The isoflavones biochanin A ( 1a), genistein ( 1b), and daidzein ( 4) at concentrations >20 microM inhibit cell growth of various cancer cell lines. To enhance the antiproliferative activities of these compounds, we synthesized three analogs, 2-[3-carboxy-(6-tert-butoxycarbonylamino)-hexylamino-propyl]-7,5-dihydroxy-4'-methoxyisoflavone ( 3a), 2-[3-[N-[6-(tert-butoxycarbonyl)-aminohexyl]]-caboxamidopropyl]-5,7,4'-trihydroxyisoflavone ( 3b), and 5-{2-[3-(4-hydroxy-phenyl)-4-oxo-4 H-chromen-7-yloxy]-acetylamino}-pentyl)-carbamic acid tert-butyl ester ( 6). When cancer cells expressing predominantly estrogen receptor mRNA of the beta- relative to alpha-subtype were treated with 3a, 3b, or 6, DNA synthesis was inhibited in a dose-dependent manner, ranging from 15 to 3000 nmol/L, with little inhibitory effect in normal vascular smooth muscle cells. Compound 6 was the most potent one, and its antiproliferative effect in cancer cells was modulated by estrogen and by the apoptosis inhibitor Z-VADFK. When tested in vivo, compound 6 decreased tumor volume of ovarian xenografts by 50%, with no apparent toxicity. Compound 6 may be a promising agent for therapy of cancer either alone or in combination with chemotherapeutic agents.