Enzyme induction is independent on liver functional status

Background and objectives: Drug-drug interactions have became an important issue in health care. Since metabolism is the major pathway of elimination of drugs, and cytochromes P450 (CYPs) are the most common enzymes involved, most drug-drug interactions arise from either inhibition or induction of CYP enzymes. Although the effect of liver disease on the magnitude of drug interactions consequent upon enzyme induction has been extensively investigated, highly discrepant results have been obtained.Human studies, because of insurmountable ethic problems (the impossibility to administer repeated doses of a non-therapeutic drug to patients with serious liver dysfunction), had to use hepatopathic patients taking inducers for therapeutic purposes and, consequently, often lack rigorous methodology. Therefore, animal studies are necessary to clarify the influence of liver disease on CYP induction. This study was designed to compare enzyme induction in animals with normal and impaired liver function,rigorously stratified according to the degree of liver dysfunction. Methods: To reach this goal, we used 3 groups of animals: normal rats and two groups of rats with experimentally-induced cirrhosis, obtained by treatment with carbon tetrachloride (CCl4) administered by inhalation; one group with compensated (corresponding to human Child grade A or B) liver cirrhosis, the other group with decompensated (human Child grade C) liver cirrhosis. Each group was divided in 2 subgroups of 6 rats, one treated with vehicle (olive oil), the other with the inducing agent benzo[α]pyrene (bap). The inducing effect was assessed by measuring the activity of the prototypical enzymes induced by bap, CYP1A1 and CYP1A2, using liver microsomes obtained from normal and CCl4-treated rats. 7-ethoxyresorufin and 7-methoxyresorufin (both metabolized to resorufin) were used as validated probes for CYP1A1 and CYP1A2, respectively.The severity of liver cirrhosis was defined on the basis of histological examination (Ishak score), the presence or absence of ascites and laboratory data (PT, albumin, AST and ALT). Results and conclusion: Total CYP content per gram of liver tissue decreased in proportion to the decline of liver function (14.04 ± 3.20 nmol/g in normal rats vs 11.00 ± 1.99 nmol/g and 9.16 ± 1.08 nmol/g in rats with compensated and decompensated cirrhosis, respectively). In vehicle-treated rats, Vmax for ethoxyresorufin-7-dealkylase (CYP1A1 marker reaction) decreased from 570.2 ± 118.5 nmol/min/mg protein in normal animals to 229.8 ± 64.3 nmol/min/mg protein and 199.6 ± 29.6 nmol/min/mg protein in rats with compensated and decompensated cirrhosis, respectively. Intrinsic metabolic clearance (CLint) decreased in a similar fashion. Treatment with bapincreased Vmax to 6282 ± 1231 nmol/min/mg protein in normal rats,and even higher induction was observed in rats with compensated and decompensated cirrhosis. In vehicle-treated rats, Vmax for methoxyresorufin-7-dealkylase (CYP1A2 marker reaction) decreased from 78.8 ± 23.9 nmol/min/mg protein in normal animals to 39.3 ± 9.2 nmol/min/mg protein and 29.5 ± 6.1 nmol/min/mg protein in rats with compensated and decompensated cirrhosis, respectively, and a similar decrease was observed for CLint. In the bap-treated rats, Vmax of normal animals was 8 times higher (635 ± 168 nmol/min/mg protein) than in the vehicle-treated groups, and a greater inducing effect was again observed in cirrhotic rats. In conclusion, our study clearly show that, at variance with the results of some previous human studies, enzyme induction does not decrease with the decline of liver function. Determination of CYP1A1, CYP1A2 and AhR mRNA and of their protein expression (by Western blot) is in progress in our laboratory to confirm these findings.