DRUG DISTRIBUTION&CLEARANCE

GUIDED BY:Dr.Satyabrata bhanjaM. Pharm., Ph. D

Presentation by: G.Shekhar (256213886011)Department of Pharmaceutics (1st year -2nd sem)

MALLA REDDY COLLEGE OF PHARMACY

DRUG DISTRIBUTION

Contents:* Introduction

* Volume of distribution

* Factors effecting on drug distribution

* Protein & tissue binding

* Kinetics

* Determination of rate constant & different plots (direct , scatchard ,&reciprocal )

Introduction

*DRUG DISTRIBUTION refers to the reversible transfer of drug from one location to another within the body (or) which involves reversible transfer of a drug between compartments.

*Definitive information on the distribution of a drug requires its measurement in various tissues. Such data has been obtained in animals, but is essentially lacking in humans.

*Much useful information on the rate and extent of distribution in humans can be derived from blood or plasma data.

*Distribution is a Passive Process, for which the

driving force is the Conc. Gradient between the blood and Extravascular Tissues.

*The Process occurs by the Diffusion of Free Drug until equilibrium is established.

* As the Pharmacological action of a drug dependsupon its concentration at the site of actionDistribution plays a significant role in the Onset,Intensity, and Duration of Action.

* Distribution of a drug is not Uniform throughoutthe body because different tissues receive thedrug from plasma at different rates and todifferent extents.

*The Volume of distribution (VD), also known asApparent volume of distribution, is used toquantify the distribution of a drug betweenplasma and the rest of the body after oral orparenteral dosing.

*It is called as Apparent Volume because all partsof the body equilibrated with the drug do nothave equal concentration.

*It is defined as the volume in which the amount ofdrug would be uniformly distributed to producethe observed blood concentration.

Redistribution

Highly lipid soluble drugs when given by i.v. or byinhalation initially get distributed to organs with high bloodflow, e.g. brain, heart, kidney etc.

Later, less vascular but more bulky tissues (muscles,fat)take up the drug and plasma concentration falls and drug iswithdrawn from these sites.

If the site of action of the drug was in one of the highlyperfused organs, redistribution results in termination of thedrug action.

Greater the lipid solubility of the drug, faster is itsredistribution.

The real volume of distribution has physiological meaning and is related to the Body Water.

The volume of each of these compartments can be determined by use of specific markers or tracers.

Physiological Fluid Compartments the

Markers Used Approximate volume (liters)

Plasma Evans Blue, Indocyanine Green

4

Extracellular fluid Inulin, Raffinose, Mannitol

14

Total Body Water D2O, Antipyrine 42

The intracellular fluid volume can be determined as the difference between total body water and extracellular fluid.

Drugs which bind selectively to Plasma proteinse.g. Warfarin have Apparent volume ofdistribution smaller than their Real volume ofdistribution.

The Vd of such drugs lies between blood volumeand total body water i.e. b/w 6 to 42 liters.

Drugs which bind selectively to ExtravascularTissues e.g. Chloroquine have Apparent volumeof distribution larger than their Real volume ofdistribution.

The Vd of such drugs is always greater than42 liters.

Several factors influence drug distribution to various tissues of the body. They are listed below .

1)Physicochemical properties of the drug * molecular size * oil\water partition coefficient (Ko\w)* degree of ionization that depends on pKa

2) Physiological factors * organ or tissue size * blood flow rate * physiological barriers to the diffusion of drugs

- blood capillary membrane - cell membrane - specialized barriers - blood brain barrier - blood cerebrospinal fluid barrier

_placental barrier

-blood testis barrier

3) Drug binding in the blood

4) Drug binding to the tissue and other macromolecules

5) Miscellaneous factors

a) age

b) Pregnancy

c) Obesity

d) Diet

e) Disease states

f) Drug interactions

Physicochemical properties of the drug

Drugs having molecular wt. less than 400 daltons easilycross the Capillary Membrane to diffuse into theExtracellular Interstitial Fluids.

