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‐PEG₂‐c(NGR), were synthesized, radiolabeled with ^99mTc, and evaluated in CD13‐positive human fibrosarcoma HT‐1080 tumor xenografts. The radiotracers, ^99mTc‐HYNIC‐c(NGR) and ^99mTc‐HYNIC‐PEG₂‐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 ^99mTc‐HYNIC‐c(NGR) and ^99mTc‐HYNIC‐PEG₂‐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 ^99mTc‐labeled HYNIC peptide could not be modulated through introduction of PEG₂ unit, thus posing a challenge for studies with other linkers towards enhanced tumor uptake and retention.