Although cytochrome P450 (CYP)-mediated metabolism continues to be of major importance for a large proportion of small molecule new drug candidates, various methods in recent decades have accelerated the development of drug candidates with significant non CYP-mediated metabolism. Increased interest in these molecules stems from the efforts to develop drug candidates possibly lacking CYP metabolism liability from drug interactions and toxicity standpoints. Regulatory agencies still have a safety interest in understanding the biochemical pathways involved in the metabolism of these drugs.

In general, uridine diphosphate glucuronosyltransferases (UGTs) are the most common enzymes involved in metabolism after CYP enzymes, followed by numerous hydrolases, carbonyl reductases, aldehyde oxidase and other enzymes. Reaction phenotyping of many of these enzymes is not as straightforward as with CYPs due to their inherent diversity and limited research tools available. With UGTs, commercially available recombinant human preparations are available for the most relevant enzymes, although selective inhibitors may be lacking. In the case of aldehyde oxidase, a recombinant human enzyme and relatively selective inhibitors are available. For hydrolases, carbonyl reductases and other enzymes such as FMO, strategies may rely on the use of various subcellular fractions, plasma, and inhibitors to elucidate a plurality of the enzymes that may contribute to drug metabolism. The complexity of elucidation of non CYP-mediated metabolism is reflected in regulatory safety requirements.