Chapter 4 Drug Metabolism ( 药物代谢 ). Shanghai Jiao Tong University 1.Introduction 1.1 What is...

Chapter 4 Drug Metabolism

( 药物代谢 )

Shanghai Jiao Tong University

1.Introduction

1.1 What is drug metabolism The enzymatic biotransformations of drugs is known as drug metabolism that is human body evolved to protect itself against low molecular weight environmental pollutants. The principal mechanism is the use of nonspecific enzymes that transform the foreign compounds (often highly nonpolar molecules) into polar molecules that are excreted by the normal bodily processes.


1.2 Site of Drug Metabolism and First-Pass Effect

The principal site of drug metabolism is the liver, but the kidneys, lungs, and

GI tract also are important metabolic sites.

When a drug is taken orally (the most common route of administration), it is usually absorbed through the mucous membrane of the small intestine or from the stomach. Once out of the GI tract it is carried by the bloodstream to the liver where it is usually first metabolized. Metabolism by liver enzymes prior to the drug reaching the systemic circulation is called the presystemic or first-pass effect, which may result in complete deactivation of the drug.


1.3 Purpose of Drug Metabolism Studies

Drug metabolism studies are essential for evaluating the potential safety and efficacy of drugs.

Exploration of new drugs. Based on the mechanisms of biotransformation, it is possible to design new drugs with longer half-lives and fewer side-effects.

Once the metabolic products are known, it is possible to design a compound that is inactive when administered, but which utilizes the metabolic enzymes to convert it into the active form. These compounds are known as prodrugs, and are discussed in Chapter 5


1.4 Classfication of Drug metabolism

Drug metabolism reactions have been

divided into two general categories, termed phase I and phase II reactions.

Phase I transformations

involve reactions that introduce or unmask a functional group, such as oxygenation ， reduction or hydrolysis.

Phase II transformations

mostly generate highly polar derivatives (known as conjugates), such as glucuronides and sulfate esters, for excretion in the urine.


2. Phase I transformations

Phase I or functionalization reaction, include oxdative, reductive, and hydrolytic biotransformations

The purpose of these reaction is to introduce a polar functional group (e.g., OH, COOH, NH2, SH) into the xenobiotic molecule. This can be achieved by direct introduction of functional group or by modifying or “unmasking” existing functionalities

Although Phase I reaction may not produce sufficiently hydrophilic or inactive metabolites, they generally tend to provide a functional group that can undergo subsequent phase II reactions


2.1 Oxidative Reactions

Oxidative biotransformations processes are, by far, the most common and important in drug metabolism.

Mixed function oxidase:

molecular oxygen O2

NADPH (reduce from of nicotinamide adenosine dinucleotide phosphate)

cytochrome P450.

RH + NADPH + H + O2P450

ROH NADP H2O+ +


Catalytic reaction cycle involving cytochrome P-450 in oxidation

(substrate)

(NADPH)CYP450 Reductase

co [CYP450(Fe+2)][RH]

CO

cytochrome P-450(Fe+3)

Chromophore absorbs at 450 nm

(NADPH) CYP450 Reductase

Activated oxygen

Oxidized product


So far, 17 families of CYPs with about 50 isoforms have been characterized in the human genome.

classification: CYP 3 A 4

Family >40% sequence-homology

sub-family>55% sequence-homology

isoenzyme

The super-family of cytochrome P450 enzymes

The following families were confirmed in humans:

CYP1-5, 7, 8, 11, 17, 19, 21, 24, 26, 27, 39, 46, 51

Main CYPs concern with the metabolism of drug :

CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4


Metabolic Contribution

CYP 2D630%

CYP 1A22%CYP 2C9

10%

other3%

CYP 3A455%

CYP 3A4

CYP 2D6

CYP 2C9

CYP 1A2

other

hepatic only

also small intestine


Classes of substrates for cytochrome P450Functional Product

R R OH

R

R'

R' R

R'

R'

O

R

RCH2R'

R

RCHR'

O

R CH2R'

OH

O

R CHR'

OH

ArCH2RArCHR

OH

RCH2R' RCHR'

OH

RCH2-X-R'

(X=N,O,S,halogen)

RCH-XR'

OH

RCHO + RXH

R-X-R'(X=NR,S)

N R'R

O


Other oxidases

a) flavin monooxygenases

Classes of substrates for flavin monooxygenase see next page

b) Monoamine oxidase

These enzymes exist in mitochondria(腺粒体 ).

