A drug has bioavailability of 40% after oral dosing due to first-pass hepatic metabolism. If the hepatic extraction ratio (ER) is 0.6 and the drug shows linear kinetics, what fraction of the oral dose actually reaches systemic circulation?
- A 40% ✓
- B 60%
- C 16%
- D 24%
Correct answer: A. 40%
Explanation
Oral bioavailability (F) for a drug subject to first-pass hepatic extraction is given by F = 1 – ER (assuming complete intestinal absorption and no gut-wall metabolism). With ER = 0.6, F = 1 – 0.6 = 0.40 = 40%. This means 40% of the absorbed dose reaches systemic circulation, consistent with the given bioavailability of 40%. The 60% extracted on first pass undergoes hepatic biotransformation before entering systemic circulation.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.