DRUG ELIMINATION 08122011 1 Drug Elimination Learning Objectives

DRUG ELIMINATION 08/12/2011 1

Drug Elimination Learning Objectives • At the end of this session each student should be able to describe: Ø Phases I and II of drug metabolism, emphasizing the importance of the cytochrome P 450 monooxygenase system Ø Processes of biliary excretion and enterohepatic recirculation of drugs, and of drug & drug metabolite elimination by the kidney 2

Drug Elimination • Drug elimination is the irreversible loss of drug from the body; it occurs by two processes: 1. metabolism and 2. Excretion • Metabolism consists of anabolism (build-up) and catabolism (breakdown) of substances by enzymic conversion of one chemical entity to another within the body. • Excretion consists of elimination from the body of chemically unchanged drug or its metabolites 3

Drug Elimination… • The main routes by which drugs and their metabolites leave the body are the: vkidneys vhepatobiliary system vlungs (important for volatile/gaseous anaesthetics) • Most drugs leave the body in the urine, either unchanged or as polar metabolites 4

Drug Elimination… • some drugs are secreted into bile via the liver, but most of these are then reabsorbed from the intestine. • There are, however, instances (e. g. rifampicin) where faecal loss accounts for the elimination of a substantial fraction of unchanged drug in healthy individuals, and • faecal elimination of drugs such as digoxin that are normally excreted in urine becomes progressively more important in patients with advancing renal failure 5

Drug Elimination… • Excretion via the lungs occurs only with highly volatile or gaseous agents (e. g. general anaesthetics) • Small amounts of some drugs are also excreted in secretions such as milk or sweat. • Elimination by these routes is quantitatively negligible compared with renal excretion, although excretion into milk can sometimes be important because of effects on the baby 6

Drug Elimination… • Lipophilic substances are not eliminated efficiently by the kidney • Consequently, most lipophilic drugs are metabolized to more polar products, which are then excreted in urine. • Drug metabolism occurs predominantly in the liver, especially by the cytochrome P 450 (CYP) system 7

DRUG EXCRETION 8

Drug Excretion Kidneys • Drugs differ greatly in the rate at which they are excreted by the kidney, ranging from penicillin, which is cleared from the blood almost completely on a single transit through the kidney, to diazepam, which is cleared extremely slowly. • Most drugs fall between these extremes, and metabolites are nearly always cleared more quickly than the parent drug. 9

Drug Excretion Kidneys… • Three fundamental processes account for renal drug excretion: Ø Glomerular filtration Ø Active tubular secretion Ø Passive diffusion across tubular epithelium 10

Drug Excretion Kidneys… • Excretion by the kidneys depends on : 1. The rate of filtration of the drug at the gromerulus: • -Is determined by glomerular filtration rate (GFR) which is about 125 ml/ min in man. • (a) Only the unbound drug is filtered. • (b) the rate of filtration is not affected by lipid solubility or by the degree of ionization of the drug 11

• (c) Renal clearance of a drug of > 125 ml/min implies that some active secretion is occurring • while clearance less than creatinine clearance [i. e. <125 ml/min] implies that some re-absorption of drug is taking place. 12

Excretion by the kidneys depends on : … 2. Tubular reabsorption. a-Passive –Drug molecules diffuse back into the peritubular plasma, down a concentration gradient. The rate of reabsorption depends on the same physicochemical properties of a drug that govern absorption b- Active-Active tubular re-absorption occurs for drugs that resembles essential endogenous metabolites like [ amino acids, methyldopa, thyroxine) • Re-absorption of some drugs can be inhibited by others e. g. Uric Acid re-absorption is inhibited by Probenecid and may enhance excretion of uric acid • Probenecid may be clinically useful in gout arthritis 13

Excretion by the kidneys depends on : … 3. Tubular Secretion. (a) If there is active secretion, the renal clearance usually exceeds the GFR and can be as large as the renal plasma flow (b) Clearance can exceed renal plasma flow if the drug is concentrated in RBCs and is available for secretion [prilocaine in highly acidic urine] (c) Renal clearance of actively secreted drugs is not reduced by protein binding, as the drug/albumin complex dissociates 14 very rapidly as free drug is secreted

