chapter 4 drug metabolism ( 药物代谢 ). shanghai jiao tong university 1.introduction 1.1 what is...
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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.
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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.
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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
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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.
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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
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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+ +
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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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)
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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 +
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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
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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
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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
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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。
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4.3 Other factors
1) Species difference.
2) Sex, age, nutrition conditions have effects on drug metabolism.
3) Hepatic functions.
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5. Application in new drug research
1) Lead discovery
2) Prodrug design
3) Soft drug design
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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。