Phenesterin, an experimental anticancer agent, is a steroidal alkylating agent composed of the carboxylic acid ester of cholesterol and an aryl nitrogen mustard.
A bioassay of phenesterin for possible carcinogenicity was conducted by administering the chemical by gavage to Sprague-Dawley rats and B6C3F1 mice.
Groups of 35 rats of each sex were administered phenesterin at one of two doses, either 5 or 10 mg/kg body weight, three times per week for 52 weeks, then observed for an additional 32 or 33 weeks. The vehicle used was 0.05% polysorbate 80 in buffered saline. Controls consisted of groups of 10 rats of each sex which received the vehicle (vehicle control) and 10 rats of each sex which were untreated (untreated control). All surviving rats were killed at 84 or 85 weeks.
Groups of 35 mice of each sex were administered the chemical at one of two doses, either 15 or 30 mg/kg body weight, three times per week for 52 weeks. The males receiving 15 mg/kg were observed for an additional period of 29 weeks, and those surviving to this time were then killed; the animals of the remaining groups were observed for additional periods of only 10-22 weeks, due to early deaths. Seventy-seven weeks after the foregoing groups were started, additional groups of 40 mice of each sex were started and were administered the chemical at 7 mg/kg body weight three times per week; administration of the chemical terminated at week 102 for the males and at week 88 for the females, due to deaths of all females at this time. Controls for the low-dose (7 mg/kg) groups of mice consisted of groups of 20 mice of each sex which received the vehicle (vehicle control) and 20 mice of each sex which were untreated (untreated control); controls for the mid-dose (15 mg/kg) and the high-dose (30 mg/kg) controls consisted of groups of 15 mice of each sex similarly receiving the vehicle or untreated. All surviving low-dose controls were killed at 104 weeks, and all surviving mid- and high-dose controls were killed at 81-84 weeks.
Phenesterin was toxic to rats and mice at the doses used, as shown by reduced mean body weights and survival. Time-adjusted analyses were used for evaluation of incidences of tumors in the female mice.
In female rats, a dose-related trend (P=0.019) was present in adenocarcinoma of the mammary gland, using the pooled controls, and the incidences of the tumor in the individual dosed groups were significant (P<0.009) when compared with those in the pooled controls (controls 1/18, low-dose 12/29, high-dose 12/30).
In male mice, the incidence of alveolar/bronchiolar carcinomas or combined alveolar/bronchiolar adenomas and carcinomas in the low-dose group (18/40) was significantly higher (P<0.020) than that in the low-dose vehicle-control group (0/16). In female mice, seven low-dose animals hadalveolar/bronchiolar adenomas and eight other low-dose animals had alveolar/bronchiolar carcinomas. When these tumors were combined, their time-adjusted incidence was significant (P=0.004) when compared with that in the low-dose vehicle controls (controls 1/18, low-dose 15/35). The lower and nonsignificant incidences of these tumors observed in the mid- and high-dose groups may be due to the earlier mortality in these groups compared with the low-dose groups.
In each sex of mid- and high-dose mice, incidences of lymphoma and leukemia were dose related (P<0.005), using vehicle controls; they were also significant (P<0.018) in direct comparisons of mid- and high-dose groups of both sexes with respective vehicle controls (males: controls 0/14, mid-dose 9/29, high-dose 11/25; females, time-adjusted: controls 0/15, mid-dose 14/18, high-dose 17/19). The significance of the incidence of lymphoma and leukemia in the mid- and high-dose groups of males was increased (P<0.001) when the pooled-control group was used, both in the test for dose-related trend and in tests for direct comparisons of dosed groups with the controls.
In each sex of mice, sarcomas of the myocardium were found in all groups of dosed animals, but in no control animals (males: low-dose 5/40, mid-dose 7/29, high-dose 2/25; females: low-dose 8/34, mid-dose 2/7, high-dose 3/7). In males, the incidence in the mid-dose group was significant when compared with that in the pooled controls (P=0.006); in females, the incidences in the low- and high-dose groups were significant (P<0.023).
It is concluded that under the conditions of this bioassay, phenesterin was carcinogenic in female Sprague-Dawley rats, producing adenocarcinomas of the mammary gland, and in both sexes of B6C3F1 mice, producing alveolar/bronchiolar carcinomas, hematopoietic tumors, and myocardial sarcomas.