Persistent effects of Libby amphibole and amosite asbestos following subchronic inhalation in rats.

BACKGROUND: Human exposure to Libby amphibole (LA) asbestos increases risk of lung cancer, mesothelioma, and non-malignant respiratory disease. This study evaluated potency and time-course effects of LA and positive control amosite (AM) asbestos fibers in male F344 rats following nose-only inhalation exposure. METHODS: Rats were exposed to air, LA (0.5, 3.5, or 25.0 mg/m(3) targets), or AM (3.5 mg/m(3) target) for 10 days and assessed for markers of lung inflammation, injury, and cell proliferation. Short-term results guided concentration levels for a stop-exposure study in which rats were exposed to air, LA (1.0, 3.3, or 10.0 mg/m(3)), or AM (3.3 mg/m(3)) 6 h/day, 5 days/week for 13 weeks, and assessed 1 day, 1, 3, and 18 months post-exposure. Fibers were relatively short; for 10 mg/m(3) LA, mean length of all structures was 3.7 µm and 1% were longer than 20 µm. RESULTS: Ten days exposure to 25.0 mg/m(3) LA resulted in significantly increased lung inflammation, fibrosis, bronchiolar epithelial cell proliferation and hyperplasia, and inflammatory cytokine gene expression compared to air. Exposure to 3.5 mg/m(3) LA resulted in modestly higher markers of acute lung injury and inflammation compared to AM. Following 13 weeks exposure, lung fiber burdens correlated with exposure mass concentrations, declining gradually over 18 months. LA (3.3 and 10.0 mg/m(3)) and AM produced significantly higher bronchoalveolar lavage markers of inflammation and lung tissue cytokines, Akt, and MAPK/ERK pathway components compared to air control from 1 day to 3 months post-exposure. Histopathology showed alveolar inflammation and interstitial fibrosis in all fiber-exposed groups up to 18 months post-exposure. Positive dose trends for incidence of alveolar epithelial hyperplasia and bronchiolar/alveolar adenoma or carcinoma were observed among LA groups. CONCLUSIONS: Inhalation of relatively short LA fibers produced inflammatory, fibrogenic, and tumorigenic effects in rats which replicate essential attributes of asbestos-related disease in exposed humans. Fiber burden, inflammation, and activation of growth factor pathways may persist and contribute to lung tumorigenesis long after initial LA exposure. Fiber burden data are being used to develop a dosimetry model for LA fibers, which may provide insights on mode of action for hazard assessment.

Genotoxicity of intermittent co-exposure to benzene and toluene in male CD-1 mice.

Benzene is an important industrial chemical. At certain levels, benzene has been found to produce aplastic anemia, pancytopenia, myeloblastic anemia and genotoxic effects in humans. Metabolism by cytochrome P450 monooxygenases and myeloperoxidase to hydroquinone, phenol, and other metabolites contributes to benzene toxicity. Other xenobiotic substrates for cytochrome P450 can alter benzene metabolism. At high concentrations, toluene has been shown to inhibit benzene metabolism and benzene-induced toxicities. The present study investigated the genotoxicity of exposure to benzene and toluene at lower and intermittent co-exposures. Mice were exposed via whole-body inhalation for 6h/day for 8 days (over a 15-day time period) to air, 50 ppm benzene, 100 ppm toluene, 50 ppm benzene and 50 ppm toluene, or 50 ppm benzene and 100 ppm toluene. Mice exposed to 50 ppm benzene exhibited an increased frequency (2.4-fold) of micronucleated polychromatic erythrocytes (PCE) and increased levels of urinary metabolites (t,t-muconic acid, hydroquinone, and s-phenylmercapturic acid) vs. air-exposed controls. Benzene co-exposure with 100 ppm toluene resulted in similar urinary metabolite levels but a 3.7-fold increase in frequency of micronucleated PCE. Benzene co-exposure with 50 ppm toluene resulted in a similar elevation of micronuclei frequency as with 100 ppm toluene which did not differ significantly from 50 ppm benzene exposure alone. Both co-exposures - 50 ppm benzene with 50 or 100 ppm toluene - resulted in significantly elevated CYP2E1 activities that did not occur following benzene or toluene exposure alone. Whole blood glutathione (GSH) levels were similarly decreased following exposure to 50 ppm benzene and/or 100 ppm toluene, while co-exposure to 50 ppm benzene and 100 ppm toluene significantly decreased GSSG levels and increased the GSH/GSSG ratio. The higher frequency of micronucleated PCE following benzene and toluene co-exposure when compared with mice exposed to benzene or toluene alone suggests that, at the doses used in this study, toluene can enhance benzene-induced clastogenic or aneugenic bone marrow injury. These findings exemplify the importance of studying the effects of binary chemical interactions in animals exposed to lower exposure concentrations of benzene and toluene on benzene metabolism and clastogenicity. The relevance of these data on interactions for humans exposed at low benzene concentrations can be best assessed only when the mechanism of interaction is understood at a quantitative level and incorporated within a biologically based modeling framework.

