Pharmacokinetics - Past Paper PDF

Pharmacokinetics Dr. D. BY K. Brahma Department of Pharmacology DR. JOSEPH, OYEPATA SIMEON NEIGRIHMS, Shillong What is Pharmacokinetics how the human body act on the drugs? Pharmacokinetics is the quantitative study of drug movement in, through and out of t...

Pharmacokinetics Dr. D. BY K. Brahma Department of Pharmacology DR. JOSEPH, OYEPATA SIMEON NEIGRIHMS, Shillong What is Pharmacokinetics how the human body act on the drugs? Pharmacokinetics is the quantitative study of drug movement in, through and out of the body. Intensity of effect is related to concentration of the drug at the site of action, which depends on its pharmacokinetic properties Pharmacokinetic properties of particular drug is important to determine the route of administration, dose, onset of action, peak action time, duration of action and frequency of dosing Relationship – Dynamics and Kinetics Dosage Regimen Absorpti on Pharmacokineti Distributi cs on Metaboli Concentration sm in Excretio Plasma Pharmacodynam n ics Concentration at the site of action Effect The Pharmacokinetic Process The Pharmacokinetic Process Biological Membrane - image Drug Transportation Drug molecules can cross cell membrane by: Passive Diffusion Protein – mediated transport (carrier mediated) Facilitated Transport Active trnsport  Primary  Secondary Passive transport (down hill movement) Most important Mechanism for most of the Drugs Majority of drugs diffuses across the membrane in the direction of concentration gradient No active role of the membrane Proportional to lipid : water partition coefficient Lipid soluble drugs diffuse by dissolving in the lipoidal matrix of the membrane Characteristics Not requiring energy Having no saturation Having no carriers Not resisting competitive inhibition Passive transport Affecting factors : the size of molecule lipid solubility polarity degree of ionization the PH of the environment such as: fluid of body fluid in cell blood, urine Remember The drugs which are Unionized, low polarity and higher lipid solubility are easy to permeate membrane. The drugs which are ionized, high polarity and lower lipid solubility are difficult to permeate membrane. pH Effect Most of drugs are weak acids or weak bases. The ionization of drugs may markedly reduce their ability to permeate membranes. The degree of ionization of drugs is determined by the surrounding pH and their pKa. Henderson–Hasselbalch Equation pKa = negative logarithm of acid dissociation constant [A-] = ionized Drug [HA] = unionized drug pH Vs ionization Implications Acidic drugs re absorbed are largely unionized in stomach and absorbed faster while basic drugs are absorbed faster in intestines Ion trapping Acidic drugs are excreted faster in alkaline urine – urinary alkalizers Basic drugs are excreted faster in acidic urine – urinary acidifiers Filtration Passage of Drugs through aqueous pores in membrane or through Para cellular space Lipid insoluble drugs can cross – if the molecular size is small Majority of intestinal mucosa and RBCs have small pores and drugs cannot cross But, capillaries have large paracellular space and most drugs can filter through this Filtration Carrier Mediated Transport Involve specific membrane transport proteins know as drug transporters or carriers – specific for the substrate Drug molecules bind to the transporter, translocated across the membrane, and then released on the on other side of the membrane. Specific, saturable and inhibitable Depending on Energy requirement - Can be either Facilitated (passive) or Active Transport Facilitative transporters Move substrate of a single class (uniporters) down a concentration gradient No energy dependent Similar to entry of glucose into muscle (GLUT 4) Active Transport – energy dependent Active (concentrative) transporters can move solutes against a concentration gradient energy dependent Primary active transporters - generate energy themselves (e.g. ATP hydrolysis) Secondary transporters - utilize energy stored in voltage and ion