Lipid membrane permeability of 2'-modified derivatives of phosphorothioate oligonucleotides

Antisense oligonucleotides have the ability to inhibit gene expression in viral infections, malignancy, and other diseases. Even though much work has been accomplished with oligonucleotides demonstrating in vitro therapeutic effects, little work has been done to address how these molecules gain access to the cell. One of the plausible means of entrance could be through passive diffusion of the oligonucleotides through the cellular lipid bilayer. To enhance membrane permeability of oligonucleotides lipophilic moieties at the 2' position of the ribose ring have been added. To evaluate the effect of this modification, a liposome system was used. The oligonucleotides evaluated were a series composed of poly A 10mers phosphorothioates labeled at the 5' end with fluorescein and modified at the 2' position of the ribose ring with lipophilic alkyl chains ranging from methyl to nonyl. Efflux studies were accomplished by monitoring the appearance of the oligonucleotide in the incubation medium. There were modest but significant differences between the efflux half-life times of the 2'-modified compounds and the control compound. The values ranged from approximately 6 days for the control, unmodified compound to 4.6 days for the propyl modification. The nonyl derivative had a longer efflux half-life time (8.3 days) compared with the control, unmodified phosphorothioate oligonucleotide.