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LCRS Year 2 - Pharmacology and Therapeutics > Lecture 4 - Pharmacokinetics > Flashcards

Lecture 4 - Pharmacokinetics Flashcards

1
Q

What are the 6 steps in the journey of a drug through the body?

A 
Administration
Absorption
Distribution
Metabolism
Excretion
Voiding
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2
Q

What does the acronym ADME define?

A 
The 4 steps in the body the drug goes through
Absorption
Distribution
Metabolism
Excretion
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3
Q

What are the two different types of administration regarding the proportion of the body affected?

A

Systemic and Local

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4
Q

What do the terms systemic and local administration mean and what is the difference?

A

Systemic administration - the entire organism is exposed to the drug after administration
Local administration - drug exposure is restricted to one area of the organism

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5
Q

What do the terms “enteral” and “parenteral” mean in terms of administration?

A

Enteral - administration is via the GI tract

Parenteral - administration is via anywhere but the GI tract

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6
Q

What are the 7 different routes of drug administration?

A 
Moving down the body:
Inhalation
Ingestion
Intramuscular
Dermal
Subcutaneous
Intravenous
Intraperitoneal
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7
Q

What is the general aim of any form of administration of drug?

A

To get the drug into the bloodstream

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8
Q

What are the general paths of ingested and inhaled drugs into the bloodstream?

A

Ingestion - Drug enters GI tract, gets absorbed and taken to the liver, travels to the liver via hepatic portal system and then enters systemic circulation.
Inhalation - Lungs are well perfused, drug enters lungs then diffuses across pleural membrane into circulation.

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9
Q

What are the two ways drug molecules move around the body?

A

Bulk flow transfer - moves in bulk in the bloodstream to the tissues
Diffusion transfer - moves molecule by molecule over short distances

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10
Q

What are the two environments drugs have to traverse?

A

Lipid

Aqueous

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11
Q

What are barriers and compartments defined as in the body regarding environments?

A

Lipid environments = barriers

Aqueous environments = compartments

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12
Q

What are the drugs’ methods of crossing barriers inside the body?

A

Diffusing through lipid (if they are lipid-soluble)
Diffusing through aqueous pores in the lipid (if they are polar)
Carrier molecules
Pinocytosis (engulfing of the molecule and taking it in by the cell, not quite the same as endocytosis)

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13
Q

What are the preferable environments of polar and non-polar drugs?

A 
Polar = aqueous environments
Non-polar = lipid environments
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14
Q

What type of equilibrium do most drugs exist in and why is this?

A

Dynamic equilibrium, they exist in the body as both their polar (acid/base form) and their non-polar (neutralised) forms

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15
Q

What environmental features affect the ratio of polar (ionised) and non-polar (non-ionised)?

A

The pH of the environment

The pKa of the molecules

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16
Q

Is aspirin ionised or non-ionised in the stomach and why?

A

Non-ionised
Aspirin is an acidic molecule and has a pKa of 3.4. The stomach has a very low pH, around 1, which is much lower than the pKa of 3.4 thus the majority of aspirin molecules are non-ionised.

17
Q

Is aspirin ionised or non-ionised in the small intestine and why?

A

Ionised

The pH is basic, 7+ in the small intestine and is higher than the pKa of 3.4.

18
Q

Where is aspirin better absorbed and why?

A

Aspirin is much better absorbed in the stomach as a much greater proportion of the molecules exist in an unionised state, due to the low pH, allowing it to easily cross lipid membranes and enter the cells. Aspirin is absorbed in the small intestine but it is very slow due to a higher proportion of its molecules existing in an ionised state, due to the high pH, making it difficult for it to cross lipid membranes.

19
Q

Why is both soluble and enteric-coated aspirin used?

A

Soluble aspirin is absorbed much more quickly since the majority of it is absorbed in the stomach very quickly due to its unionised form majority. e.g. treating headache fast
Enteric-coated aspirin is used to treat longer-term conditions as its coating survives the stomach acid environment and the aspirin is then slowly absorbed over a long period of time in the small intestine. e.g. treating arthritis

20
Q

What is the concept of “ion trapping” and why is it useful?

A

When the drug enters the blood, it exists as both ionised and unionised molecules. The unionised molecules get “mopped up” or bound to proteins in the blood in a protein-drug complex while ionised molecules continue free. This means the drug is effectively “trapped” since it can only enter its target cells when in the free unionised form. This is useful in prolonging a drug’s half-life such as aspirin.

21
Q

What are the four factors influencing drug distribution?

