Rethinking toxicity testing: Influence of aging on the outcome of long-term toxicity testing and possible remediation

Traditionally, toxicity testing is conducted at fixed dose rates (i.e., mg/kg/day) without considering life-changing events, e.g., stress, sickness, aging- and/or pregnancy-related changes in physical, physiological and biochemical parameters. In humans, life-changing events may cause systemic dose non-proportionality requiring modulation of drug dosage; similar changes occur in animals altering systemic dose during chronic/carcinogenic testing leading to "late-occurring" effects in some studies. For example, propylene monomethyl ether, an industrial chemical, initially induced sedation in rats and mice with recovery upon induction of hepatic CYPs after ~1 week. Sedation reappeared in rats but not in mice after ~12 months of exposure due to decreased CYP activity in rats, elderly mice were able to maintain slightly higher CYP activity avoiding recurrence of sedation. The systemic dose of two pharmaceuticals (doxazosin and brimonidine tartrate) increased up to 6-fold in ≥12-month old rats with no toxicity. In a rat reproductive toxicity study, systemic dose of 2,4-D, an herbicide, rapidly increased due to increased consumption of 2,4-D-fortified diet during pregnancy, lactation and neonatal growth, requiring adjustment to maintain the targeted systemic dose. Ideally, toxicological studies should be based on systemic dose with the option of modulating external dose rates to maintain the targeted systemic dose. Systemic dose can easily be monitored in selected core study animals at desired intervals considering recent developments in sampling and analysis at a fraction of the overall cost of a study.