Consequences of Drug Metabolism
Enzyme (E)
Substrates (s)
Drug
(D)
MAJOR
Active
Influence of Substrate on Enzyme Activity
E + S ↔ ES → P
Inactive
MINOR
Inactive
Less Lipid Soluble
more readily excreted
Active
Toxic
Non-toxic
Non-toxic
Toxic
Active
Equal, less
or more active
Effect of Enzyme Level on Activity
Amount of substrate transformed
Dr. Robert G. Lamb
Professor
Pharmacology & Toxicology
Metabolites
(DS)
E + S ↔ ES → P
4x
3x
2x
1x
Increasing enzyme concentration
Drug Metabolism
Time of reaction
Cellular Location of Drug Metabolizing Enzymes
Overall Metabolism Scheme
reduction
1
Hydrolysis of Aspirin
O
R
–
E
C – OR
H2N
O C CH3
COOH
E
OH
COOH
O
C
R - COOH + ROH
acid
alcohol
O
aspirin
Hydrolysis of Procaine
E = plasma esterase
C2H5
O
CH2
CH2
Short-acting local
anesthetic
N
C2H5
procaine
(novocaine)
E = plasma esterase
O
+ HO C CH3
salicylic acid
Hydrolysis of Aspirin
acetic acid
H2N
O
C
C2H5
O
H
para-aminobenzoic
acid (PABA)
+ HO
CH2
CH2
N
C2H5
diethylaminoethanol
Procaine Hydrolysis
Amide Hydrolysis
Lidocaine Hydrolysis
Reduction of Chloral Hydrate
2
Overall Scheme of Oxidative Metabolism
P450-Dependent Drug Oxidation
NADP+
Non-specific system associated with ER
NADPH+
1- Substrate Binding
P 450
Reductase
Multiple forms of CYP-P450 [enzyme]
1A2[12%] induced by Smoking and Charcoal Cooking
2B6 [20%] induced by Phenobarbital (PB) and Rifampin
2E1 [6%] induced by Alcohol and Isoniazid
3A4 [28%] induced by PB, Phenytoin, Rifampin, etc.
Flavoprotein
(reduced)
Flavoprotein
(oxidized)
2- Substrate Reduction
2
e-
3- Substrate Oxygenation
P -4 5 0 -F e 2 +
RH
3
NADPH Cytochrome P450 reductase [enzyme]
RH
P -4 5 0 -F e 3 +
O2
RH
P -4 5 0 -F e 2 +
O2
H 2O
Substrates: Oxygen, NADPH and Drug
1
Regulation of Oxidative Metabolism
1. Level of CYP-P450 and Reductase Enzymes
Higher in alcoholics and smokers (more drug)
Higher with drug intake (PB etc.) [more drug]
Lower in elderly, infants (less drug)
2. Level of substrates (drugs, oxygen and NADPH)
Examples of Oxidative Metabolism I
N-Oxidation
Primary amines
Secondary amines
RNH 2
S
S
R2
Deamination
O
R2
OH
RCHCH 3
R
NH2
Desulfuratio
n
Thioridazine,
cimetidine,
chlorpromazine
R1
C CH3
R
NH2
R1
Amphetamine,
diazepam.
NH
S
Thiopental.
C
N
OH
2-Acetylaminofluorene,
acetaminophen.
R2
R1
Nicotine,
methaqualone
R1
N
R2 N
O
R3
Role of Phase I and II Reactions
Phase I reactions usually precede Phase II reactions.
Phase I reactions produce chemically reactive sites.
Phase II reactions occur at reactive sites.
O
R1
C
R2
CCH 3 + NH 3
R1
R2
Tertiary amines
Aniline
chlorphentermine
RNHOH
R1
R3
Examples of Oxidative Metabolism II
4- Product Dissociation
R-OH
(oxidized product)
R2
R1
4- Substrate Rearrangement
4
P -4 5 0 -F e 3 +
R-H
(parent drug)
S-Oxidation
3- Substrate Reduction
e-
Phase II metabolites are usually inactive.
O
R2
3
Glucuronidation of Aspirin
Acetylation of Sulfanilamide
NH2
NH CO
+
UDPG is UDP-glucuronic acid
SA = salicylic acid
metabolite of aspir
E is glucuronosyl transferase
OH
E
COOH UDPG
CH3
CH3
N - AT
SO2NH2
SO2NH2
Sulfanilamide
OC6H9O6
COOH
HOOC
Acetic Acid
Acetylsulfanilamide
N-AT is N-Acyltransferase
ether glucuronide of SA
10%
SA
Methylation Reactions
Glycine Conjugation of Aspirin Metabolite (SA)
OH
+ H 2N
COOH
CH2 COOH
N - AT
OH
CONH
CH 2 COOH
HO
HO
OH
CH3
CH CH2 NH
OMT
SAM
epinephrine
SA
(salicylic acid)
CH3
OH
CH3O
HO
CH CH2 NH
metanephrine
salicyluric acid
glycine
75% major
metabolite of Aspirin
HO
HO
OH
CH CH2 NH2
norepinephrine
NMT
SAM
HO
HO
OH
CH3
CH CH2
NH
epinephrine
O-, N-methyltransferase (OMT & NMT)
S-Adenosylmethionine (SAM)
Acetaminophen Hepatotoxicity
ACETAMINOPHEN
HNCOCH3
HNCOCH3
PAPS
SULFATE
OH
45 - 50%
HNCOCH3
UDPGA
P-450 MIXED FUNCTION OXIDASE
GLUCURONIDE
45 - 50%
HO-N-COCH3
OXIDATIVE STRESS (•OH, O 2 •–)
4 - 5%
OH
POSTULATED
TOXIC
INTERMEDIATES
NCOCH3
HIGH DOSE (10-15g)
LOW DOSE (1-2g)
Key Factor
GLUTATHIONE
1+
HNCOCH3
Diseases: Hyperthyroidism
Conditions: smoking, alcoholism
HNCOCH3
O
CELL
MACROMOLECULES
GLUTATHIONE
MERCAPTURIC
ACID
Enzyme Induction (slow) increases drug clearance
Drugs [many]: PB, Rifmpin, Phenytoin, etc.
NUCLEOPHILIC CELL
MACROMOLECULES
OH
Factors Influencing Drug Metabolism I
Higher doses of drugs are required
OH
Alcoholic
N-Acetylcysteine
Only one induction period then stable level
CELL
DEATH
4
Factors Influencing Drug Metabolism II
Enzyme Inhibition (fast) reduces drug clearance
Diseases:Hypothyroidism, Liver Disease
Drugs (many): Chloramphenicaol, Cimetidine,
Disulfiram, Ethanol (acute), etc.
Conditions: Pregnancy, Aging, Newborn
Factors Influencing Drug Metabolism III
Age: low metabolism in elderly and newborn
start low and go slow with drug dose
Nutrition: high metabolism with chronic intake of alcohol
and charcoal cooked food and lower with high
acute alcohol intake.
Factors Influencing Drug Metabolism IV
Genetic Variations:
Isoniazid [prophylaxis of tuberculosis] produces liver
injury in slow acetylators.
Succinylcholine [surgical muscle relaxant] produces
prolonged respiratory depression (apnea) in
patients with abnormal plasma cholinesterase
which reduces the hydrolysis of succinylcholine.
5