In November of 1999, the FDA issued a guidance on the study design, data analysis, and recommendations for dosing and labeling of in vivo metabolic drug interaction studies . The basic concepts that are behind the recommendations in this guidance are as follows:
1. An understanding of the metabolic fate of a drug and the contribution of metabolism to the overall elimination of the drug is essential in the assessment of its safety and efficacy profile.
2. It is important to elucidate whether the investigational drug affects the metabolism of currently marketed drugs and conversely whether the metabolism of the investigational drug is also affected by currently available drugs.
3. Sometimes even though a drug might not be metabolized, it still can be a potent inhibitor of a certain metabolic pathway. Thus it is important to elucidate its effect on the metabolism of currently marketed drugs metabolized by the inhibited enzymes.
4. The clinical importance of a drug interaction sometimes depends on the genetic polymorphism (whether a patient is considered a slow or fast metabolizer) of the individual. Moreover, other covariates such as age, race, and gender can be of prime importance in the clinical outcome of the interaction.
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