A 90-day chloroform inhalation study in F-344 rats: profile of toxicity and relevance to cancer studies.

Chloroform acts via a nongenotoxic-cytotoxic mode of action to produce cancer if given in doses and at dose rates sufficiently high to produce organ-specific toxicity. In a recent study, chloroform failed to induce cancer in male or female F-344 rats when administered by inhalation for 2 years at 90 ppm, 5 days/week. The present study was undertaken to define the concentration-response curves for chloroform-induced lesions and regenerative cell proliferation in the F-344 rat when exposed by inhalation and to correlate those patterns of toxicity with the results from the inhalation cancer bioassay. Male and female F-344 rats were exposed to airborne concentrations of 0, 2, 10, 30, 90, or 300 ppm chloroform 6 hr/day, 7 days/week for 4 days or 3, 6, 13 weeks. Additional treatment groups were exposed 5 days/week for 13 weeks or were exposed for 6 weeks and held until Week 13. Bromodeoxyuridine was administered via osmotic pumps implanted 3.5 days prior to necropsy and the labeling index (LI, percentage of nuclei in S-phase) was evaluated immunohistochemically. A full-screen necropsy identified the kidney, liver, and nasal passages as the only target organs. This study confirmed that 300 ppm is extremely toxic and would be inappropriate for longer-term cancer studies. The primary target in the kidney was the epithelial cells of the proximal tubules of the cortex, with significantly elevated increases in the LI at concentrations of 30 ppm and above. However, only a marginal increase in the renal LI in the males was seen after exposures of 90 ppm, 5 days/week. Chloroform induced hepatic lesions in the midzonal and centrilobular regions with increases in the LI throughout the liver, but only at 300 ppm exposures. An additional liver lesion seen only at the highly hepatotoxic concentration of 300 ppm was numerous intestinal crypt-like ducts surrounded by dense connective tissue. Enhanced bone growth and hypercellularity in the lamina propria of the ethmoid turbinates of the nose occurred at the early time points at concentrations of 10 ppm and above. At 90 days there was a generalized atrophy of the ethmoid turbinates at concentrations of 2 ppm and above. Cytolethality and regenerative cell proliferation are necessary but not always sufficient to induce cancer because of tissue, sex, and species differences in susceptibility. A combination of a lack of direct genotoxic activity by chloroform, only a marginal induction of cell proliferation in the male rat kidney, and lower tissue-specific susceptibility in the female rat is apparently responsible for the reported lack of chloroform-induced cancer in a long-term inhalation bioassay with F-344 rats.