This document discusses drug metabolism and the clinical relevance of biotransformation. It begins by defining biotransformation as the biochemical reactions that alter drugs within the body, and discusses the roles of microsomal enzymes like cytochrome P450 enzymes and UDP glucuronosyl transferases. It then covers the phases of drug metabolism and the enzymes involved in each phase. Specific cytochrome P450 isoforms and their drug substrates are discussed. The effects of enzyme induction and inhibition on drug metabolism and potential drug interactions are summarized. Factors influencing individual differences in drug metabolism like genetic polymorphisms and environmental factors are briefly mentioned in the conclusion.
3. • Substances absorbed across lungs or skin
• Ingested either –
-Unintentionally as compounds present in food &
drinks
-Deliberately as drugs for therapeutic purposes
• Taken for recreational purposes
4. • The processes by which biochemical reactions alter the
drugs/xenobiotics within the body
• Chemical alteration of the drugs in body
• Renders Non-polar(Lipid Soluble) compounds Polar (Lipid
Insoluble)
• So that they are not reabsorbed & are excreted
5. Biotransformation of drugs may lead to the following-
A. Active drug Inactive metabolite
Eg. 1) Lidocaine
2) Ibuprofen
3) Chloramphenicol
4) Propranolol
11. ENZYMES REACTIONS
Phase I “Oxygenases"
Cytochrome P450s (P450 or CYP) C & O oxidation, dealkylation,
Flavin-containing monooxygenases (FMO) N, S, and P oxidation
Epoxide hydrolases (mEH, sEH) Hydrolysis of epoxides
Phase II “Transferases"
Sulfotransferases (SULT) Addition of sulfate
UDP-glucuronosyltransferases (UGT) Addition of glucuronic acid
Glutathione-S-transferases (GST) Addition of glutathione
N-acetyltransferases (NAT) Addition of acetyl group
Methyltransferases (MT) Addition of methyl group
Other enzymes
Alcohol dehydrogenases Reduction of alcohols
Aldehyde dehydrogenases Reduction of aldehydes
12.
13. • Located in Endoplasmic Reticulum
• Microsomes –
ER isolated by homogenisation & fractionation of cell
reform into
vesicles known as Microsomes
15. • Abbreviated as CYP or P450
• The CYPs are a superfamily of enzymes, all of which contain a molecule of
heme non-covalently bound to the polypeptide chain Hemoproteins
16. • The term P450 because the reduced hemoprotein binds with CO to form a
complex that absorbs light maximally at 450 nm
• Located in Endoplasmic Reticulum ( In its lipid bilayer )
17. • CYP’s are involved in synthesis and metabolism of some endogenous
substances
Eg. Synthesis of steroid hormones, bile acids
Metabolism of retinoic acid, fatty acids (PG’s & eicosanoids)
• CYP’s carry out the Oxidative reactions (Phase I) of the drug metabolism
• Along with CYP’s, microsomal drug oxidations also require- P450 reductase,
NADPH, and molecular oxygen
18.
19. • About 100 different isoforms identified in humans
• Using Gene arrays, immunoblotting analyses, selective functional
markers & P450 inhibitors
• Divided into Families & Subfamilies
21. • CYP isoforms found in human liver are
CYP1A2
CYP2A6, CYP2B6, CYP2C9, CYP2C18, CYP2C19, CYP2D6, CYP2E1
CYP3A4, CYP3A5
CYP4A11
CYP7
• The most active CYPs for drug metabolism are those in the CYP2C, CYP2D,
and CYP3A sub-families
25. A) Increased Expression
Inducer drug binds a cytoplasmic/nuclear receptor
↓
Translocation of inducer-receptor complex to nucleus
↓
Dimerisation of complex with other nulclear receptor (RXR, Arnt)
↓
This heterodimer binds to response
elements in promoter regions of specific P450 genes
↓
Gene expression induced
↓
↑ Enzyme levels ↑ Enzyme expression
27. B) Enzyme Stabilisation/↓ Degradation
Inducer binds to active site of enzyme
↓
Enzyme stabilised
↓
↓ Degradation of enzyme
↓
Accumulation of enzyme
↓
↑ Enzyme activity
28. • Also known as Ethanol type induction
