Mechanistic Insights Aid the Search for CFC Substitutes: Risk Assessment of HCFC-123 as an Example

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Abstract
An international consensus on the need to reduce the use of chlorofluorocarbons (CFCs) and other ozone-depleting gases such as the halons led to the adoptions of the 1987 Montreal Protocol and Title VI of the 1990 Clean Air Act Amendments, "Protecting Stratospheric Ozone." These agreements included major provisions for reducing and eventually phasing out production and use of CFCs and halons as well as advancing the development of replacement chemicals. Because of the ubiquitous use and benefits of CFCs and halons, an expeditious search for safe replacements to meet the legislative deadlines is of critical importance. Toxicity testing and health risk assessment programs were established to evaluate the health and environmental impact of these replacement chemicals. Development and implementation of these programs as well as the structural-activity relationships significant for the development of the replacement chemicals are described below. A dose-response evaluation for the health risk assessment of the replacement chemical HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane) is also presented to show an innovative use of physiologically based pharmacokinetic (PBPK) modeling. This is based on a parallelogram approach using data on the anesthetic gas halothane, a structural analog to HCFC-123. Halothane and HCFC-123 both form the same metabolite, trifluoroacetic acid (TFA), indicative of the same metabolic oxidative pathway attributed to hepatotoxicity. The parallelogram approach demonstrates the application of template model structures and shows how PBPK modeling, together with judicious experimental design, can be used to improve the accuracy of health risk assessment and to decrease the need for extensive laboratory animal testing.