gradients generated by a primary active transporter (e.g. Na+/K+-ATPase) Symporters (Co-transporters) Antiporters (Exchangers) Major Drug Transporters ATP-Binding Cassette Transporters (ABC) Super family – Primary active transport P-glycoprotein (P-gp encoded by MDR1) Intestinal mucosa, renal tubules and blood brain barrier etc. Mediate only efflux of solute from cytoplasm - detoxification Solute Carrier (SLC) transporters – Secondary active transport Organic anion transporting polypeptides (OATPs) Organic cation transporters (OCTs) Expressed in liver and renal tubules – metabolism and excretion of drugs Pinocytosis It involves the invagination of a part of the cell membrane and trapping within the cell of a small vesicle containing extra cellular constituents. The vesicle contents can than be released within the cell, or extruded from the other side of the cell. Pinocytosis is important for the transport of some macromolecules (e.g. insulin through BBB).  1. Absorption of Drugs  Absorption is the transfer of a drug from its site of administration to the blood stream  Most of drugs are absorbed by the way of passive transport  Intravenous administration has no absorption  Fraction of administered dose and rate of absorption are important Factors affecting absorption Drug properties: lipid solubility, molecular weight, and polarity etc Blood flow to the absorption site Total surface area available for absorption Contact time at the absorption surface Affinity with special tissue Routes of Administration (important): Factors affecting absorption – contd. Route of administration: Topical: Depends on lipid solubility – only lipid soluble drugs are penetrate intact skin – only few drugs are used therapeutically Examples – GTN, Hyoscine, Fentanyl, Nicotine, testosterone and estradiol Organophosphorous compounds – systemic toxicity Abraded skin: tannic acid – hepatic necrosis Cornea permeable to lipid soluble drugs Mucus membranes of mouth, rectum, vagina etc, are permeable to lipophillic drugs Factors affecting absorption – contd. Route of administration: Subcutaneous and Intramuscular: Drugs directly reach the vicinity of capillaries – passes capillary endothelium and reach circulation Passes through the large paracellular pores Faster and more predictable than oral absorption Exercise and heat – increase absorption Adrenaline – decrease absorption Factors affecting absorption – contd. Route of administration: Oral Route Physical properties – Physical state, lipid or water solubility Dosage forms: Particle size Disintegration time and Dissolution Rate Formulation – Biopharmaceutics Physiological factors: Ionization, pH effect Presence of Food Presence of Other agents Oral Administration – 1st pass metabolism Before the drug reaches the systemic circulation, the drug can be metabolized in the liver or intestine. As a Result, the concentration of drug in the systemic circulation will be reduced. 1st pass Elimination – Metabolism in liver Buccal cavity Stomach Vena Intestine cava Portal vein Rectum Buccal and Rectal – bypasses liver Vena cava Absorption – contd. Intravenous administration has no absorption phase According to the rate of absorption: Inhalation→Sublingual→Rectal→intramusc ular→subcutaneous→oral→transdermal Example – Nitroglycerine: IV effect – immediate, SL – 1 to 3 min and per rectal – 40 to 60 minute Bioavailability Bioavailability refers to the rate and extent of absorption of a drug from dosage form as determined by its concentration-time curve in blood or by its excretion in urine. It is a measure of the fraction (F) of administered dose of a drug that reaches the systemic circulation in the unchanged form Bioavailability of drug injected i.v. is 100%, but is frequently lower after oral ingestion, because: The drug may be incompletely absorbed The absorbed drug may undergo first pass metabolism in intestinal wall and/or liver or be excreted in bile. Bioequivalent Practical Significance – low safety margin drugs Biovailability - AUC AUC – area under the curve AUC p.o. Plasma