Cytochrome P450 enzymes - Drug Interactions

1
Cytochrome P450 (CYP) enzymes - Drug
Interactions

• P. Naina Mohamed
• Pharmacologist

2
Introduction

• Cytochrome P450 (CYP) is a family of isozymes
  responsible for the oxidation of organic
  substances.
• CYP enzymes have been identified in animals,
  plants, fungi, protists, bacteria, archaea, and
  even in viruses.
• In man there are around 30 CYP enzymes which are
  responsible for drug metabolism and these belong
  to families 14.
• But, 90 of drug oxidation can be attributed to
  six main enzymes CYP 1A2, 2C9, 2C19, 2D6, 2E1
  and 3A4.
• The most significant CYP isoenzymes in terms of
  quantity are CYP3A4 and CYP2D6.
• CYP3A4 is found not only in the liver but also in
  the gut wall, where it may serve as a primary
  defence mechanism.
• The present system of nomenclature for the
  various CYP isozymes employs a three-tiered
  classification, an Arabic numeral (family), a
  capital letter (subfamily) and another Arabic
  numeral (gene), e.g. CYP1A2 (Here CYP inmplies
  Cytochrome P450, 1 indicates family, A
  indicates subfamily and 2 indicates the gene).
• These enzymes are heme-containing membrane
  proteins.
• CYP3A4 isozyme has resulted in several clinically
  significant drug-drug interactions.

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Introduction (Contd)

• In humans, Cytochrome P450 enzymes are present in
  most tissues of the body, and they are located
  either in the inner membrane of mitochondria or
  in the endoplasmic reticulum of cells.
• They play important roles in hormone synthesis
  and breakdown, cholesterol synthesis, and vitamin
  D metabolism.
• They also function to metabolize potentially
  toxic compounds, including drugs and products of
  endogenous metabolism such as bilirubin,
  principally in the liver.
• CYPs are the major enzymes involved in drug
  metabolism, accounting for about 75 of the total
  metabolism.
• Most drugs undergo deactivation by CYPs, either
  directly or by facilitated excretion from the
  body.
• Also, many substances are bioactivated by CYPs to
  form their active compounds.
• Although a majority of the isozymes are located
  in the liver, extrahepatic metabolism also occurs
  in the kidneys, skin, gastrointestinal tract, and
  lungs.
• Significant inactivation of some orally
  administered drugs is due to the extensive
  first-pass metabolism in the gastrointestinal
  tract by the CYP3A4 isozyme.

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Introduction (Contd)

• CYP2C19 and CYP2D6 are affected by genetic
  control.
• Individuals lacking the gene for these isozymes
  are poor metabolizers.
• Individuals possessing it are capable of normal
  drug metabolism and are considered extensive
  metabolizers.
• Hepatic disease which affects organ function, and
  congestive heart failure which decreases blood
  flow to the liver affect the metabolism.
• The cytochrome P450 monooxygenase system is more
  affected by aging than any other metabolic
  pathway .
• Decreased biotransformation occurs in newborns
  due to underdevelopment of hepatic microsomal
  components.
• In the elderly, decreases in hepatic blood flow,
  enzyme activity, and liver mass result in reduced
  metabolic activity.

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CYP1A subfamily

• The CYP1A family consists of two enzymes, 1A1 and
  1A2.
• CYP1A1 is found mainly in the lungs, mammary
  glands, placenta and lymphocytes.
• It is an enzyme involved in the inactivation of
  procarcinogens and is highly induced by
  polycyclic aromatic hydrocarbons (PAHs), which
  are found in cigarette smoke.
• There is a strong association between the
  activity of CYP1A1 and the risk of lung cancer.
• CYP1A2 is expressed mainly in the liver.
• It is induced by cigarette smoking.
• It is also induced by the ingestion of some
  foodstuffs such as cruciferous vegetables as well
  as barbecued or charbroiled food.
• Some drugs such as omeprazole may induce CYP1A2
  activity.
• Drugs such as theophylline, caffeine, imipramine,
  paracetamol and phenacitin are metabolized by
  CYP1A2.
• Alteration in CYP1A2 activity, for example by
  smoking, may alter the requirements for
  theophylline among asthmatics and haloperidol
  among psychiatric patients.
• Smokers have increased tolerance to caffeine due
  to smoking induced caffeine metabolism.

