Radiosynthesis and preclinical evaluation of Al[18F]F-NOTA-C-(D-VAP) as a potential tumor PET probe targeting GRP78

Introduction: Glucose-regulated protein 78 (GRP78) was low/not expressed in normal cells, while microenvironmental changes such as hypoxia, glucose depletion, and low pH due to rapid tumor cell proliferation in various malignant tumors (lung, breast, colorectal, head and neck, and liver tumors) can cause up-regulation of GRP78 expression. GRP78 was an attractive biomarker for the diagnosis and therapy of related tumors^[1]. D-VAP (peptide sequence: sntrvap) was a GRP78-targeting polypeptide ligand screened by phage display random polypeptide library technology^[2]. The aim of this study was to develop ¹⁸F-labeled D-VAP as a PET tracer for targeting GRP78 imaging.

Methods: D-VAP peptide analogues NOTA-C-(D-VAP) was synthesized and manual radiolabeled with 18F using an Al[¹⁸F]F complex. The partition coefficients, stability in vitro, and metabolic stability of Al[¹⁸F]F-NOTA-C-(D-VAP) were carried out. Biodistribution and static microPET studies of Al[¹⁸F]F-NOTA-C-(D-VAP) were evaluated in xenograft tumor mouse models.

Results: Al[¹⁸F]F-NOTA-C-(D-VAP) can be produced in 25 min without decay corrected yield of 51 ± 16% (n = 6), radiochemical purity of 99% and specific activity of 20–50 GBq/µmol (figure 1 A). The Log P value of Al[¹⁸F]F-NOTA-C-(D-VAP) was -3.41 ± 0.03 (n = 4). The probe was stable in PBS and FBS solution. The biodistribution and PET imaging data of 60 min postinjection revealed that Al[¹⁸F]F-NOTA-C-(D-VAP) exhibited rapid blood clearance through renal excretion. The tumor uptake of Al[¹⁸F]F-NOTA-C-(D-VAP) in tumor xenografts was proportional to GRP78 protein expression (figure 1 B). ROI quantitative data of each organ were in complete agreement with the biodistribution data. The tumor-to-muscle ratio of A549 tumor-bearing mice was as high as 5.69 ± 0.32 at 60 min PET imaging, which has a good tumor imaging effect (figure 1 C).

Conclusions: Our results demonstrate the potential of using Al[¹⁸F]F-NOTA-C-(D-VAP) as a PET agent for tumor imaging of GRP78-positive expression.

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