A CFD-PBPK hybrid model for simulating gas and vapor uptake in the rat nose

In laboratory studies of rodents, the inhalation of organic vapors often results in preferential damage to olfactory epithelium. Such focal lesion formation may be due either wholly or in part to a corresponding nonuniformity in the spatial distribution of vapor uptake within the nasal cavities. As a tool for determining this dose distribution, a mathematical model based on a combination of computational fluid dynamics (CFD) and physiologically based pharmacokinetic (PBPK) modeling was developed for simulating toxicant vapor uptake in the rat nose. The nasal airways were subdivided into four distinct meatuses selected such that each contained a major air flow stream. Each meatus was further divided into four serial regions attached to separate tissue stacks containing mucus, epithelial, and subepithelial compartments. Values for the gas-phase mass transfer coefficients and gas flows in the 16 airway regions were determined by a solution of the Navier-Stokes and convection-diffusion equations using commercially available CFD software. These values were then input to a PBPK simulation of toxicant transport through the 16 tissue stacks. The model was validated by using overall uptake data from rodent inhalation studies for three "unreactive" vapors that were either completely inert (i.e., acetone), reversibly ionized in aqueous media (i.e., acrylic acid), or prevented from being metabolized by an enzyme inhibitor (i.e., isoamyl alcohol). A sensitivity analysis revealed that accurate values of the mass transfer coefficient were not necessary to simulate regional concentrations and uptake of unreactive vapors in the rat nose, but reliable estimates of diffusion coefficients in tissue were crucial for accurate simulations.