Steviol glycoside safety: is the genotoxicity database sufficient?

The safety of steviol glycoside sweeteners has been extensively reviewed in the literature. National and international food safety agencies and approximately 20 expert panels have concluded that steviol glycosides, including the widely used sweeteners stevioside and rebaudioside A, are not genotoxic. However, concern has been expressed in recent publications that steviol glycosides may be mutagenic based on select studies representing a small fraction of the overall database, and it has been suggested that further in vivo genotoxicity studies are required to complete their safety profiles. To address the utility of conducting additional in vivo genotoxicity studies, this review evaluates the specific genotoxicity studies that are the sources of concern, and evaluates the adequacy of the database including more recent genotoxicity data not mentioned in those publications. The current database of in vitro and in vivo studies for steviol glycosides is robust and does not indicate that either stevioside or rebaudioside A are genotoxic. This, combined with a lack of evidence for neoplasm development in rat bioassays, establish the safety of all steviol glycosides with respect to their genotoxic/carcinogenic potential.

Overview: the history, technical function and safety of rebaudioside A, a naturally occurring steviol glycoside, for use in food and beverages.

Rebaudioside A is a sweet tasting steviol glycoside extracted and purified from Stevia rebaudiana (Bertoni). Steviol glycosides can currently be used as a food ingredient in only a handful of countries. Questions on specifications, safety and special population effects have prevented steviol glycosides from obtaining a legal status permitting their use as a sweetener in most countries. A set of papers reporting results of research studies and reviews has been compiled in this Supplement to definitively answer unresolved questions. Specifically, recently completed studies on the general and reproductive toxicity of rebaudioside A corroborate studies carried out with purified steviol glycosides demonstrating safety at high dietary intake levels. Comparative metabolism studies provide further affirmation of the common metabolic pathway for all steviol glycosides and the common metabolism between rats and humans. Finally, clinical studies provide further evidence that purified rebaudioside A has no effect on either blood pressure or glucose homeostasis. This paper summarizes the information used to conclude that high purity rebaudioside A (rebiana) produced to food-grade specifications and according to Good Manufacturing Practices is safe for human consumption under its intended conditions of use as a general purpose sweetener.

A critical review of the genetic toxicity of steviol and steviol glycosides.

Extracts of the leaves of the stevia plant (Stevia rebaudiana Bertoni) are used to sweeten food and beverages in South America, Japan and China. The components responsible for the sweet properties of the plant are glycosides of steviol, primary stevioside (ent-13-hydroxykaur-16-en-18-oic acid), which is 250-300 times sweeter than sucrose and rebaudiosides A and C. Stevioside and steviol have been subjected to extensive genetic testing. The majority of the findings show no evidence of genotoxic activity. Neither stevioside nor its aglycone steviol have been shown to react directly with DNA or demonstrate genotoxic damage in assays relevant to human risk. The mutagenic activity of steviol and some of its derivatives, exhibited in strain TM677, was not reproduced in the same bacteria having normal DNA repair processes. The single positive in vivo study measuring single-strand DNA breaks in Wistar rat tissues by stevioside, was not confirmed in experiments in mice and appears to be measuring processes other than direct DNA damage. Neither stevioside nor steviol-induced clastogenic effects at extremely high dose levels in vivo. Application of a Weight-of-Evidence approach to assess the genetic toxicology database concludes that these substances do not pose a risk of genetic damage following human consumption.

Reevaluation of the cancer potency factor of toxaphene: recommendations from a peer review panel.