Tubular secretion… (d) Compounds which undergo tubular secretion includes: ANIONS: • Penicillins, Probenecid, Thiazide and Loop diuretics, Salicylates, Drug conjugates. Glucuronides, Glycine-conjugates [ salicyluric Acid] Sulphates. CATIONS: • Onium salts and strong bases [Choline, Neostigmine, Morphine, Quinine etc. ] 15

• Drug Disposition in renal insufficiency -Water soluble and intermediate soluble drugs e. g. Gentamicin and Digoxin accumulate -Lipid Soluble drugs e. g. . Phenytoin do not accumulate -Actively transported drugs show intermediate accumulation -Frusemide can be given in large doses [it is actively secreted] without incidence of systemic toxicity 16

ARTIFICIAL KIDNEY& PLASMA DIALYSIS. • Is a scientific technique for separating substances in solution by taking advantage of their differences in their diffusability through a porous membrane. This method can be used to remove toxins from the body in renal failure • Effective for small water soluble drug molecules that are little protein bound [Chlorate ion, Ethylene Glycol] • Ineffective for highly lipid soluble drugs [Phenytoin, Thiopentone] • Ineffective for highly protein bound drugs like Warfarin or tissue bound drugs like digoxin • Blood may be pumped through column of activated charcoal for rapid clearance of lethal 17 compounds e. g. paraquat and paracetamol

EXCRETION BY OTHER ROUTES. LIVER • Liver cells transfer various substances, including drugs, from plasma to bile by means of transport systems similar to those of the renal tubule including organic cation transporters, organic anion transporters and P-glycoproteins. • Various hydrophilic drug conjugates (particularly glucuronides) are concentrated in bile and delivered to the intestine, where the glucuronide is usually hydrolysed, releasing active drug once more; free drug can then be reabsorbed and the cycle repeated (enterohepatic circulation) 18

EXCRETION BY OTHER ROUTES. LIVER… • The effect of this is to create a reservoir of re-circulating drug that can amount to about 20% of total drug in the body and prolongs drug action. • E. g. where this is important include morphine and ethinylestradiol • Several drugs are excreted to an appreciable extent in bile. • Vecuronium is an example of a drug that is excreted mainly unchanged in bile • Rifampicin is absorbed from the gut and slowly deacetylated, retaining its biological activity. Both forms are secreted in the bile, but the deacetylated form is not reabsorbed, so eventually most of the drug leaves the body in this form in the faeces 19

EXCRETION BY OTHER ROUTES. LIVER… • Bile flow is about 0. 5 m. L/minute. v Biliary clearance of unmetabolized drugs which enter by simple diffusion is negligible -Active transport exist for polar compounds with MW>250 daltons. These mechanisms are applicable for carbohydrates [ dextran, inulin, sucrose, mannitol] and acid compounds [ bile acids, bilirubin, iodine contrast media, glucuronide, glycine and sulphate conjugates and penicillins]. 20

- Excreted compounds are often reabsorbed from the gut, usually after deconjugation brought about by the normal bacteria in the gut. The drug may be re-excreted by the liver to produce enterohepatic recycling [e. g. Contraceptive Steroids, Phenothiazine] -the reabsorbed drug, metabolite or conjugate is finally excreted by the kidney. -Biliary obstruction and hepatocellular failure produce impairment of biliary excretion 21

Excretion by Other routes… LUNGS—Volatile Anaesthetics: • Drug molecules may diffuse across alveolar membrane. The lungs are the major organs of elimination of volatile anaesthetic agents. Excretion in expired air may be obvious to smell but quantitatively insignificant [ Ethanol, Paraldehyde, Thiols]. SALIVA, MILK, SWEAT, SEBUM: • Amounts of drug excreted are small but relevant to the breast fed infant and the treatment of acne with antibacterial drugs that partition into sebum [e. g Tetracycline]. 22

Summary • Elimination is the removal of the original drug molecule from the body by excretion or by metabolism • Excretion in the urine is important for water soluble drugs and metabolism by the liver important for lipid-soluble drugs • Clearance is the volume of blood stripped of drug in unit time • Renal clearance of a drug is the sum of glomerular filtration plus active secretion minus re-absorption • Lipid-soluble drugs are passively re-absorbed from kidney tubules, while water-soluble drugs are not; acids and bases exhibit p. H-dependent renal excretion 23

