Figure 1. Schematic representation of the selected pharmacokinetic model.

Two compartments represent the amount of free phenytoin (unbound) and of bound drug in plasma. V₁ is the volume of distribution, θ_prop is the proportionality constant between the bound and unbound drug amounts, k_m is the Michaelis-Menten elimination rate constant (mg/L), V_max is the maximum velocity of metabolism (mg/h), V_max0 is the time-invariant maximum velocity of metabolism at baseline (mg/h), V_maxi is the time-dependent velocity defined by the rate constant k_ind (h⁻¹) and t is time (h). The total amount of phenytoin in plasma is the sum of unbound and bound phenytoin.