They catalyze oxidation of amines into aldehyde and ammonia.

For example, degradation of

RCH2-NH2 RCH=NH RCHO + NH3

c) Alcohol and aldehyde oxidases

R-CHOH R-CHO R-COOH


Classes of substrates for flavin monooxygenase

Functional ProductR-NR'2 R-NR'2

O

R-NHR' R-NR'

OH

R-NR'

OH

R=NR'

O

NH NHOH

R-N-NHR'

R''

R-N-NHR'

R''

O

R-CNH2

S

R-CNH2

SO

SHHSR

R'HNSO2

HNR

R'HN

2RSH RSSR

RSSR 2RSO2


1) Aromatic Hydroxylation There are electron-donating groups in Aromatic ring

Oxidation take place easily at para position

R

O

OH

NH

R

NH

NH

NH

NH2

NH

Propranol( 普萘洛尔 ) Phenformin( 苯乙双胍 )

R=H R=OH


There are electron-withdrawing groups in Aromatic ring

Oxidation can not teak place

Higher electron

Cloud Density

Diazepam

Probenecid( 丙磺舒 )

lower electron

Cloud Density

R=H R=OH

N

N

R

Cl

CH3 O

SN

OHO

O

O


代谢与毒性：亲电反应性活泼的代谢中间体可与 DNA 、 RNA的亲核基团以共价键结合对机体产生毒性

Epoxides of aromatic compounds

OH

H

R

R

OHR

OH

OH

SG

OH

R

X

OH

R

rearrangement

epoxide hydrolase

H2O

glutathione S -transf erase

GSH

macromolecular

nucleophiles X


RNA adduct with benzo(a)pyrene metabolite

Metabolic activation of polyaromatic hydrocarbons can lead to the formation of covalent adducts with RNA,

O

epoxide

hydrolase

OH

HO

OH

HO

O

78

910

OH

HO

HO

NH

NH

N

N

N

O

R


2) Alkene EpoxidationBecause alkenes are more reactive than aromatic π-bonds, it is not surprising that alkenes also are metabolically epoxidized. An example of a drug that is metabolized by alkene epoxidation is the anticonvulsant agent carbamazepine

Carbamazepine ( 卡马西平 )

CONH2

P450

CONH2

O

epoxide

hydrolase

CONH2

HO OH


3) Oxidations of Alkynes

如攻击的是端基碳，则氢原子迁移，形成烯酮，水解后生成羧酸。如攻击的是非端位，则一酶中的卟啉的氮原子发生 N- 烷基化 ( 毒性 )

CR

Fe3+O

CYP450

CYP450

CHC

Fe3+O

CYP450

CH

R

R

O

Nprophyrin

R CH

C O

R COOH

R

O

HN

protein


4) Oxidation at Aliphatic and Alicyclic Carbon Atoms

Metabolic oxidation at the terminal methyl group of an aliphatic side chain is referred to

as ωoxidation and oxidation at the penultimate carbon isω-1 oxidation.

a. An saturated aliphatic side chain is oxide

at both ω andω-1 oxidations.