Now, the penetration of drug from the Extracellular fluid(ECF) is a function of :- Molecular Size:

Small ions of size < 50 daltons enter the cell through Aq. filledchannels where as larger size ions are restricted unless aspecialized transport system exists for them.

Ionisation:A drug that remains unionized at pH values of blood and ECFcan permeate the cells more rapidly.Blood and ECF pH normally remains constant at 7.4, unlessaltered in conditions like Systemic alkalosis/acidosis.

Lipophilicity:

Only unionized drugs that are lipophilic rapidlycrosses the cell membrane.

e.g. Thiopental, a lipophilic drug, largely unionizedat Blood and ECF pH readily diffuses the brain whereas Penicillins which are polar and ionized at plasmapH do not cross BBB.

Effective Partition Coefficient for a drug is given by:

Effective K o/w = Fraction unionized at pH 7.4

X K o/w of unionized drug

Perfusion Rate is defined as the volume of blood thatflows per unit time per unit volume of the tissue.

Greater the blood flow, faster the distribution.

Highly perfused tissues such as lungs, kidneys, liver,heart and brain are rapidly equilibrated with lipidsoluble drugs.

The extent to which a drug is distributed in aparticular tissue or organ depends upon the size ofthe tissue i.e. tissue volume.

A stealth of endothelial cells lining the capillaries.

It has tight junctions and lack large intra cellular pores.

Further, neural tissue covers the capillaries.

Together , they constitute the BLOOD BRAIN BARRIER.

Astrocytes : Special cells / elements of supporting tissueare found at the base of endothelial membrane.

*The blood-brain barrier (BBB) is a separation of circulatingblood and cerebrospinal fluid (CSF) maintained by thechoroid plexus in the central nervous system (CNS).

Since BBB is a lipoidal barrier

*It allows only the drugs having high o/w partition coefficient to diffuse passively where as moderately lipid soluble and partially ionized molecules penetrate at a slow rate.

*Endothelial cells restrict the diffusion of microscopic objects (e.g. bacteria ) and large or hydrophillic molecules into the CSF, while allowing the diffusion of small hydrophobic molecules (O2, CO2, hormones).

*Cells of the barrier actively transport metabolic products such as glucose across the barrier with specific proteins.

Various approaches to promote crossing BBB:

Use of Permeation enhancers such as Dimethyl Sulfoxide.

Osmotic disruption of the BBB by infusing internal carotid artery with Mannitol.

Use of Dihydropyridine Redox system as drug carriers to the brain ( the lipid soluble dihydropyridine is linked as a carrier to the polar drug to form a prodrug that rapidly crosses the BBB )

PENETRATION OF DRUGS THROUGHPLACENTAL BARRIER

*Placenta is the membrane separating Fetal blood from theMaternal blood.

*It is made up of Fetal Trophoblast Basement Membrane and theEndothelium.

*Mean thickness in early pregnancy is (25 µ) which reduces to (2 µ)at full term.

*Many drugs having mol. wt. < 1000 Daltonsand moderate to high lipid solubility e.g.ethanol,sulfonamides,barbiturates,steroids,anticonvulsants and some antibiotics cross thebarrier by simple diffusion quite rapidly .

*Nutrients essential for fetal growth aretransported by carrier mediated processes.

Blood – Cerebrospinal Fluid Barrier:

The Cerebrospinal Fluid (CSF) is formed mainly bythe Choroid Plexus of lateral, third and fourthventricles.

The choroidal cells are joined to each other bytight junctions forming the Blood – CSF barrierwhich has permeability characteristics similar tothat of BBB.

Only high lipid soluble drugs can cross the Blood –CSF barrier.

Blood – Testis Barrier:

It has tight junctions between the neighbouringcells of sertoli which restricts the passage ofdrugs to spermatocytes and spermatids.

Miscellaneous Factors

• Diet: A Diet high in fats will increase the free fatty acid levels incirculation thereby affecting binding of acidic drugs such asNSAIDS to Albumin.

• Obesity: In Obese persons, high adipose tissue content can takeup a large fraction of lipophilic drugs.

