Initial investigations of Tc-99m-labeled morpholinos for radiopharmaceutical applications

This laboratory is evaluating phosphorothioate deoxyribonucleic acids (DNAs ) and peptide nucleic acids (PNAs) for a variety of nuclear medicine applic ations. Morpholinos (MORFs) are a new class of oligomers with a nuclease-re sistant, nonionic and water-soluble phosphorodiamidate backbone. We now rep ort on the in vitro and in vivo properties of MORFs labeled with technetium -99m. Both 15-mer and 18-mer MORFs were obtained. each with a primary amine attached to the 3' equivalent end via a three-carbon beta-alanine linker. The amine was used to conjugate with NHS-MAG3 for Tc-99m radiolabeling. By surface plasmon resonance at room temperature, the association rate constan t for hybridization of the 18-mer MORF to its complementary oligomer (cMORF ) was equivalent to that of DNAs and PNAs of comparable length. Hybridizati on of Tc-99m-MORF in vitro to free cMORF, to a cMORF polymer and to cMORF b eads was nearly quantitative under a variety of conditions. Kinetic studies in vitro at room temperature showed rapid (2-5 min) and nearly quantitativ e (90%) binding to cMORF beads. Using size-exclusion high-performance liqui d chromatography, the stability of the 99mTc-MORF was found to be greater t han 85% over 24 h in 37 degreesC serum with minimal protein binding. In nor mal mice, the Tc-99m-MORF showed rapid pharmacokinetics, with only 21% and 8% remaining in the whole body at 3 and 24 h post administration, respectiv ely. In vivo targeting with Tc-99m-MORF of cMORF beads in one thigh of norm al mice compared to control beads in the other thigh showed target/control thigh ratios of 2-10 between 3 and 24 h. These results demonstrate that MOR F oligomers are capable of in vivo hybridization. Their properties of hybri dization affinity and kinetics and their in vivo stability and pharmacokine tics make them suitable subjects for in vivo studies.