Quantitative correlation between carcinogenicity and sister chromatid exchange induction in vivo for a group of 11 N-nitroso derivatives

Sister-chromatid exchange: second report of the Gene-Tox Program

This paper reviews the ability of a number of chemicals to induce sister-chromatid exchanges (SCEs). The SCE data for animal cells in vivo and in vitro, and human cells in vitro are presented in 6 tables according to their relative effectiveness. A seventh table summarizes what is known about the effects of specific chemicals on SCEs for humans exposed in vivo. The data support the concept that SCEs provide a useful indication of exposure, although the mechanism and biological significance of SCE formation still remain to be elucidated.

Induction of sister chromatid exchange in multiple murine tissues in vivo by various methylating agents

In order to study the reliability of in vivo sister chromatid exchange (SCE) assays for predicting carcinogenicity, several known animal carcinogens were tested in a multicellular in vivo SCE assay and an in vivo/in vitro murine lymphocyte assay. The methylating agents 1,2-dimethylhydrazine.2 HCl (DMH), dimethylnitrosamine (DMN), methylnitrosourea (MNU), methyl methane-sulphonate (MMS), and methylazoxymethanol acetate (MAM) were tested for SCE induction in several murine tissues in vivo, including bone marrow, alveolar macrophages, regenerating and intact liver, and kidney from B6D2F1 mice. In all cell types, clear dose-responses were observed following exposure of mice to subcytotoxic fractions of the LD50 dose of DMH, MNU, or MMS. DMN (0.03-0.27 mmol/kg) produced small, although not dose-related, increases in SCE in all cell types. At the doses tested (0.06 and 0.08 mmol/kg), MAM did not induce elevated SCE in the various cell types. Following a series of multiple i.p. injections of low, non-toxic doses of DMH (0.15 mmol/kg, once a week, for 10 weeks), significant differences were observed in intact vs. regenerating liver and in single vs. multiple injections in regenerating liver. Following exposure of B6D2F1 mice to a single i.p. injection of 0.25 mmol/kg DMN, DMH, or MMS or 0.19 mmol/kg MNU, SCE responses were evaluated in Concanavalin A (Con A)- and LPS-stimulated blood and spleen lymphocytes. Considerable cytotoxicity was observed in blood lymphocytes. In Con A- and LPS-stimulated spleen lymphocytes, DMH-, and DMN- and MMS-induced SCE frequencies were approximately 1.5-2 x baseline levels and MNU-induced SCE were approximately three- to fourfold higher than baseline values in cultures initiated at 1 and 24 h postexposure. At 48 and 72 h after an i.p. injection of 0.131 mmol/kg MNU, SCE responses in lymphocytes were approximately 2 x baseline levels. At 24 h following one, two, or four injections (one/week) of 0.075 mmol/kg MNU dose-related increases in SCE were observed in spleen lymphocytes. These studies illustrate that carefully designed in vivo SCE assays may have the capacity to predict the tumorigenic potential of chemical agents.

Evaluation of sister chromatid exchange as an indicator of sensitivity to N-ethyl-N-nitrosourea-induced carcinogenesis in rats

Sister chromatid exchange (SCE) frequencies in peripheral lymphocytes are a frequently used endpoint to indicate exposures to genotoxins in groups of humans. The aim of this study was to ascertain, in an experimental design, whether or not SCE rates have any association with the risk of cancer at the individual level in rats exposed to a known carcinogen. Individual SCE rates were determined in three consecutive analyses in cultured blood lymphocytes of 50 adult male Wistar rats. Analyses were done before as well as 24 hr and 7 days after a single intraperitoneal administration of 0, 25, 50, or 75 mg/kg of N-ethyl-N-nitrosourea (ENU). The animals were followed until death; also, the relationship between SCEs and carcinogenic outcome, i.e., the presence or absence of tumors, and their latency period were examined. ENU significantly decreased the life expectancy of the rats. The tumor types most clearly associated with ENU treatment were various gliomas and thyroid-gland and testicular tumors. ENU induced a moderate (maximally 1.6-fold) increase in the mean frequency of SCEs/cell at both sampling times after treatment. The effect was somewhat more pronounced 1 day rather than 1 week after treatment. The mean SCE rates in rats with ENU-specific cancers or in animals with early or multiple tumors did not differ from those in animals that survived no less than 65 weeks or longer without developing tumors. In ENU-treated animals with tumors, no relationship was found between the mean SCE rate and survival time. It is concluded that in outbred Wistar rats the SCE response in cultured lymphocytes does not indicate individual susceptibility to the carcinogenic action of ENU. On a group basis, however, animals with high SCE rates were shown to have increased risk of cancer.

