some regulatory regions but met with uncertainty in others. To address these human safety concerns, previously available in vivo acute oral and acute dermal toxicity data was evaluated from a total of 597 agrochemical active ingredients and products. It was identified that all active ingredients and 99.6% of products had an acute dermal classification equal to or less toxic than their acute oral classification, confirming that waiving the acute dermal study and basing the outcome on the acute oral toxicity result has no impact on human health assessment. Additionally, automated Acute Toxicity Estimate (ATE) calculations were conducted on 440 products to evaluate if the predicted dermal toxicity resulted in the same in vivo classification. 93% of ATE predictions provided excellent correlation to the in vivo result and 6.4% resulted in a more conservative prediction. It is therefore clear that the results of this investigation, should remove any regulatory concerns and that OECD 402 can be confidently eliminated in its entirety from testing requirements globally.

Compounded conservatism in European re-entry worker risk assessment of pesticides

We review the risk parameters and drivers in the current European Union (EU) worker risk assessment for pesticides, for example considering crop maintenance, crop inspection or harvesting activities, and show that the current approach is very conservative due to multiple worst-case default assumptions. As a case study, we compare generic exposure model estimates with measured worker re-entry exposure values which shows that external cumulative exposure is overpredicted by about 50-fold on average. For this exercise, data from 16 good laboratory practice (GLP)-compliant worker exposure studies in 6 crops were evaluated with a total number of 184 workers. As generic overprediction does not allow efficient risk management or realistic risk communication, we investigate how external exposure can be better predicted within the generic model, and outline options for possible improvements in the current methodology. We show that simply using averages achieves more meaningful exposure estimates, while still being conservative, with an average exposure overprediction of about 9-fold. Overall, EU risk assessment includes several numerically unaccounted "hidden safety factors", which means that workers are well protected; but simultaneously risk assessments are biased towards failing due to compounded conservatism. This should be considered for further global or regional guidance developments and performing more exposure-relevant risk assessment.

A cross-industry collaboration to assess if acute oral toxicity (Q)SAR models are fit-for-purpose for GHS classification and labelling

This study assesses whether currently available acute oral toxicity (AOT) in silico models, provided by the widely employed Leadscope software, are fit-for-purpose for categorization and labelling of chemicals. As part of this study, a large data set of proprietary and marketed compounds from multiple companies (pharmaceutical, plant protection products, and other chemical industries) was assembled to assess the models' performance. The absolute percentage of correct or more conservative predictions, based on a comparison of experimental and predicted GHS categories, was approximately 95%, after excluding a small percentage of inconclusive (indeterminate or out of domain) predictions. Since the frequency distribution across the experimental categories is skewed towards low toxicity chemicals, a balanced assessment was also performed. Across all compounds which could be assigned to a well-defined experimental category, the average percentage of correct or more conservative predictions was around 80%. These results indicate the potential for reliable and broad application of these models across different industrial sectors. This manuscript describes the evaluation of these models, highlights the importance of an expert review, and provides guidance on the use of AOT models to fulfill testing requirements, GHS classification/labelling, and transportation needs.

Determination of preclinical safety of oil obtained from Pachira aquatica Aublet (Malvaceae) seeds: histopathological, biochemical, hematological, and genetic toxicity studies in rats

Pachira aquatica is a species used for medicinal and food purposes and has numerous phytochemicals that may have systemic toxic effects and damage to genetic material. This study aimed to evaluate acute and short-term oral toxicity, as well as genotoxic and clastogenic effects of oil extracted from P. aquatica (PASO) seeds in rats and Drosophila melanogaster. The results obtained with biochemical and hematological analyses did not show significant changes in any evaluated parameters when compared with reference values for the species used in the study. Data from the histopathological analysis corroborated results found in this study. These findings indicate low acute and short-term toxicity following oral PASO exposure in rats under the experimental conditions tested. Tests performed in rats showed that PASO did not present significant genotoxic or clastogenic effects on the cells analyzed with the three doses tested. Treatment with PASO in the offspring of HB crossing, which showed high cytochrome P450 levels, did not exhibit genotoxic activity, as demonstrated by the SMART test. These results suggest that products from the hepatic oil metabolism did not show genotoxicity under the conditions tested. Together, the results indicate that, under the experimental conditions tested, PASO is safe for repeated intake. As PASO exhibited low potential to cause harmful effects on living organisms, our study encourages further research aimed at assessing its pharmacological activity, since it is a widely consumed plant.