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CYP2 family

• The CYP2 family consists of CYP2A6, CYP2C9,
  CYP2C19, CYP2D and CYP2E1.
• CYP2A6
• It was previously known as coumarin hydroxylase.
• It is a relatively unimportant enzyme in terms of
  the number of substrates which it metabolizes,
  one of the substrates broken down by this enzyme
  is nicotine.
• CYP2C9
• S-isoform of Warfarin is metabolized by CYP2C9.
• There are a number of polymorphisms of the gene
  that encode this enzyme resulting in poor
  metabolic status.
• Other drugs metabolized by CYP2C9 include
  non-steroidal anti-inflammatory drugs (NSAIDs)
  (including COX-2 selective inhibitors), the
  hypoglycemic agent tolbutamide, phenytoin and the
  angiotensin-II receptor antagonist losartan.
• CYP2C19
• Diaazepam (Benzodiazepam), Omeprazolle (Proton
  pump inhibitor), Propanolol (Beta adrenergic
  blocker) and the Amitriptyline (Antidepressant)
  are the main substrates of CYP2C19.

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CYP2 family (Contd)

• CYP2D6
• Anti-arrhythmics such as flecanide and encainide,
  tricyclic antidepressants, some beta-blockers and
  a number of selective serotonin re-uptake
  inhibitors are metabloised (Substrates) by this
  enzyme.
• Commonly used analgesics, including codeine and
  tramadol, are also broken down by this enzyme.
• Previously it was named as debrisoquine
  hydroxylase,
• CYP2E1
• CYP2E1 was named previously as dimethylnitrosamine
  N-demethylase.
• It is responsible for the metabolism of small
  organic compounds such as alcohol and carbon
  tetrachloride as well as the halogenated
  anaesthetic agents halothane, enflurane, diethyl
  ether, trichloroethylene, chloroform, isoflurane
  and methoxyflurane.
• It is also responsible for the breakdown of many
  low molecular weight toxins and carcinogens, many
  of which are used in manufacturing and dry
  cleaning industry, including benzene, styrene,
  acetone, vinyl chloride and N-nitrosamines.
• Some of these substances are pro carcinogens
  which are activated by CYP2E1.
• There are gender differences in the expression of
  the enzyme, obesity and fasting may also affect
  its activity.
• In China an association was detected between
  polymorphisms of CYP2E1 and oesophageal and
  gastric cancer.
• There is also mounting evidence that CYP2E1 may
  be a key factor in the pathogenesis of alcoholic
  liver disease.

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CYP3A subfamily

• CYP3A4 is the most abundantly expressed drug
  metabolizing enzyme in man responsible for the
  breakdown of over 120 different medications.
• It is an important area for study with respect to
  enzyme based drug interactions.
• Among the drugs metabolized are sedatives such as
  midazolam, triazolam and diazepam, the
  antidepressives amitriptyline and imipramine, the
  anti-arryhthmics amiodarone, quinidine,
  propafenone and disopyramide, the antihistamines
  terfenadine, astemizole and loratidine, calcium
  channel antagonists such as diltiazem and
  nifedipine and various antimicrobials and
  protease inhibitors.

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Mechanism of action of CYP enzymes

• Substrate binds to the active site of the enzyme
• Stimulation of conformational changes of the
  active site
• Transfer of an electron from NAD(P)H
• Reduction of the iron(III) (Heme center) of CYP
  enzyme to the ferrous state
• Molecular oxygen binds covalently to the heme
  iron
• Formation of dioxygen intermediate of P450
  enzymes
• Reduction of the dioxygen adduct to a negatively
  charged peroxo group
• Formation of a highly reactive species (P450
  Compound 1)
• Catalyse a wide variety of reactions

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Drug interaction

• Changes in cytochrome P450 system determine the
  occurrence of several drug interactions that can
  result in drug toxicities, reduced
  pharmacological effect, and adverse drug
  reactions.
• Many drugs may increase or decrease the activity
  of various CYP isozymes either by inducing the
  biosynthesis of an isozyme (enzyme induction) or
  by directly inhibiting the activity of the CYP
  (enzyme inhibition).
• By recognizing whether the drugs involved act as
  enzyme substrates, inducers, or inhibitors, it is
  possible to prevent clinically significant
  interactions from occurring.
• Avoiding coadministration or anticipating
  potential problems and adjusting a patient's drug
  regimen early in the course of therapy can
  provide optimal response with minimal adverse
  effects.

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Enzyme induction

• The effect of induction is simply to increase the
  amount of P450 present and speed up the oxidation
  and clearance of a drug.
• Smoking and long-term alcohol or drug consumption
  and can reduce the duration of action of a drug
  by increasing its metabolic elimination.
• The CYP1A2 enzyme can be induced by exposure to
  polycyclic aromatic hydrocarbons found in
  char-grilled foods and cigarette smoke. Since its
  activity is enhanced by smoking and eating
  grilled meat or cruciferous vegetables, it is
  difficult to obtain therapeutic effects of drugs
  like theophylline and propranolol.
• Most human CYP2C and 3A subfamily proteins are
  induced by barbiturates.
• Human CYP2E1 is inducible by ethanol and
  isoniazid.
• Most drugs can exhibit decreased efficacy due to
  rapid metabolism, but drugs with active
  metabolites can display increased drug effect
  and/or toxicity due to enzyme induction.
• The interactions due to enzyme induction take 2
  to 3 weeks to develop completely and are slow to
  resolve after stopping the enzyme inducer.

