A biologically based risk assessment for vinyl acetate-induced cancer and noncancer inhalation toxicity

Dosimetric adjustment factors for methyl methacrylate derived from a steady-state analysis of a physiologically based clearance-extraction model

Cells within the epithelial lining of the nasal cavity metabolize a variety of low-molecular-weight, volatile xenobiotics. In common with terminology developed for other metabolizing organs, the nose extracts these chemicals from the airstream, thereby clearing some portion of the total nasal airflow. In this article, a physiologically based clearance-extraction (PBCE) model of nasal metabolism is used to predict extraction for steady-state conditions. This model, developed by simplification of existing physiologically based pharmacokinetic (PBPK) nasal models, has three tissue regions in two flow paths. A dorsal flow stream sequentially passes over a small area of respiratory epithelium and then over the entire olfactory epithelial surface within the nose. A ventral airstream, consisting of most of the total flow, passes over the larger portion (>80%) of the respiratory epithelium. Each underlying tissue stack has a mucus layer, an epithelial tissue compartment, and a blood exchange region. Metabolism may occur in any of the subcompartments within the tissue stacks. The model, solved directly for a steady-state condition, specifies the volumetric airflow over each stack. Computational fluid dynamic (CFD) solutions for the rat and human for the case with no liquid-phase resistance provided a maximum value for regional extraction, E(max)'. Equivalent air-to-liquid phase permeation coefficients (also referred to as the air-phase mass transfer coefficient) were calculated based on these E(max)' values. The PBCE model was applied to assess expected species differences in nasal extraction and in localized tissue metabolism of methyl methacrylate (MMA) in rats and in humans. Model estimates of tissue dose of MMA metabolites (in micromol metabolized/h/ml tissue) in both species were used to evaluate the dosimetric adjustment factor (DAF) that should be applied in reference concentration (RfC) calculations for MMA. For human ventilation rates equivalent to light exercise, the DAF was estimated to be 3.02 at 28.4 ppm, the benchmark concentration for nasal lesions. Depending on specific assumptions about distribution of esterase activities in human tissues, the range of DAF values was 1.56-8.00. The DAF for heavy exercise with a ventilation rate of 42 L/min was still 2.98. Estimated DAFs were concentration dependent, varying between 2.4 and 4.76 in the inhaled concentration range from 1 and 400 ppm. Present default methods utilize a DAF of 0.145. These steady-state calculations with this PBCE model should be useful in risk assessment calculations for a variety of vapors and gases that are converted to toxic metabolites in cells in the respiratory tract.

Clearance concepts applied to the metabolism of inhaled vapors in tissues lining the nasal cavity

Some inhaled vapors are metabolized by tissues in the nasal cavity or carried away in nasal venous blood after diffusing from the lumen through the nasal epithelial tissues. These processes remove chemical from the airstream. Clearance (volume/time) is the volumetric airflow from which chemical would have to be completely removed to account for the net loss. We present here a steady-state analysis of a series of physiologically based clearance-extraction (PBCE) models for nasal clearance of inhaled vapors, consisting of one, two, three, or four subcompartments. A two-compartment model is the simplest representation of tissues in the nasal cavity, with an air and a tissue compartment. The three-compartment model had air, mucus, and tissue phases. The four-compartment model included both epithelial and submucosal tissues in addition to the air and mucus compartments. For the two-, three-, and four-compartment models, the airstream clearance (Cl(sys)) equation has a common form. Cl(sys) = Cl(tot)H(m:a)PA(gas)Q divided by Cl(tot)H(m:a)(Q + PA(gas)) + PA(gas)Q. In this equation, Cl(tot) is the total tissue clearance, PA(gas) is the gas-phase diffusional clearance, Q is the airflow, and H(muc:a) is the mucus air partition coefficient. Cl(tot) varies in complexity for the different models since it encompasses tissue diffusion, tissue clearance due to metabolism, and blood flow. A physiologically based clearance-extraction (PBCE) model for the whole nose with three nasal tissue regions, each containing a four-compartment tissue stack, was used to simulate nasal uptake of three vapors-acetone, methyl methacrylate (MMA), and vinyl acetate (VA)-to show the dependence of clearance on different parameters for specific compounds. Acetone is not metabolized in the nose, MMA is metabolized at a moderate rate by nasal tissues, and VA is metabolized at a high rate in mucus and tissues. Equations derived from steady-state analyses show the importance of the specific biochemical and physiological parameters for clearance of each of these chemicals and permit calculation of airstream clearance from simple algebraic relationships.