Synthesis of an universal linker to label oligonucleotides via Click Chemistry

For more than 3 decades, oligonucleotides have been used for therapies, imaging and diagnostics. They are known to hybridize specifically with RNA of a complementary sequence on tissue sections and more recently to block the expression of target mRNA when administered in vivo (1).
Positron emission Tomography (PET) is a sensitive and non invasive imaging technique, and is the most advanced technology currently available for studying in vivo molecular interactions and therapeutic agents. It is a method of choice to assess the pharmacokinetics of new therapeutics agents such as modified oligonucleotides. Among positron-emitting nuclides, fluorine-18 (t1/2 = 109.8 min) appears to be the best candidate due to its favourable physical and nuclear properties. Several of the methods described in the literature to label oligonucleotides present a number of disadvantages (time of synthesis, low overall radiolabelling yield, non-universal).
Due to the speed, selectiveness and the relatively mild experimentals conditions, “Click” chemistry seems a powerful technique. The most explored Click reaction is Huisgen 1,3 dipolar cycloaddition. In our case, this reaction occurs between an alkyne group presents on the oligonucleotide and an azide group on the 18F labelled prosthetic group.
The originality of our strategy is the use of a universal linker diverted from the trans-4-hydroxy-proline directly connected to the oligonucleotide. This linker mimics a sugar of the oligonucleotide sequence and should improve their resistance to exonucleases. Synthesis of this compound will be presented.