Utilization of the quantitative component of the information obtained from short term tests

We are referring in this paper to traditional tests of genotoxicity that are essentially detecting properties correlated with the initiating activity of chemical and physical agents. In classical initiation-promotion experiments, promotion is changing dramatically tumor frequency, but a better correlation between initiation and tumor frequency may exist when carcinogenesis experiments in rodents utilize only an initiator agent. End points, metabolism and target organs of short term test may be not optimal in respect to the real initiation process. In conclusion, we have to expect a very high level of statistical noise in the correlation between short term tests and carcinogenicity in rodents. Correlations of this type are not unique in science. For instance, the input data for weather forecasting and the real weather are often related with a high level of statistical noise. But this is not considered a good reason for throwing away the quantitative component of the input information. Similarly, in this context the utilization of the quantitative component of the information improves in a moderate, but not negligible way the predictive value of the information offered from short term tests, especially when they are used in a battery. In this review we discuss the possibility that, when studying correlations between genotoxicity tests and carcinogenicity in rodents, a quantitative approach could replace with important advantages the qualitative approach generally adopted up to the present time. The qualitative approach appeared as a more modest but realistic approach to the complexities of these correlation studies. What we suggest is that a quantitative approach mathematically not less correct than the qualitative approach is feasible. More precise and useful information for risk assessment evaluations can be obtained.

Induction of sister-chromatid exchange (SCE) and cell-cycle inhibition in mouse peripheral blood B lymphocytes exposed to mutagenic carcinogens in vivo

To determine the sensitivity of the mouse peripheral blood lymphocyte (PBL) culture system, male B6C3f1 mice were injected i.p. with either 2-acetylaminofluorene (AAF) (20, 40, 80, 160 mg/kg), benzo[a]pyrene (BP) 25, 75, 150, 300 mg/kg), dichlorvos (DCV) (5, 15, 25, 35 mg/kg), ethyl methanesulfonate (EMS) (10, 30, 90, 180, 270 mg/kg), or N-nitrosomorpholine (NM) (37.5, 75, 150, 300 mg/kg) dissolved in either RPMI 1640 (DCV, EMS, NM) or sunflower oil (AAF, BP). 24 h later blood was removed by cardiac puncture, and the lymphocytes were cultured in the presence of lipopolysaccharide for analysis of SCE in B lymphocytes. All 4 mutagenic carcinogens (AAF, BP, EMS, NM) induced significant dose-related increases in SCE frequency. DCV, a potent neurotoxicant, caused no change in the baseline SCE frequency. At the highest concentration of each chemical examined, AAF caused a 1.6-fold increase, EMS a 1.8-fold increase, NM a 3.0-fold increase, and BP a 3.1-fold increase in SCE frequency compared to concurrent controls. A comparison of these results for PBLs with those reported in the literature for bone marrow cells indicates that PBLs offer a good quantitative and qualitative estimate of the SCE-inducing potential for these 5 compounds in bone marrow cells.

Stage-specific induction of sister-chromatid exchanges in utero

In order to correlate the induction of sister-chromatid exchanges (SCE) to biological endpoints, and elucidate aspects of this relationships, 7,12-dimethylbenz[a]anthracene (DMBA) and methyl methanesulfonate (MMS), chemicals with different biological actions at different stages in development, have been evaluated for their ability to induce SCE at different gestational ages in the Sprague-Dawley rat. Transplacental exposure to these agents was accomplished by a recently developed intraperitoneal infusion technique to replenish metabolically degraded 5-bromo-2'-deoxyuridine used for SCE visualization. Maternal bone marrow and whole fetal tissue, fetal liver and fetal brain were compared. Day-9 embryo was found to be very sensitive to the effects of both agents, with the ability to induce SCE declining in development in whole fetus and fetal organs. The embryotoxic effects of the agents seem to be ones best correlated with the capacity of the agents to induce SCE. Also, fetal liver is more sensitive than fetal brain to the effects of DMBA compared with MMS, suggesting fetal metabolic activation may have occurred. Measurement of the amount of radiolabelled DMBA reaching the fetal tissue used to estimate SCE indicates that the amount of chemical reaching the fetus does not account for the increased sensitivity, especially at Day 9. Some factor(s) in development, such as differentiation stage, rather than the fetal accessibility to chemical, seem to be important in the induction of SCE in utero.