Certain A-ring substituted estrogens are excellent inhibitors of hormone dependent breast cancer in 3 different animal models. Four-substitution on estrone by either nitro- or amine groups imparts carcinostatic properties to the molecule while diminishing the estrogenic and toxic properties of this hormone. It is proposed in this renewal application; 1) to continue the investigations of growth inhibition of hormone dependent breast tumors by A-ring substituted estrogens. These studies will extend and refine the present understanding of the effect of A-ring substitutions on tumor inhibition as well as establish their influence on the cell cycle with flow cytometric studies; 2) to carry out formulation and pharmacokinetic studies directed toward improving the potency of these agents, particularly following oral administration; 3) to investigate the effects of combination therapy in rodent models of hormone dependent breast cancer. These studies will take advantage of the apparently separate mechanisms of activity shown by the A-ring substituted estrogens (4-nitro- and 4-aminoestrone) that are capable of receptor binding, and the A-ring substituted 3- deoxyestrogens which inhibit mammary tumors without receptor involvement. Furthermore these studies will examine the efficacy of utilizing the progesterone receptor inductive capacity of the tumor inhibitory 4-nitroestrone 3-methyl ether in treatment regimens involving progesterone and the antiprogestin (RU486). Combinations with tamoxifen and chemotherapeutic agents will also be examined; 4) to examine the characteristics, fate and activity of the estrogen receptor in MXT tumors during treatment with A-ring substituted estrogens and classical anti- estrogens. In every incidence, the A-ring substituted estrogens are more efficiacious than diethylstilbestrol or tamoxifen when examined in the rodent tumor systems. Although effectual, tamoxifen exhibits some undesirable side effects and this antiestrogen is, apparently, inactive in certain human tumors that are otherwise responsive to hormonal treatment. Thus, new hormonal agents are needed in the armamentarium to combat breast cancer: a) to expand the spectrum of neoplasms that respond to hormonal therapy and; b) to provide insight into the characteristics of hormone dependent cancers.