The performance, reliability and potential application of in silico models for predicting the acute oral toxicity of pharmaceutical compounds

Acute oral toxicity (AOT) information is utilized to categorize compounds according to the severity of their hazard and used to inform risk assessments for human health and the environment. Given the wealth of historical AOT information and technological advances, in silico models are being created and evaluated as potential tools to predict the AOT of compounds and reduce reliance on animal testing. Utilizing a historical database of AOT data on 371 Bristol Myers Squibb pharmaceutical compounds (PCs) (195 pharmaceutical intermediates and 176 active pharmaceutical ingredients), we evaluated two pioneering in silico AOT programs: the Leadscope Acute Oral Toxicity Model Suite and the Collaborative Acute Toxicity Modeling Suite. These models demonstrated a high degree of agreement with the in vivo results as well as a high level of sensitivity. We found that these models can be effectively utilized to identify PCs which are of low acute oral toxicity (LD₅₀ > 2000 mg/kg), PCs which should not be classified as Dangerous Goods (LD₅₀ > 300 mg/kg), and can assist in identifying a starting dose for in vivo AOT studies. This manuscript provides an evaluation of the performance of these in silico models and proposes use cases where these in silico models can be most confidently and effectively employed.

Towards a tiered test strategy for plant protection products to address mixture toxicity by alternative approaches in human health assessment

Plant protection products (PPPs) consist of pesticide active substances and co-formulants. Generally, active substance effects are assumed to dominate in PPP toxicity. Nevertheless, co-formulants may well affect the toxicity of PPPs via toxicodynamic and toxicokinetic interaction. To account for potential mixture effects and improve PPP data requirements for application in risk assessment, a tiered test strategy is proposed. The strategy is based on a comparison of PPP and active substance toxicity, which enables the prioritisation of PPPs for further testing, adaptation of the toxicological threshold value or removal of toxic co-formulants from the PPP. Moreover, it focuses on the integrative assessment of existing information and newly generated data using alternative test methods. The proposed strategy will improve PPP toxicological assessment by accounting for mixture toxicity, providing a set of regulatory options for risk assessment and the necessary data for hazard assessment. The predictivity of alternative methods for PPPs will improve by evaluation of their reliability and uncertainty. © 2020 The Authors. Pest Management Science published by JohnWiley & Sons Ltd on behalf of Society of Chemical Industry.

In vitro Alternatives to Acute Inhalation Toxicity Studies in Animal Models-A Perspective

When assessing the risk and hazard of a non-pharmaceutical compound, the first step is determining acute toxicity, including toxicity following inhalation. Inhalation is a major exposure route for humans, and the respiratory epithelium is the first tissue that inhaled substances directly interact with. Acute inhalation toxicity testing for regulatory purposes is currently performed only in rats and/or mice according to OECD TG403, TG436, and TG433 test guidelines. Such tests are biased by the differences in the respiratory tract architecture and function across species, making it difficult to draw conclusions on the potential hazard of inhaled compounds in humans. Research efforts have been therefore focused on developing alternative, human-relevant models, with emphasis on the creation of advanced In vitro models. To date, there is no In vitro model that has been accepted by regulatory agencies as a stand-alone replacement for inhalation toxicity testing in animals. Here, we provide a brief introduction to current OECD test guidelines for acute inhalation toxicity, the interspecies differences affecting the predictive value of such tests, and the current regulatory efforts to advance alternative approaches to animal-based inhalation toxicity studies. We then list the steps that should allow overcoming the current challenges in validating In vitro alternatives for the successful replacement of animal-based inhalation toxicity studies. These steps are inclusive and descriptive, and should be detailed when adopting in house-produced 3D cell models for inhalation tests. Hence, we provide a checklist of key parameters that should be reported in any future scientific publications for reproducibility and transparency.

Non-relevance of acute dermal toxicity testing for assessing human health protection in the regulatory decision-making for agrochemical formulated products

Compared to oral toxicity tests, dermal toxicity tests offer little or no additional scientific information or public health protection for agrochemical-formulated products (US EPA, 2016). Based on that, a retrospective analysis of the results of acute oral and dermal LD₅₀ studies of agrochemical products registered in Brazil was carried out by the Technical Group on Toxicological Risk Assessment (GT-ART) of the Brazilian Crop Protection Association (ANDEF). The data were obtained from 6 agrochemical industries that are associated to ANDEF, following these considerations: only rat studies were selected; only paired studies were chosen; only studies performed with top doses ≥2,000 mg/kg were selected; biological products were excluded. The dataset includes 342 formulated products in 21 formulation types. Among these 342 formulated products, 228 have a single active ingredient, 107 have 2 and 7 have 3 or more. The comparison of acute oral to dermal toxicity studies of agrochemical-formulated products registered in Brazil corroborates the United States Environmental Protection Agency (US EPA) conclusion on waiving acute dermal toxicity tests, which will result in avoiding unnecessary use of time and resources, data generation costs and animal testing.

