CPSA Shanghai 2011
Changing Paradigm in Drug Discovery & Development:
East Meets West
April 13 - 16, 2011
Renaissance Shanghai Pudong Hotel
Shanghai, China
Abstract
Thursday Evening - Keynote
Stable and Reactive Metabolites in Drug Research
R. Scott Obach
Pharmacokinetics, Pharmacodynamics, and Drug Metabolism
Pfizer Inc., Groton, CT, USA
In almost all instances, when a drug is administered the patient is exposed not only to the parent drug, but to metabolites as well. The metabolites can potentially have implications for safety and efficacy.
Regarding safety, there is considerable interest in knowledge of human metabolite profiles of new chemical entities and comparison of these profiles to those in animal toxicology species. This interest has ultimately resulted in the release of regulatory guidance that defines the determinants of which metabolites merit a closer consideration and what assurances are needed to demonstrate that the metabolite has been adequately assessed for risk. However, examples of circulating chemically stable metabolites causing a toxicity unrelated to effects already possessed by the parent compound are unknown. Gaining a profile of circulating metabolites generally requires a radiolabelled study and careful interpretation of data must be done to ensure that abundant metabolites are addressed while low concentration metabolites are not unnecessarily scrutinized. Less definitive approaches that can be used to address whether human circulating metabolites are present in animals will be illustrated. Chemically reactive metabolites which react with tissue macromolecular nucleophiles can be responsible for toxicities that are unrelated to the target effect of the parent compound. To address the possibility that a new compound could generate a chemically reactive metabolite, in vitro assays are used. However, while such assays can raise the possibility that a compound is capable of being bioactivated, when used alone they are a poor indicator of the potential for toxicity. Other factors must be considered.
Regarding efficacy, since metabolites frequently bear close structural and physicochemical resemblance to the parent drug, it is not uncommon for them to bind to the same protein target as the parent was designed to bind with similar potency. Thus, such pharmacologically active metabolites can contribute to the efficacy (or even safety concerns) associated with the target. Early knowledge of active metabolites and their dispositional profiles is critical to an understanding of concentration-effect relationships. A process that is used to identify active metabolites and follow up on them to determine their true contribution to in vivo activity will be described.