- Assess the likelihood of a reduced or enhanced response to drugs that are metabolized by CYP3A4 and CYP3A5. Some opioids are among the drugs that are metabolized by CYP3A4 and CYP3A5.
- Determine the genotype of individuals with a personal or family history of adverse drug reactions to medications metabolized by CYP3A4 and CYP3A5.
CYP3A4 and CYP3A5 are closely related and act on many of the same drugs. CYP3A4 plus CYP3A5 comprise approximately 30% of the total cytochrome enzymes in the liver. Both are also present in intestinal mucosa. Genetic variations in CYP3A4 and CYP3A5 may cause dramatic differences in enzyme activity between individuals. These genetic variations cause differences in metabolism of substrates of CYP3A4 and CYP3A5, which includes many opioid drugs. Such differences in metabolism can cause toxic effects or loss of therapeutic efficacy and make drug choice and dose optimization challenging.
CYP3A4 acts on over 40% of the drugs approved for use in the USA. CYP3A4 also has a role in metabolizing some steroids and some carcinogens. CYP3A4 is a protein of 502 amino acids with a molecular weight of 57,290. It is membrane bound in endoplasmic reticulum of liver and other tissues.
CYP3A5 contribution to drug metabolism has been reported to vary from 6 – 99% of the total CYP3A activity. Much of the variability of CYP3A5 activity is due to genetics. CYP3A5 metabolizes a number of opioids, other medications, steroids, cholesterol and some lipids.
The CYP3A4 gene is located on chromosome 7. The gene is 27,592 bp long, and has 13 exons. A number of single nucleotide polymorphisms (SNPs) of this gene are known. A number of these variants affect the activity of the protein produced by the CYP3A4 gene. The *2 and *17 SNPs are on exon 7, the *11 and *12 SNPs are on exon 11 of the CYP3A4 gene.
The CYP3A5 gene is located on the long (q) arm of chromosome 7 from base pair 99,648,189 to base pair 99,679,997. The CYP3A5*3 allele is the most frequent and has a substitution of a G for an A at bp position 6986.
The phenotype frequency is not currently well characterized.
These assays were developed using CLSI guidelines. Control DNA samples of known genotype are tested together with each patient sample to ensure correct results. Genomic DNA is extracted from the submitted buccal swab sample and subjected to real time polymerase chain reaction (real time PCR). The following CYP3A4 alleles were tested: *2, *11, *12, *17. In the CYP3A5 test the *3 allele was tested. The wild type or normal allele (*1) was assigned by default if none of the variant alleles were detected.
- Buccal swab
- Buccal swab:
i. Physical damage
ii. Specimen appears to have microbial contamination or other visible contamination
iii. The name on the tube does not match the name on the paperwork.
iv. It is older than 10 days.
There are 3 phenotypic categories for CYP3A4 and for CYP3A5: Normal (extensive) Metabolizer, Intermediate Metabolizer, and Poor Metabolizer.
Note that the metabolic capacity of CYP3A4 and CYP3A5 is also influenced by concomitant medications and dietary components that may inhibit or induce these enzymes. In addition concomitant medications that are CYP3A4 and CYP3A5 substrates and disease status may affect metabolic capacity. All of these factors must be considered to determine drug choice and dose for the best efficacy and safety.
The detection of genetic variants does not replace the need for therapeutic drug monitoring or other appropriate clinical monitoring by the health care provider. Additional mutations for the tested genes that are not described in the methodology section will not be detected. CYP3A4 and CYP3A5 metabolism is also influenced by concomitant medications, inhibitors, inducers, diet and disease states. These tests were developed and the performance characteristics were determined by MDL. The CYP3A4 and CYP3A5 tests have not been cleared or approved by the US Food and Drug Administration. The FDA has determined that such approval is not necessary.
These tests are approved for use on New York state residents.
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