Selection of Appropriate Experimental Systems

1 In silico Advances continue to be made in the prediction of clearance mechanisms using in silico approaches (Ekins et ah, 2001), although the status of the technology at this time cannot replace the conduct of experiments in the laboratory. 

Perhaps the most important contribution to error in use of expressed cytochrome P450s is in the interpretation and extrapolation of generated data. The increased metabolic rate per milligram protein or per nanomole P450 relative to human liver microsomes can be overinterpreted in some cases where experimental conditions are not appropriately controlled. For example, there is potential for false positive results in some cases, where the rates of metabolic turnover for one enzyme may overestimate the contribution predicted from a single enzyme, based on subsequent data from other experimental systems such as human liver microsomes with selective chemical inhibitors. 

Preferred and accepted inhibitors for P450s (adapted from Tucker 

CYP2C8 Montelukast (0.2 ji,M) Sertraline (also inhibits CYP2D6) 

CYP2C9 Sulfaphenazole (2.5-25 ji,M) Ticlopidine (also inhibits CYP2D6) 

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