NH

R

perhexiline (R=H)b. Alicyclic carbon is oxide to the alicyclic alcohol (R=OH).

valproic acid ( 丙戊酸 )

扩冠药哌克西林

COOH

H3C

H3C

¦Ø

¦Ø-1


5) Oxidations of Carbons Adjacent to sp2 Centers

a. Allyl carbon oxidation

HO

N CH3

CH3CH3

CH3

HO

N CH3

CH2OHCH3

CH3

HO

N CH2OH

CH3CH3

CH3

+

Pentazocin ( 喷他佐辛）

b. Benzyl carbon is oxide to a alcohol further to a aldehyde

O

HN

HN

SH3C

CH3

OO O

HN

HN

SH3C

CH2OH

OO

O

HN

HN

SH3C

OO

O

OH

Tolbutamine

甲磺丁脲的氧化


ω oxidations

ω-1 oxidations

benzyl carbon oxidation

COOH

CH3

H3C

H3C

Ibuprof en

COOH

CH3

H3C

HOH2C

COOH

CH3

H3C

H3C

COOH

CH3

H3C

H3C

HO

OH

Oxidation of ibuprofen


6) Dealkylation

Dealkylations include N-, O- and S-dealkylation.

R-X-CH2-R’

[R-X-CH(OH)-R’]

R-XH + O=CH-R’X = O, N, S


a. N-dealkylation

Dealkylation of secondary or tertiary amines will produce primary amines

and aldehydes

C HNR

RC ON

R

RH NH

R

R+ O

CH3

O

HN

CH3

N

CH3

CH3

CH3

O

HN

CH3

NH

CH3

CH3

O

HN

CH3

NH2

N N

N NH

Imipramine

Desimipramine

( 去甲丙咪嗪 )

Lidocaine


b. O-dealkylation and S-dealkylation

Dealkylation of ethers will produce phenols

Codeine Morphine

S-dealkylation usually produces sulfhydryl group and aldehyde.

R-S-CH3 [R-S-CH2OH] R-SH + HCHO[o]

6-methylthiopurine 6-thiopurine

OH3CO OH

NCH3

OHO OH

NCH3

N

N NH

N

SCH3

N

N NH

N

SH


7) Oxidative Deamination

By Flavin monooxygenase

For example, deamination of amphetamine ( 安非他明，苯丙胺）

R'

R NH2 cytochrome

P450

OH

NH3

R'

R

R'

RO

+ NH4

NH2

flavin

monooxygenase HN

H

OH

B:

b

b

-H2O

NH

NOHHO

NO

HB:

a

b

NO2N

OHN

OHOH B:

B-H

O+NH2OH

H2O

For primary aliphatic and arylalkyl amines

By CYP450 enzyme


8) N-oxidation

tertiary amines gives chemically stable tertiary amine N-oxides that do not undergo further oxidation unlike N-oxidation of primary and secondary amines

For example, N-oxidation of chlorpheniramine （氯苯那敏，扑尔敏）

N

N

Cl

N

N

Cl

O

For secondary amines leads to a variety of N-oxygenated products. Secondary hydroxylamine formation is common, but these metabolites are susceptible to further oxidation to give nitrones

For example, N-oxidation of fenfluramine( 氟苯丙胺）

F3C CH3

HN CH3

F3C CH3

N CH3HO

F3C CH3

N CH3O


9) S-oxidation

For example, N-oxidation of chlorpromazine( 氯丙嗪）

SR'

RS

R'

R O

N

S

Cl

N

N

S

Cl

N

O


2.2 Reductions Reactions

Oxidative processes are, by far, the major pathways of drug metabolism,

but reductive reactions are important for biotransformations of the functional groups listed in Table

Reductive reactions are important for the formation of hydroxyl and amino groups that render the drug more hydrophilic and set it up for phase II conjugation

Classes of substrates for reductive reactions

Functional group Product

O

R R' R

OH

R'

R'

R'

O

O

R

R R'

R'

OH

O

R

R

RNO2 RNHOH

RNO RNHOH

RNHOH RNH2

RN=NR' RNH2 + R'NH2

R3N-O R3N

R-X R + X


Stereospecific: Ketone reductases exhibit (pro-S)-hydrogen specificity.

Stereoselectivity for enantiomer substrate:

The reduction of the anticoagulant drug warfarin( 抗凝药华法林 ) is selective for the R-(+)-enantiomer; reduction of the S-(−)-isomer occurs only at high substrate concentrations.

