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Yr 2 LCRS - Pharm > 4 - Pharmacokinetics > Flashcards

4 - Pharmacokinetics Flashcards

1
Q

What is Pharmacokinetics?

A

The branch of pharmacology concerned with the movement of drugs within the body.

‘What the body does to the drug.’

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2
Q

What is suitable formulation?

A

The need for a drug to be soluble in some form.

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3
Q

How can the journey of a drug through the body be summarised?

A

ADME

  • Absorption
  • Distribution
  • Metabolism
  • Excretion

Starts with Administration and ends with Removal.

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4
Q

What are the different methods of which a drug can be administered?

A
  • Ingestion
  • Inhalation
  • Dermal
  • Intramuscular
  • Intraperitoneal
  • Subcutaneous
  • Intravenous
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5
Q

With regards to drug administration, what do the words ‘Systemic’, ‘Local’, ‘Enteral’ and ‘Parenteral’ mean?

A

Systemic - to the entire organism
Local - restricted to one area

Enteral - Gastro-Intestinal admin
Parenteral - Outside the GI tract

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6
Q

What is the main method of drug excretion?

A

Kidneys filter out the drug into the urine.

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7
Q

In what situations would you use parenteral administration instead of enteral?

A

When the drug has a very short half life, so it needs to be absorbed very quickly.

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8
Q

How would you describe the following medications in terms of their administration:

  • Ventolin
  • Aspirin
  • Betnovate (Steroid Cream)
  • Cannabis
  • Antacid
  • Nicotine
A

Ventolin
- systemic and local

Aspirin
- systemic

Betnovate
- mainly local (but skin also has capillaries so a little bit systemic)

Cannabis
- Systemic (acts of neurological tissue so it needs to get into the blood)

Antacid
- Local because only in the GI tract

Nicotine
- Systemic (applied locally but it can react with many receptors)

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9
Q

How do drugs get into the systemic circulation (get absorbed)?

A

Either:

Bulk Flow Transfer
- whole dose in bulk into the blood, in the bloodstream

Diffusional Transfer
- molecule by molecule over short distances

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10
Q

Why is the solubility of drugs important to consider?

A

This is because they have to traverse both aqueous and lipid environments.

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11
Q

What is the difference between body compartments and barriers?

A

Body compartments are aqueous
- e.g. blood, lymph, EC fluid, IC fluid

Barriers are lipid
- e.g. cell membranes (epithelium/endothelium)

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12
Q

Describe the absorption and movement of aspirin in the body.

A 
Oral tablet
GI tract
Stomach
Small Intestine (Absorbed)
- needs to transverse both aqueous and lipid environments
- travels by bulk flow transfer
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13
Q

Describe the absorption and movement of an IV administered drug in the body.

A

Material straight into blood
Cuts out a lot of absorption
Diffusional and Bulk Flow transfer

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14
Q

Name examples of a polar and a non-polar solvent

A 
Non-polar = Benzene
Polar = Water
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15
Q

What kind of substances can easily penetrate lipid membranes?

A

Non-polar substances
(they easily dissolve in non-polar solvents too)

Polar drugs struggle to get into a lipid environment.

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16
Q

What form do drugs naturally exist in?

A

Most drugs are either weak acids or weak bases

Therefore drugs exist in ionised (polar) and non-ionised (non-polar) forms – the ratio depends on the pH

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17
Q

What happens to drugs, based on the pH of their environment, to help with absorption?

A

Drugs are rapidly becoming ionised or non-ionised in a dynamic equilibrium based on the pH of the environment.

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18
Q

Explain the pH Partition Hypothesis

A

Brodie et al. (Shore, et al. 1957) proposed the pH - partition theory to explain the influence of GI pH and drug pKa on the extent of drug transfer or drug absorption. Brodie reasoned that when a drug is ionized it will not be able to get through the lipid membrane, but only when it is non ionized and therefore has a higher lipid solubility.

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19
Q

Explain Aspirin absorption with reference to the ‘pH Partition Hypothesis’

A

Aspirin is a weak acid (-COOH)

  • in the stomach, it is mainly unionised (-COOH) but some is ionised (-COO-ve)
  • in the s. intestine, it is mainly ionised
  • therefore, most aspirin is absorbed in the stomach
  • this is because mainly only the unionised form can be absorbed through the lipid membranes
  • a very little bit of the ionised form can be absorbed in the stomach through water-filled junctions
20
Q

What are the different forms of aspirin and in which situation would you administer each one?

A

SOLUBLE ASPIRIN

  • if you want fast absorption (e.g. hangover)
  • quick absorption in stomach
  • fast effect

COATED ASPIRIN

  • if you want slow absorption (e.g. arthritis)
  • absorbed in small intestine
  • long-lasting effects
21
Q

What is ion trapping in pharmacology?

A

At a steady state, an acidic drug will accumulate on the more basic side of a membrane and vice versa.

A drug can be trapped within protein complexes in its ionised form. This prolongs the half-life of the drug.

22
Q

What overall factors affect drug distribution in the body?

A
  • Regional blood flow
  • Extracellular binding (Plasma-protein binding)
  • Capillary permeability (tissue alterations – renal, hepatic, brain/CNS, placental)
  • Localisation in tissues
23
Q

What does plasma-protein binding do to drug half-life?

