Question

Five hundred milligrams of a pharmacological compound was administered to a 70-kg subject. Later the measured in the plasma to be 1 μg/mL. Calculate the volume of distribution in Liters.

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Answer 1

Dose administered = 500mg

Plasma concentration = 1 mcg/mL = 1000mcg/L = 1mg/L

(1000mL = 1L and 1000microgram = 1 milligram)

Volume of distribution = dose administered /Plasma concentration = (500/1) L = 500 L

Volume of distribution of drug = 500 Liters.

The volume of distribution of a drug depends on its:
lipid solubility, ionization at physiological pH, extent of binding to plasma and tissue proteins, presence of tissue-specific transporters and differences in regional blood flow.

Lipid insoluble drugs are incapable of entering cells hence their Volume of distribution is often equal to extracellular fluid volume. Highly Plasma protein bound drugs have low volume of distribution as they are restricted to the vascular plasma fluid compartment.

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