Application of metal-free triazole formation in the synthesis of cyclic RGD-DTPA conjugates

S. van Berkel, A. Dirks, S. Meeuwissen, D. Pingen, O. Boerman, P. Laverman, F. van Delft, J. Cornelissen and F. Rutjes

Institute for Molecules and Materials, Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, NL.
Jul, 2008



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-delta-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 alpha(v)beta(3) (IC(50)=192 nM) and favorable pharmacokinetics.