This reevaluation of the current U.S. EPA cancer potency factor for toxaphene is based upon a review of toxaphene carcinogenesis bioassays in mice conducted by Litton Bionetics (unpublished report, 1978) and the National Cancer Institute (NCI) (Technical Report Series No. 37, conducted by Gulf South Research Institute, 1979). The mechanistic data available for toxaphene, including consideration of the potential of the compound to induce genotoxicity, was examined with an emphasis on whether this information supports a change in the cancer potency factor. If a quantitative dose-response assessment for toxaphene is to be performed, the data from both the NCI and Litton cancer bioassays should be used. Additionally, liver tumor results from female mice, rather than male mice, should be used for estimating potential human cancer risk because the background rate of liver tumors in females is lower and less variable than that exhibited by males. An ED(10) was estimated as the point of departure. The mechanistic data were not sufficient to fully support a margin of exposure approach. Therefore, we believe that applying a linear extrapolation from the ED(10) to the origin is an appropriate means to estimate risk at low doses. This is a highly conservative approach and, when it is applied, we conclude that the current EPA cancer potency factor should be reduced from 1.1 (mg/kg/day)(-1) to 0.1 (mg/kg/day)(-1).

Lessons learned in applying the U.S. EPA proposed cancer guidelines to specific compounds.

An expert panel was convened to evaluate the U.S. Environmental Protection Agency's "Proposed Guidelines for Carcinogen Risk Assessment" through their application to data sets for chloroform (CHCl3) and dichloroacetic acid (DCA). The panel also commented on perceived strengths and limitations encountered in applying the guidelines to these specific compounds. This latter aspect of the panel's activities is the focus of this perspective. The panel was very enthusiastic about the evolution of these proposed guidelines, which represent a major step forward from earlier EPA guidance on cancer-risk assessment. These new guidelines provide the latitude to consider diverse scientific data and allow considerable flexibility in dose-response assessments, depending on the chemical's mode of action. They serve as a very useful template for incorporating state-of-the-art science into carcinogen risk assessments. In addition, the new guidelines promote harmonization of methodologies for cancer- and noncancer-risk assessments. While new guidance on the qualitative decisions ensuing from the determination of mode of action is relatively straightforward, the description of the quantitative implementation of various risk-assessment options requires additional development. Specific areas needing clarification include: (1) the decision criteria for judging the adequacy of the weight of evidence for any particular mode of action; (2) the role of mode of action in guiding development of toxicokinetic, biologically based or case-specific models; (3) the manner in which mode of action and other technical considerations provide guidance on margin-of-exposure calculations; (4) the relative roles of the risk manager versus the risk assessor in evaluating the margin of exposure; and (5 ) the influence of mode of action in harmonizing cancer and noncancer risk assessment methodologies. These points are elaborated as recommendations for improvements to any revisions. In general, the incorporation of examples of quantitative assessments for specific chemicals would strengthen the guidelines. Clearly, any revisions should retain the emphasis present in these draft guidelines on flexibility in the use of scientific information with individual compounds, while simultaneously improving the description of the processes by which these mode-of-action data are organized and interpreted.

Acrylamide: a review of its genotoxicity and an assessment of heritable genetic risk.

An updated review of the genotoxicity studies with acrylamide is provided. Then, using data from the studies generating quantitative information concerning heritability of genetic effects, an assessment of the heritable genetic risk presented by acrylamide is presented. The review offers a discussion of the reactions and possible mechanisms of genotoxic action by acrylamide and its epoxide metabolite glycidamide. Several genetic risk approaches are discussed, including the parallelogram, direct (actually a modified direct), and doubling dose approaches. Using data from the specific-locus and heritable translocation assays, the modified direct and doubling dose approaches are utilized to quantitate genetic risk. Exposures of male parents to acrylamide via inhalation, ingestion, and dermal routes are also quantitated. With these approaches and measurements and their underlying assumptions concerning extrapolation factors (including germ cell stage specificity, DNA repair variability, locus specificity), number of human loci associated with dominant disease alleles, and spontaneous mutation rates, an assessment of heritable genetic risk for humans is calculated for the three exposure scenarios. The calculated estimates for offspring from fathers exposed to acrylamide via drinking water are up to three offspring potentially affected with induced genetic disease per 10(8) offspring. Estimates for inhalation or dermal exposures suggest higher risks for induced genetic disease in offspring from fathers exposed in occupational settings.

