How accurate is in vitro prediction of carcinogenicity?

Abstract

Positive genetic toxicity data suggest carcinogenic hazard, and this can stop a candidate pharmaceutical reaching the clinic. However, during the last decade, it has become clear that many non-carcinogens produce misleading positive results in one or other of the regulatory genotoxicity assays. These doubtful conclusions cost a lot of time and money, as they trigger additional testing of apparently genotoxic candidates, both in vitro and in animals, to discover whether the suggested hazard is genuine. This in turn means that clinical trials can be put on hold. This review describes the current approaches to the 'misleading positive' problem as well as efforts to reduce the use of animals in genotoxicity assessment. The following issues are then addressed: the application of genotoxicity testing screens earlier in development; the search for new or improved in vitro genotoxicity tests; proposed changes to the International Committee on Harmonisation guidance on genotoxicity testing [S2(R1)]. Together, developments in all these areas offer good prospects of a more rapid and cost-effective way to understand genetic toxicity concerns. AbbreviationsCA, chromosome aberrations; ECVAM, European Centre for the Validation of Alternative Methods; GLP, Good Laboratory Practice; ICH, International Committee on Harmonisation; RCC, relative cell counts; RICC, relative increase in cell counts; RPD, relative population doubling Chemical carcinogenicity and genetic toxicologyChildren working as chimney sweeps often died from cancers. It was subsequently discovered that a chemical in soot, benzo[a]pyrene, causes animal cancers, as well as mutations in bacteria (reviewed by Phillips, 1983). Thus, the concept of chemical carcinogenicity and the discipline of genetic toxicology were born. Difficulties soon emerged. First, it was recognized that not all animal carcinogens are bacterial mutagens (McCann et al., 1975). This was thought to reflect the differences between animals and bacteria, including xenobiotic metabolism in the liver and other organs. So, mammalian cell lines were developed, and liver extracts were included in test protocols. This combination of bacterial and mammalian cell tests produces the 'desired' positive results for about nine out of 10 carcinogens (Kirkland et al., 2005). This is not, however, as good a result as it seems, as this same combination also produces positive results for as many as nine out of 10 non-carcinogens (Kirkland et al., 2005). Finally, it was recognized that many carcinogens do not actually have a genotoxic mode of action (Shaw and Jones, 1994). Thus, the unanticipated consequence of a focus on sensitivity during genotoxicity assay development was that many of the compounds now producing positive results are either not carcinogens, or carcinogens that do not have a genotoxic mode of action. This means that there are many compounds carrying positive genotoxicity information that is not relevant to the assessment of cancer risk. Some of these are potentially useful pharmaceuticals. It is the...

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