Iodoform, a halogenated alkane with antiseptic and anti-infective properties, was selected for bioassay by the National Cancer Institute because of its similarity to methyl iodide, which has produced sarcomas in BD rats and to chloroform, a compound which has been found to induce hepatomas in NLC mice.
A bioassay for possible carcinogenicity of technical-grade iodoform was conducted using Osborne-Mendel rats and B6C3F1 mice. Iodoform in corn oil was administered by gavage, at either of two dosages, to groups of 50 male and 50 female animals of each species. Administration of the chemical occurred 5 days per week, for a period of 78 weeks, followed by an observation period of 34 weeks for rats and 13 or 14 weeks for mice. The high and low time-weighted average dosages of iodoform were, respectively, 142 and 71 mg/kg/day for male rats, 55 and 27 mg/kg/day for female rats, and 93 and 47 mg/kg/day for male and female mice. For each species, 20 animals of each sex were placed on test as vehicle controls. These animals were gavaged with pure corn oil at the same rate as the high dose group of the same sex. Twenty animals of each sex were placed on test as untreated controls for each species. These animals were not intubated.
A significant positive association between dosage and mortality was observed in male rats but not in female rats or in mice of either sex. Adequate numbers of animals in all groups survived sufficiently long to be at risk from late-developing tumors.
No statistical significance could be attributed to the incidences of any neoplasms in rats or mice of either sex when compared to their respective controls.
Under the conditions of this bioassay, no convincing evidence was provided for the carcinogenicity of iodoform in Osborne-Mendel rats or B6C3F1 mice.