• Inducers are known as Ethanol type inducers
INDUCERS ENZYME INDUCED
Ethanol CYP 2E1
Troleandomycin
Clotrimazole
CYP 3A4
Isosafrole CYP 1A2
29. • Enzyme induction increases the rate of metabolism by 2-4 fold
• Reaches its peak by 4-14 days
• Maintained till inducing agent is administered
• Enzymes return to their original value over 1-3 weeks
30. • Decreased intensity and/or duration of action of drugs inactivated
by metabolism, eg. oral contraceptives failure
• Increased intensity of action of drugs activated by metabolism,
eg. Acute paracetamol toxicity due to its metabolite N-acetyl-p-
benzoquinoneimine (NAPQI)
• Tolerance due to auto induction, eg. carbamazepine, rifampin
• Endogenous substrates (steroids, bilirubin) are metabolized faster
31. • Precipitation of acute intermittent porphyria: enzyme induction
increases porphyrin synthesis
• Adjustment of dose of drugs taken regularly given with intermittent use
of inducer drugs, eg. oral anticoagulants, oral hypoglycaemics,
antiepileptics, antihypertensives
• Interference with chronic toxicity testing in animals
32. • Congenital nonhaemolytic jaundice due to deficient glucuronidation
of bilirubin – Phenobarbitone hastens clearance of jaundice
• Cushing’s syndrome: phenytoin reduces manifestations by ↑
degradation of adrenal steroids produced in excess
• Chronic poisonings: by faster metabolism of the accumulated poisonous
substance
• Liver disease
33. • Enzyme inhibition takes place by action of inhibitor drugs directly on
enzymes
• As the inhibitors act directly on the enzymes, it has a fast time course
(within hours) compared to enzyme induction
• Inhibitor drugs functionally inactivate the enzymes
↓
Inhibition of drugs metabolism
↓
Toxicity of the object drug
38. Due to CYP 3A4 induction –
• Oral contraception failure
• St. John’s Wort reduce the plasma level of cyclosporin to subtherapeutic
levels rejection of a transplant
• St. John’s Wort also decreases the statin levels raised cholesterol levels
Due to CYP 3A4 inhibition –
• QT prolongation due to inhibition of metabolism of substrate drugs –
Terfenadine, Astemizole, Cisapride
• Grapefruit juice -
Hypotension due to inhibiton of metabolism of substrate drug Felodipine
Dizziness & Serotonin syndrome due to ↑ levels of Buspirone
42. Due to CYP 2C9 induction –
• Doses of substrate drugs like phenytoin, losartan, glimepiride need to be
increased
Due to CYP 2C9 inhibition –
• Risk of bleeding due to enhanced Warfarin activity
43. Substrates
• Omeprazole
• Lansoprazole
• Diazepam
• Phenytoin
• Naproxen
• Propranolol
Inducers
• Barbiturates
• Carbamazepine
• Rifampin
Inhibitors
• Omeprazole
• Fluoxetine
• Ritonavir
• Sertraline
Drug interactions :
• Inhibitors like Ketoconazole, INH, Omeprazole co-administered with
substrate drugs like Anticonvulsants, Diazepam, TCA, Omeprazole
leads to more ADR’s of substrate drugs
44. Substrates
• Ethanol
• Halothane
• Paracetamol
Inducers
• Ethanol
• INH
Inhibitors
• Disulfiram
Drug interactions :
• Induction of enzyme by alcohol leads to increased formation of
N-acetyl-p-benzoquinoneimine (NAPQI), hepatotoxic metabolite of
Paracetamol
45. Substrates
• Theophylline
• Caffeine
• Paracetamol
• Carbamazepine
• R-Warfarin
Inducers
• Smoking
• Charcoal-broiled
meat
• Rifampicin
• Carbamazepine
Inhibitors
• Fluvoxamine
o Compared to its drug metabolising role, its role in activation of
procarcinogens is more important
46. • Catalyse the Phase II reaction – ‘Glucuronidation’
• Glucuronidation is the only conjugation reaction that takes place in ER
(other conjugation in cytosol)
• Substrate drug Glucuronide metabolite
47. • Glucuronides excreted via – 1. Kidney in urine (minor)
2. Intestines with bile (major)
• Glucuronides are cleaved by ẞ-Glucuronidase which is found in bacteria of
lower GI tract Enterohepatic circulation of drugs Eg. OCP’s
48.
