A drug is dosed orally with F = 0.5; to achieve a target Css,inf with Vd = 40 L and Cl = 5 L/h, calculate the required continuous oral dose rate. If Css,inf is 4 mg/L, what is the rate?

• A. 20 mg/h
• B. 30 mg/h
• C. 35 mg/h
• D. 40 mg/h

Answer: (D)

At steady state, the amount entering the system equals the amount being eliminated. For an oral dose with bioavailability F, the input rate is F times the dose rate, while the elimination rate is Cl times the steady-state concentration (Css,inf). Setting these equal gives F × Dose_rate = Cl × Css,inf, so Dose_rate = (Css,inf × Cl) / F.

Plugging in the numbers: Css,inf = 4 mg/L, Cl = 5 L/h, F = 0.5. Dose_rate = (4 × 5) / 0.5 = 40 mg/h.

The distribution volume (Vd) doesn’t affect the steady-state input rate; it influences how long it takes to reach Css,inf, not the rate needed to maintain it.