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DRUG METABOLISM

Transformation of Xenobiotics by Biological Systems

Metabolisme Obat adalah :

Mengubah (memodifikasi) atau mendegradasi obat,

melalui sistem enzimatik.

mengubah senyawa lipofilik menjadi produk polar yang

siap diekskresikan.

Waktu yang diperlukan, menunjukkan durasi dan

intensitas aksi farmakologik suatu obat.

Hasilnya :

Toksikasi

Detoksikasi (yang utama)

IMPLICATIONS FOR DRUG METABOLISM

IMPLICATIONS FOR DRUG METABOLISM

1. Termination of drug action

2. Activation of prodrug

3. Bioactivation and toxication

4. Carcinogenesis

5. Teratogenesis

Termination of Drug Action

Conversion of drug from active metabolite to

inactive metabolite

Parent compound metabolite

propranolol hydroxypropranolol (active) (active)

(active) (active)

Iproniazid Isoniazid

(active) (active)

Activation of Prodrug

Inactive Terfenadine is Converted to its Active

Metabolite Fexofenadine

terfenadine

fexofenadine

activation of prodrug

Some Xenobiotics Are Metabolized to Carcinogenic Agents

• 3,4 Benzopyrene

• Aflatoxin

• N-Acetylaminofluorene

Metabolites of these agents interact with DNA

carcinogenesis

Small Amounts of Acetaminophen is Converted to the

Reactive Metabolite N-Acetylbenzoquinoneimine

Bioactivation of acetaminophen; under certain conditions, the electrophile N-

acetylbenzoquinoneimine reacts with tissue macromolecules, causing liver necrosis.

bioactivation

N-Hydroxylation of AAF

N-Hydroxylation of AAF is the first metabolic step towards

the development of a carcinogenic agent

Further Metabolism of N-HydroxyAAF Produces Cancer

N-HydroxyAAF undergoes phase II metabolism to the

ultimate carcinogen. The glucuronide pathway is also

involved in carcinogenesis

CYP1A1 Converts Benzopyrene to a Carcinogen

Thalidomide is a Teratogen

– THALIDOMIDE: Fetal malformations in

humans, monkeys, and rats occur due to

metabolism of the parent compound to a

teratogen. This occurs very early in gestation.

teratogensis

FACTORS AFFECTING DRUG METABOLISM

Factors Affecting Drug Metabolism

• Age

• Diet

• Genetic Variation

• State of Health

• Gender

• Degree of protein binding

• Species Variation

• Substrate competition

• Enzyme inhibition

• Enzyme Induction

• Route of Drug Administration


• Age

– Neonates

– Children

– Elderly


• Diet

– Charcoal broiled foods (contain polycyclic

hydrocarbons that increase certain enzyme protein in

cells)

– Grapefruit juice (the active component is the

furancoumarin 6,7-dihydroxybergamottin which

inhibits a certain a group of microsomal enzymes)

Genetic variation

Some Enzymes That Exhibit Genetic Variation

– Pseudocholinesterase

• typical enzyme

• atypical enzyme

– N-Acetyltransferase (isoniazid is a substrate)

• fast acetylation

• slow acetylation

– Cytochrome P450 2D6

– Cytochrome P450 2C19

– TMPT -Thiomethylpurinetransferase

– Dihydropyrimidine Dehydrogenase

:

Asetilator

1.Asetilator cepat

2.Asetilator lambat.

