Serum concentrations of the opioid peptide leucine-enkephalin were measured by radioimmunoassay in 30 patients with histologically confirmed bronchial carcinoma and 10 control subjects. This peptide, which is present in greatest amounts in the central and autonomic nervous systems, has previously been found in bronchial neoplasms. The mean serum concentration of leucine-enkephalin was significantly greater in the patients with carcinoma (1035 pg/ml) than in the control subjects (426 pg/ml). In the 23 patients with a tumour in non-apical regions of the lung, however, the mean concentration of the peptide (422 pg/ml) did not differ significantly from that in control subjects; serum concentrations in the seven patients with an apical neoplasm (mean 3050 (range 1259-5820) pg/ml) were significantly greater than values in either the control subjects or the patients with non-apical lung tumours. All seven subjects with an apical tumour had one or more features of Horner's syndrome and the three with all four components of the syndrome had the highest serum concentrations. Serum concentrations of leucine-enkephalin were unrelated to tumour type or presence of metastatic disease. No patient had evidence of metastases in the central nervous system or adrenal glands. Raised serum concentrations of leucine-enkephalin in patients with an apical tumour probably reflect invasion of cervical sympathetic ganglia with release of the peptide into the circulation rather than elaboration of the peptide by the neoplasm.
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