• Pregnancy: During pregnancy the growth of the uterus, placentaand fetus increases the volume available for distribution ofdrugs.

• Disease States: Altered albumin or drug – binding protein conc.• Altered or Reduced perfusion to organs /tissues• Altered Tissue pH

Protein Molecular Weight (Da)

concentration

(g/L)

Drug that bind

Albumin 65,000 3.5–5.0 Large variety of drug

α1- acid glycoprotein

44,000 0.04 – 0.1 Basic drug - propranolol, imipramine , and lidocaine . Globulins, corticosteroids.

Lipoproteins 200,000–3,400,000 .003-.007 Basic lipophilic drug Eg- chlorpromazine

α1 globulin

α2 globulin

59000

13400

.015-.06 Steroid , thyroxineCynocobalamineVit. –A,D,E,K

BIND TO BLOOD PROTEIN

PLASMA PROTEIN- DRUG BINDING

◦ α1 globulin bind to a number of steroidal drug cortisone , prednisolone $ thyroxine , cynocobalamine

◦ α2 globulin

◦ (ceruloplasmin ) bind to Vit. A D E K

◦ β1-globulin

◦ (transferrin ) bind to ferrous ion

◦ β2-globulin

◦ bind to carotinoid

γ- globulin

bind to antigen

majority of drug bind to extravascular tissue- the order of binding -: liver > kidney > lung > muscle

liver – epoxide of number of halogenated hydrocorban ,paracetamol

lung – basic drug imipramine , chlorpramazine , antihistaminis ,

kidney – metallothionin bind to heavy metal , lead, Hg , Cd ,

skin – chloroquine $ phenothiazine eye - chloroquine $ phenothiazine Hairs- arsenicals , chloroquine, $ PTZ bind to hair

shaft . Bone – tetracycline Fats – thiopental , pesticide- DDT

Factor affecting drug protein binding 1. factor relating to the druga) Physicochemical characteristic of drugb) Concentration of drug in the bodyc) Affinity of drug for a particular component 2. factor relating to the protein and other binding

componenta) Physicochemical characteristic of the protein or

binding componentb) Concentration of protein or binding componentc) Num. Of binding site on the binding site 3. drug interaction 4. patient related factor

Physicochemical characteristics of drug

Protein binding is directly related to lipophilicity

lipophilicity = the extent of binding

e.g. The slow absorption of cloxacilin in compression to ampicillin after i.m. Injection is attributes to its higher lipophilicity it binding 95% letter binding 20% to protein

Highly lipophilic thiopental tend to localized in adipose tissue .

Anionic or acidic drug like . Penicillin , sulfonamide bind more to HSA

Cationic or basic drug like . Imepramine alprenololbind to AAG

Physicochemical property of protein / binding component – lipoprotein or adipose tissue tend to bind lipophilic drug by dissolving them to lipid core .

The physiological pH determine the presence of anionic or cationic group on the albumin molecule to bind a variety of drug

Concentration of protein / binding component

Mostly all drug bind to albumin b/c it present a higher concentration than other protein

number of binding sites on the protein

Albumin has a large number of binding site as compare to other protein and is a high capacity binding component

*Tissue binding , apparent volume of distribution and drug storage

A drug that bind to blood component remains confined to blood have small volume of distribution.

Drug that show extra-vascular tissue binding have large volume of distribution .

the relationship b/w tissue drug binding and apparent volume of distribution-

Vd = amount of drug in the body = X

plasma drug concentration C

the amount of drug in the body X = Vd . C

SIMILAR , amount of drug in plasma = Vp . S

Amount of drug in extravascular tissue = Vt .Ct

The total amount of drug in the body

Vd . C = Vp.C+Vt. Ct

where , Vp is volume of plasma

Vt is volume of extravascular tissue

Ct is tissue drug concentration

Vd = Vp + Vt Ct/C ………………….(1)

Dividing both side by C in above equation

The fraction of unbound drug in plasma (fu)

