Bioassay of isophosphamide for possible carcinogenicity.
A bioassay of the anticancer drug isophosphamide for possible carcinogenicity was conducted by injecting the test chemical intraperitoneally into Sprague-Dawley rats and B6C3F1 mice. Groups of 35 rats and 35 mice of each sex were given the injections at one of two doses three times per week for 52 weeks. Doses for rats were either 6 or 12 mg/kg, and for mice either 10 or 20 mg/kg. After the period of administration of the isophosphamide, the surviving rats were observed for 31 weeks and the mice for 28 weeks. Untreated controls as well as vehicle controls (buffered saline) were used. The matched vehicle-control groups each consisted of 10 rats or 15 mice of each sex; pooled vehicle-control groups, used for statistical evaluation, consisted of the matched vehicle controls of each species combined with 20 male and 20 female matched vehicle-control rats or 15 male and 15 female matched vehicle-control mice from a similar bioassay of another test chemical. All surviving rats were killed at 79-84 weeks, all surviving mice at 79-81 weeks. Mean body weights of the high-dose rats of either sex were lower than those of the matched vehicle controls after approximately 25 weeks on study. Survival was low among the high-dose male and female rats, but in the low-dose groups it was adequate for meaningful statistical analyses of the incidences of tumors. Mean body weights of the mice did not show any consistent effect from the isophosphamide treatment. Survival was adequate for meaningful statistical analyses in both groups of female mice, while survival of the males was 31% for both treated groups at the end of the study. In male rats, tumors of the hematopoietic system included six malignant lymphomas and two granulocytic leukemias. With the unadjusted analysis, these tumors showed a dose-related trend in male rats using pooled vehicle controls (controls 0/29, low-dose 3/32, high-dose 5/34, P = 0.032) and a higher incidence in the high-dose males than in the pooled vehicle controls (P = 0.040). These tumors were not significant when compared with matched vehicle controls using adjusted analyses, and they cannot clearly be associated with treatment. However, it should be noted that five rats with these tumors were observed in the high-dose group whose median survival was only 35 weeks. In female rats, the incidence of uterine leiomyosarcoma was significant in the low-dose group using pooled vehicle controls (controls 0/27, low-dose 15/32, P<0.001). There was also a significant incidence of mammary fibroadenoma among low-dose females using pooled vehicle controls (controls 8/28, low-dose 28/33, P<0.001). The incidence of each tumor was also significant when compared with matched vehicle controls using time-adjusted analyses. The low survival of the high-dose group (median time on study, 35 weeks) may explain the lower incidences of the uterine leiomyosarcoma and the mammary fibroadenoma in this group. In some rats, the leiomyosarcomas metastasized to the lungs, urinary bladder, spleen, and other abdominal sites. In female mice, the incidence of malignant lymphoma of the hematopoietic system showed a significant dose-related trend using either matched vehicle controls (controls 0/14, low-dose 3/32, high-dose 13/34, P=0.001) or pooled vehicle controls (controls 1/29, P<0.001). Further, incidences of this tumor in the high-dose females were significantly higher than incidences in the matched vehicle (P=0.005) or pooled vehicle (P=0.001) controls. It is concluded that under the conditions of this bioassay, isophosphamide was not carcinogenic in male Sprague-Dawley rats or in male B6C3F1 mice. However, the incidence of leiomyosarcomas of the uterus indicates that isophosphamide was carcinogenic in female Sprague-Dawley rats, and the incidence of fibroadenoma of the mammary gland in female rats was associated with treatment with isophosphamide. Isophosphamide was carcinogenic in female B6C3F1 mice, producing malignant lymphomas of the hematopoietic system.