Mechanistic Applicability Domain Classification of a Local Lymph Node Assay Dataset for Skin Sensitization

The goal of eliminating animal testing in the predictive identification of chemicals with the intrinsic ability to cause skin sensitization is an important target, the attainment of which has recently been brought into even sharper relief by the EU Cosmetics Directive and the requirements of the REACH legislation. Development of alternative methods requires that the chemicals used to evaluate and validate novel approaches comprise not only confirmed skin sensitizers and non-sensitizers but also substances that span the full chemical mechanistic spectrum associated with skin sensitization. To this end, a recently published database of more than 200 chemicals tested in the mouse local lymph node assay (LLNA) has been examined in relation to various chemical reaction mechanistic domains known to be associated with sensitization. It is demonstrated here that the dataset does cover the main reaction mechanistic domains. In addition, it is shown that assignment to a reaction mechanistic domain is a critical first step in a strategic approach to understanding, ultimately on a quantitative basis, how chemical properties influence the potency of skin sensitizing chemicals. This understanding is necessary if reliable non-animal approaches, including (quantitative) structure-activity relationships (Q)SARs, read-across, and experimental chemistry based models, are to be developed.

Organs-on-chips: into the next decade

Organs-on-chips (OoCs), also known as microphysiological systems or 'tissue chips' (the terms are synonymous), have attracted substantial interest in recent years owing to their potential to be informative at multiple stages of the drug discovery and development process. These innovative devices could provide insights into normal human organ function and disease pathophysiology, as well as more accurately predict the safety and efficacy of investigational drugs in humans. Therefore, they are likely to become useful additions to traditional preclinical cell culture methods and in vivo animal studies in the near term, and in some cases replacements for them in the longer term. In the past decade, the OoC field has seen dramatic advances in the sophistication of biology and engineering, in the demonstration of physiological relevance and in the range of applications. These advances have also revealed new challenges and opportunities, and expertise from multiple biomedical and engineering fields will be needed to fully realize the promise of OoCs for fundamental and translational applications. This Review provides a snapshot of this fast-evolving technology, discusses current applications and caveats for their implementation, and offers suggestions for directions in the next decade.

High concordance of drug-induced human hepatotoxicity with in vitro cytotoxicity measured in a novel cell-based model using high content screening

To develop and validate a practical, in vitro, cell-based model to assess human hepatotoxicity potential of drugs, we used the new technology of high content screening (HCS) and a novel combination of critical model features, including (1) use of live, human hepatocytes with drug metabolism capability, (2) preincubation of cells for 3 days with drugs at a range of concentrations up to at least 30 times the efficacious concentration or 100 microM, (3) measurement of multiple parameters that were (4) morphological and biochemical, (5) indicative of prelethal cytotoxic effects, (6) representative of different mechanisms of toxicity, (7) at the single cell level and (8) amenable to rapid throughput. HCS is based on automated epifluorescence microscopy and image analysis of cells in a microtiter plate format. The assay was applied to HepG2 human hepatocytes cultured in 96-well plates and loaded with four fluorescent dyes for: calcium (Fluo-4 AM), mitochondrial membrane potential (TMRM), DNA content (Hoechst 33,342) to determine nuclear area and cell number and plasma membrane permeability (TOTO-3). Assay results were compared with those from 7 conventional, in vitro cytotoxicity assays that were applied to 611 compounds and shown to have low sensitivity (<25%), although high specificity ( approximately 90%) for detection of toxic drugs. For 243 drugs with varying degrees of toxicity, the HCS, sublethal, cytotoxicity assay had a sensitivity of 93% and specificity of 98%. Drugs testing positive that did not cause hepatotoxicity produced other serious, human organ toxicities. For 201 positive assay results, 86% drugs affected cell number, 70% affected nuclear area and mitochondrial membrane potential and 45% affected membrane permeability and 41% intracellular calcium concentration. Cell number was the first parameter affected for 56% of these drugs, nuclear area for 34% and mitochondrial membrane potential for 29% and membrane permeability for 7% and intracellular calcium for 10%. Hormesis occurred for 48% of all drugs with positive response, for 26% of mitochondrial and 34% nuclear area changes and 12% of cell number changes. Pattern of change was dependent on the class of drug and mechanism of toxicity. The ratio of concentrations for in vitro cytotoxicity to maximal efficaciousness in humans was not different across groups (12+/-22). Human toxicity potential was detected with 80% sensitivity and 90% specificity at a concentration of 30x the maximal efficacious concentration or 100 microM when efficaciousness was not considered. We conclude that human hepatotoxicity is highly concordant with in vitro cytotoxicity in this novel model and as detected by HCS.

