Which of the following is an expression for the amount of drug in tissue in terms of tissue concentration and tissue volume?

• A. Vt * Ct
• B. Vt * Kp * Cv
• C. Vt + Cv
• D. Ct * Cv

Answer: (A)

In pharmacokinetics, the amount of drug in a tissue comes from multiplying the drug’s concentration within that tissue by the tissue’s volume. This is the straightforward way to convert a concentration (how much drug per unit tissue) into a total amount (how much drug is present overall in that tissue). If Ct is the tissue concentration (for example mg/L) and Vt is the tissue volume (L), the product Ct × Vt gives the total amount of drug in the tissue (mg).

The other forms don’t fit this relationship: multiplying two concentrations (Ct × Cv) doesn’t yield an amount, and introducing a partition coefficient with a concentration (Vt × Kp × Cv) or summing volumes and concentrations (Vt + Cv) mixes incompatible dimensions.