Despite a limited number of compounds in series, it provides significant novel insight into the structure–activity relationship of heterosteroids as anticancer agents. Biological studies show that annulation of androst-5-ene core with pyrimidine is efficient for development of novel selective compounds for treatment of hormone-dependent breast cancer. Moreover, installation of dihydrotriazine pendant at A- and D-rings of estrane and androst-5-ene cores results in strong antiproliferative activities against breast and prostate cancer cells comparable with cisplatine. The two-fold gain in cytotoxicity of 16-C dihydrotriazine-modified estrane against ERα-positive breast cancer cells vs. ERα-negative cells and its high activity as ERα inhibitor were shown while similar androstene derivative was less selective.