Verification of a false positive in a two-year rat carcinogenicity study using dual control groups.

There is sometimes controversy over whether or not statistically significant responses produced in carcinogenicity studies have biologically significance. Ambiguous results from our previous two-year oral carcinogenicity study on acotiamide hydrochloride hydrate (acotiamide-HH), a prokinetic drug for functional dyspepsia, in rats made it unclear whether the drug may exhibit uterine carcinogenicity. To check this finding, we performed a second long-term carcinogenicity study using two identical control groups to more accurately evaluate uterine carcinogenesis by considering the incidence of spontaneous neoplasms. Female Fischer 344 rats were divided into three groups: the two control groups (control 1 and 2) were administered vehicle (0.5% w/v methylcellulose) and the acotiamide-HH-treated group was administered 2,000 mg/kg/day of acotiamide-HH by oral gavage for two years. Among all groups, the incidence of endometrial adenocarcinoma (EmA) was highest in the control 2 group, followed by the acotiamide-HH-treated group and the control 1 group. Moreover, acotiamide-HH did not affect the incidence of precursor lesions of EmA. In cases where an ambiguous difference is observed, the use of two control groups allows for a more informed interpretation of the findings in the drug-treated groups. The outcomes in this study strongly support the hypothesis that the increase in EmA in rats treated with acotiamide-HH in our previous study is unrelated to administration of the drug.