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Flashcards in Pharmacokinetics Deck (25)
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1
Q

Give some examples of site of administration of drugs.

A

Focal (eye, skin, inhalation)
Systemic - enteral (sublingual, oral, rectal)
- parenteral (subcutaneous, intramuscular, IV)

2
Q

Why is the site of administration important when prescribing drugs?

A

Ease, practicality, localisation
Allows less systemic absorption and less off-target effects, as well as increasing the liklihood of proper use by the patient.

3
Q

What is oral bioavailability

A

The proportion of a dose which reaches the systemic circulation in an unchanged form.

4
Q

What does the amount of drug available after oral administration depend on?

A

Gut absorption, altered by food/disease.

First pass metabolism

5
Q

What does the rate of availability of a drug after oral administration depend on?

A

Pharmaceutical factors and gut absorption.

6
Q

How do you calculate bioavailability (%)?

A

AUC oral/AUC injected

AUC is the area underneath a plasma concentration curve.

7
Q

What is the therapeutic ratio for a drug?

A

The maximum tolerated dose over the minimum effective dose.

8
Q

What is first pass metabolism

A

The blood from the gut passes through the portal system so is delivered to the liver before systemic circulation, greatly reducing the concentration of the drug reaching the body.

9
Q

How can first pass metabolism be avoided?

A
Parenteral route (e.g. subcutaneous/IV)
Sublingual or rectal route
10
Q

What is volume of distribution for a drug?

A

The theoretical volume into which a drug has been distributed assuming this has occurred instantaneously.

amount given/plasma concentration at 0s

11
Q

The free level of a drug is what exerts an effect. When is this especially important for a drug? (i.e. a drug with specific properties)

A

If it is highly bound to albumin, has a small volume of distribution, has a low therapeutic index.

12
Q

Describe the difference between an object drug and a precipitant drug.

A

Object - class I. Used at a dose much lower than the number of albumin binding sites.

Precipitant - class II. Used at a dose much greater than the available binding sites.

13
Q

Give an example of an object drug and it’s corresponding precipitant drug.

A

Warfarin - sulphonamides, aspirin, phenytoin

Tolbutamide - sulphonamides, aspirin

Phenytoin - valproate

14
Q

What is first order kinetics when referring to a drug?

A

When the rate of decline of plasma drug level is proportional to the level of the drug. It’s half-life can be defined.

15
Q

What is zero order kinetics when referring to a drug?

A

When the rate of decline of a plasma drug level is constant.

16
Q

Where does metabolism and excretion of drugs take place?

A

Liver - metabolism

Kidney - excretion

17
Q

Describe phase I of liver metabolism.

A

Carried out by mixed function oxidases to oxidise, reduce or hydrolyse drugs.
- NADPH cytochrome P450 reductase
- cytochrome P450 in liver microsomes
They have low substrate specificity and have an affinity for lipid-soluble drugs.

The enzymes are inducible and inhabitable (competitive, non-competitive).

18
Q

Give an example of an enzyme inducer in the liver and the drug that it affects.

A

Phenobarbitone - warfarin, phenytoin

Rifampin - oral contraceptive

Cigarettes - theophylline

19
Q

Give an example of an enzyme inhibitor in the liver and the drug that it affects.

A

Cimetidine - warfarin, diazepam

20
Q

Give a drug which interacts with warfarin and how it does so.

A

Alcohol - inhibits metabolism
Aspirin/sulphonamides/phenytoin - displace from plasma proteins
Broad spectrum antibiotics - decrease vitamin K synthesis by bacteria in the gut
Aspirin - reduces platelet function
Barbituates/rifampicin - induce liver metabolism enzyme

21
Q

In liver disease, what drugs must you be careful administering?

A

Those with a low therapeutic ratio

22
Q

What feature of urine will affect how much of a drug is excreted?

A

pH

23
Q

For a weakly acidic drug, what effect will increasing alkalinity of the urine have on its excretion?

A

Decreases excretion because it ionises the drug, decreasing tubular absorption as the charged drug stays in the lumen of the blood vessel.

24
Q

For a weakly basic drug, what effect will increasing acidity of the urine have on its excretion?

A

Inceases excretion

25
Q

In renal disease, what effect does this have on the maintenance dose and loading dose of a drug?

A

Maintenance drug must be reduced

Loading dose remains unchanged unless the volume of distribution changes.