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PoD > Drug Distribution > Flashcards

Drug Distribution Flashcards

1
Q

Define drug distribution

A

Reversible transfer of a drug between the blood and the tissues of the body (fat, muscle, brain etc)

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2
Q

What factors determine distribution?

A
  • Plasma protein biding
  • Tissue perfusion
  • Transport mechanisms
  • Diseases and other drugs
  • Elimination
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3
Q

What proteins do drugs bind to?

A

Albumin or alpha1-glycoprotein (phenytoin)

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4
Q

Is a drug active when it is unbound or bound to a plasma protein?

A

Unbound

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5
Q

Factors that can increase concentration of unbound drug

A
  • Renal failure
  • Hypoalbuminaemia
  • Pregnancy
  • !!Other drugs!!
  • Saturability of binding
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6
Q

Affects of other drugs on unbound drug concentration?

A

Some drugs can displace drugs from bound state, therefore increasing that drug’s concentration and becoming toxic

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7
Q

Why is protein binding important?

A

Drug must be more than 90% bound and tissue distribution small

i.e. 96% bound, so 4% free for action. If unbound levels change to 6% - then levels of free drug increase by 50%

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8
Q

What is important to know bout a drug to determine therapeutic range?

A
  • Volume of distribution
  • Clearance
  • Half-life
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9
Q

What is the volume of distribution (Vd)?

A

Volume of fluid required for the total amount of drug at a concentration equal to that in plasma - should be 42L

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10
Q

What does a high Vd indicate?

A

More drug in tissues therefore greater ability to diffuse through membrane

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11
Q

What does a 12L Vd indicate?

A

Stays in extracellular fluid but cannot penetrate cells

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12
Q

What does a low Vd indicate?

A

Drug is highly protein bound

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13
Q

What is clearance?

A

Theoretical volume from which a drug is completely removed over a period of time (elimination)

Rate at which the drug is being removed from the body

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14
Q

What does renal clearance depend on?

A

Concentration and urine flow rate

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15
Q

What does hepatic clearance depend on?

A

Metabolism and biliary excretion

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16
Q

What does high and low clearance indicate?

A
  • High: removed from bloodstream rapidly

- Low: removed slowly

17
Q

What is the half life (t1/2) of a drug?

A

The time taken for the drug conc. in blood to decline by half

18
Q

What does half life depend on?

A

Volume of distribution and rate of clearance

T1/2 = 0.693Vd/Cl

19
Q

Affect of clearance on half life and toxicity

A

If Cl is low, then half life is increased which increases toxicity of drug

20
Q

Why is it important to know the half life of a drug?

A

Work out how often a drug needs to be administered to keep levels in therapeutic range

21
Q

Define drug elimination

A

The removal of active drug and metabolite from the body

22
Q

What does drug elimination determine

A

The length of action of the drug

23
Q

What two parts are involved in drug elimination?

A
  • Drug metabolism - liver

- Drug excretion - usually kidneys but also biliary system/gut, lung, milk (pregnancy)

24
Q

What are the primary organs for drug excretion?

A

Kidneys

25
Q

Mechanisms used in kidneys for excretion

A
  • Glomerular filtration
  • Passive tubular reabsorption
  • Active tubular secretion
26
Q

What does renal damage cause?

A

Drug toxicity

27
Q

Importance of glomerular filtration

A

Unbound drugs are filtered as long as size, charge or shape are not large and re-enetered into circulation

Factors affecting the GF rate reduce the clearance of a drug

28
Q

GF and clearance

A

Factors that affect GF rate will reduce the clearance of a drug

29
Q

Describe active tubular secretion

A

Drugs are secreted into the proximal tubule (acid and basic compounds ) - important for cationic and anionic drugs

30
Q

Describe passive tubular reabsorption

A

Drug present in renal tubule is concentrated and passive diffusion allows drug to move back through tubule into circulation - only unionised drugs such as weak acids

Also affected by renal failure

31
Q

Where does passive diffusion occur in tubular reabsorption?

A

Distal tubule and collecting duct

32
Q

What type of drugs are reabsorbed from kidney tubules?

A

Un-ionised drugs such as weak acids

33
Q

Describe biliary secretion

A

Liver secretes bile and drugs may be passively secreted into the bile

34
Q

What is entero-hepatic circulation?

A

Process of reabsorbing drugs from the bile into the circulation

35
Q

What does metabolism of drug in the liver lead to?

A

Conjugation of drug which is not reabsorbed from the intestine - activity terminated and ready for excretion

36
Q

Affect of liver damage

A

Reduces rate of conjugation (excretion) and biliary secretion - this allows build up or reabsorption causing toxicity

PoD (35 decks)

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