Inhalation developmental neurotoxicity study of ethylbenzene in Crl-CD rats

BACKGROUND: This study was conducted to evaluate the potential adverse effects of whole-body inhalation exposure of F-0 and F-1 parental animals from a 2-generation reproduction study of ethylbenzene on nervous system functional and/or morphological end points in the F-2 offspring from four groups of male and female CrI:CD (R) (SD)IGS BR rats. METHODS: Thirty rats/sex/group for F-0 and 25/sex/group for F-1 were exposed to 0, 25, 100, and 500 ppm, ethylbenzene for six hours daily for at least 70 consecutive days prior to mating for the F-0 and F-1 generations. Inhalation exposure for the F-0 and F-1 females continued throughout mating and gestation through Gestation Day (GD) 20. On lactation days (LD) 1-4, the F-0 and F-1 females received no inhalation exposure, but instead were administered ethylbenzene in corn oil via oral gavage at dosages estimated to result in similar internal maternal exposure based upon PBPK modeling estimates (0, 26, 90, and 342 mg/kg/day, respectively, divided into three equal doses, approximately two hours apart). Inhalation exposure of the F-0 and F-1 females was reinitiated on LD 5 and continued through weaning on postnatal day (PND) 21. Survival, body weights, and physical landmarks were assessed in selected F-2 offspring. Neurobehavioral development of one F-2-generation treatment derived offspring/sex/litter was assessed in a functional observational battery (FOB; PND 4, 11, 22, 45, and 60), motor activity sessions WND 13, 17, 21, and 61), acoustic startle testing (PND 20 and 60), a Biel water maze learning and memory task (initiated on PND 26 or 62), and in evaluations of whole-brain measurements and brain morphometric and histologic assessments (PND 21 and 72). RESULTS: There were no adverse effects on reproductive performance in either the F-0 or F-1 parental generations exposed to up to 500 ppm ethylbenzene [Faber et al. Birth Defects Res Part B 77:10-21, 2006]. In the current developmental neurotoxicity component, parental ethylbenzene exposure did not adversely affect offspring survival, clinical condition, body weight parameters, or acquisition of developmental landmarks of the F-2-generation treatment derived offspring. There were no alterations in FOB parameters, motor activity counts, acoustic startle endpoints, or Biel water maze performance in offspring attributed to parental ethylbenzene exposure. A few isolated instances of statistically significant differences obtained in the treatment-derived groups occurred sporadically, and were attributed to unusual patterns of development and/or behavior in the concurrent control group. There were no exposure-related differences in any neuropathology parameters in the F-2-generation treatment derived offspring. CONCLUSIONS: The no observed adverse effect level (NOAEL) for maternal reproductive toxicity, developmental toxicity, and developmental neurotoxicity in this study was considered to be 500 ppm/342 mg/kg/day ethylbenzene, the highest exposure level tested in the study.