Pharmacology: Student Notes -1

PHARMACOLOGY---- Student Notes

PHARMACOLOGY LECTURE 1

INTRODUCTION

• a xenobiotic is a drug that is not synthesied within the body

• pharmacodynamics is the effect of the drug on the body (effect of drug at

receptor site)

• pharmacokinetics is the effect of the body on the drug (absorption, distribution

and elimation of drug)

• toxicology deals with the side effects of drugs

• the therapeutic index ratio is the minimum dose that produces toxicity divided

by the minimum dose that produces the therapeutic response (therefore you want

a high therapeutic index)


RECEPTORS 1 - TARGETS FOR DRUG ACTION

• affinity constant = K(D) - a low KD = a high affinity

o a K(D) of 10E-6 or more is considered too weak

• drugs act at the following sites:

o cell surface receptors

o ion channels

o carrier pumps

� e.g. some diurectics act on the Na+/Cl- cotransporter in the distal

tubules

� e.g. some diurectics act on the Na+/K+ antiporter in the collecting

ducts

� e.g. some antiulcer/reflux drugs work on the H+/K+ antiporter

o enzymes

� e.g. NSAIDs inhibit cyclo-oxygenase which blocks prostaglandin

production

� e.g. ACE inhibitors inhibit angiotensin converting enzyme to

decrease blood pressure

� e.g. HMGCoA reductase inhibitors inhibit this to reduce lipid

concentration

o nuclear receptors/RNA/DNA

o intracellular structural proteins

• Question: Aspirin and ibuprofen are NSAIDs that inhibit cyclo-oxygenase in

platelets (ibuprofen - competitive; aspirin - irreversible). What are the

clinical consequences of this difference in inhibition? o Aspirin will be longer acting. Hence, people undergoing surgery must stop

taking aspirin 1 week before so clotting can occur during. whereas people

must only stop taking ibuprofen 1 day before surgery

• inhibition constant = K(I) - measures degree of inhibition of an enzyme

• therapeutics: treatment of disease can be by different targets:

o e.g. to treat gastric hyperacidity:

� neutralising drug (NaHCO3)

� proton pump inhibitor (omeprazole)

� histamine 2 receptor blocker (ranitidine)

o some drugs act on more than one target, e.g. caffeine

• levels of mecahnisms of action of drugs

o e.g. propranolol:

-1. molecular: is a competitive inhibitor by binding to beta 1

receptors

-1. cellular: prevents increase in cAMP, protein phosphorylation

-1. physiological: reduces cardiac heart rate and contractile force

-1. therapeutic: used to treat angina


RECEPTORS 2 - INTRACELLULAR MECHANISMS

(PHARMACODYNAMICS)

• there are 4 basic mechanisms for transmembrane signalling

0. ligand gated channels

1. G protein-coupled receptors/second messengers

2. intracellular receptors

3. ligand-regulated transmembrane enzymes

• 1. ligand gated channels: o example: nicotinic acetylcholine receptor

� where: found on skeletal muscle

� function: when activated causes contraction

� structure: pentamer crossing lipid bilayer

� mechanism of action: acetycholine binds to alpha subunit -->

conformational change --> trransient opening of central aqueous

channel --> Na+ flows down concentration gradient -->

depolarisation --> contraction

• 2. G protein-coupled receptors/second messengers o signalling system involves:

0. extracellular drug binds to cell surface receptor

1. receptor triggers activation of a G protein on cytoplasmic face of

plasma membrane

2. activated G protein alters activity of an effector element (e.g.

enzyme or ion channel)

3. effector element changes concentration of intracellular second

messenger

o G protein structure:

� contains alpha, beta and gamma subunits

� the beta-gamma anchors the G protein to the membrane

� the G protein is not normally bound to the receptor but is free

floating in the cell membrane (cytoplasmic side

o how G proteins work: see Alberts Molecular Biology of the Cell, Figure

15-23

0. when the receptor is not occupied by a drug: the G protein is in a

resting state, where beta-gamma anchors G protein to membrane,

and GDP occupies site on alpha subunit

1. the ligand binds to the receptor, which alters the conformation of

the receptor, exposing the binding site for G protein

2. the G protein binds to the receptor, greatly weakening the affinity

of the G protein for GDP

3. GDP dissociates, allowing GTP to bind to the alpha subunit

4. this causes the alpha subunit to change conformation, it now

dissociates from the receptor and the beta and gamma subunits

5. the free alpha subunit now changes conformation so that it can

bind with its target enzyme

6. this target enzyme acts, e.g. converts ATP-->cAMP, ion channel

etc.

7. hydrolysis of the GTP by the alpha subunit returns the subunit to

its original conformation, causing it to dissociate from the target

enzyme and reassociate with the beta and gamma subunits

o Gs is a G protein that acts on adenyl cyclase, hence converts ATP--

>cAMP

o Gi is a G protein that inhibits adenyl cyclase, hence prevents ATP--

>cAMP

o note that the slower the hydrolysis of the GTP (step 8), the longer for

which the G protein will act

o SECOND MESSENGERS - commonly used second messenger systems in

the body include:

0. cAMP

� many drugs/hormones act by increasing or decreasing the

adenyl cyclase which in turn increases or decreases cAMP

1. calcium-phosphoinositide pathway (IP3/DAG)

� mechanims of this pathway:

1. Gs stimulates phospholipase C (a membrane

enzyme) -->

2. hydrolysis of IP2 into DAG (diacyclglycerol) and

IP3 (inositol-1,4,5-triphosphate)

� DAG activates protein kinase C -->

phosphorylates intracellular proteins -->

altered cellular function (e.g. contraction of

smooth muscle)

� IP3 triggers release of Ca++ from storage

vesicles (in smooth muscle this will increase

contraction)

3. these signals are terminated by the following:

0. dephosphorylation of IP3

1. phosphorylating DAG to arachidonic acid

2. actively removing the Ca++

o How does ACh affect the ion channels in the heart? � the vagus nerve releases ACh --> binds to receptor in cardiac

muscle --> enhances K+ permeability --> hyperpolarisation -->

decreases activity

• 3. intracellular receptors - regulate gene expression o ligands for intracellular receptors include: steroids, vitamin D, thyroid

hormone

o mechanism:

-1. ligand is lipid soluble, hence can cross plasma membrane

-1. binds with intracellular receptor

-1. intracellular receptors increases gene transcription by directly or

indirectly acting on promoter region

o drugs that act on intracellular receptors are slow acting because gene

transcription takes time, but endure hours after the drug has been

eliminated from the body

• 4. ligand-regulated transmembrane enzymes (protein tyrosine kinases) o mechanism:

-1. ligand (includes insulin, growth factors) binds to receptor

-1. enzymes (tyrosine kinases) located in the cytoplasmic domains

(receptors have large extracellular and intracellular domains) are

activated and cause phosphorylation of certain targets

-1. these targets are activated or inactivated by this

SPECIFIC EXAMPLES DISCUSSED IN LECTURES, NOT ALLUDED TO ON HANDOUT

2. SURGERY

o during surgery, give curare to block nicotinic receptors (found on skeletal

muscle)

o therefore, surgeon can cut through muscle (it is relaxed)

o now, the curare stops breathing, therefore, must respirate patient

o after operation, give acetylcholinesterase inhibitor (stops

acetylcholinesterase from breaking down ACh in the synaptic clefts from

being broken down) which will increase the amount of ACh in the

synaptic clefts which is now in a great enough concentration to overcome

the curare, and hence, breathing will return quickly

2. CHOLERA

o is caused by a bacterium - cholerae

o bacterium secretes a toxin

o toxin enters intestinal cells

o toxin prevents GTP hydrolysing to GDP

o this prevents G stimulatory protein from shutting itself off

o therefore, intestinal cells accumate cAMP

o this leads to a Cl-/Na+ imbalance (more Cl- and Na+ in the lumen)

o hence, water is lost to the lumen

o this results in diarrhoea

2. CAFFEINE

o this inhibits phosphoresterase

o normally phosphoresterase hydrolases cAMP, which decreases cAMP

o hence, caffeine causes the following effect: increased cellular cAMP in

smooth muscle --> vasodilatation in kidney --> increases GFR --> diuresis


PHARMAKINETICS - GENERAL PRINCIPLES

• pharmacokinetics is the characterisation of the time course of drug/metabolite

concentrations in the body

• clinical pharmacokinetics is the application of pharmacokinetic principles to the

therapeutic management of patients

• disposition o is defined as those pharmacokinetc processes that occur after absorption of

the drug into the body (i.e. dristribution and elimination, and includes:

0. movement of drug from the site of administration to site of action

0. movement of drug to other organs

0. excretion of drug

• absorption: most drugs are administered orally, and the gastrointestinal

absorption of drugs is a function of its physico-chemical properties (pKa,

lipophilicty, molecular weight) and the physiology of the gastrointestinal tract

(pH, gastric emptyimg)

• distribution: is a function of:

o affinity of drug for tissue and plasma

o blood flow rates to organs

o drug's physico-chemical properties

• elimination: is a function of

o biotransformation in the liver to metabolites

o excretion of the drug by the kidney

• various parameters: o bioavailability = F

o volume of distribution = V

o half-life = T(1/2)

o clearance = CL

• first order kinetics = the rate of absorption, distribution and elimination is

directly proportional to the concentration

• zeroth order kinetics = the rate of absorption, distribution and elimination are

independent of the concetration

• the therapeutic window = the range of concentrations that give both adequate

therapy without unacceptable toxicity


PHARMACOKINETIC CONCEPTS

• the two basic processes that modify pharmacokinetics (how the body deals with a

drug) are:

0. clearance

0. volume of distribution

CLEARANCE = CL

• INTRODUCTION

o CL = rate of elimination/concentration (mg/h)

o elimination of drug can occur at:

� CL(renal) = rate of elimination at kidneys / C

� CL(liver) = rate of elimination at liver / C

� CL(other) = rate of elimination not at the kidney or liver / C

� CL(total) = CL(renal) + CL (liver) + CL (other)

o first order elimination (constant CL) � for most drugs the rate of elimination is directly proportional to the

concentration, and so clearance is constant

o zeroth order elimination (variable CL) - capacity limited elimination � some durgs do not have a constant clearance; these are capacity

limited drugs (e.g. ethanol)

� for these drugs, an increase in concentration, does not see a

corresponding increase in elimination because the pathways of

elimination are saturated

� note: actually, most drug elimination pathways will become

saturated if the concentration is high enough, and so eventually, at

a high enough concentration, all drugs will become capacity

limited BUT most drugs are given at concentrations that aren't

nearly high enough for this

� a formula describing rate of elimination is:

� rate of elimination = (Vmax + C) / (Km + C)

o flow dependent elimination: for drugs that are cleared very readily,

elimination is dependent on blood flow

• MEASUREMENT OF TOTAL BODY CLEARANCE:

o area under concentration curve = AUC (this is the total area under the

blood/plasma concentration time curve) - see handout for diagram

o F = bioavailabilty (how much drug reaches the circulation after ingestion)

o Css = steady state concentration

o tau = the AUC within the dosing interval divided by the dosing interval

o single dose: � IV: CL = dose / AUC

� oral: CL = dose x F / AUC

o chronic dosing: � IV constant infusion into body: CL = dose rate / Css

� IV giving at separate intervals: CL = dose / (tau x Css)

