A drug follows two-compartment pharmacokinetics. After IV bolus, the drug's plasma concentration-time curve shows a rapid initial decline (alpha/distribution phase) followed by a slower decline (beta/elimination phase). Which parameter is most useful for calculating the loading dose for this drug?

• A Volume of the central compartment (Vc) because loading dose fills only the initial dilution space
• B Clearance (CL) because it determines how much drug is needed per unit time
• C Volume of distribution calculated from the terminal (beta) phase (Vd-beta or Vz) multiplied by the desired steady-state concentration
• D Volume of distribution at steady state (Vdss) because it accounts for both central and peripheral compartments ✓

Explanation

For loading dose calculation in two-compartment pharmacokinetics, the volume of distribution at steady state (Vdss) is the most physiologically relevant parameter because it represents the equilibrium volume when drug distribution between central (plasma) and peripheral (tissue) compartments is complete. Loading dose = Vdss × target Css. Using only Vc would underestimate the dose needed for full distribution. Vd-beta (terminal phase Vd) overestimates Vd and would lead to an excessively high loading dose with potential toxicity. Clearance determines maintenance dose rate, not the loading dose for a given target concentration.

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

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