Urinary corticosterone as an indicator of stress-mediated immunological changes in rats

Drugs that target the CNS or doses of drugs near the maximum tolerated dose may cause a non-specific stress response during routine safety testing in rodents that leads to the release of corticosterone and changes immunological parameters. In situations with mild clinical signs of stress and changes to immune organs, it may be difficult to differentiate direct immunotoxicity from changes mediated by stress. To address this concern, studies were conducted to identify potential biomarker of stress in rats that could be used in routine toxicology studies. Since serial blood collections for corticosterone levels are not practical, studies were conducted to evaluate urine corticosterone and its metabolites as a potential biomarker of stress in male Sprague-Dawley rats. Exogenous corticosterone was used as a reference to identify immune system targets and determine their relative sensitivity to corticosterone. The data from rats treated with exogenous corticosterone and from rats treated with drug or chemical stressors produced linear relationships between urine corticosterone and most immunological parameters, with r-squared values greater than 0.6. Thus, quantitatively similar effects on immunological end points are produced by exogenous corticosterone and by corticosterone induced by chemical stressors with regard to their correlation to selected immunological changes. In preclinical safety testing for a new drug, the combined findings of increased urinary corticosterone and changes of the predicted magnitude and direction in blood lymphocyte and neutrophil differentials and thymus weight or cellularity would strongly suggest that the immunological effects are secondary to a drug-induced stress response. Because these results can be obtained reliably during routine preclinical evaluations, they should be useful for the weight-of-evidence approaches often used in regulatory settings.