Antisense stabilization

The therapeutic utility of systemically administered ASON had been limited by their short plasma half life (sometimes even less than 3 min). This is due to their sensitivity to nuclease digestion. When the first-generation ASON were chemically modified, e.g., by replacing the oxygen in the phosphodiester bond with sulfur (phosphorothiorate) they obtained an increased stability in biological fluids while their antisense effect has been maintained. First-generation agents can be delivered via intravitreal injection, parenterally, by topical cream, enema, and inhaled aerosol. These antisense... 

The development of nucleic acid-based therapeutics is not as straightforward as researchers had initially anticipated. Stability, toxicity, specificity, and delivery of the compounds continue to be challenging issues that need further optimization. In recent years, researchers have come up with intricate solutions that have greatly improved the efficacy of potential antisense, ribozyme, as well as RNAi-based therapeutics. Clinical trials for all these types of nucleic acid-based therapeutics are underway. So far, data from several trials and studies in animal models look promising, in particular, the therapies that trigger the RNAi pathway. However, history has shown that compounds that do well in phase I or phase II clinical trials may still fail in phase III. A striking example is the nonspecific suppression of angiogenesis by siRNA via toII-Iike receptor 3 (Kleinman et al. 2008). It will become clear in the near future which compounds will make it as a new class of antiviral therapeutics. 

Figure 5.12. Chemical structure of antisense oligodeoxynucleotides (AS-ODN). Phosphorothioate and methylphosphonate AS-ODN have a sulfur atom and a methyl group respectively, substituted for a nonbridging oxygen atom to increase stability to nucleases.

In most cases, the DNA inserted into the MCS for expression is not a genomic gene with the original exon/intron configuration, but a cDNA. In exceptional cases, antisense RNA or ribozymes may be expressed. cDNA lacks intronic sequences, but still has the 5 and 3 UTR sequences. As discussed above, the presence of the 3 UTR may reduce the stability of mRNA transcribed from the cDNA insert. Furthermore, the length and secondary structure of the 5 UTR may influence the efficiency of translation. Therefore, it is generally recommended to use cDNA with only short 5 and 3 UTR sequences for expression with expression vectors. 

Agrawal, S., Zhang, X., Zhao, H., Lu, Z., Van, J., Cai, H. et al. (1995) Absorption, tissue distribution and in vivo stability in rats of a hybrid antisense oligonucleotide following oral administration. Biochem. Pharm., 50, 571-576. 

Miyao, T., Takakura, Y., Akiyama, T., Yoneda, F., Sezaki, H. and Hashida, M. (1995) Stability and pharmacokinetic characteristics of oligonucleotides modified at terminal linkages in mice. Antisense Res. Develop., 5,115-121. 

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