onecompartment

Onecompartment, in pharmacokinetics often referred to as the one-compartment model, is a simplified representation of how a drug distributes within the body and is eliminated. It treats the body as a single, homogeneous compartment in which the drug mixes instantly after administration, and from which elimination occurs. This model is commonly used for teaching, initial data interpretation, and quick, first-pass analysis of dosing regimens.

Mathematical formulation commonly assumes first-order processes. After an intravenous bolus, the drug concentration C(t) in plasma decays exponentially: C(t) = (Dose/Vd) e^{-kt}, where Vd is the apparent volume of distribution and k is the elimination rate constant, k = CL/Vd, with CL representing clearance. For oral dosing with first-order absorption, the concentration can be described by C(t) = (F·Dose/Vd) · [ ka/(ka − k) ] · (e^{-kt} − e^{-k a t}), where ka is the absorption rate constant and F is bioavailability. These equations yield a monoexponential decline and allow estimation of Vd, CL, and sometimes ka.

Applications include preliminary pharmacokinetic analyses, dose optimization in simple cases, and educational demonstrations of kinetic principles. The model is most appropriate for drugs that distribute rapidly and uniformly, with linear kinetics and minimal tissue-specific binding.

Limitations: the one-compartment model cannot capture multi-phase distribution or nonlinear kinetics seen with many agents, and it oversimplifies complex tissues' involvement. For drugs that exhibit slower tissue distribution, saturable binding, or multiple distribution phases, multi-compartment models provide a more accurate description. Extensions often add an absorption compartment or move to more complex multi-compartment frameworks.