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. 2014 May 24;4(8):770-7.
doi: 10.7150/thno.7759. eCollection 2014.

(64)Cu labeled sarcophagine exendin-4 for microPET imaging of glucagon like peptide-1 receptor expression

Zhanhong Wu  1 , Shuanglong Liu  2 , Indu Nair  1 , Keiko Omori  1 , Stephen Scott  1 , Ivan Todorov  1 , John E Shively  3 , Peter S Conti  2 , Zibo Li  2 , Fouad Kandeel  1
Affiliations

(64)Cu labeled sarcophagine exendin-4 for microPET imaging of glucagon like peptide-1 receptor expression

Zhanhong Wu et al. Theranostics. .

Abstract

The Glucagon-like peptide 1 receptor (GLP-1R) has become an important target for imaging due to its elevated expression profile in pancreatic islets, insulinoma, and the cardiovascular system. Because native GLP-1 is degraded rapidly by dipeptidyl peptidase-IV (DPP-IV), several studies have conjugated different chelators to a more stable analog of GLP-1 (such as exendin-4) as PET or SPECT imaging agents with various advantages and disadvantages. Based on the recently developed Sarcophagin chelator, here, we describe the construction of GLP-1R targeted PET probes containing monomeric and dimeric exendin-4 subunit. The in vitro binding affinity of BarMalSar-exendin-4 and Mal2Sar-(exendin-4)2 was evaluated in INS-1 cells, which over-express GLP-1R. Mal2Sar-(exendin-4)2 demonstrated around 3 times higher binding affinity compared with BaMalSar-exendin-4. After (64)Cu labeling, microPET imaging of (64)Cu-BaMalSar-exendin-4 and (64)Cu-Mal2Sar-(exendin-4)2 were performed on subcutaneous INS-1 tumors, which were clearly visualized with both probes. The tumor uptake of (64)Cu-Mal2Sar-(exendin-4)2 was significantly higher than that of (64)Cu-BaMaSarl-exendin-4, which could be caused by polyvalency effect. The receptor specificity of these probes was confirmed by effective blocking of the uptake in both tumor and normal positive organs with 20-fold excess of unlabeled exendin-4. In conclusion, sarcophagine cage conjugated exendin-4 demonstrated persistent and specific uptake in INS-1 insulinoma model. Dimerization of exendin-4 could successfully lead to increased tumor uptake in vivo. Both (64)Cu-BaMalSar-exendin-4 and (64)Cu-Mal2Sar-(exendin-4)2 hold a great potential for GLP-1R targeted imaging.

Keywords: 64Cu; PET; Sarcophagine; exendin-4 dimer; insulinoma.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interest exists.

Figures

Figure 1
Figure 1
Synthesis scheme of monomeric and dimeric exendin-4 analogs: (A) BaMalSar-exendin-4 and (B) Mal2Sar-(exendin-4)2.
Figure 2
Figure 2
(A) Inhibition of 125I-exendin(9-39) binding to GLP-1R on INS-1 cells by BaMalSar-exendin-4, Mal2Sar-(exendin-4)2 and Exendin-4. Data are mean ± SE (n = 3). (B) Western blot analysis of GLP-1R expression in INS cells, INS-1 tumor tissue and major organs. Data is shown as mean ± SE (n=3), * p<0.001 vs. Tumor.
Figure 3
Figure 3
Small-animal PET studies of INS-1 tumor-bearing mice. Decay-corrected whole-body coronal images of NOD/SCID mice bearing INS-1 tumor at 1, 4 and 24 h after injection of 64Cu-BaMalSar-exendin-4 without (control, left) and with unradiolabeled exendin-4 (blocking, right).
Figure 4
Figure 4
Small-animal PET studies of INS-1 tumor-bearing mice. Decay-corrected whole-body coronal images of NOD/SCID mice bearing INS-1 tumor at 1, 4 and 24 h after injection of 64Cu-Mal2Sar-(exendin-4)2 without (control, upper panel) and with unradiolabeled exendin-4 (blocking, lower panel).
Figure 5
Figure 5
Quantitative analyses of small-animal PET data. Time activity curves (TACs) of 64Cu-BaMalSar-exendin-4 and 64Cu-Mal2Sar-(exendin-4)2 on (A) tumor, (B) kidney, and (C) liver.
Figure 6
Figure 6
Immunofluorescence staining of GLP-1R expression in INS-1 tumor tissue. INS-1 tumor was stained with hematoxylin/eosin (blue), insulin (green) and GLP-1R (red) (top panel: 20X; bottom panel: 40X).
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