Question

Drug A has the following characteristics: Volume of distribution (V_d) = 40 liters; Half-Life = 15 hours. Assuming a dose of 150 mg given every twenty-four hours and a 70% absorption rate, what is the estimated steady state concentration of the drug?

Answer 1

Kinetics of this drug is first order, because reaches steady state

Calculate elimination rate constant K by using first order kinetics.

K = ln2/t_1/2

Half-Life = 15 hours (given)

K = 0.693/15hours = 0.0642/ hours

Calculate clearance CL by using formula

K = CL/Vd

Where Vd = volume of distribution = 40 liters (given)

CL = K x Vd = 0.0642/ hours x 40 liters = 2.568 liters/hour

Now calculate steady state concentration of the drug by using formula

C = F D/CL x t

D = dose = 150 mg (given)

t = dosing interval = 24 hours (given)

CL = clearance = 2.568 liters/hour

F = fraction absorbed = 70/100 = 7/10

Steady state concentration = (7/10 *150 mg) /(2.568 liters/hour x 24 hour) = 1.70 mg/liter

The estimated steady state concentration of the drug is 1.70 mg/liter