A comprehensive review of published and previously unpublished studies was performed to evaluate the neonicotinoid insecticides for evidence of developmental neurotoxicity (DNT). These insecticides have favorable safety profiles, due to their preferential affinity for nicotinic receptor (nAChR) subtypes in insects, poor penetration of the mammalian blood–brain barrier, and low application rates. Nevertheless, examination of this issue is warranted, due to their insecticidal mode of action and potential exposure with agricultural and residential uses. This review identified in vitro, in vivo, and epidemiology studies in the literature and studies performed in rats in accordance with GLP standards and EPA guidelines with imidacloprid, acetamiprid, thiacloprid, clothianidin, thiamethoxam, and dinotefuran, which are all the neonicotinoids currently registered in major markets. For the guideline-based studies, treatment was administered via the diet or gavage to primiparous female rats at three dose levels, plus a vehicle control (≥20/dose level), from gestation day 0 or 6 to lactation day 21. F1 males and females were evaluated using measures of motor activity, acoustic startle response, cognition, brain morphometry, and neuropathology. The principal effects in F1 animals were associated with decreased body weight (delayed sexual maturation, decreased brain weight, and morphometric measurements) and acute toxicity (decreased activity during exposure) at high doses, without neuropathology or impaired cognition. No common effects were identified among the neonicotinoids that were consistent with DNT or the neurodevelopmental effects associated with nicotine. Findings at high doses were associated with evidence of systemic toxicity, which indicates that these insecticides do not selectively affect the developing nervous system.
The tissue distribution and macromolecular binding of benzene was studied over a dose range spanning nine-orders of magnitude to determine the nature of the dose-response and to establish benzene's internal dosimetry at doses encompassing human environmental exposures. [14C]-Benzene was administered to B6C3F1 male mice at doses ranging between 700 pg/kg and 500 mg/kg body wt. Tissues, DNA and protein were analyzed for [14C]-benzene content between 0 and 48 h post-exposure (625 Ng/kg and 5 microg/kg dose) by accelerator mass spectrometry (AMS). [14C]-Benzene levels were highest in the liver and peaked within 0.5 h of exposure. Liver DNA adduct levels peaked at 0.5 h, in contrast to bone marrow DNA adduct levels, which peaked at 12-24 h. Dose-response assessments at 1 h showed that adducts and tissue available doses increased linearly with administered dose up to doses of 16 mg/kg body wt. Tissue available doses and liver protein adducts plateau above the 16 mg/kg dose. Furthermore, a larger percentage of the available dose in bone marrow bound to DNA relative to liver. Protein adduct levels were 9- to 43-fold greater than DNA adduct levels. These data show that benzene is bioavailable at human-relevant doses and that DNA and protein adduct formation is linear with dose over a dose range spanning eight orders of magnitude. Finally, these data show that the dose of bioactive metabolites is greater to the bone marrow than the liver and suggests that protein adducts may contribute to benzene's hematoxicity.
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