Isoniasid cepat mengalami

asetilasi menjadi asetilisoniasid

yang diekskresikan, yang berarti

kerjanya cepat (pendek)

Sulfamezatin ada

kemungkinan obat tersebut

tidak berefek

Isoniasid dapat terjadi

keracunan

Isoniasid dengan asetilasi

menjadi asetilisoniasid yang tidak

aktif

Bangsa Jepang dan bangsa

Eskimo 90% merupakan

asetilator cepat

Faktor genetik mempunyai peranan dalam laju metabolisme obat termasuk : warfarin ,

dikumarol , fenilhidantoin, fenilbutazon , dsb

Bangsa Eropa timur dan Mesir

State of health


• State of health

– Hepatitis

– Liver cancer

– Cardiac insufficiency

– Uremia

• degree of protein binding

Changes In Drug Metabolism As A Consequence Of Hepatic Disease

From Principles of Drug Action

Gender


• Gender

– Most studies are performed in the rat. In general,

male rats metabolize drugs faster than female rats

DEGREE OF PROTEIN BINDING


• Degree of protein binding

– Conditions that displace bound drug from protein

allows more of the drug to be accessible to the

enzyme for which it serves as a substrate e.g.

uremia, low plasma albumin

Species variation

:

Variasi

spesies

1. Perbedaan biotransformasi

dan konjugasi obat tergantung

dari spesies

2. Di dalam spesies hewan yang

berlainan metabolisme obat

bisa sama, dapat pula berbeda

3. Asam fenilasetat : Pada

manusia berkonjugasi dengan

glutamin. Pada unggas

berkonjugasi dengan ornitin.

Pada anjing berkonjugasi

dengan glisin.

4. Asam benzoat pada ayam

dieksresikan dalam bentuk asam

orniturat

5. Asam benzoat pada anjing di

ekskresi dalam bentuk asam

hiourat

Kualitatif

:

Variasi spesies

Kucing tidak mampu

membentuk konjugasi

dengan glukoronid, Anjing

tidak mampu mengasetilkan

aromatik amin seperti

sulfonamid

Amfetamin : pada kelinci

mengalami deaminasi,

pada anjing –

hidroksilasi cincin

arimatik

Asetanilid : anjing-

hidroksilasi kedudukan

para. Kucing – hidroksilasi

kedudukan orto.

Kualitatif

Kuantita

tif

SUBSTRATE COMPETITION

Factors Affecting Drug Metabolism

• Substrate competition

– Two or more drugs competing for the same

enzyme can affect the metabolism of each other;

the substrate for which the enzyme has the

greater affinity would be preferentially metabolized

Enzyme inhibition

Enzyme induction

Route of drug administration

Many Drugs Undergo First Pass Metabolism

Upon Oral Administration

• Oral administration

• Drug travels from gut to portal vein to liver

• Vigorous metabolism occurs in the liver. Little drug

gets to the systemic circulation

• The wall of the small intestine also contributes to first

pass metabolism

ORGAN SITES OF DRUG METABOLISM

Organ Sites of Drug Metabolism

• Liver

• Small intestine

• Kidney

• Skin

• Lungs

• Plasma

• All organs of the body

CELLULAR SITES OF DRUG

METABOLISM

Cellular Sites Of Drug Metabolism

• Cytosol

• Mitochondria

• Lysosomes

• Smooth endoplasmic reticulum

(microsomes)

KINETICS OF DRUG METABOLISM

Velocity Of Metabolism Of A Drug

0 10 20 30 40 50 60 700

10

20

30

40

50

60

70

80

[Drug] mM

Ve

locity

(ng/g

tis

sue/m

in)


0 5 10 15 20 25 30 35 40 45 50 55 600

10

20

30

40

50

60

70

80

first order metabolism

zero order metabolism

[Drug] mM

Ve

locity

(ng/g

tis

sue/m

in)

First Order Metabolism

v = Vmax [C]

Km + [C]

When Km >>> [C],

then v = Vmax [C] ,

Km

and v [C]

Metabolism of the drug is a first order process. A constant

fraction of the remaining drug is metabolized per unit time.

Most drugs are given at concentrations smaller than the Km

of the enzymes of their metabolism.

A drug may be given in doses that produce blood

concentrations less than the Km of the enyzme for the drug.


0 5 10 15 20 25 30 35 40 45 50 55 600

10

20

30

40

50

60

70

80



[Drug] mM

Ve

locity

(ng/g

tis

sue/m

in)

Zero Order Metabolism

v = Vmax [C]

K m + [C]

When [C] >>> Km,

then v = Vmax [C] ,

[C]

and v = Vmax

Metabolism of the drug is a zero order process. A constant

amount of the remaining drug is metabolized per unit time.

