68Ga-labelled exendin-3, a new agent for the detection of insulinomas with PET

Glucagonlike peptide-1 receptor: an example of translational research in insulinomas: a review

Glucagonlike peptide-1 receptors (GLP-1R) play an increasingly important role in endocrine gastrointestinal tumor management. In particular, virtually all benign insulinomas express GLP-1R in high density. Exendin-4 is a GLP-1 analog that has a longer half-life than GLP-1. Targeting GLP-1R by (111)In-DOTA-exendin-4 or (111)In-DPTA-exendin-4 offers a new approach that permits the successful localization of small benign insulinomas. It is likely that this new noninvasive technique has the potential to replace the invasive localization by selective arterial stimulation and venous sampling.

Target-specific delivery of peptide-based probes for PET imaging

Positron emission tomography (PET) is one of the most rapidly growing areas of medical imaging, with many applications in the clinical management of patients with various diseases. The principal goal of PET imaging is to visualize, characterize, and measure biological processes at the cellular, subcellular, and molecular level in living subjects with non-invasive procedures. PET imaging takes advantage of the traditional diagnostic imaging techniques and introduces positron-emitting probes to determine the expression of indicative molecular targets at different stages of disease. During the last decade, advances in molecular biology have revealed an increasing number of potential molecular targets, including peptide receptors and peptide-related biomolecules. With the help of sophisticated bioconjugation and radiolabeling techniques, numerous peptide-based agents have been developed and evaluated for delivery of PET radionuclides to the specific molecular targets in preclinical and clinical studies. As compared to macromolecules, such as proteins or antibodies, low-molecular-weight peptides have their distinctive advantages and predominantly demonstrate their favorable pharmacokinetics for in vivo PET applications. This review summarizes the criteria of peptide-based PET probes design, the selection of radioisotopes, labeling methods, and provides an overview of the current status and trends in the development of target-specific peptide-based probes with respect to their unique PET imaging applications.

Exendin-4-based radiopharmaceuticals for glucagonlike peptide-1 receptor PET/CT and SPECT/CT

Strong overexpression of glucagonlike peptide-1 (GLP-1) receptors in human insulinoma provides an attractive target for imaging. The first clinical trials demonstrated that GLP-1 receptor SPECT/CT using [Lys(40)(Ahx [6-aminohexanoic acid]-DOTA-(111)In)NH(2)]-exendin-4 can localize hardly detectable insulinomas. However, [Lys(40)(Ahx-DOTA-(111)In)NH(2)]-exendin-4 imaging has drawbacks related to the use of (111)In in that it is costly and carries a relatively high radiation burden for the patient. The aim of this study was the preclinical evaluation of [Lys(40)(Ahx-DOTA-(68)Ga)NH(2)]-exendin-4 for PET/CT and [Lys(40)(Ahx-hydrazinonicotinamide [HYNIC]-(99m)Tc)NH(2)]-exendin-4 for SPECT/CT.

METHODS

Internalization, biodistribution, dosimetry, and imaging studies were performed in the Rip1Tag2 mouse model of pancreatic beta-cell carcinogenesis and compared with our gold standard [Lys(40)(Ahx-DOTA-(111)In)NH(2)]-exendin-4. Poly-glutamic acid and Gelofusine, a gelatin-based plasma expander, were used for renal uptake reduction studies.

RESULTS

The tumor uptake of [Lys(40)(Ahx-DOTA-(68)Ga)NH(2)]-exendin-4 was 205 +/- 59 percentage injected activity per gram of tissue at 4 h. Other GLP-1 receptor-positive organs showed more than 4.8 times lower radioactivity uptake. [Lys(40)(Ahx-HYNIC-(99m)Tc/ethylenediaminediacetic acid [EDDA])NH(2)]-exendin-4, compared with its (111)In- and (68)Ga-labeled sister compounds, showed significantly less tumor and organ uptake. The significantly lower tumor and organ uptake of [Lys(40)(Ahx-HYNIC-(99m)Tc/EDDA)NH(2)]-exendin-4 did not result in inferior tumor-to-organ ratios or reduced image quality. All radiopeptides tested showed a high tumor-to-background ratio, resulting in the visualization of small tumors (maximum diameter between 1.0 and 3.2 mm) by SPECT and PET. The only exception was the kidneys, which also showed high uptake. This uptake could be reduced by 49%-78% using poly-glutamic acid, Gelofusine, or a combination of the 2. The estimated effective radiation dose was 3.7 muSv/MBq for [Lys(40)(Ahx-HYNIC-(99m)Tc/EDDA)NH(2)]-exendin-4, which was 8 times less than that for [Lys(40)(Ahx-DOTA-(68)Ga)NH(2)]-exendin-4 and 43 times less than that for [Lys(40)(Ahx-DOTA-(111)In)NH(2)]-exendin-4.

CONCLUSION

These promising pharmacokinetic and imaging data show that [Lys(40)(Ahx-DOTA-(68)Ga)NH(2)]-exendin-4 and [Lys(40)(Ahx-HYNIC-(99m)Tc/EDDA)NH(2)]-exendin-4 are suitable candidates for clinical GLP-1 receptor imaging studies.