A drug with 80% oral bioavailability is given 100 mg every 8 hours. Its total clearance is 450 mL/hour. What is the approximate average steady-state plasma concentration (Css,avg)?

• A 11.1 mcg/mL
• B 44.4 mcg/mL
• C 33.3 mcg/mL
• D 22.2 mcg/mL ✓

Explanation

Css,avg = (F × Dose / τ) / CL. F = 0.80, Dose = 100 mg = 100,000 mcg, τ = 8 h, CL = 450 mL/h = 0.45 L/h. Rate in = 0.80 × 100,000 mcg / 8 h = 10,000 mcg/h. Css,avg = 10,000 mcg/h ÷ 450 mL/h = 22.2 mcg/mL. This equation shows that steady-state concentration is determined solely by the rate of drug input (dose × F / interval) divided by clearance, independent of volume of distribution. A higher clearance or lower bioavailability will proportionally reduce Css,avg.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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