DEVELOPMENT OF NOVEL RADIOLABELED POLYDENTATE LIGANDS AS POTENTIAL DIAGNOSTIC PROBES

Abstract

The technetium ( 99m Tc) radiopharmaceuticals have been used for diagnosis of many diseases especially cancer in nuclear medicine for many years ago. Currently, target specific radiopharmaceuticals containing peptides have become more interesting due to increasing the knowledge of receptor binding affinity of specific peptides as well as ease of chemical modification of peptide sequences. Many chelators have been developed as bifunctional chelators for 99m conjugation to peptides as well as for chelation with Tc. Small size chelators are desireable for labeling the small peptides. One of the most important requirements of radiopharmaceuticals is an aqueous phase labeling the biomolecules to 99m Tc. These requirements must meet in order to use for nuclear medicine. In this context, cyclopentadiene (Cp - ) is obviously as a good choice of chelator due to its small size and low molecular weight. The 99m Tc radiolabeled peptide comprising Cp - core have not been synthesized in an aqueous phase. Instead, they have been synthesized by double ligand transfer approach in an organic solvent using harsh reaction conditions which are still not suitable for routing use. In this study we describe the syntheses of half-sandwich complexes of the type [ 5 -Cp(CONH-R)M(CO) 3 ] with M=Re or 99m Tc. The R group represents different tri-peptides which display high binding affinities for oligopeptide transporters PEPT2 and cyclic RGD peptides which have high binding affinity with integrin family receptor. The 99m Tc complexes [( 5 - CpCONH-R) 99m Tc(CO) 3 ] were prepared directly from [ 99m Tc(OH 2 ) 3 (CO) 3 ] + and Diels-Alder dimerized, cyclopentadienyl derivatized peptides in an aqueous phase. This approach corroborates the feasibility of metal-mediated retro- Diels-Alder reactions for the preparation of not only small molecules but also of peptides for carrying a [( 5 -Cp) 99m Tc(CO) 3 ] tag. The Diel-Alder products [(HCpCONH-R) 2 ] containing selected peptides were synthesized from Thiele’s acid [(HCp-COOH) 2 ] via double peptide coupling. The peptides of different sequences in this study were prepared through solid phase peptide synthesisxiv (SPPS). The Re-complexes [( 5 -CpCONH-R)Re(CO) 3 ] were synthesized by attaching [(Cp-COOH)Re(CO) 3 ] directly to the N-terminus amino group of peptides as prepared from SPPS. The authenticity of the 99m Tc-complexes is confirmed by HPLC retention time comparison with the corresponding rhenium complexes, fully characterized by spectroscopic techniques.