Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro–in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Inhibition and induction of human cytochrome P450 enzymes: current status.

Phenotyping for drug metabolizing enzymes and transporters is used to assess quantitatively the effect of an intervention (e.g., drug therapy, diet) or a condition (e.g., genetic polymorphism, disease) on their activity. Appropriate selection of test drug and metric is essential to obtain results applicable for other substrates of the respective enzyme/transporter. The following phenotyping metrics are recommended based on the level of validation and on practicability: CYP1A2, paraxanthine/caffeine in plasma 6 h after 150 mg caffeine; CYP2C9, tolbutamide plasma concentration 24 h after 125 mg tolbutamide; CYP2C19, urinary excretion of 4'-OH-mephenytoin 0-12 h after 50 mg mephenytoin; CYP2D6, urinary molar ratio debrisoquine/4-OH-debrisoquine 0-8 h after 10 mg debrisoquine; and CYP3A4, plasma clearance of midazolam after 2 mg midazolam (all drugs given orally).

Appropriate phenotyping procedures for drug metabolizing enzymes and transporters in humans and their simultaneous use in the "cocktail" approach.

Human cytochrome P450 (CYP) 3A subfamily members (mainly CYP3A4 and CYP3A5) mediate the metabolism of approximately half all marketed drugs and thus play a critical role in the drug metabolism A huge number of studies on CYP3A-mediated drug metabolism in humans have demonstrated that CYP3A activity exhibits marked ethnic and individual variability, in part because of altered levels of CYP3A4 expression by various environmental factors and functionally important polymorphisms present in CYP3A5 gene Accumulating evidence has revealed that CYP3A4 and CYP3A5 have a significant overlapping in their substrate specificity, inducers and inhibitors Therefore, it is difficult to define their respective contribution to drug metabolism and drug-drug interactions Furthermore, P-glycoprotein and CYP3A are frequently co-expressed in the same cells and share a large number of substrates and modulators The disposition of such drugs is thus affected by both metabolism and transport In this review, we systematically summarized the frequently used CYP3A probe drugs, inducers and inhibitors, and evaluated their current status in drug development and research

Drugs as CYP3A probes, inducers, and inhibitors.

CYP3A4 is the most important drug metabolizing enzyme in adult humans because of its prominent expression in liver and gut and because of its broad substrate specificity, which includes drugs from most therapeutic categories and many endogenous substances. Expression and function of CYP3A4 vary extensively both intra- and interindividually thus contributing to unpredictable drug response and toxicity. A multitude of environmental, genetic, and physiological factors are known to influence CYP3A4 expression and activity. Among the best predictable sources of variation are drug-drug interactions, which are either caused by pregnane X-receptor (PXR), constitutive androstane receptor (CAR) mediated gene induction, or by inhibition through coadministered drugs or other chemicals, including also plant and food ingredients. Among physiological and pathophysiological factors are hormonal status, age, and gender, the latter of which was shown to result in higher levels in females compared to males, as well as inflammatory processes that downregulate CYP3A4 transcription. Despite the influence of these non-genetic factors, the genetic influence on CYP3A4 activity was estimated in previous twin studies and using information on repeated drug administration to account for 66% up to 88% of the interindividual variation. Although many single nucleotide polymorphisms (SNPs) within the CYP3A locus have been identified, genetic association studies have so far failed to explain a major part of the phenotypic variability. The term "missing heritability" has been used to denominate the gap between expected and known genetic contribution, e.g., for complex diseases, and is also used here in analogy. In this review we summarize CYP3A4 pharmacogenetics/genomics from the early inheritance estimations up to the most recent genetic and clinical studies, including new findings about SNPs in CYP3A4 (*22) and other genes (P450 oxidoreductase (POR), peroxisome proliferator-activated receptor alpha (PPARA)) with possible contribution to CYP3A4 variable expression.

/pdf/pharmacogenomics-of-cytochrome-p450-3a4-recent-progress-efiofvrt9u.pdf

Pharmacogenomics of Cytochrome P450 3A4: Recent Progress Toward the "Missing Heritability" Problem.

We have proposed that 4β-hydroxycholesterol (4β-OHC) may be used as an endogenous marker of CYP3A activity. The cholesterol metabolite 4β-OHC is formed by CYP3A4. Treatment of patients with strong inducers of CYP3A enzymes, e.g. anti-epileptic drugs, resulted in 10-fold increased concentrations of plasma 4β-OHC, while treatment with CYP3A inhibitors such as ritonavir or itraconazole resulted in decreased plasma concentrations. There was a relationship between the 4β-OHC concentration and the number of active CYP3A5*1 alleles showing that 4β-OHC was not only formed by CYP3A4, but also by CYP3A5. The concentration of 4β-OHC was higher in women than in men, confirming previous studies indicating a gender difference in CYP3A4/5-activity. The rate of elimination of 4β-OHC is slow (half-life 17 days) which results in stable plasma concentrations within individuals, but limits its use to study rapid changes in CYP3A activity. In short-term studies exogenous markers such as midazolam or quinine may be superior, but in long-term studies 4β-OHC is a sensitive marker of CYP3A activity, especially to assess induction but also inhibition. Under conditions where the cholesterol concentration is changing, the ratio of 4β-OHC : cholesterol may be used as an alternative to 4β-OHC itself. The use of an endogenous CYP3A marker has obvious advantages and may be of value both during drug development and for monitoring CYP3A activity in patients.

https://bpspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2125.2010.03773.x

4β-hydroxycholesterol, an endogenous marker of CYP3A4/5 activity in humans

6β-Hydroxycortisol (6β-OHF) urinary excretion has, for a long time, been considered a marker of drug induction and, more recently, of drug inhibition in humans and in laboratory animals, but its specificity is still under debate. In this work, we review 277 papers devoted to 6β-OHF urinary excretion. We have evaluated factors that could modify 6β-OHF excretion and, thus, could explain contradictory results. We have examined the effect of the analytical techniques on physiological values. Intra- and inter-individual variability and the effect of circadian rhythms on urinary excretion of 6β-OHF as well as cortisol and 17-hydroxycorticosteroids have been evaluated. We also give an overview of drugs that induce, inhibit or have no effect on 6β-OHF. For inducing and inhibiting drugs, we calculated the ranges of variation of 6β-OHF excretion from the results indicated in the different papers. This work was done for well-known inducers, such as anticonvulsants, but also for other inducing or inhibiting drugs found in the literature. The time-course of variation in 6β-OHF excretion when different drugs are co-administered was also investigated. The potential relationship between cytochrome P
 450 3A4 (CYP3A4) polymorphism and 6β-OHF excretion was studied. Finally, the interest of 6β-OHF urinary excretion was compared with that of other tests proposed to measure CYP3A4 activity. This review demonstrates that 6β-OHF urinary excretion is a good test to evaluate drug-metabolising enzyme inducing or inhibiting properties of drugs when the subjects are their own controls, but this test is not reliable enough to measure actual CYP3A4 activity.

M. M. Galteau

Papers

Urinary 6β-hydroxycortisol: a validated test for evaluating drug induction or drug inhibition mediated through CYP3A in humans and in animals