Cytochrome P450 2C19 (CYP2C19) is the main (or partial) cause for large differences in the pharmacokinetics of a number of clinically important drugs. On the basis of their ability to metabolise (S)-mephenytoin or other CYP2C19 substrates, individuals can be classified as extensive metabolisers (EMs) or poor metabolisers (PMs). Eight variant alleles (CYP2C19*2 to CYP2C19*8) that predict PMs have been identified. The distribution of EM and PM genotypes and phenotypes shows wide interethnic differences. Nongenetic factors such as enzyme inhibition and induction, old age and liver cirrhosis can also modulate CYP2C19 activity. In EMs, approximately 80% of doses of the proton pump inhibitors (PPIs) omeprazole, lansoprazole and pantoprazole seem to be cleared by CYP2C19, whereas CYP3A is more important in PMs. Five-fold higher exposure to these drugs is observed in PMs than in EMs of CYP2C19, and further increases occur during inhibition of CYP3A-catalysed alternative metabolic pathways in PMs. As a result, PMs of CYP2C19 experience more effective acid suppression and better healing of duodenal and gastric ulcers during treatment with omeprazole and lansoprazole compared with EMs. The pharmacoeconomic value of CYP2C19 genotyping remains unclear. Our calculations suggest that genotyping for CYP2C19 could save approximately 5000 US dollars for every 100 Asians tested, but none for Caucasian patients. Nevertheless, genotyping for the common alleles of CYP2C19 before initiating PPIs for the treatment of reflux disease and H. pylori infection is a cost effective tool to determine appropriate duration of treatment and dosage regimens. Altered CYP2C19 activity does not seem to increase the risk for adverse drug reactions/interactions of PPIs. Phenytoin plasma concentrations and toxicity have been shown to increase in patients taking inhibitors of CYP2C19 or who have variant alleles and, because of its narrow therapeutic range, genotyping of CYP2C19 in addition to CYP2C9 may be needed to optimise the dosage of phenytoin. Increased risk of toxicity of tricyclic antidepressants is likely in patients whose CYP2C19 and/or CYP2D6 activities are diminished. CYP2C19 is a major enzyme in proguanil activation to cycloguanil, but there are no clinical data that suggest that PMs of CYP2C19 are at a greater risk for failure of malaria prophylaxis or treatment. Diazepam clearance is clearly diminished in PMs or when inhibitors of CYP2C19 are coprescribed, but the clinical consequences are generally minimal. Finally, many studies have attempted to identify relationships between CYP2C19 genotype and phenotype and susceptibility to xenobiotic-induced disease, but none of these are compelling.
"D. Yu et al. / Biochemical Pharmacology xxx (2015) xxx–xxx 5 negatively regulating the expression of CYP2C19 in liver cells and elucidated the underlying biological mechanism. Since CYP2C19 is involved in the biotransformation of many clinical drugs and environmental toxicants  "
"insufficient for identifying all poor metabolizers (PMs) of CYP2C19 in the general population. Conventional in vivo CYP2C19 phenotype tests (e.g., S-mephenytoin 4-hydroxylation or omeprazole 5-hydroxylation) are attractive tools because they can capture changes in CYP2C19 activity caused by both genetic and nongenetic factors (Desta et al., 2002). However, their routine clinical use has been limited because these procedures are time and resource intensive and invasive. "
[Show abstract][Hide abstract] ABSTRACT: The phenotype pantoprazole-<13>C breath test (Ptz-BT) was used to evaluate the extent of phenoconversion of cytochrome P450 2C19 (CYP2C19) caused by commonly prescribed proton pump inhibitors (PPI) omeprazole and esomprazole. A single center open label three visit study with 26 healthy volunteers and 8 stable cardiovascular patients was conducted from March 2014 to December 2014 in an institutional review board (IRB) approved study. The Ptz-BT was administered to each volunteer/patient on their first two visits two weeks apart to ascertain the baseline CYP2C19 enzyme activity using the biomarker 13CO2 in breath. After their second visit the volunteers/patients took either omeprazole or esomeprazole for 28 days, and the Ptz-BT was administered on their third visit to evaluate changes in CYP2C19 phenotype. Phenotype conversion resulted in 25 of 26 (96%) non-poor metabolizer (PM) volunteers as measured by the Ptz-BT both before and after PPI therapy. In our study population the incidence of PM status by phenotype following administration of omeprazole/esomeprazole (known inhibitors of CYP2C19) was 10-fold higher than those who are genetically PMs in the general population which could have critical clinical implications for personalizing medications primarily metabolized by CYP2C19 such as clopidogrel, PPI, cyclophosphamide, thalidomide, citalopram, clonazepam, diazepam, phenytoin etc. The Ptz-BT can rapidly (30 min) evaluate CYP2C19 phenotype and, more importantly, can identify patients with phenoconversion in CYP2C19 enzyme activity caused by non-genetic factors such as concomitant drugs.
The American Society for Pharmacology and Experimental Therapeutics.
Journal of Pharmacology and Experimental Therapeutics 07/2015; 354(3). DOI:10.1124/jpet.115.225680 · 3.97 Impact Factor
"Relative risk of developing MI and stroke between patients with and without variant CYP2C19 were estimated to be 1.34 and 3.66, respectively.3,19 The prevalence of CYP2C19 polymorphism varies between races24, and thus a weighted average of 30.54% was estimated for the overall patients and used in the model.25 "
[Show abstract][Hide abstract] ABSTRACT: Prasugrel is recommended over clopidogrel in poor/intermediate CYP2C19 metabolizers with acute coronary syndrome (ACS) and planned percutaneous coronary intervention (PCI), reducing the risk of ischemic events. CYP2C19 genetic testing can guide antiplatelet therapy in ACS patients.
Pharmacy Practice 07/2014; 12(3):438. DOI:10.4321/S1886-36552014000300007
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