Glucagon-like peptide-1 vs. somatostatin receptor targeting in malignant insulinoma

Objectives Glucagon-like peptide-1 (GLP-1) receptor targeting is a novel approach for imaging of benign insulinoma and was found to be superior compared to established methods including somatostatin receptor subtype 2 (sst2) imaging (Christ et al. JCEM 2009). Our goal was to prospectively evaluate GLP-1 and sst2 receptor targeting in patients with endogenous hyperinsulinemic hypoglycemia suspicious for having a malignant insulinoma.

Methods Eleven patients (6 females, 5 males, age 62 ± 11 years) were included after positive fasting test and positive dual-phase CT imaging. All patients underwent imaging with 111In-DTPA-exendin-4 (GLP-1 analogue) plus sst2 imaging (7 patients) and/or histological assessment of their tissue samples including GLP-1 and sst receptor quantification using in vitro autoradiography (7 patients).

Results Two of eleven patients with CT data suspicious for malignancy were histologically diagnosed as benign insulinoma. The remaining 9 patients had malignant insulinomas confirmed by histology. GLP-1 receptor targeting was positive in only 3/9 patients (33%), whereas sst2 receptor targeting was positive in 7/9 patients (78%). At least one of the two receptors was expressed in the tumor tissue of each patient.

Conclusions Sst2 receptor imaging is better in detecting malignant insulinoma than GLP-1 receptor imaging. In benign insulinoma however the reverse is found (Christ et al. JCEM 2009). Importantly, one of the two imaging methods is always positive in either type of insulinoma. Finally, sst2 receptor mediated radiotherapy might be effective in about 80% of patients with malignant insulinoma