Transcript Drug metabolism

Drug metabolism
 Refers to enzyme-mediated biotransformations
(detoxication) that alter the pharmacological activity
of both endogenous and exogenous compounds.
 Metabolism may result in:
 Pharmacologically inactive drug (detoxification).
 Pharmacologically active drug (bioactivation…prodrug
approach).
 Change the pharmacological activity (toxic effect).
Drug metabolism
 Drugs undergo a variety of chemical changes in the
animal organism by enzymes of the liver, intestine,
kidney, lung, plasma and other tissues.
 The importance of studying drug metabolism:
 Understanding the pharmacological and toxicological
activity of drugs.
 The importance of shortening the drug’s duration of action.
 The complications of drug-drug interactions mainly
depends on the induction or inhibition of metabolic
enzymes
Drug metabolism
 Can be divided into two distinct categories:
 Phase-I:
Reactions which introduce or unmask hydrophilic
groups in the drug structure (functionalisations).
 Phase-II:
Reactions which conjugate the drug or its phase-I
metabolite with a hydrophilic, endogenous species
(conjugation reactions).
Phase-I reactions
 Aliphatic hydroxylation.
 Oxidation:
 Oxidative Dealkylation.
 Oxidative deamination.
 N and S oxidation.
 Alcohol/aldehyde dehydrogenase.
 Reduction.
 Hydrolysis.
Phase-I reactions
 Two enzyme systems take part in these reactions:
 Cytochrome P-450 dependent mono-oxygenase: catalyze
the oxidation at carbon, nitrogen and sulfur.
 Flavin mono-oxygenase: catalyze oxidation at nitrogen
and sulfur.
Phase-I reactions
 Cytochrome P-450 dependent mono-oxygenase:
 Membrane–bound mixed function oxidase found in the
smooth endoplasmic reticulum of the liver.
 has the ability to activate molecular oxygen permitting
the incorporation of one oxygen atom into the organic
(drug) molecule and the reduction of the other atom
into water molecule.
Oxidative Phase-I involving
cytochrome P-450 enzymes:
 Aromatic hydroxylation:
Oxidative Phase-I involving
cytochrome P-450 enzymes:
 Aliphatic hydroxylation:
Oxidative Phase-I involving
cytochrome P-450 enzymes:
 Aliphatic hydroxylation:
 Mainly occur on the ultimate (ω) or penultimate (ω-1)
carbon atom in the structure.
 Also it occurs at an activated carbon atom, that is next to
sp , sp2 carbons:
Oxidative Phase-I involving
cytochrome P-450 enzymes:
 Aromatic epoxidation:
Polycyclic aromatic
Hydrocarbons: highly toxic ,
Obtained from burning,
industrial process and
Cigarette smoking
Oxidative Phase-I involving
cytochrome P-450 enzymes:
 Alkene epoxidation:
Glutathione conjugation
 For electrophilic drugs and metabolites:
Detoxication by glutathione adduct
formation
Glutathione conjugation
 Toxicity of aromatic compounds came from the
formation of arene oxide during the metabolism that
will be attacked by endogenous nucleophile such as
proteins, DNA or RNA.
Oxidative Phase-I involving
cytochrome P-450 enzymes:
 O-dealkylation:
Oxidative Phase-I involving
cytochrome P-450 enzymes:
 N-dealkylation:
Oxidative Phase-I involving
cytochrome P-450 enzymes:
 Oxidative deamination:
Oxidative Phase-I involving
cytochrome P-450 enzymes:
 N-oxidation:
 Mostly for primary and secondary amines as well as
aromatic amines:
 This gives N-oxide that will be rapidly converted to
hydroxylamines.
Reductive phase-I metabolism
 Mainly for carbonyl and nitro groups.
 Carbonyl reduction by aldo-keto reductase:
Reductive phase-I metabolism
 Nitro reduction by cytochrome P-450 or the bacterial
nitroreductase:
Hydrolytic phase-I metabolism
 By non-specific esterase and amidase enzymes that
present in plasma, gut, liver and kidney.
