Transcript
0:00What are CYP enzymes?
Lesson 4.2 of our metabolism module: CYP enzymes. Metabolism is the chemical change of a drug by enzymes, proteins that make chemical reactions faster. The molecule that an enzyme converts is its substrate. Elimination is the irreversible loss of the drug from the body, by metabolism or by excretion in urine or bile. For many drugs, metabolism is the main route of elimination. Metabolism usually ends the drug action. Cytochrome P450 enzymes, or CYP enzymes, form a superfamily, a large group of proteins from genes with a common origin. Each CYP enzyme has a heme group, a ring structure with an iron atom at its center. The iron atom binds oxygen. In most reactions, the enzyme oxidizes the substrate: it adds one oxygen atom to the molecule.
1:08CYP names
The human genome contains 57 functional CYP genes. In each name, the number after CYP gives the family, the letter gives the subfamily, and the last number gives the individual gene. CYP3A4 is the enzyme from gene 4 of subfamily A in family 3, and CYP3A is the whole subfamily. Of these 57, about a dozen enzymes from families 1, 2 and 3 metabolize 70 to 80 percent of all drugs in clinical use. This lesson compares these enzymes in four ways. They are the amount in the liver, the share of drug metabolism, the substrate preference and the effect of other drugs and foods.
1:58Amount in the liver
The first is the amount in the liver. The liver contains the highest amounts of CYP3A4, CYP2C9, CYP2C8, CYP2E1 and CYP1A2. CYP2A6, CYP2D6, CYP2B6, CYP2C19 and CYP3A5 occur in lower amounts. CYP2J2, CYP1A1 and CYP1B1 occur mainly outside the liver.
2:30Share of drug metabolism
The second is the share of drug metabolism. A review listed the major metabolic enzymes of the 200 most prescribed drugs in the United States in 2014. Among the major metabolic routes of these drugs that run through CYP enzymes, CYP3A4 accounts for 40 percent. Among drugs approved in the United States from 2005 to 2016, the CYP3A4 share is higher, at 64 percent. An enzyme with a lower amount in the liver can still have a large share. Among these 200 drugs, CYP2D6 ranks second, although the liver contains less CYP2D6 than CYP2C9 or CYP1A2. CYP2C19 is third, CYP1A2 and CYP2C9 are equal fourth, and CYP2C8 is sixth.
3:33Substrate preference
The third is the substrate preference. CYP2D6 prefers basic molecules, which take up a proton. CYP2C9 prefers slightly acidic molecules, which give up a proton. CYP3A4 accepts a wide range of lipophilic molecules, which dissolve well in fat.
3:57Inhibitors and inducers
The fourth is the effect of other drugs and foods. An inhibitor is a substance that lowers the activity of an enzyme. The antifungal drug ketoconazole binds to CYP3A4 and lowers the metabolism of other CYP3A4 substrates. For a drug that CYP3A4 mainly metabolizes, the concentration in plasma, the liquid part of the blood, then rises. Grapefruit juice lowers the amount of CYP3A in the gut wall, and it raises the plasma concentration of oral midazolam, a sedative. An inducer is a substance that raises the amount of an enzyme. Most inducers activate a nuclear receptor, a protein that binds to the DNA of the enzyme gene. The cell then makes more of the enzyme. The antibiotic rifampicin activates the pregnane X receptor, which controls the CYP3A genes. Rifampicin induces CYP3A4, and the plasma concentration of CYP3A4 substrates can fall.
