Babylon university college of pharmacy

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1 Metabolic Changes of Drugs Babylon university college of pharmacy drug metabolism 3 rd class second semester Dr. Abdulhussien Aljebory 2015 Books: 1. Wilson and Gisvold s Textbook of rganic Medicinal and Pharmaceutical Chemistry 11 th ed. Lippincott, Williams & Wilkins ed. 2. Foye s Principles of Medicinal Chemistry

2 METABLISM R BITRASFRMATI Metabolism: It is a general term used for chemical transformation of xenobiotics and endogenous nutrients (e.g., proteins, carbohydrates and fats) within or outside the body. It is an essential pharmacokinetic process, which renders lipid soluble and non-polar compounds to water soluble and polar compounds so that they are excreted by various processes. This is because only water-soluble substances undergo excretion, whereas lipid soluble substances are passively reabsorbed from renal or extra renal excretory sites into the blood by virtue of their lipophilicity. The term metabolism is commonly used probably because products of drug transformation are called metabolites. Metabolism is a necessary biological process that limits the life of a substance in the body.

3 Biotransformation: It is a specific term used for chemical transformation of xenobiotics in the body/living organism. a series of enzyme-catalyzed processes that alters the physiochemical properties of foreign chemicals (drug/xenobiotics) from those that favor absorption across biological membranes (lipophilicity) to those favoring elimination in urine or bile (hydrophilicity ) Xenobiotics :These are all chemical substances that are not nutrient for body (foreign to body) and which enter the body through ingestion, inhalation or dermal exposure. They include : drugs, industrial chemicals, pesticides, pollutants, plant and animal toxins, etc.

4 Functions of Biotransformation It causes conversion of an active drug to inactive or less active metabolite(s) called as pharmacological inactivation. It causes conversion of an active to more active metabolite(s) called as bioactivation or toxicological activation. It causes conversion of an inactive to more active toxic metabolite(s) called as lethal synthesis

5 Functions of Biotransformation.contd It causes conversion of an inactive drug (pro-drug)to active metabolite(s) called as pharmacological activation It causes conversion of an active drug to equally active metabolite(s) (no change in pharmacological activity) It causes conversion of an active drug to active metabolite(s) having entirely different pharmacological activity (change in pharmacological activity)

6 Drug Metabolism Extrahepatic microsomal enzymes (oxidation, conjugation) Hepatic microsomal enzymes (oxidation, conjugation) Hepatic non-microsomal enzymes (acetylation, sulfation, GSH, alcohol/aldehyde dehydrogenase, hydrolysis, ox/red)

7 Introductory Concepts Biochemically speaking: Metabolism means Catabolism (breaking down of substances) + Anabolism (building up or synthesis of substances) But when we speak about drug metabolism, it is only catabolism That is drug metabolism is the break down of drug molecules So what is building the drug molecules? We use the word synthesis, then Drugs are synthesized in laboratory and thus is not an endogenous event. Lipid soluble drugs require more metabolisms to become polar, ionizable and easily excretable which involve both phase I and phase II mechanisms.

8 What Roles are Played by Drug Metabolism? ne of four pharmacokinetic parameters, i.e., absorption, distribution, metabolism and excretion (ADME) Elimination of Drugs: Metabolism and excretion together are elimination Excretion physically removes drugs from the body. The major excretory organ is the kidney. The kidney is very good at excreting polar and ionized drugs without any major metabolism. In general, by metabolism drugs become more polar, ionizable and thus more water soluble to enhance elimination. It also effect deactivation and thus detoxication or detoxification Many drugs are metabolically activated (Prodrugs) Sometimes drugs become more toxic and carcinogenic

9 Really fundamental concepts in drug biotransformation Lipid soluble drugs are poorly excreted in the urine. They tend to store in fat and/or circulate until they are converted (phase I biotransformation) to more water soluble metabolites or metabolites that conjugate (phase II biotransformation) with water soluble substances. Water soluble drugs are more readily excreted in the urine. They may be metabolized, but generally not by the CYP enzyme systems.

