General pharmacology Lecture(3)

Transcription

1 General pharmacology Lecture(3) 2. Specialized transport: It may be: i. Carrier-mediated transport ii. Pinocytosis. i. Carrier-mediated transport: The drug combines with a carrier (a specialized protein molecule) present in the membrane. The complex thus formed translocates from one face of the membrane to the other. Such transport requires expenditure of energy. So it is called active transport. The transport is against concentration gradient. It is a specific transport. It is a saturable process. It is competitively inhibited by analogues which utilize same carrier. Sometimes non-diffusible substances are translocated along their concentration gradient (e.g. vitamin B12). It is called facilitated diffusion. It is not dependent on energy. ii. Pinocytosis: In this case, the substance is transported across the cell in particulate form by formation of vesicles. Proteins and other big molecules are transported by this process. This is rarely applicable to drugs. Carrier-mediated transport Important features are: Active process Occurs against concentration gradient Specific Energy dependent Saturable Competitive inhibition by, analogues Absorption & Bioavailability Absorption means the movement of drug from its site of administration into the bloodstream. Clinical efficacy of drug depends on: The route of administration that determines the latent period between administration and onset of action; The drug has to cross biological membranes except when given i.v. So its absorption is governed by the above described principles. Other factors affecting absorption are: 1. Physical properties 1

2 Concentrated solution of drug is absorbed faster than from diluted solution because passive transport depends on concentration gradient. Drugs are absorbed in aqueous phase. So liquids are better absorbed than solids and crystalloids are better absorbed than colloids. Drug is removed from the site of absorption by blood circulation. It is also responsible for the maintenance of concentration gradient across the membrane. So increased blood flow hastens drug absorption. 2. Dosage form Smaller the particles of the drug in a tablet better is the absorption. So by reducing the particle size, the dosage of the active drug can be reduced without lowering efficacy, e.g. corticoids, chloramphenicol, griseofulvin, tolbutamide and spironolactone. On the other hand in order to reduce absorption of anthelmintic (bephenium hydroxynaphthoate), the particle size should be large. To formulate powders or tablets, lactose, sucrose, starch and calcium phosphate or lactate are used as inert diluents. "Disintegration time" (rate of break up of the tablet or the capsule into the drug granules) and the dissolution rate (rate at which drug granules goes to solution) are important factors in determining the absorption of a drug. 3. Larger the area of absorbing surface faster is the absorption. 4. Each route of administration has its own peculiarities. It, therefore, affects drug absorption as under. Oral Epithelial lining of the gastrointestinal tract is lipoidal. So it acts as effective barriers to orally administered drugs. The rate of absorption of non-ionized lipid soluble drugs (e.g. ethanol) from stomach as well as intestine is proportional to their lipid: water partition coefficient. Acidic drugs (e.g. salicylates,barbiturates,etc.) are absorbed from the stomach because they remain unionized in the gastric juice while basic drugs (e.g. morphine, quinine) are poorly absorbed as they remain ionized in the gastric juice. They are absorbed only on reaching the duodenum (alkaline ph). However, even absorption of acidic drugs from stomach is slower because of the following reasons: a. Thick mucosa b. Mucous on mucosa c. Small surface area Presence of food retards/aids the absorption of drug by altering the gastric emptying time. It is observed that food retards the absorption of aspirin, ampicillin, captopril, digoxin, isoniazid, levodopa, penicillin G, and rifampicin, tetracycline while it aids the absorption of carbamazepine, chloroquine, griseofulvin, lithium carbonate, nitrofurantoin, riboflavin, and spironolactone. However, rapid absorption occurs if most drugs are given on empty stomach. Certain drugs are ineffective orally because of the following reasons: a. Insulin and adrenocorticotrophic hormone (ACTH) are polypeptides. They undergo enzymatic degradation within the lumen of gastrointestinal tract. 2

