Identification of Metabolising EnzymesIn vitro Metabolism
• Liver microsomes and CYP-selective inhibitors
• Recombinant CYP, FMO, MAO, CE, AOX, UGT, SULT and NAT
• UPLC with Q-TOF-MS or QE-Orbitrap-MS
Information on the enzymes involved in the metabolism of a new chemical entity is needed to evaluate the drug interaction vulnerability of the compound as a “victim-drug”. The identification of metabolising enzymes is also needed to evaluate the risk for highly variable clearance between individuals or populations due to the involvement of enzymes with high degree of polymorphism. A thorough evaluation of metabolising enzymes also requires metabolite identification.
The most common approach for enzyme identification is the use of recombinant enzymes and extrapolation of the results based on the in vivo protein levels, and the use of liver microsomal incubations together with cytochrome P450 (CYP)-selective inhibitors under linear metabolite formation kinetics. In both methods, monitoring of metabolite formation is clearly more reliable than monitoring substrate depletion only, although the latter is often used for high throughput evaluation. The clever use of HR-MS technology allows metabolite monitoring later on from the very same LC/MS data, even if only the substrate depletion would have been analysed in the first place. For more thorough evaluation of in vivo role of each enzyme, evaluation of kinetic parameters Vmax and Km with different enzymes would be advisable.