Phenytoin undergoes zero order metabolism at the doses

given.

A drug may be given in doses that produce blood concentrations

greater than the Km of the enyzme for the drug.


0 5 10 15 20 25 30 35 40 45 50 55 600

10

20

30

40

50

60

70

80



[Drug] mM

Ve

locity

(ng/g

tis

sue/m

in)

Velocity Of Metabolism Of Three Drugs

By The Same Enzyme

0 10 20 30 40 50 60 70 80 900

10

20

30

40

50

60

70

Drug A

Drug B

Drug C

[Drug] mM

Velo

cit

y(n

g/g

tis

su

e/m

in)

PHASES OF DRUG METABOLISM

Drug interactions involving CYP enzymes

• About 50-60% therapeutic drugs are metabolized by CYP

enzymes.

• Of these, about 50% by CYP3A whose have highest

concetration among other CYP enzymes.

• CYP2C9, 2C19 and 2D6 have lower concentrations, but

each metabolizes many important drugs.

• These phenomena may cause harmful drug interactions

Proportionality of drug metabolizing

enzymes

Phase I Metabolism

R R OH R R COOH

R R SH R R NH 2

Polar groups are exposed on or introduced to a molecule

Phase I Reactions

OXIDATION

REDUCTION

HYDROLYSIS

:

Fase I

(reaksi

fungsionalis

asi)

a.Oksidasi 4-dimetil amino azobenzen

4-dimetil aminoanilin

Nitrobenzen fenil

hidroksilamin anilin

Kloraldhirat 2,2,2-trikloro

etanol

Asetofenon fenil metil

karbinol

Asam arsenilat p-

aminofenil arsenoksid

Reduksi

senyawa

azo

Reduksi

senyawa

nitro

Reduksi

aldehida dan

keton

Reduksi As

(v) menjadi

As (III)

Deesterifikasi

Deaminasi

Prokain asam p-amino

benzoat + dietil amino etanol

Salisilamid asam

salisilat

b. Reduksi:

reduksi azo,

reduksi nitro,

reduksi aldehida

c. Hidrolisa:

deesterifikasi,

deaminasi

Phase II Metabolism

D+ENDOX DX+ENDO

A molecule endogenous to the body donates a portion

of itself to the foreign molecule

Fase II

(Reaksi

konjugasi)

1. Konjugasi

dengan

glukoronat

Contoh : alkohol, karboksilat, amino

Aktif sulfat : 3-fosfodenosin 5-fosfosulfat

(PAPS)

Metionin + ATP S-adenosil 5-adenosil

metion + RZH RZ-metil

Turunan N-asetil sistein disintesis dari glutation

(GSH)

turunan asam merkapturat

2. Konjugasi

dengan sulfat

3. Konjugasi

dengan metil

4. Pembentukan

merkapturat

Patterns of Drug Metabolism

• Parent molecule Phase 1 metabolism

• Phase 1 metabolite Phase 2 metabolism

• Parent molecule Phase 2 metabolism

• Phase 2 metabolite Phase 1 metabolism

Some drugs are not metabolized, for example, gallamine and

decamethonium. Atracurium undergoes spontaneous

hydrolysis.