Microarrays and toxicology: the advent of toxicogenomics

The availability of genome-scale DNA sequence information and reagents has radically altered life-science research. This revolution has led to the development of a new scientific subdiscipline derived from a combination of the fields of toxicology and genomics. This subdiscipline, termed toxicogenomics, is concerned with the identification of potential human and environmental toxicants, and their putative mechanisms of action, through the use of genomics resources. One such resource is DNA microarrays or "chips," which allow the monitoring of the expression levels of thousands of genes simultaneously. Here we propose a general method by which gene expression, as measured by cDNA microarrays, can be used as a highly sensitive and informative marker for toxicity. Our purpose is to acquaint the reader with the development and current state of microarray technology and to present our view of the usefulness of microarrays to the field of toxicology.

Advances in 3D cell culture technologies enabling tissue-like structures to be created in vitro

Research in mammalian cell biology often relies on developing in vitro models to enable the growth of cells in the laboratory to investigate a specific biological mechanism or process under different test conditions. The quality of such models and how they represent the behavior of cells in real tissues plays a critical role in the value of the data produced and how it is used. It is particularly important to recognize how the structure of a cell influences its function and how co-culture models can be used to more closely represent the structure of real tissue. In recent years, technologies have been developed to enhance the way in which researchers can grow cells and more readily create tissue-like structures. Here we identify the limitations of culturing mammalian cells by conventional methods on two-dimensional (2D) substrates and review the popular approaches currently available that enable the development of three-dimensional (3D) tissue models in vitro. There are now many ways in which the growth environment for cultured cells can be altered to encourage 3D cell growth. Approaches to 3D culture can be broadly categorized into scaffold-free or scaffold-based culture systems, with scaffolds made from either natural or synthetic materials. There is no one particular solution that currently satisfies all requirements and researchers must select the appropriate method in line with their needs. Using such technology in conjunction with other modern resources in cell biology (e.g. human stem cells) will provide new opportunities to create robust human tissue mimetics for use in basic research and drug discovery. Application of such models will contribute to advancing basic research, increasing the predictive accuracy of compounds, and reducing animal usage in biomedical science.

Functional human corneal equivalents constructed from cell lines

Human corneal equivalents comprising the three main layers of the cornea (epithelium, stroma, and endothelium) were constructed. Each cellular layer was fabricated from immortalized human corneal cells that were screened for use on the basis of morphological, biochemical, and electrophysiological similarity to their natural counterparts. The resulting corneal equivalents mimicked human corneas in key physical and physiological functions, including morphology, biochemical marker expression, transparency, ion and fluid transport, and gene expression. Morphological and functional equivalents to human corneas that can be produced in vitro have immediate applications in toxicity and drug efficacy testing, and form the basis for future development of implantable tissues.

Criteria for development of a database for safety evaluation of fragrance ingredients

Over 2000 different ingredients are used in the manufacture of fragrances. The majority of these ingredients have been used for many decades. Despite this long history of use, all of these ingredients need continued monitoring to ensure that each ingredient meets acceptable safety standards. As with other large databases of existing chemicals, fulfilling this need requires an organized approach to identify the most important potential hazards. One such approach, specifically considering the dermal route of exposure as the most relevant one for fragrance ingredients, has been developed. This approach provides a rational selection of materials for review and gives guidance for determining the test data that would normally be considered necessary for the elevation of safety under intended conditions of use. As a first step, the process takes into account the following criteria: quantity of use, consumer exposure, and chemical structure. These are then used for the orderly selection of materials for review with higher quantity, higher exposure, and the presence of defined structural alerts all contributing to a higher priority for review. These structural alerts along with certain exposure and volume limits are then used to develop guidelines for determining the quality and quantity of data considered necessary to support an adequate safety evaluation of the chosen materials, taking into account existing data on the substance itself as well as on closely related analogs. This approach can be considered an alternative to testing; therefore, it is designed to be conservative but not so much so as to require excessive effort when not justified.

