Can In silico and/or In vitro Testing be used for Toxicity ...

(How) Can In silico and/or In vitro Testing be used for Toxicity Assessment instead of in vivo Approaches? Timothy J. Shafer 2012 SOT In vitro Lunch Disclaimer: The views expressed in this presentation are solely those of the Author. They do not represent Agency Policy. What is the Current Toxicity Testing Paradigm? Batteries of in vivo tests in various mammalian specieswhen agencies have regulatory authority to require testing 870.1100 Acute oral toxicity rat 870.1200 Acute dermal toxicity 870.1300 Acute inhalation toxicity rat 870.2400 Primary eye irritation rabbit 870.2500 Primary dermal irritation 870.2600 Dermal sensitization 870.6100 Delayed neurotoxicity (acute) hen 870.6200 Acute neurotoxicity rat Subchronic Testing 870.3100 90day Oral rodent 870.3150 90day Oral - non-rodent 870.3200 21/28day Dermal 870.3250 90day Dermal 870.3465 90day Inhalation rat

870.6100 28day Delayed neurotoxicity-hen 870.6200 90day Neurotoxicity rat Chronic Testing 870.4100 Chronic oral rodent Carcinogenicity - two rodent species 870.4200 rat and mouse preferred Developmental Toxicity and Reproduction Prenatal Developmental toxicity - rat 870.3700 and rabbit, preferred 870.3800 Reproduction and fertility effects 870.6300 Developmental neurotoxicity Mutagenicity Testing 870.5100 Bacterial reverse mutation assay 870.5300 In vitro mammalian cell assay 870.5375 870.5385 In vivo cytogenetics 870.5395 Special Testing 870.7485 Metabolism and pharmacokinetics 870.7200 Companion animal safety 870.7600 Dermal penetration 870.7800 Immunotoxicity

27 Different StudiesDoes not include Eco Are there any success stories? Draize eye test (Ocular Toxicity) Use validated alternatives Consideration of chemical properties Alternative tests for corrosive compounds (acids/bases) Alternative in vitro tests (hens egg chorioallantoic membrane (HET-CAM) assay; human corneal cells in culture In vitro tests prior to in vivo Genetic Tox assays: Ames Test (Mutagenicity/Carcinogenicity) Tests mutagenicity using bacteria Is 70% predictive of carcinogens for general classes Nearly 100% prediction for PAH, Aromatic and heterocyclic amines, nitro aromatics

Poorly predictive for chlorinated organic (chlorophenols chlorinated hydrocarbons) Rat and mouse are 70% predictive of humans; and 70% predictive of each other Why do we need a new testing paradigm? Large number of chemicals for which we do not have complete toxicity information Need to assess mixtures Need to assess all endpoints and life stages Efficiency Developmental Neurotoxicity < Costs associated with in vivo testing 1% $15-20 M to register 1 food use pesticide

$1M and 1.5yr for a Developmental Neurotoxicity (DNT) guideline study Humane reasons- use fewer animals and minimize distress Who says we need a new testing paradigm? REACH legislation Registration, Evaluation, Authorisation and Restriction of Chemical substances Under REACH, animal testing is to be avoided in favour of alternative methods and registrants can only carry out tests involving the use of animals as a last resort. ( NAS Toxicity in the 21st Century report Limitations of current paradigm are high to low dose extrapolations and animal to human extrapolations = limited ability to predict what happens in humans exposed to low doses. Stakeholders

Public wants safe use of chemicals Industry wants reduced costs Animal welfare groups want reduced use of animals What kinds of decisions do regulators have to make? Allow a new compound to be produced. Risk/Benefit (e.g. new drug) vs safety (food additive) Replace or choose between compounds (Risk/Risk). Regulate levels released into the environment or use conditions for a compound (e.g. pesticide labels). Extent of clean up at a hazardous waste site. Setting exposure levels (RfD, RfC, etc), ambient air quality standards, allowable levels of contamination (e.g. in drinking water). What kind of information do regulators need to make these decisions? Who is exposed and to how much? What is the hazard? Organ system Lifestage

Ecological What are the properties of the chemical? Will it move in the environment? Will it bioaccumulate? Is it reactive? What is the chemical used for and are there alternatives? How do we improve the current paradigm? Source: Figure 2-3, NAS, 2007 What is a Toxicity Pathway anyhow? Acute Neurotoxicity of Pyrethroid Insecticides Key Event Exposure ADME

Altered Na Channel Na Channel Function Function Cell Level Tissue Level Organ Level Altered Neuronal Neuronal Excitability Excitability Altered Network Network Firing Patterns Altered

Neuronal Neuronal Pathways Pathways Adverse Normal Clinical Function Outcome ?? Adverse Clinical Outcome ? ?? ?? Developmental Neurotoxicity of Pyrethroid Insecticides Developing Predictive Models for Toxicity

Existing Chemicals Chemical Characteristics Hepatotoxicity HTS Assays for Toxicity Pathways Quantitative StructureActivity Relationships (QSAR) Cardiotoxicity QSA R Neurotoxicity Developmental toxicity Renal toxicity Carcinogenic New Chemical

Immunotoxicit y Questions 1. Is it a realistic goal to replace all animal testing? (In what timeframe?) 2. What criteria must be fulfilled for an in vitro approach to replace an in vivo approach? 3. Does the context of the decision to be made, or the level of information required matter? 4. What are the challenges to human risk? How can these be addressed? 5. (How) Can an in vitro approach be useful if the toxicity pathway is not completely understood? 6. If we replace, what are the scientific questions (uncertainties) that we need to be concerned about? 7 Would the public accept and be comfortable with decisions made using in vitro data? What if the decision was made entirely on the basis of in vitro data?

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