3 b. Poor absorption from the gastrointestinal tract, e.g. aminoglycoside antibiotics. c. Sex hormones and aldosterone are readily absorbed from the gut. However, they are inactivated in the gut wall as well as during the passage through liver before reaching to their site of action. d. Concurrently administered drugs may alter the absorption of a drug due to: Luminal effect: Insoluble complexes are formed, e.g. tetracycline with iron and antacids, phenytoin with sucralfate, cholesterol with liquid paraffin. Gut wall effects: Number of drugs may alter motility of gut and thus alter the absorption of a drug, e.g. opioids, tricyclic antidepressants, metadopramide, anticholinergics, etc. Subcutaneous and Intramuscular Many drugs are not absorbed on oral administration. However, they are absorbed on subcutaneous or intramuscular administration because they are deposited directly in the vicinity of capillaries on perenteral administration. Capillaries are highly porous. Drug absorption is accelerated by application of heat and exercise by increasing blood flow. Vasoconstrictors, e.g. adrenaline retard absorption when injected along with the drug. Hyaluronidase facilitates drug absorption from subcutaneous site by promoting spread. Many depot preparations, e.g. benzathine penicillin, protamine zinc insulin, depot progestins can be given by these routes. Pellets and implants can be inserted subcutaneously for prolonged action. Topical Sites (Skin, Cornea, Mucous Membranes) On topical application, systemic absorption depends primarily on lipid solubility of the drugs. Mucous membranes of cornea, mouth, rectum and vagina absorb lipophylic drugs. Lipid soluble unionized drugs are absorbed but lipid soluble ionized drugs are not absorbed. Abraded surfaces readily absorb drugs. Few drugs such as corticosteroids, hyoscine, nitroglycerin, organophosphorus insecticides, etc. can be absorbed through intact skin. Absorption can be promoted by rubbing the drug incorporated in an oleaginous base or by use of occlusive dressing. Bioavailability Bioavailability of a drug means availability of biologically active drug. It is a determination of the amount or fraction of administered dose of the given dosage form that reaches the systemic circulation in the unchanged form. On intravenous administration, all the drugs are available for biological activity, i.e. bioavailability is 100%. However, it is lower after oral ingestion because it depends on the following factors: Rate of absorption Gastrointestinal tract degradation First pass metabolism by gut wall and hepatic enzymes Enterohepatic circulation Presence of food and other drugs 3

4 Incomplete bioavailability after subcutaneous or intramuscular injection is less common but may occur due to local binding of the drug. Clinical Significance of Bioavailability Bioavailability variation is of practical significance under following circumstances: For drugs with low safety margin (digoxin) For precise control of doses of drugs (oral hypoglycemics, oral anticoagulants, etc.) For success or failure of antimicrobial regimen. Distribution of Drugs Drug distribution is the process by which a drug reversibly leaves the blood stream and enters the interstitium (extracellular fluid) and/or the cells of the tissues. Fig. 2.1: Distribution of a drug 4

5 The delivery of a drug from the plasma to the interstitium primarily depends on blood flow, capillary permeability, the degree of binding of the drug to plasma and tissue proteins, and the relative hydrophobicity of the drug. A. Blood flow The rate of blood flow to the tissue capillaries varies widely as a result of the unequal distribution of cardiac output to the various organs. Blood flow to the brain, liver, and kidney is greater than that to the skeletal muscles, whereas adipose tissue has a still lower rate of blood flow. B. Capillary permeability Capillary permeability is determined by capillary structure and by the chemical nature of the drug. 1. Capillary structure: Capillary structure varies widely in terms of the fraction of the basement membrane that is exposed by slit (tight) junctions between endothelial cells. In the brain, the capillary structure is continuous, and there are no slit junctions (Figure 1.8). This contrasts with the liver and spleen, where a large part of the basement membrane is exposed due to large discontinuous capillaries, through which large plasma proteins can pass. a. Blood-brain barrier: In order to enter the brain, drugs must pass through the endothelial cells of the capillaries of the central nervous system (CNS) or be actively transported. For example, the large neutral amino acid carrier transports levodopa into the brain. Lipid-soluble drugs readily penetrate into the CNS, since they can dissolve in the membrane of the endothelial cells. Ionized or polar drugs generally fail to enter the CNS, since they are unable to pass through the endothelial cells of the CNS, which have no slit junctions. These tightly juxtaposed cells form tight junctions that constitute the so-called blood-brain barrier. 2. Drug structure: The chemical nature of the drug strongly influences its ability to cross cell membranes. Hydrophobic drugs, which have a uniform distribution of electrons and no net charge, readily move across most biological membranes. These drugs can dissolve in the lipid membranes and therefore permeate the entire cell's surface. The major factor influencing the hydrophobic drug's distribution is the blood flow to the area. By contrast, hydrophilic drugs, which have either a non uniform distribution of electrons or a positive or negative charge, do not readily penetrate cell membranes and must go through the slit junctions C. Binding of drugs to proteins Plasma albumin is the major drug-binding protein and may act as a drug reservoir, for example, as the concentration of the free drug decreases due to elimination by metabolism or excretion, the bound drug dissociates from the protein. This maintains the free drug concentration as a constant fraction of the total drug in the plasma. 5