� oral taken at separate intervals: CL = F x dose / (tau x Css)

• CLINICAL RELEVANCE

o clearance is the parameter which determines the maintenance of dose rate

required

o rearrangement of the IV constant infusion into body formula gives:

� maintenance dose rate = desired concentration x clearance (i.e.

dose rate = Css x CL)

o clearance is an independent pharmacokinetic parameter

VOLUME OF DISTRIBUTION (V)

• INTRODUCTION o V = amount of drug in body / concentration in plasma

o example: a high V --> a low blood concentration, and therefore a high

drug concentration in extravascular tissue

• MEASUREMENT OF V

o V = dose/C(0) = dose/(AUC x terminal slope)

o C(0) = plasma concentration at time 0

o C(0) can be determined by drawing a log plasma concentration vs. time

and extending the line to time 0; note that this is most accurate using log

paper since the line will probably be straight with log papers

• CLINICAL RELEVANCE

o in some clinical settings, it is important to achieve the target concentration

instantly, rather than waiting for some time - the following formula can be

used

� loading dose = Css x V

o volume of distribution is an independent pharmacokinetic parameter

HALF LIFE (t(1/2))

• is the time taken for the blood concentration to fall by 50%

• clearly, half life will be constant for first order drugs

• MEASUREMENT OF HALF LIFE

o look at the concentration vs. time graph

o OR calculate using:

� half life = ln2 x V / CL

• half life is a dependent pharmacokinetic parameter derived from the

indepdent parameters CL and V • CLINICAL RELEVANCE

o for continuous dosing, steady state is said to be achieved after 5 half lives

BIOAVAILABILITY (F)

• is defined as the frction of drug dose reaching the systemic circulation following

administration by any route

o F = 1 for IV administration

o 0 < F < 1 for oral administration

• obviously therefore, F = (AUC[oral] / AUC[iv]) x (dose[iv] / dose[oral])

• estimation of area under curve: bloody obvious


ADVERSE DRUG REACTIONS (ADR)

Adverse drug reaction: noxious and unintended effect of drug, occurring at doses used for

prophylaxis, diagnosis or therapy.

CLASSIFICATION: note that it is assumed drug dosage is within therapeutic window

Type A - explainable

• type A ADR is the result of an exaggerated, but otherwise expected

pharmacological effect of drug

• are predictable and dose dependent

• examples:

o antihypertensive drugs - postural hypotension

o morphine - constipation

o caffeine - palpitations

o anti-epileptic drugs - sedation

• some causes:

-1. pharmacokinetic (what body does to drug) - common

� due to variability between patients in: bioavailability, first pass

effect, metabolism and renal excretion

� predisposing factors:

� age

� pregnancy

� diseases affecting pharacokinetics (e.g. kidney disease)

and/or pharmacodynamics, and pharmacogenetics

(oxidation, acetylation)

� dose reduction will avoid ADR

-1. pharmacodynamic (what drug does to body)

� result from

� altered end-organ sensitivity (e.g. changes in receptors)

� predisposing factors:

� eldery (impaired homeostasis; e.g. postural hypotension

with diuretics)

� drug interactions

� unavoidable toxicity (e.g. cancer chemotherapeutic drugs

causes bone marrow depression)

-1. pharmaceutical formulation - not common

� changes in the way the drug is distributed to the body can increase

concentration (e.g. by increasing bioavailability), so that the

concentration is at toxic levels

� example: in 1968, changes with phenytoin increased bioavailability

leading to toxicity

Type B - unexplainable

• type B ADR is not expected from known pharmacological action of drug

• example:

o skin rash - penicillin

o haemolysis - antimalarials

• some causes:

-1. pharmaceutical

� due to in vitro degradation of products or additives

-1. pharmacokinetic

� due to formation of electrophilic drug metabolite binding

covalently to tissue macromolecules

� exmple: sulphonamides - major route of metabolism is n-

acetylation (non-toxic), but minor route is via cytochrome P450

metabolism (forms a reactive metabolite which binds covalently to

tissue and causes cellular toxicity)

-1. pharmacodynamic

� example: haemolysis with sulphonamides in patients with red

blood cell glucose-6-phosphate dehydrogenase deficiency

Comparison between A and B

• feature = type A / type B • pharmacology = augmented / bizarre

• predictable = yes / no

• dose dependent = yes / no

• morbidity = high / low

• mortality = low / high

Other

6. teratogenic, e.g. warfarin

6. carcinogenic, e.g. diethylstilboestrol

6. drug allergy

o previous exposure - sensitisation

o antigen can be drugs, drugs + protein, metabolites, impurities

o 4 posible mechanisms

0. immediate anaphylactic

0. antibody dependent cytotoxicity

0. antigen-antibody complexing

0. cell-mediated

EPIDEMIOLOGY

• accurate informatino hard to get

o 5-35% of hospital admissions involve ADR

o 5% of hospital admissions are primarily due to ADR

o 10-20% of inpatients suffer ADR

o 0.1% of deaths on wards directly caused by ADR

o communicty prevalence unknown

FACTORS PREDISPOSING TO ADR

1. presence of severe disease, especially renal, hepatic and heart

1. polypharmacy and prolonged treatment

1. extremes og age

1. previous ADR

1. gender: females more susceptible than males

1. atopic history (more likely to have an allergic reaction)

1. genetic polymorphism

ADVERSE DRUG REACTION MONITORING - may be drug-oriented, disease

oriented, complication oriented

Methods

7. spontaneous - case reports (only a small proportion are reported)

7. intensive hospital monitoring - research based (high costs, limited numbers of

patients)

7. record linkage - collection of medical records of patients (confidentiality is a

major issue in Australia)

7. mandatory/compulsory - must report by law (e.g. in Sweden) or to gain

accreditation (e.g. Canada)

7. regulatory - manufacturer must keep record of all ADRs reported to it by doctor

or pharmacist

ADRs and Australia (spontaneous reporting)

• Adverse Drug Reaction Advisory Committe set up by Australian Drug Evaluation

Committee

• ADRAC regularly reviews all Australian reports of suspected ADRs (5000 each

year) and publishes cases and sends them to WHO Collaborating Centre for

International Drug Monitorings

• ADRAC reports on suspected adverse drug reactions are sent free to all docts,

dentists and pharmacists

What to report? Anything, no matter how trivial, particularly for

3. new drugs

3. suspected drug interactions

3. reactions signifantly affecting patient management (death, life in danger, hospital

admission or prolongation, birth defect)

Note that premarketing clinical trials provide no guarantee that all relatively common

adverse effects will be adequately domcumated (e.g. persistent cough in 1 in 100 ACE

inhibitor users)


DRUG ABSORPTION

• drug absorption = the process by which drug proceeds from the site of

administration to the site of measurement (blood stream) within the body

• absorption occurs for the following drug fomrulations:

0. oral

1. sublingual (will avoid breakdown in stomach and liver)

2. transdermal (e.g. niotine) (very slow absorption)

3. rectal (use when patient is vomiting)

4. intramuscular injection

5. subcutaneous injection

6. miscellaneous - inhalation, intranasal, eye, nose, ear drops

• gastrointestinal absorption = extent to which drug is absorbed from gut lumen

into portal circulation

• drug absorption occurs by following 2 mechanisms:

0. passive diffusion � rate of transfer across membrane = concentration difference x

surface area x permeability constant

� permeability constant = diffusion coefficient x partition coefficient

/ membrane thickness

� diffusion coefficient = k / Sqrt (molecular weight)

� molecular weight: this is realtively constant for most drugs because

molecular weights usually vary between about 100 and 500

� membrane thickness: this is relatively constant

� partition coefficient:

� this is a marjor source of variation in drug diffusion

� drugs which are lipophilic have high partition coefficients,

and drugs which are lipophobic have low partition

coefficients

� a value above 5 indicates high lipid solubility

� degree of ionisation:

� only a non-ionised nonpolar drug diffuses across the

membrane, hence this is an important factor

� degree of ionisation depends on pKa of drug (and pH of

body fluid)

1. carrier mediated transport � unlike passive diffusion, this exhibits saturability, specificity,

competition, requires energy, and involves movement against a

concentration gradient

� for GI absorption - facilitated diffusion

� is important for a few drugs based on endogenous compounds (e.g.

levo-dopa, which is used for Parkinson's disease) and vitamins

• physiological factors influencing gastrointestinal absorption

0. blood flow � the blood flow is responsible for removal of drug from the other

side of the membrane

� for some very lipophilic drugs, the limiting factor in

gastrointestinal absorption is rate of blood flow (e.g. ethanol)

1. surface areas/blood flow � the surface area of intestine is 1000 x greater than stomach

� blood flow of intestine is 8 x greater than stomach

� hence, although physico-chemical properties would dictate that

weak acids are absorbed in stomach, and weak bases in intestine,

in reality, most drugs are absorbed across intestinal wall

2. gastric emptying � affects the rate of drug absorption (i.e. decreased gastric emptying

slows down absorption)

� gastric emptying can be decreased by drugs (opioids), disease

(migraine, shock) and food

• solid oral dosage forms o for tablets/capsules to be absorbed, they must undergo:

0. distintegration to form granules

1. dissolution in gastrintestinal fluids

2. absorption into body

o some special formulas are:

� enteric coated tablets:

� contain a coat over the tablet preventing breakdown in acid

� overcome

� local side effects (e.g. coated aspirin will not cause

gastritic bleeding)

� degradation (e.g. coated penicillin will not undergo

acid hydrolysis)

� sustained release tablets:

� are formulated to decrease rate of dissolution --> decrease

rate of absorption

� are used to reduce the frequency of dosing (e.g. morphine)

o hence, formulation factors can affect drug absorption

o therapeutic failures have occurred when one manufacturer's brand has

been substituted by another

o the principal mechanism affecting drug absorption is dissolution - which

can be influenced by

� particle size

� solubility

� fluid pH

� gastric emptying

• pharmacokinetic measurement of absorption: o bioavailability (F) = amount of (oral) dosewhich reaches the systemic

circulation (between 0% and 100%)

� reasons why some drugs do not have bioavailability of 100%

-1. insufficient time for absorption

-1. decomposition in gut wall and lumen

� examples include:

� acid hydrolysis (penicillin)

� complexation (tetracycline with Ca++)

� metabolism by gut microflora

� metabolism in gut wall (oral contraceptive)

-1. liver and the first pass effect = when drug is absorbed, it

must first pass through portal circulation and liver

� i.e. drug absorbed across GI tract --> portal

circulation --> systemic circulation

� hence, in liver there may be metabolism of drug

before it can enter the systemic circulation (e.g.

morphine)

� the major determinant of oral bioavailability of high

extraction drug is hepatic enzyme activity

� some drugs have low extraction (e.g. caffein), and

hence high bioavailability

� some drugs have high extraction (e.g. morphine)

� note: F = fraction absorbed x (1-E) [E = extraction

ratio]


DRUG DISTRIBUTION

• general concepts of distribution: o distribution = process of reversible transfer of trug to and from

circulation

o distribution rate and extent is determined by:

