Working Procedures


3. Studies of cancer prevention in experimental animals

(a) Types of study considered

Animal models are an important component of research into cancer prevention. Models that permit evaluation of the effects of cancer-preventive interventions on the occurrence of cancer in most major organ sites are available. Animal models for such studies include: those in which cancer is produced by the administration of a chemical or physical carcinogen; those involving genetically engineered animals; and those in which tumours develop spontaneously. Most cancer-preventive interventions investigated in such studies can be placed into one of three categories: interventions that prevent molecules from reaching or reacting with critical target sites (blocking agents); interventions that decrease the sensitivity of target tissues to carcinogenic stimuli; and interventions that prevent evolution of the neoplastic process (suppressing agents). There is increasing interest in the use of combinations of interventions as a means of improving efficacy and minimizing toxicity; animal models are useful in evaluating such combinations. The development of optimal strategies for intervention trials in humans can be facilitated by the use of animal models that mimic the neoplastic process in humans.

Specific factors to be considered in such experiments are: (1) the temporal requirements of administration of the cancer-preventive interventions; (2) dose-response effects; (3) the site specificity of cancer-preventive activity; and (4) the number and structural diversity of carcinogens whose activity can be reduced by the intervention being evaluated.

An important variable in the evaluation of the cancer-preventive response is the time and duration of administration of the intervention in relation to any carcinogenic treatment, or in transgenic or other experimental models in which no carcinogen is administered. Furthermore, concurrent administration of an intervention may result in a decreased incidence of tumours in a given organ and an increase in incidence in another organ of the same animal. Thus in these experiments it is important that multiple organs be examined.

For all these studies, the nature and extent of impurities or contaminants present in the cancer-preventive intervention or interventions being evaluated are given when available. Also, consideration is given to the possibility of changes in the physicochemical properties of the test substance during collection, storage, extraction, concentration and delivery. Chemical and toxicological interactions of the components of mixtures may result in nonlinear dose-response relationships.

As certain components of commonly used diets of experimental animals are themselves known to have cancer-preventive activity, particular consideration should be given to the interaction between the diet and the apparent effect of the intervention being studied. Likewise, restriction of diet may be important. The appropriateness of the diet given relative to the composition of human diets may be commented on by the Working Group.

(b) Quality of studies considered

An assessment of the experimental prevention of cancer involves several considerations of qualitative importance, including: (1) the experimental conditions under which the test was performed (route and schedule of exposure, species, strain, sex and age of the animals studied, duration of the exposure, and duration of the study); (2) the consistency of the results, for example across species and target organ(s); (3) the stage or stages of the neoplastic process studied, from preneoplastic lesions and benign tumours to malignant tumours; and (4) the possible role of modifying factors.

In the interpretation and evaluation of a particular study, the Working Group takes into consideration: (1) how clearly the intervention was defined and, in the case of mixtures, how adequately the sample composition was reported; (2) the composition of the diet and the stability of the intervention in the diet; (3) whether the source, strain and quality of the animals was reported; (4) whether there were adequate numbers of animals, of appropriate age, per group; (5) whether males and female were used, if appropriate; (6) whether animals were allocated randomly to groups; (7) whether appropriate respective controls were used; (8) whether the dose and schedule of treatment with the known carcinogen were appropriate in assays of combined treatment; (9) whether the doses of the cancer-preventive intervention were adequately monitored; (10) whether the agent(s) was absorbed, as shown by blood concentrations; (11) whether the survival of treated animals was similar to that of controls; (12) whether the body and organ weights of treated animals were similar to those of controls; (13) whether the duration of the experiment was adequate; (14) whether there was adequate statistical analysis; and (15) whether the data were adequately reported.

(c) Quantitative aspects

The incidence of tumours may depend on the species, sex, strain, and age of the animals, the dose of carcinogen (if any), the dose of the agent and the route and duration of exposure. A decreased incidence and/or decreased multiplicity of tumours in adequately designed studies provide evidence of a cancer-preventive effect. A doseĀ­related decrease in incidence and/or multiplicity further strengthens this association.

The nature of the dose-response relationship can vary widely, depending on the agent and the target organ. Saturation of steps such as absorption, activation, inactivation, and elimination may produce non-linearity in the dose-response relationship (Hoel et al., 1983; Gart et al., 1986), as could saturation of the detoxication processes. The dose-response relationship can also be affected by differences in survival between the treatment groups.

(d) Statistical analyses

Factors considered in the statistical analysis by the Working Group include: (1) the adequacy of the data for each treatment group; (2) the initial and final effective numbers of animals studied and the survival rate; (3) body weights; and (4) tumour incidence and multiplicity.

The statistical methods used should be clearly stated and should be the generally accepted techniques defined for this purpose. In particular, the statistical methods should be appropriate for the characteristics of the expected data distribution and should account for interactions in multifactorial studies. Consideration is given as to whether the appropriate adjustment was made for differences in survival.

If available, recent data on the incidence of specific tumours in historical controls, as well as in concurrent controls, are taken into account in the evaluation of tumour response.

Posted 5 July 2016