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Drug MetabolismDrug Metabolism
Drug metabolism is the transformation of foreign compounds ( xenobiotics) into a water soluble derivatives which can be easily eliminated in the urine.
ExampleExample
In General , the metabolism of In General , the metabolism of xenobiotics takes place in two steps xenobiotics takes place in two steps
known asknown as phase I & phase I &
phase II reactionsphase II reactions
Phase I ( functionalization Phase I ( functionalization reaction )reaction )
Is the process of increasing of the hydrophilicity of lipophilic drug by introducing a polar functional group eg; OH,COOH,NH2,SH to the molecule through oxidative, reductive & hydrolytic biotransformations.
Phase II ( conjugation Phase II ( conjugation reactions )reactions )
Is Linking of an endogenous solubilizing moiety either to the original drug (if polar function is already present) or to the phase I metabolite.
Common solubilizing groups are glucuronic acid, various amino acids or sulphate groups.
The conjugate molecule, being more polar and water-soluble, is usually excreted via the renal route
Effect of metabolism on the Effect of metabolism on the therapeutic activity of drugstherapeutic activity of drugs
Factors affecting drug Factors affecting drug metabolismmetabolism
Genetic factors Physiological factors Pharmaceutical factors Pharmacodynamic factors. Enviromental factors.
Genetic factorsGenetic factors
Biological half –life (t1/2) of various drug
Genetic Polymorphism:
Different expression of metabolizing enzymes according to the Race (ethnicity)
Physiological factorsPhysiological factorsAge ,Gender,maternity status,liver function & Nutritional Age ,Gender,maternity status,liver function & Nutritional
status.status. eg: Age which is the ability of the body to metabolize the eg: Age which is the ability of the body to metabolize the
drug lower in v. young & elderlydrug lower in v. young & elderly..
Pharmacodynamic factorsPharmacodynamic factors
The dose, the route and the frequency of administration of drugs & Drug interaction can affect their metabolic profiles.
Drugs given too frequently may overload the metabolic system available to it, leading to elevated blood and tissue levels of the drugs. The effect of protein binding also influences the metabolism.
Drug interactions for example:-Phenobarbital stimulate the metabolism of
Diphenylhydantoin.Plasma Concentration of anticoagulants such as
Warfarin are reduced by simultaneous application of barbiturate
Enviromental FactorsEnviromental FactorsInhaled gases,toxins eg:Nicotine (cigarette – 8 to 10
mg ) -Acute nicotine exposure
(From – insecticide sprays or tobacco)Nausea, vomiting, salivation, diarrhea, dizziness,
mental confusion, weakness-Fatal exposure (60 mg fatal for adult)
Decreased blood pressure, irregular pulse, convulsions, respiratory failure and death
-Cotinine - Major metabolite-Lung – First site of metabolism
-Liver – Major site-Half-life – about 2 hours
))Phase I (Functionalization Phase I (Functionalization reactionsreactions
Oxidations (electron removal, dehydrogenation and hydroxylation)
Reduction ( electron donation, hydrogenation
and removal of oxygen )
Hydrolytic reactions of amides & esters.
-Two general types of enzyme systems take part in these reactions:
-a) Microsomal Mixed Function Oxidases (MFOs)Flavoprotein, NADPH-monooxygenaseCytochrome P450
-b) Non-cytochrome oxidizing enzymes.Xanthine oxidaseAlcohol/aldehyde dehydrogenase
I) Oxidation ReactionsI) Oxidation Reactions The main enzymes involved in the oxidation of xenobiotics
called mixed – function oxidases (MFO) or monooxygenases, found mainly in the liver but also occur to less extent in other tissues.
Cytochrom P450 ( CYP450 ) catalyze the majority of Drug metabolism oxidation reactions. MFO is an old terminology,the enzyme are most frequently
known as CYP450 Superfamily
-The enzyme systems carrying out this biotransformation are referred to as monooxygenases or microsomal (non specific enzymes in liver).
-The reaction requires both molecular oxygen and the reducing agent (Activation of O2 1 atom goes to organic molecule, the other reduced to H2O0.
-NADPH (nicotinamide adenosine dinucleotide phosphate).
-Monooxygenases are made up of several components-:
1 (Cytochrome P-450 which is the most important component and is responsible for transferring an oxygen atom to the substrate R-H.
2 (Cofactors supply the reducing equivalents )electrons (needed in the overall metabolic oxidation
a) NADPH. Dependent cytochrome P-450 reductase.
b) NADH. Linked cytochrome P-450.*Cytochrome P-450 is found in high concentration in the
liver, also present in other tissues like lung, kidney, intestine, skin, placenta and adrenal cortex.
