o-Chloropyridine is used as an intermediate in synthetic organic, pharmaceutical, and agricultural chemical (fungicides, herbicides) manufacture. It is also used as a catalyst for phase transfer and is a key intermediate in the manufacture of pyrithione-based biocides for use in cosmetics and various pharmaceutical products. o-Chloropyridine is available in purified (99%), technical (95%), or crude (80%) grades. o-Chloropyridine was nominated for testing by the NTP based on increasing production and use as a site-limited pharmaceutical and agrochemical intermediate, the potential for occupational and environmental exposures during its manufacture, its persistence in the environment (lasting longer than 6 months), evidence of mutagenicity based on results of several short-term test systems, and suspicion of carcinogenicity based on effects associated with structurally related chemicals. Male and female F344/N rats and B6C3F1/N mice received o-chloropyridine (99% pure) dermally for 2 weeks or in drinking water for 3 months. Genetic toxicology studies were conducted in Salmonella typhimurium and mouse peripheral blood erythrocytes.
In the 2-week dermal studies, groups of five male and five female rats and mice were administered o-chloropyridine in ethanol 5 days per week over a 16-day period (12 dose days) at doses of 0, 6.25, 12.5, 25, 50, or 100 mg o-chloropyridine/kg body weight. Vehicle control animals were administered ethanol alone. A constant concentration of test chemical per dose concentration was administered to each animal at volumes of 0.5 mL/kg body weight for rats and 2 mL/kg for mice. All dosed rats and mice survived to the end of the studies. The mean body weights of all dosed groups of rats and mice were similar to those of the vehicle control groups. Liver weights of 50 and 100 mg/kg male rats were significantly greater than those of the vehicle controls. No gross or microscopic lesions were considered related to o-chloropyridine administration.
In the 3-month toxicity studies, groups of 10 male and 10 female F344/N rats and B6C3F1/N mice were exposed to o-chloropyridine in drinking water at concentrations of 0, 10, 30, 100, 300, or 1,000 ppm (equal to average daily doses of approximately 1, 3, 9, 25, and 65 mg o-chloropyridine/kg body weight to male rats, 1, 3, 9, 27, and 70 mg/kg to female rats, 1.5, 4.5, 15, 41, and 110 mg/kg to male mice, and 1.3, 4, 12, 38, and 92 mg/kg to female mice) for 3 months. Additional groups of 10 male and 10 female rats designated for clinical pathology testing were exposed to the same concentrations for 22 days.
All rats survived to the end of the study. Mean body weights of male and female rats exposed to 1,000 ppm were significantly less than those of the controls. Water consumption by the 1,000 ppm rats was less than that by the control groups during the first week of the study. In the 1,000 ppm groups, thinness was noted in seven of 10 male rats and all female rats on day 8, likely due to dehydration, and also in five of 10 male rats on day 64. The absolute and relative (except 100 ppm) right kidney weights of all exposed groups of male rats and of groups of female rats (≥ 30 ppm) were greater than controls. Absolute and relative liver weights of male rats (≥ 100 ppm) and female rats (≥ 30 ppm) were significantly greater than those of the control groups. Epididymal sperm counts were significantly lower in male rats exposed to 1,000 ppm, indicating that o-chloropyridine exhibits the potential to be a reproductive toxicant. In the liver of male rats, the incidence of clear cell focus (1,000 ppm) and the incidences and severities of hepatocyte cytoplasmic vacuolization (≥ 300 ppm) were significantly higher. The incidence of hepatocyte cytoplasmic vacuolization was significantly greater in female rats also (1,000 ppm). There were also several low-magnitude histologic and hematologic responses in male and female rats suggesting an erythron effect characterized by a decreased erythron (≥ 300 ppm males and females) with a compensatory erythropoietic response: increased reticulocyte counts (≥ 300 ppm males, 1,000 ppm females) and hematopoietic cell proliferation in the spleen (≥ 300 ppm males, 1,000 ppm females). Splenic congestion (1,000 ppm males, 10 ppm and ≥ 100 ppm females) was also observed and may have been related to the erythron effect.
There were no treatment-related deaths in either sex of mice. The final mean body weight and mean body weight gain of 1,000 ppm male mice were significantly less than those of the control group; the mean body weight gain of 300 ppm female mice was significantly greater than that of the controls. Water consumption by the 1,000 ppm groups was less than that of the control groups during the first week of the study. The liver weights of all exposed groups of male mice and of 300 and 1,000 ppm female mice and the kidney weights of 1,000 ppm males (relative) and females (absolute and relative) were significantly greater than those of the controls. The incidences of hepatocyte centrilobular hypertrophy were significantly increased in the 300 and 1,000 ppm groups of male and female mice, with exposure concentration-related increases in severity. An erythron effect, similar to that observed in rats, was observed in 1,000 ppm female mice only and was characterized by a small decrease in hematocrit value, hemoglobin concentration, and erythrocyte count; no accompanying erythropoietic response was observed in mice.
o-Chloropyridine was mutagenic in S. typhimurium strains TA98 and TA100 when tested with exogenous metabolic activation enzymes from rat or hamster liver; no mutagenic activity was observed in the absence of metabolic activation. In vivo, no increases in the frequencies of micronucleated erythrocytes were observed in peripheral blood of male or female mice exposed to o-chloropyridine for 3 months in drinking water.
Under the conditions of these 3-month drinking water studies, there were treatment-related organ weight changes and lesions in male and female rats and mice. The major target tissues in rats affected by o-chloropyridine exposure included the kidney, spleen, bone marrow, and liver; the major target organ in mice was the liver. The measurement most sensitive to o-chloropyridine exposure in male rats was increased absolute (all exposure groups) and relative (all exposure groups except 100 ppm) kidney weights in the absence of histopathologic changes [lowest-observed-effect level (LOEL) = 10 ppm]. In female rats, a LOEL of 10 ppm was based on splenic congestion, observed in all treated groups (except 30 ppm), with hematopoietic cell proliferation and pigmentation in the spleen, bone marrow hyperplasia, and hematological changes at higher exposure concentrations. This pattern of erythropoietic responses in the spleen and bone marrow and hematologic changes was also observed in male rats at 300 ppm or greater doses. In male mice, absolute and relative liver weights were significantly higher than controls in all treated groups (LOEL 10 ppm), with histologic changes (centrilobular hepatocyte hypertrophy) occurring at 300 ppm and 1,000 ppm. In female mice, absolute and relative liver weights were significantly higher than controls, with increased incidences of centrilobular hepatocyte hypertrophy occurring at similar exposure concentrations (LOEL 300 ppm).