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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. Shanghai Jiao Tong University 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. Shanghai Jiao Tong University 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 sideeffects. 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 Shanghai Jiao Tong University 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. Shanghai Jiao Tong University 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 Shanghai Jiao Tong University 2.1 Oxidative Reactions Oxidative biotransformations processes are, by far, the most common and important in drug metabolism. RH + NADPH + H + O2 P450 ROH + NADP + H2O Mixed function oxidase: molecular oxygen O2 NADPH (reduce from of nicotinamide adenosine dinucleotide phosphate) cytochrome P450. Shanghai Jiao Tong University Catalytic reaction cycle involving cytochrome P-450 in oxidation Oxidized product (substrate) (NADPH) CYP450 Reductase co cytochrome P-450(Fe+3) [CYP450(Fe+2)][RH] CO Activated oxygen Chromophore absorbs at 450 nm (NADPH) CYP450 Reductase Shanghai Jiao Tong University The super-family of cytochrome P450 enzymes So far, 17 families of CYPs with about 50 isoforms have been characterized in the human genome. classification: CYP 3 A 4 isoenzyme Family >40% sequencehomology sub-family >55% sequencehomology 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 Shanghai Jiao Tong University Metabolic Contribution hepatic only CYP 2C9 10% CYP 1A2 other 2% 3% CYP 3A4 CYP 2D6 CYP 2C9 CYP 1A2 other CYP 3A4 55% CYP 2D6 30% also small intestine Shanghai Jiao Tong University Classes of substrates for cytochrome P450 Functional Product R R OH R' R O R' R' R R' ArCHR ArCH 2R OH R R CH2R' R R O O R CHR' OH R CH 2R' CHR' OH RCHR' RCH2R' OH RCH2-X-R' (X=N,O,S,halogen) R-X-R' (X=NR,S) RCH-XR' RCHO OH R N R' O + RXH Shanghai Jiao Tong University 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 Shanghai Jiao Tong University Classes of substrates for flavin monooxygenase Functional Product R-NR'2 R-NR'2 O R-NHR' R-NR' OH R-NR' R=NR' OH O NHOH NH O R-N-NHR' R-N-NHR' R'' R'' R-CNH2 R-CNH2 S S HSR O HNR SH R'HN SO2 R'HN 2RSH RSSR RSSR 2RSO2 Shanghai Jiao Tong University 1) Aromatic Hydroxylation There are electron-donating groups in Aromatic ring Oxidation take place easily at para position NH O OH N H N H R N H NH NH2 R Propranol(普萘洛尔) R=H Phenformin(苯乙双胍) R=OH Shanghai Jiao Tong University There are electron-withdrawing groups in Aromatic ring O OH Oxidation can not teak place CH 3 N lower electron Cloud Density O S N O O Probenecid(丙磺舒) N Cl Higher electron Cloud Density R Diazepam R=H R=OH Shanghai Jiao Tong University Epoxides of aromatic compounds R rearrangement OH R epoxide hydrolase H 2O R H H O OH R OH glutathione S -transf erase GSH OH SG R macromolecular nucleophiles X OH X 代谢与毒性:亲电反应性活泼的代谢中间体 亲核基团以共价键结合 对机体产生毒性 可与DNA、RNA的 Shanghai Jiao Tong University RNA adduct with benzo(a)pyrene metabolite epoxide hydrolase HO O OH O N O 10 9 HO 8 7 OH N R NH N HO NH HO OH Metabolic activation of polyaromatic hydrocarbons can lead to the formation of covalent adducts with RNA, Shanghai Jiao Tong University 2) Alkene Epoxidation Because 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 HO O OH epoxide P450 hydrolase CONH2 Carbamazepine (卡马西平) CONH2 CONH2 Shanghai Jiao Tong University 3) Oxidations of Alkynes R C CH O Fe3+O R N prophyrin CYP450 CYP450 R R C CH Fe3+O CYP450 COOH R C C O H H N R protein O 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) Shanghai Jiao Tong University 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. ¦Ø-1 ¦Ø R H3C COOH H3C valproic acid (丙戊酸) NH perhexiline (R=H) b. Alicyclic carbon is oxide to the alicyclic alcohol (R=OH). 