drug metabolism refers to enzyme-mediated biotransformations (detoxication) that alter the...
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Drug metabolism
Refers to enzyme-mediated biotransformations (detoxication) that alter the pharmacological activity of both endogenous and exogenous compounds.
DrugMetabolic enzymes
Metabolite
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.
• Many enzymes take place in such biotransformations; oxidase, hydrolase, lipase, synthetase, dehydrogenase, …etc
Blood
CNS
Muscles
Heart
Metabolism may result in:
– Pharmacologically inactive drug (detoxification).
– Pharmacologically active drug (bioactivation…prodrug approach).
– Change the pharmacological activity (toxic effect).
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-II metabolism
• Involves the following conjugation reactions that are catalyzed by transferase enzymes:
– Glucuronidation.– Sulfation.– Amino acid conjugation.– Methylation.– Acetylation.
Phase-I reactions
• Two general types of enzyme systems take part in these reactions:
– Microsomal Mixed Function Oxidases (MFOs)• Flavoprotein, NADPH-monooxygenase• Cytochrome P450
– Non-cytochrome oxidizing enzymes.• Xanthine oxidase• Alcohol/aldehyde dehydrogenase
General features of Cytochrome P-450
• 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
• some overlap in the substrate specificities.
CYP 450Families and subfamilies
Foye's principles of medicinal chemistry
CYP Biotransformations
• Chemically diverse small molecules are converted, generally to more polar compounds
• Reactions include:
– Aliphatic hydroxylation, aromatic hydroxylation– Dealkylation (N-,O-, S-)– N-oxidation, S-oxidation– Deamination– Dehalogenation
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:
RR
R
O
R R
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:
RR
R
O
R R
Oxidative Phase-I involving cytochrome P-450 enzymes:
• Aromatic hydroxylation:
HN
HNO
O
C6H6
Phenytoin
HN
HNO
O
C6H6 OH
Phase-II conjugate
• Aromatic epoxidation:
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
DNA
Oxidative Phase-I involving cytochrome P-450 enzymes:
• Alkene epoxidation:
N
O NH2
Carbamazipine
N
O NH2
O
N
O NH2
HOOH
HN
NH
O
O O
Secobarbital
HN
NH
O
O O
OH
HO
Secodiol
Glutathione conjugation
• For electrophilic drugs and metabolites:
G-SHO
R R SG
HH H HOH
G-SH O OH
SG
HS
NH
O COOH
HNH O
COOHNH2
Glutathione (tripeptide: glutamate+cysteine+glycine)
Detoxication by glutathione adduct formation
OH
O
O
N
O GSH
OH
O
O
N
GS
HO
[O]
O
Arene oxide
OH
SG
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.
Glutathione conjugation
O
O
HH
NIH
Shift
OH
H2O
Or epoxide hydrolase
OH
OH
GSHOH
SG
DNA, RNA, proteins
OH
M
• O-dealkylation:
O
N
O
OH
CH3
O
N
HO
OH
Oxidative demethylation
Codeine Morphine
• N-dealkylation:
HN R
OH
NH2R
O
H
N-dealkylation
Cl
N
N
C6H6
Cl
N
NH
C6H6
N-t-butylnorchlorcyclizine norchlorcyclizine
Br
N
Br
NH2
Br
O
OH
Brompheniramine
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.
R NH2 R NH
OHR NO2
NH2HN
OHN
O
proteins and nucleic acids
NO2
N
OH
Protein
Oxidize Fe+2 in hemoglobin to Fe+3
(methemoglobin or ferrhemoglobin)
this form is no longer capable to transport oxygen
(methemoglobinemia toxicity)
NO
HNO
2-acetylaminofluorene
NOHO
NO
-O3SO
Sulfate
conjugation
NO
NO
Nu
|Nitrenium ion
NO
Nu
Hydrolytic phase-I metabolism
• By non-specific esterase and amidase enzymes that present in plasma, gut, liver and kidney.
• It has a beneficial role in most of prodrugs that after hydrolysis inside the body release the active form of the drug.
Ester vs. Amide bond
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 hydroxide anion on ester and amide
R O
O
R NH
O
OH OH
Example
ProcaineShort acting local anesthetic
ProcainamideLong acting antiarrhythmic
T1/2 = 2.5-4.5 hrT1/2
= 40-84 second
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)
Why Dipifevrine has been prepared?
– Adrenaline is a polar drug….difficult access into the ocular cavity.
– Adrenaline has a generalized adrenergic effect…. Many side effects such as increase blood pressure, heart rate and bronchodilation.
– Dipifevrine is more lipophilic, better penetration… localized effect.
HO
HO
HN
OH
Adrenaline
Other phase-I metabolic enzymes
• Alcohol dehydrogenase and aldehyde dehydrogenase:
R
OH
R
O
H R
O
OH
Alcohol
dehydrogenase
Aldehyde
dehydrogenase
R
O
RNo oxidation
Other phase-I metabolism
• Heterocyclic ring oxidation:
• S-dealkylation:
O
NH
Phenmetrazine
O
NH
O
N
N N
HN
S
6-methylmercapropurine
N
N N
HN
SH
6-mercapropurine
Other phase-I metabolism
• Sulfoxidation: by flavin monooxygenase
N
S
N
Chlorpromazine
N
S
N
O
N
S
NH2
O
N
S
O
OHO
Deaminated metabolite Dealkylated metabolite
Other phase-I metabolism• Azoreduction:
NN
COOH
OH
S
HNN
O
O
Azulfidine
NH2
S
HNN
O
O
H2N
COOH
OH
P-aminosalicylic acidSulfapyridine
Antibacterial action Anti-inflammatory action
General notes regarding phase-I metabolism
• Hydrolysis normally catalyzed by carboxylesterases:– Cholinesterase…. Hydrolyzes choline-like esters (such
as succinylcholine), procaine and acetylsalicylic acid.
– Arylcarboxyesterase.– Liver carboxyesterase
O
O
O
O
N
N
Syccinylcholine
OH
O
OH
O
NHO
N
COOC2H5
HN
COO
HN
COOCH3
HN
COOZwitter ionicPolarEasily excreted
General notes regarding phase-I metabolism
• Esters that are sterically hindered are hydrolyzed more slowly and may be appeared unchanged in urine:
OH
O
O
N
Atropine
50% excreted unchanged in urine
OH
O
O
N
Atropine
General notes regarding phase-I metabolism
• Amides are more stable to hydrolysis than esters….large fraction of amide containing drugs are normally excreted unchanged.
H2N
NH
O
N
Procainamide
60% excreted unchanged in urine
H2N
O
ON
Procaine
HN
O
N
Lidocaine
Procaine has a short duration of anesthesia
lidocaine has a long duration of anesthesia
CH3
CH3
HN
O
N
Lidocaine
Nucleophilic attack of HO-
Aromatic hydroxylation
• The least substituted aromatic ring will be favorably oxidized, especially at the least hindered carbon atom
• The activated ring will be better oxidized (the ring bearing an electron donating group)
• The least substituted aromatic ring will be favorably oxidized, especially at the least hindered carbon atom
N
NCl
MeclizineAntiemetic agent
N
NCl
OH
Major metabolite
• The activated ring will be better oxidized (the ring bearing an electron donating group)
HN
HN
N
Cl
Cl
Clonidine
No aromatic hydroxylation