Synthesis and cellular uptake of a MR contrast agent coupled to an antisense peptide nucleic acid--cell- penetrating peptide conjugate.

In order to image mRNA transcription by in vivo magnetic resonance imaging (MRI), two intracellular MR contrast agents were developed, which are composed of a Gd-DOTA complex, a peptide nucleic acid (PNA) sequence and a cell-penetrating peptide. One was designed to bind to mRNA of dsRed (red fluorescent protein originating from Discosoma coral) by its PNA sequence, whereas the second one contains a nonsense sequence with no natural counterpart. The conjugates were synthesized using a continuous solid-phase synthesis scheme and characterized by ESI-MS. Fluorescence studies showed that both contrast agents could enter efficiently into 3T3 cells in a concentration-dependent manner from 0.5 to 9.0 microM. The contrast agent was located predominantly in vesicles around the nucleus, whereas no uptake into the nucleus was observed. The results of in vitro MR studies showed a statistically significant increase of the intracellular relaxation rate R (1,cell) at a labeling concentration of only 0.5 microM, thus contrast enhancement was detectable too. These results suggest that the synthesized contrast agents could label cells for optical as well as MR imaging and in future might be useful to prove specific accumulation in cells containing target mRNA. Copyright 2007 John Wiley & Sons, Ltd.