Question

A drug is 40% bioavailable by the oral route and 58% bioavailable by the transdermal route. If a patient is taking a 2.5-mg oral dose twice a day and is switched to the counterpart 2% ointment, how many milligrams of the ointment should be administered to provide the daily equivalent dose of the drug?

Answer 1

Method 1:

Oral drug dose = 2.5mg/day = 5mg/day

Bioavailability for oral route = 40% = 5x40/100 = 2mg (Daily quivalent dose of the drug)

Ointment form 2% = 2g/100g = 0.02g =20mg

Bioavailability of drug through transdermal route = 58%

Bioavailability of 1mg of drug (via transdermal) = 1x 58/100 = 0.58 mg

Dose of ointment required to maintain daily equivalent dose: 20mg (2% ointment) /0.58mg = 3.44mg

Method 2:

Daily quivalent dose of the drug:

Oral dosage = 2.5mg/day = 5mg/day

Bioavailability of 5mg = 5x40/100 (bioavailability -oral route) = 2mg

Ointment form 2% = 2g/100g

Bioavailability of 2g(2000mg) of ointment = 2000x58/100 (Bioavailability through transdermal route)

= 1160mg

2000mg -----> 1160mg availability

? -------> 2mg

Milligram of ointment required to provide daily equivalent dose of 2mg = 2mgx2000mg/1160mg

=3.44mg