Pharmacokinetic Phenytoin Problems

Questions provided by Michele Splinter, PharmD


Question 1. Determine an intravenous loading dose of phenytoin sodium injection for SM, 6'2", 85 Kg male, using a volume of distribution of 0.65 L/Kg and a desired level of 20 mg/L. Round to the nearest multiple of 50 mg. (1 point)


Question 2. Determine a daily maintenance dose of oral phenytoin sodium capsules for SM using a Vmax of 4.29 mg/kg/day, Km of 3.9 mg/L, Css = 18.2 mg/L. Round to the nearest dose using available phenytoin sodium capsules to determine a daily dose for SM. (1 point)


Question 3. A 26 year-old, 5'9", 94 kg female had been receiving 350 mg phenytoin chewable tablets daily for 2 weeks and her trough plasma concentration was 7.93 mg/L. Her dose was increased to 250 mg twice a day and now two weeks later her phenytoin level is 20.9 mg/L. (3 x 0.5 points)

a. Calculate the patient-specific Km.

b. Calculate the patient-specific Vmax

c. Calculate the new maintenance daily dose of phenytoin chewable tablets with a desired Cpss of 16 mg/L using both patient-specific Vmax and Km.


Question 4. A male patient with generalized tonic-clonic seizures attained a steady state serum concentration of 11 mg/L on 530 mg daily of Dilantin® kapseals. He was transferred to a rehabilitation hospital and received 600 mg Dilantin® kapseals. Two weeks later he as noted to have ataxia and a new level was 18 mg/L.

a. Calculate Km and Vmax using the graphical method of Mullen. (2 x 0.25 points)

b. Calculate the new dose per day to produce 14 mg/ml. (0.5 points)

c. Using the regression method of Ludden, determine Vmax and Km. (2 x 0.25 points)

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Some Equations:

IBW equation (male)
Equation 31.2.1 IBW for Male Patients

IBW equation (female)
Equation 31.2.2 IBW for Female Patients

Vd for Obese patient
Equation 31.2.3 Vd for Obese Patients (i.e. Actual Body Weight > 120% of Ideal Body Weight)

Loading Dose
Equation 31.2.4 Loading Dose

Dose Rate
Equation 31.2.5 Dose Rate

Km Patient Specific
Equation 31.2.6 Patient Specific Km

Patient Specific Vm
Equation 31.2.7 Patient Specific Vm

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