10 Biotransformation & urinary excretion Drugs with major first pass effect (bioavailability) [% urinary excretion] imipramine (39) [<2] lidocaine (35) [2] morphine (24) [8] propranolol (26) [<0.5] Drugs with little or no first pass effect (bioavailability) [% urinary excretion] clonidine (95) [62] flucytosine (84) [99] metronidazole (99) [10] penicillin G (0) [>90]* (when injected)

11 Metabolite activity Inactive (detoxification) Similar activity to the drug Examples and notes Routes that result in the formation of inactive metabolites are often referred to as de Phenol Cl H Phenol sulphokinase 3'-Phosphoadenosine-5'- phosphosulfate (PAPS) Ph Diazepam (Sustained anxiolytic action) Hydroxylation Cl S Ph Temazepam (Short duration) H Phenyl hydrogen sulfate The metabolite may exhibit either a different potency or duration of action or both to the original drug. H -Demethylation Cl H Ph xazepam (short duration) H CHHCH CHH 2 Different activity Ipronazid (Antidepressant) -Dealkylation H C Isoniazid (Antituberculosis) HC H 2 Toxic metabolites ther substances responsible for hepatotoxicity C 2 H 5 -Hydroxyphenacetin (Hepatotoxic) C 2 H 5 Phenacetin (Analgesic) C 2 H 5 Phenetidine Substances responsible for methemoglobinamia

12 Stereochemistry of Drug Metabolism H CH 2 C H CH 2 C H S-(-)-Warfarin H Ph H CH 2 C S-6-Hydroxywarf arin Ph H H H 2 C H H Ph R,S-(+)-alcohol derivative H Major route R-(+)-Warfarin H Ph Minor route H H 2 C H H Ph H R,R-(+)-alcohol derivative R-(-)-Ibuprofen (inactive) H CH Metabolism S-(+)-Ibuprofen (active) CH H

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14 1- Many drugs undergo several sequential biotransformation reactions. Biotransformation is catalyzed by specific enzyme systems 2. Sites of biotransformation: The liver: the major site other tissues.(ther. Lungs Plasma 3- Biotransformation of drugs can be affected by many parameters, including: A. prior administration of the drug in question or of other drugs B. diet C. hormonal status D. genetics E. disease (e.g., decreased in cardiac and pulmonary disease) F. age and developmental status G. liver function

15 Various enzymes involved in metabolism of drugs

16 Sites of Drug Metabolism Liver: Major site, well organized with all enzyme systems The first-pass effect Following drugs are metabolized extensively by first-pass effect: Isoproterenol, Lidocaine Meperidine, Morphine, Pentazocine, Propoxyphene, Propranolol, itroglycerin, Salicylamide Intestinal Mucosa: The extra-hepatic metabolism, contains CYP3A4 isozyme Isoproterenol exhibit considerable sulphate conjugation in GI tract Levodopa, chlorpromazine and diethylstilbestrol are also reportedly metabolized in GI tract Esterases and lipases present in the intestine may be particularly important carrying out hydrolysis of many ester prodrugs Bacterial flora present in the intestine and colon reduce many azo and nitro drugs (e.g., sulfasalazine) Intestinal b-glucuronidase can hydrolyze glucuronide conjugates excreted in the bile, thereby liberating the free drug or its metabolite for possible reabsorption (enterohepatic circulation or recycling)

17 Classification of biotransformation reactions 1. Reactions that involve enzyme-catalyzed biotransformation of the drug without any conjugations. Phase I reactions include: oxidations reductions hydrolysis reactions they introduce a functional group (e.g., -H) that serves as the active center for sequential conjugation in a phase II reaction.

18 2. Reactions that include conjugation reactions, which involve the enzyme-catalyzed combination of a drug (or drug metabolite) with an endogenous substance. Phase II reactions require: a functional group an active center as the site of conjugation with the endogenous substance. energy indirectly for the synthesis of activated carriers, the form of the endogenous substance used in the conjugation reaction (e.g., UDP-glucuronate).

19 Drug metabolism reactions :

20 Enzymes catalyzing phase I biotransformation reactions Enzymes catalyzing phase I biotransformation reactions include: cytochrome P-450 aldehyde and alcohol dehydrogenase deaminases esterases amidases epoxide hydratases

21 Enzymes catalyzing phase II biotransformation reactions include: glucuronyl transferase (glucuronide conjugation) sulfotransferase (sulfate conjugation) transacylases (amino acid conjugation) acetylases ethylases methylases glutathione transferase.