扩冠药哌克西林 Shanghai Jiao Tong University 5) Oxidations of Carbons Adjacent to sp2 Centers a. Allyl carbon oxidation N HO CH 3 CH3 CH3 N HO CH3 CH3 CH 3 N + CH 2OH HO CH 3 CH3 CH 2OH CH3 CH 3 Pentazocin (喷他佐辛) b. Benzyl carbon is oxide to a alcohol further to a aldehyde CH3 H3C H N H N O S O O CH2OH H3C H N H N O S O O Tolbutamine 甲磺丁脲的氧化 O H3C H N H N O S O O OH Shanghai Jiao Tong University Oxidation of ibuprofen CH3 COOH HOH2C CH3 COOH H3C H3C ω oxidations H3C CH3 COOH H 3C H 3C HO CH3 Ibuprof en COOH H3C H3C ω-1 oxidations OH benzyl carbon oxidation Shanghai Jiao Tong University 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 Shanghai Jiao Tong University a. N-dealkylation Dealkylation of secondary or tertiary amines will produce primary amines and aldehydes R R N C H R CH 3 R NH R N C O H R H N CH 3 N O CH 3 CH 3 H N O CH 3 CH3 O + CH 3 N H CH 3 H N NH2 O CH 3 Lidocaine N Imipramine Desimipramine N N N H (去甲丙咪嗪) Shanghai Jiao Tong University b. O-dealkylation and S-dealkylation Dealkylation of ethers will produce phenols N H 3CO N CH 3 O OH O HO Codeine CH 3 OH Morphine S-dealkylation usually produces sulfhydryl group and aldehyde. R-S-CH3 S [o] CH 3 N N N [R-S-CH2OH] N H R-SH + HCHO SH N N N N H 6-methylthiopurine 6-thiopurine Shanghai Jiao Tong University 7) Oxidative Deamination For primary aliphatic and arylalkyl amines By CYP450 enzyme OH R NH2 cytochrome O R P450 R' R NH3 NH4 + R' R' B: H By Flavin monooxygenase For example, deamination of amphetamine (安非他 明,苯丙胺) N a HO N OH B: b H NH2 flavin monooxygenase b HN OH -H2O b NH B-H NO2 HO N O N H B: H2O NH2OH + O OH O Shanghai Jiao Tong University 8) N-oxidation 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 F3C CH3 F3C CH3 HO N CH3 CH3 O N 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) Cl Cl N O N N N CH3 Shanghai Jiao Tong University 9) S-oxidation R R S O R' S R' For example, N-oxidation of chlorpromazine(氯丙嗪) O S S N Cl N N Cl N Shanghai Jiao Tong University 2.2 Reductions Reactions Classes of substrates for reductive reactions Oxidative processes are, by far, the major pathways of drug metabolism, Functional group Product O OH 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 R R R' R' RNO 2 RNHOH RNO RNHOH RNHOH RNH 2 RN=NR' RNH 2 + R'NH2 R3N-O R 3N OH O R R' R R' R R' R R' O O R-X R + X Shanghai Jiao Tong University 1)Carbonyl Reduction 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 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. O R-Warfarin is reduced in humans principally to OH CH3 the R,S-warfarin alcohol. Ph R O O warfarin (R=H) S-warfarin is metabolized mainly to 7hydroxywarfarin (R=OH) . Shanghai Jiao Tong University 2) Reduction for nitro or Azo compounds These reductases mainly exist in hepatic mitochondria with NADH or NADPH as coenzyme. Nitrobenzene 2H NO NO2 H N R 2H 2H NHOH NH2 O N Cl R clonazepam (R=NO 2) O N S N NH niridazole (R=NO 2) 尼立达唑(抗血吸虫药 ) Shanghai Jiao Tong University Azo HH N-N 2H N=N 2H 2 COOH O H N S O N=N 磺胺匹林 OH (抗结肠炎) sulf asalazine O H N S O COOH NH 2 NH 2 + H2N OH Shanghai Jiao Tong University 3) Azido Reductione and Tertiary Amine Oxide Reduction Azido to amine Tertiary Amine to Tertiary Amine O CH3 HN HO O O N N O CH 2CH2CH 2N(CH3) 2 X zidovudine(X=N 3) Imipramine N-oxide Shanghai Jiao Tong University 4) Reductive Dehalogenation volatile anesthetic halothane (Fluothane) is metabolized by a reductive dehalogenation mechanism by cytochrome P450 Br CHCF3 Cl e -Br H C CF3 Cl escape from enzyme covalent binding d e b H C CF2 Cl F -F a H C CF3 Cl R RH c H2C CF3 Cl ClHC CF2 Shanghai Jiao Tong University 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. O COOH O CH3 O CH 3 N OR H O H aspirin O cocaine (R=CH3) Shanghai Jiao Tong University Esters can be hydrolysis easily than amides NEt2 O H3CO O O CH3 O Cl O H N C CH 2NHC4H9 H 2N X NEt2 procainamide (X=NH) procaine (X=O) O butanilicaine propanidid 丙泮尼地 (静脉麻醉药) 布坦卡因 Shanghai Jiao Tong University 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. Shanghai Jiao Tong University 3.1 Glucuronic Acid Conjugation(葡萄糖醛酸结合) Coenzyme