Paramagnetic oligonucleotides: Contrast agents for magnetic resonance imaging with proton relaxation enhancement effects

An antisense paramagnetic oligonucleotide analogue targeted to a model macr omolecular receptor (5S rRNA) was prepared. The paramagnetic agent's relaxi vity (dependence of the relaxation rate on paramagnetic agent concentration ) in the presence and absence of the macromolecular receptor was measured a t 1.5 and 6.3 T. The relaxivity of the targeted agent increased specificall y in the presence of the macromolecular receptor (16% at 6.3 T and 15% at 1 .5 T). This effect was specific for a paramagnetic oligonucleotide targeted to the receptor and was larger than the relaxivity enhancement due simply to receptor-induced viscosity differences. Maximizing this relaxivity enhan cement of tumor targeted paramagnetic oligonucleotides will aid in contrast agent development for magnetic resonance imaging.