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ABSTRACT:Numerous studies, usually limited to male rodents, have reported an inverse relationship between the age of the animal and the activities of various multi-cytochrome P450-dependent drug-metabolizing enzymes. It has been suggested that the aging-induced decline in hepatic drug-metabolizing capacity is solely a male phenomenon. That is, whereas the levels of male-specific isoforms of P450 decline with senescence, the female-dependent isoforms remain unchanged in females and even increase in male liver. In addition to their baseline activities, induction levels of hepatic monooxygenases have also been reported to decrease with aging. To examine aging-and sex-dependent effects on drug metabolism at a more molecular level, we measured the expression (mRNA, protein, and/or catalytic activity) of a near dozen constitutive and inducible isoforms of P450 in 5-and 23-month-old male and female Sprague-Dawley rats. Moreover, we investigated the induction effects of low concentrations of phenobarbital known to reveal gender differences and the threshold sensitivities of both constitutive and inducible isoforms. With the exception of male-specific CYP2C11 (whose expression declined ϳ70% in aged male rats), we observed little senescence-associated reduction in either preinduction or induction levels of CYP2B1, CYP2B2, CYP3A1, CYP3A2, CYP2C6, CYP2C7, CYP2C12, and CYP2C13 in either male or female rats. Moreover, the sexually dimorphic expression levels apparent at 5 months of age persisted in the old rats.People over 65 years of age represent the fastest growing segment of the population and consume at least one-third of all prescription drugs. Stated another way, 82% of people over 65 use prescription medications (Willcox et al., 1994). Moreover, the high prevalence of polypharmacy in the elderly likely contributes to an abnormally high 20 to 25% incidence of adverse drug reactions and toxicities in this age group (Hunt et al., 1992;Willcox et al., 1994). The generally accepted aging-associated decline in drug disposition is a result of senescent changes in drug absorption, distribution, excretion, and/or metabolism (Schmucker and Lonergan, 1987). Direct studies on liver drug metabolism in elderly humans are scant with most evidence derived from animal studies.In this regard, early investigations using laboratory animals had to limit their analyses to nonspecific multi-P450 2 -dependent drug-metabolizing enzymes. Moreover, the important contribution of sex was often overlooked by using only the "preferred" gender in pharmacotoxicological studies, the male rodent, or worse yet, not even recording the sex of the animal. Last, few investigators considered the effect of strain, a critical factor (Chengelis, 1988) influencing age-associated changes in drug metabolism.One of the earliest studies using rats reported an inverse relationship between age and the metabolism of commonly used model substrates (Kato and Takanaka, 1968). This, as well as additional stud...