form Groups conjugated O HOOC O H HO O O HO O HO O P O P OH -OH, -COOH, -NH2, NH O N -NR2, -SH, O OH OH OH Uridine-5-diphospho-α-D-glucuronic acid (UDPGA) COOH O Ransferase enzyme: Glucueonosyl transferase ( 葡萄糖醛酸转移酶) OH OH OH OH H HO H H CHO OH H OH OH COOH Shanghai Jiao Tong University 1) O-Glucuronide OH O 2N H N CHCl2 AcHN OH PhO Acetaminophen(Phenol) OH O O OH Chloramphenicol(alcohol) Fenoprofen (Carboxyl) 2) N-Glucuronide OCONH 2 O NHCH 3 Desipramine(Amine) 3) S-Glucuronide N SH N CH 3 O NH2 Meprobamate(Amide) (眠尔通) Methimazole(甲巯基咪唑) Shanghai Jiao Tong University 3.2 Sulfate Conjugation Coenzyme form Groups conjugated -OH, -NH2 NH2 O O HO S O P O O OH N N O 3’ -Phosphoadenosine-5’ phosphosulfate (PAPS) N N Ransferase enzyme: = O3PO OH Sulfotransferase OH H N H 3C HO HO O OH CH 3 CH 3 O S O Salbufamol(沙丁胺醇) HO HO O H N CH 3 CH 3 O S O Koprenaline(异丙肾上腺素) Shanghai Jiao Tong University 3.3 Amino Acid Conjugation(Glycine and glutamine) ATP O R OH PP i acyl CoA synthetase CoASH O R AMP COOH R H 2N H O AMP R O R CoASH SCoA COOH amino acid N-acyltransf erase R H N H Groups conjugated: -COOH O Coenzyme form N R (Ar) SCoA N H2N + H N N O glycine O n-acyltransferase Br Brompheniramine(溴苯那敏) R H N OH N-oxidation Br COOH Br COOH Shanghai Jiao Tong University 3.4 Glutathione Conjugation Coenzyme form NH 2 HH O N SH Groups conjugated: H O N H Ar-X, arene oxide, epoxide COOH COOH Ransferase enzyme Glutathione S-transferase Glutathione (GSH) H 3CO2SO SG OSO2CH 3 H3CO 2SO SG S G 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 。 Shanghai Jiao Tong University 3.5 Acetyl Conjugation Coenzyme form Groups conjugated: OH, -NH2 O H 3C Ar-NH2 R-NH 2 R-OH Ransferase enzyme SCoA Acetyltransferase O CoAS O CH3 O R-O CH 3 Ar-NH + Acetyl transf erase R-SH O CH3 NH-R O R-S CH 3 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。 Shanghai Jiao Tong University Procainmide O Unchanged in Urine, 59% H 2N N N H 24% f ast 17% slow 3% H Unchanged in Urine, 85% 1% O N O NAPA N H N 0.3% H N O O N H H N O H 2N N H H N Shanghai Jiao Tong University 3.6 Methyl Conjugation Coenzyme form Groups conjugated: NH 2 N CH3 S HOOC N H NH2 O -OH, -NH2, SH, Heterocyclic N N N Ransferase enzyme Methyltransferase OH OH S-Adenosyl methionine (SAM) H2N COOH H2N H H2N H3CS COOH H ATP PPi +Pi +S CH3 O Ad methionine adenosyltransferase HO OH COOH H methyltransferase CH3-X-R + S HX-R HO O Ad OH Mammalian phase II conjugating agents Shanghai Jiao Tong University Conjugate Glucuronide Coenzyme form Groups conjugated O HOOC O H HO O O HO O HO O P O P OH OH NH O N O Ransferase enzyme -OH, -COOH, -NH2, UDPGlucuronosyltran -NR2, -SH, sferase OH OH Uridine-5-diphospho-α-Dglucuronic acid (UDPGA) NH 2 Sulfate O O HO S O P O O OH N N O H 2O3P O N -OH, -NH2 Sulfotransferase -COOH Glycine N OH 3-Phosphoadenosine-5phosphosulfate (PAPS) Glycine and glutamine O R (Ar) SCoA + H2N H COOH R Activated acyl or aroyl coenzyme A cosubstrate N-acyltransferase Glutamine N-acyltransferase Shanghai Jiao Tong University Conjugate Coenzyme form Glutathione NH 2 HH O N SH H O N H COOH Groups conjugated Ransferase enzyme Ar-X, arene oxide, epoxide, carbocation Glutathione OH, -NH2 Acetyltransferase -OH, -NH2, Methyltransferase S-transferase COOH Glutathione (GSH) Acetyl O H 3C SCoA Acetyl coenzyme A NH 2 Methyl N CH 3 S HOOC H NH 2 N O N N OH OH S-Adenosyl methionine (SAM) SH, Heterocyclic N Shanghai Jiao Tong University 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 Shanghai Jiao Tong University 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。 Shanghai Jiao Tong University 4.3 Other factors 1) Species difference. 2) Sex, age, nutrition conditions have effects on drug metabolism. 3) Hepatic functions. Shanghai Jiao Tong University 5. Application in new drug research 1) Lead discovery 2) Prodrug design 3) Soft drug design Shanghai Jiao Tong University 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。