49. • UGT proteins encoded by 19 human genes
• UGT 1 locus Chr. 2 9 Genes
UGT 2 locus Chr. 4 10 Genes
• UGT 1 Family Drug’s metabolism
UGT 2 Family Endogenous substances metabolism
51. • Involved in Phase I reactions (minor contribution)
• 6 families of FMO’s are present
• FMO3 is the most abundant in human liver
• Substrates -Nicotine, Cimetidine, Ranitidine, Clozapine
• FMOs are not induced or inhibited by any clinically used drugs
↓
Not involved in drug-drug interactions
52. • Carry out hydrolysis of epoxides, most of which are produced by CYPs
• Types – Soluble Epoxide Hydrolase (sEH)
Microsomal Epoxide Hydrolase (mEH)
• mEH metabolises very few drugs
• Substrate – Carbamazepine
• Inhibitor - Valnoctamide, Valproic acid
• Drug interaction – mEH inhibition Toxic effects of Carbamazepine
53. • Found in both ER & cytosol
• Catalyse the hydrolysis of ester- and amide-containing chemicals
• Detoxification or Metabolic activation - drugs, environmental toxicants,
carcinogens
• Eg. Chemotherapeutic Prodrug Irinotecan Active drug SN-38
54. • Found in both ER & cytosol
• 20 isoforms identified
• Microsomal Forms – Metabolism of endogenous leukotrienes &
prostaglandins
• Cytosolic forms - conjugation, reduction, isomerization reactions of
drug metabolism
55. • Dose and frequency of administration required to achieve effective
therapeutic levels vary in individuals
• Individual differences in rates of drug metabolism
• Depend on genetic and non-genetic factors
56. Genetic Factors –
• Genetic Polymorphism
i. Phase I Enzyme Polymorphism
ii. Phase II Enyme Polymorphism
Non-genetic Factors -
• Commensal gut microbiota
• Diet & environmental factors
• Age & sex
• Concurrent exposure to inhibitors or
inducers
• Diseases
57. Genetic Polymorphism –
• Definition - occurrence of a variant allele of a gene at a population
frequency of ≥ 1%, resulting in altered expression or functional activity of
the gene product, or both
• Clinically significant genetic polymorphisms seen in both phase I & II drug
metabolising enzymes
• ↑/↓ catalytic activity of enzyme
• Results in altered efficacy of drug therapy or adverse drug reactions
58. Genetic Polymorphism –
• Based on Metabolic Ratio, individuals are divided into –
1) Poor metabolisers (PM)
2) Extensive metabolisers (EM)
3) Ultra rapid metabolisers (UM)
• [Metabolic Ratio: defined as percent of dose excreted as unchanged drug
divided by the percent of dose excreted as metabolite in urine collected
over a time period after oral ingestion of drug]
59. P450 genetic polymorphism –
• Three P450 genetic polymorphisms have been particularly well
characterized –
1) CYP 2D6
2) CYP 2C19
3) CYP 2C9
60. • Debrisoquin-Spartein Oxidation type of polymorphism
• PM – 3-10% Caucasians
• UM – 33% Ethiopians & Saudi Arabians
• Inherited as autosomal recessive trait
• Faulty expression of the P450 protein due to either defective mRNA
splicing or protein folding
• CYP2D6 dependent oxidations of debrisoquin and other drugs are impaired
61. • In PM - ↓ CYP2D6-dependent metabolic activation of Tamoxifen to
Endoxifen ↑ relapse in breast cancer
• In UM –
i. ↓ plasma levels of Nortriptyline - No therapeutic effect, ↑suicide rates
ii. ↑ prodrug Codeine Morphine - ↑ s/e of Morphine, morphine-induced
death of breast-fed infant of mother taking excess Codeine
62. • 3–5% Caucasians and 18–23% Japanese
• Inherited as autosomal recessive trait
• Independent of CYP 2D6 polymorphism
• PM genotype – Due to splicing defects,
Allelic variants - CYP2C19*2, CYP2C19*3
• EM genotype – Due to increased transcription,
Allelic variant – CYP2C19*17
63. 1) S-Mephenytoin Hydroxylation Glucuronidation Inactive
metabolite excreted in urine
R-Mephenytoin N-demethylation Active metabolite Nirvanol
In PM’s - Hydroxylation of S-Mephenytoin ↓
N-demethylation to Nirvanol ↑
↑ adverse effects like sedation, ataxia
2) PM phenotype can significantly improve therapeutic efficacy of
Omeprazole in gastric ulcer & GERD
64. 3) EM’s with allele CYP2C19*17 – higher expression and higher function
Higher metabolic activation of prodrugs –
-Tamoxifen↓relapse of breast cancer
-Clopidogrel↑risk of bleeding
Higher elimination of drugs –
-Antidepressants like Imipramine
-Antifungals like Voriconazole
65. • Alleles encode single amino acid mutations which are responsible for
altered metabolic activity of the enzyme
• CYP2C9*2 ↔ Arg144Cys mutation
• CYP2C9*3 ↔ Ile359Leu mutation
• Individuals with these mutations have lower tolerance for Warfarin
adverse effect like bleeding
• Also low tolerance for drugs like Phenytoin, Losartan
66. • > 50 genetic lesions in UGT1A1 gene
• Lead to inheritable unconjugated hyperbilirubinemia –
• Crigler-Najjar syndrome I (AR)- Complete absence of bilirubin
