PRINCIPLES OF TOXICOLOGY

Clearly apoptosis is a process critical to the balance of cell populations in normal tissue. The loss of
the ability for neoplastic cells to undergo apoptosis could tip the scales in favor of cell proliferation
and uncontrolled growth. As such, apoptosis is another process that could be detrimentally affected by
the loss of p53 function.
This has been a brief and necessarily simplistic overview of some of the cellular processes that can
be subverted in the course of the carcinogenic process. It should be evident from this discussion that
the 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 (Figure
13.8). What is not often clearly articulated to the student of chemical carcinogenesis is the fact that
while 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 be
done, the knowledge that has been developed over a relatively short time regarding the mechanisms
of action of chemical carcinogens and critical cellular targets for these agents is astounding. This
knowledge, has given us valuable insights to the origins of human cancer and will lead to the
development of better tools with which to fight it.
13.5 TESTING CHEMICALS FOR CARCINOGENIC ACTIVITY
The General Chronic Animal Bioassay Protocol
13.5 TESTING CHEMICALS FOR CARCINOGENIC ACTIVITY 289
The National Toxicology Program (NTP) is the agency currently responsible for testing chemicals for
carcinogenic activity in the United States, a responsibility originally held by the National Cancer
Institute. But while the responsibility for testing chemicals has changed, the general animal testing
protocol currently used to evaluate the carcinogenic potential of a chemical has remained essentially
the 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 test
procedure (cancer) and the consequences and importance accorded any positive findings identified by
this test procedure. Furthermore, echoing an early recommendation by the FDA that testing be done
at doses and under experimental conditions likely to yield maximum tumor incidence, the use of high
doses to maximize the sensitivity of the procedure has become an area of considerable controversy. In
short, the chemical doses tested, the animal species selected, and the simple, observational nature of
this test often later become targets for criticism when the test results are applied in risk or hazard
assessments that have a large impact on public health policy. For this reason, this and subsequent
sections of this chapter will focus on the basic experimental design of the chronic animal cancer
bioassay and the scientific issues commonly raised about these procedures or what interpretation might
be given the results.
The commonly recommended requirements for a thorough assessment of carcinogenic potential in
a 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. This
helps ensure that false negative responses are not generated by selecting a non-responsive
species for the test.
• That adequate controls are run during the test procedure. Ideally, the tumor incidence in test
animals is compared to both historical and concurrent control animal responses. This helps
ensure 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 to
be missed. The goal is to test enough animals to have a sufficient statistical basis whereby
even a weak carcinogenic response should be observed and to be able to determine whether
an 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.