22 Classification of enzyme according to its location Location of these enzymes: numerous tissues some are present in plasma. Subcellular locations include: cytosol mitochondria endoplasmic reticulum nly those enzymes located in the endoplasmic reticulum are inducible by drugs

23 Metabolism of codeine (normal)

24 Enzymes Involved in Drug Metabolism CYP450, Hepatic microsomal flavin containing monooxygenases (MFM or FM) Monoamine xidase (MA) and Hydrolases Cytochrome P450 system: localized in the smooth endoplasmic reticulum. Cytochrome P450 is a Pigment that, with C bound to the reduced form, absorbs maximally at 450nm Cytochromes are hemoproteins (heme-thiolate) that function to pass electrons by reversibly changing the oxidation state of the Fe in heme between the 2+ and 3+ state and serves as an electron acceptor donor P450 is not a singular hemoprotein but rather a family of related hemoproteins. ver 1000 have been identified in nature with ~50 functionally active in humans with broad substrate specificity Simplified apoprotein portion HC HC L Fe +3 H R CH 2 CH 2 Substrate binding site Heme portion with activated xygen

25 1. Cytochrome P-450 monooxygenase (mixed function oxidase) a. General features A large number of families (at least 18 in mammals) of cytochrome P-450 (abbreviated CYP ) enzymes exists each member of which catalyzes the biotransformation of a unique spectrum of drugs some overlap in the substrate specificities. This enzyme system is the one most frequently involved in phase I reactions.

26 The cytochrome P-450 families are referred to using an arabic numeral, e.g., CYP1, CYP2, etc. Each family has a number of subfamilies denoted by an upper case letter, e.g., CYP2A, CYP2B, etc. The individual enzymes within each subfamily are denoted by another arabic numeral, e.g., CYP3A1, CYP3A2, etc. Cytochrome P-450 catalyzes numerous reactions, including: aromatic and aliphatic hydroxylations dealkylations at nitrogen, sulfur, and oxygen atoms heteroatom oxidations at nitrogen and sulfur atoms reductions at nitrogen atoms ester and amide hydrolysis

27 Cytochrome P450: aming Before we had a thorough understanding of this enzyme system, the CYP450 enzymes were named based on their catalytic activity toward a specific substrate, e.g., aminopyrine -demethylase now known as CYP2E1 Currently, all P450 s are named by starting with CYP (CYtochrome P450, 1, L, 2 - the first number is the family (>40% homology), the letter is the subfamily (> 55% homology), and the second number is the isoform. The majority of drug metabolism is by ~10 isoforms of the CYP1, CYP2 and CYP3 families in humans Major human forms of P450: Quantitatively, in the liver the percentages of total P450 protein are: CYP3A4 28%, CYP2Cx 20%, CYP1A2 12%, CYP2E1 6%, CYP2A6 4%, CYP2D6 4% By number of drugs metabolized the percentages are: CYP3A4 35%, CYP2D6 20%, CYP2C8 and CYP2C9 17%, CYP2C18 and CYP2C19-8% CYP 1A1 and CYP1A2-10%, CYP2E1 4%, CYP2B6 3%

28 Few Important CYP450 Isozymes CYP family Main functions CYP1 CYP2 CYP3 CYP7 CYP11 CYP17 CYP19 CYP21 CYP24 CYP27 Xenobiotic metabolism Xenobiotic metabolism, Arachidonic acid metabolism Xenobiotic and steroid metabolism Cholesterol 7α-hydroxylation Cholesterol side-chain cleavage, Steroid 11β hydroxylation, Aldosterone synthesis Steroid 17α-hydroxylation Androgen aromatization Steroid 21-hydroxylation Steroid 24-hydroxylation Steroid 27-hydroxylation

29 EC Recommended name Family/gene * secologanin synthase CYP72A * trans-cinnamate 4-monooxygenase CYP * benzoate 4-monooxygenase CYP * calcidiol 1-monooxygenase CYP * cholestanetriol 26-monooxygenase CYP * -monooxygenase CYP * flavonoid 3'-monooxygenase CYP * 3,9-dihydroxypterocarpan 6a-monooxygenase CYP93A * leukotriene-b 4 20-monooxygenase CYP4F * methyltetrahydroprotoberberine 14-monooxygenase CYP93A * tyrosine -monooxygenase CYP79

