Delivery of antisense oligonucleotides by means of pH-sensitive liposomes

pH-sensitive liposomes for drug delivery in cancer treatment

In recent years, liposomes have been employed with growing success as pharmaceutical carriers for antineoplastic drugs. One specific strategy used to enhance in vivo liposome-mediated drug delivery is the improvement of intracytoplasmic delivery. In this context, pH-sensitive liposomes (pHSLip) have been designed to explore the endosomal acidification process, which may lead to a destabilization of the liposomes, followed by a release of their contents into the cell cytoplasm. This review considers the current status of pHSLip development and its applicability in cancer treatment, focusing on the mechanisms of pH sensitivity and liposomal composition of pHSLip. The final section will discuss the application of these formulations in both in vitro and in vivo studies of antitumor efficacy.

Spontaneous formation of bilayers and vesicles in mixtures of single-chain alkyl carboxylates: effect of pH and aging and cytotoxicity studies

We report the observation of bilayer fragments, some of which close to form vesicles, over a large range of pH at room temperature from mixtures of single-chain biocompatible commercially available nontoxic alkyl carboxylic surfactants after neutralization with HCl. The pH at which the morphological transitions occur is varied only by changing the ratio between two surfactants: the alkyloligoethyleneoxide carboxylate and sodium laurate. The effect of aging of the mixed surfactant systems in the pH region desired for dermatologic application (4.5 < pH < 7) is also studied. Finally, we show results of cytotoxicity studies on the surfactant mixtures.

Delivery of different length poly(L-lysine)-conjugated ODN to HepG2 cells using N-stearyllactobionamide-modified liposomes and their enhanced cellular biological effects

Short (14-20-mer range) synthetic oligodeoxynucleotides (ODNs) allow specific modulation of cellular gene expression at various stages, thus providing a versatile tool for fundamental studies and a rational approach to anticancer chemotherapy. However, several problems, such as metabolic stability, efficient cell internalization of ODNs and their efficient entrapment into liposomes continue to markedly limit this approach. To improve the target specificity and biological activity of ODN, three different length of poly(L-lysine) (PLL) were conjugated to ODN and these conjugates were encapsulated in N-stearyllactobionamide (N-SLBA)-modified liposomes, N-SLBA is a ligand for the asialoglycoprotein receptor. Then, we investigated their effects on cell cycle and survivin protein levels of HepG2 cells. The results showed that the encapsulation efficiency was improved because the polycationic charges of PLL neutralized the polyanionic charges of ODN. Among them, PLL (M(W) 2000 and 10,000)-conjugated ODN encapsulated in N-SLBA liposomes induced apoptosis of HepG2 cells and highly inhibited survivin gene expression.

Uptake of liposomally entrapped fluorescent antisense oligonucleotides in NG108-15 cells: conventional versus pH-sensitive

Antisense oligodeoxynucleotides (asODN) are novel therapeutic agents designed to alter RNA metabolism, ultimately resulting in decreased production of disease-associated gene products. To investigate internalisation of liposomally delivered asODN in NG108-15 cells, a hybrid cell line of mouse neuroblastoma and rat glioma, and assure that uptake of marker corresponds to that of antisense, we compared the cellular uptake of fluorescently labelled marker (fluorescein isothiocyanate (FITC)-dextran) and antisense oligonucleotide (FITC-asODN), entrapped either in conventional soy phosphatidylcholine (SPC) liposomes or pH-sensitive liposomes (composed of dioleoylphosphatidylethanolamine and cholesteryl hemisuccinate in a molar ratio of 3 : 2). Both SPC and pH-sensitive liposomes were prepared by a modified freeze-thawing method. Entrapment efficiencies (about 20% of the original material) did not depend on the liposome compositions and fluorescent material used. Fluorescence activated cell sorting (FACS) analysis was used to quantify the association of fluorescent material with the NG108-15 cells, whereas confocal microscopy gave insight on the location of cell associated-fluorescence. Conventional liposomes failed to deliver fluorescent material into the cells, but in contrast, pH-sensitive liposomes significantly improved the uptake of both FITC-dextran and FITC-asODN, with the uptake of liposomal FITC-dextran being greater than the uptake of liposomal FITC-asODN. These results suggest that pH-sensitive liposomes can be applied as a carrier system in the delivery of genetic material into the cells.

