Molecular Imaging of αvβ6–Positive Tumors and Pancreatic β-Cell Mass by Radiolabeled Peptides
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Conventional diagnostic methodologies of lung cancer and diabetes are limited by sensitivity and specificity. In consequence, patients usually get diagnosed when the symptoms appear and the diseases are at the advanced stages. As the expressions of the avß6 integrin and glucagon-like peptide-1 receptor (GLP-1R) are highly related to aggressive tumor phenotypes and functional pancreatic ß-cells, this work has been set to develop peptide-based radiotracers that can specifically bind to avß6 or GLP-1R for noninvasively monitoring the progression of lung cancer and diabetes. By phage display, a peptide sequence that specific binds to avß6 was identified. In the evaluation of its truncated forms with similar binding affinity, a polyethylene glycol chain (PEG11) was inserted to the C-terminus and a bifunctional chelator (DOTA) was conjugated to either end of the peptides. The conjugates were labeled with 111In (t1/2: 2.8 d) under mild conditions with the highest achievable specific activity of 1.15 × 104 MBq/µmol. The in vivo evaluation was performed in a lung adenocarcinoma xenograft mouse model. Of the six conjugates, 10PD showed the best tissue contrast of the H2009 (avß6+) tumor. However, it also yielded to have the highest renal accumulation. The high kidney uptake of 10PD was found to be alleviated by conjugating DOTA at the N-terminus or reducing the peptide net charges. To evaluate GLP-1-based radiotracers for imaging pancreatic ß-cell mass (BCM), GLP-1, [D-Ala8]GLP-1, two bicyclic GLP-1 analogs (EM2196 and EM2198), and exendin-4 were synthesized and compared for their biological properties. All peptide constructs were tagged with an 6-aminohexanoic linker (Ahx) followed by DOTA conjugation at C-terminus and labeled with 64Cu (t1/2: 12.7 h). The specific activity of the labeled peptide conjugates was up to 1.0 × 106 MBq/µmol with radiochemical purity over 97%. Compared to GLP-1, [D-Ala8]GLP-1 revealed strong resistance against DPP-IV. In addition, EM2198 demonstrated high stability against NEP 24.11 presumably by the shielding effects from the two lactam bridges. All peptide conjugates were highly selective to the GLP-1R with the IC50 values in 0.1-0.4 nM. However, only 64Cu-EM2198 showed clear pancreas area on the microPET/CT studies. The signal of 64Cu-EM2198 from the pancreas was confirmed by the ex vivo imaging scans. The potential of 64Cu-EM2198 for imaging BCM was further supported by co-injecting a blocking dose of unlabeled exendin-4 and performing imaging studies in the STZ-treated diabetic mice.