Design, synthesis, and comparative evaluation of 99m Tc(CO)3 -labeled N-terminal and C-terminal modified asparagine-glycine-arginine peptide constructs.

The present study describes modification of asparagine-glycine-arginine (NGR) peptide at N-terminally and C-terminally by introduction of a tridentate chelating scaffold via click chemistry reaction. The N-terminal and C-terminal modified peptides were radiometalated with [99m Tc(CO)3 ]+ precursor. The influence of these moieties at the two termini on the targeting properties of NGR peptide was determined by in vitro cell uptake studies and in vivo biodistribution studies. The two radiolabeled constructs did not exhibit any significant variation in uptake in murine melanoma B16F10 cells during in vitro studies. In vivo studies revealed nearly similar tumor uptake of N-terminally modified peptide construct 5 and C-terminally construct 6 at 2 h p.i. (1.9 ± 0.1 vs 2.4 ± 0.2% ID/g, respectively). The tumor-to-blood (T/B) and tumor-to-liver (T/L) ratios of the two radiometalated peptides were also quite similar. The two constructs cleared from all the major organs (heart, lungs, spleen, stomach, and blood) at 4 h p.i. (<1% ID/g). Blocking studies carried out by coinjection of cCNGRC peptide led to approximately 50% reduction in the tumor uptake at 2 h p.i. This work thus illustrates the possibility of convenient modification/radiometalation of NGR peptide at either N- or C-terminus without hampering tumor targeting and pharmacokinetics.