On the mechanisms of internalization and intracellular delivery mediated by pH-sensitive liposomes

We investigated the molecular mechanisms by which pH-sensitive liposomes surpass the cytoplasmic and endosomal membranes to deliver their aqueous contents into the cytoplasm. Various liposome formulations were evaluated for their efficacy to mediate intracellular delivery of encapsulated material, including a novel sterically stabilized pH-sensitive formulation ((DOPE:CHEMS:DSPE-PEG(2000) (6:4:0.3)) that was previously developed in our laboratories. In an attempt to fully characterize the nature of liposome-cell interactions different approaches based on a dual-labeling fluorescence assay were used. Our results indicate that the efficacy of interaction of pH-sensitive liposomes, both plain and sterically stabilized, with cells is strongly determined by the inclusion of DOPE in their composition, independently of the type of the amphiphilic stabilizer used. In fact, DOPE-containing liposomes shown to be non-pH sensitive by biophysical assays, mediated cytoplasmic delivery of their contents as efficiently as well known pH-sensitive formulations (e.g. DOPE:CHEMS). However, among the different formulations studied, DOPE:CHEMS liposomes were those exhibiting the highest extent of cell association. Moreover, our results with cells pretreated with metabolic inhibitors or lysosomotropic agents clearly indicate that DOPE-containing liposomes are internalized essentially by endocytosis and that acidification of the endosomes is not the only mechanism involved in the destabilization of the liposomes inside the cell.

Therapeutic applications of colloidal drug carriers

Colloidal drug carriers such as liposomes and nanoparticles can be used to improve the therapeutic index of both established and new drugs by modifying their distribution, and thus increasing their efficacy and/or reducing their toxicity. This is because the drug distribution then follows that of the carrier, rather than depending on the physicochemical properties of the drug itself. If these delivery systems are carefully designed with respect to the target and the route of administration, they may provide one solution to some of the delivery problems posed by new classes of active molecules, such as peptides and proteins, genes and oligonucleotides. They may also offer alternative modes for more conventional drugs, such as highly hydrophobic small molecules. This review discusses the use of colloidal, particulate carrier systems (25 nm to 1 µm in diameter) in such applications.

Highly loaded nanoparticulate carrier using an hydrophobic antisense oligonucleotide complex

Antisense oligonucleotides, and particularly those with phosphorothioate backbones, have emerged as potential gene specific therapeutic agents and are currently undergoing evaluation in clinical trials for a variety of diseases. In the area of HIV-1 therapeutics, targeting of oligonucleotides to infected cells, such as macrophages, would be highly desirable. The present study was designed to prepare and characterize oligonucleotide-loaded nanoparticles for this purpose. Due to their hydrophilic characteristics, oligonucleotides are difficult to entrap in polymeric particles. Here, the oligonucleotides were first complexed with cetyltrimethylammonium bromide. The oligonucleotide-loaded nanoparticles were prepared by the emulsification-diffusion method and subsequently purified. In comparison with previous studies, a high oligonucleotide-loading was achieved; 2.5, 5 and 10% oligonucleotide loading were assessed. If the initial oligonucleotide content was 4%, this method produced a final oligonucleotide loading of 1.9% with an entrapment efficiency of 47%. The integrity of the oligonucleotide and of the polymer, in the final freeze-dried product, was retained.

Uptake of oligonucleotide-loaded nanoparticles in prostatic cancer cells and their intracellular localization

The development of antisense biotechnology is dependent, in part, on creating improved methods for delivering oligonucleotides to cells. In this study, we investigated a colloidal system (nanoparticles (NP) of poly (D,L) lactic acid) that affects the intracellular delivery of oligonucleotides. We have examined the intracellular compartmentalization in DU145 cells of fluorescein labeled phosphorothioate oligonucleotides, both in the free state and when loaded into NP. Fluorescent oligonucleotides were incubated for 18 h with DU145 cells and the mean intracellular fluorescence was determined by flow cytometry. After the addition of monensin, an increase in signal intensity was observed, indicating that free oligonucleotides were resident in an acidic intracellular environment, whereas oligonucleotides from the NP did not reside in an acidic compartment. Free and NP loaded with oligonucleotides effluxed from DU145 cells from two intracellular compartments. This preliminary report indicates that colloidal carriers such as NP could prove to be useful in affecting intracellular trafficking of oligonucleotides in DU145 and in other cells.