BACKGROUND The results of several clinical trials using various luteinizing hormone-releasing hormone agonists for treatment of advanced breast cancer are encouraging. However, only about 30% of breast cancers are estrogen-dependent and can be treated by hormonal manipulation. New therapeutic approaches combining estrogen ablation therapy with other compounds must be explored. Various studies suggest that bombesin or gastrin-releasing peptide acts as an autocrine growth factor and may play a role in the initiation and progression of some cancers, including that of the breast. METHODS Female athymic nude mice bearing xenografts of the MCF-7 MIII human breast cancer cell line were treated for 7 weeks with bombesin/gastrin-releasing peptide antagonist (D-Tpi6, Leu13 psi[CH2NH]-Leu14) bombesin(6-14) (RC-3095) injected subcutaneously daily at a dose of 20 micrograms and luteinizing hormone-releasing hormone antagonist SB-75 (Cetrorelix) administered biweekly in the form of microgranules releasing 45 micrograms/day. RESULTS After 2 weeks of treatment, a significant inhibition of tumor volume was observed in the groups treated with RC-3095 alone or in combination with SB-75 but not in those treated with SB-75 as a single agent. After 7 weeks, tumor growth as measured by tumor volume and percentage changes in tumor volume and tumor weight was greatly inhibited in all of the treated groups. Uterine and ovarian weights were reduced and serum luteinizing hormone levels decreased by administration of SB-75 alone or in combination with RC-3095. Histologically, a significant decrease in argyrophilic nucleolar organizer region count in tumor cell nuclei was observed in all of the treated groups, indicating a lower proliferation of these cells. High-affinity binding sites for bombesin were detected in cultured MCF-7 MIII cells. Chronic treatment with RC-3095 caused a significant down-regulation of epidermal growth factor receptors in tumor cell membranes, which might be related to tumor inhibition. In studies in vitro, SB-75 inhibited proliferation of MCF-7 cells in culture but not proliferation of MCF-7 MIII cells. CONCLUSIONS Because previously we demonstrated that RC-3095 inhibits the proliferation of MCF-7 MIII cells in vitro, it appears that the major antitumoral effect of RC-3095 on the MCF-7 MIII cancer line is direct, whereas that of SB-75 is indirect, and that it is mediated by suppression of the pituitary-gonadal axis. In view of its immediate and powerful inhibitory effect on MCF-7 MIII tumors, bombesin/gastrin-releasing peptide antagonist RC-3095 might be considered as a possible new agent for the treatment of breast cancer.