PHASE I METABOLIC PATHWAYS

Microsomal Oxidation

Cytochrome P450

fp = NADPH cytochrome P450 reductase, or NADH cytochrome b5

reductase

Oxidation Of Drugs By Cytochrome P450

Aliphatic Oxidation

Aromatic Hydroxylation (1)

acetanilid p-hydroxyacetanilid

Aromatic Hydroxylation (2)

N-Dealkylation

O-Dealkylation

S-Demethylation

Oxidative Deamination

S-Oxidation

N-Oxidation

N-Hydroxylation

N-Hydroxylation of AAF

N-Hydroxylation of AAF is the first metabolic step towards

the development of a carcinogenic agent

Oxidative Dehalogenation

Halotan Trifluoroasetil

klorida

Desulfuration

ISOENZMYES OF CYTOCHROME P450

CYP1A1

CYP1A2

CYP2A6

CYP2B_

CYP2C9

CYP2C19

CYP2D6

CYP2E1

CYP3A4

CYP3A5

CYP3A7

CYP4A_

Cytochrome P450 3A4

(CYP3A4)

CYP3A4

• CYP3A4 is responsible for metabolism of 60%

of all drugs

• It comprises approximately 28% of hepatic

cytochrome P450

• Metabolizes terfenadine

• Ingestion of grapefruit juice reduces expression

of this enzyme

• Inhibited by some regularly used drugs

Some Drugs That Inhibit CYP3A4

• Macrolide antibiotics

– Erythromycin

– Clarithromycin

– Other such agents

• Antifungal agents

– Ketoconazole

– Itraconazole

– Other such agents

• HIV protease inhibitors

CYP3A4

• Ketoconazole and terfenadine can produce a

drug interaction with fatal consequences.

AN INGREDIENT IN GRAPEFRUIT JUICE

INHIBITS CYP3A4

6',7', - Dihydroxybergamottin

Grapefruit Juice Increases Felodipine Oral Availability in

Humans by Decreasing Intestinal CYP3A Protein Expression

J.Clin. Invest. 99:10, p.2545-53, 1997

Hours

Grapefruit Juice Consumption Blocks Terfenadine

Metabolism to Fexofenadine

X

CYP3A4 And P-Glycoprotein

• P-Glycoprotein and CYP3A4 control oral bioavailability

of many drugs

• P-Glycoprotein and CYP3A4 share many substrates

and inhibitors

CYP2D6 is an Enzyme with Polymorphisms

• Approximately 70 nucleotide polymorphisms are

known

• Four phenotype subpopulations of metabolizers*

– Poor metabolizers (PM)

– Intermediate metabolizers (IM)

– Extensive metabolizers (EM)

– Ultrarapid metabolizers (UM)

• Variations according to racial background

• More than 65 commonly used drugs are

substrates

• Codeine is a well known substrate

* The Pharmacological Basis of Therapeutics

Codeine is a Substrate of CYP2D6

Consider the variation in codeine’s metabolism among

PM, IM, EM, UM individuals

-CH3

(methyl morphine)

CYP2C9

• Metabolizes some 16 commonly used drugs

• Warfarin and phenytoin are among the substrates

• Two allelic variants are known: metabolizes substrates

5% to 12% of the wild type enzyme

– Warfarin clearance is greatly reduced in individuals

possessing the allelic variants

• Dose adjustments are required for drugs in individuals

who have the mutant enzymes

CYP2C19

• S-mephenytoin is a substrate

– (4-hydroxylation at the phenyl ring)

• As much as eight allelic variants identified

– All are nonfunctional proteins

• Poor metabolizers of S-mephenytoin lack 4-hydroxylase

activity, but N-demethylation to nirvanol is an alternative

but slow metabolic pathway

– Dose adjustments must be made for poor

metabolizers of S-mephenytoin and for other drugs

that are substrates for this enzyme

CYP1A2

• Polycyclic hydrocarbons are among its

substrates

• Inducers include

– Polycyclic hydrocarbons such as 3,4,-benzopyrene,

3-methylcholanthrene, etc.

