Improved in Vivo Targeting Capability and Pharmacokinetics of 99mTc-Labeled isoDGR by Dimerization and Albumin-Binding for Glioma Imaging

Previously, we successfully developed the c(phg-isoDGRk) peptide as a novel integrin alpha(5)beta(1)-targeted SPECT imaging probe Tc-99m-HisoDGR for Glioma imaging. However, the fast clearance of Tc-99m-HisoDGR in blood reduced its tumor accumulation and retention, which would be the obstacles for further clinical application. Dimerization and albumin-binding strategies have been proven as effective approaches to improve tumor targeting capability and blood circulation time of radiotracers. In this study, the novel PEGylated dimeric isoDGR peptides (termed 3PisoDGR(2)) and its analogue with an albumin binder (termed AB-3PisoDGR(2)) were designed, and the corresponding radiotracers Tc-99m-3PisoDGR(2) and Tc-99m-AB-3PisoDGR(2) were fabricated and assessed for tumor-targeting and in vivo pharmacokinetics properties in subcutaneous and orthotopic tumor models. The dimerization of isoDGR peptide provided higher binding affinity to tumor cells and longer blood circulation time than the original monomeric isoDGR peptide, resulting in twice increased tumor uptake (Tc-99m-3PisoDGR(2) 2.51 +/- 0.17 % ID/g vs Tc-99m-PisoDGR 1.17 +/- 0.21 %ID/g, P < 0.01) at 0.5 h post-injection (p.i.) and enhanced tumor to nontargeting tissue ratios (T/NT) in most normal organs. The blocking study indicated that the tumor uptake was receptor-mediated specifically. NanoScanSPECT/CT imaging of Tc-99m-3PisoDGR(2) in glioma tumor-bearing model showed clear visions of tumors with low background, except high uptake in excretion system including kidneys and bladder at all detected time points (0.5, 1, and 2 h p.i.). The orthotopic glioma tumor could also be clearly visualized by nanoScanSPECT/CT imaging with Tc-99m-3PisoDGR(2). The addition of albumin-binding entity further prolonged blood circulation time and reached higher tumor uptake for Tc-99m-AB-3PisoDGR(2). However, since Tc-99m-AB-3PisoDGR(2) is less capable of passing BBB than Tc-99m-3PisoDGR(2), Tc-99m-3PisoDGR(2) is preferable for the in situ glioma imaging. In conclusion, Tc-99m-3PisoDGR(2) represents an improved molecular probe for integrin alpha(5)beta(1)-targeted tumor imaging, showing more potential for further clinical application.