Alora is a transdermal system designed to deliver estradiol

Alora is a transdermal system designed to deliver estradiol continuously and consistently over a 3-4 day period when placed on the skin. Alora provides systemic estrogen replacement theory in postmenopausal women. For a 36 cm^2 patch, the release rate of the drug is 0.1 mg of estradiol per day. Clinical trials have provided the following pharmacokinetic data. In one study, following applications of the patch, the peak concentration of the estradiol was found to be 144 pg mL^-1 (10^12 pg = 1 g) and the half life of estradiol was found to be 1 h. Over an 84 h dosing interval, the plasma clearance for estradiol from the Alora patch was found to be 61 L h^-1. What is the apparent volume of distribution in liters?

Solution

Here, the value of half life (t_1/2) is given as 1h. From this value, rate constant (K) can be calculated by using formula t_1/2 = log2/ K.

t_{1/2 = log2/ K}

1 = 0.3010/ K (since log2 = 0.3010)

Therefore K = 0.3010

Now by using formula V_d = CL/K (V_d = Volume of distribution, CL = Clearance), the volume of distribution can be calculated.

V_d = CL/K = 61/0.3010 = 202.65 L. (Since the value of clearance is given here, 61L/H)

Hence the volume of distribution (V_d) = 202.65 L