concentration (mcg/ml) F = ------------ x 100% F – bioavailability AUC i.v. 0 5 Time (h) 10 15 Biovailability – contd. MTC MEC 2. Distribution of Drugs It is the passage of drug from the circulation to the tissue and site of its action. The extent of distribution of drug depends on its lipid solubility, ionization at physiological pH (dependent on pKa), extent of binding to plasma and tissue proteins and differences in regional blood flow, disease like CHF, uremia, cirrhosis Movement of drug - until equilibration between unbound drug in plasma and tissue fluids Volume of Distribution (V) Definition: Apparent Volume of distribution is defined as the volume that would accommodate all the drugs in the body, if the concentration was the same as in plasma Expressed as: in Liters Dose administered IV V= Plasma concentration Volume of Distribution (V) Total Body Fluid = 42 L (approx.) Volume of Distribution (V) Chloroquin – 13000 liters, Digoxin – 420 L, Morphine – 250 L and Propranolol – 280 L Streptomycin and Gentamicin – 18 L (WHY ?) `Vd` is an imaginary Volume of Fluid which will accommodate the entire quantity of the drug in the body, if the concentration throughout this imaginary volume were same as that in plasma Volume of Distribution (V) Vd = IV dose/C Factors influencing Vd Lipid solubility (lipid : water partition coefficient) pKa of the drug Affinity for different tissues Blood flow – Brain Vs Fat Disease states Plasma protein Binding Redistribution Highly lipid soluble drugs – distribute to brain, heart and kidney etc. immediately followed by muscle and Fats Brain and CSF Penetration Blood brain barrier (BBB): includes the capillary endothelial cells (which have tight junctions and lack large intracellular pores) and an investment of glial tissue, over the capillaries. A similar barrier is loctated in the choroid plexus Brain and CSF Penetration – contd. BBB is lipoidal and limits the entry of non-lipid soluble drugs (amikacin, gentamicin, neostigmine etc.). (Only lipid soluble unionized drugs penetrate and have action on the CNS) Efflux carriers like P-gp (glycoprotein) present in brain capillary endothelial cell (also in intestinal mucosal, renal tubular, hepatic canicular, placental and testicular cells) extrude drugs that enter brain by other processes. (Inflammation of meanings of brain increases permeability of BBB) Dopamine (DA) does not enter brain, but its precursor levodopa does. This is used latter in parkinsonism. Placental Transfer Only lipid soluble Drugs can penetrate – limitation of hydrophillic drugs Placental P-gp serves as limiting factor But, REMEMBER, its an incomplete barrier – some influx transporters operate Thalidomide Plasma Protein Binding Plasma protein binding (PPB): Most drugs possess physicochemical affinity for plasma proteins. Acidic drugs bind to plasma albumin and basic drugs to α1-glycoprotein Extent of binding depends on the individual compound. Increasing concentration of drug can progressively saturate the binding sites The clinical significant implications of PPB are: a) Highly PPB drugs are largely restricted to the vascular compartment and tend to have lower Vd. b) The PPB fraction is not available for action. c) There is an equilibration between PPB fraction of drug and free molecules of drug. Plasma Protein Binding – contd. d) The drugs with high physicochemical affinity for plasma proteins (e.g. aspirin, sulfonamides, chloramphenicol) can replace the other drugs(e.g. acenocoumarol, warfarin) or endogenous compounds (bilirubin) with lower affinity. e) High degree of protein binding makes the drug long acting, because bound fraction is not available for metabolism, unless it is actively excreted by liver or kidney tubules. f) Generally expressed plasma concentrations of the drug refer to bound as well as free drug. g) In hypoalbuminemia, binding may be reduced and high concentration of free drug may be attained (e.g. phenytoin). Tissue storage Drugs may also accumulate in specific organs or get bound to specific tissue