PBPK modeling of canine inhalation exposures to halogenated hydrocarbons.

Human exposure guidelines for halogenated hydrocarbons (halons) and halon replacement chemicals have been established using dose-response data obtained from canine cardiac sensitization studies. In order to provide a tool for decision makers and regulators tasked with setting guidelines for egress from exposure to halon replacement chemicals, a quantitative approach, using a physiologically based pharmacokinetic model, was established that allowed exposures to be assessed in terms of the chemical concentrations in blood during the exposure. This model, which includes a respiratory tract compartment containing a dead-space region, a pulmonary exchange area, and a breath-by-breath description of respiratory tract uptake, allows successful simulation of exhaled breath concentrations of humans during the first minute of exposure to the anesthetics halothane, isoflurane, and desflurane. In the current study, the human model was modified with canine parameters and validated with data obtained from dog studies with halothane, isoflurane, desflurane, and CFC-11. With consideration of appropriate values for ventilation and cardiac output, the model successfully simulated data collected under a variety of exposure scenarios. The canine model can be used for simulating blood concentrations associated with the potential for cardiac sensitization. These target blood concentrations can then be used with the human model for establishing safe human exposure duration. Development of the canine model stresses the need for appropriate data collection for model validation.