The fraction unbound drug in tissue (fut)

fut = Cut

Ct

Assuming that equilibrium unbound or free drug conc. In plasma and tissue is equal

C t = fu

C fut

mean Cu = Cut then ,

Vd = Vp + Vt . fu

fut

substituting the above value in equa. 1

It is clear that greater the unbound or free concentration of drug in plasma larger its Vd

The kinetics of reversible drug–protein binding for a protein with one simple binding site can be described by the law of mass action, as follows:

or ………………1The law of mass action, an association constant, K a, can be expressed as the ratio of the molar concentration of the products and the molar concentration of the reactants. This equation assumes only one-binding site per protein molecule

……………………….…2

Experimentally, both the free drug [D] and the protein-bound drug [PD], as well as the total protein concentration [P] + [PD], may be determined. To study the binding behavior of drugs, a determinable ratio (r )is defined, as follows

moles of drug bound is [PD] and the total moles of protein is [P] + [PD], this equation becomes

Substituting the value of PD from equa.

This equation describes the simplest situation, in which 1 mole of drug binds to 1 mole of protein in a 1:1 complex. This case assumes only one independent binding site for each molecule of drug. If there are n identical independent binding sites per protein molecule, then the following is used:

*

Protein molecules are quite large compared to drug molecules and may contain more than one type of binding site for the drug. If there is more than one type of binding site and the drug binds independently on each binding site with its own association constant, then Equation 6 expands to

The values for the association constants and the number of binding sites are obtained by various graphic methods.

1. Direct plot

It is made by plotting r vresus (D)

2. Double reciprocal plot

The reciprocal of Equation 6 gives the following equation

A graph of 1/r versus 1/[D] is called a double reciprocal plot. The y intercept is 1/n and the slope is 1/nKa . From this graph , the number of binding sites may be determined from the yintercept, and the association constant may be determined from the slope, if the value for n is known.

3. Scatchard plot

The Scatchard plot spreads the data to give a better line for the estimation of the binding constants and binding sites

r = n Kas – r Kas[DF]

*Drug clearance is a pharmacokinetic term for describing drug elimination from the body without identifying the mechanism of the process.

*instead of describing the drug elimination rate in terms of amount of drug removed per unit time (eg,mg/min).

*Drug clearance is defined as the fixed volume of fluid (containing the drug) cleared of drug per unit time

*the units for the clearance are volume /time (ml/min,l/hr).

*for example ,if the clt of penicillin is 15mL/min in a patient and penicillin has a VD of 12 L, then from the clearance definition.15 ml of the 12 L will be cleared of drug per minute.

* Alternatively,CLT may be defined as the rate of drug elimination divided by the plasma drug concentration.

CLT= elimination rate plasma concentration(CP)

CLT=dDE/dt =mL/minCP

Elimination rate=dDE = CpClTdt

• Just as the elimination rate constant (k) represents the sum total of all the rate constants for drug elimination, including excretion and biotransformation,CLT is the sum total of all the clearance processes in the body,including clearance through the kidney (renal clearance), lung,and liver (hepatic clearance).

Rather than describing in terms of amount of drug removed per unit time, Clearance is described as volume of plasma cleared of drug per unit time (volume/time)

10 Litres

L/hr

100 mg/L1000 mg Drug

Simplest case- a beaker…

DRUG

URINE

ke

Metabolites

km

KIDNEY

Bile

kbile

LIVER

BODY

IV

Vd

But the body’s not a beaker- multiple systems involved…..

The calculation of clearance from k and VD assumes (sometimes incorrectly) a defined model, whereas clearance estimated directly from the plasma drug concentration time curve does not assume any model.

Physiologic/Organ Clearance:

Clearance may be calculated for any oxygen involved in the irreversible removal of drug from the body. Many organs in the body have the capacity for drug elimination, including drug excretion and biotransformation.