– Charcoal broiled foods (polycyclic hydrocarbons)

CIMETIDINE Inhibits CYP450 Metabolism Of Many Drugs

Warfarin

Phenytoin

Metoprolol

Labetalol

Quinidine

Caffeine

Lidocaine

Theophylline

Alprazolam

Diazepam

Flurazepam

Triazolam

Chlordiazepoxide

Carbamazepine

Quinidine

Ethanol

Tricyclic

antidepressants

Metronidazole

Calcium channel

blockers

Diazepam

Sulfonylureas

NONMICROSOMAL OXIDATIONS

ALCOHOL DEHYDROGENATION

ALDEHYDE DEHYDROGENATION

XANTHINE OXIDATION

DIAMINE OXIDATION

MONOAMINE OXIDATION

Nonmicrosomal Oxidations

Alcohol dehydrogenation is conducted by the enzyme

alcohol dehydrogenase (cytosolic)

Aldehyde dehydrogenation is conducted by the enzyme

aldehyde dehydrogenase (cytosol and mitochondria)

Xanthine oxidation is conducted by the cytosolic enzyme

xanthine oxidase.

Diamine oxidase (cytosolic) oxidizes histamine and

diamines such as cadaverine and putrescine.

Monoamine oxidation is conducted by mitochondrial

monoamine oxidase (norepinephrine, epinephrine,

dopamine and serotonin are endogenous substrates.

Monoamine Oxidase Metabolism of Serotonin

Some Popular Substrates of Monoamine Oxidase

• Serotonin

• Epinephrine

• Norepinephrine

• Dopamine

• Tyramine (found in certain foods)

Diamine Oxidase

cadaverine

Alcohol Dehydrogenase

• A soluble enzyme, found almost exclusively in the

parenchymal cells of the liver

• Converts ethanol to acetaldehyde

• Converts methanol to formaldehyde

• Converts ethylene glycol to its respective aldehyde

metabolites

• Is inhibited by pyrazole

Alcohol Dehydrogenase

CH3CH2OH + NAD+ CH3CHO + NADH + H+

ethanol acetaldehyde

Aldehyde Dehydrogenase

CH3CHO + NAD+ CH3COOH + NADH + H+

acetaldehyde acetate

Xanthine Oxidase

REDUCTION

Nitro Reduction NITRO REDUCTION

RNO2 RNH2

MICROSOMES AND CYTOSOLMicrosomes and cytosol

Nitro Reduction

Azo Reduction

RN=NR' RNH2 + R'NH2

AZO REDUCTION


Azo Reduction

Microsomes and cytosol

Alcohol Dehydrogenation

Cytosol

DIHYDROPYRIMIDINE DEHYDROGENASE

5-Fluorouracil 5-Fluoro-5,6-dihydrouracil DPYD

• DPYD

– Inactivates 5-fluorouracil by ring reduction

– Inherited deficiency of this enzyme leads to 5-fluorouracil

toxicity

– Enzyme deficiency can be detected by enzymatic or

molecular assays using white blood cells

5-fluorouracil

HYDROLYSIS

Amide Hydrolysis

RCONR'R" RCOOH+ HNR'R"

AMIDE HYDROLYSIS


Amide Hydrolysis

Ester Hydrolysis

RCOOR' RCOOH + R'OH

ESTER HYDROLYSIS


Ester Hydrolysis

Ester Hydrolysis

Microsomes and cytosol

Enalaprit

Epoxide Hydrolase

PHASE II METABOLIC PATHWAYS

D+ ENDOX D X+ENDO

PHASE 2 METABOLISM

A molecule endogenous to the body donates a portion

of itself to the foreign molecule

PHASE II REACTIONS Glucuronidation

Sulfate Conjugation

Acetylation

Glycine Conjugation

Methylation

Transulfuration

Glutathione Conjugation

Mercapturic Acid Synthesis

GLUCURONIDATION

Uridine-5’--D-glucuronic Acid

The microsomal enzyme glucuronyl transferase conducts the

donation of glucuronic acid from the endogenously synthesized

UDPGA to various substrates to form glucuronide conjugates.

Examples of such substrates are morphine and acetaminophen.

