Question

A drug is 57% bioavailable by the oral route and 72% bioavailable by the transdermal route. If a patient is taking a 5-mg oral dose twice a day and is switched to the counterpart 2% ointment, how many total grams of the ointment (nearest hundredth) should be topically administered daily (only one dose per day is needed to equal the tablet daily dosing) to provide the equivalent daily dose of the drug?

Answer 1

Drug administered by the oral route : 5mg twice per day.

Total drug administered by the oral route per day is : 5mg×2 = 10mg per day.

Bioavailability by oral dose : 57%. Therefore, drug available to the person = (57÷100)×10= 5.7mg/day

To provide equivalent daily dosing of the drug,

Let the amount of drug by transdermal route be X

since only 72% of the ointment will be available,

(72/100) times X = 5.7mg

Solving for X,

X= 7.9mg / day.

But this is the drug to be administered. The question asks the amount of ointment. Now, the ointment contains substances other than the drugs like oils etc. The question mentions, 2% ointment. Which means only 2mg drug is present in 100mg ointment.

So to calculate the total amount of ointment required, use the formula,

100* (amount of drug by transdermal route)/ percentage of drug in ointment

100* 7.9 / 2 = 395mg or 0.395g

Ans= 0.39g