Mouse specific lung tumors from CYP2F2-mediated cytotoxic metabolism: an endpoint/toxic response where data from multiple chemicals converge to support a mode of action.

It is proposed that metabolism of several structurally-related chemicals by CYP2F isoforms of the cytochromes P450 family results in a cytotoxicity-driven mode of action in organs high in CYP2F; namely, CYP2F2 in nasal and lung tissue in mice and CYP2F4 in nasal tissues in rats. Importantly, the CYP2F1 isozyme expressed in humans appears to have a low capacity to metabolize these compounds. In mice, the resultant cytotoxicity and subsequent regenerative hyperplasia is hypothesized drive an increase in lung tumors that are mostly benign and are not life shortening. Although a complete picture of the mode of action has not been developed in any one model compound, data from the individual compounds can be combined to synthesize and reinforce confidence in the CYP2F toxicity hypothesis. For coumarin, naphthalene, and styrene, inhibition of toxicity with inhibition of CYP2F2 has been demonstrated. Rat CYP2F4 appears to be equally active in metabolizing these chemicals; however, CYP2F4 occurs to a much lower extent in rat Clara cells and levels of metabolites produced are not sufficient to cause lung cytotoxicity. Human lungs contain far fewer of Clara cells than rats or mice, and human lung microsomes failed to, or only marginally, metabolize these compounds. In addition, the human lung differs markedly from the mouse lung in the morphology of its Clara cells, which make humans much less sensitive than mice to toxicity due to reactive metabolites. The absence of a role for CYP2E1-generated metabolites (primarily alkyl oxidation vs. ring-oxidation) in mouse pulmonary effects was demonstrated by the lack of protection from styrene toxicity by CYP2E1 inhibitor, or reduction of toxicity in CYP2E1-knockout mice, and lack of lung toxicity of the primary metabolite of ethylbenzene. The chemicals used as examples of this mode of action generally are negative in standard genotoxicity assays. Apart from increased SCE, no consistent pattern in genotoxicity results was found among these chemicals. Thus, while lung tumors from bronchiolar cell cytotoxicity are theoretically possible in humans, it is unlikely that metabolism by CYP2F1 would produce levels of cytotoxic metabolites in human lungs sufficient to result in lung cytotoxic responses and thus tumors. Therefore, it is unlikely several chemicals that cause mouse lung tumors via CYP2F2 metabolism will cause lung tumors in humans.

Review of reproductive and developmental toxicity studies with isopropanol.

Published studies for reproductive and developmental toxicity conducted with isopropanol have been conducted by the inhalation and oral gavage routes of administration. Interpretation of the data from these studies has resulted in discussions regarding NOAELs and additional benchmark dose modeling publications. Unpublished reproductive and developmental toxicity studies administered in the drinking water were also conducted by BIBRA, and the results of those studies are presented here. In addition, all of the reproductive and developmental toxicity studies conducted with isopropanol are summarized and evaluated for concordance of effects and NOAELs. Endpoints of concern for regulatory agencies were decreases in male mating index and reductions in postnatal pup survival. Original study reports were evaluated and data collated to address these two endpoints, and the data summarized. Data are presented suggesting that there were technical problems in the study that implied a decrease in male mating index, and based on the results from the drinking water studies, the weight of evidence suggests that isopropanol does not affect male mating or fertility at dose levels of up to 1000 mg/kg/day. The weight of evidence suggests that isopropanol can cause decreases in postnatal pup survival following oral gavage administration of 1000-1200 mg/kg/day to the dams. The NOAEL for this endpoint with oral gavage administration was 700 mg/kg/day. Indications of maternal toxicity were also an important predictor for decreased postnatal survival. Decreased postnatal pup survival was also noted in the drinking water studies with isopropanol with a LOAEL of 2278 mg/kg/day and a NOAEL of 1947 mg/kg/day.

Subchronic toxicity of ethylene glycol in Wistar and F-344 rats related to metabolism and clearance of metabolites.

Ethylene glycol (CAS RN 107-21-1) can cause kidney toxicity via the formation of calcium oxalate crystals in a variety of species, including humans. Numerous repeated dose studies conducted in rats have indicated that male rats are more susceptible than female rats. Furthermore, subchronic and chronic studies using different dietary exposure regimens have indicated that male Wistar rats may be more sensitive to renal toxicity than male Fischer-344 (F-344) rats. This study was conducted to compare the toxicity of ethylene glycol in the two strains of rats under identical exposure conditions and to evaluate the potential contribution of toxicokinetic differences to strain sensitivity. Ethylene glycol was mixed in the diet at concentrations to deliver constant target dosage levels of 0, 50, 150, 500, or 1000 mg/kg/day for 16 weeks to groups of 10 male Wistar and 10 male F-344 rats based on weekly group mean body weights and feed consumption. Kidneys were examined histologically for calcium oxalate crystals and pathology. Samples of blood, urine, and kidneys from satellite animals exposed to 0, 150, 500, or 1000 mg/kg/day for 1 or 16 weeks were analyzed for ethylene glycol, glycolic acid, and oxalic acid. Treatment of Wistar rats at 1000 mg/kg/day resulted in the death of two rats; in addition, at 500 and 1000 mg/kg/day, group mean body weights were decreased compared to control throughout the 16 weeks. In F-344 rats exposed at 1000 mg/kg/day and in Wistar rats receiving 500 and 1000 mg/kg/day, there were lower urine specific gravities, higher urine volumes, and increased absolute and relative kidney weights. In both strains of rats treated at 500 and 1000 mg/kg/day, some or all treated animals had increased calcium oxalate crystals in the kidney tubules and crystal nephropathy. The effect was more severe in Wistar rats than in F-344 rats. Accumulation of oxalic acid in the kidneys of both strains of rats was consistent with the dose-dependent and strain-dependent toxicity. As the nephrotoxicity progressed over the 16 weeks, the clearance of ethylene glycol and its metabolites decreased, exacerbating the toxicity. Benchmark dose analysis indicated a BMDL05 for kidney toxicity in Wistar rats of 71.5 mg/kg/day; nearly fourfold lower than in F-344 rats (285 mg/kg/day). This study confirms that the Wistar rat is more sensitive to ethylene glycol-induced renal toxicity than the F-344 rat and indicates that metabolism or clearance plays a role in the strain differences.