Wk2- Pharmacokinetics (Absorption, Distribution, Metabolism, Excretion) Flashcards by S F

Pharmacokinetics & Pharmacodynamics can be described as either-
a) “What the drug does to the body”
b) “What the body does to the drug”
Which describes Pharmacokinetics, & which describes Pharmacodynamics?

a) “What the drug does to the body”
= Pharmacodynamics
b) “What the body does to the drug”
= Pharmacokinetics

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What are the 4 key components of Pharmacokinetics?

Pharmacokinetics=
1) Absorption
2) Distribution
3) Metabolism
4) Excretion

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One of the 4 key components of Pharmacokinetics is ABSORPTION. What does this mean?

ABSORPTION= the process of moving an unchanged drug from the site of administration to the systemic circulation

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One of the 4 key components of Pharmacokinetics is DISTRIBUTION. What does this mean?

DISTRIBUTION= the process of transferring a drug from the circulation into other compartments of the body (eg. fluid, cells, tissues)

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One of the 4 key components of Pharmacokinetics is METABOLISM. What does this mean?

METABOLISM= the breakdown or transformation of a drug to a form which is able to be easily excreted

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One of the 4 key components of Pharmacokinetics is EXCRETION. What does this mean?

EXCRETION= the process of irreversible loss of a drug from the body

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What are the 3 key stages that Pharmacokinetics determines?
[As in, there are 3 stages of drug action which vary depending on how the drug is absorbed, distributed, metabolised, excreted]

Pharmacokinetics determines-
1) Onset of Action
2) Peak Concentration
3) Duration of Action
[Note- These all vary depending on the pharmacokinetics of drug- ie. how the drug is absorbed, distributed, metabolised, excreted]

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The Pharmacokinetics of a drug, specifically Absorption & Distribution, determines the ‘ONSET OF ACTION’. What does this mean?

Absorption & Distribution, determines the ‘ONSET OF ACTION’.
Onset of Action= period of time until the minimal effective concentration is reached

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The Pharmacokinetics of a drug, specifically Absorption & Distribution, determines the ‘PEAK CONCENTRATION’. What does this mean?

Absorption & Distribution, determines the ‘PEAK CONCENTRATION’.
Peak Concentration= period of time until the highest concentration is reached, & the concentration/intensity of the highest concentration

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The Pharmacokinetics of a drug, specifically Metabolism & Excretion, determines the ‘DURATION OF ACTION’. What does this mean?

Absorption & Distribution, determines the ‘DURATION OF ACTION’.
Duration of Action= the time period from initially reaching minimal effective concentration, to when the drug has been metabolised, excreted, & then concentration levels return to the minimal effective concentration

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Look at this graph comparing onset time, peak plasma concentration reached, & duration of action.
Q- Which route of drug administration does each line represent?

Black= IV administration
Purple= IM administration
Red= SC administration
Blue= Oral administration

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Why is it that drugs administered orally have a longer onset of action time, & lower peak concentration compared to drugs administered intravenously?

Because before entering the systemic circulation, orally administered drugs are first metabolised by the liver via a process known as ‘FIRST PASS METABOLISM’. The proportion of the drug which reaches the circulation unchanged (after going through first pass metabolism) is called the ‘BIOAVAILABILITY’ of the drug.

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A 100mg drug is administered orally. After going through First Pass Metabolism in the liver, 20mg of the drug reaches the circulation unchanged.
Q- What is the BIOAVAILABILITY of this drug?

Bioavailability= the proportion of drug which enters the systemic circulation unchanged and is available to interact at its site of activity.
A- Bioavailability of this drug is 20% [meaning 20% of the 100mg dose is then available in the systemic circulation to elicit its action at its site of activity]

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What 2 factors influences the Bioavailability of a drug?

Factors that influence Bioavailability:
1) Amount of drug absorbed [from the GI tract & into the liver- via portal vein]
2) Amount of drug which escapes first pass metabolism
[Note- Bioavailability= the proportion of drug which enters the systemic circulation unchanged and is available to interact at its site of activity]

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What does the ‘Bioavailability’ of a drug refer to?

Bioavailability= the proportion of drug which enters the systemic circulation unchanged, and is therefore available to interact at its site of activity (eg. receptor, ion channel, enzyme, carrier molecule)

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What is the Bioavailability of drugs administered Intravenously?

Bioavailability of IV drugs= 100%
[100% of the dose administered enters the systemic circulation as the route of administration means it goes directly into the systemic circulation]

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Describe the pathway of an orally administered drug.
[Start from the administration of the drug, & finish at the entering of systemic circulation]

Pathway of ORAL medication=
1. Drug administered orally
2. Drug enters GI tract
3. Drug is absorbed into the LIVER [via the portal vein]
4. Drug moves from LIVER to SYSTEMIC CIRCULATION
5. Drug undergoes remaining pharmacokinetic processes of distribution, metabolism, excretion

List the 6 factors which influence the- RATE & EXTENT to which a drug is absorbed?

6 factors which influence the- RATE & EXTENT to which a drug is absorbed:

1- Route of drug administration
2- Nature of the absorbing surface
3- Blood Flow
4- Solubility of the drug
5- Ionisation
6- Formulation

One of the 6 factors which influences the rate & extent to which a drug is absorbed is the-
NATURE OF THE ABSORBING SURFACE
Qs- 1) What is the absorbing surface?
2) How does it influence drug absorption?

Factor affecting drug absorption- NATURE OF THE ABSORBING SURFACE
1) Absorbing Surface= the amount of cells & surface area of the area where the drug is moving across
2) Influence= A smaller amount of cells separating the drug from the systemic circulation increases absorption rate. Absorption rate reduces if the passage of movement/absorption has a high surface area, such as complex membrane systems like the GI tract, muscle, skin etc.