In vitro culture systems as an alternative for female reproductive toxicology studies

Studies have shown that daily exposure to different products, whether chemical or natural, can cause irreversible damage to women’s reproductive health. Therefore it is necessary to use tests that evaluate the safety and efficacy of these products. Most reproductive toxicology tests are performedin vivo. However, in recent years, various cell culture methods, including embryonic stem cells and tissues have been developed with the aim of reducing the use of animals in toxicological tests. This is a major advance in the area of toxicology, as these systems have the potential to become a widely used tool compared within vivotests routinely used in reproductive biology and toxicology. The present review describes and highlights data onin vitroculture processes used to evaluate reproductive toxicity as an alternative to traditional methods usingin vivotests.

One science-driven approach for the regulatory implementation of alternative methods: A multi-sector perspective

EU regulations call for the use of alternative methods to animal testing. During the last decade, an increasing number of alternative approaches have been formally adopted. In parallel, new 3Rs-relevant technologies and mechanistic approaches have increasingly contributed to hazard identification and risk assessment evolution. In this changing landscape, an EPAA meeting reviewed the challenges that different industry sectors face in the implementation of alternative methods following a science-driven approach. Although clear progress was acknowledged in animal testing reduction and refinement thanks to an integration of scientifically robust approaches, the following challenges were identified: i) further characterization of toxicity pathways; ii) development of assays covering current scientific gaps, iii) better characterization of links between in vitro readouts and outcome in the target species; iv) better definition of alternative method applicability domains, and v) appropriate implementation of the available approaches. For areas having regulatory adopted alternative methods (e.g., vaccine batch testing), harmonised acceptance across geographical regions was considered critical for broader application. Overall, the main constraints to the application of non-animal alternatives are the still existing gaps in scientific knowledge and technological limitations. The science-driven identification of most appropriate methods is key for furthering a multi-sectorial decrease in animal testing.

Part I--Comparing Noncancer Chronic Human Health Reference Values: An Analysis of Science Policy Choices

The goal of this study was to systematically evaluate the choices made in deriving a chronic oral noncancer human health reference value (HHRV) for a given chemical by different organizations, specifically those from the U.S. Environmental Protection Agency, Health Canada, RIVM (the Netherlands), and the U.S. Agency for Toxic Substances and Disease Registry. This analysis presents a methodological approach for comparing both the HHRVs and the specific choices made in the process of deriving an HHRV across these organizations. Overall, across the 96 unique chemicals and 171 two-way organizational comparisons, the HHRV agreed approximately 26% of the time. A qualitative method for identifying the primary factors influencing these HHRV differences was also developed, using arrays of HHRVs across organizations for the same chemical. The primary factors identified were disagreement on the critical or principal study and differential application of the total uncertainty factor across organizations. Of the cases where the total UF was the primary factor influencing HHRV disagreement, the database UF had the greatest influence.

Alternative approaches for identifying acute systemic toxicity: Moving from research to regulatory testing

Acute systemic toxicity testing provides the basis for hazard labeling and risk management of chemicals. A number of international efforts have been directed at identifying non-animal alternatives for in vivo acute systemic toxicity tests. A September 2015 workshop, Alternative Approaches for Identifying Acute Systemic Toxicity: Moving from Research to Regulatory Testing, reviewed the state-of-the-science of non-animal alternatives for this testing and explored ways to facilitate implementation of alternatives. Workshop attendees included representatives from international regulatory agencies, academia, nongovernmental organizations, and industry. Resources identified as necessary for meaningful progress in implementing alternatives included compiling and making available high-quality reference data, training on use and interpretation of in vitro and in silico approaches, and global harmonization of testing requirements. Attendees particularly noted the need to characterize variability in reference data to evaluate new approaches. They also noted the importance of understanding the mechanisms of acute toxicity, which could be facilitated by the development of adverse outcome pathways. Workshop breakout groups explored different approaches to reducing or replacing animal use for acute toxicity testing, with each group crafting a roadmap and strategy to accomplish near-term progress. The workshop steering committee has organized efforts to implement the recommendations of the workshop participants.

Alternative acute oral toxicity assessment under REACH based on sub-acute toxicity values

The REACH Regulation requires information on acute oral toxicity for substances produced or imported in quantities greater than one ton per year. When registering, animal testing should be used as last resort. The standard acute oral toxicity test requires use of animals. Therefore, the European Chemicals Agency examined whether alternative ways exist to generate information on acute oral toxicity. The starting hypothesis was that low acute oral toxicity can be predicted from the results of low toxicity in oral sub-acute toxicity studies. Proving this hypothesis would allow avoiding acute toxicity oral testing whenever a sub-acute oral toxicity study is required or available and indicates low toxicity. ECHA conducted an analysis of the REACH database and found suitable studies on both acute oral and sub-acute oral toxicities for 1,256 substances. 415 of these substances had low toxicity in the sub-acute toxicity study (i.e., NO(A)EL at or above the limit test threshold of 1,000 mg/kg). For 98% of these substances, low acute oral toxicity was also reported (i.e., LD50 above the classification threshold of 2,000 mg/kg). On the other hand, no correlation was found between lower NO(A)ELs and LD50. According to the REACH Regulation, this approach for predicting acute oral toxicity needs to be considered as part of a weight of evidence analysis. Therefore, additional sources of information to support this approach are presented. Ahead of the last REACH registration deadline, in 2018, ECHA estimates that registrants of about 550 substances can omit the in vivo acute oral toxicity study by using this adaptation.

