Prostate cancer remains the number one cause of noncutaneous cancer, with 220,900 new cases predicted for the year 2003 alone. Of the more promising classes of compounds studied thus far for the treatment of prostate cancer, estrogens of various types have consistently exhibited antitumor activities both in vitro and in vivo. For this reason, we have synthesized and screened a library of unique 17alpha/11beta modified 17beta-estradiol (E(2)) analogues designed for estrogen receptor beta (ER-beta) specificity and a potential for cytotoxic activity directed toward prostate cancer cells. From this library, the novel compound 17alpha-20Z-21-[(4-amino)phenyl]-19-norpregna-1,3,5(10),20-tetraene-3,17beta-diol (APVE(2)) was identified as the primary lead, found to induce a high level (>90%) of cell death through an apoptotic mechanism, with an EC(50) of 1.4, 2.7, and 16 nM in the LNCaP, PC3, and DU145 cell lines, respectively. APVE(2) was found to bind to ER-beta, albeit weakly, with an EC(50) of 250 nM and a binding activity of 6.2% relative to E(2), nearly two orders of magnitude less than the concentration required to induce apoptosis. APVE(2) bound preferentially to ER-beta by 7-fold over ER-alpha, and did not induce growth in the MCF-7 cell line, thus indicating that it is not a classical ER agonist. Furthermore, the cytotoxic actions of APVE(2) were not reversed by co-treatment with a 50-fold excess E(2). In summary, a novel 17 modified estrogen APVE(2) was identified as a lead compound, capable of inducing apoptosis in three prostate cancer cell lines at low nanomolar concentrations, through a mechanism inconsistent with an ER-mediated mechanism.