Download Drug Metabolism

Drug Metabolism
• Drug Metabolism: The biochemical changes that occur on drugs
or other foreign compounds, the purpose of which is to facilitate
elimination from the body.
Body
Drug
(lipophilic)
Enzyme
Metabolite
(polar)
Excretion
• Without it, xenobiotics can remain indefinitely in the body.
• It may lead to the formation of inactive and non-toxic compounds,
hence the term detoxification.
• However, more recent studies have shown that some metabolites
are not only active, but may be toxic.
Example:
OH
Metabolism
O
N
A
F
Haloperidol
B
Cl
BCPP+
1
Categories of Drug Metabolism Reactions
A) Oxidative
1. Phase I Reactions:
(Functionalization)
B) Reductive
C) Hydrolytic
- Introduces Polar Functional Groups: e.g. –OH, -COOH, -NH2
2. Phase II Reactions:
(Conjugation)
Combination-type Reactions
Eg, A + B = AB
- Attaches Polar and Ionizable Endogenous Groups so as to achieve
complete solubility
- It also tends to lower or terminate biological activity
2
Phase I Reactions
1) Direct Attachments:
OH
Aromatic Hydroxylation
CH3
OH
Aliphatic Hydroxylation
CH3
2) Unmask Existing Functional Groups
A) Oxidation
R
R
C
H2
OH
O
Oxidation
R1
C
O
Oxidation
H
R1
C
OH
O
C
H2
O
CH3
RNHCH2CH3
O-Demethylation
N-Deethylation
R
CH2OH +
H
C
H
O
R NH2
+
H3C
C
H
3
B) Reduction
O
R1
C
OH
Reduction
R2
C
R1
R2
H
R NO2
;
Reduction
R NH2
Nitro group
Ketone
R1 N N R2
Reduction
R1NH2
+ R2NH2
AZO group
C) Hydrolysis
O
R1
C
O
R2
O
Hydrolysis
R1
C
+ R2 OH
OH
Ester
O
R1
C
N
H
R2
O
Hydrolysis
R1
C
+
OH
R2NH2
Amide
4
Phase II Reactions
1) Glucuronic Acid Conjugation
HOOC
O
R
5
OH
+ HO
H
1
HO
3
Phase I metabolite
or parent xenobiotic
O
OH O
UDP-Glucuronyl
Transferase
HOOC
O
5
UDP
Actived Uridine-5’-Diphosphoa-D-Glucuronic Acid
(Note a-linkage at C-1)
HO
HO
O
O
R
1
3
OH
H
b-Glucuronide
(Note b-linkage at C-1)
5
2) Sulfate Conjugation
OH
PAP O
+
O
Phase I metabolite
or parent xenobiotic
O
S
O
OH
S
OH
Activated 3’-PhosphoAdenosine-5’-Phospho-Sulfate
R H
N
O
+ PAP O S
OH
O
R NH2
O
O
O
S
O
OH
3) Amino Acid Conjugation
R
+
OH
CH
H2 N
R2
O
R2
O
COOH
R
N
H
CH
COOH
Amino Acid
6
4) Other Phase II Reactions
Generally, these do not increase water
Solubility but mainly terminate activity
A) Methylation
OH
OH
HO
NH2
O-Methylation
H3CO
COMT
HO
NH2
HO
Active
Inactive
B) Acetylation
O
O
NH2 N-Acetylation
N
H
N
N
H
H
N
CH3
O
N
Active
Inactive
Note: Both methylation and acetylation lead to lower solubility and
often lead to inactivation
7
Serves as a protective mechanism
against electrophilic species
C) Glutathione Conjugation (GSH)
O + +
NH2
HS CH2CH NHCOCH2CH2CHCOOH
OH
CONHCH2COOH
Arene Oxide
GSH
SG
OH
SCH2CHNHCOCH3
COOH
8
Sites of Drug Biotransformation
• Liver
- most important
- Most drug metabolizing enzymes
- Orally administered drugs first pass through the liver and thus are susceptible to
First-Pass Effect. This can lead to lower bioavilability.