Procaine
Procainamide
 It has a beneficial role in most of prodrugs that after
hydrolysis inside the body release the active form of
the drug.
Hydrolytic phase-I metabolism
 Examples of prodrugs activated by hydrolytic enzymes:
 Dipivefrine: is a di-tertbutylcarboxy ester of
adrenaline…. More lipophilic… better penetration
through the corneal membrane….then will be
hydrolyzed to give the active form (adrenaline)
Hydrolytic phase-I metabolism
 Ibuprofen as most of the non-steroidal anti-
inflammatory has a local inhibition of protective
prostaglandin synthesis in stomach which results in
gastric irritation
Hydrolytic phase-I metabolism
 -methydopa is an anti-hypertensive agent which
have poor oral bioavailability because of the zwitterionic form.
Other phase-I metabolic enzymes
 Monoamine oxidase: involved in deamination of
catecholamines.
 Alcohol dehydrogenase and aldehyde dehydrogenase:
Other phase-I metabolism
 Heterocyclic ring oxidation:
 S-dealkylation:
Other phase-I metabolism
 Sulfoxidation: by flavin monooxygenase
 Azoreduction:
Other phase-I metabolism
 Miscellaneous reductions:
 Reduction of disulfide and sulfoxide into sulfides:
 Dehydroxylation of aromatic and aliphatic hydroxyl
derivatives.
 Ketone reduction by ketone reductase rather than the
alcohol dehydrogenase (using NADPH as the cofactor)
for naltexone, naltrexone, and hydromorphone)
Other phase-I metabolism
 β-oxidation :
 This occurs for alkyl carboxylic acids .
 Involves the oxidative cleavage of two carbon units at a
time (as acetate), starting from the carboxyl terminus
until no more acetate can be removed:
General notes regarding phase-I
metabolism
 Hydrolysis normally catalyzes by carboxylesterases:
 Cholinesterase…. Hydrolyzes choline-like esters (such as
succinylcholine), procaine and acetylsalicylic acid.
 Arylcarboxyesterase.
 Liver carboxyesterase
General notes regarding phase-I
metabolism
 Esters that are sterically hindered are hydrolyzed more
slowly and may be appeared unchanged in urine:
 Amides are more stable to hydrolysis than
esters….large fraction of amide containing drugs are
normally excreted unchanged.
Phase-II metabolism
 Involves the following conjugation reactions that are
catalyzed by transferase enzymes:
 Glucuronidation.
 Sulfation.
 Amino acid conjugation.
Glucuronidation
 Involves conjugation of phenols, alcohols,
hydroxylamines, carboxylic acids (O-glucuronidation),
amines, sulfonamides, amides (N-glucuronidation)
and thiols (S-glucuronidation) with glucuronic acid:
Glucuronidation
Estradiol
Sulfation (sulfate conjugation)
 Mainly for phenols, alcohols, arylamines, and N-
hydroxy compounds, catalyzed by sulfotransferases.
Salbutamol
General notes regarding sulfation
 Sulfation and glucuronidation occur side by side, even
competing for the same substrate….phenols in many
cases.
 Sulfation mainly occur in intestine for orally
administered drugs…leads to pre-systemic first pass
elimination….decreasing the oral bioavailability.
 Co-administration of acetaminophen with oral
contraceptive ethynylestradiol may affect oral
availability of both drugs.
 The rate of sulfation is age dependent, decreasing with
age
Glycine conjugation
 Is the conjugation of carboxylic acid group with glycine
and glutamine (mainly for phenylacetic acids) in
animals) whereas in primates the conjugation take
place with glutamate or ornithine.
 The carboxylic acid should be activated by forming
CoA thioester which serve as a good leaving group.
Glycine conjugation
AMP
Coenzyme-A
General notes regarding drug
carboxylic acids
 The activation of carboxylic acids by forming the CoA
ester is a stereo-selective reaction:
Acetylation
 Is a reaction of amino groups involving the transfer of
acetyl CoA to an aromatic primary or aliphatic amine,
amino acids, hydrazine and sulfonamides.