DRUG METABOLISM 24

DRUG METABOLISM. • Most lipid-soluble compounds are metabolized to more water-soluble products • Drugs must posses some degree of lipid solubility to be able to go through liver cell membranes to gain access to enzymes • The metabolism (Biotransformation) of foreign compounds [Xenobiotics], occurs mainly in the liver, although kidneys, adrenal cortex, lungs, placenta, skin and even lymphocytes may be involved to a small extent. • Atracurium is an unusual drug in that it breaks down in peripheral blood to inactive products so that its neuromuscular blocking action is terminated. • Atracurium breaks down by spontaneous degradation in blood (at p. H 7. 4, 37 C) 25

Metabolism of most drugs occurs in two phases: Ø phase I and Ø phase II reactions • These reactions take place mainly in the liver, [though some drugs are metabolised in plasma (e. g. hydrolysis of suxamethonium by plasma cholinestrase; lung e. g various prostanoids or gut e. g. tyramine, salbutamol] • The two processes often, though not invariably, occur sequentially 26

Drug Metabolism • PHASE I METABOLISM Involves: oxidations such as the addition of an OH to a lipophilic part of the original compound. -Phase I reactions generally produce a more water soluble and less active compound. • PHASE II METABOLISM. • Comprises conjugation reactions. The conjugation occurs at the site of a reactive group (such as –OH, added perhaps by a phase I reaction). • A phase II reaction renders a compound much 27 more water soluble and totally inactive.

METABOLIC PATHWAYS-Phase IMIXED FUNCTION OXIDASE [MFO] • This enzyme system is located in the smooth Endoplasmic Reticulum [Microsomal Fraction], is relatively non specific and is also known as MONO-OXYGENASE (one atom of O 2 molecule is incorporated into each molecule of product). • This enzyme system is the most important for Phase I reactions. • A major component of the system is the haem protein cytochrome P 450 ( so called because in the reduced state it forms a complex with carbon monoxide that absorbs light at 450 nm) 28

METABOLIC PATHWAYS-Phase IMIXED FUNCTION OXIDASE [MFO] • Cytochrome P 450 comprises a superfamily of closely related isoenzymes. • Some of the subfamilies of cytochrome P 450 are involved in the metabolism of several endogenous substances (e. g. corticosteroids) • Other subfamilies are involved in the metabolism of xenobiotics (foreign substances) • The individual isoenzymes have evolved selectively for particular substrates. 29

METABOLIC PATHWAYS-Phase IMIXED FUNCTION OXIDASE [MFO] Examples Chemical Reactions 1 -Hydroxylation of aromatic ring 2. -Hydroxylation of alkyl chain 3. -Epoxidation 4. -Oxidative deamination Examples of Benzene to give Phenol of Toluene to give Benzyl alcohol. of Benzopyrene to give benzopyrene Epoxide of Amphetamine to give Phenyl- Acetone. 30

OTHER OXIDATIONS. • Some are Cytoplasmic [Ethanol metabolism], others are Mitochondrial [ e. g. Monoamine oxidase (MAO)substrates which include Noradrenaline, tyramine]. • Reductions-reductive metabolism of drugs is less common than oxidation e. g. reductive metabolism of Halothane to Trifluoroethane • Hydrlolysis 1. - Ester bonds [ in Aspirin, Atropine, Pethidine, ] and 2. -Amide bonds [Lignocaine ] can be hydrolysed. -Esterases are present in gut, liver, plasma and other tissues. 31

PHASE I REACTIONS The P 450 MONOOXYGENASE SYSTEM -Metabolism involves Cytochrome P 450 enzymes (are haem proteins, comprising a large family ‘superfamily’ of related but distinct enzymes-) -These enzymes differ from one another in amino acid sequence, in regulation by inhibitors and inducing agents, and in the specificity of the reactions that they catalyse 32

CYTOCHROME P 450 enzymes • Each referred to as CYP followed by a defining set of numbers and a letter • Each CYP metabolises specific drugs • Some examples of common drugs that are substrates for P 450 isoenzymes Isoenzyme P 450 Drug CYP 1 A 1 Theophyline CYP 1 A 2 Caffeine, paracetamol, tacrine, theophyline CYP 2 A 6 Methoxyflurane CYP 2 C 9 Phenytoin, ibuprofen, warfarin, tolbutamide CYP 2 C 19 Omeprazole CYP 2 D 6 Codeine, metoprolol, debrisoquine, CYP 2 E 1 halothane, enflurane, Alcohol CYP 3 A 4/5 Nifedipine, losartan, terfenadine etc 33