� delivery of drug to tissue

� ability of drug to pass through tissue membranes

� binding of drug to plasma and tissue components

o perfusion rate limitations

� occur when the tissue membranes are not a barrier to distribution

of a drug

� hence well perfused tissues take up the drug more rapidly than

poorly perfused tissues (e.g. lungs, kidneys > brain, muscle > fat,

bone)

� the time to convey to the tissue the amount of drug at equilibrium

depends on:

1. Kp - the equilibrium ratio (Kp = equilibrium concentration

of tissue / equilibrium concentration of venous blood)

2. perfusion rate

o permeability rate limitations

� occur when the tissue membranes act as a barrier to distribution of

a drug

� factors that act as barriers to distribution include:

1. tight capillaries (e.g. in brain)

� most capillaries are relatively leaky, and permeable

all except very large molecules, e.g. proteins and

protein bound drugs

� however, in the brain the endothelial cells have no

fenestrae and are attached by tight junctions

2. cell membranes

� physico-chemical properties (molecular weight and

solubility) determine rate at which drug penetrates

cell membranes since they determine a moluecles

ability to pass through:

1. lipid matrix of membrane

2. membrane pores

3. affinity for active transport carriers

o distribution equilibrium = unbound concentrations of drug in tissue and

plasma are equal

� the exception to this will be if tissue is involved in excretion or

metabolism or where there is active transport out of the tissue

• plasma protein binding is an important factor in determining a drug's volume of

distribution because only drugs not pound can be distributed outside the

circulation

o affinity of protein for a drug:

� [unbound drug] + [protein] <--> [drug-protein complex]

� k1 is the rate constant of the reaction (drug + protein --> drug-

protein)

� k2 is the rate constant of the reverse reaction (drug-protein -->

drug + protein)

� Ka = k1 / k2 = association constant (a measure of affinity of the

protein for the drug)

o drugs bind predominantly to two proteins:

0. albumin

� comprsises 60% of total plasma proteins

� acts as a transporting protein for numerous endogenous

substances

� binds various anions, cations, steroids, bilirubin, many

drugs (especially those which are acids)

� the drug-albumin complex usually consists of one or two

high affinity binding sites, and many low affinity binding

sites

� the 2 high affinity bindings sites are:

0. bilirubin binding site (drugs such as warfarin and

sulphonamides bind)

1. benzodiazepine binding site (drugs such as

benzodiazapines, tryptophan and medium change

fatty acids bind)

� knowledge of which site on albumin a drug binds enables

the prediction of drug-drug and drug-endogenous ligand

displacement interactions

� displacement interactions and reduction in albumin

concentrations may may make interpretation of total

(unbound + bound) concentrations difficult in therapeutic

drug monitoring

1. alpha1-acid glycoprotein

� is only in small concentrations in plasma, but does bind to

many drugs (e.g. propranolol, lignocaine, imipramine)

� is an acute phase protein, and so in inflammatory diseases

(including MI and cancer) its concentration may increase

several fold

� binding to this protein usually occurs at one site of high

affinity

o other proteins which bind drugs are:

� lipoproteins (bind triglycerides, phospholipids, cholesterol, some

drugs)

� transcortin (binds steroids)

� thyroxine binding globulin

� retinol binding globulin

� other structures (e.g. white cells bind some antimalarial drugs)

• tissue binding o tissue binding occurs at following sites:

� tissue proteins

� DNA (e.g. antimalarials)

� lipids (e.g. fat soluble drugs and environmental pollutants)

� skin/ocular melanin (e.g. quinacrine)

• kinetics of protein binding o fu = unbound fraction of drug in plasma = 1 / (1 + (Ka x Pu))

� Ka = association constant

� Pu = [unbound protein concentration] = Pt - DP

� Pt = [total protein]

� DP = [drug-protein]

� (note: fu is normally constant, i.e. independent of drug

concentration (because only a small fraction of available binding

sites are occupied))

o fu is affected by:

0. effect of drug concentration � if concentration of drug is so high that almost all drug

binding sites are filled, then a further increase in

concentration will not cause more drug-protein complexes

to form, hence fu will increase (i.e. fu will no longer be

constant)

� (in other words, as DP approaches Pt, fu will become

variable, i.e. concentration dependent)

� this is uncommon for albumin because [albumin] is high

and so large concentrations of drug are required to

saturated the binding sites

� this is more common for alpha1-glycoprotein because

[alph1-glycoprotein] is low and so lower concentrations of

drug are required to saturated the binding sites

1. effect of protein concentration � if total protein concentration decreases, then fu increases

� albumin may decrease in nephrotic syndrome or severe

liver cirrhosis (increase fu)

� alpha1-acid glycoprotein may increase in many diseases

(e.g. rheumatoid arthritis) (reudce fu)

2. effect of binding affinity: � a decrease in affinity will increase fu

� this may occur because of:

� disease states

� displacement by endogenous ligands or other drugs

• binding and drug disposition o knowledge of degree of drug-plasma binding is important because:

0. only unbound drug is pharmacologically active

1. is a determinant of drug's volume and distribution

2. assists in elucidating clearance mechanisms

o volume of distribution (V) = amount of drug in body (A) / plasma

concentration (C)

� a large volume of distribution indicates extensive tissue binding

� a simple model is:

� V = Vp + (fu x Vt / fut)

� Vp = plasma volume (about 3L)

� fu = drug's unbound fraction in plasma

� Vt = tissue volume

� fut = unbount fraction in tissue

� Vt can be extracellular water (15 L) or total body

water (45 L)

� if a drug is polar and does not penetrate

membrane easily, then Vt will be 15 L

� if a drug is lipophilic then it will distribute

into total body water, (Vt = 45 L)


DRUG CLEARANCE - HEPATIC METABOLISM

Introduction

• a major route of drug clearance involves the metabolism of lipophilic drugs to

hydrophilic derivatives that can be eliminated by the kidneys

• this process is called biotransformation

Tissues invovled in drug metabolism

• most drug metabolism invovles enzymes in the liver, many of which are

associated with the smooth endoplasmic reticulum of hepatocytes

• metabolism of drugs by liver directly after absorption into the portal system prior

to reaching circulation is refered to as first pass metabolism

• other tissues that play a role in drug metabolism include:

o gastrointestinal tract (contributes to first pass clearance)

o kidneys

o skins (relevenat following topical applications of drugs)

Classification of drug metabolism pathways

• phase I reactions

o (functionalisation = introduction of a new functional group)

o phase I reactions are functionalisation reactions, frequently adding a new

oxygen atom (e.g. -OH group) or exposing an existing functional group

(e.g. hydrolysis of ester linkage)

o are usually catalsed by oxidases located in endoplasmic reticulum, mainly

cytochome P450 enzymes

o CYP450s are mainly in hepatocytes, but also cells of the kidney, lung,

intestine, skin, testis, brain

o each CYP450 oxidises many substrates (with some selectivity for classes

of chemicals)

o nomenclature of CYP450s:

� example P4502E1

� first number (i.e. "2") refers to CYP family (>40%

homology)

� capital letter (i.e. "E") refers to subfamilies (>60%

homology)

� second number (i.e. "1") refers to a particular P450 isoform

o major human cytochrom P40s (also called hepatic mono-oxygenases)

� CYP1A2

� is expressed universally in liver tissue

� is further induced by smoking, ingestion of grilled meats

� metabolises drugs such as: caffeine, theophylline,

phenacetin by dealkylations

� also activates aromatic amino procarcinomages

� CYP2C9/10 and CYP2C18/19 (important)

� collectively, the 2C family is very important in drug

metabolism, e.g. taxol, phenytoin, diclofenac, piroxicam,

diazepam, cycloguanil, imipramine

� a genetic deficeincy (polymorphism) for CYP2C9 was

identified with tolbutamide

� a genetic polymorphism for CYP2C19 was identified with

mephenytoin

� CYP2D6 (important)

� expressed mainly in liver

� important in drug metabolism, e.g. codein, debrisoquine,

propranolol, captopril, dextromethorphan, nortryptiline

� a genetic polymorphism (identified with sparteine) affects

10% of Caucasions, 2% of Mongoloids and Negroes

� CYP2E1

� expressed mainly in liver, and also peripheral lymphocytes

� induced by ethanol (major role in ethanol metabolism in

alcoholics)

� minor role in drug metabolism, e.g. paracetamol,

chlorzoxazone

� important in metabolism of a range of industrial chemicals,

e.g. benzene, styrene, vinyl chloride, carbon tetrachloride,

chloroform, ethyl carbamate

� CYP3A4 (important)

� most abundant human isoform (liver, intestine)

� induced by barbiturates, rifampicin, glucocorticoids

� most important CYP450 in drug metabolism, e.g.

nifedipine, quinidine, taxol, erythromycin, contraceptives,

warfarin, cyclosporin, midazolam, lidnocaine

• phase II reactions

o phase II reactions are biosynthetic reactions (conjugation reactions)

o involve conjugation of a hydrophilic endogenou cofactor with a drug

o the conjugate is usually more polar than metabolites formed via phase I

oxidation, thus phase II conjugates are readily cleared by kidney or bile

o major phase II reactions include (primarily occur in liver)

� glucuronidation - transfer of glucuronic acid to drug

� enzyme involved: UDP-glucuronosyl transferase

� cofacter required: UDP-glucuronic acid

� functional group required in drug: -OH, -COOH, -NH2, -

NH, -SH

� sulphation - transfer SO3- to drug

� enzyme involved: sulfotransferase

� cofacter required: PAP-sulphate

� functional group required in drug: aromatic -OH or -NH2

� acetylation (transfer of acetyl group to drug)

� enzymes involved: N-acetyltransferase

� cofacter required: acetyl CoA

� functional group required in drug: aromatic or aliphatic -

NH2

� glutathione conjugation (add glutathione to drug)

� enzyme involved: glutathione-S-transferase

� cofacter required: glutathione

� functional group required in drug: epoxides, organic

halides

Integration of drug metabolism

• for a drug to be metabolised by phase II conjugation it requires an appropriate

function group

• such functional groups may have been introduced by a phase I enzyme

• thus, phase I and phase II reactions are tightly integrated:

o drug in plasma --> oxidised metabolite --> conjugated metabolite

o either the product of the phase I reaction (oxidised metabolite) or of the

phase II reaction (conjugated metabolite) may be excreted via urine or bile

Pharmacological implcations of drug metabolism

• modification by phase I and phase II reactions usually alters the pharmacological

properties of the agent

• pharmacological deactivation

o most drug metabolites exhibit less affinity for relevant receptors, thus

biotransformation usually implies pharmacological deactivation

o biotransformation also typically generates metabolites that are more easily

cleared from the body, further dimishing the duration of a drug's action

o example: amphetmine

� is a CNS stimulant, reversing fatigue symptoms

� inactivated by CYP450-catalysed oxidation (deamination)

o example: diazepam

� is a hypnosedative used in control of anxiety, insomnia

� undergoes oxidation by CYP2C19 to an inactive metabolite (N-

demethylation)

• pharmacological activation

o for a few drugs, inactive prodrugs undergo metabolism within the body to

active drugs

o example: sulindac

� is a NSAID used in treatment of rheumatoid arthritis, gout and

moderate pain

� converted to an active silphide metabolite by reductive phase I

metabolism occurring in the gut (catalysed by bacterial reductases

in gut contents, not gut wall)

