Eighty percent of human ovarian and endometrial cancers express receptors for luteinizing hormone-releasing hormone (LHRH). These receptors might be used for targeted chemotherapy with cytotoxic LHRH analogs such as AN-152, in which doxorubicin is linked to agonist carrier [D-Lys6]LHRH. The antiproliferative effects of doxorubicin and AN-152 were assessed in LHRH receptor-positive ovarian (EFO-21, EFO-27) and endometrial (HEC-1A, Ishikawa) cancer cell lines as well as in LHRH receptor negative ovarian SKOV-3 and endometrial MFE-296 lines. The mechanism of action of AN-152 was investigated by a blockage of receptors using an excess of the LHRH agonist [D-Trp6]LHRH. In some cases, confocal laser-scanning microscopy was used to visualize the accumulation of AN-152 or doxorubicin within the cells. In 3 of 4 LHRH receptor-positive cell lines (EFO-21, HEC-1A, Ishikawa) AN-152 was more effective than doxorubicin in inhibiting cell proliferation. The effect of AN-152 was shown to be receptor-mediated because it could be reduced by competitive blockade of the LHRH receptors with [D-Trp6]LHRH. In contrast, AN-152 was less active than doxorubicin in LHRH receptor-negative lines. Confocal laser-scanning microscopy showed an intranuclear accumulation of AN-152 and competitive inhibition thereof by [D-Trp6]LHRH in LHRH receptor-positive cell lines, but no intracellular accumulation of AN-152 could be detected in the receptor-negative SKOV-3 line. These results suggest a selective receptor-mediated action of AN-152 in receptor-positive cell lines.