Cholecystokinin receptor imaging using an octapeptide DTPA-CCK analogue inpatients with medullary thyroid carcinoma

Cholecystokinin (CCK)-B receptors have been demonstrated on a high percenta ge of medullary thyroid carcinomas (MTC) in vitro. After encouraging result s both in vitro and in animal studies, we studied the efficacy of an octape ptide [In-111-DTPA]-CCK analogue in seven patients with MTC. In four of fiv e patients in whom serum calcitonin levels were monitored, a significant ri se was found following the injection, indicating retained biological activi ty of the radiopeptide. In all patients there was visualization of the CCK- B receptor-positive stomach. In one of two patients with known MTC lesions, some of the lesions were visualized; in addition some lesions were visuali zed in one of the five other patients who had elevated serum tumour markers but negative localizing studies, Radioactivity in the presumed tumour site s was still present at 48 h p.i. The uptake in the presumed tumour sites an d stomach was low. Background radioactivity dropped rapidly owing to urinar y excretion. After 1 h, breakdown products of the labelled analogue predomi nated both in urine and in serum, and virtually no intact peptide was prese nt. In conclusion: (1) the CCK-B receptor-positive gastric mucosa and presu med MTC lesions could be visualized in patients using an octapeptide [In-11 1-DTPA]-CCK analogue that is probably internalized, proving the feasibility of CCK-B receptor imaging in vivo; (2) there was a relatively low uptake o f the CCK analogue in the strongly CCK receptor-positive stomach, and rapid degradation of the peptide in serum.