Small cell lung cancers (SCLC) and some non-small cell lung cancers (NSCLC) have neuroendocrine features which include production of a variety of neuropeptides, cell surface expression of the receptors for these peptides, and autocrine stimulation by the peptides. Previous studies showed that some peptide antagonists and anti-peptide antibodies inhibited the growth of SCLC cell lines which expressed receptors for the specific peptide. We and others showed that the heterogeneity of peptide receptor expression and responsiveness was a major potential obstacle for developing therapeutic uses of peptide antagonists. In this manuscript we evaluated the effects of 11 peptide antagonists (3 bombesin-specific, 2 cholecystokinin-specific, 1 arginine vasopressin (AVP)-specific, and 5 substance P derivatives with broad specificity) on peptide-induced calcium mobilization and growth of SCLC and NSCLC cell lines. For each antagonist, we determined the dose-response effects, specificity of peptide antagonism, and biological stability in serum using Indo-1AM-based flow cytometric assays. We found that the three bombesin antagonists, S30, SC196, and L336,175, varied in potency from 10 nm to 10 µm, varied in serum stability from 6 h to more than 24 h, and had no effect on the calcium response elicited by other peptides. None of these compounds effectively inhibited the growth of SCLC cell lines in [3H]dThd and cell growth assays in vitro. Similarly, the three cholecystokinin and AVP antagonists were highly specific for cholecystokinin and AVP, respectively, had widely varying potency, but had little inhibitory effect on SCLC growth in vitro. In contrast, the five substance P derivatives inhibited the calcium response to bombesin, AVP, bradykinin, and fetal bovine serum. None of these five antagonists were as potent as the six specific antagonists described above, but they were more effective in inhibiting the growth of SCLC cell lines in vitro. These substance P derivatives inhibited the growth of peptide-sensitive SCLC cell lines more efficiently than their inhibition of peptide-insensitive NSCLC or breast cancer cell lines. Relatively high concentrations of these substance P derivatives were required to inhibit in vitro growth, even in the absence of added peptide. It is likely that more potent broad spectrum antagonists, toxins, or radiolabeled stable antagonists will need to be developed for maximal clinical development of this type of anti-growth factor therapy.

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Supported in part by NCI Grants U01 CA 46088, 1 P30 CA 46934, and 1 P50 CA 58187 and a grant from the Lucille P. Markey Charitable Trust.

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