Inhibition of P 450 • Inhibitors of P 450 differ in their selectivity toward different isoforms of the enzyme and are classified by their mechanism of action. • Some drugs inhibit some P 450 isoenzymes by competing for active site of the isoenzyme but are not substrates for the isoenzyme Inhibiting drug Isoenzyme inhibited Quinidine CYP 2 D 6 Ketoconazole CYP 3 A 4 Gestodene CYP 3 A 4 Diethly carbamate CYP 2 E 1 34

PHASE II REACTIONS -Involves conjugation reactions. The conjugation occurs at the site of a reactive group such as – OH in the liver. -chemical groups often involved in conjugation are: • Glucuronyl • Sulphate • Methyl • Acetyl • Glycyl and Glutathione -A phase II reaction renders a compound much more water soluble and totally pharmacologically inactive than its precussor and is excreted in urine or bile 35

Phase II reaction… • Glucuronide Conjugates. • Glucose is oxidized to glucuronic acid. This in turn is combined with alcoholic or phenolic hydroxy group or amines to form glucuronide conjugates. • OTHER CONJUGATES • Products from Phase I or parent drug can form addition compounds with sulphate [Paracetamol sulphate], acetate [ Acetyl Isoniazid], Glycine [Salicyluric acid ], and a methyl group [ NA to adrenaline ]. 36

Induction of microsomal enzymes • A number of drugs increase the activity of microsomal oxidase and conjugating systems when administered repeatedly. • The effect is referred to as induction and is the result of increased synthesis of microsomal enzymes • Examples include Rifampicin, ethanol and Carbamazepine • Enzyme induction can increase or decrease drug toxicity 37

Induction of microsomal enzymes… • There are drugs, e. g. paracetamol, where the phase I metabolites are mainly responsible for their toxicity; consequently toxicity is increased following enzyme induction • The carcinogenic action of some polycyclic hydrocarbons is associated with increased hepatic formation of highly reactive oxidative products (e. g. epoxides) that can damage DNA. • However the mechanism of enzyme induction is not well understood 38

First pass (pre-systemic) metabolism • The liver (or at times the gut wall) extracts and metabolizes some drugs taken orally so efficiently that the amount reaching the systemic circulation is considerably less than the amount absorbed. • This decrease the bioavailability of the drug • This is known as the “first pass effect” • Some drugs have to be given by other routes to avoid first pass effect e. g. Glyceryl trinitrate for angina which is given sublingually 39

• First-pass metabolism is generally a nuisance in practice, because: Ø A much larger dose of the drug is needed when it is given orally than when it is given by other routes Ø Marked individual variations occur in the extent of firstpass metabolism of a given drug, resulting in unpredictability when such drugs are taken orally Ø E. g. of drugs that undergo substantial first-pass elimination include; - aspirin, glyceryl trinitrate, isosorbide dinitrate, levodopa, Lidocaine, metoprolol, morphine, propranolol, salbutamol, verapamil. 40

Pharmacologically Active Drug Metabolites • Some drugs are themselves pharmacologically inactive but become pharmacologically active only after liver metabolism these are called “prodrugs” for example: Drug active metabolite Azathioprine mercaptopurine Enalapril enalaprilat Aspirin salicylic acid Cortisone hydrocortisone Zidovudine zidovudine triphosphate prednisone prednisolone 41

• Pro-drugs are sometimes designed deliberately to overcome problems of drug delivery • Metabolism can alter the pharmacological actions of a drug qualitatively. • Aspirin inhibits some platelet functions and has antiinflammatory activity. It is hydrolysed to salicylic acid, which has anti-inflammatory but not anti-platelet activity. • In other instances, metabolites have pharmacological actions similar to the parent compound (e. g. benzodiazepines, many of which form long lived active metabolites that cause sedation to persist after the parent drug has disappeared) 42

Toxic metabolites • Some ‘pro-drugs’ produce toxic metabolites Drug toxic metabolite Paracetamol N-acetyl-p-benzoquinoneimine (reactive quinone) Halothane trifluoroacetic acid Methoxyflurane fluoride Cyclophosphamide acrolein 43

Exercise • Other examples of pro-drugs and their metabolites: • Cyclophosphamide? . . • Malathion? . . • Diazepam? . . • Methanol? . . • Aspirin? . . . • Morphine? . . • Levodopa? . . • Etc etc … 44