Rate of elimination by an organ =

Rate of presentation organ input – rate of extraction organ output

= Q.C in-Q.C out

Clearance= Q(ER)

If the drug concentration in the blood (Ca) entering is greater than the drug concentration of blood (Cv) leaving the organ, then some of the drug has been extracted by the organ. The ER is Ca-Cv divided by the entering drug concentration (Ca).

ER=Ca-Cv

Ca

Hepatic clearance:CLH= = rate of eliminated by liver

plasma concentration(C)

CLH =CLT-CLR

• Clearance is commonly used to describe first-order drug elimination from compartment models such as the one-compartment model.

• Model-independent methods are non compartment model approaches used to calculate certain pharmacokinetic parameters such as clearance and bioavailability (F).The major advantage of model-independent methods is that no assumption for a specific compartment model is required to analyze the data. Moreover, the volume of distribution and the elimination rate constant need not be determined

* Compartment model:

static volume and first-order elimination is assumed. Plasma flow is not considered. Clt=k Vd.

• Physiologic model:

clearance is the product of the plasma flow (Q) and the extraction ratio (ER).Thus CLt=Q ER

• Model independent:

volume and elimination rate constant not defined.

Renal clearance, Clr, is defined as the volume of plasma that is cleared of drug per unit of time through the kidney. Similarly, renal clearance may be defined as a constant fraction of the Vd in which the drug is contained that is excreted by the kidney per unit of time. More simply, renal clearance is defined as the urinary drug excretion rate (dDu/dt) divided by the plasma drug concentration (Cp).

CLr= rate of eliminated by kidney

plasma concentration(c)

= dDu/dt

Cp

* Rate of drug passing through kidney = rate of drug excreted

Clr Cp= Qu Cu

mL/min mg/mL = mL/min mg/mL

Comparison of drug excretion methods

renal clearance may be measured without regard to the physiologic mechanisms involved in this process. From a physiologic viewpoint, however, renal clearance may be considered as the ratio of the sum of the glomerular filtration and active secretion rates less the reabsorption rate divided by the plasma drug concentration:

clr=filtration rate+ secretion rate – reabsorptionrate

cp

* the actual renal clearance of a drug is not generally obtained by direct measurement .

Filtration only

If glomerular filtration is the sole process for drug excreationand no drug is reabsorbed. Then tha amount of drug filtered at any time (t) will always be cp*GFR likewise if the clr of the drug is by glomerular filtration only as in the case of inulin then clr=GFR otherwise clr represents all the processes by which the drug is cleared through the kidney, including any combination of filtration, reabsorption, and active secretion.

dDu=ke VD Cp (compartment) (6.24)

dt

dDu=Clr Cp (physiologic) (6.25)

dt

From above eqns:

Ke VD CP =CLR CP

KE=CLR

VD (6.26)

FILTRATION AND REABSORPTION:

for a drug with a reabsorption fraction of fr, the drug excretion rate is reduced and equation6.25 is restarted as equation 6.27:

dDu=clr(1-fr)cp (6.27)

dt

equating the right sides of equations 6.27 and 6.24 indicates that the first-order rate constant (ke) in the compartment model is equivalent to clr(1-fr)/vd.in this case, the excretion. Rate constant is affected by the reabsorption fraction (fr) and the GFR because these two parameters generally remain constant the general adoption of a first-order elimination process to describe renal drug excretion is a reasonable approach.

Filtration and active secretion

for a drug that is primarily filtered and secreted, with negligible reabsorption, the overall excretion rate will exceed GFR at low drug plasma concentrations, active secretion is not saturated and the drug is excreted by filtration and active secretion at high concentrations the percentage of drug excreted by active secretion decreases due to saturation.

Clearance decreases because excretion rate decreases .

Clearance decreases because the total excretion rate of the drug increases to the point where it is approximately equal to the filtration rate.