For example, Lidocaine is inactive when given orally due to the first-pass effect
N
N
a, b
NH2
+
O
Lidocaine
(Xylocaine)
HO
O
N
H
a. Microsomal Oxidation
b. Microsomal Amidase
• Other Sites:
Intestine; Kidneys; Lungs; Skin; Placenta; Brain; Adrenal Glands
9
Oxidative Biotransformations of Drugs
- By far the most widespread and the most important
- General Reaction:
R-H + O2 +
H+
NADPH
Enzyme
R-OH + NADP+ + H2O
- Enzyme System: Mixed Function Oxidases or
Monooxygenases, the most important component
is Cytochrome P-450.
- Cytochrome P-450 is responsible for transferring an
Oxygen atom to the substrate R-H
-NADPH: Reduced Nicotinamide Adenosine Dinucleotide
Phosphate.
10
1) Aromatic Hydroxylation
O
R
R
Arene
OH
R
Arenol
Arene Oxide
• Hydroxylation often occurs para to the substituent on the ring
• Occurs on the more activated ring, common activating groups:
-OH; -OCH3; -NH2; -NHR; and Alkyl group, e.g. -CH3, -CH2CH3, etc
• Deactivated rings are resistant to oxidation, common deactivating groups:
-F, -Cl, -Br, -NO2, -SO2NHR, -COR etc
Example:
O
OH
N
H
Propranolol (Inderal®)
O
OH
N
H
OH
11
2) Olefinic Oxidation
-Oxidation of olefinic “C=C” double bond leads to epoxide
e.g.
HO
O
N
O
N
N
NH2
O
Carbamazepine
(Anticonvulsant)
OH
O
NH2
Epoxide
NH2
Trans-diol
3) Benzylic Carbon Oxidation
e.g. 1
O
O
O
SO2NHCNHC4H9
SO2NHCNHC4H9
SO2NHCNHC4H9
CH2
CH3
Tolbutamide (Orinase)
Hypoglycemic Agent
CH2OH
Alcohol
Metabolite
COOH
Carboxylic Acid
Benzylic Carbon
12
OH
OH
OCHCHCH2NHCH(CH3)2
e.g. 2
OCHCHCH2NHCH(CH3)2
Metoprolol (Lopressor)
(b-Adrenergic Blocker)
CH2CH2OCH3
HO CHCH2OCH3
4) Allylic Carbon Oxidation
H2C C
H
CH2
Allylic Carbon
e.g.
7
7
1
6
7
7
HO
2
6
3
5
OH
3
+
6
3
+
HO
6
3
4
O
C5H11
Tetrahydrocannabinol
(D1-THC) (Hallucingen)
7-Hydro- D1-THC
6b6a(equal or more active) (minor)
(minor)
No 3-metabolite due to steric hindrance
13
5) Oxidation of Carbon Atom a to C=O or C=N
O
N
Cl
N
Diazepam
O
N
H
OH
N
Cl
Hydroxydiazepam
6) Alicyclic and Aliphatic Carbon Oxidation
A) Oxidation at a Terminal Carbon (w-Oxidation)
B) Oxidation at Penultimate Carbon (w-1-Oxidation)
e.g.
HOOC
C3H7
C3H7
A OH
CH2CH2CHCOOH
Valproic Acid
CH2CH2CH2CHCOOH
(Antiepileptic Agent)
C3H7
C3H7
B
CH3CHCH2CHCOOH
CH3CH2CH2CHCOOH
OH
14
C) Alicyclic Hydroxylation
O
O
e.g.
Acetohexamide
(Dymelor)
O O
S
NH
O N
H
4
O O
S
NH
O N
H
H
H
H
4
OH
H
H
trans-4-OH-
7) Oxidation Involving Carbon-Heteroatoms
(Also cis-4-,
trans-3- ,
cis-3- as minor)
A) Oxidative N-Dealkylation
e.g.
CH3
N
H3C
CH3
HN
C
H2
C
H2
CH3
NH2
Benzphetamine
Note: - Small alkyl group is preferentially removed
- First alkyl group is removed at faster rate than the second one
15
B) Direct Oxidation on N-Atoms
O
O
e.g.
O
O
N-Oxidation
N
N
H3C
CH3
O
Meperidine-N-Oxide
Meperidine
C) Oxidative Deamination
e.g. 1
H
H
aC
O
NH2 a-Carbon
CH3
Hydroxylation
C
CH3
NH2
-NH3
C
O
CH3
Amphetamine
Note: For endogenous amines i.e. Dopamine, Norepinephrine, Serotonin etc.
the group of enzymes responsible for deamination is called Monoamine
Oxidase (MAO)
16
e.g. 2
HO
MAO
NH2
HO
HO
C
HO
O
H
Dopamine
D) O-Dealkylation
e.g.
