NTP technical report on the toxicity studies of Methylene Bis(thiocyanate) (CAS No. 6317-18-6) Administered by Gavage to F344/N Rats and B6C3F1 Mice.

Methylene bis(thiocyanate) is used as a biocide in a number of applications. Its major use is in water cooling systems and paper mills as an inhibitor of algae, fungi, and bacteria. Methylene bis(thiocyanate) was selected for study because of the potential for human exposure to the compound and because of the interest in organothiocyanates as a chemical class. Toxicity studies of methylene bis(thiocyanate) (approximately 98% pure) were conducted with male and female F344/N rats and B6C3F1 mice; the compound was administered to the animals by gavage in an aqueous methyl cellulose vehicle for 2 weeks or 13 weeks. In addition to these studies, the genetic toxicity of methylene bis(thiocyanate) was evaluated by determining mutagenicity in Salmonella typhimurium with and without S9 activation and frequency of micronucleated normochromatic erythrocytes in the peripheral blood of mice. In the 2-week studies, groups of five rats and five mice per sex were administered methylene bis(thiocyanate) at concentrations of 0, 10, 20, 40, 80, and 160 mg/kg body weight. All animals in the two highest dose groups (80 and 160 mg/kg) died by Day 2 of the studies. Except for one female rat, all animals receiving 40 mg/kg methylene bis(thiocyanate) also died before the end of the studies. Few significant gross lesions were observed in the 80 and 160 mg/kg groups. Clinical observations were similar to those reported for cyanide toxicity and included dyspnea, tremors. and ataxia. The stomach, which was identified as the target organ in rats and mice surviving for at least 24 hours, had necrotic inflammatory lesions of the mucosal surface of both the glandular and nonglandular portions. In the 13-week studies, groups of 10 rats and 10 mice per sex were administered methylene bis(thiocyanate) at concentrations of 0, 1, 2, 4, 8, and 16 mg/kg body weight. In the rat study, deaths occurred in the 2, 4, 8, and 16 mg/kg groups, while in the mouse study, deaths occurred only in the 8 and 16 mg/kg groups. As in the 2-week studies, the stomach was identified as the primary target organ. However, the lower doses administered in the 13-week studies resulted in gastric effects that were limited to the forestomach and consisted primarily of squamous mucosal hyperplasia and hyperkeratosis. Rats receiving the higher doses of methylene bis(thiocyanate) developed a mild anemia, and sperm motility was decreased in male rats receiving 4 or 8 mg/kg. Methylene bis(thiocyanate) was not mutagenic in S. typhimurium, with or without S9 activation. The frequencies of micronucleated normochromatic erythrocytes in the peripheral blood of dosed and control mice were similar. Chemical disposition studies of [14C]-labeled methylene bis(thiocyanate) were conducted in male F344 rats. In these studies, more than 90% of the administered radioactivity was eliminated in 48 hours. However, as the dose was increased from 0.2 to 1 to 10 mg/kg, greater percentages of the administered radioactivity remained in the tissues. Blood cyanide concentrations were increased shortly after the administration of 10 mg/kg [14C]-methylene bis(thiocyanate) but were similar to control values 2 hours after dosing. Overall, the toxic effects of methylene bis(thiocyanate) were consistent with those of an irritant chemical administered by gavage. There was also some indication that the release of cyanide may result in acute toxicity at the higher dose levels used in these studies. The no-observed-adverse-effect level for forestomach lesions in the 13-week studies was 4 mg/kg for male rats and 2 mg/kg for female rats and male and female mice. Synonyms: MBT; methylene-bis-thiocyanate; methylene bisthiocyanate; methylene dithiocyanate.