Oligonucleotides in PET imaging
Nucleotides are the build blocks of DNA and RNA, consisting of a nucleobase (adenine, cytosine, guanine, thymine, or uracil), a 5-carbon sugar, and a phosphate group. Oligonucleotide can hybridize to another chain if their sequences can form G☰C and A⚌T or A⚌U pairs. Thus, oligonucleotides can be used as ligands for DNA and RNA (antisense oligonucleotides). Oligonucleotides can also be designed to specifically bind to other molecular targets (aptamer oligonucleotides). Techniques for generic labelling and protection from nucleases are available, but directing oligonucleotides into the cell cytoplasm and nucleus while still maintaining their functionality is still challenging.
In blood plasma, oligonucleotides are bound to albumin (Srinivasan et al., 1995), which is common to negatively charged molecules. Oligonucleotides tend to bind to other proteins, too, and while this may not be a problem for medicinal use of oligonucleotide, it increases the nonspecific binding and decreases the image quality.
Antisense oligonucleotide binds to its target RNA, and inhibits its translation. The secondary structures of RNAs (hairpins, stem-loops) may prevent or slow down the hybrid formation with the antisense oligonucleotide. The number of mRNAs copies in a cell may be too low for detection in vivo. Antisense imaging may therefore be most successful in detecting viral or bacterial mRNAs that are abundant in infected tissue.
DNA and RNA aptamers (usually 20-100 bases) are usually single-stranded, and can form 3D-structures, enabling them to bind to a specific target (aptatope); targets can be anything from small ions such as Zn2+ to viruses and bacteria. Libraries of oligonucleotides can be synthesized randomly and simultaneously screened for their binding affinity to a target molecule. This procedure is easier and faster than the methods for production of monoclonal antibodies, and it works with targets that are toxic or non-immunogenic. Aptamers are an order of magnitude smaller than antibodies (10-20 kDa versus 150 kDa), and therefore have faster endothelial passage and clearance. Aptamers, like antibodies, are usually targeted against molecules that are abundant enough for successful detection with the excellent sensitivity of modern PET devices.
Escort aptamers can be used for targeted delivery of drugs, and diagnostic or therapeutic radionuclides.
Riboswitches are naturally occurring aptamers, mainly found in the untranslated regions of mRNA molecules; when bound to their target molecule, the 3D structural change in mRNA affects its translational activity.
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Created at: 2017-10-28
Updated at: 2017-10-29
Written by: Vesa Oikonen