Safety evaluation of Bon-santé cleanser® polyherbal in male Wistar rats

BACKGROUND

The potential harm of medicinal herbs has been recently observed following herbal toxicity studies after ingestion of polyherbal remedies. This was the rationale for the food and drug regulatory agency decision for thorough safety evaluation of herbal medicines. Androgenic, antipyretic, analgesic and anti-inflammatory potentials as well as chemical compositions of extracts of massularia acuminata, terminalia ivorensis, anogeissus leiocarpus and macuna pruriens respectively have been documented. Thus, Bon-santé cleanser® (BSC) is formulated from these medicinal plants with the intention to boost body hormones and energizes the body. Considering the wide usage of BSC, we investigated on its safety in male Wistar rats.

METHODS

Thirty-two male Wistar rats weighing 201.9 ± 7.5 g were grouped into four treatment groups of eight per group. Group I, (control) received distilled water (10 ml/kg). Groups II-IV received 250 mg/kg, 500 mg/kg and 1000 mg/kg of BSC per oral respectively. Each group was treated for sixty days.

RESULTS

Acute toxicity test, in male Wistar albino mice, showed that LD50 was 600 mg/kg via i.p. while 4 g/kg was nonlethal after oral administration in mice. Hepatic and renal biomarker enzymes were unaltered in all rats. Increased in PCV (p <0.05) was observed at 500 mg/kg. BSC modulates antioxidants biomarkers following sub-chronic administration and increased serum Na(+) (p >0.05). BSC at 1000 mg/kg caused mild inflammation of the liver and heart but not kidneys histologically.

CONCLUSIONS

BSC has been found to be relatively safe in Wistar rats. Although, our findings indicate that herbal therapy with BSC should be done with caution as a mild alteration in the liver and heart architectures were observed.

ADMET evaluation in drug discovery: 15. Accurate prediction of rat oral acute toxicity using relevance vector machine and consensus modeling

Background

Determination of acute toxicity, expressed as median lethal dose (LD₅₀), is one of the most important steps in drug discovery pipeline. Because in vivo assays for oral acute toxicity in mammals are time-consuming and costly, there is thus an urgent need to develop in silico prediction models of oral acute toxicity.

Results

In this study, based on a comprehensive data set containing 7314 diverse chemicals with rat oral LD₅₀ values, relevance vector machine (RVM) technique was employed to build the regression models for the prediction of oral acute toxicity in rate, which were compared with those built using other six machine learning approaches, including k-nearest-neighbor regression, random forest (RF), support vector machine, local approximate Gaussian process, multilayer perceptron ensemble, and eXtreme gradient boosting. A subset of the original molecular descriptors and structural fingerprints (PubChem or SubFP) was chosen by the Chi squared statistics. The prediction capabilities of individual QSAR models, measured by q_ext² for the test set containing 2376 molecules, ranged from 0.572 to 0.659.

Conclusion

Considering the overall prediction accuracy for the test set, RVM with Laplacian kernel and RF were recommended to build in silico models with better predictivity for rat oral acute toxicity. By combining the predictions from individual models, four consensus models were developed, yielding better prediction capabilities for the test set (q_ext² = 0.669–0.689). Finally, some essential descriptors and substructures relevant to oral acute toxicity were identified and analyzed, and they may be served as property or substructure alerts to avoid toxicity. We believe that the best consensus model with high prediction accuracy can be used as a reliable virtual screening tool to filter out compounds with high rat oral acute toxicity. Graphical abstract

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: the challenge ahead

Low-dose exposures to common environmental chemicals that are deemed safe individually may be combining to instigate carcinogenesis, thereby contributing to the incidence of cancer. This risk may be overlooked by current regulatory practices and needs to be vigorously investigated.

