MicroPET imaging of brain tumor angiogenesis with 18F-labeled PEGylated RGD peptide

Abstract

We have previously labeled cyclic RGD peptide c(RGDyK) with fluorine-18 through conjugation labeling via a prosthetic 4-[18F]fluorobenzoyl moiety and applied this [18F]FB-RGD radiotracer for alphav-integrin expression imaging in different preclinical tumor models with good tumor-to-background contrast. However, the unfavorable hepatobiliary excretion and rapid tumor washout rate of this tracer limit its potential clinical applications. The aims of this study were to modify the [18F]FB-RGD tracer by inserting a heterobifunctional poly(ethylene glycol) (PEG, M.W. =3,400) between the 18F radiolabel and the RGD moiety and to test this [18F]FB-PEG-RGD tracer for brain tumor targeting and in vivo kinetics. [18F]FB-PEG-RGD was prepared by coupling the RGD-PEG conjugate with N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB) under slightly basic conditions (pH=8.5). The radiochemical yield was about 20-30% based on the active ester [18F]SFB, and specific activity was over 100 GBq/micromol. This tracer had fast blood clearance, rapid and high tumor uptake in the subcutaneous U87MG glioblastoma model (5.2+/-0.5%ID/g at 30 min p.i.). Moderately rapid tumor washout was observed, with the activity accumulation decreased to 2.2+/-0.4%ID/g at 4 h p.i. MicroPET and autoradiography imaging showed a very high tumor-to-background ratio and limited activity accumulation in the liver, kidneys and intestinal tracts. U87MG tumor implanted into the mouse forebrain was well visualized with [18F]FB-PEG-RGD. Although uptake in the orthotopic tumor was significantly lower (P<0.01) than in the subcutaneous tumor, the maximum tumor-to-brain ratio still reached 5.0+/-0.6 due to low normal brain background. The results of H&E staining post mortem agreed with the anatomical information obtained from non-invasive microPET imaging. In conclusion, PEGylation suitably modifies the physiological behavior of the RGD peptide. [18F]FB-PEG-RGD gave improved tumor retention and in vivo kinetics compared with [18F]FB-RGD.

Linker Effects on Biological Properties of 111In-Labeled DTPA Conjugates of a Cyclic RGDfK Dimer

“…To improve integrin α v β 3 binding affinity, the multivalent concept has been successfully used to prepare multimeric cyclic RGD peptides, such as E[c(RGDfK)] 2 and E{E[c(RGDfK)] 2 } 2 , for development of the integrin α v β 3 -targeted diagnostic ( 64 Cu, 99m Tc and 111 In) and therapeutic ( 90 Y and 177 Lu) radiotracers (19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36). Different PKM linkers have been used to improve excretion kinetics of the radiolabeled peptides.…”

Phage display and molecular imaging: expanding fields of vision in living subjects

“…Third, tissue biodistribution is also important, as many molecular imaging agents elicit undesired uptake in organs such as the kidneys and liver. Chemical modifications, such as PEG, have been shown to reduce non-specific liver uptake (Chen et al 2004b). In addition, modifying the chemical properties of peptide sequences (Ekblad et al 2008), or increasing specificity by altering the binding affinity to other target family members (Wang et al 2007a) have been used to improve the tissue biodistibution of in vivo molecular imaging agents.…”

“…Second, rapid clearance is desired to obtain minimal background levels for molecular imaging applications; however, this must be balanced with the ability to elicit specific uptake in desired tissue. Chemical modifications with polyethylene glycol (PEG) or albumin binding peptides have been used to increase circulation time and enhance pharmacokinetic profiles (Chen et al 2004b). In addition, peptides with higher target binding affinity (Kimura et al 2009a) and multivalent target binding (Liu 2006) have been shown to elicit increased imaging signals compared to weaker binding and monomeric peptides; yet, in some cases monovalent proteins have been shown to exhibit better tumor uptake compared to higher affinity dimeric versions (Cheng et al 2008).…”

Imaging of Vx‐2 rabbit tumors with ανβ3‐integrin‐targeted 111In nanoparticles

“…[2][3][4][5][6][7][8][9][10][11] Although these agents can be exquisitely specific for a v b 3 -integrin, their penetration beyond the circulation allows binding to a cadre of nonendothelial sources. The biodistribution of perfluorocarbon NP to reticuloendothelial (RES) organs is well-known and previously reported, 12 but the potential for higher radionuclide payloads and their intravascular distribution make them attractive agents for rapid identification of nascent tumors in non-RES regions, including the brain, head and neck, lung, breast and prostate.…”