Application of Metal‐Free Triazole Formation in the Synthesis of Cyclic RGD–DTPA Conjugates

Abstract
The tandem 1,3-dipolar cycloaddition-retro-Diels–Alder (tandem crDA) reaction is presented as a versatile method for metal-free chemoselective conjugation of a DTPA radiolabel to N-δ-azido-cyclo(-Arg-Gly-Asp-d-Phe-Orn-) via oxanorbornadiene derivatives. To this end, the behavior of several trifluoromethyl-substituted oxanorbornadiene derivatives in the 1,3-dipolar cycloaddition was studied and optimized to give a clean and efficient method for bio-orthogonal ligation in an aqueous environment. After radioisotope treatment, the resulting 111In-labeled c(RGD)-CF3-triazole-DTPA conjugate was subjected to preliminary biological evaluation and showed high affinity for αvβ3 (IC50=192 nM) and favorable pharmacokinetics.