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The cytochrome P450 (CYP450) enzyme system consists of complex mono-oxygenases that are involved in the biosynthesis of steroids, fatty acids, and bile acids, in addition to performing oxidative metabolism of endogenous and exogenous substrates such as toxins and drugs. Concentrated in liver cells, but also found in extrahepatic tissues such as the intestinal epithelium, they modify the chemical structure of drugs shortly after they are absorbed from the gastrointestinal tract. The chemical modification renders them more water soluble and hence more easily excreted. There are more than 50 CYP450 enzymes, of which some are especially clinically important. Although opinions vary as to which are most important, CYP2C9, CYP2C19, CYP2D6, CYP3A, and CYP1A2 are commonly listed.
CYP450 Enzymes and Drug–Drug Interactions
Since a patient can take several concurrent medications that are metabolized by the same CYP450 enzyme, it should come as no surprise that CYP450-related drug–drug interactions are common. Clinicians are generally aware of these interactions, which either involve competition between two or more medications for the same enzyme, or an increase in the activity of a particular CYP enzyme in response to a particular medication. In the former scenario, blood concentrations and the risk of side effects can increase to dangerous levels; in the latter, concentrations can decline to the point that therapeutic value is lost.
CYP450 Enzymes and Metabolic Capabilities
Some CYP450 enzyme genes are highly polymorphic, resulting in enzyme variants that have different metabolic capabilities. Patients can be classified as poor, intermediate, extensive (normal), or ultrarapid metabolizers. Poor metabolizers lack active gene enzyme alleles, intermediate have one active and one inactive allele, and ultrarapid have more than two copies of an active gene allele. These variants are relatively common, with prevalences that vary across different races. Patients are at risk for undesirable blood concentrations—poor metabolizers may develop serious side effects, and ultrarapid metabolizers may be insufficiently treated. For prodrugs such as clopidogrel (Plavix®, Bristol-Myers Squibb, New York, NY) that must be metabolized to become pharmacologically active, the converse is true—ultrarapid metabolizers may develop dangerous side effects, and poor metabolizers may not achieve therapeutic responses.
There are now a few commercially available genotyping tests for some CYP450 enzyme genes.
- Roche Molecular Diagnostics (Pleasanton, CA) offers the FDA-cleared, CE-marked AmpliChip® CYP450 Test, which uses a DNA microarray based on Affymetrix (Santa Clara, CA) technology. The test assays DNA purified from human blood to genotype two important CYP subtypes. Proprietary software generates customized reports with genotype and predicted phenotype information. The assay detects up to 33 CYP2D6 and 3 CYP2C19 alleles. The CYP2D6 enzyme metabolizes a number of medications, including antidepressants, antipsychotics, antiarrhythmics, and beta-blockers. The CYP2C19 enzyme metabolizes anticoagulants such as clopidogrel, anticonvulsants, proton pump inhibitors, antipsychotics, antimalarials, and other medications.
The assay is run on the Affymetrix GeneChip® System 3000Dx, a dedicated microarray system that supports the AmpliChip CYP450 Test. The system includes the Fluidics Station 450Dx, which automates hybridization, washing, and staining of the array; the GCS 3000Dx Scanner with AutoLoader Dx, which allows 48 microarrays to be scanned without operator intervention; and a Workstation with GCOS Dx software. - Nanosphere (Northbrook, IL) offers the FDA-cleared Verigene® CYP2C19 Test, which identifies the CYP2C19*2, *3, and *17 genotypes directly from a whole blood sample in less than 2.5 hr. The CYP2C19*2, *3 variants account for reduced drug metabolism and the CYP2C19*17 variant is associated with increased drug metabolism. The test has a CLIA designation of moderate complexity. With the sample-to-result Verigene system instruments, users can process tests on-demand using Verigene test cartridges, without the need for batch processing, specially trained personnel, or specialized facilities. The instruments consist of the Verigene Reader and the Verigen Processor SP, a modular benchtop analyzer that combines automated nucleic acid extraction, purification, amplification (if needed), and hybridization in each independent module. The work flow consists of loading the test cartridge, consumables, and the patient’s blood sample into the Processor SP, followed by automated sample preparation and processing; and finally placement of the slide from the test cartridge into the Verigene Reader for the results.
The Verigene Reader manages sample information, reads results from the processed cartridges, prints the results, and allows internal data storage and LIS connectivity without the need for an external PC. Because one Verigene Reader can control multiple processors, throughput can be customized to the size and demands of each laboratory. Verigene Processor SP’s can be stacked to minimize the benchtop footprint. - AutoGenomics (Vista, CA) offers the FDA-cleared INFINITI® CYP2C19 Assay, which is indicated for use as an aid to clinicians in deciding on the therapeutic strategy for drugs that are metabolized by the CYP2C19*2, *3, and *17 gene products. The assay analyzes DNA from whole blood, is available in Single and Quad formats, and uses the CYP2C19 Intellipac® Reagent Module, CYP2C19 Amp Mix, and CYP2C19 BioFilmChip® Microarray. It is automated by the INFINITI Analyzer, a multiplexing, continuous flow, random access microarray platform that integrates all the discrete test processes for the analysis of DNA, such as sample handling, reagent management, hybridization, and stringency and detection, into a totally self-contained system.
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