Pharmacokinetics And Pharmacodynamics

Question 1

What factors affect choice of route?

Answer 1

Factors affecting absorption are:
- How quickly will the drug get into the bloodstream?
- How much drug will get into the bloodstream?
- Do we need the cooperation of patient’s or care staff systemic or
local effect?

Hence, for choice of route must consider:
Speed of action
Duration of action
Bioavailability
Accuracy of dose
Adverse effects
Patient status, convenience

Question 2

Define and describe PK.

Answer 2

The passage of the drug through the body. What the body does to the drug.

Question 3

Define and describe PD.

Answer 3

How the drug acts. What the drug does to the body.

Question 4

How do bioavailability and distribution affect drug use?

Answer 4

Bioavailability (F) is the percentage of the administered dose which reaches the systemic circulation of the patient

 F x DOSE = amount of drug absorbed

F should be predictable, it is affected by:

• Dose form (bioavailability of liquids and solids are different)
• Chemical form
• First pass effect

Distribution describes what happens to the drug once it has been anbsorbed into the circulation. It can be described as the Volume of Distribution (Vd), a theoretical concept that measures the extent to which a drug moves into the tissues.
Vd uses a two compartment model (blood and tissues)
- Small Vd = most drug in blood
- Large Vd = most drug in tissue

Question 5

What are the different routes of administration?

Answer 5

PO - Enteral (Oral)
NG - Nasogastric
S/L - Sublingual
MR - Controlled release
PR - Rectal
IV, IM, S/C - Parenteral
Topical - Skin
ophthalmic - Eye
inhalation - Lungs
nasal - Nose

Question 6

What affects duration of action?

Answer 6

Half Life is the time taken for the concentration of the drug in the blood to decrease by half

• Examples of half lives:
  - Oxazepam 5-10 hours
  - Temazepam 8-20 hours
  - Lorzaepam 10-20 hours
  - Diazepam 20-80 hours

Half life’s vary per patient (longer in the elderly)

Can influence half-life pharmacologically through:
Modified Release (MR), Controlled Release (CR), Slow Release (SR) which maintains drug plasma concentrations for prolonged periods 

Advantages:
⇓ in dosing frequency ⇑ adherence
⇓ in incidence and / or intensity of adverse effects
Disadvantages:
Expensive 
Lack of standardisation

Question 7

What is the clearance of a drug?

Answer 7

Clearance is the volume of plasma completely cleared of drug per unit time

Rate of elimination = Cl x Cp (clearance x plasma concentration)

e.g. if the concentration of a drug in the body is 100mg/L and the clearance is 2L per hour then the rate of elimination is 200mg per hour

Question 8

What is steady state in PD?

Answer 8

At steady state
Rate in = Rate out

How long does it take to reach steady state?
How long does it take to eliminate all the drug from the body when a patient stops taking the drug?

Steady state should be reached within the therapeutic index (TI) of the drug, where TI is the ratio between lethal dose and therapeutic dose. The bigger the TI the safer the drug (eg penicillin has a large TI, lithium has a small TI)