PRINCIPLES OF TOXICOLOGY

13.6 INTERPRETATION ISSUES RAISED BY CONDITIONS OF THE TEST PROCEDURE 293
are not present at lower doses. It has been shown with a number of chemicals that a particular metabolic
pathway becomes saturated above a certain dose level. Once saturated either the formation of a specific
toxic metabolite begins to increase, or a detoxification–protective pathway now begins to become
overwhelmed. This leads to a cellular insult and damage that either does not occur at lower doses, or
does so at a significantly lower rate. This phenomenon is often referred to as a dose that “produces
zero order kinetics” in an otherwise “first-order reaction process.”
2. High doses produce irritation or inflammation. These conditions produce the formation of
reactive oxygen species that are capable of inducing DNA damage that simply does not occur at lower
doses.
3. High doses may produce changes in immune or endocrine systems, disrupt nutrition, or
otherwise produce stressors that induce cancer secondary to changes in the background cancer rate.
Because these same organ toxicities have thresholds and so do not occur at lower doses, low doses of
the chemical are incapable of inducing cancer secondary to these specific biochemical and molecular
changes.
4. High doses can produce damage to important DNA repair enzymes, or the DNA damage will
overwhelm the cell’s ability to withstand these genetic assaults.
5. High doses produce a recurrent injury, cell death, and cell turnover that are not induced at lower
doses. Under these conditions the cytotoxicity that is induced by high doses alters important cellular
pools of factors responsible for maintaining genetic integrity within the cell. Thus, high doses may
foster conditions within the cell that result in mutations or genetic damage indirectly. In addition, the
increase in cell turnover may now cause mutations to become fixed that would normally be repaired.
A sustained increase in cell turnover may also increase the rate at which natural errors in DNA
replication and spontaneous mutations occur.
In short, the criticism of high-dose testing is that the cancers observed may originate secondarily to
other important biochemical changes and toxicities that are induced only at high doses. In this situation,
where the chemical induces cancer indirectly and is related to conditions unique to high doses, then
low-dose conditions would not be carcinogenic. Whether such chemicals should be viewed as a
carcinogenic hazard or as chemicals without carcinogenic activity becomes a function of dose; an issue
that may raise considerable controversy when the positive carcinogenicity data are used to regulate the
exposures of such chemicals.
The possibility that the carcinogenicity of a particular chemical may be a high-dose phenomenon
has been assumed or hypothesized for a number of different chemicals and different mechanisms. For
example, a number of different chemicals cause a chronic reduction in the circulating levels of thyroid
hormone at high doses. This, in turn, causes a chronic elevation in blood levels of thyroid-stimulating
hormone, a normal response to low thyroid levels, that results in a chronic overstimulation of thyroid
follicular cells and eventually the development of thyroid follicular cell tumors. This high-dose
phenomenon has a threshold (lower doses will not decrease thyroid hormone levels), and no risk of
cancer would be associated with lower doses. Similarly, the bladder tumors observed with very high
doses of compounds such as vitamin C, saccharin, glycine, melamine, and uracil are believed to be
induced only when an excessive dose produces the depositing of insoluble calculi or crystals in the
urinary bladder. The occurrence of these physical agents produce a chronic irritation or inflammation,
thereby providing a stimulus for the proliferation of the bladder epithelium and ultimately the formation
of bladder tumors. Since none of these changes are produced at lower doses, there are clear thresholds
for these carcinogens, and their “carcinogenic hazard” can be induced only at unrealistically high
exposure levels.
As the evidence accrued that use of the MTD in bioassays frequently produced dose-dependent
results, scientists within the NTP assessed the use of the MTD and the long-held view that responses
obtained at the MTD could be extrapolated in a linear fashion to lower doses. This assessment
concluded that the following implicit assumptions underlie the current use of the MTD, and the
associated use by regulatory agencies of a linear extrapolation of the results obtained with it: