Clin Pharmacokinet
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The pharmacokinetics and pharmacodynamics of drugs are significantly altered in the burn patient, and the burn patient population shows wide inter- and intraindividual variation in drug handling. Burn injury evolves in two phases. The first phase corresponds to the burn shock, which occurs during the first 48 hours after thermal injury. ⋯ Drug concentration measurements help to take into account interindividual variability. However, adaptation of doses based on Bayesian methods is frequently not possible because the distribution of pharmacokinetic parameters is poorly characterized in this population. Methods based only on individual data or on a surrogate marker for efficacy may be used to optimize the dosing regimen in this population.
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Despite recent advances in understanding of the role of the gut as a metabolizing organ, recognition of gut wall metabolism and/or other factors contributing to intestinal loss of a compound has been a challenging task due to the lack of well characterized methods to distinguish it from first-pass hepatic extraction. The implications of identifying intestinal loss of a compound in drug discovery and development can be enormous. Physiologically based pharmacokinetic (PBPK) simulations of pharmacokinetic profiles provide a simple, reliable and cost-effective way to understand the mechanisms underlying pharmacokinetic processes. The purpose of this article is to demonstrate the application of PBPK simulations in bringing to light intestinal loss of orally administered drugs, using two example compounds: verapamil and an in-house compound that is no longer in development (referred to as compound A in this article). ⋯ Mechanistic insights provided by PBPK simulations can be very valuable in answering vital questions in drug discovery and development. However, such applications of PBPK models are limited by the lack of accurate inputs for clearance and distribution. This article demonstrates a successful application of PBPK simulations to identify and quantify intestinal loss of two model compounds in rats and humans. The limitation of inaccurate inputs for the clearance and distribution parameters was overcome by optimizing these parameters through fitting intravenous profiles. The study also demonstrated that the large interspecies differences associated with gut wall metabolism and gastric emptying, evident for the compounds studied, make animal model extrapolations to humans unreliable. It is therefore important to do PBPK simulations of human pharmacokinetic profiles to understand the relevance of intestinal loss of a compound in humans.