30 Localization The primary location of cytochrome P-450 is the liver, ther tissues, including: the adrenals ovaries and testis tissues involved in steroidogenesis and steroid metabolism. The enzyme's subcellular location is the endoplasmic reticulum. Mechanism of reaction 1. In the overall reaction: the drug is oxidized oxygen is reduced to water. Reducing equivalents are provided by nicotinamide adenine dinucleotide phosphate (ADPH), and generation of this cofactor is coupled to cytochrome P-450 reductase.

31 Genetic polymorphisms Genetic polymorphism of several clinically important cytochrome P-450s, particularly CYP2C and CYP2D, is a source of variable metabolism in humans, including differences among racial and ethnic groups. These enzymes have substantially different properties (Vmax or Km).

32 Induction of drug metabolism : Enzyme synthesis initiated within 24 h of exposure, increasing over 3 5 days Effect decreases over 1 3 weeks after inducing agent is discontinued Environmental Factors: Cigarette smoking eating BBQ meat cruciferous veggies (plants in the mustard family which includes the cabbage, radish, broccoli, and many weeds.) high protein diet Ethanol exposure to insecticides (DDT, Lindane) & PCBs (polychlorinated biphenyls)

33 Inhibition Rapid onset within 1 day Competitive or noncompetitive (clinically more likely) inhibition of P-450 enzyme activity can result in the reduced metabolism of other drugs or endogenous substrates such as testosterone. Inhibition can be caused by a number of commonly used drugs, including: Cimetidine fluconazole Fluoxetine Erythromycin grapefruit juice.

34 General Metabolic Pathways Phase II - Conjugation Glucuronic acid conjugation Sulfate Conjugation Glycine and other AA Glutathion or mercapturic acid Acetylation Methylation Hydrolytic Reactions Esters and amides Epoxides and arene oxides by epoxide hydrase Drug Metabolism Phase I - Functionalization xidation Aromatic moieties lefins Benzylic & allylic C atoms and a-c of C= and C= At aliphatic and alicyclic C C-Heteroatom system C- (-dealkylation, -oxide formation, -hydroxylation) C- (-dealkylation) C-S (S-dealkylation, S-oxidation, desulfuration) xidation of alcohols and aldehydes Miscellaneous Reduction Aldehydes and ketones itro and azo Miscellaneous

35 Phase II conjugation Reactions When phase I reactions are not producing sufficiently hydrophilic (water soluble) or inactive metabolites to be eliminated from the body, the drugs or metabolites formed from phase I reaction undergoes phas II reactions. Generally phase I reactions provide a functional groups or handle in the molecule that can undergo phase II reactions. Thus, phase II reactions are those in which the functional groups of the original drug (or metabolite formed in a phase I reaction) are masked by a conjugation reaction. Phase II conjugation reactions are capable of converting these metabolites to more polar and water soluble products. Many conjugative enzymes accomplish this objective by attaching small, polar, and ionizable endogenous molecules such as glucuronic acid, sulfate, glycine, glutamine and glutathione to the phase I metabolite or parent drug. The resulting conjugated products are very polar (water soluble), resulting in rapid drug elimination from the body.

36 Phase II Conjugation Reactions These reactions require both a high-energy molecule and an enzyme. The high-energy molecule consists of a coenzyme which is bound to the endogenous substrate and the parent drug or the drug s metabolite resulted from phase I reaction. The enzymes that catalyzed conjugation reactions are called transferases, found mainly in the liver and to a lesser extent in the intestines and other tissues. Most conjugates are biologically inactive and nontoxic because they are highly polar and unable to cross cell membrane. Exceptions to this are acetylated and methylated conjugates because these phase II reactions (methylation and acetylation) does not generally increase water solubility but serve mainly to terminate or reduce pharmacological activity (they are usually pharmacologically inactive).

37 Conjugating molecules: o 1- Glucuronic acid conjugation: o It forms -glucuronides with phenols Ar-H, alcohols R-H, hydroxylamines H2-H,and carboxylic acid RCH. o It can form -glucuronides with sulfonamides, amines, amides, and Sglucuronides with thiols. o 2-Sulfate conjugation: o It is less common. o It is restricted to phenols, alcohols, arylamines, and -hydroxyl compounds. o But primary alcohols and aromatic hydroxylamines can form unstable sulfate conjugates which can be toxic.