C) FMO is also a member of the mono-oxygenase system
*It is characterized by the substrate nonspecificity, this versatility may be attributed to the multiple forms of the enzyme.
-Consequently, the biotransformation of a parent xenobiotic to several oxidized metabolites is carried out not just by one form of P-450 by several different forms.
-It is now actually proven that the metabolism of drug is carried out by different isoforms,members of the CYP450 superfamily,eg:CYP2A1,CYP2D6,CYP3A4…etc
-A large number of families (at least 18 in mammals) of cytochrome P-450 (abbreviated “CYP”) enzymes exists as well as many subfamilies.
each member catalyzes the biotransformation of a unique group of drugs
-CYP450 SUPERFAMILY: classified according to sequence homology.
-High homology: > 90%, intermediate: ~ > 60%; Low: ~ > 40%
-FAMILY: members have > 40% homology (low). E.g.: CYP1 vs. CYP2
-SUBFAMILY: members have > 60% homology (intermediate). E.g.: CYP2A vs. CYP2B
ISOFORM: CYP2A1, CYP2A2. (High)
CLASSIFICATION
Major reactions of oxygenation Major reactions of oxygenation catalyzed by CYP450catalyzed by CYP450::
1-Carbone oxidation reaction
2-N-Oxygenation reactions.
3-S-oxidation.
2-N-Oxygenation reactions :1-Carbone oxidation reactions
a)Hydroxylation of Saturated aliphatic C atom. ring b)Hydroxylation of aromatic
aliphatic c)Oxidation of unsaturated
Major reactions of oxygenation Major reactions of oxygenation catalyzed by CYP450catalyzed by CYP450
3-S-oxidation.
Oxidation reactionsOxidation reactions
1. Carbon oxidation reaction
A) –Aliphatic hydroxylationB) -Aromatic hydroxylation
2. N-Dealkylation3. N-oxide formation4. Oxidative Deamination5. O-Dealkylation reactions6. S-Dealkylation .
7. S-oxidation reactions
A) –Aliphatic hydroxylationA) –Aliphatic hydroxylationi)saturated aliphatic carbon atomsi)saturated aliphatic carbon atoms Saturated aliphatic C-H bonds are
metabolised by hydroxylation on the penultimate carbon atom (ω-1 )and on the ultimate carbon(ω)to lesser extent.
ii)Enzymatic introduction of a hydroxyl ii)Enzymatic introduction of a hydroxyl group into cyclohexane ring generally group into cyclohexane ring generally
occurs at C-3 or C-4occurs at C-3 or C-4
-In humans the trans-4-hydroxycyclohexyl product has been reported as a major metabolite of acetohexamide ( hypoglycemic agent )
iii(TerodilineAromatic p-hydroxylation predominate
with R-isomer where as benzylic hydroxylation is preferred with S-isomer.
iivv)Tolbutamide)Tolbutamide
CYP450
Tolbutamide
Pentobarbital
CYP450
Ibuprofen
CYP450
CYP450
Phenmetrazine
Valproic Acid
CYP450
CYP450
)v
VI)Oxidation at Benzylic Carbon Atoms Benzylic carbon atoms are susceptible to oxidation forming the corresponding
alcohol or carbinol which is further oxidized to or conjugated with glucuronic acid.
CH3CN
CH3
CH2COOH
O
Tolmetin
CHOOCN
CH3
CH2COOH
O
Dicarboxylic Acid Metabolite
)Oxidation at Carbon Atoms Alpha to Carbonyl and Imines
An important class of drugs undergoing this type of oxidation is the benzodiazepines e.g. diazepam and flurazepam. The C-3 carbon atom is to both a lactam carbonyl and an immino functionality.
N3
N
Cl
OH3C
Diazepam
N3
N
Cl
O
(CH3CH2)2NCH2CH2
Flurazepam
F
N3
N
O2N
OH3C
Nimetazepam
4
NH
12
3 CH2CH3C6H5
OO
4
NH
12
3 CH2CH3C6H5
OO
HO
Glutethimide 4-Hydroxyglutethimide
Hydroxylation of the carbon atom to carbonyl group generally occurs only to a limited extent e.g. glutethimide
vvi)i) Aliphatic hydroxylation Aliphatic hydroxylation (alkene epoxidation)(alkene epoxidation)..
B) Aromatic Hydroxylation (Oxidation of aromatic rings) :Aromatic epoxidation:
It involves oxidation of aromatic compounds (arenes) to their phenolic metabolites (arenols).
