A 28-day whole-body inhalation study to evaluate octamethylcyclotetrasiloxane (D4) absorption/distribution in two rat...

To investigate the potential toxicity of Octamethylcyclotetrasiloxane (D4), studies in laboratory rats have used primarily one of two strains, Sprague-Dawley (SD) and Fischer-344 (F-344). Reproductive studies used SD rats whereas F-344 rats were used in D4 pharmacokinetics, metabolism, acute/subacute/chronic toxicity and oncogenicity studies. Here, we assessed specific endpoints related to D4 pharmacokinetics and biochemistry in SD and F-344 rats within a single study, which allows for direct comparisons between strain and sex. This assessment included determination of microsomal total P450, NADPH-cytochrome c reductase, epoxide hydrolase, CYP2B1/2, CYP1A1/2, CYP3A1/2, CYP2C11, and CYP2A1. Aside from slight brown pigment in the liver, the treated animals experienced no toxicologically significant weight loss, decrease in food consumption, or clinical signs. Concentrations of D4 in plasma and fat were generally greater in females relative to males in both strains. SD females appeared to have statistically significantly greater plasma and fat concentrations following 28 days of repeated exposure to D4 relative to F-344 rats, suggesting the existence of potential sex and strain differences in D4 pharmacokinetics. The effect of D4 exposure on liver enzyme expression was similar among and between sexes and strain and was consistent with that for phenobarbital-like inducers. Notable differences included a finding of elevated CYP2B1/2 protein levels without a similar magnitude of increase in CYP2B/1 activity and a greater degree of CYP3A1/2 induction (protein and activity) for female SD rats. The importance of these findings is unclear, however reduced CYP2B1/2 activity may give rise to lower rates of D4 metabolism and clearance, consistent with the higher tissue levels of D4 in SD relative to F-344 female rats.