A

It increases drug half-life because the drug cannot be absorbed whilst bound.

24
Q

What does drug localisation in tissue refer to?

A

It means which tissues a drug tends to accumulate in.

e. g. lipophilic drugs tend to accumulate in lipophilic tissues.
- fatty tissue is poorly perfused
- therefore, very lipophilic drugs take a long time to move out of it

25
Q

Where are drugs with a high molecular weight often excreted?

A

In the bile.

26
Q

What percentage of acidic drugs are plasma-protein bound in the body?

A

50-80%

27
Q

How much of the body’s blood supply goes to fatty tissue?

A

2% of body’s blood supply (but 15% of body’s mass on average)

28
Q

What percentage of very fat soluble drugs are partitioned in fatty tissue at any one time?

A

75% partitioned in fat at equilibrium

29
Q

What are the two main organs responsible for drug excretion?

A

The Liver and The Kidneys.

30
Q

What is often the difference between drugs excreted via the kidneys compared to those excreted via the liver?

A

The Kidneys
- ultimately responsible for the elimination of most drugs

The Liver
- some drugs are concentrated in the bile (usually large molecular weight conjugates)

31
Q

How can drugs be found in the breath of a person?

A

Any fluid that the human body makes can contain drugs they have taken because drugs can partition themselves into many tissues.

32
Q

Explain the mechanism of drug excretion in the kidneys

A
  1. Glomerulus – drug-protein complexes not filtered (only about 20%)
  2. Proximal tubule – active secretion of acids and bases (about 80%)
  3. Proximal and distal tubules – lipid soluble drugs reabsorbed
  • active excretion (the body wants to get rid of the drugs)
33
Q

How can you enhance drug excretion via urine pH?

A

You can manipulate urine pH to enhance the excretion of a drug

e. g. i.v. sodium bicarbonate will increase urine pH
- Increased urine pH ionizes the aspirin
- makes it less lipid soluble and less reabsorbed from the tubule
- increases its rate of excretion

34
Q

Which measurement can be indicative of blood drug level?

A

Amount of drug in a person’s saliva.

35
Q

Explain the mechanism of drug excretion in the liver.

A

Biliary excretion (large molecular weight molecules can concentrate)

Large Mr = >1000-1500

Active transport systems – into bile (bile acids and glucuronides)

36
Q

Other than the two main routes of drug excretion, give some other examples of excretory routes.

A
  • lungs
  • skin
  • GI secretions
  • saliva
  • sweat
  • milk
  • genital secretions

Generally of very little quantitative importance.

37
Q

What is Enterohepatic Cycling?

A
  • Highly perfused tissue
  • Drug absorbed
  • Into the bile
  • Drug/metabolite excreted into gut (via bile)
  • Reabsorbed
  • Taken back to liver and excreted again…

NOT ALL DRUGS DO THIS

38
Q

What is drug persistence?

A

The amount of time the drug can remain in the body.

39
Q

What are the effects of Enterohepatic Cycling on a drug in the body?

A
  • increases drug half-life

- leads to drug persistence

40
Q

Define ‘Bioavailability’

A

The proportion of a drug or other substance which enters the circulation when introduced into the body and so is able to have an active effect.

  • linked to absorption
41
Q

Define ‘Apparent Volume of Distribution’

A

The theoretical volume that would be necessary to contain the total amount of an administered drug at the same concentration that it is observed in the blood plasma/The volume in which a drug appears to be distributed.

  • e.g. if the physiochemical structure allowed a drug to be distributed everywhere, then it’s apparent volume=amount administered.
  • linked to distribution
42
Q

Define ‘Biological Half-Life’

A

Time taken for the concentration of drug (in blood/plasma) to fall to half its original value

  • linked to metabolism/excretion
43
Q

Define ‘Clearance’

A

Blood (plasma) clearance is the volume of blood (plasma) cleared of a drug (i.e. from which the drug is completely removed) in a unit time.

(Related to volume of distribution and the rate at which the drug is eliminated. If clearance involves several processes, then total clearance is the sum of these processes.)

44
Q

With reference to drug-elimination kinetics, explain the pattern by which most drugs are lost from the body?

A

Exponential loss of the drug (most drugs)
- first order kinetics

Some drugs (e.g. ethanol) are lost by the same amount  at each time interval 
- zero order kinetics

LOG SCALE AND LINEAR

45
Q

Why are ionised drugs less likely to cross through a lipid membrane than protein-bound drugs?

A

This is because protein-bound drugs are in dynamic equilibrium and so are always dissociating.

46
Q

Explain First Order Kinetics

A

First order kinetics (most drugs)

Amount of drug decreases at a rate that is proportional to the concentration of drug remaining in the body.

Cl (Clearance) = Vd x Kel (where Kel = Log2/t0.5 )

47
Q

Explain Zero Order Kinetics

A

Zero order kinetics (some drugs)

Amount of drug decreases at a rate that is independent of the concentration of drug remaining in the body.

Cl (Clearance) = time x Kel (where Kel = δC/δt)

Implies a saturable (usually enzymic) metabolic process.

Yr 2 LCRS - Pharm (44 decks)

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