Construction of a novel chimera consisting of a chelator-containing Tat peptide conjugated to a morpholino antisense oligomer for technetium-99m labeling and accelerating cellular kinetics

The attempt to target the limited copies of messenger RNA (mRNA) in vivo with radiolabeled nucleobase oligomers as antisense probes is challenging. Selecting an antisense molecule with superior properties, enhancing the cellular kinetics, and improving the radiolabeling chemistry would be the reasonable approach to accomplish this goal. The present study reports a method to construct a chimera of phosphorodiamidate morpholino nucleobase oligomer (MORF) covalently conjugated to a peptide containing a cell membrane transduction Tat peptide and an N2S2 chelator for technetium-99m (99mTc) radiolabeling (N2S2-Tat-MORF). The radiolabeling properties and cellular kinetics of 99mTc-N2S2-Tat-MORF were measured. As hypothesized, the preparation of 99mTc-N2S2-Tat-MORF could be achieved by an instant one-step method with labeling efficiency greater than 95%, and the 99mTc-N2S2-Tat-MORF showed distinct properties in cell culture from those of a control, the same MORF sequence without Tat but with mercaptoacetyltriglycine (MAG3) as chelator for 99mTc (99mTc-MAG3-MORF). 99mTc-N2S2-Tat-MORF achieved maximum accumulation of about 35% within 2 h, while 99mTc-MAG3-MORF showed lower and steadily increasing accumulations but of less than 1% in 24 h. These preliminary results demonstrated that the proposed chimera has properties for easy labeling, and 99mTc-N2S2-Tat-MORF prepared by this method possesses enhanced cellular kinetics and merits further investigation for in vivo mRNA targeting.