Toxicology Letters 127 (2002) 43 – 46 www.elsevier.com/locate/toxlet
Threshold of toxicological concern (TTC) in food safety assessment Robert Kroes a,*, Gunhild Kozianowski b a
IRAS, Utrecht Uni6ersity, P.O. Box 80176, NL-3508 TD Utrecht, The Netherlands b Su¨dzucker, Postfach 1127, D-67261 Gru¨ndstadt, Germany
Abstract The threshold of toxicological concern (TTC) is a principle which refers to the possibility of establishing a human exposure threshold value for all chemicals, below which there is no appreciable risk to human health. The concept that exposure thresholds can be identified for individual chemicals in the diet is already widely embodied in practice of many regulatory bodies in setting acceptable daily intakes (ADIs) for chemicals whose toxicological profile is known. However, the TTC concept goes further than this in proposing that a de minimis value can be identified for many chemicals, including those of unknown toxicity, taking the chemical structure into consideration. This concept forms the scientific basis of the US Food and Drug Administration (FDA) ‘1995 Threshold of Regulation’ for indirect food additives. The TTC principle has also been adopted by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in its evaluations of flavouring substances. The establishment of a more widely accepted TTC would benefit consumers, industry and regulators. In precluding extensive toxicity testing and safety evaluations when human intakes are below such a threshold, TTC would focus limited resources of time, cost, animal use and expertise on the testing and evaluation of substances with greater potential to pose risks to human health and contribute to a reduction in the use of animals. An International Life Sciences Institute (ILSI)— Europe expert group has examined this TTC principle, which was based on general toxicity endpoints (including carcinogenicity), for its applicability in food safety evaluation. In addition, the group examined specific endpoints, such as neurotoxicity, immunotoxicity and developmental toxicity. The results of the expert group’s considerations including the development of a guideline to apply the principle are discussed. © 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Threshold for toxicological concern; Threshold of regulation; Food safety
1. Introduction Man is exposed to thousands of chemicals, whether naturally occurring or man-made. Exten* Corresponding author. Tel.: + 31-35-253-5373; fax: + 3135-622-3442. E-mail address: [email protected]
sive and usually expensive toxicity studies are necessary to evaluate the safety of applying chemicals or to establish if the contaminants that humans are exposed to may cause harm. To assess whether a generic threshold value —threshold of toxicological concern (TTC) —or range of values can be established, a study was performed in evaluating several databases of chemicals (Kroes
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et al., 2000). This paper further explores the possibilities to apply the TTC principle.
2. Principles of the concept The concept that there is a level of exposure to a given substance below which no significant risk is expected to exist has been widely accepted, and the establishment of acceptable daily intakes (ADIs) is based on that concept (SCF, 1996; WHO, 1987). Frawley (1967) was the first to present an analysis to establish a generic threshold value (threshold of regulation) or range of values with the aim to reduce extensive toxicity studies and safety evaluations, and to address, within the available capacity, those substances for which the potential or actual intake is substantial. In 1986, Rulis conducted an analysis of the Food and Drug Administration’s (FDA) Priority-Based Assessment of Food Additives (PAFA) database containing 159 compounds with subchronic or chronic toxicity data, LD50 values from 18 000 oral rodent studies contained in the Registry of Toxic Effects of Chemical Substances (RTECS), and TD50 values of 130 compounds found in the carcinogen potency database of Gold et al. (1984). Both scientists concluded that an intake between 1 and 10 mg/person/day of various chemical substances might not pose a risk. As depicted above (Rulis, 1986; Flamm et al., 1987), the TTC concept was initially developed as a threshold of regulatory concern concept by the US FDA. The US legislation defines all substances, which will become or may reasonably be expected to become components of a food or to affect its characteristics as food additives. This definition also includes indirect food additives, i.e. substances, which are used in the manufacture of food-contact materials and may migrate to or otherwise become food components. These indirect food additives, which are usually consumed in minute traces, only require FDA’s safety evaluation and approval. Since FDA is by law obliged to set priorities on issues of tangible risk rather than on trivial ones, the so-called de minimis principle, there is a potential discrepancy between requirements for data
generation and evaluation and the de minimis principle to use resources for issues of real concern. Therefore, the FDA analysed the available toxicity data to assess whether for low exposure substances a threshold can be defined below which an intake would be of no appreciable risk— even without data. The resulting concept, the ‘Threshold of Regulatory Concern’ concept aimed at directing resources including those for animal testing towards testing and evaluation of substances with greater potential to health risks. The derivation of this threshold of regulatory concern has been based on an analysis of the Carcinogenic Potency Data Base (CPDB) by Gold et al. (1984, 1989). In this database for 500 carcinogens tested in 3500 experiments, the distribution of the chronic dose rates to be able to induce tumours in half the test animals at the end of their life span (TD50 values in mg/kg bw/day) was calculated. This distribution was then transformed via linear extrapolation to the distribution of exposure corresponding to a risk of 10 − 6 to develop cancer when life span-exposed. As a result FDA indicated that a dietary concentration below 0.5 mg/kg of food, corresponding to 1.5 mg intake/person/day is so negligible that it is not expected to present a public health concern, even if it were later found to be a carcinogen (Federal Register, 1993; Munro et al., 1996). This conclusion was implemented as the ‘Threshold of Regulation for Food Contact Materials’. It implicates that components for which a lower exposure can be established may be exempted from regulation. More recently, this concept has been refined by Munro et al. (1996, 1999). They analysed the available data for up to 900 non-carcinogenic organic chemicals assigned to three structural classes according to Cramer et al. (1978). The no-observed-effect levels (NOELs) of the most sensitive endpoint, species and sex were cumulatively plotted by class. To the lowest fifth percentile NOELs, a 100-fold safety or uncertainty factor was applied, and the acceptable daily intake for a 60 kg person was calculated. This resulted in a threshold level (TTC) of 1800 mg/day for class I compounds (innocuous), 540 mg/day for class II compounds and 90 mg/day for class III