A

Regional blood flow
Extracellular binding (plasma-protein binding)
Capillary permeability
Localisation

22
Q

Why does regional blood flow affect drug distribution?

A

Tissues with high perfusion rates are going to absorb drugs faster. Tissues may increase in perfusion when their activity increases e.g. skeletal muscle during exercise

23
Q

Why does extracellular binding (such as to plasma proteins) affect drug distribution?

A

The body only absorbs free molecules so if the molecule is bound then it cannot be absorbed. The greater the degree of binding, the longer the half-life as it will take longer to absorb all the drug into the cells

24
Q

Why does capillary permeability affect drug distribution?

A

The greater the capillary permeability, the better the drug is absorbed. Small water-filled gaps between endothelial cells lining the vessel allow ionised drug molecules (such as aspirin in blood) to pass through. Distribution can also depend heavily on the capillary architecture (fenestrated, continuous, discontinuous)

25
Q

What is the pH partition hypothesis?

A

Only the un-ionised form of a drug molecule passes through the intestinal membrane.

26
Q

How does localisation in tissues affect drug distribution?

A

Fat isn’t highly perfused so it becomes a very lipophilic environment. Drugs that are lipophilic tend to localise in fatty tissue and fat soluble drugs highly partition into fatty tissue (around 75% partitioned in fat at equilibrium)

27
Q

What are the two major routes of excretion of drugs?

A

Liver

Kidney

28
Q

What is a major way some drugs are excreted in the kidney and what type of drugs are excreted this way?

A

Excretion in the bile

Tends to be large molecular weight drugs (concentrated and excreted by liver)

29
Q

Which of the liver and the kidneys do most of the excretion of xenobiotics?

A

Kidneys

30
Q

How and why is the majority of the drug excreted into the urine, ultrafiltration or active secretion?

A

Active secretion, acids and bases are usually actively secreted into the urine in the proximal convoluted tubule and most drugs are weakly acidic or polar at least thus undergo the same process.

31
Q

What are 3 significant features of the glomerulus, PCT, and PCT/DCT regarding drug excretion/reabsorption?

A

Glomerulus - drug-protein complexes are not filtered.
PCT - Active secretion of acids and bases. Drugs that have these properties are also actively secreted into the urine.
PCT/DCT - Lipid-soluble drugs are reabsorbed!! Lipid-soluble drug molecules may be filtered by the glomerulus or molecules may become unionised by changes in the pH of the urine leading to reabsorption.

32
Q

How does the liver excrete drugs and what mechanism is used to get the drug into the excretion system?

A 
Biliary excretion (excretion in the bile)
Active transport of drugs into the bile, usually transport bile acids and glucuronides but drugs follow because they are non-polar and have a large molecular weight
33
Q

What is enterohepatic cycling and what can it cause?

A

Drugs are excreted into the bile which enters the GI tract. The drug can then be reabsorbed in the gut and returned to the liver via the enterohepatic circulation (gut to liver) and the cycle repeats. This won’t happen forever but it causes “drug persistence” as the drug is taking longer to leave the system. Can be problematic.

34
Q

What are the four pharmacokinetic characteristics and why are they important?

A 
Bioavailability
Apparent Volume of Distribution
Biological Half-Life
Clearance
- Important as they can be used to predict the time-course of drug action
35
Q

What is Bioavailability and what part of the drug action (ADME) is it part of?

A

The amount of drug that gets into the systemic circulation. Post-hepatic concentration e.g. if 1% of the administered drug amount exits the liver and enters systemic circulation = very low bioavailability
Part of ABSORPTION

36
Q

What is the Apparent Volume of Distribution and what part of the drug action (ADME) is it part of?

A

A theoretical volume which describes how far the drug has been distributed by the body. If it has characteristics which will allow it to distribute around the entire body then the AVD is the entire body, or if it’s localised to one or two organs, the AVD is those two organs.
Part of DISTRIBUTION

37
Q

What is the Biological Half-Life and what part of the drug action (ADME) is it part of?

A

The half-life of a drug in its active form in the body. (the time taken for the concentration of the drug to fall to half its original value) The half-life is important as it must be appropriate to the condition it’s treating.
Part of METABOLISM/EXCRETION

38
Q

What is Clearance (regarding excretion of drugs) and what part of the drug action (ADME) is it part of?

A

How fast the drug is effectively removed from the body (the volume of plasma cleared of a drug per unit time).
Part of EXCRETION

LCRS Year 2 - Pharmacology and Therapeutics (14 decks)

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