| Polymorphism and CYP450 To discuss PGx, we must first define two terms - polymorphism and cytochrome P450 (CYP450).A polymorphism is a variation in a gene (allele) that affects at least 1% of the population. CYP450 refers to a family of enzymes found predominantly in the liver. CYP450 enzymes work on a variety of substrates (drugs), altering their chemical structures to facilitate excretion in the urine and feces. There are many known polymorphisms in CYP450 enzymes. | View Page |
| CYP450s Many CYP450 enzymes have been characterized, and the substrates (drugs) that each can recognize have been worked out to a large extent. These subfamilies of CYP450 enzymes have all been associated with polymorphisms that can affect drug disposition: CYP1A2, CYP2C9, CYP2C19 and CYP2D6. | View Page |
| Clinical Utility The ultimate goal in measuring CYP450 function or identifying polymorphisms is to predict effective therapeutic doses and responses in patients.Polymorphisms are identified using molecular techniques (allele-specific PCR, restriction digests, sequencing, hybridization assays, bead-based systems, microarrays, pyrosequencing, et al).Although most clinical labs do not offer PGx testing, reference labs are beginning to market these tests. For example, one reference laboratory in the Midwest that offers CYP2D6 profiling measures about one dozen of the most common and significant mutation sites on this enzyme. This allows for detection of approximately 98% of the known CYP2D6 polymorphisms. The laboratory then generates a report which will advise the physician on the patient's drug-metabolizing status.Estimates show that 6-10% of the general population have a complete deficiency of CYP2D6, with the prevalence of mutations varying from <1% to as much as 21% within a given population. | View Page |
| CYP450 Induction and Inhibition Variables other than mutations also affect CYP450 enzymes. Many drugs are able to induce CYP450 enzymes, and CYP450s can be inhibited by a variety of substances. For example, CYP2D6 can be inhibited by the common medications cimetidine (Tagamet) and fluoxetine (Prozac). Since many patients are on multiple medications and since dietary and environmental factors can change, CYP450 expression levels cannot be solely predicted based on their genotype. Some CYP450 inducers and inhibitors are listed in the table on the following page. | View Page |
| Genotype versus Phenotype Genotyping can give us a definitive profile of a given CYP450 enzyme's mutations. But since there are dozens of mutations usually associated with each enzyme, a complete characterization of a CYP450 is not always realistic. Without complete sequencing of the entire allele, it may not be possible to entirely rule out a mutation in a patient who shows none of the more common polymorphisms. If we consider the number of possible mutations and the possible presence of inducing/inhibiting substances, phenotyping for drug metabolism may sound more reasonable than genotyping. | View Page |
| The Bottom Line By knowing a patient's disposition to specific drugs, the physician should be able to start the patient on an appropriate regimen rather than perfecting treatment based on trial and error. Drugs whose metabolism may prove to be problematic can be avoided, and second-line therapies that are metabolized by different, unaffected enzymes can be chosen. Clinical chemists, pharmacologists, and physicians need to translate knowledge of CYP450 polymorphisms into clinically-validated treatment algorithms. Dosing recommendations for PM, EM, IM and UM patients are beginning to appear in the literature for various classes of drugs, and the FDA is encouraging the incorporation of pharmacogenomic testing in the development process for new drugs. | View Page |
| A patient is taking cimetidine for a stomach ulcer. This drug inhibits CYP2D6. The patient is now prescribed amphetamine for narcolepsy. Amphetamine is metabolized by CYP2D6. What would you predict? | View Page |
| Warfarin cont. The genes involved in warfarin metabolism are CYP2C9 and vitamin K epoxide reductase complex subunit 1 (VKOR). Warfarin owes its anticoagulant action to its inhibition of VKOR. This enzyme recycles vitamin K, a critical element for the clotting factors II, VII, IX, and X, as well as for proteins C, S, and Z. There are six CYP2C9 alleles that are known to cause prolonged metabolism of warfarin: CYP2C9 *2, *3, *4, *5, *6, and *11. (Polymorphisms in CYP450 genes are denoted with asterisks.)One-third of the patients that receive warfarin metabolize it differently than expected and experience a higher risk of bleeding.Genetic testing for the two most common polymorphisms (CYP2C9*2 and *3) as well as for VKOR may be able to reduce the variability associated with warfarin dosing response. Labs performing PGx testing can provide general warfarin dosing recommendations based on the patient's genotype analysis. The lab report will indicate whether a patient has a normal, mild, moderate, high, or very high sensitivity to warfarin. For example, a patient who has one CYP2C9 normal wild-type allele (CYP2C9 *1), one polymorphism (CYP2C9*3), and also a VKOR polymorphism is predicted to have a moderate sensitivity to warfarin. This patient should have frequent INR monitoring and possible warfarin dose reduction. It is important to recognize that knowing a genotype does not necessarily guarantee accurate dose prediction; other drugs and/or environmental or disease factors can also alter CYP2C9 activity. Therefore, monitoring the INR is still very important. | View Page |
| CYP2D6 CYP2D6 has received the most attention: It is estimated that about 25% of common drugs are metabolized by CYP2D6. CYP2D6 accounts for only about 1% of all CYP450 enzymes, but it is important in the metabolism of about 100 drugs. There are more than 80 genetic variants that have been described in the CYP2D6 gene. The normal, wild-type allele displays normal metabolic activity whereas some of the variant forms have enhanced or diminished activity. The variants can be grouped generally according to the resulting alterations in protein function. The groupings correlate with four major enzyme metabolic capacities (phenotypes): poor, intermediate, extensive (normal), or ultra-rapid metabolizers. | View Page |
| CYP450 Induction and Inhibition CYP450 Inhibitor Inducer CYP1A2 Amiodarone Cimetidine Ciprofloxacin Tobacco CYP2C9 Amiodarone Fluvastatin Isoniazid Fluconazole Rifampin Secobarbital CYP2C19 Cimetidine Indomethacin Ketokonazole Prednisone CYP2D6 Celecoxib Cimetidine Cocaine Methadone Pentazocine Imipramine Desipramine Amitriptyline CYP2E1 Disulfiram Fluoxetine Ethanol Isoniazid CYP3A Midazolam Erythromycin Methadone Phenobarbital Dexamethasone Note: This is not an exhaustive listing of inducers and inhibitors.Reference: Tanaka E, Terada M, Misawa S. Cytochrome P450 2E1: it's clinical and toxicological role. J Clin Pharm Ther. 2000 Jun;25(3):165-75. | View Page |
| Genotype versus Phenotype Phenotyping involves measuring the metabolism of a probe drug. For example, with CYP2D6, dextromethorphan or debrisoquine can be given to a patient to see how well the drug is metabolized. Both these drugs are safe and extensively metabolized by CYP2D6. By measuring the parent drug and the metabolite in urine, the metabolic capacity of a CYP450 enzyme can be estimated. Such testing is complex and tedious, however, and has not become routine in clinical laboratories. Therefore, genotyping is likely to be the main tool that is used for assessing the PGx of a patient. | View Page |