. OECD Guidelines for the Testing of Chemicals are periodically reviewed in the light of scientific
progress, changing assessment practices and animal welfare considerations. The original Guideline
452 was adopted in 1981. Development of a revised TG 452 was considered necessary in order to
reflect recent developments in the field of animal welfare and regulatory requirements (1)(2)(3)(4).
The updating of TG 452 has been carried out in parallel with revisions of the Test Guidelines 451,
Carcinogenicity Studies and 453, Combined Chronic Toxicity/Carcinogenicity studies, with the
objective of obtaining additional information from the animals used in the study and providing further
detail on dose selection.
2. The majority of chronic toxicity studies are carried out in rodent species, and this Test Guideline is
intended therefore to apply primarily to studies carried out in these species. Should such studies be
required in non-rodent species, the principles and procedures outlined may also be applied, with
appropriate modifications, as outlined in an OECD Guidance Document on the design and conduct of
chronic toxicity and carcinogenicity studies (5).
3. The three main routes of administration used in chronic toxicity studies are oral, dermal and
inhalation. The choice of the route of administration depends on the physical and chemical
characteristics of the test substance and the predominant route of exposure of humans. Additional
information on choice of route of exposure is provided in an OECD Guidance Document on the
design and conduct of chronic toxicity and carcinogenicity studies (5).
4. This Guideline focuses on exposure via the oral route, the route most commonly used in chronic
toxicity studies. While long–term chronic toxicity studies involving exposure via the dermal or
inhalation routes may also be necessary for human heath risk assessment and/or may be required
under certain regulatory regimes, both routes of exposure involve considerable technical complexity.
Such studies will need to be designed on a case-by-case basis, although the Guideline outlined here
for the assessment and evaluation of chronic toxicity by oral administration could form the basis of a
protocol for inhalation and/or dermal studies, with respect to recommendations for treatment periods,
clinical and pathology parameters, etc. OECD Guidance is available on the administration of test
substances by the inhalation (5)(6) and dermal routes (5). The updated Guidelines TG 412, Subacute
inhalation toxicity: 28 day study (8) and TG 413, Subchronic Inhalation Toxicity: 90-Day Study (9),
together with the associated OECD Guidance Document on acute inhalation toxicity testing (7),
should be specifically consulted in the design of longer term studies involving exposure via the
inhalation route.
5. The objectives of chronic toxicity studies covered by this test guideline include:
the identification of the hazardous properties of a chemical,
the identification of target organs,
characterisation of the dose:response relationship,
identification of a no-observed-adverse-effect level (NOAEL) or point of departure for
establishment of a Benchmark Dose (BMD),
the prediction of chronic toxicity effects at human exposure levels,
provision of data to test hypotheses regarding mode of action (5).
Draft consultant’s proposal. V. 8. OECD TG 452 November, 2008
2
INITIAL CONSIDERATIONS
6. 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