Closed-chamber inhalation pharmacokinetic studies with hexamethyldisiloxane in the rat

Gas uptake methods together with physiologically based pharmacokinetic (PBPK) modeling have been used to assess metabolic parameters and oral absorption rates for a wide variety of volatile organic compounds. We applied these techniques to study the in vivo metabolism of hexamethyldisiloxane (HMDS), a volatile siloxane with low blood/air (partition coefficient PB approximately 1.00) and high fat/blood partitioning (partition coefficient PF approximately 300). In contrast to other classes of metabolized volatiles, metabolic parameters could only be estimated from closed-chamber results with confidence by evaluating both closed-chamber disappearance curves and constant concentration inhalation studies. The constant-concentration inhalation results refine the estimate of the blood/air partition coefficient and constrain model structure for storage of the lipophilic compound in blood and tissues. The gas uptake results, from Fischer 344 rats (male, 8-9 wk old) exposed to initial HMDS air concentrations from 500 to 5000 ppm, were modeled with a 5-tissue PBPK model. Excellent fits were obtained with diffusion-limited uptake of HMDS in fat and a lipid storage pool in the blood. Metabolism, restricted to the liver, was described as a single saturable process (V(max) = 113.6 micro mol/h/kg; K(m) = 42.6 micro mol/L) and was affected by inhibitors (diethyldithiocarbamate) or inducers (phenobarbital) of cytochrome P-450s. Exhalation kinetics of HMDS after oral/intraperitoneal administration showed low bioavailability and significant lag times, also quite different from results of other classes of volatile hydrocarbons. In general, estimates of metabolic clearance by gas uptake studies were improved by simultaneous examination of time-course results from constant concentration inhalation studies. This conclusion is likely to hold for any volatile lipophilic compound with low blood/air partitioning.