4-Tissue Compartments with Metabolites, K1/k2

This is a general compartment model for ligands which have recirculating metabolites entering tissue and contributing to tissue activity. Tissue kinetics of both the authentic ligand (C₁ and C₂) as well as the metabolites (C₃ and C₄) are described by a 2-tissue compartment model, with a transfer k_trans constant linking the two exchangeable compartments which represents metabolites generated in tissue.

Differential Equation of the Mass Balance

Two input curves are required: the authentic ligand in plasma C_p(t), and the metabolites exchanging with tissue C_M(t). The input curve loading menu shows two corresponding entries as soon as the model is selected.

Operational Model Curve

with the concentration C_B(t) of tracer in whole blood, and the blood volume fraction v_B.

Parameter Fitting

The model includes more parameters than can be reliably fitted at once. Therefore it has been implemented with K1/k2 and K1m/k2m as fitting parameters to allow fixing the distribution volume of the exchangeable compartment, or to use them as common parameters in a coupled fit.