Maintenance Dose Calculation — FRCA Primary MCQ

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HardPharmacology - General PrinciplesMaintenance Dose CalculationFRCA Primary

A drug shows linear, time-invariant pharmacokinetics. Its total systemic clearance is 500 mL/min and its oral bioavailability is 0.4. You wish to maintain an average steady-state plasma concentration of 10 mg/L using a continuous oral dosing regimen. What oral maintenance dose rate is required?

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Reveal the answer and explanation

Correct answer: E — 750 mg/h

Explanation lettering: B = shown as A · C = shown as B · D = shown as C · E = shown as D · A = shown as E

A is correct. At steady state the rate of systemic drug input equals the rate of elimination, CL × Css. Clearance of 500 mL/min is 30 L/h, so the required systemic input is 30 × 10 = 300 mg/h. Only 40% of an oral dose reaches the systemic circulation, so the administered rate must be 300/0.4 = 750 mg/h; equivalently, apparent oral clearance (CL/F) is 75 L/h. C (300 mg/h) is the intravenous requirement and omits the bioavailability correction. B (120 mg/h) multiplies by F instead of dividing, giving a sub-therapeutic dose. D (75 mg/h) misapplies apparent oral clearance as a dose rate. E (1200 mg/h) results from treating bioavailability as 25% rather than 40%.

Reference: Bioavailability considerations in evaluating drug-drug interactions using the population pharmacokinetic approach (apparent oral clearance CL/F), J Clin Pharmacol, 2010. https://pubmed.ncbi.nlm.nih.gov/20864622/