 Secondary amines are not acetylated.
 Acetylation normally ends with pharmacologically
inactive metabolites with some exceptions:
Acetylation
 The rate of acetylation is mainly affected by the
existence of genetic polymorphism.
 Two acetylator phenotypes:
 Slow acetylators: tend to accumulate higher blood
concentrations of un-acetylated drugs...toxicity
(isoniazid… peripheral nerve damage,
procainamide…lupus erythematous).
 Fast acetylators: eliminate drugs more rapidly, at the
same time can form toxic metabolite very fast.
80-90% of orientals are fast acetylator
100% of Eskimos are fast acetylators
Methylation
 Mainly O- and N-methylation.
 It is more important for endogenous compounds than
for drugs.
 The methylated metabolite in general is more
lipophilic than the parent compound, sometimes this
lipophilic metabolite is more active than the parent:
Methylation
 The methyl source here is the activated intermediate
S-adenosylmethionine (SAM), the enzyme catalyze
this reaction is the methyl transferase enzyme.
 Results in the formation of O-methylated, N-
methylated and S-methylated products.
Methylation
 O-methylation is catalyzed by Catechol-O-methyl
transferase (COMT):
 Regioselective….methylates m-hydroxyl group much
more than the O-hydroxyl group.
 Substrate selective… only methylates catecholcontaining compounds, does not work on monohydric
or other dihydric phenols.
 The main importance is the biological inactivation of
adrenergic neurotransmitter, norepinephrine and other
Catechol-like structures.
 Found in liver, kidney, and nervous system.
Methylation
 N-methyltransferase:
 Catalyzes the methylation of phenylethanolamines such as
histamine, norepinephrine and norephedrine.

Requires SAM as a methyl source.
 S-methyltransferase:

One of the detoxication pathways for thiol containing
compounds.

Needs SAM for methyl transfer.
General notes regarding drug
carboxylic acids
 Small, unbranched carboxylic acids normally undergo
β-oxidation.
 Branched aliphatic and aromatic carboxylic acid are
resistant to β-oxidation and form either glycine or
glucuronide conjugates.]
 Substitution at the α-carbon favors glucuronidation
rather than glycine conjugation.
 Benzoic and heterocyclic carboxylic acids favor glycine
conjugation.
General notes regarding phase-II
metabolism
 Phase-II metabolism (especially the glucuronidation
and sulfation) thought to terminate pharmacological
activity of drugs (why?):
 By transforming the drug into readily excreted ionizable
metabolite.
 This metabolite will have poor affinity for the active site
of drug’s receptors.
 This metabolite will have poor cellular diffusion.
General notes regarding phase-II
metabolism
 Phase-II metabolism sometimes give more active
metabolites:
 Morphine-6-glucuronide has more analgesic activity
than morphine.
 Minoxidil sulfate is the active form for the antihypertensive agent minoxidil.
 Sometimes, Phase-II reactions result in toxic
derivatives , especially glucuronidation and sulfation:
Enzyme induction
 Many drugs, and environmental chemicals enhances
the metabolism of themselves or other co-ingested
compounds ……this will alter the their pharmacologic
and toxicologic effects.
 It is a dose-dependent phenomenon.
 This mainly occur by inducing transcription of
CYP450mRNA which leads to overproduction of these
enzymes in the liver and other extra-hepatic tissues.
 Enzyme induction: is the process by which the
rate of synthesis of an enzyme is increased
relative to the un-induced organism
Enzyme induction
 Enzyme inducers:
 Many drugs (table attached) have the ability to stimulate the
activity of CYP450 isoforms.
 Many environmental chemicals also alter the activity of
CYP450 isoforms:




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Cigarette smoking.
Polycyclic aromatic hydrocarbons.
Xanthines and flavones in food.
Halogenated hydrocarbons in insecticides.