Lifestyle factors are responsible for a considerable portion of cancer incidence worldwide, but credible estimates from the World Health Organization and the International Agency for Research on Cancer (IARC) suggest that the fraction of cancers attributable to toxic environmental exposures is between 7% and 19%. To explore the hypothesis that low-dose exposures to mixtures of chemicals in the environment may be combining to contribute to environmental carcinogenesis, we reviewed 11 hallmark phenotypes of cancer, multiple priority target sites for disruption in each area and prototypical chemical disruptors for all targets, this included dose-response characterizations, evidence of low-dose effects and cross-hallmark effects for all targets and chemicals. In total, 85 examples of chemicals were reviewed for actions on key pathways/mechanisms related to carcinogenesis. Only 15% (13/85) were found to have evidence of a dose-response threshold, whereas 59% (50/85) exerted low-dose effects. No dose-response information was found for the remaining 26% (22/85). Our analysis suggests that the cumulative effects of individual (non-carcinogenic) chemicals acting on different pathways, and a variety of related systems, organs, tissues and cells could plausibly conspire to produce carcinogenic synergies. Additional basic research on carcinogenesis and research focused on low-dose effects of chemical mixtures needs to be rigorously pursued before the merits of this hypothesis can be further advanced. However, the structure of the World Health Organization International Programme on Chemical Safety ‘Mode of Action’ framework should be revisited as it has inherent weaknesses that are not fully aligned with our current understanding of cancer biology.

Disruptive chemicals, senescence and immortality

Carcinogenesis is thought to be a multistep process, with clonal evolution playing a central role in the process. Clonal evolution involves the repeated 'selection and succession' of rare variant cells that acquire a growth advantage over the remaining cell population through the acquisition of 'driver mutations' enabling a selective advantage in a particular micro-environment. Clonal selection is the driving force behind tumorigenesis and possesses three basic requirements: (i) effective competitive proliferation of the variant clone when compared with its neighboring cells, (ii) acquisition of an indefinite capacity for self-renewal, and (iii) establishment of sufficiently high levels of genetic and epigenetic variability to permit the emergence of rare variants. However, several questions regarding the process of clonal evolution remain. Which cellular processes initiate carcinogenesis in the first place? To what extent are environmental carcinogens responsible for the initiation of clonal evolution? What are the roles of genotoxic and non-genotoxic carcinogens in carcinogenesis? What are the underlying mechanisms responsible for chemical carcinogen-induced cellular immortality? Here, we explore the possible mechanisms of cellular immortalization, the contribution of immortalization to tumorigenesis and the mechanisms by which chemical carcinogens may contribute to these processes.

Testing Chemical Safety: What Is Needed to Ensure the Widespread Application of Non-animal Approaches?

Scientists face growing pressure to move away from using traditional animal toxicity tests to determine whether manufactured chemicals are safe. Numerous ethical, scientific, business, and legislative incentives will help to drive this shift. However, a number of hurdles must be overcome in the coming years before non-animal methods are adopted into widespread practice, particularly from regulatory, scientific, and global perspectives. Several initiatives are nevertheless underway that promise to increase the confidence in newer alternative methods, which will support the move towards a future in which less data from animal tests is required in the assessment of chemical safety.

Scientists face growing pressures to move away from using traditional animal toxicity tests to determine whether manufactured chemicals are safe. This essay explores the incentives, hurdles, and ongoing initiatives towards a reduced reliance on such tests.

Animal use in pharmacology education and research: the changing scenario

The use of animals in research and education dates back to the period when humans started to look for ways to prevent and cure ailments. Most of present day's drug discoveries were possible because of the use of animals in research. The dilemma to continue animal experiments in education and research continues with varied and confusing guidelines. However, the animal use and their handling vary in each laboratory and educational institution. It has been reported that the animals are being subjected to painful procedures in education and training unnecessarily. The extensive use of animals in toxicity studies and testing dermatological preparations has raised concerns about the ways animals are sacrificed for these “irrelevant experiments”. On the other side of the coin are scientists who advocate the relevant and judicious use of animals in research so that new discoveries can continue. In this review, we discuss the evolution of the use of animals in education and research and how these have been affected in recent times owing to concerns from animal lovers and government regulations. A number of computer simulation and other models have been recommended for use as alternatives to use of animals for pharmacology education. In this review we also discuss some of these alternatives.

Inhibition of phosphotidylinositol-3 kinase pathway by a novel naphthol derivative of betulinic acid induces cell cycle arrest and apoptosis in cancer cells of different origin

