PRINCIPLES OF TOXICOLOGY

292 CHEMICAL CARCINOGENESIS
may be more relevant to the humans because they possess alterations in genes known to be involved
in many human tumors.
13.6 INTERPRETATION ISSUES RAISED BY CONDITIONS OF THE TEST
PROCEDURE
Human health hazards and, to allow for some quantitative assessment of the risk, the reliability of the
animal-to-human extrapolation of animal cancer data is understandably an important issue. And as is
true for any animal test procedure, questions concerning the reliability with which the results of the
chronic bioassay can be extrapolated to human exposure conditions are frequently raised. However,
in addition to the obvious potential for frank differences to arise in the human response because of
species differences, an issue that can be raised with the animal test data for any other toxic endpoint
(e.g., liver injury or developmental deficits), a number of interpretation issues have been raised that
stem from the experimental conditions of the cancer bioassay test procedure itself. For example, it has
long been noted that a number of interpretation problems will simply arise out of the data collected
from this procedure because significant species differences may reasonably be anticipated and because
of the test’s relatively crude and observational approach. That is, after approximately 3 years of test
procedure, tissue collection and histopathological examination, we are largely left with a single, simple
observation, specifically, the number of tumors in a tissue following lifetime high-dose exposure.
Because the test procedure is arguably a screening test for carcinogenic activity, regardless of dose,
when a positive result is observed, little else is provided. For example, typically little or no information
is provided on dose–response or the mechanism by which the cancer was induced. Thus, it should
perhaps not be surprising that the utility of this procedure continues to foster debate in the scientific
community, or that much additional research is routinely required to be able to reliably interpret and
extrapolate the results obtained by this procedure.
In a seminal article dealing with the problems associated with chronic cancer bioassay tests and
their interpretation, Squire listed five experimental design issues that remain relevant today:
1. Use of the MTD as currently defined
2. The number of doses tested
3. The relevance of findings in certain test species
4. Route of exposure and vehicle
5. Extent of the pathological examination
These and related issues raised by this test procedure are discussed in the following paragraphs because
they have a considerable impact on the hazard evaluation of the chemical in question, and because they
are frequently raised when debates occur over the significance of the observation or the regulation of
a particular chemical.
The Doses Used to Test Chemicals Are Too High
This has become perhaps the most frequently raised criticism of chronic animal cancer tests. Because
the MTD is often selected as the highest dose the test animal can maintain for a lifetime without
shortening the length of the test, it is often a dose where chronic toxicity and biological changes occur.
The biological arguments against the applicability of positive results that are seen only at high doses,
doses that produce chronic toxicity and substantially exceed the expected human exposure level, are
as follows:
1. High doses may alter the metabolism and disposition of the chemical such that the types of
reactive, toxic metabolites that are responsible for the critical biochemical changes producing cancer