In the assessment and evaluation of the toxicological characteristics of a chemical, all available
information on the test substance should be considered by the testing laboratory prior to conducting
the study, in order to focus the design of the study to more efficiently test for chronic toxicity
potential and to minimize animal usage. Information that will assist in the study design includes the
identity, chemical structure, and physico-chemical properties of the test substance; any information on
the mode of action; results of any in vitro or in vivo toxicity tests; anticipated use(s) and potential for
human exposure; available (Q)SAR data and toxicological data on structurally-related substances;
available toxicokinetic data (single dose and also repeat dose kinetics where available) and data
derived from other repeated exposure studies. The determination of chronic toxicity may be carried
out after initial information on toxicity has been obtained from repeated dose 28-day and/or 90-day
toxicity tests. A phased testing approach to chronic toxicity testing should be considered as part of
the overall assessment of the potential adverse health effects of a particular chemical (9)(10)(11)(12).
7. The chronic toxicity study provides information on the possible health hazards likely to arise from
repeated exposure over a considerable part of the entire lifespan (in rodents). The study will provide
information on the toxic effects of the substance, indicate target organs and the possibility of
accumulation. It can also provide an estimate of the no-observed-adverse effect level which can be
used for establishing safety criteria for human exposure. The need for careful clinical observations of
the animals, so as to obtain as much information as possible, is also stressed.
8. In conducting a chronic toxicity study, the guiding principles and considerations outlined in the
OECD Guidance Document on the recognition, assessment, and use of clinical signs as humane
endpoints for experimental animals used in safety evaluation (13), in particular paragraph 62 thereof,
should always be followed.
9. Detailed guidance on and discussion of the principles of dose selection for chronic toxicity and
carcinogenicity studies can be found in an OECD Guidance Document on the design and conduct of
chronic toxicity and carcinogenicity studies (5) as well as two International Life Sciences Institute
publications (14)(15). The core dose selection strategy is dependent on the primary objective or
objectives of the study (paragraph 5). In selecting appropriate dose levels, a balance has to be
achieved between hazard screening on the one hand and characterisation of low-dose responses and
their relevance on the other. This is particularly relevant in the situation where a combined chronic
toxicity and carcinogenicity study (TG 453) is to be carried out (paragraph 10).
10. Consideration should be given to carrying out a combined chronic toxicity and carcinogenicity study
(TG 453), rather than separate execution of a chronic toxicity study (TG 452) and carcinogenicity
study (TG 451). Careful consideration should however be given to the principles of dose selection
(paragraphs 9 and 20-24) when undertaking a combined chronic toxicity and carcinogenicity study
(TG 453), and it is also recognised that separate studies may be required under certain regulatory
frameworks.
11. Definitions used are given in the Annex.