A Novel 99mTc-labeled isoDGR Peptide for Integrin αvβ6/α5β1 Bi-selectively Targeted NanoSPECT/CT Imaging of Pancreatic Cancer

Objectives Pancreatic adenocarcinoma is one of the deadliest human malignancies. However, predicting surgical resectability and prognosis of pancreatic carcinoma are very difficult even with most advanced conventional imaging modalities. Development of new method to detect pancreatic adenocarcinoma is urgently needed. Many integrin receptors, including both αvβ6 and α5β1, are proved to be over-expressed in pancreatic tumors. In this study, a novel integrin αvβ6/α5β1 bi-selective peptide sequence c(phg-isoDGRk) termed isoDGR peptide was designed as a SPECT imaging probe, and the properties of targeting subcutaneous and orthotopic pancreatic tumors was assessed.

Methods The isoDGR peptide was conjugated with 6-hydrazinonicotinyl (HYNIC) and then radiolabeled with 99mTc using tris(3-sodium sulfonatophenyl)phosphine (TPPTS) and tricine as coligands to obtain a radiotracer 99mTc-HYNIC-isoDGR (99mTc-HisoDGR). In vitro and in vivo stability of 99mTc-HisoDGR was evaluated. The expression of integrin αvβ6/α5β1 on BxPC-3 human pancreatic tumor cells were validated by flow cytometry, and cell binding was performed with 99mTc-HisoDGR. BxPC-3 human pancreatic tumor xenograft and orthotopic mode was established. Biodistribution and NanoScan SPECT/CT imaging of pancreatic tumor were performed. The blocking study was administrated with co-injection of excess cold peptide. The immunohistochemistry (IHC) stain was carried out in ex-vivo tumor tissues.

Results 99mTc-HisoDGR was prepared with more than 99% radiochemical purity. The probe showed admirable in vitro and in vivo stability. BxPC-3 human pancreatic tumor cells were validated that over-express both integrin αvβ6 and α5β1. The BxPC-3 tumor was clearly visualized by 99mTc-HisoDGR NanoScan SPECT/CT imaging with low background except relatively high kidney uptake at all detected time points (0.5 h, 1 h and 2 h p.i.). The biodistribution results consistent with the imaging results. The blocking study showing that tumor uptake can be significantly blocked by excess cold peptide. The orthotopic pancreatic tumor can be also clearly visulized by NanoScan SPECT/CT imaging, and verified by Optical imaging and IHC stain of ex-vivo orthotopic pancreatic tumor tissue.

Conclusions The imaging probe 99mTc-HisoDGR was successfully designed and prepared with high radiochemical purity and high stability. Our results suggest that SPECT imaging with 99mTc-HisoDGR would provide an effective approach for the noninvasive detection of integrin αvβ6/α5β1 positive BxPC-3 human pancreatic tumor. Since both the integrin αvβ6 and α5β1 are specifically over-expressed in many tumors, the bi-selectivity of the isoDGR peptide for integrin αvβ6 and α5β1 giving its the ability to detect more various tumor types, not only integrin αvβ6 and α5β1 double-positive tumors, but also integrin αvβ6 or α5β1 single-positive tumors.