Inhibition of growth of MKN45 human gastric‐carcinoma xenografts in nude mice by treatment with bombesin/gastrin‐releasing‐peptide antagonist (RC‐3095) and somatostatin analogue RC‐160

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Abstract
Nude mice bearing xenografts of the gastrin‐responsive human gastric carcinoma MKN45 cell line were treated for 4 to 5 weeks with bombesin/gastrin‐releasing‐peptide(GRP) antagonist (RC‐3095), somatostatin analogues RC‐160 and SMS 201‐995, or the combination of RC‐3095 and RC‐160. Tumor volumes and weights were reduced significantly to a similar extent by RC‐160 and SMS 201‐995, administered by daily s.c. injections at a dose of 100 μg/day. Bombesin/GRP antagonist RC‐3095, given s.c. at a dose of 20 μg/day, had the greatest inhibitory effect on tumor growth. The combination of RC‐3095 with RC‐160 did not further potentiate the suppression of tumor growth. Histologically, the number of mitotic cells decreased significantly in the groups treated with RC‐160 or the combination of RC‐3095 with RC‐160. Serum gastrin levels were significantly diminished in all treated groups. Therapy with RC‐160 or the combination also significantly decreased levels of serum growth hormone. Receptor assays on tumor membranes showed that bombesin was bound to 2 classes of receptor sites, one with high affinity and low capacity, the other with low affinity and high capacity. Binding sites for epidermal growth factor (EGF) were down‐regulated in tumor cells after treatment with RC‐3095, RC‐160 or the combination of RC‐3095 with RC‐160. In studies in vitro, both RC‐160 and RC‐3095 significantly inhibited the proliferation of MKN45 cells in culture as measured by cell number. These data demonstrate, for the first time, that the growth of human gastric cancer in nude mice can be inhibited not only by somatostatin analogues, but also by administration of modern bombesin/GRP antagonists, such as RC‐3095.