Plasma Drug Protein Binding

8/6/2019 Plasma Drug Protein Binding

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DistributionVolume = Amount (dose) / Concentration

Gave 60mg V = 60mg/1 mg/L

V = 60 L

6 mg

54mg

60 mg

Looks like your 60mgwas dissolved into 60Linstead of 6L due tounequal partitioning.

This is Apparent Vd!!

6 L

Concentration = 1 mg/L


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As a first approximation, the body behaves like a

well-stirred beaker, i.e., chemicals are dispersed

throughout the container (body) rather quickly.

MAJOR CONCEPT #1CONCEPT OF VOLUME OF DISTRIBUTION

(VD) OF DRUGS

(Stir)


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CONCEPT OF VOLUME OF DISTRIBUTION OF DRUGS:

DEFINITION OF VD

Add DRUG

to Beaker

Calculate Volume

Obtain Sample

Assay for [Drug]

(Stir)

[Drug] = Amount Added z Volume of Beaker

Volume of Beaker = Amount Added z [Drug]


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DEFINITION OF VD

Dose Body

with DRUG

Calculate Volume

(This volume is called VD)

Obtain Plasma Sample

Assay for [D]P

By DEFINITION: VD = A/[D]P(where A is amount of drug in body and [D]P is concentration of drug in plasma)


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DEFINITION OF VD

WARNING: VD is a calculated value that

should not be taken literally as

representing somereal volume!!!!!!

VD is:

1. a calculated value,2. a reproducible value,

3. a clinically useful value.

VD is not a real volume with

an independent existence. In Thisregard, the word volume is

used in a metaphorical sense.


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DISTRIBUTION INTO BODY

COMPARTMENTS

Plasma 3.5 litres, heparin, plasma expanders

Extracellular fluid 14 litres,tubocurarine, charged polar compounds

Total body water 40 litres,


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INTRODUCTION TO VD

B

LD = VD x [D]P(target)

KEY EQUATION #1


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INTRODUCTION TO VD

VD and [D]P(target) and B are THE determinants of loading

dose (LD)!!

In other words, the amount of drug that must be given to

achieve rapidly a target concentration

of drug in the plasma is solely determined by VD, B

and [D]P(target).

LD = (VD x [D]P(target))/B


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DETERMINANTS OF VD

Distribution into Body Compartments

Restriction of Drug to

Limited Areas of Body Free Assess of Drug to

Many Areas of Body

vs Large VDSmall VD


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DETERMINANTS OF VD

Tissue Binding

qq [D]P

oo VD

A

qq [D]P

=


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DETERMINANTS OF VD

Plasma Protein Binding

oo[D]P

qqVDoo[D]P

=

A


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DETERMINANTS OF VD

Distribution into Fat

qq [D]P

oo VDqq [D]P

=

A


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OBTAINING VD

Time (hrs)

[D]P0 is [D]P at time 0 and

is obtained by extrapolationVD is usually easy to

obtain!

1. Give bolus

of drug.

2. Measure plasma

levels over time.

3. Extrapolate to find

plasma level at time 0.

VD = Amount in body at time 0/[D]p0 = DoseIV /[D]P

0


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EXAMPLE OF USING VD TO CALCULATE LD

Pharmacokinetic

Parameters forDigoxin:

[D]P(target) = 1.5 g/L

VD = 580 L

Oral Bioavailability = 0.7

LD = (VD x [D]P(target))/B

Oral LD = (580 L x 1.5 g/L) /0.7

Oral LD = 1243 g ~ 1.2mg

Calculation of Oral LD

For Digoxin:


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Plasma drug proteinbinding


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Protein Interactions

affinity

binding

specificity


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Biological relevanceofdrugbinding

Thebindingofdrugto plasma (and tissue)

proteinsis a majordeterminantofdrug

disposition (distribution)

Bindinghas a veryimportanteffecton

drug dynamicssinceonlythefree(unbound) druginteracts withreceptors


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Relevanceofplasma and tissueproteinbinding

From a biological pointofview YES

From a clinical pointofview NO

problemofdruginteraction and

displacementhasbeenoverestimated


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The problemofdruginteraction

and displacementh

asbeenoverestimated


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Th

e classical example:Phenylbutazone/warfarininteraction

Interaction actually exists

Displacement actually exists

but the plasma binding displacement is

not the underlying mechanism of

interactionPBZ stereoselectivity inhibits the metabolism

of s-warfarin


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Why plasma bindingseldom

has clinical relevance

Because few drugs (so-called displacer) are

therapeutically used

Because when displacement exists, it has no

consequence on the receptor exposure to thefree fraction of the displaced drug which

generally remains unaffected


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Isthereoften displacementof

drugfromthebindingsite?

No

For a substantial displacement to take place, the

displacer must occupy most of the availablebinding site thereby lowering the binding siteavailable to the primary drug


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No

To take place, the molar concentration of the

drug in plasma must exceed the molar

concentration of albumin (150 g/mL for a a

drug with a MW of 250)

e.g.: PBZ, phenytoin, valproic acid

This is not true for a1-glycoprotein acid (basic

drug)

Isthereoften displacementof

drugfromthebindingsite?


