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

Risk evaluation and hazard classification for tetrahydrofuran (THF) is based partly on the incidences of renal tumors in male F344/N rats reported in a 2-year carcinogenicity study by the National Toxicology Program (NTP). A Pathology Working Group (PWG) was commissioned to conduct an independent review of the kidney slides from this bioassay (along with two subchronic studies) to assess renal changes in light of recent scientific work on pathogenesis of pre-neoplastic and neoplastic lesions in rat kidney. PWG pathologists confirmed the NTP assessment that adenomas were non-statistically increased in animals exposed to the highest level of THF. However, when pre-neoplastic and neoplastic lesions were combined, there was no difference between control and THF-exposed groups. Also, the majority of these proliferative lesions were in rats with severe chronic progressive nephropathy (CPN). Accordingly, the PWG concluded that renal lesions in the control and THF-exposed groups resulted primarily from regenerative processes associated with advanced CPN. Based on an alpha2u-globulin/hyaline droplet response observed in a 4-week study with THF, the PWG could not exclude the possibility of both advanced CPN and low-grade α2u-g nephropathy contributing to the renal proliferative lesions developing chronically in high-dose males. Neither condition has a pathologic counterpart in humans.