A drug is administered as a 400 mg loading dose and has a Vd of 40 L. What is the expected initial plasma concentration (C0)?

• A 1 mg/L
• B 10 mg/L ✓
• C 5 mg/L
• D 16 mg/L

Explanation

The initial plasma concentration is calculated as C0 = Dose / Vd = 400 mg / 40 L = 10 mg/L. The loading dose formula derives from this relationship: Loading dose = Target Cp × Vd / F (where F is bioavailability). For an IV dose, F = 1. This calculation is fundamental for designing loading doses to rapidly achieve therapeutic plasma concentrations in drugs with large volumes of distribution where multiple maintenance doses would take too long to reach steady state.

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

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