6 Plasma-protein-binding: Drugs may reversibly bind to non-specific non-functional sites on plasma proteins which serve no biological effect. Affinity of most drugs for plasma 6

7 proteins depends on their physicochemical properties as well as on the concentration of binding protein in the plasma. Thus, in pregnancy, the protein bound fraction of substances such as thyroxine increases due to a rise in the concentration of the specific binding protein in plasma. Conversely, free fraction of the drug is increased due to low plasma proteins in a patient of hypoproteinaemia. Acidic and neutral drugs bind to albumin fraction, e.g. salicylates, diazepam, phenytoin, warfarin. Basic drugs bind to orsomucoid (alfa-2-acid glycoprotein), lipoprotein, and beta-globulin. Examples of such drugs are lidocaine, propranolol and quinidine. Plasma protein binding Type of drug Type of plasma protein Examples Acidic and neutral drugs Albumin Salicylates Diazepam Phenytoin Warfarin Basic drugs Orsomucoid (α 2 acid glycoprotein) Lipoprotein β-globulin Lidocaine Propranolol Quinidine 7

8 METABOLISM (BIOTRANSFORMATION) Lecture(4) Biotransformation means chemical change of a drug within a living organism. Drugs are foreign substances to the body. So body tries to get rid of them subjecting to various mechanisms. After absorption, drugs could undergo three possible fates (Fig. 2.2): 1- Excreted unchanged 2- Metabolized by enzymes 3- Spontaneously changed into other substances because of appropriate ph of body fluids. Metabolism makes non-polar (lipid soluble) compounds to polar (lipid insoluble) substances so that they are not reabsorbed in the renal tubules and are excreted. The primary site for drug metabolism is liver. Other sites are kidney, intestine, lungs, and plasma. Biotransformation of drugs may lead to the following: a- Inactivation of drugs such as propranolol, morphine, etc. active drug inactive metabolite b- Formation of active metabolite from an active drug such as imipramine to desipramine; trimethadione to dimethadione. In this case, effect observed is due to the parent drug as well as active metabolite. active drug active metabolite c- Formation of active metabolite from inactive drug. Such a drug is called prodrug. Its effect is then due to its active metabolite. Example is levodopa to dopamine. inactive drug active metabolite Drug Phase I Metabolite Phase II Excretion Metabolite Fig. 2.2: Schematic representation of metabolism of a drug 8