Metabolic rate (Liver) • The concept of clearance can be equally applied to liver metabolism • An organ clearance can be defined in terms of an eliminating organ. • Hepatic clearance equals liver blood flow ( normally approx. 1500 ml/min) multiplied by the extraction ratio (=E=the fraction of the drug in the blood that is metabolized during a single passage through the liver) • The extraction ratio can vary from 1 (virtually all drug presented is removed) to 0 (no drug metabolized) 45

Metabolic rate (Liver) … Examples: Low Extraction ratio 1. -Diazepam 2. -Phenytoin 3. - Salicylic acid 4. -Theophylline 5. -Warfarin High Extraction ratio 1. -Lignocaine 2. -Glyceryl Trinitrate’ 3. -Isoprenaline 4. - Morphine 5. -Paracetamol. 46

Metabolic rate (Liver)… • For drugs with large extraction ratio, the liver has such a large capacity for metabolism that clearance is unaffected by changes in enzyme activity or by the fraction of drug that is protein bound, but is proportional to blood flow (changes in amount delivered), that is metabolism is perfusion limited. • Shock (with reduced hepatic blood flow) decreases metabolism. • Lignocaine and morphine are unusually persistent in patients with cardiogenic 47 shock

Metabolic rate (Liver)… • Drugs with large extraction ratios exhibit considerable first-pass metabolism. • (after absorption from the gut all the drug is presented to the liver in the hepatic portal vein. Subsequently, only a fraction of the drug presented to the liver in the systemic blood on each pass as the liver blood flow is only a fraction of the total cardiac output) 48

Metabolic rate (Liver)… • For drugs with a small extraction ratio, the rate at which drug is delivered to the liver exceeds the rate of either diffusion into the liver or the capacity of the liver to metabolize. • Therefore, clearance is sensitive to changes in the fraction of drug in the plasma that is unbound and to changes in enzyme activity, but is not sensitive to changes in blood flow. • Nevertheless, the liver can still be the major route of elimination of drugs in this group 49

Metabolic rate (Liver)… • Most drug metabolizing systems exhibit first order kinetics (rate of enzymatic elimination is proportional to the concentration of the free drug in the plasma) within therapeutic range of drug plasma concentrations. • A few drugs e. g. Salicylic Acid, Phenytoin and Ethanol. show saturation kinetics, which approximate to Zero Order Kinetics (that is the rate of enzymatic elimination is constant and independent of plasma concentration) 50

Genetic Variation in Rate Of Metabolism There is genetically determined variation in the rate of metabolism by some specific enzyme systems. Examples are: 1. -N-Acetylation. 2. -Plasma cholinesterase 3. -Drug Hydroxylation by MFO. 51

Metabolism and drug interactions • The activity of the MFO system can be increased by exposure to some drugs or foreign chemicalsthe system is said to be inducible • As a result, there is an increased rate of elimination of the inducing drug itself and many other drug substrates • The therapeutic effect of drugs can therefore be reduced if their metabolism is induced • Drugs that cause induction includes— Griseofulvin, Phenobarbitone, Phenytoin and Rifampicin. 52

Metabolism and drug interactions… • Conversely a number of the enzyme systems can be inhibited by drugs, leading to decreased elimination of other drugs or endogenous substrates. • This can lead to excessive drug effects, although often the interaction is not of major clinical significance or can be taken into account by change in dosage 53

Metabolism and drug interactions… • When exposure to the inducing or inhibiting drug or chemical ceases, MFO activity returns to normal. • This may mean that another dosage adjustment is needed to prevent over dosage (when an inducing influence is removed) or under dosage (when an inhibiting influence is removed) 54

A number of enzyme Systems can be inhibited by drugs-(examples of inhibitors of enzymes and drugs whose metabolism is inhibited): Enzyme Inhibitor Metabolism Inhibited MFO Cimetidine ciprofloxacin Phenytoin Warfarin MAO Tranylcypromine Phenelzine Tyramine-like agents Aldehyde dehydrogenase Disulfiram Chlorpropamide Ethanol Xanthine Oxidase Allopurinol Azathioprine Mercaptopurine 55

Summary • Metabolism of most drugs occur in two phases, phase I unmasks or inserts a hydrophylic functional group while phase II involves conjugation • Many drugs are oxidised by the mixed function oxidase system. This system can be induced and inhibited by drugs • Pro drugs are pharmacologically inactive compounds metabolized to active products 56

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