� resulting metabolite is 500 times more potent inhibitor of

cyclooxygenase than parent

o example: codeine

� is an inactive prodrug that required CYP2D6-catalysed O-

demethylation to form morphine, which is much more active a mu

opioid receptors

� in a subsequent reaction, morphine is converted to a potent

analgesic (morphine-6-glucuronide) via glucuronidation (note that

it is unusual for phase II metabolites to be active)

Role of drug metabolism in drug interactions

• knowing the metabolic fate of drug is important because:

3. it can help avoid adverse effects due to drug interactions during tretment

with 2 or more drugs

� example: ketoconazole is metabolised by a specific CYP450 (3A4)

and can inhibit the clearance of terfenadine which is a substrate for

the same CYP450 (hence cardiac toxicity)

3. prolonged exposure to some drugs can cause overexpression of CYP450s

(induction) causing a loss of potency of other drugs that are metabolised

via that isoform

� example: barbiturates induce specific CYP, and eththinylestradiol

(contraceptive) is metabolised by this CYP, and so is metabolised

too quickly (therefore ineffective) if patient is taking barbiturates


RENAL EXCRETION

Introduction

• renal excretion is the final step in most drug pathways (water-soluble xenobiotics

and lipophilic compounds converted to water soluble metabolites are excreted)

• fraction excreted unchanges = fe = amount in urine / dosage = renal clearance /

total clearance

• CL(total) = CL(renal) + CL (hepatic) + CL (other) {assume that CL (other) is

minimal}

• thus, if drug's total clearnace and fe are known, renal and hepatic clearances can

be calculated

• for 70% of drugs, CL(renal) < CL(nonrenal)

Mechanisms

• 1. filtration

o 20% of renal flow forms glomerular filtrate (typically, GFR = 120 ml/min)

o protein binding of a drug allows only unbount portion (fu) to filter

o rate of filtration of a drug from plasma is related directly to plasma

concentration (C)

o equation: rate at which drug is filtered at glomerulus = C x

CL(glomerular) = C x fu x GFR

• 2. active transport in the proximal tubule

o reabsorption:

� the proximal tubule performs active reabsorption of glucose and

amino acids, but drugs are not carried by these transporters

o secretion:

� the proximal tubule has 2 active transport excretory systems

(accepting organic cations and anions respectively) which are non-

selective and excrete a wide variety of xenobiotics and endogenous

substrates

� these systems:

� are energy dependent

� can be saturated at high concentrations (hence substances

handled have a tubular maximum secretion rate [Tm])

� show competition between drugs for the same transporter

� natural substrates for these transporters include metabolites

conjugated with sulphage, glucuronide and glycine, and metabolic

products such as creatinine (which is also filtered)

• 3. distal tubule and collecting duct

o as the tubular concentration of drug rises progressively along the nephron

due to reabsorption of water, hence a concentration gradient develops

which favours the passive diffusion of drug from the tubule into the

peritubular capillary

o reabsorption varies from negligible to compelte, denedning on the

physico-chemical properties of drug (molecular weight, pKa, lipophilicity)

o two physiological variables are important in this process

-1. urine flow rate

� this influences the drug concentration concentration

gradient

� low tubular flwo rate will be accompanied by a higher drug

concentration in distal tubular fluid, and more complete

reabsorption of a lipid-soluble drug

-1. distal tubular pH

� varies between 4.5 and 8

� changes within this range alter degree of ionisation of a

weak acid or base

� since non-ionic form only will be reabsorbed, a chnage in

urine pH can alter tubular reabsorption

� example: urate excretion can be enhanced by alkalinising

the tubular fluid (uric acid is a weak acid) since there will

be more ionised urate

• overall: CL (renal) = CL(GFR) + CL (secretion) + CL (reabsorption) {note that

CL (reabsorption) < 0}

Significance

• renal disease:

o renal disease alters all forms of renal excretion

o renal clearance, tubular secertion etc. are reduced in direct proportion to

the reduction in GFR

o this will be clinically relevant if the major pathway of excretion is renal

o example:

� lipiophobic beta-blocker atenolol has > 90% renal excretion and an

intermediate therapeutic index

� in renal failure, toxicity is common unless dose is reduced

• estimating drug excretion:

o normal renal function varies widely with age, body weight etc.

o plasma creatinine is not sufficiently reliable

o 24 hour urine collections to directly measure creatinine clearance is

impractical

o if a high degree of precision is needed, creatining clearance (GFR) is

estimated from plasma creatinine, body weight, age, and sex

� a good method is the Cockkroft and Gout formula

� estimated creatinine clearance (ml/min) = (140-age) x

weight / (serum creatine (mmol/L) x 815)

� multiply this value by 0.85 for females

� other estimation formulae exist

� these methods are not valid if renal function is not stable, or on

extremely high or low protein diets, or at extremes of age

o for the few drugs which have a narrow therapeutic index and are mainly

excreted renally (e.g. aminoglycosides and digoxin), individual GFR

should be estimated and then the individual's total clearance for the drug

derived; from this the correct dosage (loading and maintenance and

timing) can be calculated; therapeutic drug monitoring should also occur

o example:

� for digoxin, total clearance in normal man is 130 mil/min, fe = 0.6

� by how much should the normal maintenance dose rate (250

micrograms/day) be altered to achieve the same Css in a patient

with a GFR of 30 ml/min?

� you know that F x dose rate = CL x Css

� therefore, to keep Css constant, need to adjust dose rate in direct

proportion to Cl

� CL(nonrenal) = 130 x (1 - fe) = 130 x 0.4 = 50 ml/min (for normal

man and patient since this involves liver)

� in normal man, CL(renal) = 130 x 0.6 = 80 ml/min

� in patient, CL(renal) = 30 x 0.6 = 20 ml/min

� therefore in patient, CL(total) = CL(nonrenal) + CL (renal) = 50 +

20 = 70 ml/min

� hence, need to reduce maintenance dose to 70/130 = 55% of

normal (125 micrograms/day)

• altered tubular secretion - potential drug interaction

o anion pathway:

� handles many drugs such as penicillins, cephalosporins,

probenecid, salicylates, diuretics

� (PAH (para-amino-hipppurate) is transported avidly, and clearance

by the healthy kidney > 90% of total renal plasma flow (600

ml/min) - hence can be used to determine renal blood flow;

fortunately most drugs aren't transported so efficiently)

o cation pathway:

� handles drugs such as cimetidine, procainamide, amiloride,

pindolol, ranitidine

o drugs compete for transport, thus introducing one can reduce the renal

clearnace of another

� example: probenecid has been used to reduce excretion of

penicillin, and thus reduce the dose required (by up to 80%)

� example: cimetidine can produce a significant rise in plasma

procainamide due to competition for common transport mechanism

o note: in neonates, tubular secretory function is very poor, but develops

after birth - caution is needed in drug treatment

• distal tubule

o effect of urine volume

� drug nephrotoxicity can be worse when urine volume is low (e.g.

gentamicin toxicity)

� in drug overdoses: high fluid intake and therefore urine output

often aids recovery

o effect of urine pH

� for drugs that are weak acids (pKa 3-7) or weak bases (pKa 6-12),

the degree of ionisation in tubular fluid is dependent upon pH

� example: methamphetamine

� is a weak base (pKa = 10)

� renal excretion is 4 x faster in an acid than an alkaline urine

(because obviously a lower pH means lower ionisation and

hence less reabsorption and hence greater excretion)

� example: phenobarbital

� is a weak acid (pKa = 7.4)

� renal excretion is 7 x faster in alkaline urine (however renal

clearance is still only a small fraction of total clearance)

� clinically important example: salicylic acid

� is a weak acid (pKa = 3.5)

� at physiologic pH it is mainly ionised

� at pH of 5.0, the amount of non-ionised form is 25 x that

present at pH of 7.4

� in serious aspirin overdose, a systemic and tubular acidosis

occurs, which enhances tubular reabsorption (and therefore

prolongs half life of elimination)

� this can be reversed by giving systemic alkali and fluids

and producing an alkaline pH urine with high flow

� (note that haemodialysis is actually used in life-threatening

poisoning)

• renal handling of drug metabolites

o for a few drugs, the metabolite is toxic or active, examples include:

� clofibrate

� is a cholesterol lowering drug

� is metabolised in liver to clofibric acid glucuronide which

is excreted renally

� however, in renal failure, clofibrate treatment produced an

unexpected syndrome of muscle pain and inflammation

� it turned out that clofibric acid is a selective muscle toxin at

the plasma levels reached in renal failure

� morphine

� in renal failure, morphine has much longer duration of

action than plasma half life (the plasma half lfie is not

prolonged in renal failre)

� morphine is metabolised in part to morphine-6-glucuronide

which is renally excreted

� M-6-g has opioid properties, and elevated levels of m-6-g

account for this difference in renal failure

� hence give morphine in less frequent doses in patients with

failure

� procainamide

� dose of procainamide should be reduced in renal failure

because its excretion is reduced, and the major metabolite

N-acetyl-procainamide (renally excreted) is antiarrhythmic

� it is usual to use TDM and measure procainamide and N-

acetyl-procainamide levels to control treatment

• drugs acting on kidneys

o some drugs depend on concentration mechanisms in renal tubule for their

therapeutic effect and selectivity action

o examples:

� urinary tract antibiotics (nitrofurantoi, nalidixic acid) are present in

urine at 100 x the plasma concentration

� loop acting (freusemide) and distal vonvoluted tubule acting

(thiazides) diuretics depend on tubular concentration for their

selective effects on the luminal surface of the tubular cells

o these drugs become less effective when the tubular concentrating process

is impaired in renal disease


INTERRELATIONS

Diagrams on handout are useful

Introduction

• usually, there is a functional and reversible relationship between the concentration

of a drug at its site of action and the intensity and duration of response produced

• because it's impossible to measure concentration of drug at site of action and

response at site of action, inferences have to be made by studying plasma drug

concentrations and clinical responses

Concentration and response

• response increases with increasing concentration (see diagrams - note that the

shapes of 1a and 1b are different because 1a has a linear x axis scale, and 1b has a

logarithmic x axis scale) - note that this is not a linear relationship (i.e. doubling

the dose does not double the effect)

• Hill equation:

o intensity of effect = (Emax x c^n) / (EC50^n + c^n)

o Emax = maximum effect

o EC50 = concnetration needed to produce 50% of Emax

o c = concentration of drug in plasma

o n = shape (also called slope) factor that accommodates the shape of the

curve

� a higher n defines a steeper curve

� example: if n = 1 then EC20 = 0.25EC50 and EC80 = 4EC50

� example: if n = 2 then EC20 = 0.5EC50 and EC80 = 2EC50

� most drugs have an n value between 1 and 3

• examination of 1b shows that: between about 20% and 80% of maximum effect,

response is approximately directly proportional to the logarithm of the

concentration

Time delays

• drug effect often lags behind plasma concentrations

• example: digoxin

o used for treatment of heart failure and some arrhythmias

o has a positive inotropic effect (i.e. it increases force of heart muscle

contraction)

o the effect lags behind plasma concentrations by several hours after dosing

o this delay is related to the time taken for the drug concentration to

equilibriate between the plasma and the site of action (myocardium)

o hence TDM during the distribution phase is highly misleading

• see figure 2

o the diagram is divided into 4 parts:

1. large increase in concentration, small increase in effect

2. small increase in concentration, large increase in effect

3. small decrease in concentration, no change in effect

4. decrease in concentration, decrease in effect

• causes for the time delay in effect following concentration include:

0. time taken for drug concentration to equilibriate between plasma and site

of action

1. drug produces active metabolite(s)

2. observed effect is indirect measure of true effect

� example: allopurinol

� used to prevent gout

� acts to inhibit xanthine oxidase (the enzyme that catalyses

the formation of uric acid)

2. drug reduces the concentration of an endogenous substance, whose

concentration must fall below a critical value to produce observed

response

� example: warfarin

� is an anticoagulant

� inhibits formation of several clotting factors

2. drug acts irreversibly at site of action

� example: aspirin

� inhibits enzyme cyclooxygenase

Onset-duration-intensity relationships

• following asumptions are made:

o response can be characterised by Hill equation

o only the drug acts to produce response and does not have active

metabolites

o the drug does not influence its own kinetics (for example, by altering liver

blood flow or fu or CL)

• onset (see figure 3)

o onset occurs when concentration at site of action reaches critical value

(Cmin = minimum concentration at site of action, Amin = minimum

amount in the body needed to achieve effect)

o onset is governed by factors such as:

� dosage form (e.g. rapid release or slow release tablets)

� dose size

� route of administration

� absorption rate

� distribution kinetics

� concentration-response relationships (EC50, n)

• duration (see figure 3)

o duration of effect will last as long as Cmin is exceeded at site of action

o is a function of dose and rate of removal (elimination or redistribution)

o if a drug is eliminated by first order kinetics: doubling a single dose, will

obviously increase the duration of effect by one half life

o there is a limit to how many dose doublings can be achieved because of

toxicity

o hence multiple dosings are usually essentially

Integration of concentration-time-intensity relationships

• see sheet; it is very basic and obvious, and hence not worth writing about

PHARMACOLOGY LECTURE 12 AND 13

VARIABILITYIN DRUG RESPONSES

Description of variability

• dose(min) - dose effective at the 5 centile (of the population)

• dose(max) - dose effective at the 95 centile

• typically, dose(min):dose(max) = 1:4

o example: penicillin is effective for 5% of population at dose z, and is

effective for 95% of population as dose 2z, hence dose(min):dose(max) =

1:2

• sources of variability are:

o genetic

o drug interactions

o food interactions

o drug formulation and route

o age, weight and gender

o disease

o environmental factors

o in practice, behavioural factors are common (non compliance and dosing

errors)

Classification of variability in response

• pharmacokinetic factors

o absorption 0. formulation of drug

� this determines rate and extent of dissolution

� an unreliable oral formulation can lead to incomplete and

variable absorption

� examples where this especially a problem: digoxin,

phenytoin, cyclosporin

1. route of administration

� patient response varies with route

� example: lignocaine

� is an antiarrhythmic

� well absorbed orally, but because of high

presystemic metabolism, therapeutic plasma levels

are not reached, hence has to be given intravenously

� also has a neurotoxic metabolite, methylxylidide

2. other drugs: may interfer with absorption

3. food may alter absorption, examples include:

� fat delays gastric emptying and therefore prolongs the T1/2

of absorption - this may delay Tmax but does not usually

affect bioavailability (although a poorly absorbed drug such

as griseofulvin (antifungal) has a higher F after food)

� dietary Ca++ forms non-absorbed chelates with

tetracycline (these should therefore be taken on an empty

stomach)

� absorption of L-dopa is less reliable after food because of

competition for its active transport system (in gut wall and

blood-brain barrier) by amines in food

o bioavailability � for lipid soluble (well absorbed) drugs, F is mainly determined by

degree of presystemic hepatic metabolism (F = 1 - E(H))

� E(H) is dependent on hepatic enzymes and shows marked

interindividual variation

� clearly, for drugs with a large E(H), only a small variation in E(H)

will lead to a large change in F

� example: propranolol

� has an average E(H) = 70%, therefore average F =

30%

� a subject showing a 50% reduction in Vmax will

half E(H), hence F will become 65% and less than

one half the average oral dose will be required

� this is not true for the majority of drugs that have a

negligble first pass effect (E(H))

o volume of distribution � for a given drug, V(D) is mainly proportional to body weight

(although for non-fat soluble drugs, lean body weight is a better

predictor) (not true for neonates)

� V(D) determines the loading dose, so body weight is necessary in

calculating loading doses for drugs with narrow TIs

� example: loading dose of gentamicin and tobramicin is 2 mg/kg

BW

o clearance -1. renal excretion

� renal clearance correlates directly with GFR

� renal function is proportional to body surface area - weight

relates closely to this

� for the few drugs with a narrow therapeutic range and

predominantlly renal excretion, maintenance dose is

calculated from body weight (unless renal disease is

present)

� example: usual adult dose of gentamicin is 1 mg/kg BW (8-

hourly) for adults, 1.5 mg/kg BW for children (reflects

higher body surface area)

-1. hepatic metabolism

� is major cause of variation in clearance of most drugs and

is hard to predict, but associated with:

� genetic traits (most unknown and polygenic)

� smoking (enhance clearance of diazepam,

chlorpromazine, theophylline)

� chronic alcohol intake (induces CYP450s and

increases clearnaces of diazepam, phenytoin)

� acute alcohol intake (acts as a transient phase I

enzyme inhibitor)

� living in a city or rurual area (city dwellers have

higher hepatic clearance, ascribed to enzyme

induction from environment exposure to chemicals)

� example: theophylline (see figure c)

� shows wide and unpredictable variation between

individuals (T1/2 between 4 and 14 hours)

� is due to difference in hepatic clearance (but F does

not alter (low E(H)))

� example: propranolol (see figure a and b)

� shows wide variation between individuals (T1/2

between 2 and 6 hours)

� E(H) varies in parallel (so a subject with impaired

hepatic metabolism on propranolol will show both

increased bioavailability and reduced drug

clearance)

� 2 non-linear kinetic examples: phenytoin and theophylline

� Km is close to therapeutic levels, hence a small

change in either enzyme activity or dosage will lead

to a large change in plasma steady state levels

� note: these have very high 1st pass CL

� Ca++ channel blockers

� beta blockers

� tetracyclic (?) antidepressants

Pharmacodynamic factors

• poorly understood, although many drugs have a greater dynamic than kinetic

variability

• monitoring drug plasma levels of these drugs will not be useful in predicting

response

• for drugs that compete at site of action with endogenous ligands, plasma

concentration of ligand may determine drug response:

o example: hypotensive effect of ACE inhibitors correlates with plasma

angiotensin levels; hence may cause severe hypotension in dehydration

o example: anticoagulant effect of warfarin is partly determined by vitamin

K intake (broccoli)

• or, the pathophysiologic state may determine response

o example: BP fall after nifedipine is proportional to original blood pressure

Clinical importance of variability

5. standard dose forms o example: theoretical drug

� T1/2 = 12 hours

� dose(min) = 50 mg (12 hourly)

� dose(min) : dose(max) = 1 : 4

� if the drug was marketed in 50mg, 100mg and 200mg tablets then

� 90% of population could take a 100 mg tablet, twice a day

� the 10%:

� low clearance group take the 50 mg daily (T1/2 will

be longer)

� high clearance group take the 200 mg, 3 times a day

(T1/2 will be shorter)

6. starting treatment -1. CL determines maintenance dose

-1. for drugs that have common dose-related side effects, treatment is started

with dose(min) (with or without a loading dose)

-1. in a few subjects, an altered response can be predicted and starting must be

modified

-1. dose-escalation is then performed as appropriate - genearlly by allowing

concentrations to reach steady state before assessing response (although

the time to response for some drugs differs from the T1/2)

-1. for drugs that have minimal dose-related side effects, or treatment is

urgent, the dose(max) is generally used for simplicity (although cost

implications)

Predicting drug responses in special population groups

• in people who need treatment, pharmacokinetics/dynamics often differ greatly

from that seen in normal (because of age and disease)

• example:

o peak incidence of heart failure is age > 70

o drug absorption, distribution and effects differ with old age and with heart

failure

• the young o in kids, adjustment of dose for body weight/body surface area is usually

adequate

o however, in neonates (especially premature), drug absorpiton, distributio

and clearance differ

o premature infants:

� 85% of body weight is water (adult 55%) and body fat is 1% or

less: a lipid-soluble drug will need a lower loading dose (per kg

BW compared with adults)

� hepatic metabolic pathways and renal excretion are immature (30-

50% of adult values on a BW basis), maintenance drug dosage is

reduced accordingly

o examples:

� chloramphenicol (antibiotic) caused cardiovascular collapse - the

'grey' baby syndrome in neonates when give in standard dosage;

the drug accumulated due to reduced hepatic clearance

� penicillin: active renal tubular secretion is absent at birth, the

clearance/BW of penicillin is 20% of adult values (but can be

induced by drug exposure)

� phenobarbitone: is an effectve (but out-dated) treatment when

given to near-term women to reduce foetal haemolytic jaundice; it

crosses the placenta and induces foetal hepatic enzymes which

have low activity normally; this increases conjugation and

excretion of bilirubin

• the elderly o renal function:

� age-related reduction in GFR and renal blood flow is universal

� toxicity from standard dose of drugs eliminated mainly by kidney

(especially digoxin and gentamicin) would be expected

� estimation of GFR and dose adjustement is necessary (GFR varies

positively with 140 - age)

o hepatic function:

� hepatic clearance declines (especially phase I metabolism) - but

cannot be assessed clinically

� hence, T1/2 for many drugs increases with age (e.g. for diazepam)

� cautious drug sue, smaller doses and longer inter-dosing intervals

are recommended

o pharmacodynamic factors: elderly appear to be more sensitive to sedative

effects of diazepam even when differences in clearance have been

accounted for

• pregnancy o drug use must be minimised because of risk to foetus

o drug distribution changes by 14th week because plasma and ECF volumes

increase by 30%, and plasma proteins, including albumin, fall by 20%

o also, GFR increases and some hepatic phase I metabolism is increases

(probably induced by high levels of progesterone)

o problems arise when treatment is essential, example: phenytoin (for

epilepsy contorl)

� increased metabolism, hence phenytoin levels fall

� but, fall in albumin means that f(u) of this highly protein-bound

drug is greater

� since TDM measures total drug concentrations, increasing dose to

reach a plasma level that is in the (non-pregnant) therapeutic range

will result in toxicity due to high concentrations of unbound drug

� instead, a lower therapeutc range is desirable, and regular TDM is

necessary

� after delivery, the dose needs to be rapidly reduced to avoid

toxicity

• disease

o low cardiac output � in general:

� in shock and severe left heart failure, blood flow to most

organs is decreased

� intestinal, subcutaneous and intramuscular drug absorption

is slow and unreliable in shock therefore

� example: diabetics with hypotensive shock

� when given subcutaneous insulin, often do not respond

until the circulation improves, when delayed

hypoglycaemia may occur

� it is necessary to give insulin intravenously to ensure a

prompt response

� toxicity in brain and heart:

� blood flow to brain and heart is better maintained than to

other organs in shock

� therefore, a rapid intravenous bolus will produce very high

peak local concentrations and increased risk of toxicity in

these organs

� example: there is an increased risk of cardiac arrhythmias

with intravenous digoxin in heart failure

� this is minimised by giving drugs by slow intravenous

infusion of minutes

� impaired renal and hepatic function due to cardiac disease is

discussed below

o renal disease � for a drug with mainyl renal excretion, clearance will be prolonged

in renal disease

� example (taken from renal clearance lecture)

� for digoxin, total clearance in normal man is 130 mil/min,

fe = 0.6

� by how much should the normal maintenance dose rate

(250 micrograms/day) be altered to achieve the same Css

in a patient with a GFR of 30 ml/min?

