Example: Phenytoin is an antiepileptic drug. Its elimination follows Michaelis-Menten kinetics. In adults, the average Vmax is 700mg/day and the average Km is 8 mg/L. What is the expected steady state conc following administration of 250 mg q 12 hr to an adult patient ? Calculate phenytoin half life and total body clearance at this steady state conc. (Phenytoin Vd is 50 L and F=1).
(1)500mg
day =
700 mg/day Cpss
8 mg/L + Cpss
Cpss = 20 mg/L
CLT =
700 mg/day
8 mg/L + 20 mg/L
= 25 L/day
t1/2 =
(0.693) 50L
700 mg/day
(8 mg/L+20 mg/L) = 1.34 day