9 Enzymes for Drug Biotransformation Drug metabolizing enzymes are categorized into 2 groups: 1-Microsomal Enzymes Microsomal enzymes are mainly present in the smooth surfaced endoplasmic reticulum of the liver. Main enzymes are mixed function oxidases like cytochrome P-450. About 50 cytochromes, P-450s are functionally active in human beings. These are categorized into families and subfamilies. Term CYP (Cytochrome P-450 mono-oxygenases) is used for their identification. About 8-10 isomers of CYP1, CYP2 and CYP3 families are associated in the majority of all drug metabolism reactions in human beings. Microsomal enzymes are involved primarily with Phase-I oxidation and reduction reactions. 2-Non-microsomal Enzymes These enzymes are present in plasma, cytoplasm, mitochondria of hepatic cells and other tissues. These enzymes are involved in all Phase-II reactions (except glucuronide conjugation), certain oxidation, reduction and hydrolytic reactions. Biotransformation reactions are of two types: 1. Phase-I Non-synthetic reactions: In this case, the metabolite may be active or inactive. It includes following reactions: i. Oxidation ii. Reduction iii. Hydrolysis These reactions introduce polar groups of drugs such as hydroxyl, amino, sulfhydryl and carboxy. Due to this drugs are made water soluble and pharmacologically less active. Oxidation: It is the most significant and important drug metabolizing reaction. Oxidation is carried out by A- Microsomal "mixed function oxidase" 1- Cytochrome P-450, 2- Haemoprotein enzymes 3- NADPH B-Non-microsomal oxidases (alcohol dehydrogenase, aldehyde dehydrogenase, diamine oxidase, monoamine oxidase and xanthine oxidase enzymes) in the liver. Oxidative reactions are hydroxylation, oxidation, deamination and dealkylation. Alcohol, barbiturates, diazepam, theophylline, morphine, paracetamol, steroids, etc. are metabolized by oxidative reactions. Reduction: It is a reaction which is opposite to oxidation. It is very less common. Some of the drugs, which are metabolized by this reaction, are chloral hydrate, warfarin, halothane, chloramphenicol, naloxone and prednisone. Hydrolysis: It occurs in plasma, liver, intestines and other tissues. It is carried out by esterases (e.g. plasma cholinesterase) or amidases. During this reaction, drug molecule is broken down into its two components. Examples are pethidine, cholinesters, procaine, procainamide, lidocaine. Cyclization: In this reaction, a straight chain compound is converted to ring structure such as proguanil. 9

10 Decyclization: In this reaction, ring structure of the cyclic drug molecule opens up, e.g. phenytoin, barbiturates. Phase I non-synthetic reactions Metabolic reaction Examples of drugs metabolized Oxidation Alcohol Barbiturates Diazepam Theophylline Morphine Paracetamol Steroids Reduction Chloral hydrate Chloramphenicol Halothane Naloxone Prednisone Warfarin Hydrolysis Cholinesterase Lidocaine Pethidine Procaine Procainamide Cyclization Proguanil Decyclization Barbiturates Phenytoin 2. Phase-II Synthetic (conjugation) reactions: These reactions mostly give rise to inactive metabolites. There occurs conjugation of the drug or its phase I metabolite with an endogenous substance. The later is derived from carbohydrate or amino acid. This reaction leads to the formation of a polar, highly ionized organic acid which is easily excreted in urine or bile. Various synthetic reactions are: a. Glucuronide conjugation: Drugs with a hydroxyl or carboxylic acid group (e.g. aspirin, phenacetin, chloramphenicol, morphine, metronidazole) and endogenous substances like steroids, bilirubin and thyroxine are conjugated with glucuronic acid which is derived from glucose. It is the most important synthetic reaction. b. Glycine conjugation: Compounds having carboxylic group such as salicylates are conjugated with glycine. c. Glutathione conjugation: Certain drugs, e.g. paracetamol give rise to highly reactive quinone or epoxide intermediates during metabolism, which are inactivated by glutathione conjugation. 11

