S39 Evaluating in vivo esterase-mediated drug metabolism: Should be easy, but why do we fail?

Philip M Potter , St. Jude Children's Research Hospital, Memphis, TN
Carboxylesterases (CE) are ubiquitous enzymes that hydrolyze ester moieties into their respective alcohol and carboxylic acid. To date, no known endogenous substrates have been definitively identified and consequently, they are thought to represent a first line defense towards esterified xenobiotics. This includes numerous clinically used drugs (e.g., irinotecan, oseltamivir, methylphenidate, clopidogrel) where the hydrolysis reaction can either activate or inactive the agent. In humans, two major CE isoforms have been identified, hCE1 (CES1) and hiCE (CES2). Therefore, it should be relatively easy to: determine the enzyme involved in drug hydrolysis; to assess the contribution of each towards in vivo activity; and achieve effective drug dosing by accounting for the metabolic turnover in different tissues. However, since CEs have evolved to hydrolyze substrates with significantly different chemical structures, the plasticity of the enzyme active site results in considerable overlap of hydrolysis of substrates by hCE1 and hiCE. In addition, there are few (if any) animal models that provide informative information with regard to in vivo hydrolysis and drug disposition. This is due to the much more complex esterase genomes present in species typically used for such studies, and the differences in levels and substrate specificity of the enzymes expressed in these models. I will discuss the above issues, with respect to the contribution of CEs towards drug design and metabolism, as well as the development of novel reagents that can be used to evaluate esterified mediated drug hydrolysis, both in vitro and in vivo tools. This will include the design and synthesis of isozyme-specific inhibitors that can be used for reaction profiling, as well as the generation of a plasma esterase-deficient mouse strain that may be more predictive with regard to human pharmacokinetics for ester-containing molecules. Ultimately, these tools should allow for more rational approaches for the development and implementation of esterified agents in the drug discovery/design process.