NTP sought out alternative model systems for toxicological and carcinogenesis studies through research into genetically modified models (GMMs). The history of development of GMMs began at NIEHS and NTP in 1998 with the first Board of Scientific Counselors review.

GMMs were subsequently considered for their potential to provide additional information in understanding a substance's mode of action. The goal was to improve both the accuracy and efficacy of experimental assessment. Genetic modifications involve inactivating tumor suppressor functions or activating genes commonly observed in human cancers. The idea in using GMMs was that this would result in rapid onset tissue growth that could be observed within a shorter time frame than with conventional models.

In non-GMM studies, the high incidence of spontaneous or background tumors, which occur most often late in conventional 2-year rodent studies, hindered interpretation of the findings and their implications. The use of GMMs had the potential to improve the accuracy and effectiveness of experiments designed to determine the carcinogenic potential of substances.

Ultimately, however, the recommendation was made in 2006 to discontinue efforts to develop and refine GMMs as a routine replacement for rodents currently used in the conventional 2-year bioassay.

See Historical Controls for Genetically Modified Models.

The National Toxicology Program describes the results of individual experiments on a chemical agent and notes the strength of the evidence for conclusions regarding each study. Negative results, in which the study animals do not have a greater incidence of neoplasia than control animals, do not necessarily mean that a chemical is not a carcinogen, inasmuch as the experiments are conducted under a limited set of conditions. Positive results demonstrate that a chemical is carcinogenic for laboratory animals under the conditions of the study and indicate that exposure to the chemical has the potential for hazard to humans. Other organizations, such as the International Agency for Research on Cancer, assign a strength of evidence for conclusions based on an examination of all available evidence, including animal studies such as those conducted by the NTP, epidemiologic studies, and estimates of exposure. Thus, the actual determination of risk to humans from chemicals found to be carcinogenic in laboratory animals requires a wider analysis that extends beyond the purview of these studies.

Five categories of evidence of carcinogenic activity are used in the Technical Report series to summarize the strength of evidence observed in each experiment: two categories for positive results (clear evidence and some evidence); one category for uncertain findings (equivocal evidence); one category for no observable effects (no evidence); and one category for experiments that cannot be evaluated because of major flaws (inadequate study). These categories of interpretative conclusions were first adopted in June 1983 and then revised on March 1986 for use in the Technical Report series to incorporate more specifically the concept of actual weight of evidence of carcinogenic activity. For each separate experiment (male rats, female rats, male mice, female mice), one of the following five categories is selected to describe the findings. These categories refer to the strength of the experimental evidence and not to potency or mechanism.

Clear Evidence of Carcinogenic Activity is demonstrated by studies that are interpreted as showing a dose-related (i) increase of malignant neoplasms, (ii) increase of a combination of malignant and benign neoplasms, or (iii) marked increase of benign neoplasms if there is an indication from this or other studies of the ability of such tumors to progress to malignancy.

Some Evidence of Carcinogenic Activity is demonstrated by studies that are interpreted as showing a chemical-related increased incidence of neoplasms (malignant, benign, or combined) in which the strength of the response is less than that required for clear evidence.

Equivocal Evidence of Carcinogenic Activity is demonstrated by studies that are interpreted as showing a marginal increase of neoplasms that may be chemically related.

No Evidence of Carcinogenic Activity is demonstrated by studies that are interpreted as showing no chemical-related increases in malignant or benign neoplasms.

Inadequate Study of Carcinogenic Activity is demonstrated by studies that because of major qualitative or quantitative limitations cannot be interpreted as valid for showing either the presence or absence of carcinogenic activity.

For studies showing multiple chemical-related neoplastic effects that if considered individually would be assigned to different levels of evidence categories, the following convention has been adopted to convey completely the study results. In a study with clear evidence of carcinogenic activity at some tissue sites, other responses that alone might be deemed some evidence are indicated as "were also related" to chemical exposure. In studies with clear or some evidence of carcinogenic activity, other responses that alone might be termed equivocal evidence are indicated as "may have been" related to chemical exposure.

When a conclusion statement for a particular experiment is selected, consideration must be given to key factors that would extend the actual boundary of an individual category of evidence. Such consideration should allow for incorporation of scientific experience and current understanding of long-term carcinogenesis studies in laboratory animals, especially for those evaluations that may be on the borderline between two adjacent levels. These considerations should include:

• adequacy of the experimental design and conduct;
• occurrence of common versus uncommon neoplasia;
• progression (or lack thereof) from benign to malignant neoplasia as well as from preneoplastic to neoplastic lesions;
• some benign neoplasms have the capacity to regress but others (of the same morphologic type) progress. At present, it is impossible to identify the difference. Therefore, where progression is known to be a possibility, the most prudent course is to assume that benign neoplasms of those types have the potential to become malignant;
• combining benign and malignant tumor incidences known or thought to represent stages of progression in the same organ or tissue;
• latency in tumor induction;
• multiplicity in site -specific neoplasia;
• metastases;
• supporting information from proliferative lesions (hyperplasia) in the same site of neoplasia or in other experiments (same lesion in another sex or species);
• presence or absence of dose relationships;
• statistical significance of the observed tumor increase;
• concurrent control tumor incidence as well as the historical control rate and variability for a specific neoplasm;
• survival-adjusted analyses and false positive or false negative concerns;
• structure-activity correlations; and
• in some cases, genetic toxicology.

Earlier designations include:
P = Positive; E = Equivocal; N = Negative;

Download Explanation of Levels of Evidence of Carcinogenic Activity