Food additives
 These chemicals do not have something in common except
they are all metabolised by one or more CYP450 isoforms
Enzyme induction
 The importance of studying enzyme induction:
 Evaluating the pharmacologic, toxicologic and
explaining certain unexpected drug interaction in
patients.
 Studying the drug-drug interactions
 As a result of induction:
 The drug may be metabolized more rapidly to more
potent or more toxic metabolite.
 Or enhance the activation of procarcinogens
 Or metabolized to less active metabolite.
Enzyme induction and drug-drug
interactions
 Many drugs are CYP450 inducers of CYP450
subfamilies, at the same time may be also substrates
for the same isoforms:
 Examples:
 Phenobarbital
 Phenytoin
 Rifampicin
 Cigarette smoking
Enzyme induction and drug-drug
interactions
 Examples of drug-drug interactions result from
enzyme induction:
 Rifamipicn induce metabolism of contraceptives by
CYP3A4...... Reducing their serum levels..... Increase the
risk for pregnancy.
 Phenobarbital, cigarette smoking and dexamethasone
induce the metabolism of estrogens, vitamin D and
bilirubin in women …….decrease their biological
activity……. So cigarette smoking in premenopausal
women increases the risk of osteoporosis and early
menopause.
Enzyme induction and drug-drug
interactions
 Examples of drug-drug interactions result from
enzyme induction:
 Cigarette smoking lowers the serum levels of
theophylline, imipramine, estradiol, decrease the
urinary excretion of nicotine and decrease the
drowsiness from diazepam, chlordiazepoxide
 Administration of some drugs for a long time may
stimulate their own metabolism….. Apparent tolerance:

The sedative action of Phenobarbital becomes shorter with
repeated doses due to the increase in self metabolism.
Enzyme induction and drug-drug
interactions
 Examples of drug-drug interactions result from enzyme
induction:
 Heavy alcoholics metabolize phenobarbital, tolbutamide, and
phentoin more rapidly than non-alcoholics due to the enzyme
induction of CYP2E1.
 The usefulness of enzyme induction:
 Certain inducers have been used therapeutically for
hyperbilirubinemia in children.
 Some inducers stimulate the conversion of drugs into more
polar metabolites.
Enzyme induction and food-drug
interactions
 Examples:
 Cabbage and cauliflower stimulate monooxygenase
activity in rat intestine.
 Flavones, safrole, eucalyptol, xanthines and volatile oils
present in food and plants also have enzyme induction
properties.
Enzyme inhibition
 Can be divided into three major categories:
 Reversible inhibition.
 Metabolite intermediate complexation of CYP450.
 Mechanism-based inactivation of CYP450.
 Reversible inhibition:
 is a result of reversible interaction at the heme-iron active centre
of CYP450, the lipophilic site or both.
 This action will be abolished once the enzyme inhibitor is
discontinued.
 Examples: fluoroquinolones, cimetidine, azoles antifungal
agents.
Enzyme inhibition
 Metabolite-intermediate complexation of CYP450:
 Happens when the metabolite of certain drugs forms
stable covalent bond with the reduced ferrous heme
intermediate.
 Alkylamines are examples of such drugs due to the
formation of the nitroso metabolite:
Enzyme inhibition
 Examples of metabolite intermediate complexation drugs:
 Macrolide antibiotics.
 Eryhthromycin.
 Clarithromycin.
 Orphinadrine (anti-parkinson agent).
 The clinical significance of this inhibition is the
impairment of metabolism of many coadministerd drugs as
well as the associated changes in pharmacokinetics for
these drugs
Enzyme inhibition
 Mechanism-based inhibition (suicide inhibition):
 Some drugs contain functional groups that when
oxidized by CYP450 generate metabolites that bind
irreversibly to the enzyme.
 Examples:



Alkane and alkenes containing drugs can form radical
intermediate after oxidation with CYP450…. This will alkylates
the heme moiety…. Abnormal porphyrins….. 17-α-acetylenic
progestin, norethindrone are examples of such drugs
Cyclophosphamide…forms acrolein and phosphoramide.
Chloramphenicol….(what is the reactive intermediate?).