Animal models for protecting ischemic myocardium: results of the NHLBI Cooperative Study. Comparison of unconscious and conscious dog models

The Animal Models for Protecting Ischemic Myocardium Study was undertaken for the purpose of developing reproducible animal models that could be used to assess interventions designed to limit infarct size. This paper describes the results obtained in an unconscious dog model and in a conscious dog model, developed in three participating laboratories. The unconscious dog model, involving reperfusion after 3 hours of ischemia in open-chest dogs, was intended to determine whether therapy followed by early reperfusion would limit infarct size more than reperfusion alone. The conscious dog model used chronically instrumented dogs and permanent coronary occlusion to better mimic myocardial infarction in man. In both models, the proximal circumflex artery was occluded, and the primary experimental endpoint was infarct size, as measured by histological techniques 3 days after the initial occlusion. Infarct size was analyzed in relation to baseline variables including the anatomic area at risk, collateral blood flow to the subepicardial zone of ischemia and hemodynamic determinants of myocardial metabolic demand. Most of the variation in infarct size in control dogs could be related to variation in the area at risk, collateral blood flow, and rate pressure product. Using multivariate analysis and groups of 15 dogs, an intervention that limited infarct size by 10-13% of the area at risk would have been detected 50% of the time. Larger treatment effects would be detected more readily, and smaller effects often would be missed, unless group sizes were larger. Two drugs, verapamil and ibuprofen, were evaluated in both models, with experimental group sizes averaging 13 and 20 dogs, in the unconscious and conscious models, respectively. Three of 15 verapamil-treated dogs in the unconscious model study had much smaller infarcts than expected from baseline parameters. With these exceptions, neither drug limited infarct size in either model.

The Draize eye test

Hundreds of substances are used daily that can damage eyesight. People's eyes are open to accidental or intentional exposure during the production, transportation, use, and disposal of chemical preparations. Ensuring the safety of consumer products was born during the mid twentieth century in the aftermath of chemical warfare research, and was motivated by the hazards of unsafe cosmetics. Justified by an exigency for public protection, the Draize eye test became a governmentally endorsed method to evaluate the safety of materials meant for use in or around the eyes. The test involves a standardized protocol for instilling agents onto the cornea and conjunctiva of laboratory animals. A sum of ordinal-scale items of the outer eye gives an index of ocular morbidity. Advances in ocular toxicology are challenging the validity, precision, relevance, and need of the Draize eye test. Preclinical product-safety tests with rabbits and other mammals also raise ethical concerns of animal wellbeing. Some use the Draize test as a rallying point for how animals are treated in science and industry. A battery of cellular systems and computer models aim to reduce and ultimately to replace whole-animal testing. Molecular measures of ocular toxicity may eventually allow comprehensive screening in humans. The Draize eye test was created and refined for humanitarian reasons and has assuredly prevented harm. Its destiny is to be progressively supplanted as in vitro and clinical alternatives emerge for assessing irritancy of the ocular surface.

Mouse models in oncoimmunology

Fundamental cancer research and the development of efficacious antineoplastic treatments both rely on experimental systems in which the relationship between malignant cells and immune cells can be studied. Mouse models of transplantable, carcinogen-induced or genetically engineered malignancies - each with their specific advantages and difficulties - have laid the foundations of oncoimmunology. These models have guided the immunosurveillance theory that postulates that evasion from immune control is an essential feature of cancer, the concept that the long-term effects of conventional cancer treatments mostly rely on the reinstatement of anticancer immune responses and the preclinical development of immunotherapies, including currently approved immune checkpoint blockers. Specific aspects of pharmacological development, as well as attempts to personalize cancer treatments using patient-derived xenografts, require the development of mouse models in which murine genes and cells are replaced with their human equivalents. Such 'humanized' mouse models are being progressively refined to characterize the leukocyte subpopulations that belong to the innate and acquired arms of the immune system as they infiltrate human cancers that are subjected to experimental therapies. We surmise that the ever-advancing refinement of murine preclinical models will accelerate the pace of therapeutic optimization in patients.

Methods of in vitro toxicology

In vitro methods are common and widely used for screening and ranking chemicals, and have also been taken into account sporadically for risk assessment purposes in the case of food additives. However, the range of food-associated compounds amenable to in vitro toxicology is considered much broader, comprising not only natural ingredients, including those from food preparation, but also compounds formed endogenously after exposure, permissible/authorised chemicals including additives, residues, supplements, chemicals from processing and packaging and contaminants. A major promise of in vitro systems is to obtain mechanism-derived information that is considered pivotal for adequate risk assessment. This paper critically reviews the entire process of risk assessment by in vitro toxicology, encompassing ongoing and future developments, with major emphasis on cytotoxicity, cellular responses, toxicokinetics, modelling, metabolism, cancer-related endpoints, developmental toxicity, prediction of allergenicity, and finally, development and application of biomarkers. It describes in depth the use of in vitro methods in strategies for characterising and predicting hazards to the human. Major weaknesses and strengths of these assay systems are addressed, together with some key issues concerning major research priorities to improve hazard identification and characterisation of food-associated chemicals.