R
Arene
R
Arene oxide
O
R
OH
Arenol
It is a major route of metabolism for many drug containing phenyl groups .
Benzo[a]pyrene
ONH
N
N
N
N
O
Covalently bound Deoxyguanosine adduct(systemic toxicity)
O
HO
OH
Deoxyribose
NH
N
N
N
N
O
Deoxyribose
:Rules for Aromatic Oxidation-In most of drugs containing aromatic moieties, microsomal aromatic hdroxylation
occurs at the para-position.-Microsomal aromatic hydroxylation reactions proceed most readily in activated
(electron-rich) rings e.g. rings containing electron donating group as NH2 group.-Deactivated aromatic rings (e.g., those containing electron-withdrawing groups as
Cl, N+R3, COOH, SO2NHR are generally slow or resistant to hydroxylation.
Cl
NH
Cl
HN
NH
Clonidine hydrochloride
COOH
SO2N(CH2CH2CH3)2
Probenecid
8
7 O
O
3
2 Cl
Cl
Cl
Cl
2,3,7,8-Tetrachlorodibenzo-p-dioxin(TCDD)
Cl
For compounds in which two aromatic rings are present, hydroxylation occurs preferentially in the more electron-rich ring.
HN
NCl
O
7
N
S
3 Cl
CH2CH2CH2N(CH3)2
Chlorpromazine
Cl
p-Chlorobiphenyl
Diazepam
HN
HNO
O
C6H6
Phenytoin
HN
HNO
O
C6H6 OH
Phase-II conjugate
When para‑ position of aromatic ring is occupied the oxidation occurs in ortho‑ position.
2
3
CH3OH
C CH
HO
17-Ethinylestradiol
Estradiol
CYP450 CYP450
CYP450
MMetabolic oxid. of C-N & C-S involve hydroxylation of etabolic oxid. of C-N & C-S involve hydroxylation of alpha carbone atom attached directly to alpha carbone atom attached directly to
heteroatom(N,O,Sheteroatom(N,O,S)):General Mechanism
a) Hydroxylation of the -carbon atom attached directly to the heteroatom.
R X C
H
R X C
OH
R XH + C
O
Usually unstable
= N = O = S
R
Aldehyde or ketone
R - NH2 1 ry amineR - OH alcoholR - SH thioalcohol
2 (N-dealkylation
b) Hydroxylation or Oxidation of the Heteroatom (N, S only):
C N C N OH C N O
Hydroxylol
This reaction is catalyzed by cytochrome P-450 and N-oxide amine oxiases or N‑oxidases.
C S C S O C SO
O
Sulphoxide Sulphone
cont…..cont…..N-dealkylationN-dealkylationؤؤ
It involves oxidation of tertiary and secondary It involves oxidation of tertiary and secondary amines. amines. oxidative alpha-hydroxylation at alpha-C oxidative alpha-hydroxylation at alpha-C
then dealkylationthen dealkylation..
i) Oxidation of Tertiary Aliphatic Amines:
It is characterized by oxidative removal of alkyl group (particularly –CH3 group) form tertiary aliphatic and alicylic amines. Removal of the first alkyl group occurs more rapidly than the removal of the second alkyl group. Bisdealkylation may occur but very slowly.
CH2CH2CH2NCH3
CH3
HCH
O
CH2CH2CH2NCH3
H
HCH
O
CH2CH2CH2NH2
minor
Imipramine Desmethylimipramine(desipramine)
Bisdesmethylimipramine
i (
ii(
HN
CHCH2
CH3
CH3
HCH
O
NH2
CHCH2
CH3
NH3 CCH2
CH3
O
Methamphetamine Amphetamine Phenylacetone
-Oxidation of Secondary Amines
Amines can undergo deamination. Amphetamine for example is deaminated to phenyl acetone and ammonia
iii
Nicotine
Nornicotine
Cotinine
Norcotinine
CYP450
CYP450CYP450
-The biotransformation of amines is the same as the carbon and nitrogen oxidation reactions seen for aliphatic amines but tertiary and secondary aromatic amines are rarely encountered in medicinal agents
.
3 (N-Oxide formation :
Mephentermine Mephentermine N-Oxide
4 (Oxidative Deamination:
Amines can undergo deamination. Amphetamine for example is .. deaminated to phenyl acetone and ammonia .
Oxidative deamination of most exogenous primary amines is carried out by the mixed oxidases. However, endogenous primary amines, such as dopamine, norepinephrine, tryptamine and serotonin, are metabolized through oxidative deamination by monoamine oxidases (MAO).
