Janet A. Springer

A guide for mutagenicity testing using the dominant lethal assay

The dominant lethal assay has been used and continues to be used to provide information about the effects of chemicals on the gonadal cells of male animals. Guidelines for conducting this test are useful but as with any guideline scientists should avoid interpreting them as protocols. Thus this document is a general approach to dominant lethal testing and should be used in conjunction with other available protocols and procedures.

Protocols for the dominant lethal test, host-mediated assay, and in vivo cytogenetic test used in the food and drug administration's review of substances in the gras (generally recognized as safe) list.

Protocols are described for the dominant lethal and in vivo cytogenetics test in rats and the host-mediated assay, using Salmonella typhimurium and Saccharomyces cerevisiae in mice, as used by the Food and Drug Administration in its mutagenicity review of substances from the generally recognized as safe (GRAS) list. In addition proctolols are described for in vitro mutagenicity tests with S. typhimurium and S. cerevisiae and for statistical treatment for evaluation of data from dominant lethal tests.

Effects of prolonged chemical treatment with cyclophosphamide and 6-mercaptopurine in the dominant lethal test system

Abstract The effects of prolonged chemical treatment with cyclophosphamide and 6-mercaptopurine were investigated in the dominant lethal test system. Groups of 15 male Holtzman rats were given 5 daily i.p. injections of the compounds per week for 10 weeks; dose levels were 0 (water), 5.0, 10.0 and 20.0 mg/kg in the cyclophosphamide study and 0 (dimethylsulfoxide), 12.5, 25.0 and 50.0 mg/kg in the 6-mercaptopurine study. Each study also contained a group given i.p. injections of triethylenemelamine (0.025 mg/kg), the positive control compound, which was diluted in water. After the final injection, each male rat was housed with 2 virgin females per week for 2 weeks. Females were killed 14 days from the midweek of cohousing and uterine contents were examined to evaluate the numbers of total implants, dead implants, dead implants per total implants and females with 1 or more, or 2 or more, dead implants. Results from the cyclophosphamide study indicated a positive dominant lethal effect in both weeks of mating. Negative results were obtained for all levels of 6-mercaptopurine treatment for both weeks, with the exception of 1 aberrant value at the 25.0-mg/kg level for week 1.

A collaborative dominant lethal study of triethylenemelamine in the rat

Abstract A collaborative dominant lethal study of the known mutagen triethylenemelamine (TEM) was conducted in male rats. Six laboratories were supplied with animals from a common source and with TEM of the same lot number in an attempt to ensure uniformity of strain and drug effect. Male Sprague-Dawley rats, which were proven breeders, were divided into five groups of 15 animals each and were given a single ip injection of TEM at 0 (saline), 0.125, 0.250, 0.375, or 0.500 mg/kg. respectively. After the injection, each male was housed with two virgin females (200 g) each week for a 10-week period. The females were sacrificed by asphyxiation 14 days after the midweek of cohousing, and corpora lutea were counted and the uterine contents were examined. The results showed a highly significant difference among laboratories for the number of corpora lutea per pregnant female and for preimplantation losses over all doses and weeks. Variability among laboratories was least for total implants in both treated and control groups and dead implants in the control group. Although the variation was great between laboratories with respect to the several parameters considered, the results obtained by each laboratory did show that TEM has a significant mutagenic effect in the dominent lethal test.

Additional statistical evaluation and pharmacological considerations of hycanthone methanesulfonate‐induced dominant lethality

Investigations of the mutagenicity of hycanthone methanesulfonate (HCT) in mammals have led to varied results. To avoid ambiguous interpretation of significant results previously reported in two dominant-lethal studies of HCT a nested analysis of variance was performed on the number of corpora lutea per pregnant female, the number of implantations per pregnant female, average preimplantation losses, the number of dead implants per pregnant female, and the number of live implants. The analysis showed that variability among males and females was not responsible for the reported significant effects and consequently established the validity of the previous studies. Based on a comparison of the mammalian mutagenicity work performed utilizing hycanthone, pharmacokinetic factors are thought to be responsible for the differences seen; consideration of these factors in the future may lead to fewer false-negative results in mammalian systems.

Comparing Human and Animal Cancer Risk Models Using Multistage Theory: Exponential vs. Relative vs. Additive Risk

Comparing human and animal cancer risk assessments often results in inconsistencies due to different extrapolation methodologies. Descriptive relative risk models using cumulative dose are typically used in human epidemiological studies, whereas multistage additive risk models using dose rate are typically used in high dose animal studies to extrapolate to human exposures. Biologically motivated nonclonal and clonal multistage risk models are reviewed for their unifying role in consistently comparing human and animal risks and for suggesting alternate cumulative dose epidemiological risk models.

Carcinogenic Risk of Benzene Derived from Animal and Human Data

Benzene is widely considered to be an animal and human carcinogen but there are wide-ranging estimates of its potency. Risk assessments of benzene and human leukemia have primarily focused on exposure by inhalation over an occupational lifetime. The Food and Drug Administration (FDA) regulates benzene as a contaminant in food additives and thus is interested in the risk from lifetime exposure by the oral route. The risk from lifetime ingestion of benzene is assessed using the NTP animal gavage studies and the National Institute of Occupational Safety and Health (NIOSH) epidemiology studies, and the results are compared. The unit risks derived from the human epidemiology data for leukemia are similar to those derived from the rodent data when the animal risks are summed over all organ sites in each sex grouping. The collective unit risk values calculated from the animal data were.038 and.039 per mg/kg body weight/day exposure for male and female mice, respectively. The unit risks calculated from the human epidemiology data were.043 and.024 per mg/kg body weight/day exposure using the relative and absolute risk models, respectively.