UDP--D-Glucuronsyltransferase

• Is also called glucuronyl transferase

• A microsomal enzyme

• Substrates are called aglycones

• Conducts phase 2 metabolic reactions

• Products are called glucuronides

• Glucuronides formed

– RN-G; RO-G; RCOO-G; RS-G; RC-G

• Bilirubin is an endogenous substrate

• Induced by phenobarbital

Glucuronidation of Benzoic Acid

UGT= UDP--D-Glucuronsyltransferase

Glucuronidation of Aniline

Glucuronidation of p-Hydroxyacetanilid

Morphine Metabolism

A small amount of morphine undergoes N-demethylation

Morphine Morphine -6-glucuronide (active metabolite)

Morphine Morphine -3-glucuronide (inactive metabolite)

Morphine Metabolism

Morphine -3-glucuronide is the major metabolite

Induction Of UDP--D-Glucuronyl Transferase

• Induced by phenobarbital

• Induced by 3-methylcholanthrene

SULFATE CONJUGATION

Sulfate Conjugation

• Conducted by the soluble enzyme sulfotransferase

• Endogenous donor molecule to conjugation is

3’-phosphoadenosine-5’-phosphosulfate (PAPS)

3’-Phosphoadenosine-5’-phosphosulfate (PAPS)

The cytosolic enzyme sulfotransferase conducts the donation of

sulfate from the endogenously synthesized PAPS to various

substrates to form sulfate conjugates. An example of such substrate

is acetaminophen.

Sulfate Conjugation of p-Hydroxyacetanilid

PAP: 3’-phosphoadenosine- 5’-phosphate

MINOXIDIL METABOLISM

MINOXIDIL

(inactive)

MINOXIDIL N-O-SULFATE

(active metabolite)

MINOXIDIL N-O-GLUCURONIDE

(inactive metabolite)

N-ACETYLATION

N-Acetyltransferase

• A soluble enzyme

• Isoniazid is a substrate

• Genetic variation occurs

– Some individuals are fast acetylators

– Some individuals are slow acetylators

• Acetyl coenzyme A is the endogenous donor

molecule

Acetyl CoA

Various acetylases, for examples, choline acetylase and N-acetyl

transferase, all soluble enzymes, conduct the transfer of the acetyl

group of acetyl CoA to various substrates. For example, N-acetylation

of isoniazid. Genetic polyporphism occurs with N-acetyltransferase.

N-Acetyltransferase

SUGAR CONJUGATION

Conversion of 6-Mercaptopurine to a Nucleotide

METHYLATION

S-Adenosylmethionine

Cytosolic enzymes such as catechol-O-methyl transferase (COMT) and

phenylethanolamine-N-methyl transferase (PNMT) conducts the

donation of the methyl group from the endogenously synthesized SAM

to various substrates to form methylated conjugates. Norepinephrine is

N-methylated by PNMT to form epinephrine. Norepinephrine,

epinephrine, dopamine, and L-DOPA are O-methylated by COMT.

Methyltransferases

• A family of soluble enzymes that conducts

– N-methylation; N-CH3

– O-methylation; O-CH3

– S-methylation; S-CH3

• S-adenosylmethionine (SAM)is the endogenous donor

molecule. It is demethylated to S-adenosylhomocysteine

N-Methyltransferases

PNMT- Phenylethanolamine-N-methyltransferase

Norepinephrine Epinephrine PNMT

SAM

O-Methylation Of Catecholamines

COMT- catechol-O-methyltransferase

O-Methylation of Norepinephrine

COMT- catechol-O-methyltransferase

S-Methylation of 6-Mercaptopurine

TPMT - thiopurinemethyltransferase; some individuals are

deficient in this enzyme that is critically important for the

metabolism of this agent

AMINO ACID CONJUGATION

AMINO ACID CONJUGATION

(mitochondria)

Multiple Metabolic Pathways Exist

for Aspirin’s Metabolism

Hydolysis of aspirin produces salicyclic acid, as

seen in the next slide

Salicyluric Acid is the Glycine Conjugate of Aspirin

Salicyluric acid, the glycine conjugate of salicyclic acid, is the main

metabolite of aspirin. Approximately 76% of aspirin is metabolized

through amino acid conjugation.

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