Validation of the Embryonic Stem Cell Test in the International ECVAM Validation Study on Three In Vitro Embryotoxicity Tests

A detailed report is presented on the performance of the embryonic stem cell test (EST) in a European Centre for the Validation of Alternative Methods (ECVAM)-sponsored formal validation study on three in vitro tests for embryotoxicity. Twenty coded test chemicals, classified as non-embryotoxic, weakly embryotoxic or strongly embryotoxic on the basis of their in vivo effects in animals and/or humans, were tested in four laboratories. The outcome showed that the EST can be considered to be a scientifically validated test, which is ready for consideration for use in assessing the embryotoxic potentials of chemicals for regulatory purposes.

Zebrafish embryos as models for embryotoxic and teratological effects of chemicals

The experimental virtues of the zebrafish embryo such as small size, development outside of the mother, cheap maintenance of the adult made the zebrafish an excellent model for phenotypic genetic and more recently also chemical screens. The availability of a genome sequence and several thousand mutants and transgenic lines together with gene arrays and a broad spectrum of techniques to manipulate gene functions add further to the experimental strength of this model. Pioneering studies suggest that chemicals can have in many cases very similar toxicological and teratological effects in zebrafish embryos and humans. In certain areas such as cardiotoxicity, the zebrafish appears to outplay the traditional rodent models of toxicity testing. Several pilot projects used zebrafish embryos to identify new chemical entities with specific biological functions. In combination with the establishment of transgenic sensor lines and the further development of existing and new automated imaging systems, the zebrafish embryos could therefore be used as cost-effective and ethically acceptable animal models for drug screening as well as toxicity testing.

Liver cell models in in vitro toxicology

In vitro liver preparations are increasingly used for the study of hepatotoxicity of chemicals. In recent years their actual advantages and limitations have been better defined. The cell models, slices, and mainly primary hepatocyte cultures, appear to be the most powerful in vitro systems, as liver-specific functions and responsiveness to inducers are retained either for a few days or several weeks depending on culture conditions. Maintenance of phase I and phase II xenobiotic metabolizing enzyme activities allows various chemical investigations to be performed, including determination of kinetic parameters, metabolic profile, interspecies comparison, inhibition and induction effects, and drug-drug interactions. In vitro liver cell models also have various applications in toxicology: screening of cytotoxic and genotoxic compounds, evaluation of chemoprotective agents, and determination of characteristic liver lesions and associated biochemical mechanisms induced by toxic compounds. Extrapolation of the results to the in vivo situation remains a matter of debate. Presently, the most convincing applications of liver cell models are the studies on different aspects of metabolism and mechanisms of toxicity. For the future, there is a need for better culture conditions and differentiated hepatocyte cell lines to overcome the limited availability of human liver tissues. In addition, strategies for in vitro analysis of potentially toxic chemicals must be better defined.

Mechanistic applicability domains for nonanimal-based prediction of toxicological end points: general principles and application to reactive toxicity

In light of new legislation (e.g., the REACH program in the European Union), several initiatives have recently emerged to increase acceptance of (quantitative) structure-activity relationships [(Q)SARs] to reduce reliance on animal (in vivo) testing. Among the principles for assessing the validity of (Q)SARs is the need for a defined domain of applicability, i.e., identification of the range of compounds for which the (Q)SAR can confidently be applied for purposes of toxicity prediction. Here, we attempt to develop a "natural" classification into applicability domains based on considering how a compound and the target organism between them "decide" on the nature and extent of the toxic effect. With particular emphasis on reactive toxicity, we present rules, based on organic reaction mechanistic principles, for classifying reactive toxicants into their appropriate mechanistic applicability domains.

The Myeloid U937 Skin Sensitization Test (U-SENS) addresses the activation of dendritic cell event in the adverse outcome pathway for skin sensitization

The U-SENS™ assay, formerly known as MUSST (Myeloid U937 Skin Sensitization Test), is an in vitro method to assess skin sensitization. Dendritic cell activation following exposure to sensitizers was modelled in the U937 human myeloid cell line by measuring the induction of the expression of CD86 by flow cytometry. The predictive performance of U-SENS™ was assessed via a comprehensive comparison analysis with the available human and LLNA data of 175 substances. U-SENS™ showed 79% specificity, 90% sensitivity and 88% accuracy. A four laboratory ring study demonstrated the transferability, reliability and reproducibility of U-SENS™, with a reproducibility of 95% within laboratories and 79% between-laboratories, showing that the U-SENS™ assay is a promising tool in a skin sensitization risk assessment testing strategy.