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Why plasma protein displacement

seldomhas clinical relevance

Generally only the free (unbound) drug is

metabolized and can access to the receptorAND

the free drug concentration is controlled by the

free drug clearance which is independent of the

plasma binding


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Plasma drug proteinbindingPhysiological aspects


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Plasma binding proteins

Proteins MW Concentration

g/L mM

Albumin 67 000 35-50 500-700

E-glycoprotein 42 000 0.4-1.0 9-23acid

Lipoproteins 200 000 variableto2.4 106

Transcortin 53 000 0.03-0.07 0.6-1.4


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Thefreefraction


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Drug plasma proteinbinding

Expressed in % or by fu (free fraction)

>90% = highly bound


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Thefreefraction:fu

fu = =free concentration

total concentration

Cfree

Ctot

Definition:


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Thetotal concentration


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Ctot is a functionofCfree

Ctot = Cfree +Cbind

Ctot = Cfree +BmaxxCfree

Kd +Cfree

Dependentvariable Parameters Independentvariable

controlled byClfree


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Total concentration

Ctot

=

! When conceptualizing dependency and

functionality,thisequationshould notberearranged

Cfree

fu

!


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Total concentration:

a convenientbutillicitrearrangementwhich canbemisleading when

discussing druginteraction

Cfree = fuxCtotal

indirectlyestimated

knownfrominvitro assay

measured byanalytical technique


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Whatisthe consequenceoffuo

Ctotq = or Cfree o = fuo xCtotal

Total concentration:

a convenientbutillicitrearrangement

which canbemisleading when

discussing druginteraction

Cfree

fuo

NOYES

Displacement (fu)modifiesCtot,notCfree


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Caseforwhich weneed toknow in vivo free concentration


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Caseforwhich weneed to

know in vivo free concentration

Formechanistic purposes

Fordata analysis whenbindingto

proteinisnot linear

e.g.: cortisol,ACEI


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Forextrapolation

from in vitro to in vivo in vitro, Kd (binding) and EC50 (functional response) arefree concentrations but EC50 (for PK/PD) is total

concentration

CMI (free) vs effective plasma concentration (Ctot)

between species

comparison of EC50 between animals requires to take into

account free fraction

Caseforwhich weneed to know

in vivo freefraction


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RESTRICTIVE VS NONRESTRICTIVE

ELIMINATIONRESTRICTIVE:

ClearanceDEPENDSon Protein Binding

KIDNEY: Drug Filtration Rate = fU y GFRLIVER: CL = fU y Clint

NONRESTRICTIVE:

Clearance INDEPENDENTof Protein Binding

KIDNEY: CL = Q (renal blood flow)

LIVER: CL = Q (hepatic blood flow)


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Competitiveinteraction

Caseofrestrictivelyeliminated drug

Clfree = Clint = constant Cltot = fuxClint

perfusionrate: K0

redistribution

Ctot

CfreeClfree = cst

Cltot

Administrationofthe2nd ligand, displacement fuo


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in vitrovs.

in vivosituation


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fuvsCfree:in vitro situation

fu = 0.5

Cfree = 3/V

Ctot = 6/V

fu = 0.83

Cfree = 5/V

Ctot = 6/V

1 2

6

5

4

3

1 2

6

5

4

3

displacer

displacee

V= volumeof

thebaker


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fuvsCfree:in vivo situation:

initial steadystate

1

2

6

5

4

3

Infusion=A

A=MT-1

K12C

free

K21 Cfree

Plasma Extracellularfluid Intracellularfluid

Elimination = K10 xCfree (3) = Aequated byinfusion

TOTALCONCENTRATION = 6/V

FREECONCENTRATION = 3/V


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fuvsCfree:in vivo situation:just afteradministrationofdisplacer

1

2

65

4

3

Infusion=A

A=MT-1

K12

xCfree: increase

transitively

K21 xCfree


K10 xCfree (5) > A

displacer

Just displaced

free drug IncreasetransitorilyTOTALCONCENTRATION = 6/V



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fuvsCfree:in vivo situation:

final steadystate

1

2

63

Infusion=A

A=MT-1

K12

xCfree

K21 xCfree


Elimination = K10 xCfree (3) = A

displacer

TOTALCONCENTRATION = 4/V



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Competitiveinteraction

wheninteractionoccurs,

Ctot is altered notCfree


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Thethreemainexceptionstothe

general ruleforwhich druginteractionhasno clinical meaning

1. Rapid bolus IVinjection

2. Parenteral administrationofdisplaced drug

with a highextractionratio

3. Therapeutic drugmonitoring and drug

displacementfromthe plasma bindingsite


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Caseforwhich druginteraction

atthebindingsiteisrelevant

1. Rapid IVinjection

Ifthe displacing agentisgivenrapidly (IVbolus),theCfree could increase

dramatically duetorapid displacementof

the displaced drugbeforethe

compensatorymechanism (redistribution)takes place

p Sulfamide and bilirubin kernicterus


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Caseforwhich druginteraction at

the plasma bindingsiteisrelevant

2. Parenteral administrationofdisplaced

drug with a highextractionratio

currently,noexample


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displacementfrom plasma binding Therapeutic drugmonitoringis performed for

drugs with a narrow concentrationrange

betweentherapeutic and toxic effect

Monitoringis carried outontotal plasma

concentrations




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displacementfrom plasma binding Anexample:

Phenytoin alone: Ctot = 20 g/mL

Phenytoin+Valproic acid: Ctot = 15 g/mL

no dosage adjustmentisnecessarybecause

Ctot decreased butnotCfree duetofuincrease


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Algorithmfordetermining clinical significance

ofpotential binding displacementinteraction

Is drugofinterest >90%

proteinbound?

