The later administration of progesterone more rapidly activates dormant mouse mammary tumor cells initiated by 3'-methyl-4-dimethylaminoazobenzene.

Implantation of progesterone at 1 month of age induced the development of mammary tumors in female C57BL/6 x DS-F1 mice that had been treated with 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) neonatally, and that had undergone ovariectomy and received implants of estradiol-17beta (E2) pellets at 1 month of age, and the incidence of mammary tumors became 100% at 15 months of age. On the other hand, no mammary tumors developed in these mice with implants of E2 pellets alone. Implantation of progesterone alone also induced no mammary tumors in mice that had been treated with 3'-Me-DAB neonatally, and had undergone ovariectomy at 1 month of age. Implantation of progesterone at 4, 6, 8, and 10 months of age also caused the prompt development of mammary tumors as implantation of progesterone at 1 month of age. When ages at which the incidence became 50% were estimated on curves of the incidences, these ages on implantation of progesterone at 1, 4, 6, 8, and 10 were about 11, 13, 14, 14, and 14 months of age. These results suggest that progesterone together with estrogen promotes the development of mammary tumors induced by 3'-Me-DAB, and that the later progesterone is administered, the more rapidly it activates dormant mammary tumor cells initiated by 3'-Me-DAB.