Betulinic acid (BA) is a pentacyclic triterpenoid natural product reported to inhibit cell growth in a variety of cancers. However, the further clinical development of BA got hampered because of poor solubility and pharmacological properties. Interestingly, this molecule offer several hotspots for structural modifications in order to address its associated issues. In our endeavor, we selected C-3 position for the desirable chemical modification in order to improve its cytotoxic and pharmacological potential and prepared a library of different triazoline derivatives of BA. Among them, we previously reported the identification of a potential molecule, that is, 3{1N(5-hydroxy-naphth-1yl)-1H-1,2,3-triazol-4yl}methyloxy betulinic acid (HBA) with significant inhibition of cancer cell growth and their properties. In the present study, we have shown for the first time that HBA decreased the expression of phosphotidylinositol-3 kinase (PI3K) p110α and p85α and caused significant downregulation of pAKT and of NFκB using human leukemia and breast cancer cells as in vitro models. Further it was revealed that PI3K inhibition by HBA induced cell cycle arrest via effects on different cell cycle regulatory proteins that include CDKis cyclins and pGSK3β. Also, this target-specific inhibition was associated with mitochondrial apoptosis as was reflected by the increased expression of mitochondrial bax, downregulated bcl2 and decreased mitochondrial levels of cytochrome c, together with reactive oxygen species generation and decline in mitochondrial membrane potential. The apoptotic effectors such as caspase 8, caspase 9 and caspase 3 were found to be upregulated besides DNA repair-associated enzyme, that is, PARP cleavage caused cancer cell death. Pharmacodynamic evaluation revealed that both HBA and BA were safe upto the dose of 2000 mg/kg body weight and with acceptable pharmacodynamic parameters. The in vitro data corroborated with in vivo anticancer activity wherein Ehrlich solid tumor showed that HBA as a more potent agent than BA without any body weight loss and mortality.

Extensive review of fish embryo acute toxicities for the prediction of GHS acute systemic toxicity categories

Distribution and marketing of chemicals require appropriate labelling of health, physical and environmental hazards according to the United Nations global harmonisation system (GHS). Labelling for (human) acute toxicity categories is based on experimental findings usually obtained by oral, dermal or inhalative exposure of rodents. There is a strong societal demand for replacing animal experiments conducted for safety assessment of chemicals. Fish embryos are considered as alternative to animal testing and are proposed as predictive model both for environmental and human health effects. Therefore, we tested whether LC50s of the fish embryo acute toxicity test would allow effectively predicting of acute mammalian toxicity categories. A database of published fish embryo LC50 containing 641 compounds was established. For these compounds corresponding rat oral LD50 were identified resulting in 364 compounds for which both fish embryo LC50 and rat LD50 was available. Only a weak correlation of fish embryo LC50 and rat oral LD50 was obtained. Fish embryos were also not able to effectively predict GHS oral acute toxicity categories. We concluded that due to fundamental exposure protocol differences (single oral dose versus water-borne exposure) a reverse dosimetry approach is needed to explore the predictive capacity of fish embryos.

The added value of the 90-day repeated dose oral toxicity test for industrial chemicals with a low (sub)acute toxicity profile in a high quality dataset

A survey conducted on the EU Notification of New Substances (NONS) database suggested that for industrial chemicals with a profile of low toxicity in (sub)acute toxicity tests there is little added value to the conduct of the 90-day repeated dose study. Avoiding unnecessary animal testing is a central aim of the EU REACH chemicals legislation; therefore we sought to verify the profile using additional data. The OECD's eChemPortal was searched for substances that had both a 28-day and a 90-day study and their robust study summaries were then examined from the ECHA CHEM database. Out of 182 substances with high quality 28-day and 90-day study results, only 18 reported no toxicity of any kind in the (sub)acute tests. However, for 16 of these there were also no reported signs of toxicity at or close to the limit dose (1000mg/kgbw/d) in the 90-day study. Restricting the 'low (sub)acute toxicity in a high quality dataset' profile to general industrial chemicals of no known biological activity, whilst allowing irritant substances, increases the data set and improves the prediction to 95% (20 substances out of 21 substances). The low toxicity profile appears to be of low prevalence within industrial chemicals (10-15%), nevertheless, avoidance of the conduct of a redundant 90-day study for this proportion of the remaining REACH phase-in substances would avoid the use of nearly 50,000 animals and save industry 50million Euros, with no impact on the assessment of human health.

Assessment of cellular responses after short- and long-term exposure to silver nanoparticles in human neuroblastoma (SH-SY5Y) and astrocytoma (D384) cells

Silver nanoparticle (AgNP, 20 nm) neurotoxicity was evaluated by an integrated in vitro testing protocol employing human cerebral (SH-SY5Y and D384) cell lines. Cellular response after short-term (4-48 h, 1-100 μ g/ml) and prolonged exposure (up to 10 days, 0.5-50 μ g/ml) to AgNP was assessed by MTT, calcein-AM/PI, clonogenic tests. Pulmonary A549 cells were employed for data comparison along with silver nitrate as metal ionic form. Short-term data: (i) AgNP produced dose- and time-dependent mitochondrial metabolism changes and cell membrane damage (effects starting at 25 μ g/ml after 4 h: EC50s were 40.7 ± 2.0 and 49.5 ± 2.1 μ g/ml for SH-SY5Y and D384, respectively). A549 were less vulnerable; (ii) AgNP doses of ≤ 18 μ g/ml were noncytotoxic; (iii) AgNO3 induced more pronounced effects compared to AgNP on cerebral cells. Long-term data: (i) low AgNP doses (≤ 1 μ g/ml) compromised proliferative capacity of all cell types (cell sensibility: SHSY5Y > A549 > D384). Colony number decrease in SH-SY5Y and D384 was 50% and 25%, respectively, at 1 μ g/ml, and lower dose (0.5 μ g/ml) was significantly effective towards SH-SY5Y and pulmonary cells; (ii) cell proliferation activity was more affected by AgNO3 than AgNPs. In summary, AgNP-induced cytotoxic effects after short-term and prolonged exposure (even at low doses) were evidenced regardless of cell model types.

