Cyclic-RGD penta-peptides cRGDyK derivatized with cyclopentadienyl complexes of technetium and rhenium as radiopharmaceutical probes.

The present study reports the syntheses of half-sandwich complexes of the type [M(η5 -C5 H4 CONH-R)(CO)3 ] (M═Re,99m Tc;R═cyclic RGD peptide (cRGDyK) for potential imaging of αv β3 integrin expression. The 99m Tc complex was prepared directly from the reaction of [99m Tc(OH2 )3 (CO)3 ]+ with cRGDyK, doubly conjugated to Thiele's acid [(C5 H5 COOH)2 ] in water. This approach extends the viability of metal-mediated retro Diels-Alder reactions for the preparation of small molecules such as linear tripeptides to a more complex cyclic peptide carrying a [(η5 -C5 H4 )99m Tc(CO)3 ] tag. The Diels-Alder product [(C5 H5 CONH-cRGDyK)2 ] was prepared from Thiele's acid via double peptide coupling. The Re-complex [Re(η5 -C5 H4 CONH-cRGDyK)(CO)3 ] was obtained by attaching [Re(η5 -C5 H4 COOH)(CO)3 ] directly to the N-terminus of cRGDyK. The identity of the 99m Tc-complex is confirmed by chromatographic comparison with the corresponding rhenium complex, fully characterized by spectroscopic techniques.