FDA no longer has to require animal testing for new drugs

Agency can rely on animal-free alternatives before human trials.

In vitro skin irritation: facts and future. State of the art review of mechanisms and models

The skin is the main target tissue for exogenous noxes, protecting us from harmful environmental hazards, UV-irradiation and endogenous water loss. It is composed of three layers, whereas the outermost epidermis is a squamous epithelium that mainly consists of keratinocytes. These cells execute a terminal differentiation, which finally results in the assembly of the stratum corneum. This layer, consisting of cornified keratinocytes, is an effective barrier against a vast number of substances. Apart of this, keratinocytes play crucial roles in the immune surveillance and the initiation, modulation and regulation of inflammation in the epidermis. Regarding cutaneous inflammatory reactions, skin irritation is one of the most common adverse effect in humans. For reasons of human safety assessment new chemicals are still evaluated for irritant potentials by application to animals followed by visible changes such as erythema and oedema. Testing for skin irritation in animals potentially cause them pain and discomfort. Furthermore, the results are not always predictive for those found in humans. In order to replace animal testing and to improve the prediction of irritants, the cosmetic and toiletry industry, in Europe represented by Colipa, develops and uses several alternative in vitro test systems. In this respect, the use of in vitro reconstructed organotypic skin equivalents are mostly favored, because of their increasingly close resemblance to human skin. Due to ethical and scientific questions and on account of the 7th amendment of the European Council Directive 76/768/EEC, the authors see the requirement to drive the development of alternative tests for irritants. Therefore, this article centres on cosmetic ingredients and provides the readership an overview of the state of art of cellular mechanisms of skin irritation and summarizes the results of the commonly used skin equivalents to evaluate irritation in vitro.

Fish cell lines as versatile tools in ecotoxicology: assessment of cytotoxicity, cytochrome P4501A induction potential and estrogenic activity of chemicals and environmental samples

In vitro systems such as primary cells and cell lines are of growing importance in ecotoxicology. Cells from different tissues and species of fish are used for the assessment of toxic action of chemicals and evaluation of environmental samples. For organotins and substituted phenols, we have found that the in vitro cytotoxicity is positively correlated with the acute toxicity in vivo, and therefore cytotoxicity assays may serve as an alternative for acute fish toxicity testing. We have been using the hepatocellular carcinoma (PLHC-1) cell line for the assessment of the cytochrome P4501A (CYP1A) induction potential of polyaromatic hydrocarbons (PAHs), nitro-PAHs and azaarenes. For these compounds, the CYP1A induction potential is found to be related to the molecular structure and lipophilicity. In mixtures, CYP1A induction of individual compounds is additive. Based on the comparative investigation of the induction potential we derived an induction equivalency (IEQ) concept that can be applied for the evaluation of environmental samples such as landfill leachates, sediments and motorway runoffs. Fish cell lines are also valuable, rapid and cost-effective tools for the assessment of estrogenic activity of chemicals and environmental samples. We have developed an estrogen-responsive reporter gene system using the rainbow trout gonad cell line RTG-2, in which an estrogen receptor beta form is expressed at very low levels, but is not inducible. As the estrogenic activity is dependent on the cellular level of estrogen receptor (ER), ER has to be co-transfected in transient transfections in addition to an estrogen-responsive reporter gene. Using a dual luciferase system, the estrogenic activity of 12 compounds including alkylphenols, DDT-isomers and its metabolites have been assessed. Our system shows a high sensitivity with a detection limit of 0.05 nM estradiol and is therefore more sensitive than many other mammalian or yeast systems. The relative estrogenic activity (e.g. o,p'-DDT) and other toxicological effects may differ from those in mammalian systems, indicating that a risk evaluation for fish could only be meaningfully assessed in fish-specific systems. This paper illustrates the versatility and high potential of fish cell lines in ecotoxicology.

The use of fetal bovine serum: ethical or scientific problem?

