IN-VIVO HYBRIDIZATION OF TC-99M-LABELED PEPTIDE NUCLEIC-ACID (PNA)

Hybridization of a radiolabeled single-stranded DNA oligonucleotide wi th its single-stranded complement in vivo has not yet been convincingl y demonstrated. A contributing factor may be unfavorable in vivo prope rties of the phosphodiester and phosphorothioate DNAs. Peptide nucleic acid (PNA) oligomers have been reported to possess in vivo properties more suitable for radiopharmaceutical applications. Methods: We have radiolabeled an amine-derivatized 15-base PNA oligomer with Tc-99m thr ough a modified MAG(3) chelator. Results: The ability of the PNA to hy bridize in vitro with its complement appeared to be unimpaired after c onjugation and radiolabeling. Size-exclusion, high-performance liquid chromatography (HPLC) analysis of 37 degrees C serum after 24 hr of in cubation showed the radiolabel to be present predominately as labeled PNA with indications of labeled serum proteins and a low molecular wei ght catabolite. Whole-body clearance in mice was rapid, with 50% of th e label eliminated in about 2 hr. After 2.5 hr, the highest uptake (ki dneys) was only 1.5% of the injected dose/g; less than 0.07%/g was pre sent in all sampled tissues at 24 hr. To evaluate in vivo hybridizatio n, beads were implanted subcutaneously in both thighs of normal mice. In the left thigh only, the beads were conjugated with complementary s ingle-stranded PNA. At 23 hr following intraperitoneal administration of the labeled PNA, the left/right thigh radioactivity ratio was 6:1. Whole-body images at this time showed only bladder, kidneys and the le ft thigh. Conclusion: Unlike the radiolabeled DNAs investigated in thi s laboratory, Tc-99m-PNA displays stability and pharmacokinetic proper ties suitable for eventual use as radiopharmaceuticals.