111In-DTPA-exendin-4 SPECT/CT scans localize hardly detectable insulinomas

Objectives : Correct preoperative localization of insulinomas is often a challenging clinical problem. We have shown before that glucagon-like peptide-1 (GLP-1) receptor imaging using ¹¹¹In-DOTA-exendin-4 SPECT/CT offers a new diagnostic approach to localize small insulinomas (Wild et al. NEJM. 2008; 359: 766-8). However, the preparation of ¹¹¹In-DOTA-exendin-4 is difficult. The goal of this study is the clinical evaluation of GLP-1 receptor imaging with a novel more advanced tracer, ¹¹¹In-DTPA-exendin-4, in patients with endogenous hyperinsulinaemia.

Methods So far, two patients with endogenous hyperinsulinaemic hypoglycaemia were scanned. Whole-body images and SPECT/CT of the abdomen were obtained at 4 h, 23 h and 120 h after injection of ~90 MBq (10 microgram) ¹¹¹In-DTPA-exendin-4. During surgery tumor samples were collected for GLP-1 receptor quantification using in vitro autoradiography.

Results The radiochemical characteristics of ¹¹¹In-DTPA- and ¹¹¹In-DOTA-exendin-4 are shown in the table below. Only ¹¹¹In-DTPA-exendin-4 SPECT/CT could localize the tumor in both patients, whereas CT, MRI and endoscopic ultrasound were negative.

Conclusions Compared to ¹¹¹In-DOTA-exendin-4, ¹¹¹In-DTPA-exendin-4 has superior radiochemical characteristics. These preliminary data suggest that ¹¹¹In-DTPA-exendin-4 SPECT/CT offers a new diagnostic approach that permits the successful localization of hardly detectable insulinomas and is likely to affect medical practice.

Research Support Supported by grant (PASMP3-123269/1) from the Swiss National Science Foundation.