Subchronic Oral Toxicity 90-Day Study

Although a variety of species may be of use, the albino rat is preferred as the test animal for subchronic oral toxicity studies. Commonly used laboratory strains of young, healthy animals should be employed. Dosing of the test chemical should begin as soon as possible after weaning, ideally before the animals are 3 weeks old. Animals more than 8 weeks old are not suitable for the study. 

The weight variation of animals at the commencement of the study should not exceed 20% of the mean weight. It is also important that wherever a subchronic oral toxicity study is conducted as a preliminary to a long-term study, the same species and strain should be used in all studies. At least 20 animals (1 female, 10 male) should be used for each test dose. In view of the importance of the subchronic oral toxicity study, use of more animals would be advantageous. Females should be nulliparous and nonpregnant. 

In the subchronic toxicity study, if interim sacrifices are planned, the number of animals should be increased so that there is no shortage of animals at the termination of the study this ensures meaningful conclusions about the test chemical s toxicity. In addition, a satellite group of 20 animals (10 of each gender) may be treated with the high dose of the test chemical for 90 days. These animals should be observed for signs of adverse effects, (e.g., reversibility, persistence, delayed occurrence of toxic effects). The observation period should last for a posttreatment period of appropriate length not less than 28 days. 

For a chemical of low toxicity, it is important to ensure that when it is administered in the diet, the quantities involved do not interfere with normal nutrition of the animals. Whenever the chemical is mixed with the diet, either a constant dietary concentration (ppm) or a constant dose level in terms of the body weight (mg/kg) may be used. 

When a test chemical is administered by gavage, the dose should be given at the same specified time each day. Furthermore, the test dose should be adjusted at regular intervals (e.g., weekly or biweekly), to maintain constant dose levels in terms of the changing body weight of the animal. Also, where a subchronic oral toxicity study is used as a preliminary to a long-term study, a similar dietary regimen should be used for both studies. 

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