Synthesis of 3H-labeled nucleoside-methyl[CT3]phosphonamidite and incorporation into methylphosphonate oligonucleotides for biodistribution and biostability studies.

Antisense oligonucleotides, complementary to certain gene sequences, are designed to hybridize to mRNA and inhibit the synthesis of disease related cellular or viral proteins (1-3). Although numerous in vitro studies have established a great deal of therapeutic potential for these oligonucleotides, there are still many unresolved properties of these oligomers which are critical to drug development. Methylphosphonate and phosphorothioate oligonucleotides are among the most nuclease-resistant oligomers that still maintain the properties of natural DNA. In order to develop these potential drugs as a clinical entity, it is essential to study the biodistribution and biostability. Labelled oligonucleotides can be used as tools for in vivo distribution studies. The radiolabelled oligomers which have been used in the past are mostly labelled at problematic sites at the end of oligonucleotide. Phosphodiester and phosphorothioate oligomers were generally labeled by polynucleotide kinase which transfers [P]phosphate group from ATP to the 5'-end of the oligomer (4,5). In the case of methylphosphonate oligomers a tritiated thymidine has been attached to the 5'-end through an enzymatically labile phosphodiester linkage (6). These types of labeling methods make it difficult to draw a meaningful conclusion from metabolic studies since the label can be readily cleaved from the oligonucleotide in vivo.