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Pharmacokinetics lecture 3 Contents . Compartments Bioavailability Salt factor. Compartments. T3. Compartment 1 Blood + tissues 1 & 2. Blood. T2. T1. Compartment 2 Tissues 3 & 4. T4. Compartments.

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pharmacokinetics lecture 3 contents
Pharmacokinetics lecture 3Contents ...
  • Compartments
  • Bioavailability
  • Salt factor
compartments
Compartments

T3

Compartment 1

Blood + tissues 1 & 2

Blood

T2

T1

Compartment 2

Tissues 3 & 4

T4

compartments1
Compartments
  • The sum of all those tissues into which a drug distributes at approximately the same rate.
single compartment
Single compartment

All tissues penetrated rapidly or not at all.

Combine blood and all penetrated tissues (T1 & T2) as a single compartment.

Drug assumed to spread ‘instantly’ throughout all of this space.

T1

T4

Blood

T2

T3

two compartments
Two compartments

Drug penetrates some tissues rapidly and others slowly.

Combine blood and rapid tissues (T1 & T2) as first compartment. Drug spreads ‘instantly’ through this compart-ment.

Combine slow tissues (T3 & T4) as second compartment.

T1

T4

Blood

T2

T3

bioavailability f
Bioavailability (F)
  • The fraction of a dose that reaches the systemic circulationin a chemically unaltered form.
  • Fractional availability = F
  • Quote as percentage or decimal e.g. 25% or 0.25
  • Has no units.
incomplete oral bioavailability
Incomplete oral bioavailability

2. Chemical, enzymatic

or bacterial attack

4. First pass metabolism

in gut wall or liver

3. Failure of

absorption & Pgp efflux

Liver

1. Failure of disintegration

or dissolution

failure of absorption
Failure of absorption
  • May be due to:
  • Binding to other molecules in the gut contents
  • Too polar to undergo passive diffusion
  • Efflux due to P-glycoproteins
first pass metabolism in the gut wall
First pass metabolism in the gut wall

Intestinal epithelium is rich in drug metabolising enzymes. Main Cyt P450 is CYP3A4

Cytochrome P450 activity in intestinal epithelium relative to liver (%)

Duodenum Ileum Colon

50 30 10 2

Jejunum

slide14

Blood drainage from G.I.T.

General

circulation

Mouth

Stomach

Small

intestine

Large

intestine

Liver

Rectum

General

circulation

salt factor s
Salt factor (S)

The drug administered may not be chemically identical to the drug measured in blood.

e.g. Phyllocontin tablets contain 225mg of aminophylline. But, target plasma conc. normally quoted as a theophylline conc.

Aminophylline contains approx. 80% theophylline by weight. S = 0.80

salt factor s1
Salt factor (S)

Predict initial plasma concentration of theophylline when 500 mg aminophylline injected i.v. into 70 kg adult. (V = 0.48 L/kg)

V = 0.48 L/kg x 70 kg

= 33.6 L

C = D x S = 500 mg x 0.80

V 33.6 L

= 11.9 mg/L

sample calculation
Sample calculation

A drug is administered as a complex containing 75% (by weight) of the parent drug. The volume of distribution of the drug is 1.8 L/kg.

What dose of the complex would need to be administered (i.v) to a 90kg patient in order to achieve an initial plasma concentration of 15µg/L? The dose should be expressed in

units of mg.

model answer
Model answer

V = 1.8L/Kg x 90Kg

= 162 Litres

V = D/C0

D = V x C0

S

= 162L x 15µg/L

0.75

= 3,240 µg

= 3.24 mg

Take account

of use of a salt

terms with which you should be familiar
Terms with which you should be familiar ...

Compartment

Bioavailability

First pass metabolism

Salt factor

what you should be able to do
What you should beable to do
  • Describe 1 and 2 compartment systems
  • Relate differences in blood flow in various tissues to the behaviour of a two compartment system
  • Define ‘Bioavailability’
  • List factors that may limit oral bioavailability
  • Describe how buccal or rectal administration may increase bioavailability
  • Incorporate a salt factor into pharmacokinetic calculations