1 ） Carbonyl Reduction

O

OH

R

Ph

O

CH3

O

warfarin (R=H)

R-Warfarin is reduced in humans principally to the R,S-warfarin alcohol.

S-warfarin is metabolized mainly to 7-hydroxywarfarin (R=OH) .

Carbonyl reduction typically is catalyzed by aldo-keto reductases that require NADPH or NADH as the coenzyme.

It is not common, however, to observe reduction of aldehydes to alcohols. A large variety of aliphatic and aromatic ketones, however, are reduced to alcohols by NADPH-dependent ketone reductases


2) Reduction for nitro or Azo compounds

These reductases mainly exist in hepatic mitochondria with NADH or NADPH as coenzyme.

Nitrobenzene

尼立达唑 ( 抗血吸虫药 )

NO2 NO NHOH NH2

2H 2H 2H

N

HN

Cl

O

RN

S N NH

O

R

clonazepam (R=NO2) niridazole (R=NO2)


Azo

磺胺匹林

( 抗结肠炎 )

N=N N-N NH2

2H 2H2

H H

HN S

O

O

N=N

COOH

OH

HN S

O

O

NH2

COOH

OHH2N+

sulf asalazine


3) Azido Reductione and Tertiary Amine Oxide Reduction

Azido to amine

Imipramine N-oxide

Tertiary Amine to Tertiary Amine

ON

HO

X

O

HNCH3

O

zidovudine(X=N3)

N

CH2CH2CH2N(CH3)2

O


4) Reductive Dehalogenation

volatile anesthetic halothane (Fluothane) is metabolized by a reductive dehalogenation mechanism by cytochrome P450

Br CHCF3

Cl

e C CF3

Cl

Heb

H C

Cl

CF2

FClHC CF2

escapefromenzyme

a

C CF3

Cl

Hdcovalent

binding

RH R

H2C CF3

Cl

-Br

-F

c


2.3 Hydrolytic Reactions

The hydrolytic metabolism of esters and amides leads to the formation of carboxylic acids, alcohols, and amines.

A wide variety of nonspecific esterases and amidases involved in drug metabolism are found in plasma, liver, kidney, and intestine.All mammalian tissues may contribute to the hydrolysis of a drug; however, the liver, the gastrointestinal tract, and the blood are sites of greatest hydrolytic capacity.

COOH

O

O

CH3

O

O

H

NCH3

H

ORO

aspirin cocaine (R=CH3)


Esters can be hydrolysis easily than amides

H2NO

XNEt2

procainamide (X=NH)procaine (X=O)

H3CO

ONEt2

O

O

O

propanidid

CH3

Cl

HN C

O

CH2NHC4H9

butanilicaine

丙泮尼地 ( 静脉麻醉药 )

布坦卡因


3. Phase II Transformations:

Conjugation Reactions( 结合反应）Phase II or conjugating enzymes, in general, catalyze the attachment of

small polar endogenous molecules such as glucuronic acid, sulfate, and

amino acids to drugs or, more often, to metabolites arising from phase I

metabolic processes. This phase II modification further deactivates the

drug, changes its physicochemical properties, and produces water-soluble

metabolites that are readily excreted in the urine or bile. Phase II

processes such as methylation and acetylation do not yield more polar

metabolites, but serve primarily to terminate or attenuate biological

activity.


