What is the primary determinant of the rate at which a drug distributes from the central compartment to the peripheral compartment in a two-compartment model?

What is the primary determinant of the rate at which a drug distributes from the central compartment to the peripheral compartment in a two-compartment model?

• A. The rate constant for elimination from the central compartment.
• B. The rate constant for transfer from peripheral to central.
• C. The rate constant for transfer from central to peripheral.
• D. The rate constant for absorption from the gut.

Answer: (C)

In a two-compartment model, the speed with which the drug moves from the central to the peripheral compartment is set by the intercompartmental transfer rate constant in that forward direction. The actual instantaneous transfer rate is this constant (let’s call it k12) multiplied by the drug concentration in the central compartment. So, the higher k12, the faster the distribution to the peripheral compartment. Elimination from the central compartment (k10) reduces central levels but doesn’t drive how quickly distribution occurs. The transfer from peripheral back to central (k21) affects later redistribution, not the initial rate of forward distribution. Absorption from the gut is relevant for how the drug enters the central compartment after extravascular dosing, but once distribution between central and peripheral is underway, the forward rate constant k12 is the dominant determinant.

In a two-compartment model, the speed with which the drug moves from the central to the peripheral compartment is set by the intercompartmental transfer rate constant in that forward direction. The actual instantaneous transfer rate is this constant (let’s call it k12) multiplied by the drug concentration in the central compartment. So, the higher k12, the faster the distribution to the peripheral compartment.

Elimination from the central compartment (k10) reduces central levels but doesn’t drive how quickly distribution occurs. The transfer from peripheral back to central (k21) affects later redistribution, not the initial rate of forward distribution. Absorption from the gut is relevant for how the drug enters the central compartment after extravascular dosing, but once distribution between central and peripheral is underway, the forward rate constant k12 is the dominant determinant.