Fetal bovine serum (FBS) is a common component of animal cell culture media. It is harvested from bovine fetuses taken from pregnant cows during slaughter. FBS is commonly harvested by means of a cardiac puncture without any form of anaesthesia. Fetuses are probably exposed to pain and/or discomfort, so the current practice of fetal blood harvesting is inhumane. Apart from moral concerns, several scientific and technical problems exist with regard to the use of FBS in cell culture. Efforts should be made to reduce the use of FBS or, preferably, to replace it with synthetic alternatives.

Machine Learning of Toxicological Big Data Enables Read-Across Structure Activity Relationships (RASAR) Outperforming Animal Test Reproducibility

Earlier we created a chemical hazard database via natural language processing of dossiers submitted to the European Chemical Agency with approximately 10 000 chemicals. We identified repeat OECD guideline tests to establish reproducibility of acute oral and dermal toxicity, eye and skin irritation, mutagenicity and skin sensitization. Based on 350-700+ chemicals each, the probability that an OECD guideline animal test would output the same result in a repeat test was 78%-96% (sensitivity 50%-87%). An expanded database with more than 866 000 chemical properties/hazards was used as training data and to model health hazards and chemical properties. The constructed models automate and extend the read-across method of chemical classification. The novel models called RASARs (read-across structure activity relationship) use binary fingerprints and Jaccard distance to define chemical similarity. A large chemical similarity adjacency matrix is constructed from this similarity metric and is used to derive feature vectors for supervised learning. We show results on 9 health hazards from 2 kinds of RASARs-"Simple" and "Data Fusion". The "Simple" RASAR seeks to duplicate the traditional read-across method, predicting hazard from chemical analogs with known hazard data. The "Data Fusion" RASAR extends this concept by creating large feature vectors from all available property data rather than only the modeled hazard. Simple RASAR models tested in cross-validation achieve 70%-80% balanced accuracies with constraints on tested compounds. Cross validation of data fusion RASARs show balanced accuracies in the 80%-95% range across 9 health hazards with no constraints on tested compounds.

Computer prediction of possible toxic action from chemical structure: an update on the DEREK system

Computer-based assessment of potential toxicity has become increasingly popular in recent years. The knowledge-base system DEREK is developed under the guidance of a multinational Collaborative Group of expert toxicologists and provides a qualitative approach to toxicity prediction. Major developments of the DEREK program and knowledge-base have taken place in the last 3 years. Program developments include improvements in both the user interface and data processing. Work on the knowledge-base has concentrated on the areas of genotoxicity and skin sensitisation. DEREK's predictive capabilities for these toxicological end-points has been demonstrated. In addition to the continued expansion of the knowledge-base, a number of enhancements are planned in the DEREK program. In particular, work is in progress to develop further DEREK's ability to report the reasoning behind its predictions.

Adverse outcome pathways as a tool for the design of testing strategies to support the safety assessment of emerging advanced materials at the nanoscale

Toxicity testing and regulation of advanced materials at the nanoscale, i.e. nanosafety, is challenged by the growing number of nanomaterials and their property variants requiring assessment for potential human health impacts. The existing animal-reliant toxicity testing tools are onerous in terms of time and resources and are less and less in line with the international effort to reduce animal experiments. Thus, there is a need for faster, cheaper, sensitive and effective animal alternatives that are supported by mechanistic evidence. More importantly, there is an urgency for developing alternative testing strategies that help justify the strategic prioritization of testing or targeting the most apparent adverse outcomes, selection of specific endpoints and assays and identifying nanomaterials of high concern. The Adverse Outcome Pathway (AOP) framework is a systematic process that uses the available mechanistic information concerning a toxicological response and describes causal or mechanistic linkages between a molecular initiating event, a series of intermediate key events and the adverse outcome. The AOP framework provides pragmatic insights to promote the development of alternative testing strategies. This review will detail a brief overview of the AOP framework and its application to nanotoxicology, tools for developing AOPs and the role of toxicogenomics, and summarize various AOPs of relevance to inhalation toxicity of nanomaterials that are currently under various stages of development. The review also presents a network of AOPs derived from connecting all AOPs, which shows that several adverse outcomes induced by nanomaterials originate from a molecular initiating event that describes the interaction of nanomaterials with lung cells and involve similar intermediate key events. Finally, using the example of an established AOP for lung fibrosis, the review will discuss various in vitro tests available for assessing lung fibrosis and how the information can be used to support a tiered testing strategy for lung fibrosis. The AOPs and AOP network enable deeper understanding of mechanisms involved in inhalation toxicity of nanomaterials and provide a strategy for the development of alternative test methods for hazard and risk assessment of nanomaterials.