NHCOCH3
NHCOCH3
NHCOCH3
O
+
O CH2CH3
O CHCH3
O
Phenacetin
H
CH3CH
OH
Acetaminophen
(Tylenol)
17
E) S-Dealkylation
e.g.
SCH3
SH
N
N
N
H
N
6-(Methythio)Purine
N
N
N
N
H
6-Mercaptopurine
F) Desulfuration
e.g.
O
HN
S
N
H
O
CH2CH3
O
CHCH2CH2CH3
CH3
Thiopental
CH2CH3
HN
O
N
H
O
CHCH2CH2CH3
CH3
Pentobarbital
18
G) Direct Oxidation of Sulfur
e.g.
S
S
N
S
CH3
H2CH2C
N
N
S
H2CH2C
O
CH3
N
CH3
CH3
Thioridazine (Mellaril)
Mesoridazine (Serentil)
Active
O
S
O
N
H2CH2C
S
CH3
S
O
S
O
Sulfoxide
Sulfone
N
CH3
Sulfoxide drugs and metabolites may be further oxidized to sulfones
19
H. Oxidation of Alcohols and Aldehydes
O
Alcohol
R CH2OH
Dehydrogenases
R
C
O
Aldehyde
H
Oxidase
R
C
OH
8) Other Oxidations
A) Aromatization
Et HO C CH
O
Norgestrel (Oral Contra)
Et HO C CH
HO
17a-Ethinyl-18-Homoestradiol
(Minor Metabolite in Women)
20
B) Dehalogenation
H
H
Cl
C Cl
Cl
Chloroform
O
Cl
C Cl
H2CO3 + HCl
-HCl
O
Cl
+ H2O
C Cl
Cl
Tissue
Nucleophies
Covalent Binding
Phosgene
21
Question
Predict the metabolic products of the following compounds:
(1)
S
N
Cl
CH2CH2CH2
N
H3C
CH2CH3
(2)
CH3
CH2OCH2CH2 N C CH3
H
CH3
22
Reductive Biotransformations
- Carbonyl (C=O), Nitro (NO2), and Azo (N=N) groups susceptible to reduction
1) Reduction of C=O
e.g.
O
O
H
H
N C N S
O
C CH3
Aldo-Keto Reductases
(NADPH)
O
Acetohexamide
O
O
H
H
N C N S
O
C CH3
HO H
S(-) Hydroxyhexamide
(Note asymmetric center is introduced)
23
2) Reduction of NO2 group
e.g.
O
O2N
O
C N N
H
NH
O
Dantrolene
Nitro Reductase
O
H2N
O
H2N S
O
H2N
N N
Sulfamidochrysoidine
(Prontosil)
NH
O
Aminodantrolene
3) Reduction of AZO group
O
H2N S
O
O
C N N
H
NH2
+
NH2
Sulfanilamide
(Active)
H2N
H2N
NH2
24
Hydrolytic Reactions
1) Hydrolysis of Esters
e.g.1
COOH
O
COOH
CH3
Esterases
O
Aspirin
O
OH
+ HO
CH3
Salicylic Acid
e.g.2
Cl
O
O
Cl
O
EtO
Clofibrate (Atromid-S)
Hypolipidemic Drug
O
HO
Active
25
Note: Formation of active drugs from deliberately masked drugs is the
basis of the Prodrug concept:
e.g.1 Chloramphenicol is too bitter to use orally. The Palmitate ester is made
to mask bitterness:
O
OH NHCCHCl2
O2N
O
C CH2CH2 O C (CH2)14CH3
H
e.g.2 Carbenicillin has poor oral absorption, the Indanyl ester is made to
increase absorption:
O C6H5 O
O
C CH
C HN
N
O
COOH
26
2) Hydrolysis of Amides
- Amides are more slowly hydrolyzed than esters.