glucuronidation
• Crigler-Najjar syndrome II (AR)- Decreased bilirubin glucuronidation
• Gilbert’s syndrome (AD)- circulating bilirubin levels are 60-70% higher
than normal subjects
67. Gilbert’s Syndrome –
• Most common genetic polymorphism - mutation in the UGT1A1 gene
promoter, UGT1A1*28 allele
• Patients may be predisposed to ADRs resulting from a reduced capacity of
UGT1A1 to metabolize drugs –
Toxicities of Irinotecan, Atazanavir
68. Commensal Gut Microbiota –
• Metabolism of drug by intestinal microorganisms – nonoxidative,
predominantly reductive and hydrolytic reactions; decarboxylation,
dehydroxylation, dealkylation, dehalogenation, and deamination
• Co-treatment with antibiotics like erythromycin, tetracycline leads to death
of commensal bacteria↓bacterial enzymes; this leads to –
i. Increased levels of drugs metabolised in intestine, Eg. Digoxin
ii. Decreased entero-hepatic circulation of glucuronidated drugs
69. Age –
• Increased susceptibility to the pharmacologic or toxic activity of drugs has
been reported in very young & very old patients compared with young
adults
• Differences in absorption, distribution, excretion and drug metabolism play
a role
• Both Microsomal & Non-microsomal enzymes deficient in newborns
• More susceptible to drugs like chloramphenicol, opiods
• Sulfate conjugation is rather well developed in the newborn
• Glururonide conjugation and oxidation occurs at a lower rate
• Paracetamol metabolised by Sulfate conjugation in newborn but by
Glucuronide conjugation in adults
70. Sex –
• Sex-dependent variations in drug metabolism well documented in rats
• Young adult male rats metabolize drugs much faster than mature female
rats or prepubertal male rats
• Sex-dependent differences in drug metabolism exist in humans for
ethanol, propranolol, some benzodiazepines, estrogens, and salicylates
71. Diet & Environmental Factors –
• Charcoal-broiled meat, Smoking, Cruciferous vegetables – Inducers
• Grapefruit Juice – Inhibitor
• Alcohol - Few days of relatively high doses of Alcohol cause inhibition of
various CYP 450 enzymes
- Regular long term intake (50 gms/day) cause induction of these
CYP 450 enzymes
• Industrial workers exposed to some pesticides metabolize certain drugs
more rapidly than unexposed individuals
72. Diseases –
• Acute & chronic liver diseases affecting liver architecture or function
impare hepatic metabolism of drugs
• Alcoholic hepatitis, active or inactive alcoholic cirrhosis, hemochromatosis,
chronic active hepatitis, biliary cirrhosis, viral hepatitis
• In liver, metabolism of drugs Propranolol, Verapamil, Amitriptyline,
Isoniazid, Lidocaine etc, is blood-flow limited metabolism impaired in
cardiac diseases
73. • Understanding drug metabolism & drug interactions within the body allows
principles of biotransformation to be applied in better designing &
therapeutic usage of drugs
• Increased understanding of biotransformation based on
pharmacogenomics will also render pharmacologic treatment of disease
more individualised, efficacious and safe
Hinweis der Redaktion
Active metabolite after phase 2 reactn – acyl glucuronidation of nsaids; n-acetylation of inh; morphine glucuronidation
In humans, 12 CYPs (CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, and 3A5) are known to be important for metabolism of xenobiotics
The most active CYPs for drug metabolism are those in the CYP2C, CYP2D, and CYP3A sub-families. CYP3A4, the most abundantly expressed in liver, is involved in the metabolism of over 50% of clinically used drugs (Figure 6–3A). The CYP1A, CYP1B, CYP2A, CYP2B, and CYP2E subfamilies are not significantly involved in the metabolism of therapeutic drugs, but they do catalyze the metabolic activation of many protoxins and procarcinogens to their ultimate reactive metabolites.
CAR- anticonvulsants, mainly phenobarbital, phenytoin, carbamazepine and primidone ; 2 oxazaphosphorines, cyclophosphamide and ifosfamide
PXR- dexamethasone, rifampicin, mifepristone, atorvastatin, lovastatin, clotrimazole, and phenobarbital
Chronic poisonings – charcoal is inducer multiple-dose activated charcoal is believed to have the most potential utility in overdoses of carbamazepine, dapsone, phenobarbital, quinine, theophylline
Terfenadine, astemizole, cisapride- banned; cisapride used for investigational purpose in usa
Tca s/e – “switch over” in bipolar depression
Phenylbutazone- nsaid
Warfarin- s form – 2c9; r form – 1a, 3a4; partially glucuronidated
Oatp – organic anion transport peptide
Valnoctamide –used as sedative hypnotic in epilepsy, approved in Europe, not by fda, available in India
Debrisoquin is a guanidine derivative, similar to guanithidine
Spartein- class 1a antiarrythmic