Approaches to the risk assessment of genotoxic carcinogens in food: a critical appraisal.

The present paper examines the particular difficulties presented by low levels of food-borne DNA-reactive genotoxic carcinogens, some of which may be difficult to eliminate completely from the diet, and proposes a structured approach for the evaluation of such compounds. While the ALARA approach is widely applicable to all substances in food that are both carcinogenic and genotoxic, it does not take carcinogenic potency into account and, therefore, does not permit prioritisation based on potential risk or concern. In the absence of carcinogenicity dose-response data, an assessment based on comparison with an appropriate threshold of toxicological concern may be possible. When carcinogenicity data from animal bioassays are available, a useful analysis is achieved by the calculation of margins of exposure (MOEs), which can be used to compare animal potency data with human exposure scenarios. Two reference points on the dose-response relationship that can be used for MOE calculation were examined; the T25 value, which is derived from linear extrapolation, and the BMDL10, which is derived from mathematical modelling of the dose-response data. The above approaches were applied to selected food-borne genotoxic carcinogens. The proposed approach is applicable to all substances in food that are DNA-reactive genotoxic carcinogens and enables the formulation of appropriate semi-quantitative advice to risk managers.

Safety evaluation of phytosterol esters. Part 8. Lack of genotoxicity and subchronic toxicity with phytosterol oxides.

Vegetable oil spreads containing phytosterol-esters are marketed as a cholesterol-lowering functional food in more than 20 countries worldwide. An extensive package of safety data has shown phytosterol-esters to be safe for human use. However, even though phytosterols are very stable molecules, oxidation may occur at low levels under extreme heating conditions, resulting in phytosterol oxides. As there is some suggestion of adverse biological effects in the literature for the related cholesterol oxidation products, safety data have been generated for phytosterol oxides. A phytosterol oxide concentrate (POC) was generated by prolonged heating of phytosterol-esters in the presence of oxygen. The genotoxicity and subchronic toxicity of this mixture was assessed in a series of in vitro genotoxicity assays (bacterial mutation, chromosome aberration and micronucleus) and a subchronic feeding study in the rat. Results showed that a phytosterol oxide concentrate containing approximately 30% phytosterol oxides did not possess genotoxic potential and no obvious evidence of toxicity when administered in the diet of the rat for 90 consecutive days. In the latter study, a NOEL was established at an estimated dietary level of phytosterol oxides of 128 mg/kg/day for males and 144 mg/kg/day for females. In conclusion, these materials have been shown to raise no obvious concerns for human safety.

Safety evaluation of ice-structuring protein (ISP) type III HPLC 12 preparation. Lack of genotoxicity and subchronic toxicity.

Ice-structuring proteins (ISPs) naturally occur in a range of species (including edible plants and fish) that need to protect themselves against freeze damage. ISPs have potential applications in a number of areas including cryopreservation and frozen foods manufacture. However, these materials are not currently generally available for commercial use. ISP type III HPLC 12 is of particular interest and although it is likely to be consumed naturally, its toxicological safety has not previously been assessed. This paper presents data from a set of in vitro and in vivo genotoxicity assays (bacterial mutation, chromosome aberration, mammalian cell gene mutation and rat bone marrow micronucleus) and a 3-month repeat-dose gavage study in the rat using high levels of ISP type III HPLC 12 preparation produced by recombinant baker's yeast. No evidence was seen of a genotoxic potential (using levels accepted as limit concentrations for the assays used) or notable subchronic toxicity following oral administration for 3 months in the rat at up to 580 mg ISP type III HPLC 12/kg/day, the highest dose tested (which was considered to be a NOAEL).

Safety evaluation of phytosterol esters. Part 7. Assessment of mutagenic activity of phytosterols, phytosterol esters and the cholesterol derivative, 4-cholesten-3-one.

