Evaluation of two novel 64Cu-labeled RGD peptide radiotracers for enhanced PET imaging of tumor integrin αvβ3

Abstract

Purpose Our goal was to demonstrate that suitably derivatized monomeric RGD peptide-based PET tracers, targeting integrin αvβ3, may offer advantages in image contrast, time for imaging, and low uptake in non-target tissues. Methods Two cyclic RGDfK derivatives, (PEG)2-c(RGDfK) and PEG4-SAA4-c(RGDfK), were constructed and conjugated to NOTA for 64Cu labeling. Their integrin αvβ3-binding properties were determined via a competitive cell binding assay. Mice bearing U87MG tumors were intravenously injected with each of the 64Cu-labelled peptides, and PET scans were acquired during the first 30 min, and 2 and 4 h post-injection (p.i.). Blocking and ex vivo biodistribution studies were carried out to validate the PET data and confirm the specificity of the tracers. Results The IC50 values of NOTA-(PEG)2-c(RGDfK) and NOTA-PEG4-SAA4-c(RGDfK) were 444 ± 41, and 288 ± 66 nM, respectively. Dynamic PET data of 64Cu-NOTA-(PEG)2-c(RGDfK) and 64Cu-NOTA-PEG4-SAA4-c(RGDfK) unveiled similar circulation t1/2 and peak tumor uptake of ~4 %ID/g for both tracers. Due to its marked hydrophilicity, 64Cu-NOTA-PEG4-SAA4-c(RGDfK) provided faster clearance from tumor and normal tissues yet maintaining excellent tumor-to-background ratios. Static PET scans at later time-points corroborated the enhanced excretion of the tracer, especially from abdominal organs. Ex vivo biodistribution and receptor blocking studies confirmed the accuracy of the PET data and the integrin αvβ3-specificity of the peptides. Conclusion Our two novel RGD-based radiotracers with optimized pharmacokinetic properties allowed a fast, high-contrast PET imaging of tumor associated integrin αvβ3. These tracers may facilitate the imaging of abdominal malignancies, normally precluded by high background uptakes.

Mechanistic and quantitative insight into cell surface targeted molecular imaging agent design

“…The relative values of plasma protein binding and non-specific cell uptake can be used to rank compounds, but an absolute guideline is helpful in determining if further molecular engineering is warranted. Similar to reductions in molecular weight and increases in affinity, there are benefits of increasing hydrophilicity to reduce background (such as addition of PEG to integrin binders 53 ), but eventually there are diminishing returns with further effort. Based on the data presented here and literature values (SI), a significant reduction in background is not expected below 60–80% plasma protein binding or 10 −5 s −1 cellular uptake rate.…”

The feasibility of 18F-AlF-NOTA-PRGD2 PET/CT for monitoring early response of Endostar antiangiogenic therapy in human nasopharyngeal carcinoma xenograft model compared with 18F-FDG

“…Futhermore, 18 F-FPPRGD2 [ 37 ] has been approved by the Food and Drug Administration (FDA). Many attempts, including the cyclization of the peptides, the synthesis of multimeric RGD peptides and PEGlaytion, have been made to pursue more ideal RGD probes with high tumor targeting capability [ 38 ]. However, the monomeric cyclic RGD and time-consuming multiple-step synthetic procedure and relatively low yield had hindered their widespread use [ 39 ].…”

Synthesis and Preliminary Evaluation of 68Ga‐NOTA‐Biphenyl‐c(RGDyK) for the Quantification of Integrin αvβ3

“…The biphenyl group provides structural rigidity to the molecule . Incorporating flexible molecules, such as polyethylene glycol, do not increase hydrophilicity but do decrease binding affinity …”