H3CO
H3CO
O
N
N
N C
N
NH2
Prazosin (Minipress)
Selective a-1 Blocker
H3CO
N
O
N
NH
N
H3CO
NH2
+
O
HO C
O
27
Phase II Conjugation Reactions
- The following groups are involved:
COOH
O
OH
O
HO S OO
OH
Sulfate
CH3
Methyl
HS
O UDP
OH
Glycine
COOH
O
O
a-Glucuronic Acid
H2N C COOH
H2
NH2
H
N
N
H
COOH
Glutathione
NH2
O
H2N C CH2CH2CH COOH
Glutamine
O
C CH3
Acetyl
- These groups are transferred by the appropriate conjugation enzymes on to
the drugs or other metabolites
28
- The following functional groups are susceptible to conjugation:
OH
O
R C OH
RCH2OH
R NHCOR2
RNH2
R1
R SO2NHR2
RSH
R N R2
R X H
R CH R2
1) Glucuronic Acid Conjugation
-The most common conjugative pathway
- Steps: a. Synthesis of activated glucuronic acid (UDPGA)
b. Enzymatic transfer
COOH
O
OH
OH
+
O UDP
OH
COOH
O
OH
R X H
OH
XR
OH
UDPGA (Uridine-5’-DiPhospho-a-D-Glucuronic Acid)
29
A) O-Glucuronides
e.g.1
O
HN
C
CH3
OH + UDPGA
COOH
O
OH
OH
O
NH
C
O
OH
Acetaminophen
e.g.2
O
OH
O
C OH + UDPGA
COOH
O
OH
OH
O
OH
C
OH
Salicylic Acid
30
B) N-Glucuronides
+ UDPGA
N
(CH2)3 NHCH3
N
COOH
O
OH
OH
N
(CH2)3
CH3
OH
C) S-Glucuronides
N
N
CH3
N
SH
+ UDPGA
COOH
O
OH
OH
S
N
CH3
OH
31
- Glucuronic acid conjugates are excreted mainly through urine. If molecular
mass exceeds 300 DA, biliary route becomes important.
Enterohepatic Circling (Reflux):
1. Drug undergoes glucuronidation in the liver
2. Glucuronide excreted in the bile
3. Hydrolyzed by b-Glucuronidases in intestine
4. Hydrolyzed drug reabsorbed in intestine
LIVER
Drugs or endogenous
compounds
Glucuronide
Absorption
Excretion
Hydrolyzed Drug
b-Glucuronidases
INTESTINE
32
2) Sulfate Conjugation
- Occurs primarily in phenols, but possible in aromatic amines, alcohols
H
and N-hydroxy compounds
R N OH
- Mechanism of Transfer: 1. Activation of Sulfate (SO42-) to PAPS
2. Transfer of SO42- from PAPS to substrate (drug)
O
O
O P O S O
OH
5'
O
Adenine
O
HO
N
H
+
C(CH3)3
HOH2C
OH
3’-Phosphoadenosine-5’phosphosulfate (PAPS)
N
H
-PAP
1'
3'
H2O3PO
C(CH3)3
HO
HOH2C
OH
O
O S OH
O
Sulbutamol
(b-Adrenergic
Bronchodilator)
33
3) Conjugation with Amino Acids and Glutathione
A) Conjugation with Glycine and Glutamine
- Glycine and Glutamine are used by mammalians to conjugate –COOH,
especially aromatic acids and arylalkyl acids
- Mechanism of transfer: a. Substrate (carbonic acid) is activated with ATP
Coenzyme A to form acyl-CoA complex.
b. Then the acyl-CoA complex acylates glycine or
glutamine with specificN-acyltransferase.
CoAS
OH
O
ATP CoA
O
NH2
RH
C
HOOC
NH
RCHCOOH
O
N-acyltransferase
34
e.g.
OH
O
C(CH2)3 N
Cl
F
Haloperidol
p-Flourophenylacetic Glycine Conjugate
Acid
B) Conjugation with Glutathione
- Important to detoxify reactive electrophilic species, which may otherwise form
covalent bond with nucleophilic groups in proteins and nucleic acids
- Catalyzed by cytoplasmic enzymes known as glutathione S-transferases
- Do not require coenzyme to activate substrate.
