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Principles of Disease-Megan > Drug Distribution > Flashcards

Drug Distribution Flashcards

1
Q

What must a drug do to be active

A

Leave the blood stream and enter the inter or intracellular spaces

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2
Q

What is drug distribution

A

The Reversible Transfer of a Drug between the Blood and the Extra Vascular Fluids and Tissues of the body (e.g. fat, muscle, and brain tissue)

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3
Q

What factors determine tissue distrubution

A 
Plasma protein binding
Tissue perfusion
Membrane characteristics
Transport mechanisms
Diseases and other drugs (esp renal failure, liver disease, obesity)
Elimination
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4
Q

What type of membrane characteristics are there

A

Blood-brain barrier

Blood-testes/ovary barrier

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5
Q

Give examples of drugs which bind to proteins in plasma

A

Many drugs do this
Albumin
Alpha 1-glycoprotein (e.g. phenytoin)

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6
Q

In what sate are drugs biologically active

A

Unbound

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7
Q

Is plasma protein binding reversible

A

Yes

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8
Q

How can the amount of bound drug be changed

A 
By:
Renal failure
Hypoalbuminaemia
Pregnancy
Other drugs
Saturability of binding
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9
Q

What parameters need to be characterised when considering therapeutic range

A

Volume of distribution
Clearance
Half-life

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10
Q

What is Vd

A

Apparent volume of distribution

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11
Q

What is volume of distribution

A

The volume of plasma that would be necessary to account for the total amount of drug in a patient’s body, if that drug were present throughout the body at the same concentration as found in the plasma (L/kg)

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12
Q

What does it mean if the Vd is high

A

A high ability of the drug to diffuse into and through membranes

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13
Q

What is Cl

A

Clearance

The theoretical volume from which a drug is completely removed over a period of time

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14
Q

How is clearance measured

A

Measure of elimination in units of time (ml/min)

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15
Q

What is renal clearance dependent on

A

Concentration and urine flow rate

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16
Q

What is hepatic clearance dependent on

A

Metabolism and biliary excretion

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17
Q

What is half life (t1/2)

A

The time taken for the drug concentration in the blood to decline to half of the current value

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18
Q

Give an example of drug half life

A

If it takes 4 hours for the concentration of a drug in the blood stream to drop from 10mg/L to 5 mg/L then the half life is 4 hours

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19
Q

What does half life depend on

A

The volume of distribution and rate of clearance

20
Q

What does the prolongation of the half life of a drug do

A

Increase toxicity of a drug because of a:
Reduction in clearance
Large volume of distribution (e.g. amiodarone)

21
Q

What does the formula, t1/2 = 0.693Vd/Cl link together

A

The chemical properties of the drug, i.e. lipophilicity, and the ability of the body to metabolise and excrete the drug

22
Q

How does knowledge of the half life of a drug help us clinically

A

Helps us to work out how often a drug needs to be administered

23
Q

How should drugs be administered to have a therapeutic benefit

A

Depends on the half life
Long half life = less doses required
Short half life = more doses required

24
Q

What can chronic administration cause

A

The plasma levels of a drug to take many doses before they stabilise, usually 4-5 half-lives
This may necessitate a loading dose

25
Q

What is drug elimination

A

The removal of active drug and matabolites from the body

It determines the length of action of the drug

26
Q

What is drug elimination made up of

A

Drug Metabolism

Drug Excretion

27
Q

Where does drug metabolism normally take place

A

Liver

28
Q

Where does drug excretion normally take place

A

Mainly kidney but also biliary system/gut, lung and breast milk

29
Q

What mechanisms are used for drug excretion

A

Glomerular filtration
Passive tubular reabsorption
Active tubular secretion

30
Q

About how many liters of fluid does the glomerulus filter a day

A

190

31
Q

What happens to all unbound drug

A

It’s filtered at the glomerulus as long as their molecular size, charge or shape are not excessively large

32
Q

What effect will the factors that affect the glomerular filtration rate have on the clearance of a drug

A

Reduce the clearance of a drug

33
Q

Which system is important for eliminating protein bound cationic and anionic drugs

A

The active secretion of some drugs into the proximal tubule (acidic and basic compounds)

34
Q

What does the active secretion into the proximal tubule eliminate

A

Protein bound cationic and anionic drugs

35
Q

What does passive diffusion along the concentration gradient allow

A

The drug to move back through the renal tubule into the circulation

36
Q

Describe the features of passive tubular reabsorption

A

Occurs in the distal tubule and collecting duct
Only un-ionised drugs such as weak acids are reabsorbed
Can also be affected by renal failure

37
Q

How much bile is secreted a day

A

1 liter by the liver

38
Q

In what ways can drugs be secreted into bile

A

Passively or actively

39
Q

How much drug elimination does biliary secretion account for

A

5-95%

40
Q

What happens to the drugs after they have been secreted into the bile

A

Many are then reabsorbed from the bile into the circulation

This is called entero-hepatic circulation

41
Q

What is entero-hepatic circulation

A

Drugs being reabsorbed from the bile into the circulation

42
Q

How long does entero-hepatic circulation last for

A

Continues until the drug is metabolised in the liver or excreted by the kidneys

43
Q

What can metabolism of the drug in the liver lead to

A

Conjugation of the drug

44
Q

What happens to conjugated drugs

A

They are not reabsorbed from the intestine

45
Q

What can damage to the liver cause

A

May reduce the rates of conjugation and biliary secretion, so allow the build up or reabsorption of the drug with resultant toxicity

46
Q

Name some drugs which undergo enterohepatic recirculation

A

Chlordecone
Morphine
Phenytoin
Methadone

Principles of Disease-Megan (41 decks)

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