A World Health Organization (WHO)/International Programme on Chemical Safety (IPCS) cancer risk assessment workshop in 1999 identified a number of scientific issues that remain to be resolved involving the classification of chemical carcinogens, including potency (some chemicals are carcinogenic only at toxic doses), mouse liver tumors, peroxisome proliferation, and receptor-medicated reactions (UNECE, 2004, p. 169).
New breakthroughs in our understanding of the mechanisms and biomarkers of cancer may lead to the development of novel cellular carcinogenicity assays. Recent studies on the metastatic phenotype of melanoma cells “revealed the convergence of embryonic and tumorigenic signalling pathways” (Hendrix, et al., 2007). These pathways represent potential new therapeutic targets as well as potential new biomarkers for detecting the early stages of cell transformation in vitro.
Epigenetic mechanisms also play a role in carcinogenesis. Epigenetic refers to mechanisms that alter gene expression without actual changes to the gene/DNA sequence. DNA methylation is an example of an epigenetic mechanism involved in carcinogenesis that causes heritable changes in gene expression, independent of altering the DNA sequence. Scientists have shown that DNA methylation is an important component in a variety of chemical-induced toxicities, including carcinogenicity, and should be assessed in the overall hazard assessment (Watson & Goodman, 2002; Moggs, et al., 2004). DNA methylation evaluated along with other in vitro test data has been found useful in prioritizing chemicals for further assessment. DNA methylation coupled with cytotoxicity and genotoxicity assays strengthened the prioritization and was especially useful in identifying genotoxins that were toxic at nonlethal concentrations (Watson, et al., 2004).
US FDA scientists used the Carcinogenic Potency Database (CPDB) to develop a predictive model for organ-specific carcinogenicity (Young, et al., 2004). They added molecular structures to the CPDB to generate a database of structure-activity relationship (SAR) analyses for predicting organ-specific carcinogenicity to use in their reviews of new chemicals submitted for approval. They reported a preliminary analysis for liver-specific carcinogenicity.
The OECD and ECVAM are actively reviewing, developing, and validating alternative test methods for evaluating carcinogenicity.
ECVAM identified carcinogenicity testing as a key area for animal replacement and established the ECVAM Task Force on Carcinogenicity in 2003. The task force’s objective is to develop a testing strategy that can detect both genotoxic and nongenotoxic carcinogens. To accomplish this, the task force established a collaborative network of scientists to conduct work aimed at developing “an integrated strategy based on the use of cell transformation assay by mouse fibroblast (Balb/3T3) in combination with genotoxicity in vitro testing (Micronuclei and Comet assays).” ECVAM is also conducting cell-based studies to determine the molecular signatures of carcinogenic chemicals.
The OECD considered the performance of the three main cell transformation assays to determine the need for a formal validation study before they are recommended for development into an OECD TG. The fourth draft version of the detailed review paper (DRP) No. 31 discussed OECD’s evaluation of the performance and reproducibility of the assays. The three final recommendations in the OECD report are (OECD, 2006):
The OECD recently released a report on the regulatory uses and applications of (Q)SAR models and expert systems by member countries in chemical assessment, including their use in predicting carcinogenic/genotoxic substances (OECD, 2007). The aim of this report was “enhancing the regulatory acceptance of (Q)SAR models and expanding the opportunities for future applications of the models.” The case studies by different countries described their uses of (Q)SAR models in regulatory decision-making; illustrated the utility and difficulties of incorporating (Q)SAR models into their regulatory frameworks; and demonstrated the difficulty in defining universal (Q)SAR principles. The report also described the recent EU initiative to facilitate sharing of (Q)SAR models. This OECD report will be updated when member countries provide additional examples of (Q)SARs in regulatory assessments.
Numerous efforts are under way to address the problem of false positive results in current in vitro genotoxicity tests as a way to reduce/eliminate the Tier II in vivo genotoxicity tests that are conducted when chemicals are positive in Tier I in vitro genotoxicity tests. Recent initiatives that address this issue in some manner include work being done by the following organizations: European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC), ECVAM, European Cosmetic Toiletry & Perfumery Association (COLIPA), ICH, and ILSI-HESI.