- Electrophilic species: 1) electron-deficient carbon or heteroatom
2) electron-deficient double bond
GSH

H2C
R
-
X
GS CH2 + HX
R
35
O +
H
N
HS
COOH
N
H
COOH
Glutathione
S-Transferase
NH CH3
S
O
O
N
H
OH
Mercapturic Acid Der
COOH
- glutamyl
transpeptidase
OH
OH
S
O
COOH
O
NH2
OH
S
Glutathione (GSH)
N-Acetylase
NH2
H
N
O
O
Arene Oxide
OH
NH2
O
OH
Cysteinyl
Glycinase
NH2
S
O
N
H
COOH
Notice the Glutathione adduct undergoes further
Biotransformation: cleavage of glutamic acid
and glycine; then N-acetylation
36
4) Other Phase II Biotransformation
A) Acetylation
H2N
O
S
O
Dapsone
Antileprotic Agent
O
NH2
CH3CCoA
H2N
Acetyltransferases
O
S
O
NHC CH3
O
Acetylation Polymorphism:
- Caused by differences in N-acetyltransferase activity
- Rapid Acetylators = Eskimos & Orientals-R
- Slow Acetylators = Egyptians & W. Europe-S
Consequences:
- Rapid Acetylators -- often show inadequate therapeutic response
-Slow Acetylators -- may show adverse drug reaction but greater therapeutic
response
37
B) Methylation
- Like acetylation, leads to lower solubility
- Primary function is attenuation of activity
- Constitutes only a minor pathway
- Coenzyme: S-Adenosylmethionine (SAM)
HOOC
H
CH3
NH2
C CH2CH2 S CH2
Adenine
O
SAM
HO
OH
- Compounds which undergo methylation reaction:
OH
OH
R1
R
OH
N
R
R2
SH
N
- Enzymes involved: Catechol-O-methyltransferase (COMT);
Phenol-O-methyltransferase;
Other non-specific N- and S-methyltransferase
38
Examples of Methylation Reactions:
i) O-methylation
HO
3
H2
C
H3CO
CH3
C COOH
NH2
H2
C
COMT
CH3
C COOH
NH2
HO
HO
S(-)-a-Methyldopa
- antihypertensive
- only position 3 is methylated, and bismethylation does not occur
- COMT metabolizes only catechols
Question: Is terbutaline (Brethine) a substrate for COMT?
OH
HO
CH
NHC(CH3)3
C
H2
OH
39
ii) N-methylation
OH
OH
CH
CH
NH2
CH
CH
CH3
CH3
N
N
N
H3C
N
H3C
CH3
iii) S-methylation
SH
N
C3H7
NHCH3
SCH3
N
N
OH
C3H7
N
OH
40
Factors Affecting Drug Metabolism
1. Age;
2. Species/Strain; 3. Hereditary;
4. Sex.
Assignment: Read “Wilson & Gisvold,” Chapter on Metabolism
Effect of Other Xenobiotics
A) Enzyme induction:
When a drug causes an increase in the activity of an enzyme, often due
to increased amounts of newly synthesized enzymes this would result in:
-Result: increase in drug metabolism & decrease rate of drug action, therefore
the concomitant use of 2 drugs must be analyzed for drug interaction.
- Examples:
1. Phenobarbital is used in the treatment of hyperbilirubinemia because it induces
glucuronyltranferase -- the enzyme responsible for glucuronidation of bilirubin.
41
2. Benzo[ a ]-Pyrene (found in cigarette smoke) induces Cyt P-450 and may be
responsible for increased rate of metabolism of theophylline in smokers.
T 1/2 in smokers = 4.1 h non smokers =7.2 h
Assignment: make a table of drugs that induce the metabolism of other drugs.
B) Enzyme Inhibition:
When xenobiotics cause a decrease in the activity of a drug metabolizing enzyme,
- often due to the following:
* Substrate competition
* Interference with protein synthesis
* Inactivation of metabolizing enzymes * Hepatotoxicity
- Result:
Increase duration of action of drug possible adverse effects.
- Examples:
1) Phenylbutazone inhibits S(-)-Warfarin, Tolbutamide
2) Isoniazid
``
Phenytoin (Dilantin)
3) Chloramphenicol ``
Tolbutamide
42
- General inhibitors of microsomal enzymes:
SK & F -525 A, Metyrapone, Peperonyl butoxide, Cobaltous chloride
Other Factors
A) Diet
B) Pathologic state of liver
C) Pregnancy
D) Hormonal disturbances.
Relevance of Drug Metabolism
1. Bioavailability therefore dosage
2. Active and inactive metabolites
3. Metabolite may be toxic
4. Design of new drugs
5. Provide information on organs involved
6. Drug interactions, e.g. enzyme induction, inhibition.
43