Reaction phenotyping is used to identify the major enzyme(s) involved in the metabolism of a drug. This information allows for determination of fm, which leads to further understanding of possible interpatient pharmacokinetic variability and/or the potential for drug-drug interactions when the drug is being considered as a victim. ultimately, results from human radiolabelled mass balance studies are needed to complement in vitro reaction phenotyping studies to allow for a true determination of fm for each metabolic pathway.
Note: When to perform reaction phenotyping:
1. Metabolism has to be a major route of elimination of the drug from the body. This can be inferred from preclinical studies, based on when unchanged drug in the excreta is determined to be minimal.
2. The major metabolites in vivo are known. This is because the right enzyme system is studied in vitro. For example, if the primary route of metabolism is via glucuronidation of the unchanged drug, P450 reaction phenotyping may not be very useful.
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