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Enzyme inducers

• The main drugs responsible for induction of the
  most clinically important cytochrome P450
  isoenzymes are
• Griseofulvin
• Phenytoin
• Rifampicin
• St. Johns wort
• Carbamazepine
• Phenobarbitone
• Cigerette Smoke
• GPRS Cell Phone

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Enzyme induction

• Rifampin or Carbamazepine or Barbiturates or
  Phenytoin or St. John's wort
• Induction of hepatic metabolism (CYP3A4)
• Decreased concentration of warfarin, quinidine,
  cyclosporine, losartan, oral contraceptives and
  methadone levels
• Reduced therapeutic efficacy

14
Enzyme inhibition

• Enzyme inhibition occurs when 2 drugs sharing
  metabolism via the same isozyme compete for the
  same enzyme receptor site.
• The more potent inhibitor will predominate,
  resulting in decreased metabolism of the
  competing drug.
• For most drugs, this can lead to increased serum
  levels of the unmetabolized entity, leading to a
  greater potential for toxicity.
• For drugs whose pharmacological activity requires
  biotransformation from a pro-drug form,
  inhibition can lead to decreased efficacy.
• Enzyme inhibition is more common than enzyme
  induction.
• Enzyme inhibition can occur within 2 to 3 days,
  resulting in the rapid development of toxicity.

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Enzyme inhibition

• There are three basic types of enzyme inhibition
• 1. Competitive inhibition
• 2. Non-competitive inhibition
• 3. Uncompetitive inhibition
• competitive inhibition
• Competition occurs between inhibitor and
  substrate for the same binding site on an enzyme.
  
• For example, when single oral doses of metoprolol
  (50 mg), a beta-adrenoceptor blocking agent
  and/or propafenone (150 mg) were administered, an
  approximately two-fold reduction in the oral
  clearance of metoprolol was observed when
  propafenone was included. The dose of metoprolol
  should be reduced when propafenone is also given.
• Similar drug-drug interactions are seen in the
  combined administration of thioridazine and
  propranolol (CYP2D6), fluoxetine and desipramine
  (CYP2D6), omeprazole and diazepam (CYP2C19),
  tolbutamide and phenytoin (CYP2C9), and diltiazem
  and cyclosporin (CYP3A).

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Enzyme inhibition

• Erythromycin or Ketoconazole
• Inhibition of CYP3A
• Blockade of Terfenadine metabolism
• Increased plasma concentrations of Terfenadine
• Prolongation of QT intervals
• Cardiac dysrhythmia (Torsades de pointes)
• In consequence, it has been reported that the
  metabolism of drugs like carbamazepine, midazolam
  and cyclosporin are catalysed by CYP3A, and their
  plasma concentrations are increased when its
  metabolism is inhibited by combined use with
  erythromycin.

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Enzyme inhibition

• Non-competitive inhibition
• The inhibitor binds at a site on the enzyme
  distinct from the substrate, to produce the
  inhibition of enzyme.
• Example Interactions between cimetidine and a
  number of drugs.
• Cimetidine
• Bound to haem portion of P450
• Formation of a stable cytochrome-substrate
  complex
• Inhibition of phase I drug metabolism reactions
  (i.e. hydroxylation, dealkylation)
• Prevention of access of other drugs to the P450
  system
• Cimetidine does not inhibit conjugation
  mechanisms including glucuronidation, sulphation
  and acetylation, or deacetylation or ethanol
  dehydrogenation.

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Enzyme inhibitors

• The main drugs responsible for inhibition of the
  most clinically important cytochrome P450
  isoenzymes are
• Sulphonamides
• Antifungals ( Itraconazole, Ketoconazole,
  Fluconazole)
• Macrolide Antibiotics (Clarithromycin,
  Azithromycin, Erythromycin)
• Ciprofloxacin
• Sertraline
• Cimetidine
• Omeprazole
• Metronidazole
• Antivirals (Ritonavir, Indinavir, Nelfinavir,
  Amprenavir)
• Antiarrhythmics (Amiodarone, Quinidine)
• Antidepressants (Fluoxetine, Paroxetine)
• Isoniazid
• Alcohol
• SICK FACES.COM

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Clinical examples of P450-based interactions

• Terfenadine and Macrolides (Erythromycin) or
  Azole antifungals (Ketoconazole)
• Cimetidine and Disopyramide
• Grape Fruit Juice and Felodipine
• Grape Fruit Juice and benzodiazepines (e.g.
  midazolam, triazolam) or antihistamines (e.g.
  terfenadine) or immuno-suppressive drugs (e.g.
  cyclosporin).
• Omeprazole and Phenytoin or Diazepam