38 Conjugating molecules: 3-Amino acid conjugation: By the formation of peptide link. With glycine or glutamine. 4- Glutathione conjugation: It reacts with epoxides, alkylhalides, sulfonates, disulfides, radical species. These conjugates are converted to mercapturic acid and mostly are excreted in bile. It is important in detoxifying potentially dangerous environmental toxins.

39 Conjugating molecules: 5,6- Methylation and acetylation reactions: These decrease the polarity of the drugs except tertiary amines which are converted to polar quaternary salts. The groups susceptible for these reactions are phenols, amines, and thiols. -methylation of meta-phenolic H in catecholamines does not generally increase water solubility but serve mainly to terminate or reduce pharmacological activity (they are usually pharmacologically inactive). 7- Cholesterol conjugation: For carboxylic acids by ester link formation or for drug with ester group by trans esterification. 8- Fatty acid conjugation: For drugs with alcohol functional groups by ester link.

40 There are six conjugation pathways: 1)-Glucuronidation: by glucuronyl transferase. H H HC H + H UDP R X H H HC H X H R Glucuroinc acid UDP X = H, R 2, C 2 H, SH, acidic carbon atom Glucuronyl Transferease catalyses this conjugation reaction 2)-Sulfate conjugation H S P H Adenine H 2 3 P H PAPS + R X H S X X = H, arylamine, HH R Sulfotransferease catalyses this conjugation reaction

41 There are six conjugation pathways: 3)-Amino acid conjugation: Y Y H H C R S CoA + C H 2 C 2 H R C 2 H Acyl coenzyme A H Y = H or CH 2 CH 2 C 2 H -acyltransferase catalyses the conjugation reaction 4)-Glutathione conjugation R C S CoA Acyl coenzyme A + H H 2 Y C 2 H R Y = H or CH 2 CH 2 C 2 H C H H Y C 2 H Glutathione S-transferase catalyses this conjugation reaction 41

42 5)-Methylation H 2 C H 2 S + Adenine + R X R X H 2 3 P H X = H, H 2, SH SAM Methyltransferase catalyses this conjugation reaction 6)-Acetylation H 3 C C S CoA + R X R C X R Aceyl CoA Y =H 2, HH 2, S 2 H 2, CH 2 -acyltransferase catalyses the conjugation reaction

43 Factors influencing Drug Metabolism 43

44 Factors influencing Drug Metabolism 1-Chemical Structure : The chemical structure (the absence or presence of certain functional groups) of the drug determines its metabolic pathways. 2-Species differences (Qualitative & Quantitative): Qualitative differences may result from a genetic deficiency of a certain enzyme while quantitative difference may result from a difference in the enzyme level. 3-Physiological or disease state: 1-For example, in congestive heart failure, there is decreased hepatic blood flow due to reduced cardiac output and thus alters the extent of drug metabolism. 2-An alteration in albumin production can alter the fraction of bound to unbound drug, i.e., a decrease in plasma albumin can increase he fraction of unbound free drug and vice versa. 3-pathological factors altering liver function can affect hepatic clearance of the drug.

45 Factors influencing Drug Metabolism 4-Genetic variations: Isoniazid is known to be acetylated by - acetyltransferase into inactive metabolite. The rate of acetylation in asian people is higher or faster than that in eurpoean or north american people. Fast acetylators are more prone to hepatoxicity than slow acetylator. 5-Drug dosing: 1- An increase in drug dosage would increase drug concentration and may saturate certain metabolic enzymes. 2- when metabolic pathway becomes saturated, an alternative pathway may be pursued. 45

46 Factors influencing Drug Metabolism 6-utritional status: 1-Low protein diet decreases oxidative reactions or conjugation reactions due to deficiency of certain amino acids such as glycine. 2-Vitamin deficiency of A,C,E, and B can result in a decrease of oxidative pathway in case of vitamin C deficiency, while vitamin E deficiency decreases dealkylation and hydroxylation. 3-Ca, Mg, Zn deficiencies decreases drug metabolism capacity whereas Fe deficiency increases it. 4-Essential fatty acid (esp. Linoleic acid) deficiency reduce the metabolism of ethyl morphine and hexobarbital by decreasing certain drug-metabolizing enzymes.

