Drug/xenobiotic metabolism and
pharmacogenetics
George Howell III, Ph.D
Sites of drug metabolism
• Liver – responsible for the
majority of drug
metabolism
– First pass metabolism
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Kidney
GI
Lung
Skin
Brain
Drug metabolism and its effects
• Drug metabolism – the processes by which
biochemical reactions alters drugs within the
body
• 4 ways a drug can be altered:
1. Active drug can be inactivated
2. Active drug can be converted to an active
metabolite or toxic metabolite
3. Prodrug can be converted to an active drug
4. Unexcretable drug can be converted to an
excretable metabolite
Major types of biotransformation reactions
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Oxidation/reduction reactions (Phase I)
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Typically transform drug into more hydrophilic metabolites by
adding or exposing a polar functional group
Catabolic
Can be more reactive and toxic than the parent compound
Can be excreted is sufficiently polar
Conjugation/hydrolysis reactions (Phase II)
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Further modifications to compounds to improve hydrophilicity
Anabolic
Conjugate drug with an endogenous substrate such as glucuronic
acid, sulfuric acid, acetic acid, or an amino acid to form a highly
polar compound
Oxidation/reduction reactions (Phase I)
• More than 95% of oxidative
biotransformations are performed by
the cytochrome P450
monoxygenases
• ~75% of all drugs currently used are
oxidized by the P450s
• Cytochrome P450 enzymes
– Broad substrate specificity
– Metabolize xenobiotics
– Can play a role in formation of
endogenous substances (steroid
production)
• Alcohol/aldehyde dehydrogenases
• Monoamine oxidase
• Esterases
– Acetylcholinesterase (AchE)
– Butylcholinesterase (BchE)
– Carboxylesterase (CES)
Percentage of total liver
P450 content
CYP3A4 = 30%
CYP2C9 = 20%
CYP1A2 = 15%
CYP2E1 = 10%
CYP2D6 = 5%
CYP2A6 = 4%
CYP2B6 = 1%
Cytochrome P450s
• Located within the endoplasmic
reticulum
• 74 CYP gene families
• Three main ones involved in drug
metabolism in the liver
– CYP1, CYP2, CYP3
– ~50% of all rx drugs metabolized by
CYP3A4
• P450 3A4 (CYP3A4)
– 3 = number of the enzyme family
– A = letter of the subfamily
– 4 = specifies specific enzyme
• Broad substrate specificity due in part
to the activated oxygen of the complex
(powerful oxidizing agent that can
easily react)
• Can be induced or inhibited by a variety
of compounds
– Leads to significant drug interactions
~50%
P450 cycle
Drug + O2 + NADPH + H+
Microsomal drug oxidations require:
1.
P450
2.
P450 reductase
3.
NADPH
4.
Molecular oxygen
Steps of P450 mediated oxidation:
1.
Oxidized P450 binds with drug
to form a complex
2.
P450 reductase reduces the
P450/drug complex
3.
P450 reductase reduces
molecular oxygen to form an
“activated oxygen”-P450/drug
complex
4.
Activated oxygen is transferred
to drug to form oxidized
product
5.
One molecule of water is
produced
Drug-OH + H2O + NADP+
Electron donated
Fe3+ reduced to Fe2+
P450 substrates, inducers, and inhibitors
•
P450 induction
– Increase expression by
increased synthesis or
decreased degradation
– Results in increased
metabolism of
substrates
• Decreased substrate
plasma
concentrations
•
P450 inhibition
– Decrease enzyme
activity
– Decrease rate of
metabolism of other
substrates
• Increase substrate
plasma
concentrations
Conjugation/hydrolysis reactions (Phase II)
•
Glucuronidation (highest % of drug
metabolism of phase II)
– Addition of UDP glucuronic acid catalyzed
by UDP glucuronosyltransferase (UGT)
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Acetylation
– Addition of acetate by N-acetyltransferase
(NAT)
•
Glutathione conjugation
– Addition of glutathione by glutathione-Stransferase (GST)
•
Glycine conjugation
– Addition of glycine by Acyl-CoA
glycinetransferase
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Sulfation
– Addition of a sulfate by sulfotransferase
(SULT)
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Methylation
– Addition of a methyl group by
transmethylases
•
Water conjugation
– Addition of water by epoxide hydrolase
Conjugation reactions
Factors affecting drug metabolism
• Genetic variability (pharmacogenomics)
– Certain populations have polymorphisms or mutations in metabolic enzymes with
make them rapid or poor metabolizers
• Race and ethnicity
– Polymorphisms in metabolic genes among races
• CYP2D6 polymorphisms among races
• Age
– Many biotransformations are slowed in young and elderly
– Neonates can carry out most but not all oxidative reactions
• Enzyme systems mature over the first two weeks and through childhood
– Neonates can have decreased conjugating ability
• Jaundice as a result of deficient bilirubin conjugation by UGT
• Gray baby syndrome – decreased conjugation of chloramphenicol metabolite
• Gender
– Males metabolize ethanol, propranolol, some benzodiazepines, estrogens, and
salicylates more rapidly
Factors affecting drug metabolism (cont.)
