Clinical studies of CYP3A4 and P-glycoprotein

Abstract: In the field of pharmacokinetics, cytochrome P450 enzymes have been extensively studied. The CYP450 enzyme with the most drug substrates is CYP3A4, but despite its significance, administration of a specific substrate to subjects is required to determine its activity, by measuring the metabolic ratio. For reasons of feasibility and safety it would be preferable to have an endogenous biomarker instead of a probe drug. The activity of CYP3A4 varies considerably as it can be induced and inhibited by certain compounds. For example, it is induced by ligands to the Pregnane X receptor (PxR). Activation of this receptor also induces the drug transporter P-glycoprotein. Regarding genetic variability, it is well known that CYP3A4 does not display genetic polymorphism in activity, but results from studies of P-gp in that aspect have been inconclusive. In the first study of this thesis, which was also aiming to evaluate the inductive property of rifampicin on four other CYP450 enzymes, a probe drug and an endogenous biomarker were compared regarding their properties for measuring CYP3A4 activity. Induction was achieved by administration of rifampicin in three different doses (20-500 mg once daily) to healthy volunteers. The endogenous biomarker 4β-hydroxycholesterol had a linear relationship with the metabolic ratio of the CYP3A4 probe drug quinine. In this study four other probe drugs were also used simultaneously, each specific for a different CYP450 enzyme. All enzymes except CYP2D6 were induced by rifampicin. This cocktail had been designed not to cause any drug-drug interactions among the probes, which are also specific for each enzyme. In the second study, 4β-hydroxycholesterol was used to investigate whether exposure to triclosan in the doses reached by normal use of toothpaste can induce the enzyme. Triclosan activates PxR in vitro, but the results from our clinical study indicate that this effect is not relevant when subjects are exposed in the dose range achieved by normal use of toothpaste. In the third clinical study two common haplotypes of MDR1, which encode P-gp, were compared by phenotyping with the substrate digoxin. The haplotypes were chosen because they are common and because their clinical relevance has been under discussion for many years. Digoxin is the most commonly used drug for investigating P-gp activity as it is not metabolized and because it is a known substrate of this transporter. The difference in plasma digoxin kinetics was not statistically significant in our study and the study does not support that genetic variability in MDR1 has an impact on P-gp activity in the haplotypes investigated. Ligands to PxR also induce the drug transporter P-glycoprotein, which has largely overlapping substrate specificity with CYP3A4. Drug transporters have been considerably less studied than CYP450 enzymes but the knowledge is rapidly increasing and the focus has been mainly on P-gp because of its broad specificity. Even though CYP3A4 does not have polymorphic activity, it is well known that some CYP450 enzymes do, and the data on whether P-gp has such variability has been inconclusive.