Example: Ciprofloxacin is a quinolone antibiotic that has a bioavailability of 70% after oral administration. After oral administration of 500 mg only 175 mg is recovered unchanged in urine. If the average dose administered in patients with normal kidney function is 500 mg q 12 hr what is the appropriate dose for a patient with 10% of the normal kidney function ? What is the average s.s. conc in a patient with normal kidney function taking 500 mg q 12 hr and a patient with only 10% of the normal KF taking the dose you recommended ? Vd = 140 L and half life = 4.2 hr)

fraction (f)

A e∞

F Dose =

175 mg

(0.75) 500 mg = 0.5

Answer:

Dosefailure = 500 mg[ 1-0.5(1-0.1) ]
Dosefailure = 275 mg q 12 hr
t1/2 failure

4.2 hr

[ 1 - 0.5 (1-0.1) ] = 7.6 hr
kfailure = 0.693/7.6 hr = 0.091 hr -1
Cpnormal average

(0.7) 500mg

140L 0.165hr -1 12hr = 1.26 mg/L
Cpfailure average

(0.7) 500mg

140L 0.165hr -1 12hr = 1.26 mg/L

Both regimens achieve similar average plasma drug concentration at steady state.

  • Page 4 of 4
  • 1
  • 2
  • 3
  • 4