Incorporation of tissue reaction kinetics in a computational fluid dynamics model for nasal extraction of inhaled hydrogen sulfide in rats.

Rodents exposed to hydrogen sulfide (H2S) develop olfactory neuronal loss. This lesion has been used by the risk assessment community to develop occupational and environmental exposure standards. A correlation between lesion locations and areas of high H2S flux to airway walls has been previously demonstrated, but a quantitative dose assessment is needed to extrapolate dose at lesion sites to humans. In this study, nasal extraction (NE) of 10, 80, and 200 ppm H2S was measured in the isolated upper respiratory tract of anesthetized rats under constant unidirectional inspiratory flow rates of 75, 150, and 300 ml/min. NE was dependent on inspired H2S concentration and air flow rate: increased NE was observed when H2S exposure concentrations or inspiratory air flow rates were low. An anatomically accurate, three-dimensional computational fluid dynamics (CFD) model of rat nasal passages was used to predict NE of inhaled H2S. To account for the observed dependence of NE on H2S exposure concentration, the boundary condition used at airway walls incorporated first-order and saturable kinetics in nasal tissue to govern mass flux at the air:tissue interface. Since the kinetic parameters cannot be obtained using the CFD model, they were estimated independently by fitting a well-mixed, two-compartment pharmacokinetic (PK) model to the NE data. Predicted extraction values using this PK-motivated CFD approach were in good agreement with the experimental measurements. The CFD model provides estimates of localized H2S flux to airway walls and can be used to calibrate lesion sites by dose.

Olfactory transport: a direct route of delivery of inhaled manganese phosphate to the rat brain.

Experiments examining the dosimetry of inhaled manganese generally focus on pulmonary deposition and subsequent delivery of manganese in arterial blood to the brain. Growing evidence suggests that nasal deposition and transport along olfactory neurons represents another route by which inhaled manganese is delivered to certain regions of the rat brain. The purpose of this study was to evaluate the olfactory uptake and direct brain delivery of inhaled manganese phosphate ((54)MnHPO(4)). Male, 8-wk-old, CD rats with either both nostrils patent or the right nostril occluded underwent a single, 90-min, nose-only exposure to a (54)MnHPO(4) aerosol (0.39 mg (54)Mn/m(3); MMAD 1.68 microm, sigma(g) 1.42). The left and right sides of the nose, olfactory pathway, striatum, cerebellum, and rest of the brain were evaluated immediately after the end of the (54)MnHPO(4) exposure and at 1, 2, 4, 8, and 21 d postexposure with gamma spectrometry and autoradiography. Rats with two patent nostrils had equivalent (54)Mn concentrations on both sides of the nose, olfactory bulb, and striatum, while asymmetrical (54)Mn delivery occurred in rats with one occluded nostril. High levels of (54)Mn activity were observed in the olfactory bulb and tubercle on the same side (i.e., ipsilateral) to the open nostril within 1-2 d following (54)MnHPO(4) exposure, while brain and nose samples on the side ipsilateral to the nostril occlusion had negligible levels of (54)Mn activity. Our results demonstrate that the olfactory route contributes to (54)Mn delivery to the rat olfactory bulb and tubercle. However, this pathway does not significantly contribute to striatal (54)Mn concentrations following a single, short-term inhalation exposure to (54)MnHPO(4).