5:13Midazolam
Midazolam shows inhibition and induction of CYP3A in people. Exposure is the AUC, the area under the plasma concentration-time curve. CYP3A enzymes metabolize most of a midazolam dose. Transporters, membrane proteins that move drugs, do not carry midazolam into or out of liver cells. A change in midazolam exposure therefore measures a change in CYP3A activity. Nine volunteers took part in three phases, four weeks apart. They took 400 milligrams of ketoconazole, 200 milligrams of itraconazole or placebo, a dose with no active drug, once a day for four days. On day four, each took 7.5 milligrams of midazolam by mouth. Both antifungal drugs raised the midazolam AUC to 10 to 15 times the placebo value. They raised the peak concentration, the highest plasma concentration, to 3 to 4 times the placebo value. With both drugs, midazolam impaired performance in psychomotor tests, tests of attention and movement, more than with placebo for at least six hours. In a second study, ten healthy volunteers took 600 milligrams of rifampicin or placebo once a day for five days. On day six, they took 15 milligrams of midazolam by mouth. Compared with placebo, rifampicin lowered the midazolam AUC by 96 percent and the peak concentration by 94 percent. The half-life, the time in which the plasma concentration falls by half, fell from 3.1 to 1.3 hours. These changes most likely come from induction of CYP3A4 in both the gut wall and the liver. After rifampicin, midazolam had much smaller effects on psychomotor tests than after placebo.
7:32Summary
In summary, about a dozen CYP enzymes metabolize most drugs in clinical use, and CYP3A4 is among those with the highest amounts in the liver. Among the 200 most prescribed drugs, it has the largest share of the major metabolic routes. CYP2D6 is second, although its amount in the liver is lower. CYP3A4 accepts a wide range of lipophilic substrates, and midazolam is one of them. An inhibitor raises the exposure to a drug that the enzyme mainly metabolizes, and an inducer can lower it. Compared with placebo, ketoconazole and itraconazole raised the midazolam AUC 10 to 15 times, and rifampicin lowered it by 96 percent. This concludes lesson 4.2 on CYP enzymes.
Sources
- Zhao M, Ma J, Li M, et al. Cytochrome P450 enzymes and drug metabolism in humans. Int J Mol Sci. 2021. PMID 34884615
- Guengerich FP. Mechanisms of cytochrome P450-catalyzed oxidations. ACS Catal. 2018. PMID 31105987
- Nelson DR, Koymans L, Kamataki T, et al. P450 superfamily: update on new sequences, gene mapping, accession numbers and nomenclature. Pharmacogenetics. 1996. PMID 8845856
- Zanger UM, Schwab M. Cytochrome P450 enzymes in drug metabolism: regulation of gene expression, enzyme activities, and impact of genetic variation. Pharmacol Ther. 2013. PMID 23333322
- Saravanakumar A, Sadighi A, Ryu R, et al. Physicochemical properties, biotransformation, and transport pathways of established and newly approved medications: a systematic review of the top 200 most prescribed drugs vs. the FDA-approved drugs between 2005 and 2016. Clin Pharmacokinet. 2019. PMID 30972694
- Greenblatt DJ, Zhao Y, Venkatakrishnan K, et al. Mechanism of cytochrome P450-3A inhibition by ketoconazole. J Pharm Pharmacol. 2011. PMID 21235585
- Lown KS, Bailey DG, Fontana RJ, et al. Grapefruit juice increases felodipine oral availability in humans by decreasing intestinal CYP3A protein expression. J Clin Invest. 1997. PMID 9153299
- Kupferschmidt HH, Ha HR, Ziegler WH, et al. Interaction between grapefruit juice and midazolam in humans. Clin Pharmacol Ther. 1995. PMID 7628179
- Negishi M, Kobayashi K, Sakuma T, et al. Nuclear receptor phosphorylation in xenobiotic signal transduction. J Biol Chem. 2020. PMID 32788213
- Patilea-Vrana G, Unadkat JD. Transport vs. metabolism: what determines the pharmacokinetics and pharmacodynamics of drugs? Insights from the extended clearance model. Clin Pharmacol Ther. 2016. PMID 27448198
- Olkkola KT, Backman JT, Neuvonen PJ. Midazolam should be avoided in patients receiving the systemic antimycotics ketoconazole or itraconazole. Clin Pharmacol Ther. 1994. PMID 8181191
- Backman JT, Olkkola KT, Neuvonen PJ. Rifampin drastically reduces plasma concentrations and effects of oral midazolam. Clin Pharmacol Ther. 1996. PMID 8549036
Illustrations adapted from Servier Medical Art (smart.servier.com), CC BY 4.0. Structures drawn with RDKit.
