M2021: Pharmacogenetic Testing
Pharmacogenetic Testing Policy Number: AHS – M2021 – Pharmacogenetic Prior Policy Name and Number, as applicable: Testing • M2021 – Cytochrome P450 Initial Presentation Date: 06/16/2021 Revision Date: N/A I. Policy Description Pharmacogenetics is defined as the study of variability in drug response due to heredity (Nebert, 1999). Cytochrome (CYP) P450 enzymes are a class of enzymes essential in the synthesis and breakdown metabolism of various molecules and chemicals. Found primarily in the liver, these enzymes are also essential for the metabolism of many medications. CYP P450 are essential to produce many biochemical building blocks, such as cholesterol, fatty acids, and bile acids. Additional cytochrome P450 are involved in the metabolism of drugs, carcinogens, and internal substances, such as toxins formed within cells. Mutations in CYP P450 genes can result in the inability to properly metabolize medications and other substances, leading to increased levels of toxic substances in the body. Approximately 58 CYP genes are in humans (Bains, 2013; Tantisira & Weiss, 2019). Thiopurine methyltransferase (TPMT) is an enzyme that methylates azathioprine, mercaptopurine and thioguanine into active thioguanine nucleotide metabolites. Azathioprine and mercaptopurine are used for treatment of nonmalignant immunologic disorders; mercaptopurine is used for treatment of lymphoid malignancies; and thioguanine is used for treatment of myeloid leukemias (Relling et al., 2011). Dihydropyrimidine dehydrogenase (DPD), encoded by the gene DPYD, is a rate-limiting enzyme responsible for fluoropyrimidine catabolism. The fluoropyrimidines (5-fluorouracil and capecitabine) are drugs used in the treatment of solid tumors, such as colorectal, breast, and aerodigestive tract tumors (Amstutz et al., 2018). A variety of cell surface proteins, such as antigen-presenting molecules and other proteins, are encoded by the human leukocyte antigen genes (HLAs).
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