Preparation and comparative evaluation of 99mTc-HYNIC-cNGR and 99mTc-HYNIC-PEG2-cNGR as tumor-targeting molecular imaging probes

The tripeptide sequence asparagine-glycine-arginine (NGR) specifically recognizes aminopeptidase N (APN or CD13) receptors highly expressed on tumor cells and vasculature. Thus, NGR peptides can precisely deliver therapeutic and diagnostic compounds to CD13 expressing cancer sites. In this regard, 2 NGR peptide ligands, HYNIC-c(NGR) and HYNIC-PEG(2)-c(NGR), were synthesized, radiolabeled with Tc-99m, and evaluated in CD13-positive human fibrosarcoma HT-1080 tumor xenografts. The radiotracers, Tc-99m-HYNIC-c(NGR) and Tc-99m-HYNIC-PEG(2)-c(NGR), could be prepared in approximately 95% radiochemical purity and exhibited excellent in vitro and in vivo stability. The radiotracers were hydrophilic in nature with log P values being -2.33 +/- 0.05 and -2.61 +/- 0.08. The uptake of 2 radiotracers Tc-99m-HYNIC-c(NGR) and Tc-99m-HYNIC-PEG(2)-c(NGR) was similar in nude mice bearing human fibrosarcoma HT-1080 tumor xenografts, which was significantly reduced (P<.05) during blocking studies. The 2 radiotracers being hydrophilic cleared rapidly from blood, liver, and intestine and were excreted through renal pathway. The pharmacokinetics of Tc-99m-labeled HYNIC peptide could not be modulated through introduction of PEG(2) unit, thus posing a challenge for studies with other linkers towards enhanced tumor uptake and retention.