ACTH precursors characterize the ectopic ACTH syndrome.

Abstract

OBJECTIVE: ACTH is secreted by the pituitary following processing of larger molecular weight precursors, proopiomelanocortin and pro-ACTH. Ectopic ACTH syndrome refers to the secretion of ACTH by non-pituitary tumours, but the predominant circulating form of proopiomelanocortin-related peptides remains unclear. PATIENTS: Fifteen patients with ectopic ACTH syndrome were compared to 20 patients with pituitary-dependent Cushing's syndrome, 22 patients with small cell lung carcinoma but no evidence of Cushing's syndrome, and 25 controls. DESIGN AND MEASUREMENTS: Measurement of plasma ACTH and ACTH precursors using specific monoclonal-based immunoradiometric assays at 0900 h and, in five patients with ectopic ACTH syndrome, at 15-minute intervals for 6-24 hours. RESULTS: ACTH precursors were grossly elevated in patients with ectopic ACTH syndrome (median 2194, range 139-18000 pmol/l) compared to patients with Cushing's disease (median 33, 8-73 pmol/l, P < 0.001), patients with small cell lung carcinomas (38, 8-117 pmol/l, P < 0.001) and controls (26, 10-39 pmol/l, P < 0.001). ACTH levels were also elevated in ectopic ACTH syndrome (0900 h median 34, 11-152 pmol/l) compared to patients with Cushing's disease (0900 h median 8, 3-19 pmol/l), but not to the same degree as ACTH precursors. In contrast with Cushing's disease, ACTH was secreted in a non-pulsatile fashion. ACTH precursors but not ACTH itself correlated with plasma cortisol in patients with ectopic ACTH syndrome (r = 0.65, P < 0.05). Chromatographic analysis of plasma from a patient with ectopic ACTH syndrome confirmed ACTH precursors and not ACTH to be the predominant circulating form. With the cross-reactivity of proopiomelanocortin and pro-ACTH in the ACTH IRMA of < 1 and < 10% respectively, ACTH precursors could represent all the ACTH immunoreactivity in patients with ectopic ACTH syndrome. CONCLUSIONS: Ectopic 'ACTH' is characterized by aberrant processing of proopiomelanocortin and should be more accurately referred to as 'ectopic ACTH precursor syndrome'.