Elevated plasma concentrations of C-flanking gastrin-releasing peptide in small-cell lung cancer

Many small-cell lung cancers (SCLCs) produce gastrin-releasing peptides (GRPs) (mammalian bombesin) but the plasma concentration of GRP is rarely elevated, possibly because of its rapid elimination. We developed a radioimmunoassay for the C-terminal flanking peptide of proGRP and measured its concentration in plasma from 71 patients with SCLC, in 27 healthy subjects and in 49 patients with other diseases including lung carcinomas. In addition, we studied the molecular size of immunoreactive C-flanking peptide in two SCLC cell lines and in plasma from SCLC patients. The concentration of immunoreactive C-flanking peptide in normal subjects and in control patients did not exceed 10 pmol/L and 26 pmol/L, whereas 72% of the SCLC patients had C-flanking peptide concentrations above 10 pmol/L. In patients with extensive disease (n = 35) the median concentration was 71 pmol/L (range, 10 to 940). ProGRP C-flanking peptide levels paralleled the clinical course in 12 patients. The molecule(s) responsible for the immunoreactivity had a molecular size of about 8 to 10 kd in both patient plasma and tumor cell lines, suggesting that the measured peptide(s) represented major fragment(s) if not the entire C-flanking peptide of proGRP. Thus this peptide(s) seems to be a useful marker for SCLC.