Target visualization by MRI using the avidin/biotin amplification route: synthesis and testing of a biotin-Gd-DOTA...

To design efficient targeting strategies in magnetic resonance (MR) molecular imaging applications, the formation of supramolecular adducts between (strept)avidin ((S)Av) and tribiotinylated Gd-DOTA-monoamide complexes (DOTA=1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid) was explored. Two compounds based on the trivalent core of tris(2-aminoethyl)amine each containing three biotin molecules and one (L1) or three (L2) DOTA-monoamide (DOTAMA) ligands were synthesized. In these tribiotinylated derivatives the biotins are spaced far enough apart to allow the formation of the supramolecular adduct with the protein and to host the chelating units in between the (S)Av layers. Size exclusion HPLC analyses indicated complete formation of very high molecular weight polymers (>2 MDa) with (S)Av in solution. A (1)H NMR spectroscopy relaxometric study on the obtained polymeric adducts showed a marked increase of the relaxivity at 35-40 MHz as a consequence of the lengthening of the tumbling time due to the formation of Gd-chelates/(S)Av polymers. The most efficient Gd(3)L2/(S)Av polymeric system was used for a test in cell cultures. The target is represented by a neural cell adhesion molecule (NCAM), which is overexpressed in Kaposi's sarcoma cells and tumor endothelial cells (TEC) and that is efficiently recognized by a biotinylated tetrameric peptide (C3d-Bio). In vitro experiments showed that only cells incubated with both C3d-Bio and Gd(3)L2/SAv polymer were hyperintense with respect to the control. Relaxation rates of cell pellets incubated with Gd(3)L2/SAv alone were not significantly different from the untreated cells demonstrating the absence of a specific binding.