Carcinogenicity evaluation: comparison of tumor data from dual control groups in the CD-1 mouse

Current regulatory thinking allows for the use of single control groups for rodent carcinogenicity testing although there has been a trend until recently to use dual control groups. To date, virtually nothing has been published on whether a shift from dual to single control groups will affect the identification of tumorigenic risk potential in these studies. A recent evaluation of dual control carcinogenicity data in the rat (Baldrick, Toxicol Pathol 2005, 33: 283-291) showed that although no major differences in tumor incidences between the control groups were found, some interstudy variation occurred and in cases were a notable difference was seen, the use of 2 control groups, as well as robust, contemporary background data, allowed an easier interpretation of findings in drug-treated groups. In this paper, the results of 10 mouse carcinogenicity studies, performed between 1991 and 2004, with 2 control groups, are presented. As in the rat, interstudy variation was seen and in some cases, the use of dual control groups assisted in the tumor risk assessment. Thus, the continued use of 2 control groups can have a vital role in mouse carcinogenicity studies. The paper also presents an update on survival, on the range and extent of background spontaneous neoplasms and comments on genetic drift in this commonly used mouse strain.