Human liver microsomes batches

An alternative approach to normalizing rates of drug metabolism by recombinant CYP enzymes is the application of a relative activity factor (RAF), in which the correction is not based on specific content but on specific activity, which requires a comparison of the rate of metabolism of a selective marker substrate by each recombinant CYP enzyme and human liver microsomes (75,194). The RAF is then multiplied by the observed rates of drug metabolism by each recombinant CYP enzyme before assessing the relative contribution of each enzyme to the metabolism of the drug. This approach has not been well validated. For example, it is not known whether the relative activity factor remains constant for several marker substrate reactions catalyzed by the same CYP enzyme. If the relative activity factor varies in a substrate-dependent manner, it would be difficult to know which RAF value to apply to the drug candidate under investigation. Another limitation of this approach is that the relative activity factor must be empirically determined for each lot of recombinant CYP enzyme (and preferably each batch of pooled human liver microsomes). 

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