11 d. Acetylation: Drugs with amino or hydrazine residues (e.g. sulfonamides, hydralazine, PAS, isoniazid) are acetylated with the help of acetyl coenzyme-a. Rate of acetylation of these drugs is genetically controlled (slow and fast acetylators). e. Methylation: The amines and phenols (e.g. adrenaline, histamine, nicotinic acid) undergo methylation. The endogenous methyl group for this reaction is derived from methionine and cysteine. f. Sulfate conjugation: The phenolic compounds and steroids (e.g. chloramphenicol, adrenal and sex steroids) undergo sulfate conjugation by sulfokinases. g. Ribonucleoside/nucleotide synthesis: It plays an important role in the activation of purine and pyrimidine antimetabolites used in cancer therapy. Phase II synthetic (conjugation) reactions Metabolic reaction Examples of drugs metabolized Glucuronide conjugation Aspirin Chloramphenicol Phenacetin Metronidazole Morphine Thyroxine Glycine conjugation Salicylates Glutathione conjugation Acetylation Methylation Sulphate conjugation Ribonucleoside/ nucleotide Quinone Epoxide intermediates of certain drugs such as paracetamol Hydralazine Isoniazid Para-amino-salicylic acid Adrenaline Histamine Nicotinic acid Chloramphenicol Steroids Purine Pyrimidine 3. First-pass effect: On oral administration, a drug has to pass through the gut, gut wall and liver before reaching the systemic circulation. Some drugs may undergo substantial pre systemic metabolism during their passage through these organs. This is called "firstpass effect". This is not seen when the same drug is given parenterally. First-pass effect consists of (a) intestinal first pass effect-and (b) hepatic first-pass effect. a. Intestinal first-pass effect : Drugs may be metabolized by gastric acid, digestive enzymes or by enzymes in gut wall (e.g. catecholamine). 11

12 b. Hepatic first-pass effect: If a drug is rapidly metabolized in the liver, very little or no quantity of the orally administered drug reaches the systemic circulation. So to have desired therapeutic effect the drug has to be given either parenterally or orally in very large doses. Some drugs (e.g. propranolol, imipramine) give rise to active metabolites during hepatic metabolism. These drugs can be given orally. The bioavailability of drugs, which undergo extensive first-pass effect, may vary widely by number of factors. It may be increased and cause drug toxicity in case of hepatic disease, hepatic enzyme inhibition and saturation of metabolizing enzymes while it may be decreased in case of hepatic enzyme induction. Some of the drugs which undergo extensive first pass hepatic metabolism are sex hormones, morphine, labetalol, verapamil, terbutaline, lignocaine. 4.Inhibition of drug metabolism: Drug can inhibit metabolizing enzyme activity competitively if it utilizes the same enzyme or cofactors. Often enzyme inhibition produces, undesirable drug-drug interactions. However, sometime specific enzyme inhibitors are used for therapeutic purpose, e g. allopurinol, MAO inhibitors, disulfiram, cholinesterase" inhibitors, captopril, etc. Some other clinically important examples are: Cimetidine inhibits metabolism of propranolol, theophylline and lidocaine. Isoniazid, warfarin, chloramphenicol inhibit the metabolism of phenytoin. Ethanol inhibits methanol metabolism. Metronidazole and chlorpropamide interfere with alcohol metabolism. 5.Microsomal enzyme induction: On repeated administration, certain drugs stimulate the synthesis of microsomal metabolizing enzymes (generally mixed function oxidase enzymes and rarely conjugases). Drugs or chemicals which induce enzymes are called enzyme inducers. Some important enzyme inducers are ethanol, barbiturates, rifampicin, griseofulvin, tobacco, phenytoin, carbamazepine, etc. Drugs, whose metabolism is significantly affected by enzyme induction, are phenytoin, warfare imipramine, ealbutamide, doxicycline, griseofulvin, oral contraceptives, chloramphenicol, phenylbutazone, theophylline. Possible Uses of Enzyme Induction i. Phenobarbitone is useful in congenital non-haemolytic jaundice because it causes rapid clearance of jaundice. ii. Phenytoin may reduce the manifestations of Cushing's syndrome. iii. Chronic poisoning. iv. Liver disease. There are number of factors which effect the biotransformation of a drug such as: a. At the extreme of life (old people and children) the rate of drug metabolism is slow. So the drug tends to produce greater and more prolonged effects. b. In human beings, sex dependent variations in drug metabolism are less important. However, males may metabolize salicylates, benzodiazepines and oestrogens quicker than females. c. Genetic factors and environment such as diet, weight, race, body temperature and specific genetic variation may influence the metabolism of drugs. 12

