PRINCIPLES OF TOXICOLOGY - Biology East Borneo

290 CHEMICAL CARCINOGENESISIncreasing the number of animals tested might increase the sensitivity of the test, but as thenumber of animals is increased, the cost of the experiment rises and could render the testcost-prohibitive.• The exposure and observation periods should last a lifetime, if possible, so that the latencyof the response does not become an issue.• At least two doses should be tested. One should be the maximally tolerated dose (MTD), thesecond dose should be some fraction (usually 50% or 25%) of the MTD. The MTD is definedas the highest dose that can be reasonably administered for the lifetime of the animal withoutproducing serious, life-threatening toxicity to the animal that might compromise completionof the study. In the past the MTD has been defined as a dose that causes no more than a 10percent decrease in body weight gain and does not lead to lethality over time.• A detailed pathologic examination of all tissues should be held at termination of theexperiment (and sometimes at 6-month intervals).In addition to these recommended guidelines, this test is normally performed following good laboratorypractice (GLP) procedures. These and other procedures ensure proper animal care during theextended period of the test, that no cross-contamination with other chemicals being tested will occur,and the possibility of having infectious agents or disease affect the outcome of the test is limited.Using these basic guidelines, any positive result obtained in at least one sex of one species isgenerally considered sufficient evidence to classify the chemical, for regulatory and public healthpurposes, as a carcinogen. Four different types of tissue response might be observed in a chronic testand considered positive evidence of carcinogenicity:1. An increase in the incidence of a tumor type that occurs in control animals but at a significantlylower rate2. The development of tumors at a significantly earlier period than is observed in the controlanimals3. The presence of tumor types that are not seen in control animals4. An increased multiplicity of tumors (although generally speaking, differences in total tumorload between exposed and unexposed animals is not considered reliable evidence)Positive results in a test with a more limited power to detect carcinogenicity (e.g., tests of shorterduration or fewer animals), but where the overall test procedures employed are considered adequate,may also become accepted as sufficient evidence of carcinogenicity, particularly where other relevantevidence (e.g., mechanistic data, structural alerts, structure–activity relationships) are also available.In contrast, because it is well recognized that important species differences exist in regard to response,negative results (an observed lack of a tumorigenic response), might not be considered definitiveevidence that a chemical is not a carcinogen in other species that were not tested.The Issue of Generating False-Negative or False-Positive ResultsBoth false-negative and false-positive results are a potential problem in carcinogen bioassays. Ideally,the number of animals required to provide adequate negative evidence should be great enough thateven a false-negative test (a test failing to detect existent carcinogenicity) will not allow an excessiverisk to go unnoticed. The likelihood that such a risk will not be detected during the evaluation ofbioassay data is dependent on two factors (excluding species differences in response): the number ofanimals tested and the extent to which the test dose exceeds the usual level of human exposure, thereforeincreasing either parameter tends to lessen the chance of obtaining a false negative response (withrespect to humans).The probability that a test will generate a false-negative result is also affected by the backgroundtumor rate in the control animals. As the background incidence of tumorigenesis increases, so does the