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Test ID: 3A4B    
Cytochrome P450 3A4 Genotype, Blood

Useful For Suggests clinical disorders or settings where the test may be helpful

Aids in determining therapeutic strategies for drugs that are metabolized by CYP3A4, including atorvastatin, simvastatin, and lovastatin

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

The cytochrome P450 (CYP) 3A4 enzyme is responsible for the metabolism of approximately 50% of drugs that undergo hepatic metabolism and first-pass metabolism in intestinal epithelial cells, including lipid-lowering drugs. The CYP3A4 enzyme activity is highly variable.(1) While polymorphisms and mutations have been described for the CYP3A4 gene, they do not explain the highly variable enzymatic activity of the encoded protein.(2) A CYP3A4 intron 6 polymorphism, CYP3A4*22 (c.522-191C>T), affects hepatic expression of CYP3A4 and response to statin drugs. The CYP3A4*22 allele is associated with reduced CYP3A4 activity, resulting in a better response to lipid-lowering drugs, such as simvastatin, atorvastatin, or lovastatin. Studies show that in livers with the wild-type genotype (CC) the CYP3A4 mRNA level and enzyme activity were 1.7- and 2.5-fold greater than in heterozygous CYP3A4*22 (CT) and homozygous CYP3A4*22 (TT) carriers, respectively. In 235 patients taking stable doses of drugs for lipid control, carriers of the T allele required significantly lower statin doses for optimal lipid control than did non-T carriers.(3) These results indicate that CYP3A4*22 markedly affects expression of CYP3A4 and could serve as a biomarker for CYP3A4 metabolizer phenotype. The reported allele frequency of CYP3A4*22 in Caucasians was 5% to 8%. The allele frequency is 4.3% in African Americans and in Chinese.

 

See Cytochrome P450 3A4 and 3A5 Known Drug Interaction Chart in Special Instructions.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

Extensive metabolizer:

The CYP3A4*22 allele was not detected. Therefore, this patient is expected to be an extensive metabolizer. Rapid metabolism of drugs that are inactivated or activated by CYP3A4 is expected. Coadministration of CYP3A4 inhibitors may increase the risk of toxicity for drugs inactivated by CYP3A4, or may cause lack of efficacy for drugs activated by CYP3A4.

 

Intermediate to extensive metabolizer:

One copy of the CYP3A4*22 allele was detected. Therefore, this patient is expected to be an intermediate to extensive metabolizer. 

Reduced metabolism of drugs that are inactivated or activated by CYP3A4 is expected when compared to patients who are *1/*1. Coadministration of CYP3A4 inhibitors may increase the risk of toxicity for drugs inactivated by CYP3A4, or may cause lack of efficacy for drugs activated by CYP3A4.

 

Intermediate metabolizer:

Two copies of the CYP3A4*22 allele were detected. Therefore, this patient is expected to be an intermediate metabolizer. Drugs that are inactivated or activated by CYP3A4 are metabolized at reduced rate when compared to patients who are *1/*1. Coadministration of CYP3A4 inhibitors may increase the risk of toxicity for drugs inactivated by CYP3A4, or may cause lack of efficacy for drugs activated by CYP3A4.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Patients who have received a heterologous blood transfusion within the preceding 6 weeks, or who have received an allogeneic blood or marrow transplant, can have inaccurate genetic test results due to presence of donor DNA. 

 

CYP3A4 genetic test results in patients who have undergone liver transplantation may not accurately reflect the patient's CYP3A4 status.

 

This test does not detect polymorphism or mutations other than the specific intron 6 polymorphism.

 

This test is not indicated for stand-alone diagnostic purposes.

 

This test is not intended to be used to predict drug response.

 

Drug-drug interactions and drug/metabolite inhibition must be considered.

 

Drug/metabolite inhibition can occur, resulting in inhibition of CYP3A4 catalytic activity.

 

Patients may also develop toxicity problems if liver and kidney function are impaired.

 

CYP3A4 genotyping should not be ordered for managing patients receiving fluvastatin, rosuvastatin, or pravastatin since these drugs are not metabolized appreciably by CYP3A4.

 

Rare polymorphisms exist that could lead to false-negative or false-positive results. If results obtained do not match the clinical findings, additional testing could be considered.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Evans WE, Relling RV: Pharmacogenomics: translating functional genomics into rational therapeutics. Science 1999;486:487-491

2. Lamda JK, Lin YS, Schuetz EG, Thummel KE: Genetic contribution to variable human CYP3A-mediated metabolism. Adv Drug Deliv Rev 2002;18:1271-1294

3. Wang D, Guo Y, Wrighton SA, et al: Intronic polymorphism in CYP3A4 affects hepatic expression and response to statin drugs. Pharmacogenomics J 2011;11:274-286

4. Elens L, Becker ML, Haufroid V, et al: Novel CYP3A4 intron 6 single nucleotide polymorphism is associated with simvastatin-mediated cholesterol reduction in the Rotterdam study. Pharmacogenet Genomics 2011;21(12):861-866

5. Elens L, Van Schaik RH, Panin N, et al: Effect of a new functional CYP3A4 polymorphism on calcineurin inhibitor’ dose requirements and trough blood levels in stable renal transplant patients. Pharmacogenomics 2011;12(10):1383-1396

Special Instructions and Forms Describes specimen collection and preparation information, test algorithms, and other information pertinent to test. Also includes pertinent information and consent forms to be used when requesting a particular test