1. 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)
Metabolite
(polar)
Enzyme
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:
Metabolism
BCPP+
Haloperidol

2. 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

3. Phase I Reactions 1) Direct Attachments:
Aromatic Hydroxylation
Aliphatic Hydroxylation
2) Unmask Existing Functional Groups
A) Oxidation
Oxidation
Oxidation
O-Demethylation +
N-Deethylation +

4. ; B) Reduction C) Hydrolysis Reduction Reduction Nitro group Ketone+
AZO group
C) Hydrolysis
Hydrolysis +
Ester
Hydrolysis +
Amide

5. Phase II Reactions 1) Glucuronic Acid Conjugation UDP-Glucuronyl
Transferase +
Phase I metabolite
or parent xenobiotic
Actived Uridine-5’-Diphospho-
a-D-Glucuronic Acid
(Note a-linkage at C-1)
b-Glucuronide
(Note b-linkage at C-1)

6. 3) Amino Acid Conjugation
2) Sulfate Conjugation +
Phase I metabolite
or parent xenobiotic
Activated 3’-Phospho-
Adenosine-5’-Phospho-Sulfate +
3) Amino Acid Conjugation +
Amino Acid

7. 4) Other Phase II Reactions
Generally, these do not increase water
Solubility but mainly terminate activity
A) Methylation
O-Methylation
COMT
Active
Inactive
B) Acetylation
N-Acetylation
Active
Inactive
Note: Both methylation and acetylation lead to lower solubility and
often lead to inactivation

8. Serves as a protective mechanism
against electrophilic species
C) Glutathione Conjugation (GSH) +
Arene Oxide
GSH

9. 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
a, b
a. Microsomal Oxidation
b. Microsomal Amidase
Lidocaine
(Xylocaine)
Other Sites:
Intestine; Kidneys; Lungs; Skin; Placenta; Brain; Adrenal Glands

10. Oxidative Biotransformations of Drugs
By far the most widespread and the most important
General Reaction:
NADPH
R-H + O2 + H+
R-OH + NADP+ + H2O
Enzyme
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.

11. 1) Aromatic Hydroxylation
Arenol
Arene
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:
Propranolol (Inderal®)

12. 3) Benzylic Carbon Oxidation
2) Olefinic Oxidation
Oxidation of olefinic “C=C” double bond leads to epoxide
e.g.
Carbamazepine
(Anticonvulsant)
Epoxide
Trans-diol
3) Benzylic Carbon Oxidation
e.g. 1
Benzylic Carbon
Tolbutamide (Orinase)
Hypoglycemic Agent
Alcohol
Metabolite
Carboxylic Acid

13. 4) Allylic Carbon Oxidation
e.g. 2
Metoprolol (Lopressor)
(b-Adrenergic Blocker)
4) Allylic Carbon Oxidation
Allylic Carbon
e.g. + +
7-Hydro- D1-THC
(equal or more active)
6b-
(minor)
6a-
(minor)
Tetrahydrocannabinol
(D1-THC) (Hallucingen)
No 3-metabolite due to steric hindrance

14. 5) Oxidation of Carbon Atom a to C=O or C=N
Diazepam
Hydroxydiazepam
6) Alicyclic and Aliphatic Carbon Oxidation
Oxidation at a Terminal Carbon (w-Oxidation)
Oxidation at Penultimate Carbon (w-1-Oxidation)
e.g. A
Valproic Acid
(Antiepileptic Agent) B

15. 7) Oxidation Involving Carbon-Heteroatoms
C) Alicyclic Hydroxylation
e.g.
Acetohexamide
(Dymelor)
trans-4-OH-
(Also cis-4-,
trans-3- ,
cis-3- as minor)
7) Oxidation Involving Carbon-Heteroatoms
A) Oxidative N-Dealkylation
e.g.
Benzphetamine
Note: - Small alkyl group is preferentially removed
- First alkyl group is removed at faster rate than the second one

16. B) Direct Oxidation on N-Atoms
e.g.
N-Oxidation
Meperidine-N-Oxide
Meperidine
C) Oxidative Deamination
e.g. 1
Amphetamine
Note: For endogenous amines i.e. Dopamine, Norepinephrine, Serotonin etc.
the group of enzymes responsible for deamination is called Monoamine
Oxidase (MAO)