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compounds (least innocuous) as depicted in Table 1. From this analysis it was concluded that the level of toxicity is clearly influenced by structural class, evidenced by the distinct separation of the cumulative distributions. In addition it provides the option to integrate structural knowledge into the TTC concept. It shows furthermore that higher threshold values may be appropriate for compounds without structural alerts for genotoxicity. The principles of the Munro approach are already applied by the Joint FAO/WHO Expert Committee on Food Additives to the evaluation of flavours (WHO, 1996). This threshold concept was reviewed by the Scientific Committee for Food (SCF) of the European Commission. The Committee questioned whether neurotoxic, developmental, immunotoxic, allergic reactions or endocrine activities could occur at low dose levels (SCF, 1996). The International Life Sciences Institute (ILSI) Europe initiated an Expert Group to compile and evaluate the available literature for the endpoints listed in the previous paragraph and to explore whether there are reasons to assume that such endpoints may have thresholds below the proposed generic threshold of 1.5 mg/person/day (Kroes et al., 2000). For neurotoxic, neurodevelopmental and developmental endpoints cumulative distributions of the NOELs were plotted and compared to the distributions as developed by Munro et al. (1996, 1999) and Gold et al. (1989). The analysis indicated that these endpoints were similar (neurodevelopmental and developmental toxicity) or slightly more sensitive (neurotoxicity) than the class III compounds, for which a threshold of 90 mg/person/day has been proposed. This increased sensitivity for the neurotoxic endpoint was solely due to the organic phosphate esters, which may be
considered as a separate class with a threshold value of 18 mg/person/day. All these endpoints are, however, well accommodated by the generic 1.5 mg/person/day threshold. For immunotoxicity sufficient substances for a similar analysis could not be retrieved from literature databanks (n= 37). However, the distributions of their non-immunotoxic NOELs and immunotoxic NOELs could be compared. From this comparison it was concluded that immunotoxicity was not a more sensitive endpoint. For substances exhibiting endocrine effects, it was argued that at the proposed threshold levels effects are not to be expected in light of the overall exposure to environmental (man-made as well as natural phytochemicals) and endogenous oestrogens. For similar reason it was also argued that small non-proteinous molecules without structural alerts, would not elicit allergic reactions, which affect subsets of susceptible individuals and are controlled mainly by labelling. Thus it was concluded that an intake below 1.5 mg/person/day provides adequate safety assurance. Pending further analysis, even higher TTC levels might be appropriate for compounds without structural alerts for genotoxicity or carcinogenicity (Kroes et al., 2000).
3. How to apply the TTC concept ILSI Europe is currently developing general recommendations about how to apply the TTC principle for several classes of substances to which humans may be exposed at low levels. These guidelines will recommend a stepwise approach, in which the identification of structural alerts for genotoxicity will be the first criterion to determine if in their absence a higher TTC level of exposure
Table 1 Fifth percentile NOELs and human exposure thresholds for each structural class (Munro et al., 1996) Structural class
No. of chemicals
Fifth percentile NOEL (mg/kg/day)
Human exposure threshold (mg/person/day)
I II III
137 28 447
2993 906 17
1800 540 90
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than the generic 1.5 mg/person/day can be accepted. For specific structural alerts (i.e. aflatoxin-, azoxy-, N-nitroso-like structures and in addition dibenzodioxin- and dibenzofurane-like structures) even a lower threshold may be proposed. If, however, structural alerts can be excluded, one may, in the next step, question if the compound is an organophosphate. This being the case it will be proposed to establish a TTC, which accommodates the threshold level for exposure established for this class of substances (18 mg/person/day). If in this step also this latter possibility can be excluded, a TTC could be applied following the structural classes according to Cramer et al. (1978): class III chemicals may then be accepted at a TTC level of exposure of 90 mg/person/day, whereas for Class II chemicals a level of exposure of 540 mg/person/day and for Class I a level of exposure of 1800 mg/person/day could be acceptable.
4. Conclusions It does not need emphasis that the TTC principle, if applied, will be an important tool, not only for scientists, but also for regulators and industry. It will accelerate the evaluation process of substances and will lead to appropriate priority setting. It will provide a much faster evaluation for those substances to which humans are exposed at low levels, such as certain flavours, migrating food packaging materials (indirect food additives) and other food contact materials, processing aids or even food additives used at low levels. The TTC concept may provide risk managers with a tool even to exempt uses of low exposure from regulation. It will allow us to focus efforts and resources on those chemicals, which are really important as a risk factor to humans, and thus may help to decide on adequate preventive measures in the future.
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