A single-parameter logistic equation for fitting concentration-response curves from standard acute ecotoxicity assays

A set of 69 concentration-response curves from 5 acute ecotoxicity assays was fitted with a 2-parameter logistic equation. High correlation between values of regression parameters suggested similar slopes of the curves. This enabled derivation of the empirical single-parameter logistic equation with the sole median effective concentration (EC50) parameter. Such an equation might be useful in the evaluation of lower-quality (preliminary) experimental data and for the reduction of the number of test organisms and of testing costs.

In vitro toxicity evaluation of engineered cadmium-coated silica nanoparticles on human pulmonary cells

Cytotoxicity of cadmium-containing silica nanoparticles Cd-SiO2NPs (0.05-100 µg/mL) versus SiO2NPs and CdCl2 was evaluated by an in vitro test battery in A549 by assessing (i) mitochondrial function, (ii) membrane integrity/cell morphology, (iii) cell growth/proliferation, (iv) apoptotic pathway, (v) oxidative stress, after short- (24-48 h) and long-term (10 days) exposure. Both Cd-SiO2NPs and CdCl2 produced dose-dependent cytotoxic effects: (i) MTT-assay: similar cytotoxicity pattern was observed at both 24 and 48 h, with a more Cd-SiO2NPs pronounced effect than CdCl2. Cd-SiO2NPs induced mortality (about 50%) at 1  μ g/mL, CdCl2 at 25  μ g/mL; (ii) calcein-AM/PI staining: decrease in cell viability, noticeable at 25  μ g/mL, enhanced markedly at 50 and 100  μ g/mL, after 24 h. Cd-SiO2NPs induced higher mortality than CdCl2 (25% versus 4%, resp., at 25  μ g/mL) with further exacerbation after 48h; (iii) clonogenic assay: exposure for longer period (10 days) compromised the A549 proliferative capacity at very low dose (0.05  μ g/mL); (iv) a progressive activation of caspase-3 immunolabelling was detected already at 1  μ g/mL; (v) GSH intracellular level was modified by all compounds. In summary, in vitro data demonstrated that both Cd-SiO2NPs and CdCl2 affected all investigated endpoints, more markedly after Cd-SiO2NPs, while SiO2NPs influenced GSH only.

Safety evaluation of engineered nanomaterials for health risk assessment: an experimental tiered testing approach using pristine and functionalized carbon nanotubes

Increasing application of engineered nanomaterials within occupational, environmental, and consumer settings has raised the levels of public concern regarding possible adverse effects on human health. We applied a tiered testing strategy including (i) a first in vitro stage to investigate general toxicity endpoints, followed by (ii) a focused in vivo experiment. Cytotoxicity of laboratory-made functionalized multiwalled carbon nanotubes (CNTs) (i.e., MW-COOH and MW-NH2), compared to pristine MWCNTs, carbon black, and silica, has been assessed in human A549 pneumocytes by MTT assay and calcein/propidium iodide (PI) staining. Purity and physicochemical properties of the test nanomaterials were also determined. Subsequently, pulmonary toxic effects were assessed in rats, 16 days after MWCNTs i.t. administration (1 mg/kg b.w.), investigating lung histopathology and monitoring several markers of lung toxicity, inflammation, and fibrosis. In vitro data: calcein/PI test indicated no cell viability loss after all CNTs treatment; MTT assay showed false positive cytotoxic response, occurring not dose dependently at exceedingly low CNT concentrations (1 μg/mL). In vivo results demonstrated a general pulmonary toxicity coupled with inflammatory response, without overt signs of fibrosis and granuloma formation, irrespective of nanotube functionalization. This multitiered approach contributed to clarifying the CNT toxicity mechanisms improving the overall understanding of the possible adverse outcomes resulting from CNT exposure.