17. e.g. 2
Dopamine
D) O-Dealkylation
e.g.
Acetaminophen
(Tylenol)
Phenacetin

18. E) S-Dealkylation
e.g.
6-(Methythio)Purine
6-Mercaptopurine
F) Desulfuration
e.g.
Thiopental
Pentobarbital

19. G) Direct Oxidation of Sulfur
e.g.
Thioridazine (Mellaril)
Mesoridazine (Serentil)
Active
Sulfoxide
Sulfone
Sulfoxide drugs and metabolites may be further oxidized to sulfones

20. 8) Other Oxidations H. Oxidation of Alcohols and Aldehydes Alcohol
Dehydrogenases
Oxidase
8) Other Oxidations
A) Aromatization
Norgestrel (Oral Contra)
17a-Ethinyl-18-Homoestradiol
(Minor Metabolite in Women)

21. B) Dehalogenation
Phosgene
Chloroform

22. Question Predict the metabolic products of the following compounds:
(1)
(2)

23. Reductive Biotransformations
- Carbonyl (C=O), Nitro (NO2), and Azo (N=N) groups susceptible to reduction
1) Reduction of C=O
e.g.
Aldo-Keto Reductases
(NADPH)
Acetohexamide
S(-) Hydroxyhexamide
(Note asymmetric center is introduced)

24. 3) Reduction of AZO group
2) Reduction of NO2 group
e.g.
Nitro Reductase
Dantrolene
Aminodantrolene
3) Reduction of AZO group
Sulfanilamide
(Active)
Sulfamidochrysoidine
(Prontosil)

25. Hydrolytic Reactions 1) Hydrolysis of Esters e.g.1 Esterases Aspirin
Salicylic Acid
e.g.2
Clofibrate (Atromid-S)
Hypolipidemic Drug
Active

26. 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:
e.g.2 Carbenicillin has poor oral absorption, the Indanyl ester is made to
increase absorption:

27. 2) Hydrolysis of Amides
- Amides are more slowly hydrolyzed than esters. +
Prazosin (Minipress)
Selective a-1 Blocker

28. Phase II Conjugation Reactions
- The following groups are involved:
Sulfate
a-Glucuronic Acid
Glutathione
Methyl
Glycine
Glutamine
Acetyl
These groups are transferred by the appropriate conjugation enzymes on to
the drugs or other metabolites

29. 1) Glucuronic Acid Conjugation
- The following functional groups are susceptible to conjugation:
1) Glucuronic Acid Conjugation
The most common conjugative pathway
Steps: a. Synthesis of activated glucuronic acid (UDPGA)
b. Enzymatic transfer
UDPGA (Uridine-5’-DiPhospho-a-D-Glucuronic Acid)

30. A) O-Glucuronides
e.g.1
Acetaminophen
e.g.2
Salicylic Acid

31. B) N-Glucuronides
C) S-Glucuronides

32. Glucuronic acid conjugates are excreted mainly through urine
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
b-Glucuronidases
Hydrolyzed Drug
INTESTINE

33. 2) Sulfate Conjugation +
Occurs primarily in phenols, but possible in aromatic amines, alcohols
and N-hydroxy compounds
Mechanism of Transfer: 1. Activation of Sulfate (SO42-) to PAPS
2. Transfer of SO42- from PAPS to substrate (drug) +
-PAP
3’-Phosphoadenosine-5’-
phosphosulfate (PAPS)
Sulbutamol
(b-Adrenergic
Bronchodilator)

34. 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.

35. e.g.
Haloperidol
p-Flourophenylacetic
Acid
Glycine Conjugate
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

36. Notice the Glutathione adduct undergoes further
S-Transferase
Arene Oxide
Glutathione (GSH)
- glutamyl
transpeptidase
N-Acetylase
Cysteinyl
Glycinase
Notice the Glutathione adduct undergoes further
Biotransformation: cleavage of glutamic acid
and glycine; then N-acetylation
Mercapturic Acid Der

37. 4) Other Phase II Biotransformation
A) Acetylation
Dapsone
Antileprotic Agent
Acetyltransferases
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

38. B) Methylation
- Like acetylation, leads to lower solubility
- Primary function is attenuation of activity
- Constitutes only a minor pathway
- Coenzyme: S-Adenosylmethionine (SAM)
SAM
- Compounds which undergo methylation reaction:
- Enzymes involved: Catechol-O-methyltransferase (COMT);
Phenol-O-methyltransferase;
Other non-specific N- and S-methyltransferase

39. Examples of Methylation Reactions:
i) O-methylation
COMT
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?

40. ii) N-methylation
iii) S-methylation

41. Factors Affecting Drug Metabolism
1. Age; Species/Strain; Hereditary; 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.

42. 2. Benzo[  ]-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

43. Relevance of Drug Metabolism
- 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.