Effects of dose and time in a long-term, low-dose carcinogenic study.

Large numbers of female BALB/c StCrlfC3Hf/Nctr mice were exposed for up to 33 months to low doses of 2-acetylaminofluorene (2-AAF) under controlled conditions. The study design consisted of sacrifice intervals, life span, and discontinued dosing groups. Two separate and distinct endpoints, that is urinary bladder neoplasms and liver neoplasms, resulted in 2 different types of dose response relationships. Although bladder neoplasms exhibited a minimum effect level (or a nonlinear response) for specific conditions, the total results were consistent with a "no threshold concept." The late appearing liver neoplasms displayed a nearly linear type response that extrapolated directly to zero dose. Time of exposure was shown to be an important factor in that, as animals were sacrificed at 18, 24, and 33 months, a positive response was noted at the next lower dose as time was extended. Discontinuing dosing and sacrificing at 18 and 24 months also demonstrated the effects of exposure to the carcinogen 2-AAF. Induction of bladder neoplasms was shown to occur early in the study, but was dependent upon the continuous presence of 2-AAF. The liver neoplasms appeared very late in the study but were shown to be induced at a very early point in the exposures and did not require the continuous presence of the carcinogen in order to develop. A standard 18 month bioassay study, if conducted under the same conditions, would have classified this chemical as a weak acting carcinogen. These studies demonstrate the importance of the time factor in safety evaluation or risk assessment in carcinogenesis.