Model-Independent Methods

*Clearance rates may also be estimated by a single (nongraphical) calculation from knowledge of the (AUG) 0 to infinite, the total amount of drug absorbed,FD0, and the total amount of drug excreted in the urine, D u to infinite. For example, if a single IV bolus drug injection is given to a patient and the (AUG) 0 to infinite is obtained from the plasma drug level-time curve, then total body clearance is estimated by

Clt= Do

[AUG] 0 to infinite

*If the total amount of drug excreted in the urine D u to infinite, has been obtained, then renal clearance is calculated by

Clt= D 0 to infinite

[AUG] 0 to infinite

*Clearance can also be calculated from fitted parameters. If the volume of distribution and elimination constants are known, body clearance (Clt), renal clearance (Clr), and hepatic clearance (Clh) can be calculated according to the following expressions:

Clt = kVd (6.35)

Clr = keVd (6.36)

Clh = kmVd 6.37)

*Total body clearance (Clt) is equal to the sum of renal clearance and hepatic clearance and Is based on the concept that the entire body acts as a drug-eliminating system.

Clt = Clr+Clh (6.38)

• By substitution of equations 6.35 and 6.36 into equation 6.38,

kVd = keVd+kmVd

Dividing by Vd on both sides of equation 6.39,

k = ke+km

Total body clearance :

it is estimated of dividing the rate of elimination by each organ with concentration of drug present.

CLT = CLH+ CLR+CLBILE +...

Protein-bound drugs

*Protein-bound drugs are not eliminated by glomerular filtration. The bound drugs are usually excreted by active secretion, following capacity-limited kinetics. The determination of clearance that separates the two component results in a hybrid clearance.

*There is no simple way to overcome this problem.

*Clearance values for a Protein-bound drug is therefore calculated with the following equation:

Clr= rate of unbound drug excretion

conc of unbound drug in the plasma

*plasma protein binding has very little effect on the renal clearance of actively secreted drugs such as penicillin.

*drugs and their metabolites may also be excreted by routes other than the renal route ,called as the extra renal or non renal routes of drug excretion .

* Biliary excretion

* pulmonary excretion

* salivary excretion

* Mammary excretion

* Skin/dermal excretion

* Gastrointestinal excretion

* Genital excretion

*Biliary excretion:

The ability of liver to excrete the drug in the bile is expressed by biliary clearance.

biliary clearance =biliary clearance rate

plasma drug concentration

= bile flow*biliary drug clearance

plasma drug concentration

*pulmonary excretion:

Gaseous & volatile substances such as the general anaesthetics(e.g. halothane) are absorbed through the lungs by simple diffusion.

*salivary excretion:

Excretion of drugs in saliva is also a passive diffusion process & therefore predictable on the basis of pH-partition hypothesis.

*Mammary excretion:

*Excretion of a drug in milk is a important since it can gain entry into the breast – feeding infant.

* Excretion of drugs in milk is a passive process & is dependent upon pH-partition behaviour, molecular weight, lipid solubility & degree of ionisation.

*Skin/dermal excretion :

*Drugs excreted through the skin via sweat also follow pH-partition hypothesis.

*compounds such as benzoic acid , salicylic acid , alcohol & antipyrine & heavy metals like lead , mercury & arsenic are excreted in sweat.

*Gastrointestinal excretion:

* Excretion of drugs into the GIT usually occurs after parenteral administration when the concentration gradient for passive diffusion is favourable.

* The process is reverse of GI absorption of drugs.

*Genital excretion :

* reproductive tract & genital secretions may contain the excreted drugs. Some drugs have been detected in semen.

* Drugs can also get excreted via the lachrymal fluid.

Relationship of clearance to elimination half life & volume of distribution

CLT = KVd &

Therefore,by substitution,

CLT = 0.693 *Vd/ t½

t½ = 0.693 *Vd / CLT

• t½ inversely related to CLT

• t½ also dependent on volume of distribution.• K and t½ are dependent on both CLT &V

K = 0.693/ t½

REFERENCES

* Leon shargel , Susanna WU-Pong , Andrew B.C. YU , Applied Biopharmaceutics & Pharmacokinetics

* V Venkateswarulu Biopharmaceutics & Pharmacokinetics Second Edition

*Brahmankar, D.M., Jaiswal, S.B., Biopharmaceutics & Pharmacokinetics