• Diet
– Chargrilled foods and cruciferous vegetables induce CYP1A enzymes
– Grapefruit juice inhibits CYP3A
• Environment
– Cigarette smoke induces P450 enzymes via Ahr dependent mechanism
– Industrial workers exposed to some pesticides metabolize more rapidly
• Drug interactions
– See tables 4-5 and 4-6 in Lange for known inducers and inhibitors
• Disease
– Liver diseases (hepatitis, cirrhosis, cancer, hemochromatosis, fatty liver)
can impair P450 activity
– Cardiac disease can slow blood flow to liver and therefore decrease
metabolism
– Thyroid disease
• Hyperthyroid – increase metabolism
• Hypothyroid – decrease metabolism
Acetaminophen toxicity
•Normally undergoes glucuronidation and/or
sulfation
•Remaining drug undergoes P450 mediated
metabolism
•Excess acetaminophen saturates conjugation
pathways…..shunts to P450 mediated
metabolism
•Role of ethanol
•Hepatic glutathione (GSH) is depleted faster
than is regenerated and Nacetylbenzoiminoquinone (toxic metabolite
that reacts with proteins) accumulates
•N-acetylcysteine is administered w/I 8-16
hours to protect from hepatotoxicity
Pharmacogenetics
How genetic variability affects drug
metabolism
GENETIC POLYMORPHISMS
• Major factor accounting for differences in
pharmacokinetic and pharmacodynamic
parameters of individuals
• Approximately 25 polymorphisms identified
• Clinically important
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N-acetylation
debrisoquine/sparteine hydroxylation
mephenytoin oxidation
aldehyde oxidation
butyrylcholinesterase (BchE) deficiency
HYDROXYLATION POLYMORPHISMS
• Debrisoquine (old
antihypertensive) –
CYP2D6
– 5-10% most populations
are poor metabolizers (12% Chinese, Japanese)
– Predominant enzyme for
amines with hydrophobic
planer unit
– Approximately 15 variants
of CYP2D6
• 4 - no activity, 5 - reduced
activity, 3 - increased
activity,
2 - no effect
• Variation of 1000 fold can
be found in extensive
metabolizers (heterozygous,
allelic variants)
IMPACT of DEFICIENT CYP2D6
• Debrisoquine – single dose
– Primary effect on first pass metabolism
– Little change in ½ life
– Increased peak plasma concentration (Clinical effects)
• Sparteine – single dose
– Primary effect – increased ½ life
– No appreciable change in peak plasma concentration
– (no observable clinical effects)
GENETIC POLYMORPHISMS
• S-mephenytoin hydroxylation – CYP2C19
Caucasians (3%) Orientals (15-20%)
• Results in poor metabolizer phenotype
• Substrates
– Acids, bases or neutral compounds
• Diazepam, imipramine, propranolol
• Proguanil (antimalarial) activated by CYP2C19
12th Edition of Basic and Clinical has a more extensive table…….look up metabolizer
phenotype for the prevalent polymorphisms (can be inferred from clinical consequence)
N-ACETYLATION
• Incidence – slow
acetylators
– 90% Moroccans, 5%
Canadian Eskimos,
30-67% Caucasians and
persons from African
– Slow acetylators
• Phenytoin-isoniazid inhibition of CYP450
• Arylamine – induced
bladder cancer –
benzidine
– Rapid acetylators
• Drug ineffective – dose
must be increased
• Hepatitis (insignificant)
N-ACETYLATION
• N-acetylation
– Slow acetylators
• Isoniazid- induced peripheral polyneuropathy
• Drug - induced lupus erythematosus – 35 drugs with
primary amino group
ALDEHYDE DEHYDROGENASE
– About 50% of people of Oriental descent are slow
metabolizers of acetaldehyde
– Rare outside the Oriental population
• Significant acetaldehyde build up associated with
ethanol intake – flushing, increased heart rate, nausea
Butyrylcholinesterase deficiency
• Autosomal recessive
• Succinylcholine is metabolized by BchE
• Increased accumulation of succinylcholine
(depolarizing neuromuscular blocker)
• Increased muscle paralysis including respiratory
paralysis (succinylcholine apnea)