One of the 6 factors which influences the rate & extent to which a drug is absorbed is- BLOOD FLOW
Qs- 1) What does this refer to?
2) How does it influence drug absorption?

Factor affecting drug absorption- BLOOD FLOW
1) Blood Flow= circulation to the site of administration
2) Influence= Increased blood flow increases absorption rate

One of the 6 factors which influences the rate & extent to which a drug is absorbed is- BLOOD FLOW.
Q- What is 1 example of how blood flow to the site of drug administration can impact on absorption
[Hint- think about IM injections]

Example- The muscle area selected for IM injections can influence absorption rate due to changes in blood flow to different areas of the blood. Blood flows faster through the deltoid muscle compared to the gluteal muscle. Therefore, administering IM injections to the deltoid will have an increased rate of absorption

One of the 6 factors which influences the rate & extent to which a drug is absorbed is- SOLUBILITY OF THE DRUG
Qs- How does this influence drug absorption?

SOLUBILITY OF DRUGS influences absorption as more soluble drugs will increase absorption rate & extent (amount)

One of the 6 factors which influences the rate & extent to which a drug is absorbed is- IONISATION
Q- What does ionisation refer to?

IONISATION influences drug absorption
1) Ionisation refers to whether molecules are lipid soluble [Lipophilic] or water soluble [Hydrophilic]

One of the 6 factors which influences the rate & extent to which a drug is absorbed is- IONISATION
Q- How does ionisation of a drug influence its absorption?
[Hint- Compare Ionised & Unionised molecules]

a) IONISED MOLECULES [Lipophilic Molecules]= Absorption Increases- they are lipid soluble so are better able to diffuse across biological membranes (which are composed of lipids/fatty acids) & then enter the systemic circulation
b) UNIONISED MOLECULES [Hydrophilic Molecules]= Absorption is poor- they arent lipid soluble so cannot diffuse across biological membranes (composed of lipids), & therefore cant enter the systemic circulation. Are rapidly excreted from the body

One of the 6 factors which influences the rate & extent to which a drug is absorbed is- FORMULATION Qs- 1) What does this refer to? 2) How does it influence drug absorption?

FORMULATION- 1) Formulation= the way in which a drug is formed/made- eg. as a tablet, capsule, liquid, slow release, special coatings 2) Different forms absorb differently (eg. Eccentric coated drugs dont dissolve immediately in the stomach & instead release the active ingredients in the small intestine. Slow release drugs are formulated to allow for a sustained release of the active ingredient)

The 3rd Pharmacokinetic process is drug METABOLISM. Where is the primary site of drug metabolism?

The primary site of drug metabolism is in the LIVER

The 3rd Pharmacokinetic process is drug METABOLISM. How many phases of drug Metabolism are there?

There are TWO phases of drug Metabolism

The 3rd Pharmacokinetic process is drug METABOLISM. Identify the names of the TWO phases of drug Metabolism?

Drug Metabolism- 1) Phase 1= Functionalisation 2) Phase 2= Conjugation

There are TWO phases of drug Metabolism. What is PHASE 1 called & what happens during this phase?

PHASES OF DRUG METABOLISM- PHASE 1= Functionalisation= Reactions involve the introduction of a polar functional group into the molecule

There are TWO phases of drug Metabolism. What is PHASE 2 called & what happens during this phase?

PHASES OF DRUG METABOLISM- PHASE 2= Conjugation= Reactions involve the conjugation with the polar group of an endogenous substance

There are TWO phases of drug Metabolism. What is PHASE 2 called & what happens during this phase?

PHASES OF DRUG METABOLISM- PHASE 2= Conjugation= Reactions involve the conjugation with the polar group of an endogenous substance

What is the enzyme called which is of the greatest significance in drug metabolism?

The Cytochrome P450 Enzymes [CYP] are of the greatest significance in drug metabolsm

The 4th Pharmacokinetic process is drug EXCRETION. Identify the FIVE (5) main routes of Drug Excretion?

Five Routes of Drug EXCRETION- 1- Urine [from kidneys] 2- Faeces [from gut] 3- Sweat [from sweat glands] 4- Breast Milk [from breasts] 5- Expired Air [from lungs]

Excretion of water-soluble drugs occurs primarily via organ & substance?

Excretion of water-soluble drugs occurs primarily via the Kidneys in urine

Are lipid-soluble drugs excreted by the kidneys?

No- lipid soluble drugs are poorly excreted by the kidneys

What is the definition of a drugs 'half-life'?

HALF LIFE= the time taken for half the total amount of a drug to be excreted from the body

In this graph, which pharmacokinetic processes are shown by the GREEN and BLUE lines?

GREEN= Absorption BLUE= Elimination

What is the correlation between plasma concentration & activation of receptors?

The number of receptors activated is determined by the plasma concentration

What is the term which describes- 'the point at which drug absorption is equal to elimination, whereby plasma concentration is constant and within the desired therapeutic range'?

Term= Steady State Definition= 'the point at which drug absorption is equal to elimination, whereby plasma concentration is constant and within the desired therapeutic range'

What does the term 'Steady State' mean? [context- the 'Steady state of plasma concentration']

Steady State= the point at which drug absorption is equal to elimination, whereby plasma concentration is constant and within the desired therapeutic range

How is a 'Steady State' of drug plasma concentration achieved? [Hint- theres 2 ways]

A 'Steady State' of drug plasma concentration achieved through- 1- continuous administration (eg. IV infusion) 2- repeated dosing (eg. every 4hrs, every 24hrs etc.)