47 Factors influencing Drug Metabolism 7-Age: 1- Metabolizing enzymes (sp.glucuronide conjugation)are not fully developed at birth, so infants and young children need to take smaller dosesthan adults to avoid toxic effects. 2-In elderly, metabolizing enzyme systems decline. 8-Gender (sex): Metabolic differences between females and males have been observed for certain compounds Metabolism of Diazepam, caffiene, and paracetamol is faster in females than in males while oxidative metabolism of lidocaine, chordiazepoxide are faster in men than in females

48 Factors influencing Drug Metabolism 9-Drug administration route: 1-rally administered drugs are absorbed from the GIT and transported to the liver before entering the systemic circulation. Thus the drug is subjected to hepatic metabolism (first pass effect) before reaching the site of action. 2-Sublingually and rectally administered drugs take longer time to be metabolized than orally taken drugs.itroglycerine is ineffective when taken orally due to hepatic metabolism. 3-IVadministration avoid first pass effect because the drug is delivered directly to the blood stream.

49 Factors influencing Drug Metabolism 10-Enzyme induction or inhibition Several antibiotics are known to inhibit the activity of cytochrome P450. Phenobarbitone is known to be cytochrome P450 enzyme inducer while cimetidine is cyt. P450 inhibitor. If warfarin is taken with phenobarbitone, it will be less effective. While if it is taken with cimetidine, it will be less metabolized and thus serious side effects may appear.

50 Strategies to manage drug metabolism 50

51 Strategies to manage drug metabolism 1)-Pharmaceutical strategies: by using different dosage forms to either avoid or compensate for rapid metabolism. 1-Sublingual tablets (through mucous mermbrane) by delivering drugs directly to blood and bypassing first-pass effect as nitroglycerine (antiangina drug). 2-Transdermal patches and ointments: provide continuous supply of drug over extended period of time and are useful for rapidly metabolizing drugs suchj as prophylactic nitroglycerine. 3- Intramuscular injections provide a continuous supply of drug over extended period of time such as`lipid soluble esters of estradiol and testosterone. Hydrolysis of these prodrugs produce a steady supply of rapidly metabolized hormones. 4-Enteric coated formulation can protect acid-sensitive drugs as erythromycin. 5-asal administration allows for the delivery of peptides such as aerosols since they need only to penetrate the thin epithelial

52 Strategies to manage drug metabolism a β lactamase inhibitor used in conjunction with penicillin 2)-Pharmacological strategies These involve the concurrent use of enzyme inhibitors to decrease drug metabolism. Sometimes the concurrent use of an additional agent doesn t prevent metabolism but prevents the toxicity caused by metabolites of the therapeutic agent. 1- Levodopa, the aminoacid precursor of dopamine, in the treatment of Parkinson s disease. Carbidopa, a dopa decarboxylase inhibitor 2- β-lactam antibiotics activity is reduced by micoorganisms capable of secreting β lactamase enzymes. Clavulanic acid is

53 Strategies to manage drug metabolism 3)-Chemical strategies These are molecular modifications involving the addition, deletion or isosteric modification of functional groups. Examples are: 1-chlorpropamide was designed from tolbutamide 2-Methyl testosterone was designed from testosterone. 53

54 Prodrugs strategies Prodrugs are used instead of active form of the drug to: a) Enhance membrane permeability, b) Reduce drug toxicity c) vercome /mask bad taste d) vercome acid sensitivity e) Prolong (short) duration. 54

55 Advantages of Prodrugs 1- An increase in water solubility by using sodium succinate esters as chloramphenicol succinate IV injection. 2- An increase in lipid solubility a-increase duration of action by using lipid soluble esters b-increase oral absorption as by using esters of the highly polar drugs or -methylation C-Increase topical absorption of steroids by masking H group as esters or acetonides. 3-A decrease in water solubility to increase palatability as in chloramphenicol palmitate 4- Decrease GI irritation (side effect) as in aspirin 5- Site specificity as in methyl dopa 6- Increased half-life and chemical stability as in cefamandole acetate a stable prodrug, while the parent cefamandole is unstable solid dosage form. Hetacillin is another prodrug (for

