R1128 substances, novel non-steroidal estrogen-receptor antagonists produced by a Streptomyces. I. Taxonomy, fermentation, isolation and biological properties.

New non-steroidal estrogen-receptor antagonists, R1128 A, B, C and D, were isolated from the cultured broth of Streptomyces sp. No. 1128 by solvent extraction, silica gel chromatography, reverse phase chromatography and preparative HPLC. These compounds inhibited estrogen binding to its receptor. The IC50 values of R1128 A, B, C and D for partially purified rat uterine cytosol receptor were 1.1 x 10(-7) M, 1.2 x 10(-7) M, 2.6 x 10(-7) M and 2.7 x 10(-7) M, respectively.