Abstract
1. The purpose of this study was to evaluate drug clearance measured by the metabolic intrinsic clearance (CLint) in a substrate depletion assay in comparison with the in vivo clearance (CLtot) observed in adjuvant-induced arthritis (AA) rats. 2. After intravenous administration of diclofenac as a model drug, CLtot was 2.8-fold higher in AA rats than in control rats. In two different substrate depletion assays with liver microsomes for glucuronidation and hydroxylation, the CLint values for glucuronidation was significantly decreased in AA rats to 60% of the value in control rats, whereas the CLint values for hydroxylation were similar. The unbound fraction of diclofenac in plasma (fu, plasma) was significantly higher (2.8-fold) in AA rats than in control rats. 3. Hepatic clearance predicted from the CLint values for both biotransformation pathways and fu, plasma was higher in AA rats than in control rats, with good consistency between predicted and observed values. The same results were obtained for experiments using hepatocytes. 4. The plasma protein-binding activities, rather than metabolic clearance, in both types of rats would be a determining factor in the pharmacokinetic behaviour differences between control and AA rats. 5. In summary, substrate depletion assays with liver microsomes and hepatocytes in combination with protein binding assessment can help to predict changes in pharmacokinetics under AA conditions.