Assessing the enzymes responsible for metabolizing a test article can highlight potential liabilities such as metabolism routes catalyzed by single enzymes or by polymorphically-expressed enzymes.

Reaction phenotyping studies assess the impact of enzyme isoform-specific inhibitors on the disappearance of a parent test article incubated in pooled human liver microsomes.Incubation with recombinant individual drug metabolizing enzyme isoforms provides confirmation of enzyme activities.

The investigation of CYP enzymes (CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2D6 and CYP3A) is recommended as an initial investigation to better understand the clearance pathways of the drug according to the drug-drug interaction (DDI) guidelines.

If these major CYP enzymes are not involved in the metabolism of the drug, then other Phase I and Phase II enzymes should be evaluated. Reaction phenotyping provides information regarding the fraction of test articles metabolized and can be used to assess victim DDI potential, inform clinical study design, predict individual variability in pharmacokinetics and evaluate the impact of genetic polymorphism.