Biodegradable poly( l-lactic acid) matrices for the sustained delivery of antisense oligonucleotides

Abstract

Antisense oligonucleotides have emerged as exciting novel therapeutic agents which can inhibit gene expression in a sequence-specific manner and are currently undergoing clinical trials evaluation in the treatment of cancer and viral diseases. The therapeutic application of these molecules is limited by their poor biological stability and rapid in vivo elimination kinetics which necessitates frequent administration of oligonucleotides for sustained efficacy. The potential use of a sustained-release biodegradable delivery system, that would protect oligonucleotides from degradation by nucleases whilst delivering the nucleic acid in a controlled or sustained manner to the site of action, may circumvent these problems. In this study, we report on the biological stability, hybridization potential and in vitro release kinetics of antisense phosphodiester (D-oligos) and phosphorothioate (S-oligos) oligonucleotides entrapped within biodegradable poly( l-lactic acid) (PLA) matrices. The in vitro release profiles of 5′-end radiolabelled oligonucleotides entrapped within solvent-cast polymer film matrices (100±10 μm thickness) were monitored over a period of at least 28 days in either serum, citrate buffer (pH 5.5) or phosphate buffer (pH 7.4). The release profiles over 28 days suggested that the entrapped oligonucleotide was released biphasically from the polymer films, characterized by a rapid burst release during the first 48 h followed by a more sustained release. Oligonucleotide release from PLA matrices was dependent on oligomer chemistry (S-oligos were released more slowly than D-oligos) and on oligomer length (a 20-mer S-oligo was released more slowly than a 7-mer). During the release experiments, little or no degradation of the polymer matrices was observed by SEM or by DSC methods. Oligonucleotide release could be described by the mathematical model for drug release from a solution within a monolithic polymer slab. In all cases, the polymer-entrapped oligonucleotides were resistant to degradation from serum nucleases over the entire study period whereas free phosphodiester oligonucleotides were completely degraded within 1 h. Gel mobility shift analyses and duplex melting point determinations suggested that the hybridization capability of antisense oligonucleotides released from the polylactide matrices was unaffected by the solvent-casting procedure for preparing sustained release polymer devices. These results suggest that biodegradable polymer matrices may be suitable delivery systems for the sustained administration of antisense oligonucleotides.