PRINCIPLES OF TOXICOLOGY - Biology East Borneo

13.5 TESTING CHEMICALS FOR CARCINOGENIC ACTIVITY 289Clearly apoptosis is a process critical to the balance of cell populations in normal tissue. The loss ofthe ability for neoplastic cells to undergo apoptosis could tip the scales in favor of cell proliferationand uncontrolled growth. As such, apoptosis is another process that could be detrimentally affected bythe loss of p53 function.This has been a brief and necessarily simplistic overview of some of the cellular processes that canbe subverted in the course of the carcinogenic process. It should be evident from this discussion thatthe formation of a malignant tumor is a multistage process that involves multiple molecular mechanisms,including the activation of oncogenes and the inactivation of tumor suppressor genes (Figure13.8). What is not often clearly articulated to the student of chemical carcinogenesis is the fact thatwhile different types of chemical carcinogens (e.g., genotoxic and epigenetic) differ mechanistically,these mechanisms have impacts on similar molecular pathways. While there is still much work to bedone, the knowledge that has been developed over a relatively short time regarding the mechanismsof action of chemical carcinogens and critical cellular targets for these agents is astounding. Thisknowledge, has given us valuable insights to the origins of human cancer and will lead to thedevelopment of better tools with which to fight it.13.5 TESTING CHEMICALS FOR CARCINOGENIC ACTIVITYThe General Chronic Animal Bioassay ProtocolThe National Toxicology Program (NTP) is the agency currently responsible for testing chemicals forcarcinogenic activity in the United States, a responsibility originally held by the National CancerInstitute. But while the responsibility for testing chemicals has changed, the general animal testingprotocol currently used to evaluate the carcinogenic potential of a chemical has remained essentiallythe same for more than two decades. The basic procedure is relatively simple in experimental design,given the complexity of the disease process that constitutes the observational endpoint of this testprocedure (cancer) and the consequences and importance accorded any positive findings identified bythis test procedure. Furthermore, echoing an early recommendation by the FDA that testing be doneat doses and under experimental conditions likely to yield maximum tumor incidence, the use of highdoses to maximize the sensitivity of the procedure has become an area of considerable controversy. Inshort, the chemical doses tested, the animal species selected, and the simple, observational nature ofthis test often later become targets for criticism when the test results are applied in risk or hazardassessments that have a large impact on public health policy. For this reason, this and subsequentsections of this chapter will focus on the basic experimental design of the chronic animal cancerbioassay and the scientific issues commonly raised about these procedures or what interpretation mightbe given the results.The commonly recommended requirements for a thorough assessment of carcinogenic potential ina test animal that mold the basic experimental design of a chronic animal bioassay are• That two species of rodents, both sexes of each species, should be tested as a minimum. Thishelps ensure that false negative responses are not generated by selecting a non-responsivespecies for the test.• That adequate controls are run during the test procedure. Ideally, the tumor incidence in testanimals is compared to both historical and concurrent control animal responses. This helpsensure that the observed response is not an aberration of that specific study.• A sufficient number of animals should be tested so that a positive response is not likely tobe missed. The goal is to test enough animals to have a sufficient statistical basis wherebyeven a weak carcinogenic response should be observed and to be able to determine whetheran observed increase in tumors, or lack thereof, was a chance or real observation. Typically,50–100 animals of each sex and species are considered to be an appropriate-sized test.