3.1 Glucuronic Acid Conjugation （葡萄糖醛酸结合）

Coenzyme form

Uridine-5-diphospho-α-D-glucuronic acid (UDPGA)

Groups conjugated

-OH, -COOH, -NH2,

-NR2, -SH,

Ransferase enzyme: Glucueonosyl transferase (葡萄糖醛酸转移酶）

O N

OHOH

NH

O

OO

P

O

O

OH

P

O

O

OH

OHO

HOHO

HHOOC

O

OHOH

OH

OH

COOH

H OH

COOH

H OH

HO H

H OH

CHO


O2N

OH

OHO

HN CHCl2

1) O-Glucuronide

Acetaminophen(Phenol)

Chloramphenicol(alcohol) Fenoprofen (Carboxyl)

2) N-Glucuronide

3) S-Glucuronide

Desipramine(Amine)

Meprobamate(Amide)（眠尔通）

Methimazole （甲巯基咪唑）

OHAcHN PhO

OOH

NHCH3

O

OCONH2

O

NH2

N

N SH

CH3


3.2 Sulfate Conjugation

Coenzyme form

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

Groups conjugated -OH, -NH2

Ransferase enzyme:

Sulfotransferase

Salbufamol( 沙丁胺醇 )

Koprenaline( 异丙肾上腺素 )

O

OHHN

HOH3C

CH3

CH3

SO O

HO

HO

O

OHHN CH3

CH3

SO O

HO

N

NN

N

NH2

O

OH= O3PO

OP

O

O

OH

S

O

HO

O


3.3 Amino Acid Conjugation(Glycine and glutamine)

Brompheniramine( 溴苯那敏 )

Groups conjugated: -COOH

Coenzyme form

O

OHR

O

AMPR

O

SCoAR

ATP PPi

acyl CoA synthetase

CoASH AMP CoASH

COOH

H2NR

H

amino acidN-acyltransf erase

O

R NH

COOH

RH

O

R SCoA

H2N COOH

RH(Ar)

+

N

N

Br

N

Br

OH

O

NHN

Br

ON-oxidation glycine

n-acyltransferase

COOH


3.4 Glutathione Conjugation

Coenzyme form

Glutathione (GSH)

Groups conjugated:

Ar-X, arene oxide, epoxide

Ransferase enzymeGlutathione S-transferase

与某些有亲电倾向的药物结合形成 S- 取代的谷胱甘肽结合物。与带强亲电基团的结合对正常细胞中的亲核基团的物质如蛋白质、核

酸等起保护作用。

SH

NH

O COOH

HN

H

COOH

NH2

H

O

H3CO2SOOSO2CH3

SG SGH3CO2SOS G


3.5 Acetyl Conjugation

Coenzyme form Groups conjugated:

OH, -NH2

Ransferase enzyme

Acetyltransferase

有效的解毒途径，一般药物经 N- 乙酰化代谢后，生成无活性或毒性较小的产物。

N- 乙酰化转移酶的活性受遗传因素的影响较大，故有些药物的疗效、毒性和作用时间在不同民族的人群中有种族差异。

乙酰化产物溶解度减小。

O

SCoAH3C

Ar-NH2

R-NH2

R-OH

R-SH

CoASO

Ar-NH

O CH3 R-OCH3

O

O CH3 R-SCH3

OAcetyl transf erase+

NH-R


H2N

O

N

H

N

N

O

N

H

N

H

O

3%

24% f ast17% slow

Unchangedin Urine, 59%

1%

Unchangedin Urine, 85%

N

O

N

H

N

HH

O

H2N

O

N

H

N

H

0.3%NAPA

Procainmide


3.6 Methyl Conjugation

Coenzyme form

S-Adenosyl methionine (SAM)

Groups conjugated: -OH, -NH2, SH,

Heterocyclic N

Ransferase enzyme

Methyltransferase

N

NN

N

NH2

O

OHOH

S

CH3

HOOC

NH2

H

H2NH

COOH

H3CS

ATP PPi +Pi

methionineadenosyltransferase

H2N

H

COOH

S CH3+

O

HO OH

Admethyltransferase

HX-R

H2N

H

COOH

S

O

HO OH

AdCH3-X-R +


N

NN

N

NH2

O

OHO

OP

O

O

OH

S

O

HO

O

H2O3P

Conjugate Coenzyme form Groups conjugated Ransferase enzyme

Glucuronide -OH, -COOH, -NH2,

-NR2, -SH,

UDPGlucuronosyltransferase

Uridine-5-diphospho-α-D-glucuronic acid (UDPGA)

