| Side chain to side chain cyclization has been considered to be an effective technique for improving therapeutic potential of many peptides including GLP-1. Despite promising therapeutic benefits towards type 2 diabetes, clinical application of GLP-1 is greatly hampered by short half life in vivo due to rapid proteolytic degradation. Herein, a series of GLP-1 analogues in which side chains of Glu and Lys have been covalently bound were synthesized in order to stabilize alpha-helical conformations on both N- and C-terminal of GLP-1. Peptide analogues containing one, two or three lactam constraints were evaluated for their biological activity and enzyme stability towards DPP-4 and NEP 24.11 enzymes. Peptide 2, containing a lactam constraint between residues 18 and 22, showed enhanced receptor potency (7 fold increase) and enzyme towards NEP 24.11. Similarly, peptides containing two lactam bridges showed increased biological activity (up to 4 fold increase), resulting from their highly constrained structures. A GLP-1 analogue containing three lactam constraints exhibited improved receptor efficacy and proteolytic stability towards both NEP 24.11 and DPP-4, despite the native N-terminal residues (His-Ala-Glu). Due to the improved receptor potency and enzyme stability, two bicyclic peptides were investigated on their ability to image pancreatic beta-cell mass non-invasively. DOTA was tagged at the C-terminus of a GLP-1 analogue to provide a PET imaging probe. The GLP-1 receptor specificity and enzyme stability of the DOTA conjugates gave a high accumulation in the pancreas, hence exhibiting clear pancreas images from microPET/CT scans. |
