Lack of effect of carcinogen treatment on development of tumors arising spontaneously in Fischer 344 rats

Review of mononuclear cell leukemia in F-344 rat bioassays and its significance to human cancer risk: A case study using alkyl phthalates

Elevated incidences of mononuclear cell leukemia (MNCL) have been observed in a number of chronic bioassays in the F-344 rat. As this tumor type is unique to the rat and is only common in the F-344 strain, its significance for human cancer risk is unclear. For this reason, a survey of the published literature was undertaken to assess the occurrence and etiology of MNCL in F-344 rats and to evaluate its potential significance to humans using alkyl phthalate data as an example. It was found that MNCL occurs in untreated, aged F-344 rats at a high and variable rate, it is uncommon in most other rat strains, and its background incidence has increased significantly over time. This complicates retrospective data interpretation. MNCL has not been found in other mammalian species and no histologically comparable tumor is found in humans. In general, a statistically significant increase in frequency of a common tumor in the F-344 rat is an insufficient basis for determining that a chemical presents a carcinogenic hazard to humans, particularly when that tumor is not observed in other species. As one example, the alkyl phthalates constitute one group of substances which has been associated with increased MNCL frequency in the F-344 rat after high dietary doses. In evaluating the significance of this increase in MNCL, an extensive toxicological database for phthalates indicates that toxicological effects occur only at relatively high doses, and tumor development (including MNCL) occurs only after an apparent threshold is exceeded. Phthalates are not genotoxic as a class, further supporting the hypothesis of the existence of a threshold. When these considerations are collectively evaluated, it can be concluded that a finding of increased MNCL in F-344 rats exposed for a lifetime to a nongenotoxic chemical is not toxicologically relevant to humans, even when MNCL is observed at an increased incidence that is statistically significant. Thus, the increased incidence of MNCL observed in F-344 rats exposed to some alkyl phthalates is likely a strain-specific effect of little or no relevance for humans, and characterization of these chemicals as carcinogens based on increased MNCL in F-344 rats is not scientifically supported.

Short- and intermediate-term carcinogenicity testing--a review. Part 2: available experimental models

Numerous experimental protocols for short- and intermediate-term carcinogenicity assays have been available for many years. This paper surveys various of these test systems in rodents, fish species, non-vertebrates and avian embryos in ovo. The mouse skin tumour assay and the rat liver foci assay were used to introduce the basic concepts of short- and intermediate-term carcinogenicity testing in the previous part of the review. The focus of this second part of the review is on rodent assays for carcinogenicity testing in the lung, kidney, urinary bladder, pancreas, stomach, oral cavity, small intestine, colon, and on the possibility to combine several target organs in multi-organ models. The potential use of various fish species, non-vertebrates and hatching eggs for carcinogenicity testing is outlined and the advantages and limitations are discussed. This review also presents the problem of validation of any carcinogenicity test system and proposes a strategy for contemporary safety assessment of chemicals with regard to the detection and evaluation of carcinogenicity.

Studies of initiation and promotion of carcinogenesis by N-nitroso compounds

Complete carcinogens must possess both initiating and promoting properties. Most N-nitroso compounds are mutagens and are considered to be initiators, but some are not mutagenic and yet are complete carcinogens. To investigate the two activities, brief treatments of male F344 rats with each of three mutagens, nitrosodimethylurea, nitrosodiethylurea and nitrosobis-(2-oxopropyl)-amine, were followed by chronic treatment (40 weeks) with one of four non-mutagens, nitrosomethyl-3-carboxypropylamine, nitrosodiethanolamine, nitrosomethyl-2-hydroxypropylamine or phenobarbital, the last being a well-known promotor of liver tumors in rats. Each treatment group consisted of 18 animals and there were control groups of 15 animals without initiation and 15 animals without promotion. All surviving rats were sacrificed at week 78. There were almost no tumors in untreated controls or in groups treated with the promotors, other than bladder tumors in one group. Certain tumors were numerous in the initiated groups, but there were only a few instances of increased incidences after treatment with the promotors. The action of the initiators appeared to be the dominant factor and there was scant indication in this experiment of the induction of tumors by the promotors of promotion of initiated cells in most organs (e.g. the liver). This indicates that it is unlikely that non-genotoxic carcinogens induce tumors by promotion of already-initiated cells, but that some other mechanism prevails.

U.S. Environmental Protection Agency guidelines for carcinogen risk assessment: past and future

The U.S. Environmental Protection Agency (USEPA) recently proposed new guidelines to update and replace the 1986 USEPA Guidelines for Carcinogen Risk Assessment. Today, there is a better understanding of the variety of modes by which carcinogens can operate that did not exist when the 1986 USEPA guidelines were published. Many laboratories are adding new test protocols in their programs directed at questions concerning the mechanisms of action of carcinogens. In response to the evolving science of carcinogenesis, the new guidelines provide an analytical framework for incorporating all relevant biological information and recognizing a variety of situations regarding cancer risk. In addition, the guidelines are flexible enough to allow consideration of future scientific advances.

Effects of inhibition of O6-alkylguanine-DNA alkyltransferase in rats on carcinogenesis by methylnitrosourea and ethylnitrosourea

Many alkylating agents are potent carcinogens and there is considerable evidence that the formation of O6-alkylguanine in DNA can lead to mutations and the initiation of neoplastic growth. The repair of O6-methyl- or O6-ethylguanine in DNA is known to be brought about by the action of a protein termed O6-alkylguanine-DNA alkyltransferase. In order to investigate the role of this activity in the carcinogenic action of methylnitrosourea and ethylnitrosourea, O6-benzylguanine, a potent inhibitor of the alkyltransferase, was used. Groups of 20 female F344 rats were treated with the nitrosourea (0.2 mmol) by gavage in 10 weekly doses and a parallel group was also treated with 4 mg of O6-benzylguanine, 2 h prior to each dose of the nitrosourea. This dose of O6-benzylguanine was sufficient to reduce the alkyltransferase activity to zero in the liver for at least 8 h but activity had returned to about 60% of normal within 24 h. Animals were maintained until they became moribund, when they were killed, or until death related to tumors. The median week of death in the animals receiving methylnitrosourea was reduced from 60 wk to 52 wk by co-treatment with O6-benzylguanine. There was a smaller reduction from 55 to 50 wk in the rats receiving ethylnitrosourea. The treatment with O6-benzylguanine caused no significant change in the incidence of the principal tumors induced by the alkylnitrosoureas and there were no liver tumors produced by the combined treatments. These results show that the level of inactivation of alkyltransferase produced by this dose of O6-benzylguanine was not sufficient to greatly alter the potent carcinogenic effect of these doses of alkylnitrosoureas in this system.