Phytosterol esters are phytosterols derived from vegetable oils following esterification to fatty acids. When phytosterols are added to foods, they inhibit the absorption of dietary and endogenous cholesterol and thereby reduce blood cholesterol concentrations. As part of a comprehensive programme of safety assessment, the mutagenic potential of phytosterols and phytosterol esters has been assessed in a bacterial mutation assay and an in vitro chromosome aberration assay. In addition, an in vitro mammalian cell gene mutation assay and two in vivo mutagenicity studies, namely rat bone marrow micronucleus and liver unscheduled DNA synthesis (UDS) assays, were conducted on phytosterol esters only. Phytosterols and phytosterol esters did not show any evidence of mutagenic activity in any of these assays. A breakdown product of cholesterol is 4-cholesten-3-one and thus the amount of 4-cholesten-3-one in the gut may increase following supplementation of foods with phytosterol-esters. 4-cholesten-3-one had been previously reported as mutagenic but, due to various shortcomings, these data could not be used to assess the mutagenic activity of 4-cholesten-3-one. The mutagenic activity of 4-cholesten-3-one and its major faecal by-products, 5beta-cholestan-3-one, was assessed in two in vitro assays, a bacterial mutation assay and an in vitro chromosome aberration assay. Neither 4-cholesten-3-one nor 5beta-cholestan-3-one showed evidence of mutagenic activity in these assays.

Frequencies of hprt(-) mutations and bcl-2 translocations in circulating human lymphocytes are correlated with United Kingdom sunlight records.

Between 1983 and 1995 we have monitored human populations for evidence of exposure to environmental mutagens, taking blood samples to measure hprt(-) mutant frequency in T cells and more recently bcl-2 t(14:18) translocation frequency in B cells. We have now analysed data from 785 assays on 448 blood samples from 308 normal subjects and find that there is a highly significant statistical correlation between hprt(-) mutant frequency and the sunlight record for the 3 weeks prior to taking the blood sample. We discuss the weaknesses in retrospective studies of this nature and the possibility of spurious epidemiological correlations that may result. More controlled experiments can be envisaged that would give a firmer basis to the statistical associations observed. hprt(-) mutations in T cells show little evidence of a UV fingerprint, so that the correlation may be due to immunomodulation rather than mutation. We also find a correlation between the sunlight record and bcl-2 translocation. This translocation is found at a low frequency in the B cells of many normal subjects and is the commonest translocation observed in non-Hodgkin's lymphoma. Our results strengthen the case for a link between sunlight and this increasingly common cancer.

The ability of the Comet assay to discriminate between genotoxins and cytotoxins.

The Comet assay has been used widely in genetic toxicology, radiation biology and medical and environmental research. This assay detects single-strand breaks and alkali-labile sites in DNA and DNA degradation due to necrosis or apoptosis. It may also be modified to detect DNA cross-linking. Although a considerable number of chemicals have been tested in the assay there are many aspects of validation to be considered before the method could be considered to provide definitive evidence of genotoxic potential. For example, very few non-genotoxins have been tested to assess specificity of the Comet assay and there has been only one reported study which investigated whether the in vitro Comet assay is prone to false positive responses due to cytotoxicity. We have investigated the response of the alkaline Comet assay in TK6 human lymphoblastoid cells to cytotoxic damage and genotoxic damage. Several compounds which are toxic by different mechanisms were tested in the assay. Cycloheximide and trypsin gave a negative comet response at a highest dose of 5 mg/ml and no toxicity was observed. Sodium lauryl sulphate and potassium cyanide produced a significant increase in DNA migration at cell survival levels of < or = 75%. The distribution of damaged cells indicated that cells at various stages of necrotic cell death were present. Hydrogen peroxide, 4-nitroquinoline oxide, 9-aminoacridine, ethyl methanesulphonate, N-nitroso-N-ethylurea and glyoxal gave a positive comet response. Mitomycin C was negative at survival levels of approximately 70%. These results indicate that the maximum concentration of test substance tested should produce viabilities > 75% in order to avoid false positive responses due to cytotoxicity. The assay was able to detect DNA damage induced by an alkylating agent, an intercalating agent and oxidative damage. The cross-linking agent mitomycin C was not detected if a cut-off point of 75% viability is used as the criterion of a positive response.