1 Intro to pharmacokinetics

Question 1

Describe how the dosage of aminophylline is given

Answer 1

Loading dose - IV injection
- (250-500mg)

Maintenance Dose - Followed by IV infusion
- 500-700mcg/kg/hour, adjusted according to plasma theophylline concentration

Question 2

Describe how IV infusion works (with aminophylline as an example)

Answer 2

• Administration of drug (aminophylline) at a constant rate in the absence of removal -> 0 order process (elimination is a 1st order reaction
• So, in order to keep the concentration of drug within the therapeutic window, whilst not letting it get too high where elimination is unachievable

Question 3

Describe steady-state concentration (Css)

Answer 3

Defined as the concentration at which:

Administration rate = elimination rate

(to keep the dosage within the therapeutic window)

Question 4

Describe maintenance dose

Answer 4

Once achieved Css, then:
input = output

So, maintenance dose = amount removed (top-up)
- (which is eliminated from the body, hence this must be replaced in order to maintain the steady-state concentration, inside the therapeutic window)

Question 5

Give the formula to work out the infusion rate

Answer 5

Infusion rate = Clearance x steady state concentration

Question 6

In order to work out the infusion rate, what are the things to keep in mind?

Answer 6

• Salt factor (s)
• Clearance (CL)
• Css (steady state concentration)

Question 7

Describe how salt factor is an important factor to keep in mind when calculating infusion rate

Answer 7

Salt factor (s)
- if the drug is salt-like theophylline, it is administered as an infusion in the form of aminophylline, so take into account the infusion factor

(as the total drug given will be a diluted version of the salt, and in order to compensate, account for s to get right dose to achieve full therapeutic effect) –> divide answer at end (infusion rate) by salt factor (s)

Question 8

Describe how clearance is an important factor to keep in mind when calculating infusion rate

Answer 8

CL (clearance)
- [units are L/h]
- Can be calculated by
CL = k x Vd

Or it can be given in terms of L/h/kg, which shows the clearance per kg (for different weights)