56 Basic Concepts of Prodrugs Carrier-linked prodrugs: a pro-moiety is attached, which is not necessary for activity but may impart some desired property to the drug, such as increased lipid or water solubility, or site-directed delivery Advantages may include: 1. increased absorption 2. alleviation of pain at the site of injection if the agent is given parenterally 3. elimination of an unpleasant taste associated with the drug 4. decreased toxicity 5. decreased metabolic inactivation 6. increased chemical stability 7. prolonged or shortened action Bioprecursor prodrugs contain no pro-moiety but rather rely on metabolism to introduce the functionality necessary to create an active species

57 2 2 H H H H Cl Cl Cl Cl a Prodrug: Chloramphenicol Hemisuccinate a Salt Inactive as it is and activated by hydrolysis by plasma esterases to chloramphenicol/ prednisolon Increased water solubility for parenteral administration, which otherwise would precipitate and cause pain by damaging surrounding tissues Prodrug: Chloramphenicol Palmitate H 3 C H H H Prodrug: Clindamycin Palmitate Cl S (CH 2 ) 14 Inactive as it is; activated by hydrolysis by intestinal esterases to chloramphenicol/ clindamycin Minimize their bitter taste and improve their palatability in pediatric liquid suspensions H H - a + Prodrug: Prednisolon Hemisuccinate Sodium Salt

58 H S Ca Prodrug: Carbenicillin Indanyl Ester Inactive as it is and activated by hydrolysis by plasma esterases to carbenicillin Lipophilic indanyl ester furnish improved oral bioavailability F H F H F H H 3 C S H H 3 C S H H 3 C S H Sulfide (Active) Sulindac (Inactive) Sulfone (Inactive)

59 Carboxylic Acids and Alcohols Converted to ester prodrugs which are often hydrolyzed to active drug by different types of esterase enzymes: Ester hydrolase Lipase Cholesterol esterase Drug Promoiety Drug H + H Promoiety Acetylcholinesterase Carboxypeptidase Cholinesterase Microflora in the gut or Drug Promoiety Esterase Drug H + H Promoiety Manipulation of steric and electronic properties of promoiety allows control of rate and extent of hydrolysis

60 Advantage of Prodrug Formation I: Increased absorption of hydrophilic drugs by making less hydrophilic or more lipophilic H 3 C H H H + H H H H + Esterase H Epineprine H 3 C Dipivefrin H 3 C H Pivalic Acid Prodrug of Epinephrine: Dipivefrin More lipophilic, thus achieve higher intraocular concentration Hydrolysis occur in cornea, conjunctiva, and aqueous humor after ophthalmic application

61 Advantage of Prodrug Formation II: Masking unpleasant taste Chloramphenicol palmitate and Clindamycin palmitate has already been shown. ther drugs include H 2 S -Acetyl sulfisoxazole H 3 C H 3 C H 3 C H 3 C H 3 C H H H 3 C H 3 C H H 3 C H H Erythromycin estolate H 3 C H 3 C Troleandomycin

62 ot all carboxylic esters hydrolyzed in vivo where double ester approach is used R 1 R 1 H H S CR 2 S R 3 CR 2 Esterase (R 2 = Ethyl, Propyl, Butyl, Phenyl) Penicillin Esters (R 2 = Ethyl, Propyl, Butyl, Phenyl) Cephalosporin Esters Esterase o Reaction o Reaction

63 H 2 S H Cefpodoxime Proxetil (Prodrug) S H 2 S H H H Esterase S + C 2 + H 2 S H S H + H 3 C H 3 C

64 Advantage of prodrug formation III: Increase hydrophilicity and thus water solubility to apply parenterally or also orally when compounds are too lipophilic to formulate in liquid dosage form Drug C H 2 C H 2 - a + Drug H + H C H 2 C H 2 - a + Succinates Drug P - a + Drug H + H P - a + H H Phosphates - Drug H + Drug Rapid and thus the prodrug is unstable