� you know that F x dose rate = CL x Css

� therefore, to keep Css constant, need to adjust dose rate in

direct proportion to Cl

� CL(nonrenal) = 130 x (1 - fe) = 130 x 0.4 = 50 ml/min (for

normal man and patient since this involves liver)

� in normal man, CL(renal) = 130 x 0.6 = 80 ml/min

� in patient, CL(renal) = 30 x 0.6 = 20 ml/min

� therefore in patient, CL(total) = CL(nonrenal) + CL (renal)

= 50 + 20 = 70 ml/min

� hence, need to reduce maintenance dose to 70/130 = 55%

of normal (125 micrograms/day)

� note:

� if there are active metaabolites which are renally excreted,

these may accumulate

� example: morphine-6-glucuronide

� Vd is sometimes lower in renal failure

� example: for digoxin

� non-renal clearance may vary in renal failure

� pharmacodynamics may differ

� despite the above points, a general rule of thumb is that if Fe <

30% (i.e. renal clearance is less than 30%) then no dose reduction

is needed even in severe renal disease

o liver disease � bioavailability of drugs with high hepatic extraction is increased in

severe liver enzyme because either:

� depressed gepatic enzymes

� or

� shunting of blood from portal to systemic venous

circulations

� many drugs, particularly those invovled with phase I clearance,

have decreased clearance in liver disease (but the relationship with

severity of liver disease is poor)

� drugs that undergo mainly phase II metabolism do not show

impaired clearance unless liver failure is extreme

� also, Vd may be increased for drugs with low Fu when albumin

levels are low

• specific examples: see handout for more details

o hepatic disease � cirrhosis

� theophylline (bronchodilator) - reduced CL (slower fall in

plasma concentration)

� propranolol (beta blocker) - F increased (higher peak


� viral hepatitis

� warfarin (anticoagulant) - CL reduced (excessive response)

o cardiovascular disease � congestive cardiac failure

� lignocaine (anti-arrhythmic) - CL and V reduced (elevated


o renal disease � uraemia

� gentamicin (antibiotic) - reduced CL

� thiopental (anaesthetic) - prolonged anaesthesia (unknown

reason)

o gastrointestinal disease � coeliac disease

� ferrous sulphate (haematinic) - absorption reduced

(anaemia does not respond)

� Crohn's disease

� propranolol (beta blocker) - increased plasma binding:

elevated alpha1-acid glycoprotein

o respiratory disease � emphysema

� morphine (analgesic) - increased sensitivity to respiratory

depression (unknown reason)

� chest infection

� theophylline (bronchodilator) - metabolic CL decreased

(elevated plasma concentrations)

o endocrine disease � thyroid disease

� digoxin (cardioactive agent) - altered pharmacodynamics

(dimished response in hyperthyroidism; increased response

in myxoedema)

o fever � quinine (antimalarial) - impaired metabolism (plasma

concentration elevated)


THERAPEUTIC DRUG MONITORING

What is therapeutic drug monitoring (TDM)?

• = measurement of drug in plasma to assist in indivualising dosage regimen for a

particular patient (especially when therapeutic effect is not easily measured)

What is the theoretical basis for TDM?

• major assumption is that there is a direct and predicatble relationship between:

o concentration of drug in plasma

o unbound concentration of drug in plasma

o tissue concentration of drug (inc. concentration at site(s) of action)

o pharmacological effect, either efficacy and toxicity

What is the therapeutic concentration rage?

• = range of concentrations associated with a high degree of efficacy and a low risk

of dose related predictable toxicity derived from populations of patients

• note: a small proportion of patients will show excellent responses be;pw the lower

limit of the range, and others will derive benefit only a concentration above the

range

When is TDM useful?

• TDM is only useful when there is a known relationship between plasma drug

concentration and pharacological response

• for some drugs, the therapeutic response is difficult to measure (e.g. prophylactic

agents such as anticonvulstants, cardiac anti-arrhythmic agents)

• some drugs have unpredictable relations between dose and plasma concentration

(e.g. dose dependnet or "non linear" kinetic drugs such as phenytoin)

• some drugs have very narrow therapeutic index (e.g. digoxin, aminoglycosides,

anticonvulsants, anti-arrhytmic agents, theophylline, lithium, salicylic acid)

• where poor patient compliance is suspected

• for some drugs, adverse effects of drug may mimic disease state (e.g. digoxin can

cause arrhythmias, cyclosporin)

• when there is suspected toxicity

• when there is reason to suspect there has been a change in pharmacokinetics

(renal function, hepatic function etc.)

What do you need to know to intrepret the TDM results?

• exact knowledge of sampling time of plasma in relation to time of dose

administered; in general, it is bets to take a pre-dose (trough) sample, because this

is the most easily predictable part of the concentration-time curve (i.e. you know

that this will be the minimum value)

• knowledge of the sampling time in relation to "steady state" (how many half lives

have elapsed since the dose was started or changed)

• an understanding of the drug's (and the formulation) plasma concentraion versus

time profile (e.g. immediate or slow release)

• knowledge of any possible non-linear kinetics of the drug

o example: protein binding - valproate, salicylate

o example: clearance - phenytoin

• knowledge of patient specific changes, examples include

o pregnancy

o extremes of age

o genetics

o renal, hepatic, cardiac disease

o hypoalbuminaemia

o concomitant drug therapy

What is the proper clinical role of TDM?

• plasma drug concentrations must always be seen in a clinical context

CASE 1

• 20 year old has had epilepsy since age 12, but been seizure free since age 14

• over this 6 year period, he has taken phenytoin (260 mh daily) and

carrbamazepine (400 mg daily)

• he attended his family doctor, who ordered TDM of both drugs, giving these

values:

o plasma phenytoin concentration = 24 umol/L (therapeutic range 40-80)

o plasma carbamazepine concentration = 12 umol/L (therapeutic range 20-

50 umol/L)

• on the basis of these, doctor increased dose of phenytoin to 400 mg daily and

carbamazepine to 800 mg daily

• one week later, the patient required admission to hospital because of marked

drowsiness, ataxia, vomiting, nystagmus

• plasma concentrations were:

o phenytoin = 130 umol/L

o carbamazepine = 24 umol/L

• COMMENTS

o phenytoin- nonlinear kinetics

o carbamazepine - linear kinetics

o there was no need to increase dosage because he was seizure free; if

anything, the dose should have been reduced because the doctor should

have considered that the epilepsy may have left

o note, even if the doses should have been increased, phenytoin dose should

have only been increased slightly, because of its non-linear kinetics

CASE 2

• 66 year old man admitted to hospital due to severe breathlessness and swollen

ankles over past week

• has been heavy smoker and has ischaemic herat disease and COAD

• at time of admission, there is evidence of congestive heart failure, and his airways

disease appears to be no worse than usual

• he has been on theophylline at home, receiving 800 mg daily

• on admission, he complains of severe nausea, and shakes

• plasma theophylline concentration is 39 mg/L (therapeutic range 10-20 mg/L)

• his heart failure is treated and he improves

• dose of theophylline is reduced to 400 mg daily

• on 5th day he becomes breathless, and is found to have extensive bronchospasm,

but no evidence of heart failure

• his plasma theophylline concentration is now 6 mh/L

• COMMENTS

o theophylline has linear kinetics in this range

o hence, halving dose normally would have plasma levels

o but perhaps - congestive heart failure --> decreased liver function

(reduced liver flow and liver congestion) --> decreased theophylline

clearance --> theophylline rises during CCF

o also, smoking induces P450s (maybe he recently began smoking)

CASE 3

• 74 year old woman, admitted for inguinal hernia repair

• found to have atrial fibrillation (ventricular rate at rest of 105 beats/minute)

• digoxin is started at a dose of 125 micron daily (with no loading dose)

• a very conscientious intern orders a serum digoxin concentration on the next day

• the result if 0.4 nmol/L (therapeutic range 0.6 - 2.3 nmol)

• COMMENTS

o should have given loading dose (because her surgery will be soon and

there will not be enough time to give for 5 half-lives)

o not until 1 week will steady state be reached (half life = 24 hours for

digoxin)

o note: 0.4 is a trough level after 1 half life, therefore is too high, therefore

should decrease dosage


INTEGRATION OF PHYSIOLOGICAL CONCEPTS

AND PHARMACOKINETICS

"~" means approximately

CLEARANCE BY THE LIVER

• hepatic extraction ratio (E) o = fraction of dose entering liver from blood which is irreversibly

eliminated (metabolised) during one pass through the liver

o rate of entry = flow x concentration in = Q x C(in)

o rate of exit = flow x concentration out = Q x C(out)

o rate of elimination = rate of entry - rate of exit = Q(Cin - Cout)

o extraction ratio = E = (how much is extracted / how much goes in)

� = (Cin - Cout) / Cin

o i.e. E = 1 --> complete extraction, whereas E = 0 --> no extraction

o note also, for CL

� rate of elimination = CL x Cin

� CL = rate of elimination / Cin

� CL = Q(Cin - Cout) / Cin

� CL = Q x E

� (i.e. clearnance is a function of blood flow rate and extraction ratio

- this applies to any organ)

o the hepatic extraction ratio can also be described physiologically by:

� E = (fu x CLint) / (Q + (fu x CLint))

� fu = unbound fraction in blood (since only unbound can diffuse

into liver cells; there are exceptions)

� CLint = intrinsic clearance of liver (the ability of liver to

metabolise the drug in the absence of restrictions imposed by blood

flow or binding)

o note: F = 1 - E

o i.e. F = Q / (Q + (fu x CLint) o hence, it can be seen that hepatic clearance and bioavailability are

described in terms of:

� Q

� fu

� CLint

o there are two limiting cases:

� low hepatic extraction ratio drugs - when Q >>> fu x CLint

� i.e. the unbound fraction x the intrinsic clearance is very

small (therefore, using logic, bioavailability is very high)

� i.e. F ~ Q / (Q + 0)

� i.e. F ~ 1

� by the same logic, hepatic CL will be very low, regardless

of flow rate

� the clearance of such drugs depends directly on the degree

of binding and the activity of the drug's metabolising

enzymes

� these drugs have a very low first pass, and bioavailability

will be close to 100%

� examples:

� caffeine

� ibuprofen

� increasing or decreasing rate of supply by altering Q will

make very little difference to hepatic clearance

� also called capacity limited drugs

� summary: these drugs will have a high bioavailability, and

clearance will vary depending on hepatic enzyme activity

and plasma binding

� high hepatic extraction ratio drugs - when Q <<< fu x CLint

� i.e. the unbound fraction x the intrinsic clearance is very

high (therefore, using logic, bioavailability is very low)