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Enzyme inhibition

• Ketoconazole, Erythromycin, or Grapefruit juice
• Inhibition of CYP3A4
• Blocks metabolism of Terfenadine
• Increased plasma levels of Terfenadine
• Fatal cardiac arrhythmias (torsades de pointes)
• Withdrawn from the market

21
Enzyme inhibition

• Gemfibrozil (and other fibrates)
• CYP3A inhibition
• Prevents metabolism of Statins (HMG-CoA reductase
  inhibitors)
• Increased plasma levels of Statins
• Rhabdomyolysis

22
Enzyme inhibition

• Ritonavir, indinavir, nelfinavir, amprenavir or
  saquinavir
• Inhibit CYP3A4
• Blocks metabolism of Phosphodiesterase type-5
  inhibitors (Sildenafil, Tadalafil, Vardenafil)
• Increased serum levels PDE5 inhibitors
• Excessive vasodilation
• Hypotension

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Enzyme inhibition

• Cimetidine
• Inhibitor of multiple CYPs
• Increased concentration of warfarin, theophylline
  and phenytoin
• Toxicity

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Enzyme inhibition

• Amiodarone
• Inhibitor of many CYPs and of P-glycoprotein
• Decreased clearance (risk of toxicity) for
  warfarin, digoxin and quinidine
• Increased Plasma levels
• Toxicity

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Inhibition or induction of first-pass metabolism

• The gut wall contains metabolising enzymes,
  principally cytochrome P450 isoenzymes.
• Some drugs can have a marked effect on the extent
  of first-pass metabolism by inhibiting or
  inducing the cytochrome P450 isoenzymes in the
  gut wall or in the liver.
• Grapefruit juice
• Inhibits the cytochrome P450 isoenzyme CYP3A4
• (mainly in the gut)
• Reduction of the metabolism of oral
  calcium-channel blockers
• Increased Plasma levels of CCBs

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Genetic factors in drug metabolism

• Some of the population have a variant of the
  isoenzyme with different (usually poor) activity.
  
• For example, a small proportion of the population
  have low CYP2D6 activity and are described as
  poor or slow metabolisers (about 5 to 10 in
  white Caucasians, 0 to 2 in Asians and black
  people). The majority who possess the isoenzyme
  are called fast or extensive metabolisers.
• CYP2D6, CYP2C9 and CYP2C19 also show
  polymorphism, whereas CYP3A4 does not.

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Cytochrome P450 isoenzymes and predicting drug
interactions

• Prediction of drug interactions may reduce the
  numbers of expensive clinical studies in subjects
  and patients and avoids waiting until
  significant drug interactions are observed in
  clinical use.
• If a new drug is shown to be an inducer, or an
  inhibitor, and/or a substrate of a given
  isoenzyme, we can predict likely drug
  interactions by using the list of enzyme
  inducer/inhibitor/substrate.
• For example, ciclosporin is metabolised by
  CYP3A4, and rifampicin (rifampin) is a known,
  potent inducer of this isoenzyme, whereas
  ketoconazole inhibits its activity. Hence,
  rifampicin reduces the levels of ciclosporin and
  ketoconazole increases them.

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Conclusion

• Understanding the mechanism underlying drug
  interactions is useful, not only in preventing
  drug toxicity or adverse effects, but also in
  devising safer therapies for disease.

29
References

• Stockleys Drug Interactions, 9th Edition
• Karen Baxter
• Goodman Gilman's The Pharmacological Basis of
  Therapeutics, 12e Laurence L. Brunton, Bruce A.
  Chabner, Björn C. Knollmann
• Basic Clinical Pharmacology, 12e Bertram G.
  Katzung, Susan B. Masters, Anthony J. Trevor
• Tintinalli's Emergency MedicineA Comprehensive
  Study Guide, 7e Judith E. Tintinalli, J. Stephan
  Stapczynski, David M. Cline, O. John Ma, Rita K.
  Cydulka, and Garth D. MecklerThe American
  College of Emergency Physicians
• Harrison's OnlineFeaturing the complete contents
  of Harrison's Principles ofInternal Medicine,
  18e Dan L. Longo, Anthony S. Fauci, Dennis L.
  Kasper, Stephen L. Hauser, J. Larry Jameson,
  Joseph Loscalzo, Eds
• CURRENT Diagnosis Treatment in Family Medicine,
  3eJeannette E. South-Paul, Samuel C. Matheny,
  Evelyn L. Lewis

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References

• http//www.ncbi.nlm.nih.gov/pmc/articles/PMC131224
  7/
• http//www.aafp.org/afp/2007/0801/p391.html
• http//www.edhayes.com/CYP450-2.html
• http//www.nutritionandmetabolism.com/content/5/1/
  27
• http//www.ncbi.nlm.nih.gov/pubmed/16248836