# Pharmacokinetic Phenytoin Problems

## Questions provided by Michele Splinter, PharmD

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Question 1. Determine an intravenous loading dose of phenytoin sodium injection for IX, 5'11", 81 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)

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Question 2. Determine a daily maintenance dose of oral phenytoin sodium capsules for IX using a Vmax of 4.67 mg/kg/day, Km of 11.2 mg/L, Css = 13.8 mg/L. Round to the nearest dose using available phenytoin sodium capsules to determine a daily dose for IX. (1 point)

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Question 3. A 24 year-old, 5'10", 81 kg male had been receiving 350 mg phenytoin chewable tablets daily for 2 weeks and his trough plasma concentration was 7.22 mg/L. His dose was increased to 250 mg twice a day and now two weeks later his phenytoin level is 17.1 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 15 mg/L using both patient-specific Vmax and Km.

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Question 4. A female patient with generalized tonic-clonic seizures attained a steady state serum concentration of 12 mg/L on 560 mg daily of Dilantin® kapseals. She was transferred to a rehabilitation hospital and received 600 mg Dilantin® kapseals. Two weeks later she 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 15 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:

Equation 31.2.1 IBW for Male Patients

Equation 31.2.2 IBW for Female Patients

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

Equation 31.2.5 Dose Rate

Equation 31.2.6 Patient Specific Km

Equation 31.2.7 Patient Specific Vm

Last update: Sat 31 Aug 2019 10:15:01 pm
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