constituents, e.g.: Heart and skeletal muscles – digoxin (to muscle proteins) Liver – chloroquine, tetracyclines, digoxin Kidney – digoxin, chloroquine Thyroid gland – iodine Brain – chlorpromazine, isoniazid, acetazolamide Retina – chloroquine (to nucleoproteins) Iris – ephedrine, atropine (to melanin) Bones and teeth – tetracyclines, heavy metals (to mucopolysaccharide of connective tissue) Adipose tissues – thiopental, ether, minocycline, DDT 3. Biotransformation Metabolism of Drugs What is Biotransformation? Chemical alteration of the drug in the body Aim: to convert non-polar lipid soluble compounds to polar lipid insoluble compounds to avoid reabsorption in renal tubules Most hydrophilic drugs are less biotransformed and excreted unchanged – streptomycin, neostigmine and pancuronium etc. Biotransformation is required for protection of body from toxic metabolites Results of Biotransformation 1. Active drug and its metabolite to inactive metabolites – most drugs (ibuprofen, paracetamol, chlormphenicol etc.) 2. Active drug to active product (phenacetin – acetminophen or paracetamol, morphine to Morphine-6-glucoronide, digitoxin to digoxin etc.) 3. Inactive drug to active/enhanced activity (prodrug) – levodopa - carbidopa, prednisone – prednisolone and enlpril – enlprilat) 4. No toxic or less toxic drug to toxic metabolites (Isonizide to Acetyl isoniazide) (Mutagenicity, teratogenicity, carcinogenicity, hepatotoxicity)  Biotransformation - Classification 2 (two) Phases of Biotransformation: Phase I or Non-synthetic – metabolite may be active or inactive Phase II or Synthetic – metabolites are inactive (Morphine – M-6 glucoronide is exception) Phase I - Oxidation Most important drug metabolizing reaction – addition of oxygen or (–ve) charged radical or removal of hydrogen or (+ve) charged radical Various oxidation reactions are – oxygenation or hydroxylation of C-, N- or S-atoms; N or 0- dealkylation Examples – Barbiturates, phenothiazines, paracetamol and steroids Phase I - Oxidation Involve – cytochrome P-450 monooxygenases (CYP), NADPH and Oxygen More than 100 cytochrome P-450 isoenzymes are identified and grouped into more than 20 families – 1, 2 and 3 … Sub-families are identified as A, B, and C etc. In human - only 3 isoenzyme families important – CYP1, CYP2 and CYP3 CYP 3A4/5 carry out biotransformation of largest number (30–50%) of drugs. In addition to liver, this isoforms are expressed in intestine (responsible for first pass metabolism at this site) and kidney too Inhibition of CYP 3A4 by erythromycin, clarithromycin, ketoconzole, itraconazole, verapamil, diltiazem and a constituent of grape fruit juice is responsible for unwanted interaction with terfenadine and astemizole Rifampicin, phenytoin, carbmazepine, phenobarbital are inducers of the CYP 3A4 Oxidation - CYP CYP3A4/5 Nonmicrosomal Enzyme Oxidation Some Drugs are oxidized by non- microsomal enzymes (mitochondrial and cytoplsmic) – Alcohol, Adrenaline, Mercaptopurine Alcohol – Dehydrogenase Adrenaline – MAO and COMT Mercaptopurine – Xanthine oxidase Phase I - Reduction This reaction is conversed of oxidation and involves CYP 450 enzymes working in the opposite direction. Examples - Chloramphenicol, levodopa, halothane and warfarin DOPA-decarboxylase Levodopa (DOPA) Dopamine Phase I - Hydrolysis This is cleavage of drug molecule by taking up of a molecule of water. Similarly amides and polypeptides are hydrolyzed by amidase and peptidases. Hydrolysis occurs in liver, intestines, plasma and other tissues. Examples - Choline esters, procaine, lidocaine, pethidine, oxytocin Esterase Ester + H20 Acid + Alcohol Phase I – contd. Cyclization: is formation of ring structure from a straight chain compound, e.g. proguanil. Decyclization: is opening up of ring structure of the cyclic molecule, e.g. phenytoin, barbiturates Phase II metabolism Conjugation of the drug or its phase I metabolite with an endogenous substrate - polar highly ionized organic acid to be excreted in urine or bile - high energy requirements Glucoronide conjugation - most important synthetic reaction Compounds with hydroxyl or carboxylic acid group are easily conjugated with glucoronic acid - derived from glucose Examples: Chloramphenicol, aspirin, morphine, metroniazole, bilirubin, thyroxine Drug glucuronides, excreted in bile, can be hydrolyzed in the gut by bacteria, producing beta-glucoronidase - liberated drug is reabsorbed and undergoes the same fate - enterohepatic recirculation (e.g. chloramphenicol, phenolphthalein, oral contraceptives) and prolongs their action Phase II metabolism – contd. Acetylation: Compounds having amino or hydrazine residues are conjugated with the help of acetyl CoA, e.g.sulfonamides, isoniazid Genetic polymorphism (slow and fast acetylators) Sulfate conjugation: The phenolic compounds and steroids are sulfated by sulfokinases, e.g. chloramphenicol, adrenal and sex steroids Phase II metabolism – contd. Methylation: The amines and phenols can be methylated. Methionine and cysteine act as methyl donors. Examples: adrenaline, histamine, nicotinic acid. Ribonucleoside/nucleotide synthesis: activation of many purine and pyrimidine antimetabolites used in cancer chemotherapy Factors affecting Biotransformation Factors affecting biotransformation Concurrent use of drugs: Induction and inhibition Genetic polymorphism Pollutant exposure from environment or industry Pathological status Age Enzyme Inhibition One drug can inhibit metabolism of other – if utilizes same enzyme However not common because different drugs are substrate of different CYPs A drug may inhibit one isoenzyme while being substrate of other isoenzyme – quinidine Some enzyme inhibitors – Omeprazole, metronidazole, isoniazide, ciprofloxacin and sulfonamides Microsomal Enzyme Induction CYP3A – antiepileptic agents - Phenobarbitone, Rifampicin and glucocorticoide CYP2E1 - isoniazid, acetone, chronic use of alcohol Other inducers – cigarette smoking, charcoal broiled meat, industrial pollutants – CYP1A Consequences of Induction: Decreased intensity – Failure of OCPs Increased intensity – Paracetamol poisoning (NABQI) Tolerance – Carbmazepine Some endogenous substrates are metabolized faster – steroids, bilirubin 4. Excretion Organs of Excretion Excretion is a transport procedure which the prototype drug (or parent drug) or other metabolic products are excreted through excretion organ or secretion organ Hydrophilic compounds can be easily excreted. Routes of drug excretion Kidney Biliary excretion Sweat and saliva Milk Pulmonary  Bile duct Hepatic Excretion Drugs can be excreted in Intesti Portal bile, especially when the are nes vein conjugated with – glucuronic Acid Drug is absorbed → glucuronidated or sulfatated in the liver and secreted through the bile → glucuronic acid/sulfate is cleaved off by bacteria in GI tract → drug is reabsorbed (steroid hormones, rifampicin, amoxycillin, contraceptives) Anthraquinone, heavy metals – directly excreted in colon Renal Excretion Glomerular Filtration Tubular Reabsorption Tubular Secretion Glomerular Filtration Normal GFR – 120 ml/min Glomerular capillaries have pores larger than usual The kidney is responsible for excreting of all water soluble substances All nonprotein bound drugs (lipid soluble or insoluble) presented to the glomerulus are filtered Glomerular filtration of drugs depends on their plasma protein binding and renal blood flow - Protein bound drugs are not filtered ! Renal failure and aged persons Tubular Re-absorption Back diffusion of Drugs (99%) – lipid soluble drugs Depends on pH of urine, ionization etc. Lipid insoluble ionized drugs excreted as it is – aminoglycoside (amikacin, gentamicin, tobramycin) Changes in urinary pH can change the excretion pattern of drugs Weak bases ionize more and are less reabsorbed in acidic urine. Weak acids ionized more and are less reabsorbed in alkaline urine Utilized clinically in salicylate and barbiturate poisoning – alkanized urine (Drugs with pKa: 5 – 8) Acidified urine – atropine and morphine etc. Tubular Secretion Energy dependent active transport – reduces the free concentration of drugs – further, more drug dissociation from plasma binding – again more secretion (protein binding is facilitatory for excretion for some drugs) OATP – organic acid transport OCT – organic base transport P-gp Bidirectional transport – Blood Vs tubular fluid Utilized clinically – penicillin Vs probenecid, probenecid Vs uric acid (salicylate) Quinidine decreases renal and biliary clearance of digoxin by inhibiting efflux carrier P-gp Renal Excretion Acidic urine alkaline drugs eliminated acid drugs reabsorbed Alkaline urine - acid drugs eliminated - alkaline drugs absorbed Kinetics of Elimination Pharmacokinetics - F, V and CL Clearance: The clearance (CL) of a drug is the theoretical volume of plasma from which drug is completely removed in unit time CL = Rate of elimination (RoE)/C Example = If a drug has 20 mcg/ml and RoE is 100 mcg/min CL = 100/20 = 5 ml /min Kinetics of Elimination First Order Kinetics (exponential): Rate of elimination is directly proportional to drug concentration, CL remaining constant Constant fraction of drug is eliminated per unit time Zero Order kinetics (linear): The rate of elimination remains constant irrespective of drug concentration CL decreases with increase in concentration Alcohol, theophyline, tolbutmide etc.  Kinetics of Elimination Zero Order 1st Order conc. Time  CL = RoE/C Plasma half-life V = dose IV/C Defined as time taken for its plasma concentration to be reduced to half of its original value – 2 phases rapid declining and slow declining t1/2 = In2/k In2 = natural logarithm of 2 (0.693) k = elimination rate constant = CL / V t1/2 = 0.693 x V / CL Plasma half-life 50 + 25 + 12.5 + 1 half-life …………. 50% 6.25 = 93.75 2 half-lives………… 25% 93.75 + 3.125 + 1.56 = 98% 3 half-lives …….…..12.5% after 5 HL 4 half-lives ………… 6.25% Excretion - The Platue Principle Repeated dosing: When constant dose of a drug is repeated before the expiry of 4 half-life – peak concentration is achieved after certain interval Balances between dose administered and dose interval  CL = Roe/C Repeated Dosing At steady state, elimination = input Cpss = dose rate/CL Dose Rate = target Cpss x CL In oral administration Dose rate = target Cpss x CL/F In zero order kinetics: follow Michaelis Menten kinetics RoE = (Vmax) (C) / Km + C Vmax = max. rate of drug elimination, Km = Plasma conc. In which elimination rate is half maximal Target Level Strategy Low safety margin drugs (anticonvulsants, antidepressants, Lithium, Theophylline etc. – maintained at certain concentration within therapeutic range Drugs with short half-life (2-3 Hrs) – drugs are administered at conventional intervals (6-12 Hrs) – fluctuations are therapeutically acceptable Long acting drugs: Loading dose: Single dose or repeated dose in quick succession – to attain target conc. Quickly Loading dose = target Cp X V/F Maintenance dose: dose to be repeated at specific intervals Monitoring of Plasma concentration Useful in Narrow safety margin drugs – digoxin, anticonvulsants, antiarrhythmics and aminoglycosides etc Large individual variation – lithium and antidepressants Renal failure cases Poisoning cases Not useful in Response mesurable drugs – antihypertensives, diuretics etc Drugs activated in body – levodopa Hit and run drugs – Reseprpine, MAO inhibitors Irreversible action drugs – Orgnophosphorous compounds Summary – Must Know Definition of Pharmacokinetics Transport of Drugs across Biological Membrane – different processes with example Factors affecting absorption of drugs Concept of Bioavailability Distribution of Drugs – Vd and its concept Biotransformation Mechanisms with examples Enzyme induction and inhibition concept and important examples Routes of excretion of drugs Orders of Kinetics Definition and concept of drug clearance Definition of half life and platue principle Prolongation of Drug action By prolonging absorption from the site of action – Oral and parenteral By increasing plasma protein binding By retarding rate of metabolism By retarding renal excretion