13 d. Many drugs may affect the biotransformation of other drugs either by enzyme induction (acceleration) or enzyme inhibition (delay). e. Drug metabolic processes are inhibited in malnourished individuals as well as in patients with hepatic diseases (e.g. alcoholic hepatitis, biliary cirrhosis, viral hepatitis, and cancer liver). Oxidation reactions are affected the most because they are rate limiting. However, conjugation reactions are well preserved as they are less rapidly saturated. f. Hepatic metabolic efficiency 1- Is impaired in congestive heart failure because it limits blood flow to the liver. 2- It is enhanced in patients suffering from thyrotoxicosis. 3- It is decreased in hypothyroidism and the half lives of practolol, digoxin and methimazole are increased. EXCRETION Drugs or their metabolites are eliminated through channels of excretion from the body. Important channels of excretion are: 1. Kidney: Most of the drugs are excreted through kidney. So this is the most important route of drug elimination. Following processes contribute to the excretion of a drug in the urine. i. Passive glomerular filtration: All non-protein bound drugs presented to the glomerulus are filtered. The rate of elimination of drug is dependent upon:- 1-The glomerular filtration rate 2- Molecular size of the drug 3- Concentration of free drug in the plasma. ii. Acute tubular secretion: This occurs at proximal tubules. This is the active transfer of organic acids and bases by organic acid transport and organic base transport respectively (non-selective saturable carrier systems). This transport is against electrochemical gradient. Further, this carrier system transports both free and protein bound drugs. Hence, this is the most important mechanism of drug elimination by the kidney. There can be competitive inhibition if two drugs utilize the same carrier system for tubular secretion, e.g. simultaneous use of probenecid and penicillin prolongs the plasma half life of the later. iii. Passive reabsorption across the tubules: This depends on:- 1- Lipid solubility 2- Ionization of the drug at existing urinary ph. Since 99% of the glomerular filtrate is reabsorbed, only 1% is excreted in the urine, there is a concentration gradient of solutes between tubular fluid and plasma, which allows passive diffusion of weak acids and weak bases. Weak acids (e.g. barbiturates, aspirin) are reabsorbed in acidic urine but eliminated in alkaline urine. On the other hand, weak bases (e.g. amphetamine, pethidine) are reabsorbed in alkaline urine but excreted in acidic urine. Strong acids and strong bases are not reabsorbed because they remain ionized in the urine at all ph ranges. Similarly, highly water soluble drugs (e.g. mannitol, quaternary ammonium compounds, penicillin, aminoglycosides) irrespective of urinary ph are not reabsorbed. 13

14 2. Gastrointestinal tract: Orally administered unabsorbed drugs and drugs excreted in the bile are eliminated in the faeces. 3. Lungs: Gasses and volatile liquids (general anaesthetics, paraldehyde, alcohol) are eliminated by lungs irrespective of their solubility. 4. Bile: Unchanged drugs and their metabolized products may be excreted in bile. In gut, some of the metabolites specially glucuronides are deconjugated by intestinal bacteria and the released lipid-soluble drug is reabsorbed into circulation (enterohepatic circulation). Due to this, duration of action of the drug is prolonged. Examples of such drugs are rifampicin, benzodiazepines, stilboestrol and morphine. However, some amount of the drug or its metabolite is eliminated in the faeces. 5. Breast milk: Excretion of drugs in milk is important for the suckling infant who inadvertently receive the drugs. Most of the drugs are detectable in breast milk, but usually their concentration is low. However, relatively significant concentrations of lipid soluble drugs enter into breast milk. So a few drugs, such as sulfonamides, tetracyclines, sedative, hypnotics, etc. should be avoided or breast feeding should be suspended. 6.Skin and saliva: Metalloids like arsenic and heavy metals like mercury are excreted in small quantity through skin. Certain drugs like iodides and metallic salts are excreted in the saliva. 14