High-affinity no-carrier-added 99mTc-labeled chemotactic peptides for studies of inflammation in vivo

Na-for-Nle-Leu-Phe-Nle-Tyr-Lys, a chemotactic peptide that binds with high affinity to the chemoattractant receptor on granulocytes and monocytes, was labeled with Tc-99m using the diaminedithiol (DADT) chelating system to coordinate the Tc. Tc-99m labeling of the DADT-coupled peptide was accomplished in 84% overall yield (room temperature for 10 min) using [Tc-99m]glucoheptonate as the donor of prereduced Tc. HPLC analysis showed two major Tc-99m-labeled peptide peaks, Tc-99m-DADT-Pep-I and Tc-99m-DADT-Pep-II, were obtained in a ratio of 1:0.85. Using an iodoacetamide-derivatized gel to remove unlabeled peptide from the Tc-99m labeling mixtures, essentially no-carrier-added (nea) high-specific activity Tc-99m-labeled chemotactic peptides were obtained. The Tc-99 analogues of the peptides were synthesized (72% yield) in a similar fashion and correlated with Tc-99m complexes I and II by HPLC. In vitro competitive receptor binding assays of the isolated Tc-99 analogues were per formed against the tritiated chemotactic peptide [H-3]N-for-Met-Leu-Phe ([H-3]fMLF) using isolated granulocytes. The Tc-99-derivatized peptides showed similar binding affinities to the chemoattractant receptor as the unlabeled N-a-for-Nle-Leu-Phe-Me-Tyr-Lys. The nca Tc-99m-labeled peptides gave high contrast images of experimental inflammation in rabbits without causing neutropenia. Thus, it is feasible to attach the Tc-DADT chelate to low-molecular weight receptor binding chemotactic peptides and retain substantial binding to the receptor. Chemotactic peptides labeled with Tc-99m via the DADT ligand system have the potential for imaging focal sites of inflammation without toxic effects, an important consideration in the successful utilization of chemotactic peptide agonists.