Quantitative evaluation of the improvement in the pharmacokinetics of a nucleic acid drug delivery system by dynamic PET imaging with (18)F-incorporated oligodeoxynucleotides

Recently, we demonstrated the utility of positron emission tomography (PET) imaging-based pharmacokinetic evaluation studies for preclinical experiments and microdose clinical trials, mainly focused on low molecular weight compounds. In order to investigate the pharmacokinetics of nucleic acid drugs and their drug delivery systems (DDSs) in vivo by using PET imaging, we developed a novel and efficient method for radiolabeling oligodeoxynucleotides with the positron-emitting radionuclide (18)F (stoichiometry-focused Huisgen-type (18)F labeling). By using this method, we succeeded in synthesizing a variety of (18)F-labeled oligodeoxynucleotides with not only phosphodiesters (PO) in natural forms, but also phosphorothioate (PS) and bridged nucleic acid (BNA) in artificial forms, and then performed PET studies and radioactive metabolite analyses of these (18)F-labeled oligodeoxynucleotides. The tissue-distribution and dynamic changes in radioactivity showed significantly different profiles between these antisense oligodeoxynucleotides. The radioactivity of (18)F-labeled PO-DNA and PO-BNA rapidly accumulated in the kidneys and liver and then moved to the renal medulla, ureter, bladder, and intestine. However, the radioactivity of (18)F-labeled PS-DNA and PS-BNA, possessing PS backbone structures, was retained in the blood for relatively long periods and then gradually accumulated in the liver and kidneys. The metabolite analysis showed that (18)F-labeled PO-DNA rapidly degraded by 5min and (18)F-labeled PO-BNA gradually degraded over time by 60min. Conversely, (18)F-labeled PS-DNA and PS-BNA were shown to be much more stable. To demonstrate the usefulness of the PET imaging technique for evaluating the improved targeting potential of the DDS, we designed and synthesized a cholesterol-modified oligodeoxynucleotide, that we developed as an antisense nucleic acid drug against proprotein convertase subtilisin/kexin type 9 (PCSK9) for hypercholesterolemia therapy, and evaluated its pharmacokinetics using PET imaging. As expected, the (18)F-labeled cholesterol-modified PS-BNA-type oligodeoxynucleotide showed much higher and more rapid accumulation in the delivery target organ, that is, the liver, which encourages us to develop this drug. These results suggest that dynamic PET studies using (18)F-incorporated oligodeoxynucleotide synthesized by stoichiometry-focused Huisgen-type labeling is useful for quantitative pharmacokinetic evaluation of nucleic acid drugs and their delivery systems.