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Inhibitory Effect of Bombesin Receptor Antagonist RC-3095 on the Growth of Human Pancreatic Cancer Cells in Vivo and in Vitro¹

Endocrine, Polypeptide and Cancer Institute, Veterans Affairs Medical Center, and Section of Experimental Medicine, Department of Medicine, Tulane University School of Medicine, New Orleans, Louisiana 70146 [Y. Q., T. E., R-Z. C., G. H., A. V. S.]

² To whom requests for reprints should be addressed, at Veterans Affairs Medical Center, 1601 Perdido Street, New Orleans, LA 70146.

In this study, we investigated the effect of bombesin/GRP antagonist RC-3095 on the growth of CFPAC-1 human pancreatic cancer cells transplanted to nude mice or cultured in vitro. Nude mice bearing xenografts of the CFPAC-1 cell line received s.c. injections of RC-3095 (10 µg twice a day) or the vehicle (control) for 25 days. Chronic administration of RC-3095 inhibited the growth of CFPAC-1 tumors in nude mice as shown by a significant decrease in tumor volume throughout the period of treatment. Tbrnor volume doubling time was prolonged by RC-3095 treatment from 7.2 days to 10 days, and the tumor growth rate was decreased by 49%. In mice treated with RC-3095, the tumor growth delay time was 5.8 days. Treatment with RC-3095 decreased the final tumor weight by 37% and reduced DNA and protein contents in tumor tissues by 44 and 39.9%, respectively, compared to the controls. In cultures of the CFPAC-1 cell line, the addition of bombesin(1–14) (1 pM-0.1 µM) to the medium induced a dose-dependent increase in cell number. RC-3095 at 1 nM concentration effectively inhibited the bombesin-stimulated growth of CFPAC-1 cells in cultures. In the presence of 1 µM RC-3095 in the culture medium, the bombesin-induced growth of CFPAC-1 cells was totally suppressed. Bombesin was also shown to stimulate the DNA synthesis in CFPAC-1 cells in vitro as based on [³H]thymidine incorporation assay. When the cells were cultured in the presence of 1–100 nM bombesin, the uptake of [³H]thymidine by the cells was increased by 89-131%. RC-3095 inhibited both the basal and bombesin-stimulated DNA synthesis of CFPAC-1 cells. Addition of RC-3095 (10µ100 nM) alone to the cultures caused a 39µ40% decrease in the [³H]thymidine incorporation by the cells. Concomitant addition of RC-3095 (1 µM) and bombesin (1µ100 nM) to the cultures induced a significant reduction in the uptake of [³H]thymidine by the cells compared to the values obtained with bombesin alone. Receptor binding assays showed the presence of two classes of specific binding sites for bombesin on CFPAC-1 cells, one with high affinity (K_d = 4.25 ± 0.77 nM) and low capacity (B_max = 0.268 ± 0.052 pmol/10⁶ cells) and the other with low affinity (K_d = 321.70 ± 68.46 nM) and high capacity (B_max = 3.991 ± 0.374 pmol/10⁶ cells). Antagonist RC-3095 inhibited the binding of ¹²⁵I-Tyr⁴-bombesin to CFPAC-1 cell membranes in a dose-dependent manner. These observations suggest that bombesin acts as a growth factor and stimulates proliferation of CFPAC-1 human pancreatic cancer through specific receptors for bombesin/GRP present on the cells. RC-3095 appears to inhibit the growth of CFPAC-1 cells by blocking the interaction of bombesin with its receptors. Bombesin/GRP receptor antagonist RC-3095 could be considered for the development of new approaches for treatment of human pancreatic cancers.

¹ This work was supported by NIH Grant CA 40077 and the Medical Research Service of the Veterans Affairs (A. V. S.).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 9/20/93. Accepted 12/17/93.

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