Factors affecting drug metabolism
 Species may differ in details of the reaction and
enzyme control.
 Factors influencing drug metabolism:
 Genetic factors: results in differences in the expression
of metabolizing enzymes and genetic polymorphism).
 Physiologic factors: including age, hormonal changes,
sex differences, pregnancy and nutritional status.
 Pharmacodynamic factors: including dose, frequency,
route of administration and protein binding.
 Environmental factors: this depends on the competition
with other drugs for the metabolizing enzymes by toxic
chemicals such as CO and pesticides.
Drug metabolism and age
 It is well documented that the metabolism of many drugs
and their elimination is impaired in the elderly.
 In elderly there are many physiologic changes that affect
the plasma concentration and renal clearance….these will
decrease the hepatic blood flow, glomerular filtration,
hepatic enzymes activity and plasma protein binding.
 First pass metabolism of many drugs is reduced in elderly
patients: such drugs are diazepam, theophylline,
morphine, propranolol and amitriptyline.
 All of the common phase-II enzymes are affected by aging
Drug metabolism and age
 The human fetus has only the cytochrome P450
monooxygenase 3A (CYP 3A)which is capable of
metabolizing xenobiotics during the first part of gestation.
 Placenta of tobacco smokers has shown increase of CYP1A
activity that will form toxic metabolites which will
covalently bind to fetus macromolecules….. Teratogenic
and hepatotoxic effect.
 Phase-II enzymes are found in low to negligible
concentration in the fetus….. High risk of toxicity by
pregnants metabolites
Drug metabolism and age
 The ability to carry out metabolic reactions increases
after birth and approaches adult levels in about 1 to 2
months.
 The inability of infants to conjugate chloramphenicol
with glucuronic acid appears to be responsible for the
accumulation of toxic levels of this drug….. This will
lead to what is called gray-baby syndrome.
 Neonatal hyperbilirubinemia results from the
newborn baby to glucuronide bilirubin.
Species and strain differeneces
 There are metabolic differences between species, such
as between human and dogs, rabbit, pigs, cats and
birds.
Species and strain differences
 The conjugation with amino acids differs between
species as well:
 Glycine conjugation is common in most animals.
 Birds normally use ornithine amino acid for conjugation.
 Strain differences in mice and rabbit have been noted:
mainly due to genetic variations that affect the amount
of metabolizing enzymes
Hereditary or genetic factors
 Genetic factors in human is the main cause for the
differences in the rate o drug metabolism.
 the difference in the rate of acetylation is one example:
 Rapid acetylators have more hepatic acetyl N-transferase
than the slow acetylators.
 90% of Asians and Eskimos are rapid acetylator.
 Egyptians and Mediterranean are slow acetylators.
 The rate of acetlations is clinically important in terms of
therapeutic response and toxicity.
 Also, genetic factors affect the arte of oxidation.
Sex differences
 The rate of metabolism also varies according to sex in
some animal species.
 Generally it is species dependant:
 Rabbit and mice do not show a significant sex
differences in drug metabolism.
 In humans, few repots of sex differences have been
observed:

Nicotine and aspirin seem to be metabolized more rapidly in
male compared to female.
Genetic Polymorphism
 Can be defined as the genetic differences in the natural
expression of enzyme isoforms.
 Resulted in inter-individual variation in the
metabolism of drugs.
 Three families of CYP450 exist(CYP1-3).
 Two phenotypes:
 Extensive metabolizer (EM).
 Poor metabolizer (PM).
Genetic Polymorphism
 Poor metabolizer s(PM) are normally associated with
higher risk of serious side effects due to the
accumulation of drugs in the body.
 Poor metabolizer (PM) also experience loss of activity
in some drugs (codeine is an example)
Genetic Polymorphism
 Poor metabolizer phenotype is inherited as an
autosomal recessive gene while the extensive
metabolizer is the predominant one.
 Extensive metabolizers may be more susceptible than
Poor metabolizers to develop cancers because they are
better able to activate the pro-carcinogens into the
active carcinogen.