Comparison of Exendin-4 and Its Single Amino Acid Substitutions as Parent Peptides for GLP-1 Receptor Imaging Probes
Naoya Kondo1,2, Maiko Yonezawa1, Fuko Hirano1
1Department of Biofunctional Analysis, Graduate School of Pharmaceutical Sciences, Osaka Medical and Pharmaceutical University, 4-20-1 Nasahara, Takatsuki 569-1094, Osaka, Japan.
Abstract:
Glucagon-like peptide-1 receptor (GLP-1R) is an emerging critical target for the diagnosis and treatment of various diseases. Radiolabeled exendin-4 (Ex-4), a GLP-1R agonist, has been widely used as an imaging probe. However, its potential to induce hypoglycemia, especially in patients with insulinoma, limits its applicability. This study evaluated whether Ex-D3, a Glu3Asp substitution of Ex-4 with a higher internalization rate, could enhance the imaging efficacy of Ex-4 while reducing its hypoglycemic effects. We synthesized derivatives with an additional C-terminal Cys (Ex-D3-C40) for site-specific 125I labeling. Surface plasmon resonance analysis revealed that C-terminus modification did not significantly alter the binding affinity of Ex-D3-C40 to GLP-1R. In vivo studies in mice demonstrated that Ex-D3-C40 induced weaker hypoglycemic effects than Ex-4-C40. Biodistribution studies showed that 125I-labeled Ex-D3 ([125I]I-Ex-D3) achieved significantly higher pancreatic accumulation and higher pancreas-to-blood and pancreas-to-muscle ratios than [125I]I-Ex-4. Ex vivo autoradiography confirmed the binding specificity of [125I]I-Ex-D3 to GLP-1R-expressing pancreatic β-cells. These findings indicate that Ex-D3 is a promising parent peptide for the development of superior GLP-1R imaging probes with reduced hypoglycemic risk, highlighting the importance of considering pharmacological effects in designing molecular imaging probes.
Insights
A new peptide, Ex-D3, shows promise for imaging glucagon-like peptide-1 receptors (GLP-1R) with reduced risk of hypoglycemia compared to existing probes.
Area of Science:
- Biomedical imaging
- Molecular imaging
- Radiopharmaceutical chemistry
Background:
- Glucagon-like peptide-1 receptor (GLP-1R) is a key target for disease diagnosis and treatment.
- Radiolabeled exendin-4 (Ex-4) is used for GLP-1R imaging but can cause hypoglycemia.
- Hypoglycemic effects limit the clinical utility of Ex-4, especially in insulinoma patients.
Purpose of the Study:
- To evaluate Ex-D3, a modified Ex-4 with higher internalization, for enhanced GLP-1R imaging efficacy.
- To assess if Ex-D3 can reduce the hypoglycemic effects associated with Ex-4.
- To develop a superior GLP-1R imaging probe with improved safety profile.
Main Methods:
- Synthesized Ex-D3 derivatives (Ex-D3-C40) for site-specific 125I labeling.
- Assessed binding affinity of Ex-D3-C40 to GLP-1R using surface plasmon resonance.
- Evaluated hypoglycemic effects and biodistribution of 125I-labeled Ex-D3 ([125I]I-Ex-D3) in mice.
- Confirmed binding specificity via ex vivo autoradiography.
Main Results:
- C-terminus modification did not significantly affect Ex-D3-C40 binding affinity to GLP-1R.
- Ex-D3-C40 exhibited weaker hypoglycemic effects than Ex-4-C40 in vivo.
- [125I]I-Ex-D3 showed higher pancreatic accumulation and improved pancreas-to-blood/muscle ratios compared to [125I]I-Ex-4.
- Autoradiography confirmed specific binding of [125I]I-Ex-D3 to GLP-1R in pancreatic β-cells.
Conclusions:
- Ex-D3 is a promising peptide for developing advanced GLP-1R imaging probes.
- Ex-D3 offers enhanced imaging efficacy with a reduced risk of hypoglycemia.
- Considering pharmacological effects is crucial for designing effective molecular imaging agents.
