National Toxicology Program

National Toxicology Program

Acute Systemic Toxicity

Use of High-Throughput Screening Data to Reduce Animal Use for Acute Oral Toxicity Testing

NICEATM evaluated the use of high-throughput screening (HTS) data from the interagency Tox21 and the U.S. Environmental Protection Agency’s (EPA) ToxCastTM programs to reduce animal use for acute oral toxicity testing. The HTS data were analyzed for correlation and model fit to rat oral LD50 data to determine which tests or combinations of tests best characterized the rat oral toxicity data. The analysis suggested that, for some substances, combinations of in vitro assays and data from small model organisms such as zebrafish offer promise for predicting outcomes of rat acute oral toxicity tests. This project was described in a poster presentation (Polk et al.) at the 2015 Annual Meeting of the Society of Toxicology.

NICEATM Evaluation of Use of Acute Oral Systemic Toxicity Data to Predict EPA Acute Dermal Hazard Classification

Acute oral and dermal systemic toxicity test data are used to determine hazard label requirements needed to protect pesticide users, workers, and handlers. Dermal systemic toxicity test data are also used to establish the type of personal protective equipment required for occupational pesticide users.

In an effort to determine if acute oral toxicity data can be used to reliably assign EPA acute dermal hazard classifications, NICEATM collected acute oral toxicity data for pesticide active ingredients and EPA provided similar information for pesticide formulations. NICEATM plans to perform quality control reviews of the data and use defined criteria to evaluate the usefulness and limitations of the acute oral toxicity data in estimating dermal toxicity. The outcome of this effort may result in reduced animal use for pesticide testing. The current status of this project was summarized in a poster (Paris et al.) presented at the 2015 Annual Meeting of the Society of Toxicology.

To assist with ongoing efforts, NICEATM formed an interest group with members from federal agencies, nongovernmental organizations, international validation organizations, and national and international animal welfare organizations. The focus of this group is to identify alternative methods for acute oral toxicity testing.

EURL ECVAM Validation Study in the Field of Toxicokinetics and Metabolism

The European Union Reference Laboratory for Alternatives to Animal Testing (EURL ECVAM) coordinated a validation study of two in vitro assays to evaluate human liver metabolism and toxicity. Representatives from NICEATM and ICCVAM participated on the management team for this validation study.

In humans and other animals, chemical metabolism often involves liver enzymes called cytochrome P450s (CYPs). Interactions between CYP enzymes and chemicals or drugs can alter CYP enzyme activity, causing hormonal disturbances, increased liver weight, drug–drug interactions, and other toxic effects. Alteration of CYP enzyme activity can potentially increase or decrease chemical toxicity, so measuring CYP enzyme activity is important to the development of in vitro toxicity assays.

EURL ECVAM investigated a human hepatoma cell line (HepaRG®) and cryopreserved human liver cells to assess the potential for CYP enzyme induction at clinically relevant doses of pharmaceuticals. The study provides a starting point for the development of novel in vitro platforms for assessing chemical and drug metabolism and toxicity, ultimately providing a metabolically competent in vitro alternative for long-term toxicity studies.

The EURL ECVAM study is complete and the validation study report was submitted to the EURL ECVAM Scientific Advisory Committee for peer review in 2014. A draft test guideline for in vitro measurement of CYP enzyme induction is currently under consideration by the Organisation for Economic Co-operation and Development (OECD).

Evaluation and Validation Study of In Vitro Cytotoxicity Test Methods

In vitro methods that use mammalian cell cultures and various cytotoxicity endpoints have been proposed as alternatives to in vivo acute oral systemic toxicity tests that use rodents. In vitro test methods that measure basal cytotoxicity (general cytotoxicity that affects structures or processes intrinsic to all cell types) are not currently regarded as suitable replacements for rodent acute oral toxicity tests. However, some methods may be useful for establishing the starting dose for acute oral toxicity tests.

ICCVAM recommends, where appropriate, the use of validated in vitro basal cytotoxicity tests in a weight-of-evidence approach to determine starting doses for acute oral systemic toxicity tests before using animals. Using these in vitro methods where appropriate is expected to reduce the number of animals required for each toxicity test.

The use of in vitro cytotoxicity test methods to reduce animal use in acute oral systemic toxicity testing was evaluated at a Workshop on In Vitro Methods for Assessing Acute Systemic Toxicity (2000). A Guidance Document on Using In Vitro Data to Estimate In Vivo Starting Doses for Acute Toxicity was prepared by ICCVAM with the assistance of the workshop participants.

Development of OECD Test Guidelines for the Evaluation of Acute Systemic Toxicity

NICEATM and ICCVAM actively participate in the development of OECD Test Guidelines and Guidance Documents relevant to toxicology and safety testing. OECD Test Guidelines are based on the most relevant internationally agreed-upon testing methods used by government, industry, and independent laboratories.

Scientific Workshop on Acute Chemical Safety Testing: Advancing In Vitro Approaches and Humane Endpoints for Systemic Toxicity Evaluations

NICEATM, ICCVAM, and international partners sponsored a workshop to explore alternative methods for acute chemical safety testing. The goals of this February 2008 meeting of international experts in the fields of in vitro and in vivo toxicology and human and veterinary medicine were to:

  1. Review the state-of-the-science and identify knowledge gaps (at the whole organism, organ system, cellular, and/or molecular levels) relevant to key in vivo pathways involved in acute systemic toxicity
  2. Recommend how these knowledge gaps can be addressed by collecting mechanistic biomarker data during currently required in vivo safety testing
  3. Recommend how in vivo key pathway information can be used to develop more predictive mechanism-based in vitro test systems and to identify biomarkers that may serve as earlier, more humane endpoints for in vivo test methods
  4. Recommend how mechanism-based in vitro test systems and earlier, more humane endpoints can be used to further reduce, refine, and eventually replace animal use for acute systemic toxicity testing while ensuring the protection of human and animal health

Oral Up-and-Down Procedure

The oral up-and-down procedure (UDP) is an alternative to the traditional oral median lethal dose (LD50) test that reduces animal use. In the oral UDP, one animal is tested at a time, and the response of each animal to a test substance determines whether the next animal receives a higher or lower dose. A peer review panel sponsored by NICEATM and ICCVAM met in 2000 and 2001 to evaluate the revised oral UDP and develop recommendations. U.S. regulatory agencies adopted the ICCVAM recommendations on the revised oral UDP in 2003 and now accept oral UDP methods issued by OECD and the EPA, which have replaced the conventional acute oral systemic toxicity test method.

NTP is located at the National Institute of Environmental Health Sciences, part of the National Institutes of Health.