65 H 3 C H H H H H 2 H 3 P 4 Cl Phosphatase S CH P 3 H - H 3 C H H H Clindamycin Cl H S Clindamycin Phosphate

66 Chemical Delivery System The site specific delivery of drugs is an important way of increasing drug s therapeutic index. The knowledge of prodrug and drug metabolism is used to concentrate drugs at its target site thus minimizing the systemic toxicity. H H CH 2 CH CH H 2 BBB; Active transport to CS by L-Amino acid delivery system H H H CH 2 CH CH H CH 2 CH 2 H 2 H 2 L-Dopa Dopamine (Active)

67 Patient case, Heidi Raines, day 1 Heidi Raines: 33-year-old white female, previously healthy. Two days ago developed a bad headache with nausea and vomiting. Yesterday developed a stiff neck and was told over the telephone to take Extra-Strength Excedrin according to directions on the bottle. Today made an ER visit (Dr. W) and was treated with Demerol for headache pain and, finally, was able to rest. She was felt to have a migraine and a skeletal headache and was sent home with a prescription for Demerol tablets (50 mg tabs, 1-2 q 3-4 h prn pain). Her husband was very concerned about her and wanted to do all that he could to help.

68 Heidi Raines, day 2 Mrs. Raines was seen (by Dr. X) in the Clinic with headache, stiff neck, nausea, and vomiting. Demerol was stopped, and she was treated with Darvon without help. Continue Extra- Strength Excedrin, 2 e 4 h prn pain. Heidi Raines, day 5 Day 5: Mrs. Raines was seen in the Clinic (by Dr Y) and was given a prescription of Midrin * (1 stat, then 1 q 3 h, prn). Her husband was beginning to show increased anxiety concerning her welfare. He said she had not really eaten for days because of this 'sick headache'. nce again he was reassured, and was told that he could help by taking care of her at home. He promised to be helpful. *Isometheptene, 65 mg Dichloralphenazone, 100 mg Acetaminophen, 325 mg

69 Heidi Raines, day 6 Day 6: Mrs Raines continued her medications and seemed to improve, although she had been unable to keep any food down for about one week. Her headache worsened and on a Clinic visit (Dr. Z) she was treated with Meprobamate and three Extra Strength Tylenol and was sent home with instructions to take three Extra Strength Tylenol every 3-4 hours, prn. Same old story with the husband. Heidi Raines, day 7 Day 7: Continuing medications as prescribed, Mrs. Raines felt somewhat better on the day prior to admission with little headache, nausea or vomiting. However, she then developed crampy abdominal pain, anorexia, and nausea and, as per a telephone conversation, was treated with DiGel, but, because of persistent symptoms, was admitted to the hospital.

70 Heidi Raines, day 8 Physical exam on admission showed a thin, 33-year-old, cigarette smoking, white female complaining of abdominal pain, nausea, vomiting, and headache. She talked in a weak voice. Her neck was supple. The chest was clear. The heart showed a sinus rhythm. There was some abdominal tenderness with no rigidity and no masses. The patient appeared pale and rather drowsy, but had no focal neurologic findings.

71 Heidi Raines, hospital course Liver enzymes were massively elevated with SGT over 20,000 units. Fibrinogen level became nondetectable and platelet count continued to fall. Hematocrit and hemoglobin dropped. Bilirubin stayed stable around 3.2 mg%. Creatinine rose from 2.5 to 3.5 mgm%. The BU was 9 or 10 mg%. Total protein was 4.5 with albumin of 3.1 grams%. Blood ammonia was 239 mg%. eurologically, the patient deteriorated and became unresponsive. The cause of death was felt to be a respiratory arrest with subsequent cardiac arrest on the basis of severe massive hepatic necrosis (emphasis added) with metabolic changes and metabolic encephalopathy.

72 Heidi Raines: Autopsy 1. Acute massive necrosis of the liver 2. Severe metabolic acidosis, lactic acidosis, treated. 3. Acute pulm. edema, with interalveolar hemorrhage 4. Diffuse intravascular coagulation. 5. Acute renal failure secondary to acute tubular necrosis. 6. Metabolic encephalopathy, secondary to hepatic coma 7. Metabol. acidosis severe lactic acidosis, H 2 0 depletion 8. Anemia,? hemolytic,? due to blood loss into lungs.

73 Why did Heidi Raines die? Failure to properly diagnose and treat