Example: Phenytoin is an antiepileptic drug that is eliminated by a saturable metabolic pathway. What is the expected steady state plasma conc achieved following repeated administration of 125 mg q 12 hr and following repeated administration of 250 mg q 12 hr if the average Vmax is 700mg/day and the average Km is 8 mg/L ? (Phenytoin Vd is 50 L and F=1).
Doubling the dose results in
more than 4 times increase in
the steady state conc.
(1)250mg
day =
700 mg/day Cpss
8 mg/L + Cpss
Cpss = 4.44 mg/L
(1)500mg
day =
700 mg/day Cpss
8 mg/L + Cpss
Cpss = 20 mg/L