NH2
CHCH2
CH3
Amphetamine
-Carbon
Hydroxylation NH2
CCH2
O
Carbinolamine
CH3
H NH3
CCH2
CH3
O
Phenylacetone.
This process is similar to N-dealkylation, in that it involves an initial ‑carbon hydroxylation reaction to form a carbinolamine intermediate, followed by carbon-
nitrogen cleavage to the carbonyl metabolite and ammonia in primary amines
Mechanism:
Oxygen alkyl groups are removed by liver microsomal preparation by a mechanism involves α-hydroxylation of the alkyl groups
5 )Oxidative Dealkylation
i)The metabolism of these systems occurs through oxidative O‑dealkylation by microsomal enzymes.
NH3C
H
O
OH
Codeine
O-demethylation byCyt P-450 (O2)
NH3C
H
O
OH
Morphine
HO
O-demethylation byCyt P-450 (O2)
NH3C
H
O
OH
Normorpthine decrease
HO
N
CH2CO2H
CH3
C O
Cl
CH3O
CH3HN
OC2H5
O
Phenacetin
N
N
NH2
CH3O
CH3O
N N C
OO
Prazosin
. 6) S-dealkylation
N
N NH
N
SCH3
N
N NH
N
SCH2 OH
N
N NH
N
SCH2 OH
+ HCH
O
6-(Methylthio)-purine
6-Mercapto-purine
HN
NH
O
CH2CH2S
Methitural
O
S
CHCH2CH2CH3CH3
CH3
MethituralS-demethylated metabolite
Oxidation of S SulfoxideO
S SulfoneO
SO
HN N
CH2H3CS
CH2CH2
HN
CNHCH3
X HN N
CH2H3CS
CH2CH2
HN
CNHCH3
XO
Cimetidine X N C N
Metiamide X S
Sulfoxide Metabolite
N
S
2
CH2
S CH3
NCH3
CH2
Thioridazine
i-Oxidation of Sulfur:
Thioethers or sulfides, for example, Chlorpromazine and Cimetidine are oxidized to their sulphoxides
ii- Desulfuration It is the conversion of thione (C = S) to the corresponding (C = O).
HN
NH
OCH3
CHCH2CH2CH3
O
S
CH2CH3HN
NH
OCH3
CHCH2CH2CH3
O
O
CH2CH3
Thiopental Pentobarbital
II.Reduction
-Play an important role in the metabolism of compunds containing azo,nitro,carbonyl.
-Bioreduction of nitro & azo lead to amino derivatives ,where as carbonyl compounds reductions lead to alcohol analogs…
1-Azo-reduction
2-Nitro reduction
E.g.: The opioid receptor antagonist Naltrexone is reduced in humans to it’s secondary alcohol metabolite
3-Reduction of Carbonyl group
Bio reduction of sedative – hypnotic Chloral hydrate yields trichloroethanol. This oxidation is non- microsomal is believed to take place by alcohol dehydrogenase.
4-Reduction of Sulphur containing group
.activeprodrug inactive
III-Hydrolytic Reactions:Metabolism of ester & amide linkage in many drugs catalyzed by hydrolytic enzyme(esterase and amidasea).
ProcaineamideProcaine
ExampleExample
ProcaineShort acting local anesthetic
ProcainamideLong acting antiarrhythmic
T1/2 = 2.5-4.5 hrT1/2
= 40-84 second
Ester vs. Amide bondEster vs. Amide bond•The duration of actions of ester drugs are less than the amide
analogues.why?Procaine (ester type) injection or topical is usually shorter
acting than its amide analogue procainamide administered similarily
Ester bond is relatively weaker than amide bond, it will be rapidly hydrolyzed by esterase enzyme
R O
O
R NH
O
The reactivity of ester and amide bond depend on how much the carbonyl carbon is electropositive
Nitrogen atom is less electronegative than oxygen, so it will be waeker electron withdrawing atom
therefore, the crabonyl carbon attached to oxygen atom will be more electropositive, and more reactive toward nucleophilic attack of water molecule during hydrolysis.
Nucleophilic attack of Nucleophilic attack of hydroxide anion on ester and hydroxide anion on ester and
amideamide
R O
O
R NH
O
OH OH
Esterases and Amidases:
Esters are more prone to hydrolysis.
Converted to more W.soluble carboxylic acids.
E.g.: Meperidine, Succinylcholine.
Sterically hindered esters might be excreted unchanged??? Why?
Amides:
M ore resistant to hydrolysis than esters why,?
A dvantage: Procaine vs. Procainamide.
Procaine: ester, very short half life, destroyed shortly after entering circulation
P rocainamide :l onger half life than procaine.
more than 60% excreted unchanged
