Cancer risk assessment: historical perspectives, current issues, and future directions

FEMA GRAS assessment of natural flavor complexes: Citrus-derived flavoring ingredients

In 2015, the Expert Panel of the Flavor and Extract Manufacturers Association (FEMA) initiated a re-evaluation of the safety of over 250 natural flavor complexes (NFCs) used as flavoring ingredients. This publication is the first in a series and summarizes the evaluation of 54 Citrus-derived NFCs using the procedure outlined in Smith et al. (2005) and updated in Cohen et al. (2018) to evaluate the safety of naturally-occurring mixtures for their intended use as flavoring ingredients. The procedure relies on a complete chemical characterization of each NFC intended for commerce and organization of each NFC's chemical constituents into well-defined congeneric groups. The safety of the NFC is evaluated using the well-established and conservative threshold of toxicological concern (TTC) concept in addition to data on absorption, metabolism and toxicology of members of the congeneric groups and the NFC under evaluation. As a result of the application of the procedure, 54 natural flavor complexes derived from botanicals of the Citrus genus were affirmed as generally recognized as safe (GRAS) under their conditions of intended use as flavoring ingredients based on an evaluation of each NFC and the constituents and congeneric groups therein.

The sub-chronic toxicity of a naphthenic hydrocarbon solvent in rats

Cycloalkanes/naphthenes are constituents of complex hydrocarbon solvents, and hence an understanding of their toxicological profile is critical to establish safe limits for occupational exposures to these solvents. Although naphthenes are structurally related to and share a common metabolic fate with the straight and branched chain analogues, some toxicokinetic differences have been noted. The acute central nervous system response to volatile naphthenes in rodents has been shown to be slightly different compared to other alkane analogues. To determine whether these differences may extend to systemic effects with less volatile naphthenes, rats were exposed to 1500, 3000 or 6000 mg/m³ of a C₉-C₁₁ aliphatic solvent containing 70% naphthenes, for 90 days. Effects were limited to adaptive liver enlargement in both sexes and kidney toxicity in the male rat. For comparative purposes, the results from this study were compared to published reports of a complex hydrocarbon solvent with a higher proportion of volatile C₅/C₆ naphthenes and a mono-constituent naphthene (decahydronaphthalene). The results indicate that the systemic effects of naphthenes are similar to the straight and branched chain analogues and that the effects that are most relevant for human health evaluations of alkanes are acute central nervous system effects.

Evaluating the mechanistic evidence and key data gaps in assessing the potential carcinogenicity of carbon nanotubes and nanofibers in humans

In an evaluation of carbon nanotubes (CNTs) for the IARC Monograph 111, the Mechanisms Subgroup was tasked with assessing the strength of evidence on the potential carcinogenicity of CNTs in humans. The mechanistic evidence was considered to be not strong enough to alter the evaluations based on the animal data. In this paper, we provide an extended, in-depth examination of the in vivo and in vitro experimental studies according to current hypotheses on the carcinogenicity of inhaled particles and fibers. We cite additional studies of CNTs that were not available at the time of the IARC meeting in October 2014, and extend our evaluation to include carbon nanofibers (CNFs). Finally, we identify key data gaps and suggest research needs to reduce uncertainty. The focus of this review is on the cancer risk to workers exposed to airborne CNT or CNF during the production and use of these materials. The findings of this review, in general, affirm those of the original evaluation on the inadequate or limited evidence of carcinogenicity for most types of CNTs and CNFs at this time, and possible carcinogenicity of one type of CNT (MWCNT-7). The key evidence gaps to be filled by research include: investigation of possible associations between in vitro and early-stage in vivo events that may be predictive of lung cancer or mesothelioma, and systematic analysis of dose-response relationships across materials, including evaluation of the influence of physico-chemical properties and experimental factors on the observation of nonmalignant and malignant endpoints.

Unexpected tumour findings in lifetime rodent bioassay studies--what to do?

Currently, the majority of substances tested in lifetime bioassays in rodents are not mutagenic and, therefore, at the most weakly carcinogenic, generally by epigenetic mechanisms. It thus appears obvious that only marginal increases of tumour incidences can be expected in lifetime bioassays and that, therefore, every aspect of a potential carcinogenic effect must be thoroughly evaluated. This paper describes a series of key factors, which should be looked at in order to exclude that the lifetime bioassay in question is flawed for design, technical or qualification reasons. It also provides some hints whether there is indeed a real effect and not just a variation of the spontaneous tumour incidences. Tumour findings must be seen in the context of the animal model, the pharmcokinetics and pharmcodynamics of the test substance, as well as any other observation in the present or other studies with the test substance, including non-tumour findings and--in particular--potential precursor lesions and effects on feed intake and survival. The possibility that the observed carcinogenic effects may be species-specific and not relevant for man is discussed. It is also important to check what findings are reported with similar substances or substances with the same pharmacological effect. Data from additional investigations on material of the same study and/or mechanistic studies are often needed to support the final risk assessment.

Genetic toxicology data in the evaluation of potential human environmental carcinogens

In 1969, the International Agency for Research on Cancer (IARC) initiated the Monographs Programme to evaluate the carcinogenic risk of chemicals to humans. Results from short-term mutagenicity tests were first included in the IARC Monographs in the mid-1970s based on the observation that most carcinogens are also mutagens, although not all mutagens are carcinogens. Experimental evidence at that time showed a strong correlation between mutagenicity and carcinogenicity and indicated that short-term mutagenicity tests are useful for predicting carcinogenicity. Although the strength of these correlations has diminished over the past 20 years with the identification of putative nongenotoxic carcinogens, such tests provide vital information for identifying potential human carcinogens and understanding mechanisms of carcinogenesis. The short-term test results for agents compiled in the EPA/IARC Genetic Activity Profile (GAP) database over nearly 15 years are summarized and reviewed here with regard to their IARC carcinogenicity classifications. The evidence of mutagenicity or nonmutagenicity based on a 'defining set' of test results from three genetic endpoints (gene mutation, chromosomal aberrations, and aneuploidy) is examined. Recommendations are made for assessing chemicals based on the strength of evidence from short-term tests, and the implications of this approach in identifying mutational mechanisms of carcinogenesis are discussed. The role of short-term test data in influencing the overall classification of specific compounds in recent Monograph volumes is discussed, particularly with reference to studies in human populations. Ethylene oxide is cited as an example.