Increased Plasma and Tumor Somatostatin-Like Immunoreactivity in Medullary Thyroid Carcinoma and Small Cell Lung Cancer*

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Abstract
To investigate the possible elaboration of soma-tostatin-like immunoreactivity (SLI) by medullary thyroid carcinoma (MTC) and/or by small cell lung cancer, we have ra-dioimmunoassayed extracts of plasma and tumor tissue from persons with these forms of cancer. These results have been compared with blood and tissue SLI measurements in patients with epidermoid lung cancer and people without detectable neoplastic disease. The upper limit of plasma SLI in 15 control subjects was 37 pg/ml. None of the 19 patients with epidermoid lung cancer had significantly elevated plasma SLI. By contrast, significantly elevated (136–6150 pg/ml) plasma SLI was noted in 3 of 7 cases of MTC and in 4 of 26 cases of small cell lung cancer. When SLI concentrations determined by direct RIA of plasma extracts were compared to SLI values determined by RIA of SLI isolated by immunoadsorbent chromatography, a highly significant direct correlation was noted. At least three SLI species (13,000, 4,000, and 1,600 daltons) were resolved by gel filtration of plasma extracts. SLI was detectable in thyroid tissue (18–77 pg/mg protein); SLI was undetectable (<12 pg/mg) in normal lungs, epidermoid lung cancers, kidneys, and livers. Tissue SLI was detectable in three of five medullary carcinomas (60-9,200 pg/mg) and five of nine small cell lung cancers (14–441 pg/mg). These results suggest that somatostatin is elaborated by certain nonpancreatic cancers. The plasma and tissue SLI heterogeneity detected in thyroid and lung cancers probably reflects the presence of somatostatin, its biosynthetic intermediate(s), and/or their metabolites. While plasma SLI measurement in MTC seems to be of little immediate clinical importance, the presence of SLI may benefit an understanding of MTC and its origin. Despite the relatively low incidence (15%) of SLI elevation in patients with small cell lung cancer, this finding could have clinical importance for gauging the progression of this common neoplastic disease.