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 ZJ, 5'8", 77 Kg female, using a volume of distribution of 0.65 L/Kg and a desired level of 21 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 ZJ using a Vmax of 6.06 mg/kg/day, Km of 5.5 mg/L, Css = 13.4 mg/L. Round to the nearest dose using available phenytoin sodium capsules to determine a daily dose for ZJ. (1 point)

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Question 3. A 20 year-old, 6'2", 88 kg female had been receiving 350 mg phenytoin chewable tablets daily for 2 weeks and her trough plasma concentration was 7.58 mg/L. Her dose was increased to 250 mg twice a day and now two weeks later her phenytoin level is 17.8 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 12 mg/L using both patient-specific Vmax and Km.

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