Yes

Doesthe drughave a

narrow therapeutic index?

Yes

Whatisthehepatic extraction

ratioofthe drug?

High

Isthe druggiven IV?

Clinicallysignificantinteraction likely.

Perform a clinical studytoquantifyeffects

Clinicallysignificant

interactionnot likely

Would a transientincrease

infree drug concentration

be clinicallyrelevant?

no

no

low

no

no

Yes

Yes

Roslan 1994, B.J.Clin Pharmacol. 37, 125


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Age groups V d (L/kg) t (hr)

Newborns

Infants

Children

Adults

Elderly people

0.47

0.36

0.20

0.22

0.26

136

54

51

63

98

Sulfamethoxypyridazine; Apparent volume of distribution and Half-Lives.

WHY BE CONCERNED ABOUT

VARIABILITY ?


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Premature

Newborn

4 mos

12 mos

24 mos

36 mos

Adult

Percentage of Total Body WeightPercentage of Total Body Weight

Extracellular WaterExtracellular Water Intracellular WaterIntracellular Water ProteinProtein FatFat OtherOther


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WHY BE CONCERNED ABOUT

VARIABILITY ?

Digoxin doses

For infants; 15-20g/kg

For children; 10-15g/kg

For adults; 4 - 5 g/kg

How Drugs Distribute: Determines dose


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The requirement for larger doses in children than

adults is related in part to the fact that TBW andECW make up larger percentage of the total

body weight in children than adults

Total body water Extra cellular water

78% 45%(N.b)

60% 20%(Adults)


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Drug Free fraction Volume (L/Kg)

Neonates Adults Neonates Adults

Digoxin

PhenobarbitolPhenytoin

Sulfamethoxy

- pyrazine

0.8

0.680.2

0.43

0.7

0.530.1

0.38

5 -10

1.01.3

0.47

7

0.550.63

0.24

Proteinbinding (fractionfreein plasma),Apparent volumeof

Distribution


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Pregnancy

The volume available for drug distribution

increases due to pregnancy, with the growth of

uterus,placenta,and fetus.

Maternal plasma volume and ECF volume also

increase.

The concentration of plasma proteins to fallduring pregnancy, leads to reduced binding.

Glomerular filtration rate increases


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BODY WEIGHT AND SIZE

The apparent volume of distribution of a drug is determined by theanatomic space into which it distributes and its relative degree of

vascular and extra vascular binding.

The TBW and ECF is directly proportional to body weight.

Weight adjustments are generally thought unnecessary unless theweight of an individual differs by more than 50% from average

adult weight(70kg).


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OBESITY

IBW (men)= 501kg/2.5 cmabove or below in 150 cm inheight.

IBW (women) =451kg/2.5cm above or below in 150cmin height.

Percent fat =

90-2( Height Girth )

Lean body mass =(100- Percent fat )x Weight

*Girth is measuredininchesusing theumbilicallevelatexhalation.


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There is no need of modification of dosageregimen for obese persons if the Vd ,Cl,t values

are similar to that normal adults.

The dose calculation for children is based on

surface area (weight & height).


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Total BodyWater

Individual variability

= F(leanbodymass) 55 - 60% ofbody weightin adultmales

50 - 55% ofbody weightin adultfemale

~42LFora 70 Kgman.


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MajorExtracellularFluid

Compartments (11LofECF)

Plasma (blood minusthered and white

cells)

~3 Lin a 70 Kgman

~4.5% ofbody weight.

Interstitial space (betweenorgan cells)

~8 Lin a 70 Kgman ~11.5% ofbody weight.


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Blood isComposed ofCells

and Plasma.

Hematocrit (Hct). Fractionofblood thatis cells.

Oftenexpressed as percentage.

Plasma volume

= Blood volumex (1-Hct).


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Ingress and Egress

Plasma water

Ingested nutrients passthrough plasma on

wayto cells

Cellularwaste products passthrough plasma

beforeelimination

Interstitial space.

Direct access pointforalmost all cellsofthe

body

Exception -- red and whiteblood cells


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SoluteOverview:

Intracellularvs. Extracellular

Ionic composition very different Total ionic concentration verysimilar

Total osmotic concentrations virtually

identical


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MajorIonic Species

Principlecations

Extracellular: Na+

Intracellular: K+

Principleanions Extracellular: chlorideand bicarbonate.

Intracellular: proteins, aas,and phosphates

inorganic (HPO42-,H2PO4

-)

organic (amino acids and ATP).

Documents

Plasma Drug Protein Binding