General description of the approach
Currently, there is a requirement for a conjugate covalently attached to a phosphorothioate (PS) antisense oligonucleotide (ASO) (16- to 20-mer) that can be detected with a radionuclide-labeled probe. The interaction between the conjugate and the probe should be highly selective and strong. The well-known example of conjugate-probe interaction is that of streptavidin (conjugate)-biotin (probe) binding. Indeed, for streptavidin-biotin non-covalent binding both Kd and Bmax/Kd are appropriate and defined. The molecular weight (MW) of biotin is 244.3 g/mol (244.3 daltons), whereas the MW of streptavidin is ~55,000 daltons. There are numerous examples of such interaction pairs, where small ligand (probe) interacts with large protein or enzyme. However, if the following limitations are imposed on the conjugate (MW<950 g/mol) and the probe (MW≤500 g/mol), then no such conjugate-probe pair exists for which Kd and Bmax/Kd are appropriate and defined. In other words, there is no such conjugate-probe pair that would provide selective and high-affinity interaction in non-covalent manner.
In this Solution, NextGenRnD describes a chemical biology approach that should allow selective and efficient covalent modification of ASO–conjugate (conjugate MW<950 g/mol) by a specialized radionuclide-labeled probe (MW≤500 g/mol). Site-specific and enzyme-free covalent reaction is exploited to enable the visualization and quantification of PS ASOs in the central nervous system using positron emission tomography technique. This reaction is as efficient as azide-alkyne click chemistry and works at physiologic conditions.