Estrogen receptor-binding affinity and cytotoxic activity of three new estrogen-nitrosourea conjugates in human breast cancer cell lines in vitro.

The hypothesis that more selective antitumor activity may be achieved by cytotoxic agents which selectively bind to estrogen receptors (ER) in human cancer cells was tested. We have synthesized three nitrosourea derivatives of estradiol or hexestrol, and compared the ER binding affinity and cytotoxic activity of these compounds against ER-positive and -negative breast cancer cell lines in vitro. Specific binding to ER in the cytosol of MCF-7 human breast cancer cells was demonstrated in these conjugates: 17 alpha-CNU greater than 17 beta-CNU greater than HEX-CNU greater than lomustine (CCNU). The order of cytotoxicity of these derivatives against human breast cancer cells appeared to correlate with their binding affinity to ER. All three estrogen nitrosourea conjugates were more cytotoxic than CCNU, a clinically useful antitumor nitrosourea which does not bind to ER. The contribution of the estrogen moiety to the cytotoxicity of 17 alpha-CNU was demonstrated by the greater activity of the conjugate than that of a combination of estrogen and CCNU. However, cytotoxicity of these compounds against the receptor-positive MCF-7 and receptor-negative Evsa-T human breast cancer cell lines was similar. The latter finding suggested that cytotoxicity of these conjugates may not be mediated through ER. The difference in stability of these nitrosourea conjugates in aqueous buffer may partly explain their differences in cytotoxicity.