Chapter 4 Drug Metabolism ( 药物代谢 )

1. Chapter 4 Drug Metabolism(药物代谢)

2. 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.

3. 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 mucousmembrane 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.

4. 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

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.

6. 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

7. 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.

8. Catalytic reaction cycle involving cytochrome P-450 in oxidation (substrate) Oxidized product (NADPH) CYP450 Reductase cytochrome P-450(Fe+3) co [CYP450(Fe+2)][RH] CO Activated oxygen Chromophore absorbs at 450 nm (NADPH) CYP450 Reductase

9. 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% sequence-homology sub-family>55% sequence-homology 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

11. Classes of substrates for cytochrome P450 Functional Product

12. Other oxidases • 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

13. Classes of substrates for flavin monooxygenase Functional Product

14. 1) Aromatic Hydroxylation There are electron-donating groups in Aromatic ring Oxidation take place easily at para position Propranol(普萘洛尔) Phenformin(苯乙双胍) R=H R=OH

15. There areelectron-withdrawing groups in Aromatic ring Oxidation can not teak place lower electron Cloud Density Probenecid(丙磺舒) Higher electron Cloud Density Diazepam R=H R=OH

16. Epoxides of aromatic compounds 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

17. RNA adduct with benzo(a)pyrene metabolite Metabolic activation of polyaromatic hydrocarbons can lead to the formation of covalent adducts with RNA,

18. 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 Carbamazepine (卡马西平)

19. 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)

20. 4) Oxidation at Aliphatic and Alicyclic Carbon Atoms Metabolic oxidation at the terminal methyl group of an aliphatic side chain is referred to as ωoxidationand oxidation at the penultimate carbon isω-1 oxidation. a. An saturated aliphatic side chain is oxide at both ω andω-1 oxidations. valproic acid (丙戊酸) b. Alicyclic carbon is oxide to the alicyclic alcohol (R=OH). 扩冠药哌克西林

21. 5) Oxidations of Carbons Adjacent to sp2 Centers a. Allyl carbon oxidation Pentazocin (喷他佐辛） b. Benzyl carbon is oxide to a alcohol further to a aldehyde Tolbutamine 甲磺丁脲的氧化

22. Oxidation of ibuprofen ω oxidations ω-1 oxidations benzyl carbon oxidation

23. 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

24. a. N-dealkylation Dealkylation of secondary or tertiary amines will produce primary amines and aldehydes Lidocaine Desimipramine (去甲丙咪嗪) Imipramine

25. b. O-dealkylation and S-dealkylation Dealkylation of ethers will produce phenols Codeine Morphine S-dealkylation usually produces sulfhydryl group and aldehyde. [o] R-S-CH3 [R-S-CH2OH] R-SH + HCHO 6-methylthiopurine 6-thiopurine

26. 7) Oxidative Deamination For primary aliphatic and arylalkyl amines By CYP450 enzyme By Flavin monooxygenase For example, deamination of amphetamine (安非他明，苯丙胺）

27. 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） 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）

28. 9) S-oxidation For example, N-oxidation of chlorpromazine(氯丙嗪）

29. 2.2 Reductions Reactions Classes of substrates for reductive 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 Functional group Product

30. 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. R-Warfarin is reduced in humans principally to the R,S-warfarin alcohol. S-warfarin is metabolized mainly to 7-hydroxywarfarin (R=OH) .

31. 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 )

32. Azo 磺胺匹林 (抗结肠炎)

33. 3) Azido Reductione and Tertiary Amine Oxide Reduction Azido to amine Tertiary Amine to Tertiary Amine Imipramine N-oxide

34. 4) Reductive Dehalogenation volatile anesthetic halothane (Fluothane) is metabolized by a reductive dehalogenation mechanism by cytochrome P450

35. 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.

36. Esters can be hydrolysis easily than amides 丙泮尼地 (静脉麻醉药) 布坦卡因

37. 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.

38. 3.1 Glucuronic Acid Conjugation（葡萄糖醛酸结合） Coenzyme form Groups conjugated -OH, -COOH, -NH2, -NR2, -SH, Uridine-5-diphospho-α-D-glucuronic acid (UDPGA) Ransferase enzyme: Glucueonosyl transferase (葡萄糖醛酸转移酶）

39. 1) O-Glucuronide Acetaminophen(Phenol) Chloramphenicol(alcohol) Fenoprofen (Carboxyl) 2) N-Glucuronide Desipramine(Amine) Meprobamate(Amide)（眠尔通） 3) S-Glucuronide Methimazole（甲巯基咪唑）

40. 3.2 Sulfate Conjugation Coenzyme form Groups conjugated -OH, -NH2 3’ -Phosphoadenosine-5’ -phosphosulfate (PAPS) Ransferase enzyme: Sulfotransferase Salbufamol(沙丁胺醇) Koprenaline(异丙肾上腺素)

41. 3.3 Amino Acid Conjugation(Glycine and glutamine) Groups conjugated: -COOH Coenzyme form Brompheniramine(溴苯那敏)

42. 3.4 Glutathione Conjugation Coenzyme form Groups conjugated: Ar-X, arene oxide, epoxide Ransferase enzyme Glutathione S-transferase Glutathione (GSH) • 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 。

43. 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。

45. 3.6 Methyl Conjugation Coenzyme form Groups conjugated: -OH, -NH2, SH, Heterocyclic N Ransferase enzyme Methyltransferase S-Adenosyl methionine (SAM)

46. Mammalian phase II conjugating agents

48. 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

49. 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。

50. 4.3 Other factors 1) Species difference. 2) Sex, age, nutrition conditions have effects on drug metabolism. 3) Hepatic functions.