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A Physiological Pharmacokinetic Model Based on Tissue Lipid Content for Simulating Inhalation Pharmacokinetics of Highly Lipophilic Volatile Organic Chemicals

The highly lipophilic volatile organic chemicals (HLVOCs) are distributed almost uniquely in the neutral lipid fraction of tissues and blood. As suggested by their high n-octanol:water partition coefficient (>1000), their solubility in water fraction of tissues and blood is negligible. Hypothetically, then, the kinetics of HLVOCs can be simulated solely with the consideration of their solubility and distribution in neutral lipid-equivalent (NLE) fractions of the tissues and blood. The objectives of the present study were therefore (i) to develop a physiological pharmacokinetic model based on NLE content of tissues and blood, and (ii) to apply this model framework for simulating the inhalation pharmacokinetics of HLVOCs (i.e., d-limonene, α-pinene, and 1,2,4-trimethylbenzene) in humans. The PBPK model developed in this study consisted of tissue compartments that represented only their NLE content. All biological parameters, except alveolar ventilation rate, were expressed on the basis of their NLE content. Tissue:blood partition coefficients were not used since the solubility of HLVOCs in tissue neutral lipids and blood neutral lipids is considered to be the same. The NLE-based physiological pharmacokinetic model was then used to simulate the uptake and disposition kinetics of α-pinene, d-limonene, and 1,2,4-trimethylbenzene in humans. The NLE-based model developed in this study represents a novel tool for simulating the lipid concentrations and pharmacokinetics of HLVOCs without the use of tissue:blood partition coefficients.