Comparison of Technetium-99m-Labeled Pentapeptides as Bone Imaging Agents: Influence of Different Types of Acidic Amino Acids

Bone-seeking radiopharmaceuticals are essential for the early detection of bone metastases. In this study, we developed three Technetium-99m (^99mTc)-labeled oligopeptides composed of acidic amino acids and evaluated their bone-targeting ability. Hydrazinonicotinamide (HYNIC)-conjugated oligopeptides with five residues of aspartic acid (Asp), glutamic acid (Glu), or γ-carboxyglutamic acid (Gla) were synthesized and radiolabeled with ^99mTc using tricine and 3-acetylpyridine as co-ligands. Their hydroxyapatite binding, in vitro stability, biodistribution, and single-photon emission computed tomography (SPECT)/CT imaging in normal mice were evaluated. Among the three tracers, [^99mTc]Tc-HYNIC-(tricine)(AcP)-(dl-Gla)₅ ([^99mTc]3) showed the highest hydroxyapatite binding and bone uptake, with clear visualization in SPECT/CT. All compounds exhibited high radiochemical purity and in vivo stability. Oligopeptides containing Gla residues exhibited superior bone affinity and imaging performance, suggesting that Gla-based oligopeptides are promising scaffolds for the development of ^99mTc-labeled bone imaging agents.