An assessment of the genetic toxicity of atrazine: relevance to human health and environmental effects.

The genetic toxicity of atrazine, a member of the s-triazine herbicides, was reviewed with the objective of classifying the chemical. Atrazine has been subjected to a broad range of genetic tests with predominantly negative results. Some publications, specifically those measuring dominant lethality in mice and bone marrow clastogenicity in rodents, reported conflicting results across two or more independent tests. Two approaches were employed to evaluate and interpret the results. The first approach attempts to classify each type of genetic endpoint as positive or negative and resolve test conflicts by critical assessment of the study and detailed data. This is the more traditional "expert judgement" approach to hazard assessment. The second approach employs a computer-assisted weight-of-evidence method of data analysis. This approach does not require resolution of conflicts but uses all data sets to arrive at a classification of hazard. The first approach was able to resolve some conflicts but not all. Use of the "expert judgement" results in an equivocal conclusion and classification. Use of the weight-of-evidence method resulted in a conclusion that atrazine does not pose a mutagenic hazard. The weight-of-evidence scheme is proposed to be a more practical and relevant approach for assessing complex data sets.

Genetic toxicology evaluation of commercial beers, III. SCE, chromosome aberrations, and forward mutation (HGPRT) of commercial beer products in CHO cells.

Concentrated organic residues extracted from 5 blended aliquots of commercial beers were evaluated for their ability to induce sister chromatid exchange (SCE), chromosomal aberrations and forward mutation in Chinese hamster ovary (CHO) cells. Each extract was prepared by blending 4 commercial beers of similar ingredients and brewing method, passing the beer pool over XAD-2 resin, extracting the resin and concentrating the extract. Studies were performed both with and without metabolic activation using variable amounts of reconstituted residues from 225-fold concentrates of the blended samples. CHO cultures were treated with 0.75 microliters/ml through 10.0 microliters/ml of the concentrates in the SCE assays, 1.0 microliters/ml through 10.0 microliters/ml of the extracts in the aberration assays and 2.5 microliters/ml up to 20 microliters/ml for forward mutation assays. In preliminary screening for SCE as an indicator of potential DNA damage, a significant increase was observed for 3 of 5 concentrated samples; however, no increase in SCE was induced by any of the 5 samples when S9 was added as a source of exogenous metabolic activation. More definitive tests for induction of genetic events, i.e., chromosome aberrations and forward HGPRT mutations, were negative for all 5 extracts whether or not S9 mix was present. Since SCE were not induced in tests with metabolic activation and since there was no concordant aberration or point mutation induction, the preliminary indication of potential DNA damage shown by elevated SCE under conditions without metabolic activation appears to have little biological significance.

Mutagenicity studies on two triphenylmethane dyes, bromophenol blue and tetrabromophenol blue.

Bromophenol blue and tetrabromophenol blue are two triphenylmethane dyes. Triphenylmethane derivatives and their structurally related compounds, such as fluoresceins and xathenes, are widely used as industrial dyes for foods, drugs, cosmetics, textiles, printing inks or laboratory indicators. Since a number of these types of dyes have been reported to be genotoxic, safety concerns on these two dyes of interest have been raised. Consequently, a battery of genetic toxicology assays, including the Ames Salmonella/microsome assay, L5178Y TK+/- mouse lymphoma assay, mouse micronucleus test and mitotic recombination assay with yeast Saccharomyces cerevisiae strain D5, has been performed on each of the two dyes. The results of the evaluations indicate that both bromophenol blue and tetrabromophenol blue were not active and can be considered non-genotoxic for the three genetic endpoints assessed (gene mutation, chromosome aberrations and primary DNA damage). Genetic activities in some structurally related compounds of these dyes have been reported but may be attributed to the presence of mutagenic impurities rather than the compound itself.

