Subchronic toxicity studies rodents

Chronic and subchronic toxicity studies are conducted to define the dose level, when given repeatedly, that cause toxicity, and the dose level that does not lead to toxic findings. In Japan, such studies are referred to as repeated-dose toxicity studies. As with single-dose studies, at least two animal species should be used, one rodent and one nonrodent (rabbit not acceptable). In rodent studies, each group should consist of at least 10 males and 10 females in nonrodent species, 3 of each sex are deemed adequate. Where interim examinations are planned, however, the numbers of animals employed should be increased accordingly. The planned route of administration in human subjects is normally explored. The duration of the study will be dictated by the planned duration of clinical use (Table 2.14). 

Guzman, A., Garcia, C., and Demestre, I. Acute and subchronic toxicity studies of the new quinoline antibacterial agent irloxacin in rodents, Arzneim. Forsch., 49(5) 448-456, 1999. 

The EMTD is determined on the basis of a subchronic toxicity study in which small groups of test animals are exposed to a fairly broad dose range of the test substance for a period of approximately 10% of the life span of the animals (90 days for rodents). In turn, the dose levels which are tested in the 90 day subchronic study are themselves based on information generated in acute toxicity studies of the type described in the preceeding chapter. In addition to providing the EMTD the acute and subchronic tests also are utilized to obtain information on the metabolism of the test substance, the extent to which the test substance and its metabolites are distributed and bioaccumulated in the host, and which host organ systems may be affected by exposure. This information may be of importance in choosing the species or strains to be used in the chronic study as well as the route of administration. 

The traditional acute, subchronic, and chronic toxicity studies performed in rodents and other species also can be considered to constitute multiple endpoint screens. Although the numerically measured continuous variables (body weight, food consumption, hematology values) generally can be statistically evaluated individually by traditional means, the same concerns of loss of information present in the interrelationship of such variables apply. Generally, traditional multivariate methods are not available, efficient, sensitive, or practical (Young, 1985). 

Metabolism and pharmacokinetic studies have greater relevance when conducted in both sexes of young adult animals of the same species and strain used for other toxicity tests with the test substance. The number of animals used in metabolism and pharmacokinetic studies would be sufficient to reliably estimate population variability. This usually means a separate (but parallel) set of groups of animals in rodent studies. A single set of intravenous and oral dosing results from adult animals, when combined with some in vitro kinetic results, may provide an adequate data set for the design and interpretation of short-term, subchronic and chronic toxicity studies. 

Four-week toxicology studies in rodent and nonrodent The four-week studies are designed for subchronic exposure of rodents and nonrodents to the test article. These studies also look at reversibility of any toxicity observed. Many times these are the pivotal studies used to support the first in human dosing. Toxicokinetic assessments are generally included in repeat-dose toxicity studies. When testing biopharmaceuticals, studies also include assessment and characterization of immune response (immunogenicity). 

The use of the subchronic rat study for developing an oral RfD for GD is complicated by the fact that rodents have a mnch lower RBC-AChE activity level compared to hnmans (ElUn, 1981, see Table 1). By itself, this could cause rats to be relatively more sensitive than hnmans to anticholinesterase compounds however, the lower RBC-ChE activity may be offset by the presence of aliesterase in rat blood. Aliesterase, which is not present in hnmans (Cohen et al., 1971), is known to bind to and thereby rednce the toxicity of cholinesterase inhibitors (Fonnnm and Sterri, 1981). Other species differences, snch as the rates of aging of the nerve agent-ChE complex, the rates of synthesis of plasma cholinesterase in the liver, and the levels of AChE in various parts of the nervous system (see Ivanov et al., 1993) may also resnlt in differences in species sensitivities. There are insufficient data to determine the relative snsceptibilities of humans and rodents to GD therefore, for the pnrpose of this assessment, the EPA method will be followed which assumes that humans may be as mnch as ten times more sensitive to a chemical than laboratory animals. 

The toxicologist usually moves from studies of a single exposure to ones in which animals are exposed on each of 90 consecutive days. The 90-day subchronic study has become a convention in the field. Rodents usually live 2-3 years in the laboratory, so 90 days is about 10% of a lifetime. An enormous amount of 90-day rodent toxicity data have been collected over the past several decades and have played key roles in judging the risks of environmental chemicals. 

Klimisch, H.-J., Deckardt, K., Gembardt, C., Hildebrand, B., Kuttler, K. Roe, F.J.C. (1997b) Subchronic inhalation and oral toxicity of V-vinylpyrrolidone-2. Studies in rodents. Food chem. Toxicol., 35, 1061-1074... 

Chronic and subchronic exposure of rats to propylene oxide by inhalation induced respiratory cell hyperplasia, irritation and toxicity in the nasal epithelium, at a concentration of 300 ppm or higher. No adverse effects on reproduction were observed in rats or rabbits exposed to propylene oxide at up to 500 ppm. In vivo studies in rodents of dominant lethal mutations, sperm abnormalities, micronuclei, chromosomal aberrations, and sister chromatid exchanges... 

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