Sulfate

3-Phosphoadenosine-5-phosphosulfate (PAPS)

-OH, -NH2 Sulfotransferase

Glycine and

glutamine

Activated acyl or aroyl coenzyme A cosubstrate

-COOH Glycine

N-acyltransferase

Glutamine

N-acyltransferase

Mammalian phase II conjugating agents

O

R SCoA

H2N COOH

RH(Ar)

+

ON

OHOH

NH

O

OO

P

O

O

OH

P

O

O

OH

OHO

HOHO

HHOOC


Conjugate Coenzyme form Groups conjugated Ransferase enzyme

Glutathione

Glutathione (GSH)

Ar-X, arene oxide, epoxide, carbocation

Glutathione

S-transferase

Acetyl

Acetyl coenzyme A

OH, -NH2 Acetyltransferase

Methyl

S-Adenosyl methionine (SAM)

-OH, -NH2,

SH,

Heterocyclic N

Methyltransferase

O

H3C SCoA

N

NN

N

NH2

O

OHOH

S

CH3

HOOC

NH2

H

SH

NH

O COOH

HN

H

COOH

NH2

H

O


4. Factors that affect drug metabolism

4.1 Inducers

Inducers are those that promote drug metabolism in the body. Most inducers are lipophilic compounds and have no specificity in actions.

苯巴比妥：催眠药作用酶： P450 中的多个亚族诱导剂。相互作用的药物：洋地黄、氯丙嗪、苯妥因、地塞米松、保泰松等结果：加速代谢，半衰期缩短


4.2 Inhibitors

Inhibitors are those that inhibit drug metabolism in the body. Include competitive and non-competitive inhibitors.

西咪替丁：抗溃疡药作用酶： CYP2C 、 CYT1A2 抑制剂相互作用的药物：华法林、苯妥英钠、氨茶碱、苯巴比妥、安定、普萘洛尔等。而雷尼替丁几乎不会抑制上述酶的活性。溃疡患者在服用上述药物时，应避免使用西咪替丁。


4.3 Other factors

1) Species difference.

2) Sex, age, nutrition conditions have effects on drug metabolism.

3) Hepatic functions.


5. Application in new drug research

1) Lead discovery

2) Prodrug design

3) Soft drug design


本章重点内容一、药物代谢概念：药物代谢，又称药物的生物转化，是机体在长期进化中形成的一种自我

保护功能。药物分子被机体吸收后，在体内非特异性酶的作用下发生化学转化，使非极性分子转化成极性分子，使之易于排出体外。

分类：可分为Ⅰ相代谢和Ⅱ相代谢，Ⅰ相代谢又称官能团化反应，Ⅱ相代谢又称结合反应。

药物代谢研究的目的及意义：目的是揭示药物进入人体后的结构转化及这种转化对药物的毒性和活性的影响。研究药物代谢对新药的发现、先导化合物的结构优化及前药的设计都具有重要意义。

Ⅰ相代谢：又称官能团化反应包括氧化、还原、水解等化学反应，使药物分子在酶的催化下引入或转化成一些极性较大的官能团如羟基、羧基、氨基和巯基等，代谢产物的极性增大。包括：氧化代谢、还原代谢、水解反应等。

Ⅱ相代谢：又称结合反应，是指药物原型或经官能团化反应后产生的极性基团与内源性的水溶性小分子如萄糖醛酸、硫酸盐、氨基酸等在酶的催化下，以酯、酰胺或苷的形式结合，形成水溶性结合物，通过肾脏经尿排出体外。包括与葡萄糖醛酸结合、与硫酸结合、氨基酸结合、谷胱甘肽结合等。乙酰化和甲基化结合虽不能形成水溶性化合物，但对药物灭活起重要作用。

Chapter 4 Drug Metabolism ( 药物代谢 ). Shanghai Jiao Tong University 1.Introduction 1.1 What is...

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