Genotoxicity hazard assessment of Caramel Colours III and IV.

Results from a battery of short-term tests in vitro and in vivo used to assess the genotoxicity of caramel colours are presented and discussed in relation to reports from the literature. No evidence of genotoxicity was found in the Salmonella plate incorporation test using five standard strains or in the Saccharomyces cerevisiae gene conversion assay using strain D4, either with or without S-9 for activation. A weak clastogenic effect for a sample of Caramel Colour III in CHO cells was abolished in the presence of S-9. Two samples of Caramel Colour IV were not clastogenic in CHO cells. Salmonella pre-incubation tests without S-9 also failed to reveal any mutagenic activity for any of the caramel colours tested. The Caramel Colour III sample that showed clastogenic activity in CHO cells in vitro did not induce micronuclei when evaluated in a mouse bone marrow assay. These results are in general agreement with reports in the literature regarding the genotoxicity of caramel colours, and support the conclusion that caramel colours do not pose a genotoxic hazard to humans.

Safety of Sanguinaria extract as used in commercial toothpaste and oral rinse products.

This report represents the findings of an Expert Panel on the safety of Sanguinaria extract used in Viadent oral rinse and toothpaste products and represents an independent review of the Sanguinaria extract toxicologic data base. It is based on reviews and discussions of the data base by all members of the Expert Panel on Sanguinaria extract. The Panel concluded that the data base on Sanguinaria extract is substantial and indicates that Sanguinaria extract is safe in its present use in Viadent products based on a large margin of safety between levels of human exposure and levels found to produce minimum effect or to be without adverse effect in animals. The panel further concluded that published literature suggesting an association between human exposure to Sanguinaria extract and potential reproductive, cardiovascular, or ocular toxicity, or carcinogenicity is largely anecdotal, unfounded, and not corroborated by or consistent with the substantial data base that was subjected to peer review.

Genetic toxicology evaluation of commercial beers. I. Development of procedures for the preparation of extracts from commercial beer products.

Commercial beer was subjected to an investigation in order to establish standard conditions for preparing organic solvent extracts to be used in short-term genetic screening assays. Test samples for use in the evaluation were prepared by mixing several brands of commercially available beer into a composite pool which was then spiked with the mutagen, 2-nitrofluorene. The composite sample was then concentrated using varying ratios of beer to XAD-2 resin in a 1.5 cm X 30 cm column. Dry-weight analyses indicated that significant amounts of residue could be trapped by XAD-2 resin. Columns were sequentially eluted by methylene chloride, acetone and methanol followed by evaporation of the solvents under nitrogen gas. Residues from commercial products were not mutagenic, but mutagenic activity could be detected in residues from spiked beer, yielding nearly 90% of the expected biological activity in S. typhimurium TA98. A standard method amenable to processing large volumes of beer products was devised for application to other projects.

Genetic toxicology evaluation of commercial beers. II. Mutagenic activity of commercial beer products in Salmonella typhimurium strains TA98, TA100 and TA102.

5 concentrated extracts of commercial beers were prepared using XAD-2 resin. The residues were subjected to evaluation for mutagenic activity in Salmonella typhimurium strains TA98, TA100 and TA102. The tests were conducted using preincubation protocols including provisions for S9 metabolic activation. Although the extracts did produce moderate toxicity to the Salmonella organisms used in the assays, none of the residues were found to induce mutation up to their maximum testable concentrations.

Implications of treatment-condition-induced genotoxicity for chemical screening and data interpretation.

Ionic and pH alterations appear to be directly responsible for the induction of genotoxic effects in cultured mammalian cells. In vivo studies also associate high ion concentrations and pH changes with tumor enhancement of the glandular stomach and urinary bladder of rats. The implications of these findings are directly relevant to the design of in vitro and in vivo tests and to the interpretation of results from tests using materials likely to produce alterations in ionic and/or pH levels.