� i.e. F ~ Q / (0 + (CLint x fu))

� i.e. F ~ Q / (CLint x fu)

� by the same logic, hepatic CL will be very high, regardless

of changes of CLint x fu

� the clearance of such drugs depends directly on the hepatic

flow rate

� these drugs have a very high first pass, and bioavailability

will be close to 0%

� examples:

� morphine

� lignocaine

� also called flow limited drugs

� summary: these drugs will have a low bioavailability, and

clearance will vary depending on flow rate

� most hepatically cleared drugs can be classified as either low

extraction or high extraction

� by classifying a drug in this way, one can determine the critical

physiological factors which determine hepatic clerance and

bioavailability, and therefore the plasma concentrations and

response to the drug after oral and intravenous administrations

• SUMMARY: bioavailability and first pass effect o F = fraction absorbed x (1 - E)

o some drugs have low E (e.g. caffeine) --> high F (a change in E therefore

has a minor effect on F)

o some drugs have high E (e.g. morphine) --> low F (a change in E therefore

has a major effect on F)

o the major determinant of oral bioavailability of high extraction drugs is

hepatic enzyme activity

• EXERCISE: determine for some low and high extraction drugs, the effect of

changes in drug binding on

o clearance

o volume of distribution

o half life

o bioavailability

o unbound stead state plasma concentrations on chronic dosing (IV and oral)

• APPENDIX

o variability in response - for high extraction drugs, a small change in E will

lead to a large change in F, leading to more variability in plasma

concentrations after oral vs IV doses

o dosage (oral vs. IV) - for a drug with high hepatic extraction, an oral dose

will need to be much higher than an IV dose to elicit the plasma

concentrations

o other routes - to avoid some first pass effect, use sublingual, transdermal,

inhalation, rectal

o drug interactions - other drugs may induce or inhibit hepatic enzymes, and

so for high extraction drugs, this can lead to significant changes in

bioavailability

o liver disease - if shunts exist because of liver disease, bioavailability will

increase because blood flow through liver will be less; therefore, for high

extraction drugs given to patients with liver disease, there is the potenital

for increases adverse effects


PHARMACOGENETICS

• genetic polymorphism: trait determined by a single gene, occurring in normal

population in a least two phenotypes, neither of which is rare (i.e. at least 1%)

• extensive metabolisers - these people metabolise the drug normally

• poor metabolisers - these people do not metabolise the drug at the same rate as

extensive metabolisers

GENETICALLY DETERMINED VARIATION IN PHARMACOKINETICS

Debrisoquine-type polymorphism of oxidative metabolism

4. History

o was observed that excretion of unchanged debrisoquine in urine was very

variable

o extensive metabolisers could be heterzygos or homozygous for the normal

allele

o poor metaboliser phenotype occurs in 10% of Caucastion population;

lower frequency in other populations

5. Metabolic defect

o impaired drug hydroxylation by CYP 2D6

o no enzyme protein is detectable in homozygotes

o detection:

� phenotype - enzyme function tested by ratio of parent drug to

hydroxylated metabolite in urine after standard dose of drug

� genotype - gene sequences

6. Clinical relevance

o poor metabolisers have impaired metabolism of a number of drugs,

therefore:

0. more prone to adverse drug reactions if given standard doses

1. reduces drug effect if metabolite is active (e.g. codeine -->

morphine)

o there seems to be a possible link with other diseases

� poor metabolisers have lower rate of cancers of lung, liver and

gastrointestinal tract

� poor metabolisers may have a higher rate of Parkinson's disease

6. Other drugs of clinical relevance (common examples only; there are > 50)

o beta-adrenoceptor antagonists, e.g. metoprolol

o tricyclic antidepressants, e.g. amitriptyline

o anti-anginal vasodilator - perhexiline (can have bery severe adverse effects

including irreversible peripheral neuropathy)

Polymorphism of drug acetylation

2. History

o observed bimodal distribution of plasma concentration after standard dose

of isoniazid (used to treat TB) --> higher risk of peripheral neuropathy

o slow acetylators are homozygous for mutant allele

2. Metabolic defect

o reduced activity of hepatic N-acetyltransferase

o detection:

� phenytype by measuring plasma concentration of isoniazid or

caffeine after standard dose (slow acetylators have higher

concentration)

� frequency in different populations varies widely: about 10% in

Eskimos, 20% in Asians, 50% in Caucasions, 80% in Egyptians


o reduced clearance and increased risk of adverse effects from drugs which

are usually acetylated

o epidemiological associated with other diseases:

� slow acetylators - bladder cancer, Gilbert's disease (gemetic

problem in liver), SLE (autoimmune against various organs)

� fast acetylators - breast cancer, diabetes mellitus

2. Some examples of drugs of clinical relevance (slow acetylators at ncreased risk of

ADR)

o vasodilators - hydralazine --> lupus-like syndrome (auto-immune disease)

o anti-arrhythmic - procainamide --> lupus-like syndrome

o some benzodiazepines - e.g. nitrazepam --> sedation

Atypical plasma pseudocholinesterase (a rare genetic trait)

4. Metabolic defect

o there are many variants of plasma pseudocholinesterase (each under

separate genetic control)

� these have much lower than normal affinity for succinylcholine

(muscle relaxant used in general anaesthesia)

� people with variant enzyme have much longer duration of action

following usual doses (50-100 mg) of the drug (hours vs. minutes)

o enzyme activity can be tested in vitro, but in practice, most cases are

detected following anaesthetia

o incidence in Caucasians is about 0.05%; not found in Japanese, Eskimos

or South American Indians


o succinylcholine produces muscle paralysis (including respiratory muscles)

and is often used during induction of anaesthesia to allow tracheal

intubation

o effect usually lasts only a few minutes, but may last for hours in patients

with atypical cholinesterase - these patients wake up paralysed and require

prolonged mechanical ventilation

GENETICALLY DETERMINED VARIATION IN PHARMACODYNAMICS

Drug induced haemolytic anaemia (glucose-6-phosphate dehydrogenase deficiency)

2. Clinical problem

o sometimes there is an unexpected occurrence of haemolytic anaemia after

treatment with oxidant drugs

o such drugs can produce haemolytic crises in about 20% of males in some

populations

2. Mechanism

o normally, glutathione is involved in the mopping up of free radicals (it can

chemically detoxify H2O2)

o in this disorder, haemolysis caused because there is less reduced

glutathione (G-SH) within red cell due to a deficiency of enzyme G6PD

o lack of reduced glutathione makes cell vulnerable to oxidation, with

damage to cell membrane and lysis; therefore drugs that in someway

increase this oxidation can lead to haemolytic anaemias

o G6PD occurs in many different abnormal variants and genetic

transmission is X-linked

o (note heterozygotes are more resistant to malaria than homozygotes of

normal allele)

o about 200 million people are affected world-wide

2. Some drugs involved (should be avoided in people with G6PD deficiency)

o urinary antiseptic - nitrofurantoin

o anti-malarial drugs - primaquine, chloroquine

o analgesics - aspirin


INTRODUCTION TO TOXICOLOGY

BIOACTIVATION OF FOREIGN COMPOUNDS TO TOXIC METABOLITES

• normally, phase I and II reactions dimish biological properties

• but, sometimes bioactivation occurs and products are more toxic

• toxic metabolites cause cellular dysfunction by reacting with components such as

DNA or proteins

• note that toxicity only occurs in organs that possess the necessary enzymes that

cause the bioactivation (unless the metabolites are stable enough to migrate to

other organs via the blood)

• hence most toxic drugs tend to preferentially affect a limited numbers of organs

• if the metabolite causes DNA damage, then the patient is at a greater risk of

cancer

• general outcomes:

o cleared via urine/bile <-- non-toxic metabolite <-- (detoxification) <--

drug/foreign chemical --> (bioactivation) --> chemically reactive

metabolite -->

� DNA damage --> cancer

� protein and cell damage --> organ pathology

CHEMICALLY INDUCED ORGAN DAMAGE: THE LIVER AS A TARGET

• liver is vulnerable to damage by ingested chemicals because:

o of its close proximity to blood supply from digestive dract

o it actively concentrates foreign chemicals

o it plays a major role in biotransformation

• a number of toxic responses are known within the liver, including hypersensitivity

reactions (e.g. drug induced hepatitis), cirrhosis, intrahepatic cholestasis and

necrosis

• PARACETAMOL-INDUCED HEPATOTOXICITY

o is an intrinsic hepatotoxicant, i.e. produces a predictable, dose-dependent

toxic response

o is very safe at therapeutic doses

o normally:

� 90% is metabolised via phase II pathways (sulphation and

glucuronidation)

� 10% is oxidised via phase I (hepatic CYP2E1, CYP1A2, CYP3A4)

to a chemically reactive quinoneimine toxic metabolite (NAPQI),

which reacts with glutathione to form a non-toxic metabolite which

is excreted in the urine

o biochemistry under overdose conditions:

� conjugative phase II metabolism is saturated and used up, and

therefore NAPQI overproduction occurs

� this results in depletion of glutathione

� in absence of glutathione, NAPQI reacts with nucleophilic sites on

critical cellular proteins, interfering with cell function and

initiating hepatocellular necrosis

o symptoms, signs, tests and treatment of paracetamol-induced

hepatotoxicity:

� symptoms:

� early symptoms of poisoning include nausea and vomiting

(requires 10-30 g in adults)

� some patients progress to fulminant hepatic necrosis,

involving progressive jaundice with hepatic

encephalopathy (headache, drowsiness, confusion are early

signs of the rapid onset of cerebral oedema [which requires

CNS monitoring])

� signs:

� liver tenderness may appear afer 12 hours, and may persist

for up to 72 hours

� serum enzymes:

� hepatic necrosis is evident after 3 days when huge (up to

500 fold) elevations in serum hepatic enzymes occur (e.g.

aspartate and alanine transaminases)

� serum enzymes peak on day 3, and return to normal after 7-

21 days in survivors

� clotting:

� liver toxicity is accompanied by impaired hepatic

production of clotting factors (esp. III, V, VII), increasing

the prothrombin time and prothrombin time ratio

� treatment:

� thiol therapy (oral or IV N-acetylcysteine [NAC])

ameliorates toxicity by replenishing hepatic glutathione

stores, facilitating NAPQI detoxification

� NAC therapy is most effective if administered within 8 10

hours of ingestion

� effective management requires knowledge of plasma

paracetamol concentrations

� note that anaphylaxis occurs in 5% of IV NAC recipients

o see diagram on sheet

CHEMICALLY INDUCED ORGAN DAMAGE: THE BRAIN AS A TARGET

• blod brain barrier protects brain against toxic ionised and bulky water soluble

agents

• but, some toxicants can penetrate CNS, and can lead to diffuse encephalopathy

with global or dysfunction or a localised encephalopathy

• MTTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)

o in California in 1983, unusual motor defecits resembling Parkinson's

disease were seen in drug addicts using MPPP, a designer drug analogue

of meperidine opioid

o symptoms:

