PK Module 15: In-Class Activity 1

Front

A 32 year old male DJ (75 kg) requires initiation of phenytoin treatment. What loading dose of phenytoin sodium in mg is required to produce a peak plasma concentration of 18 mg/L? (Vd = 0.65 L/kg) Round to the nearest mg.

Back

955

Answer explanation

Vd = 0.65 L/kg x 75 kg = 48.8 L

LD = (Vd x Cdesired)/S

LD = (48.8 L x 18 mg/L)/0.92 = 955 mg

Front

Calculate Vd for a 90-kg (IBW = 68 kg), 35-year-old male patient given fosphenytoin (S = 0.92, F = 1) sodium IV 1000 mg as a loading dose (treat as bolus).

Back

63.2

Answer explanation

V = 0.65*[IBW + 1.33*(ABW - IBW)]

V = 0.65*[68 + 1.33*(90-68) = 63.2 L

Front

Estimate the post-load phenytoin concentration in mg/L for a 110-kg male patient given a loading dose of fosphenytoin sodium IV 1000 mg, with a Vd of 77.94 L.

Back

11.8

Answer explanation

C = (S x F x D)/V

C = (0.92 x 1 x 1000)/77.94

C = 11.8 mg/L

Front

If we wish to increase the steady-state plasma concentration to 20 mg/L, what dose of phenytoin sodium (S = 0.92, F = 1) in mg should be used?

Back

475.5

Answer explanation

Xd = (700 x 20/12 + 20) = 14000/32 = 437.5 mg/day phenytoin acid

437.5/0.92 = 475.5 mg phenytoin sodium

Front

Calculate the Vmax for a patient administered phenytoin with a plasma concentration of 10 mg/L at 300 mg/day and 18 mg/L at 400 mg/day, assuming a Km = 12.5 mg/L and a volume of distribution of 45 L.

Back

678

Answer explanation

Vmax = (Xd x S)(Km+Css)/Css

Vmax = (400)(12.5+18)/18 = 678 mg/day

Alternatively:

daily dose = -Km (daily dose/C) + Vmax

400 mg = -12.5 mg/L (400 mg/L/18 mg/L) + Vmax

Vmax = 678 mg/day

Front

What is the Km for phenytoin in this patient in mg/L?

Back

10

Answer explanation

-Km = 350 - 450/((350/10 - 450/18)

-Km = -100/(35-25)

Km = 10 mg/L