The introduction of in vitro tools to predict clearance in the early discovery process has led to new ways of working. Combined with metabolite identification, such tools have allowed design of compounds with low clearance. Encouraged by the success of such an approach and by the better knowledge of the enzyme involved in the metabolism, in vitro teams have begun to develop a plethora of assays to assess the metabolic clearance, understand the route of metabolism, and predict the human clearance. Although the diversity of assays may have allowed a more thorough approach to addressing specific issues, in the time of budget constrictions, limited access to resources and materials in vitro teams have now to decide what are the ‘must have’ and ‘nice to have’ assays to enable them to help as efficiently as possible projects at the discovery stage. Reducing the number of assays and focusing on the most relevant ones is an option to consider. Knowledge of the main enzymes involved in the drug metabolism should help to select the most relevant in vitro tools. Although the systems presented here have their merits, the author proposes that hepatocytes should be considered as the in vitro tool of choice.
Keywords: In vitro drug metabolism, P450, UGT, GST, SULT, NAT, FMO, hepatocytes, microsomes, fraction S9
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