� developed within hours to days

� included difficulty in initiating and terminating movement, resting

tremors, rigidity (which sometimes progressed to immobility)

o biochemistry

� toxicity was due to a synthetic by-product in the drug, MPTP

0. MPTP crosses blood brain barrier

0. is activated by monoamine oxidase B in astrocytes to

MPP+ which is a toxic metabolite (note that MPP+ couldn't

enter CNS, and therefore only the MPP+ produced in the

CNS caused symptoms)

0. MPP+ is a substrate for dopaminergic re-uptake pathwyas,

and therefore accumulates within dopaminergic neurons in

the substantia nigra

0. within the neurons, MPP+ is actively accumulated within

the mitochondria

0. MPP+ interferes with electron transport chain at NADH

dehydrogen

0. hence oxidative phosphorylation is impaired, and ATP is

not produced

0. cell death ensues

o later it was observed that in individuals exposed to low levels of MPTP

that didn't initially develop Parkinsons, the disease did manifest some

years later - this suggested that idiopathic Parkinson's is due to exposure to

MPTP like xenobiotics; two classes of endogenous analogues present in

foodstuffs and also human tissue (beta carbolines and isoquinolines) have

been suggested to play a role in idiopathic Parkinson's, although they are

much less potent neurotoxins than MPTP

CHEMICALLY INDUCED DNA DAMAGE: CANCER AS A TOXIC OUTCOME

• genotoxic = a substance that has the ability to damage DNA

• DNA possesses nucleophilic atoms (e.g. oxygen) that react with electrophilic

metabolites

• DNA adducts = the products of reactions between toxic metabolites and a purine

or pyrimidine

• DNA adducts promote misreplication (mutations) during DNA synthesis

• hence, if mutations in oncogenes or tumour suppressor genes occur, the cell

becomes one step closer to a neoplastic state

• often carcinogens are produced by cytochrome P450s making reactive metabolites

(e.g. benzene)

• note that cytotoxic anticancer drugs are often drugs that cause DNA damage

• TAMOXIFEN

o is very effective in treatment of oestrogen receptor positive breast cancer;

may also decrease risk of developing a second primary tumour

o mechanism of therapeutic action:

� blocks effects of oestrogen by binding to oestrogen receptor

� blockage of growth promoting effects of oestrogen suppresses

breast tumour growth

o adverse reactions: drug is generally well tolerated, with hot flushes and

menstrual disorders most problematic

o study was done to test effectiveness of tamoxifen as a chemopreventive

agent:

� 1992: clinical trial of 16,000 women in US commenced; results

promised for 1998

� 1995: trial was temporarily halted, when it seemed drug may

double risk of endometrial cancer in women who received more

than 20 mg/day for > 2 years

� 1996: tamoxifen was shown to cause liver cancer in rats (not

known whether this was relevant to human endometrial cancer)

� recently: tamoxifen has been shown to undergo bioactivation to a

genotoxic metabolite that forms up to 12 different DNA adducts in

rodent liver

� see diagram on sheet


DRUG TOLERANCE AND DEPENDENCE

Introduction

• tolerance and physical dependence often develop together, but although the

mechanisms are similar, they are not identical

Tolerance

• = decreased responsiveness to drug resulting from its administration

• acute tolerance (densensitisation, tachyphylaxis) = tolerance that develops by a

single administration

• cross tolerance (i.e. tolerance of oen drug leads to tolerance of another drug)

occurs due to similar mechanisms of action

• mechanisms of tolerance action:

0. dispositional (pharmacokinetic, metabolic) tolerance = decreased drug

effect due to reduced drug concentration at site of action (e.g. due to

increased metabolism)

0. pharmacodynamic tolerance = decreased responsiveness at site of action

due to:

� for agonists:

� reduction in number of receptors

� uncoupling of receptor and G protein

� decreased sensitivity of second messenger system

� decreased production and release of endogenous ligand

� for antagonists: the opposite (i.e. increased receptors, increased

sensitivity of second messenger system and increased production

and release of endogenous ligand)

Physical dependence

• = altered physiological state resulting from repeated or prolonged drug

administration; cessation of administration results in a withdrawal syndrome

• withdrawal syndrome depends on pharmacological action of drug taken - many

symptoms/signs are opposite to direct effect of drug

• examples:

0. opioids

� opioid symptom/sign : withdrawal symptom/sigh

� sedation : restlessness and insomnia

� analgesia : joint and bone pain

� constipation : diarrhoea

� pupil constriction : pupil dilation

� reduced BP, temp : increased BP, temp

� both drug and withdrawal of drug cause vomiting and nausea

1. sedatives

� sedative symptom/sign : withdrawal symptom : sign � reduced anxiety : increased anxiety

� sedation : restlessness

� decreased seizures : increased seizures

� muscle relaxation : tremor

� withdrawal of drug causes hallucinations

• withdrawal symptoms bary depending:

o dose change

o duration of use

o rate of decrease in drug concentration


DRUG-DRUG INTERACTIONS

Introduction

• drug-drug interaction = dose-response relation of one drug is altered by another

drug

• synergism = enahnces therapeutic efficacy

• antagonism = reduces therapeutic efficacy

• risk ADR is increased with multiple drug regimes

• a drug-drug interaction (DI) is likely to have therapeutic consequences (TDI) if it

alters the

3. concentration of a drug with a steep dose-response relationship

3. drug response for a condition which unstable (e.g. cardiac arrhytmias,

epilepsy)

• drugs usually involved in TDIs include anticoagulatns, cardiac glycosides, anti-

arrhythmics, sympathomimetic amines, antihypertensives, anticonvulsants, oral

hypoglycaemics, cytotoxics

• revision: phase I enzymes are cytochrome P450s, phase II enzymes are

conjugative enzymes

Major mechanisms of drug interaction

8. pharmacokinetic

1. absorption 1. interference with gut absorption:

0. complex formation in gut

� example: cholestyramine binds many drugs

� example: Ca++/Mg++ antacids complex with iron,

tetracycline, quinolones

1. delayed/accelerated gut emptying

� example: anticholinergics delay gastric emptying

and therefore delay drug absorption

2. changes in gut flora

� example: antibiotics interrupt enterohepatic

recycling of the oestrogen component of oral

contraceptives, causing contraceptive failure

� example: erythromycin prevents bacterial

inactivation of digoxin in the gut, increases

bioavailability, plasma digoxin levels rise 50%

2. alteration in first pass metabolism

� (note: high clearance drug have > 30% extraction from

hepatic blood (F < 0.7))

� a drug that inhibits hepatic metabolism will increase

bioavailability of high clearance drug (provided it is

metabolised by the enzyme(s) inhibited) and vice-versa

� examples:

� cimetidine inhibits CYP450s, therefore doubles oral

propranolol bioavailability

� phenytoin induces enzymes, therefore decreases

felodipine bioavailability

� acute alcohol intake inhibits a CYP, therefore

amitrptiline bioavailability is higher

2. distribution 1. drug-protein binding displacement

0. in plasma

� the drug displaces another drug from a binding site

on plasma proteins leading to:

0. transient rise in f(u) (unbound

concentration) of displaced drug

1. compensatory rise in drug clearance

� therefore a new steady state f(u) is reached that is

similar to the initial level - therefore this is usually

clinically unimportant (unless the transient rise in

f(u) is toxic)

� example of clinical importance

� 99% of warfarin binds to albumin

� NSAIDs bind to same site, and therefore

displace albumin, causing a transient rise in

warfarin, which can be dangerous while it

lasts

1. in tissue

� example:

� digoxin binds to intracellular sites in muscle

� qiunidine displaces digoxin from tissue sites,

leading to an 80% rise in plasma dioxin

levels

3. elimination 1. enzyme induction

� if this metabolic route is a major pathway of elimination,

drug kinetics will change (reduce Css and T(1/2)) and

therefore drug response will change

� enzyme induction takes about 2 weeks to develop, and on

cessation of inducer, about 2 weeks to revert to normal

� examples

� smoking causes 100% increase in clearance of

theophylline

� phenoobarbital and rifampicin cause 200% increase

in clearance of warfarin

� carbamazepine causes 200% increase in clearance

of clonazepam

� chronic alcohol

2. enzyme inhibition

� (drugs that reduce hepatic blood flow also inhibit

metabolism of high clearance drugs)

� if this metabolic route is a major pathway of elimination,

drug kinetics will change (increase Css and T(1/2)) and

therefore drug response will change

� enzyme inhibition is immediate, and on cessation of

inhibitor, reversion to normal is immediate

� examples:

� metronidazole decreases clearance of warfarin by

40%

� cimetidine decreases clearance of phenytoin by 35%

� propranolo decreases clearance of lignocaine by

50% (by reducing hepatic blood flow)

� omeprazole decreases clearance of warfarin

3. examples with regards to enzymes other than cytochrome P450s

� example 1: allopurinol

� is a xanthine oxidase inhibitor (used as an anti-gout

agent)

� also inhibits metabolism of cytotoxic agent 6-

mercaptopurine (6-MP)

� therefore concurrent use of allopurinol and 6-MP

leads to elevated plasma levels of 6-MP and toxicity

� example 2: disulfiram

� inhibits aldehyde dehydrogenase

� therefore is used to give alcoholics a nasty

"aldehyde reaction" when they take alcohol

4. alteration of liver blood flow:

� for high first pass clearance drugs only, a fall in liver blood

flow will cause a clear reduction in systemic clearance

� example: lignocaine toxicity can occur when patients are

given a beta-blocker which reduces liver blood flow

5. interference with renal excretion

� intereference can occur if there is competition for tubular

secretion; this will only be clinically significant if drug has

a narrow therapeutic ratio and a large fraction is excreted

unchanged

� (note also sometimes interactions with tubular

reabsorption)

� example 1: penicillin

� probenicid is a competitive inhibitor for tubular

secretion

� once (years ago when penicillin was not readily

available) probenecid was used to reduce dose

requirements by 80%

� example: procainamide

� procainamide competes with cimetidine for renal

tubular excretion through the organic cation

transporter

� using both at once may result in procainamide

toxicity

� example: lithium

� diuretics and NSAIDs both enhance proximal

tubular reabsorption of Na+ and Li+

� therefore these elevate Li+ levels

9. pharmacodynamic (may occur at receptor, cell or physiological level)\

o synergistic effects

� commonly used therapetucally; examples of synergy used

therapeutically:

� Parkinson's disease: L-dopa + monoamine oxidase B

inhibitor

� hypertension: vasodilator + negative inotrope

� problems can occur, the following show synergy:

� CNS sedatives - e.g. alcohol + barbiturates

� hypertension - monoamine oxidase A inhibitor +

sympathomimetic

� digoxin toxicity - diurectics that cause hypokalaemia +

digoxin

o antagonistic effects

� occasionally used therapeutically to reduce a side-effect; example:

� benzhexol (anticholinergic) used to treat Parkinsonism

induced by chlorpromazine (tranquiliser)

� problems can occur:

� NSAIDs reduce antihypertensive efficacy (reduce renal

sodium excretion)

Clinical approach to drug-drug interactions

• predict and avoid if possible, that is

o become familar with drug TDIs

o take full drug history (including alcohol, smoking)

o avoid multiple drug use when possible

o if multiple drugs need to be used, select drugs to avoid TDIs

• if unavoidable, carefully monitor drug response

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Health & Medicine

Pharmacology: Student Notes -1