Histopathologic changes in the kidneys of male F344 rats from a 2-year inhalation carcinogenicity study of tetrahydrofuran: A pathology working group review and re-evaluation

A review of the toxicological and environmental hazards and risks of tetrahydrofuran

We present in this paper a review of the toxicological and environmental hazards, exposures and risks of tetrahydrofuran (THF; CASRN 109-99-9). THF is a polar solvent and monomer that is easily absorbed by all routes of exposure. The acute toxicity of THF is low to moderate by all routes. Irreversible corrosive damage to the eye can result from direct contact. However, THF is neither a skin irritant, nor sensitizer. Studies in vitro and in vivo have shown that THF is not mutagenic. Chronic studies have found benign tumors in the kidneys of male rats and in the livers of female mice. These findings have been examined, and although a mode of action is not known, the weight of evidence suggests that these tumors are likely not relevant to human health, but instead secondary to rodent-specific modes of action. THF produces transient sedative effects in rats at high concentrations but no significant neurobehavioral changes or neuropathology in sub-chronic studies. There were no specific effects reported on reproduction or developmental toxicity in rats or mice, with non-specific developmental toxicity observed only in the presence of significant maternal toxicity. The log K(ow) value for THF is less than 3, indicating a low potential for bioaccumulation. THF is inherently biodegradable, thus is not expected to be environmentally persistent. THF does not present an ecotoxicity hazard based on test results in fish, aquatic invertebrates and plants. Exposures to THF in the workplace, to consumers and via environmental releases were modeled and all found to fall below the derived toxicity thresholds.

Consideration of rat chronic progressive nephropathy in regulatory evaluations for carcinogenicity

Chronic progressive nephropathy (CPN) is a spontaneous renal disease of rats which can be a serious confounder in toxicology studies. It is a progressive disease with known physiological factors that modify disease progression, such as high dietary protein. The weight of evidence supports an absence of a renal counterpart in humans. There is extensive evidence that advanced CPN, particularly end-stage kidney, is a risk factor for development of a background incidence of atypical tubule hyperplasia and renal tubule tumors (RTT). The likely cause underlying this association with tubule neoplasia is the long-term increased tubule cell proliferation that occurs throughout CPN progression. As a variety of chemicals are able to exacerbate CPN, there is a potential for those exacerbating the severity up to and including end-stage kidney to cause a marginal increase in RTT and their precursor lesions. Extensive statistical analysis of National Toxicology Program studies shows a strong correlation between high-grade CPN, especially end-stage CPN, and renal tumor development. CPN as a mode of action (MOA) for rat RTT has received attention from regulatory authorities only recently. In the absence of toxic effects elsewhere, this does not constitute a carcinogenic effect of the chemical but can be addressed through a proposed MOA approach for regulatory purposes to reach a decision that RTT, developing as a result of CPN exacerbation in rats, have no relevance for human risk assessment. Guidelines are proposed for evaluation of exacerbation of CPN and RTT as a valid MOA for a given chemical.

Chemically exacerbated chronic progressive nephropathy not associated with renal tubular tumor induction in rats: an evaluation based on 60 carcinogenicity studies by the national toxicology program

Chronic progressive nephropathy (CPN) is a common age-related degenerative-regenerative disease of the kidney that occurs in both sexes of most strains of rats. Recently, claims have been made that enhanced CPN is a mode of action for chemically induced kidney tumors in male rats and that renal tubular tumors (RTTs) induced by chemicals that concomitantly exacerbate CPN are not relevant for human cancer risk assessments. Although CPN is an observable histopathological lesion that may be modified by diet, the etiology of this disease and the mechanisms for its exacerbation by chemicals are unknown, and it fails to meet fundamental principles for defining carcinogenic modes of action and human relevance. Our comprehensive evaluation of possible relationships between exacerbated CPN and induction of RTTs in 58 carcinogenicity studies, conducted by the National Toxicology Program, in male and 11 studies in female F344 rats using 60 chemicals revealed widespread inconsistency in the claimed association. Because the proposed hypothesis lacks evidence of biological plausibility, and due to inconsistent relationships between exacerbated CPN and kidney tumor incidence in carcinogenicity studies in rats, dismissing the human relevance of kidney tumors induced by chemicals that also exacerbate CPN in rats would be wrong.