PharmacokineticsDr. D. K. Brahma

Department of Pharmacology

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

Concentration in Plasma

Concentration at the site of action

AbsorptionDistributionMetabolismExcretion

Pharmacokinetics

Pharmacodynamics

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 transportAffecting 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

Intestine

Rectum

Portal vein

Vena cava

Buccal and Rectal – bypasses liver

Vena cava

Absorption – contd. Intravenous administration has no

absorption phase According to the rate of absorption:Inhalation→Sublingual→Rectal→intramuscular→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 - AUCPl

asm

a co

ncen

tratio

n (m

cg/m

l)

Time (h)0 5 10 15

AUC p.o.F = ------------ x 100% AUC i.v.

AUC – area under the curveF – bioavailability

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

V = Dose administered IV

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 entirequantity of the drug in the body, if the concentration throughoutthis 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

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

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 storageDrugs 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. BiotransformationMetabolism 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 Biotransformation1. 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 - Classification2 (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

Levodopa (DOPA) DopamineDOPA-decarboxylase

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

Ester + H20 Acid + AlcoholEsterase

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

Hepatic Excretion Drugs can be excreted in bile, especially when the are conjugated with – glucuronicAcid

• 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

Portal vein

Bile duct

Intestines

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 EliminationZero Order 1st Order

conc

.

Time

Plasma half-life 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/kIn2 = natural logarithm of 2 (0.693)

k = elimination rate constant = CL / V

t1/2 = 0.693 x V / CL

CL = RoE/C

V = dose IV/C

Plasma half-life

1 half-life …………. 50%

2 half-lives………… 25%

3 half-lives …….…..12.5%

4 half-lives ………… 6.25%

50 + 25 + 12.5 + 6.25 = 93.75

93.75 + 3.125 +

1.56 = 98% after 5 HL

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

Repeated Dosing At steady state, elimination = inputCpss = dose rate/CLDose Rate = target Cpss x CLIn oral administration Dose rate = target Cpss x CL/FIn zero order kinetics: follow Michaelis Menten

kineticsRoE = (Vmax) (C) / Km + CVmax = max. rate of drug elimination, Km = Plasma

conc. In which elimination rate is half maximal

CL = Roe/C

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 measurable drugs – antihypertensives, diuretics etc Drugs activated in body – levodopa Hit and run drugs – Reseprpine, MAO inhibitors Irreversible action drugs – Orgnophosphorous compounds

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

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