Our goal was to demonstrate that suitably derivatized monomeric RGD peptide-based PET tracers, targeting integrin α vβ 3, may offer advantages in image contrast, time for imaging, and low uptake in non-target tissues.
Two cyclic RGDfK derivatives, (PEG) 2-c(RGDfK) and PEG 4-SAA 4-c(RGDfK), were constructed and conjugated to NOTA for 64Cu labeling. Their integrin α vβ 3-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.
The IC 50 values of NOTA-(PEG) 2-c(RGDfK) and NOTA-PEG 4-SAA 4-c(RGDfK) were 444 ± 41, and 288 ± 66 nM, respectively. Dynamic PET data of 64Cu-NOTA-(PEG) 2-c(RGDfK) and 64Cu-NOTA-PEG 4-SAA 4-c(RGDfK) unveiled similar circulation t 1/2 and peak tumor uptake of ~4 %ID/g for both tracers. Due to its marked hydrophilicity, 64Cu-NOTA-PEG 4-SAA 4-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 α vβ 3-specificity of the peptides.