Example: A 75 kg male epileptic patient is taking phenytoin to control his seizures. The average conc was 10 mg/L when he was taking 300 mg/day, and 20 mg/L when he was taking 400 mg/day. Using the linear transformation method calculate the dose required to achieve steady state plasma conc of 15 mg/L in this patient. (phenytoin bioavailability is 100%).

Slope = -Km
Vmax/F
Dose/τ              Cpss

D/τ

Cpss
300 mg/day    10 mg/L    30L/day
400 mg/day    20 mg/L    20L/day

D

τ

D/τ

Cpss

Answer:

Graphically:

Km = 10 mg/L
Vmax = 600 mg/day

F Dose

τ =

600 mg/day 15 mg/L

10 mg/L + 16 mg/L
Dose = 360 mg/day