Pharmacokinetics Ii

Front 1

Use a drug's half-life to determine the number of hours that it will take for the drug to reach a steady-state concentration

Back 1

With constant rate of drug infusion, steady-state is reached after about 4 half lives:
1 half life - 50%
2 half lives - 75%
3.3 half lives - 90%
4 half lives - 92.5%

Front 2

Use a drug's half-life and a measurement of Cp to predict the steady state Cp.

Back 2

Given the number of half lives, we can determine the percept of the steady state Cp (or we can derive it)

Example: Cp = 45ng/ml after 6 hrs influsion; half life=3hrs
6hrs infusion/3hr per half life = 2 half lives
2 half lives = 75% of steady state Cp
45ng/ml = 0.75Cp(steady state)
Thus, Cp(steady state) = 60ng/ml

Front 3

Calculate loading and maintenance dosage regimens for oral or intravenous administration of a drug when given the following information: target steady-state concentration; bioavailability; clearance; and volume of distribution.

Back 3

Loading dose: (Vd x desired Cp) / F
Maintenance dosing rate: (CL x desired Cp)/ F

F = bioavailability

note: lower bioavailability requires greater dosage

Front 4

Describe how the concept of therapeutic window influences decisions about the frequency of dosing.

Back 4

Cp should be within a therapeutic window to prevent toxicity and to ensure that the drug causes the desired effect.

Intermittent dosing: peak levels and troughs are important because we want to stay within the therapeutic window while minimizing doses (fewer is better for patient adherence)

Narrow therapeutic windows: smaller and frequent doses. caution: monitor frequently because changes in clearance/dosing can result in Cp outside this window.