Can acute dermal systemic toxicity tests be replaced with oral tests? A comparison of route-specific systemic toxicity and hazard classifications under the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)

Acute systemic toxicity data (LD50 values) and hazard classifications derived in the rat following oral administration and dermal application have been analysed to examine whether or not orally-derived hazard classification or LD50 values can be used to determine dermal hazard classification. Comparing the oral and dermal classifications for 335 substances derived from oral and dermal LD50 values respectively revealed 17% concordance, and indicated that 7% of substances would be classified less severely while 76% would be classified more severely if oral classifications were applied directly to the dermal route. In contrast, applying the oral LD50 values within the dermal classification criteria to determine the dermal classification reduced the concordance to 15% and the relative 'under-classification' to 1%, but increased the relative 'over-classification' to 84%. Both under- and over-classification are undesirable, and mitigation strategies are discussed. Finally, no substance with an oral LD50 of >2000mg/kg was classified for acute systemic toxicity by the dermal route, suggesting that dermal testing for acute systemic toxicity of such substances adds nothing to the hazard characterisation and should be removed from routine regulatory data requirements.

Novel in vitro and mathematical models for the prediction of chemical toxicity

The focus of much scientific and medical research is directed towards understanding the disease process and defining therapeutic intervention strategies. The scientific basis of drug safety is very complex and currently remains poorly understood, despite the fact that adverse drug reactions (ADRs) are a major health concern and a serious impediment to development of new medicines. Toxicity issues account for ∼21% drug attrition during drug development and safety testing strategies require considerable animal use. Mechanistic relationships between drug plasma levels and molecular/cellular events that culminate in whole organ toxicity underpins development of novel safety assessment strategies. Current in vitro test systems are poorly predictive of toxicity of chemicals entering the systemic circulation, particularly to the liver. Such systems fall short because of (1) the physiological gap between cells currently used and human hepatocytes existing in their native state, (2) the lack of physiological integration with other cells/systems within organs, required to amplify the initial toxicological lesion into overt toxicity, (3) the inability to assess how low level cell damage induced by chemicals may develop into overt organ toxicity in a minority of patients, (4) lack of consideration of systemic effects. Reproduction of centrilobular and periportal hepatocyte phenotypes in in vitro culture is crucial for sensitive detection of cellular stress. Hepatocyte metabolism/phenotype is dependent on cell position along the liver lobule, with corresponding differences in exposure to substrate, oxygen and hormone gradients. Application of bioartificial liver (BAL) technology can encompass in vitro predictive toxicity testing with enhanced sensitivity and improved mechanistic understanding. Combining this technology with mechanistic mathematical models describing intracellular metabolism, fluid-flow, substrate, hormone and nutrient distribution provides the opportunity to design the BAL specifically to mimic the in vivo scenario. Such mathematical models enable theoretical hypothesis testing, will inform the design of in vitro experiments, and will enable both refinement and reduction of in vivo animal trials. In this way, development of novel mathematical modelling tools will help to focus and direct in vitro and in vivo research, and can be used as a framework for other areas of drug safety science.

Selection of test methods to be included in a testing strategy to predict acute oral toxicity: an approach based on statistical analysis of data collected in phase 1 of the ACuteTox project

More than 50 different in vitro and in silico methods assessing specific organ- and system-toxicity, such as haemato-, neuro-, nephro- and hepatotoxicity, as well as intestinal absorption, distribution and metabolism, have been used in the first phase of the ACuteTox project to test a common set of 57 chemicals. This paper describes the methods used for statistical evaluation of concentration-response data collected for each of the endpoint assays, and for the development of a testing strategy applicable for acute toxicity classification of chemicals based on the achieved results of the concentration-response analysis. A final list of in vitro test methods considered to be promising candidates for building blocks of the testing strategy is presented. Only these selected test methods were further investigated in the prevalidation phase of the project. The test methods were chosen according to their reproducibility and reliability and most importantly, according to their potential to classify chemicals into the official acute oral toxicity categories of the EU Classification, Labelling and Packaging (CLP) Regulation. The potential of the test methods to correctly classify the chemicals was assessed by Classification and Regression Trees (CART) analysis.

Strategies for the optimisation of in vivo experiments in accordance with the 3Rs philosophy

There are a large number of chemicals in current use for which adequate toxicity data are not available. Whilst there are clear ethical and legal obligations to obtain data from sources other than in vivo experiments wherever possible, in certain cases in vivo assays may be deemed necessary. In such circumstances, it is essential to ensure that the maximum amount of high quality data is obtained from the minimum number of animals, using the most humane procedures, in accordance with the philosophy of reduction, refinement and replacement (3Rs). The aim of this report is to provide a strategy for anyone involved in animal experimentation, for either toxicological or pharmacological purposes, as to how in vivo experiments may be optimised. The impact of generic and endpoint specific sources of variability has been highlighted in a proof-of-principle analysis considering the variation in protocols for assays for four human health endpoints (skin sensitisation, reproductive/developmental toxicity, mutagenicity and carcinogenicity). Other factors such as operator training, experimental/statistical design, use of lower species and use of combined assays are also discussed. Recommendations for optimisation of in vivo assays, in terms of the 3Rs philosophy, applied to performing tests, harvesting data and appropriate reporting are summarised as a checklist of issues to be addressed prior to undertaking such assays.