Physiological Modeling Reveals Novel Pharmacokinetic Behavior for Inhaled Octamethylcyclotetrasiloxane in Rats

Octamethylcyclotetrasiloxane (D4) is an ingredient in selected consumer and precision cleaning products. Workplace inhalation exposures may occur in some D4 production operations. In this study, we analyzed tissue, plasma, and excreta time-course data following D4 inhalation in Fischer 344 rats (K. Plotzke et al., 2000, Drug Metab. Dispos. 28, 192–204) to assess the degree to which the disposition of D4 is similar to or different from that of volatile hydrocarbons that lack silicone substitution. We first applied a basic physiologically based pharmacokinetic (PBPK) model (J. C. Ramsey and M. E. Andersen, 1984, Toxicol. Appl. Pharmacol. 73, 159–175) to characterize the biological determinants of D4 kinetics. Parameter estimation techniques indicated an unusual set of characteristics, i.e., a low blood:air (Pb:a ≅ 0.9) and a high fat:blood partition coefficient (Pf:b ≅ 550). These parameters were then determined experimentally by equilibrating tissue or liquid samples with saturated atmospheres of D4. Consistent with the estimates from the time-course data, blood:air partition coefficients were small, ranging from 1.9 to 6.9 in six samples. Perirenal fat:air partition coefficients were large, from 1400 to 2500. The average Pf:b was determined to be 485. This combination of partitioning characteristics leads to rapid exhalation of free D4 at the cessation of the inhalation exposure followed by a much slower redistribution of D4 from fat and tissue storage compartments. The basic PK model failed to describe D4 tissue kinetics in the postexposure period and had to be expanded by adding deep-tissue compartments in liver and lung, a mobile chylomicron-like lipid transport pool in blood, and a second fat compartment. Model parameters for the refined model were optimized using single-exposure data in male and female rats exposed at three concentrations: 7, 70, and 700 ppm. With inclusion of induction of D4 metabolism at 700 ppm (3-fold in males, 1-fold in females), the parameter set from the single exposures successfully predicted PK results from 14-day multiple exposures at 7 and 700 ppm. A common parameter set worked for both genders. Despite its very high lipophilicity, D4 does not show prolonged retention because of high hepatic and exhalation clearance. The high lipid solubility, low blood:air partition coefficient, and plasma lipid storage with D4 led to novel distributional characteristics not previously noted for inhaled organic hydrocarbons. These novel characteristics were only made apparent by analysis of the time-course data with PBPK modeling techniques.