Obesity has increased markedly over the last few decades and is now a major public health crisis in the U.S. affecting over 1/3 of the US population. Optimization of dosing in obese individuals is a challenge due to the lack of knowledge regarding changes in the pharmacokinetics (PK) of therapeutic agents in obese individuals. Thus the aim of this thesis was to determine the effect of obesity on drug metabolism and evaluate methods that could potentially predict changes in pharmacokinetics in the obese population.
The impact of obesity on drug metabolism in children has not been determined and our clinical study (Chapter 2) was the first of its kind to examine the effect of childhood obesity on CYP1A2, CYP2D6, CYP3A4, xanthine oxidase, and NAT2 activity using caffeine and dextromethorphan as probe drugs. Our results conclusively indicate that obesity results in an elevation of xanthine oxidase and NAT2 enzyme activities in obese children as compared to lean children, whereas there was no difference in CYP1A2, CYP2D6 and CYP3A4 activity between obese and lean children. This study provides a potential mechanism of altered 6-mercaptopurine exposure in the obese pediatric cancer population.
While clinical studies would provide the most optimum method to predict clearance of therapeutic agents in humans, studies have reported that clearance can also be predicted using animal data. In Chapter 3, we examined mouse, rat and porcine model of obesity in order to determine the utility of these animal models to predict PK in obese humans and to identify a model that would best reflect the human obesity mediated changes in drug metabolism. The study indicated species dependent differences in CLint of various drugs that were due to, either changes in expression of drug metabolizing enzymes or changes in enzyme substrate affinity. The study demonstrated that obesity can alter enzyme activity in a species and model dependent manner. Furthermore this study identified that the rat High Fat Diet animal model of obesity is the best representation of the obesity mediated alterations in humans.
In Chapter 4, in collaboration with Drs. Scott Rector and Jim Perfield, University of Missouri, Columbia, we demonstrated obesity mediated alterations of drug metabolism enzyme activity can be prevented by sterculic oil dietary supplementation. These effects are mediated through signal transduction pathways which regulate CAR and PXR transcription factors. These results establish that obesity mediated changes are biochemically dependent and not weight dependent.
In Chapter 5, we developed a proof of concept that would help generate biochemically obese hepatocytes. In absence of hepatocytes from obese individuals, these hepatocytes can be used as a tool to predict obesity mediated changes in drug clearance. Our studies indicate that individually, leptin, resistin, IL-6 and TNF-α can modulate expression of various DMEs in a concentration dependent and isoform specific manner. This study demonstrates that the obesity microenvironment is important in obesity mediated changes in drug metabolism. Additional studies would help establish a more robust method to generate and validate these obese hepatocytes.
In summary, the work in this thesis has helped identify the drug metabolism enzymes that are altered in the obese children, the utility of using animal models of obesity as tools to study the impact of obesity on pharmacokinetics/pharmacodynamics, proven that it is possible to reverse obesity mediated changes in drug metabolism and developed an in vitro model that may be used to predict changes in drug disposition in the obese population. These findings are important for to better develop dosing strategies in obese humans with concomitant disease.