Lipid-based nanoparticles for drug delivery

Liposomes, maybe easiest described as hollow fat particles built from closed lipid bilayers, have during the last three decades found important medical and pharmaceutical applications. Their unique architecture, allowing dissolution of hydrophobic, hydrophilic as well as amphiphilic compounds, together with the fact that they may easily be constructed from biocompatible molecules, makes liposomes very attractive as vehicles for drug delivery. Several liposomal drug formulations are today in late stage clinical trials or already on the market. In addition, liposomes and other closely related lipid structures have shown promising results as artificial self-assembling vectors for gene transfection.

Despite the encouraging results obtained with liposomes to date, there is room for improvement before liposomes completely fulfil the task of being safe, efficient and specific carriers for gene and drug delivery. So far, the design of successful lipid-based delivery systems has been based largely upon empirical experiences. Systematic physico-chemical investigations of structure and stability do not only help to speed up the development of new and improved formulations, but may also aid in the understanding of the complex mechanisms governing the interaction between the lipid carriers and the living cells.

Liposomes are hollow spheres built from self-closed bilayers

Since the late 90s we have devoted a substantial part of our research activities to projects aiming at the development of novel or improved strategies for drug delivery. Many of the projects are carried out in national, or international, collaboration with pharmaceutically/medically oriented research groups. The majority of the work has concerned lipid carriers intended for administration via the parenteral route, and our activities have primarily been directed towards the development of liposomes for nuclide- and chemotherapy of cancer.

Liposomes loaded with the chemotherapeutic agent doxorubicin.

A multidisciplinary project aiming at the development of a novel strategy for radionuclide therapy of spread cancer deserves special attention. We have for some years been exploring a new concept for targeted nuclide-therapy. The concept, which utilizes liposomal carriers in combination with a double targeting principle, aims at the delivery of large quantities of short-range radionuclides to DNA in the nucleus of tumor cells. With the aid of active loading procedures we have developed protocols to fill the liposomes with high concentrations of nuclide-carrying molecules.

Schematic picture showing two-step targeting with nuclisome-particles.

Targeting of the nuclide-filled liposomes (nuclisomes) to specific cells is accomplished by coupling bio-molecules, such as small proteins, antibody-fragments, or synthetic ligands, to the surface of the liposomes. In order to achieve internalization of the drug-loaded liposomes the bio-molecule, or ligand, is directed towards receptors with endocytotic ability. The second step in the targeting process involves the direction of the nuclides to the cell nucleus. This is crucial since for nuclide-therapy the nuclear DNA is the critical target and severe damage to DNA is necessary in order to kill the cell. Our strategy is to achieve nuclear localization by linking the nuclides to substances with high affinity for DNA.

Sometimes liposomes are not the most suitable carriers for drug delivery. For example, when transporting hydrophobic drugs the aqueous core of the liposome becomes unnecessary. We have found that a different type of lipid-based nanoparticles, termed lipodisks, constitute a very interesting alternative to liposomes for formulation and delivery of  hydrophobic, as well as amphiphilic drugs. In contrast to the spherical, hollow liposomes, the lipodisks are composed of a flat, circular bilayer stabilized with a rim of PEG-lipids.

Ongoing projects include investigation and evaluation of the lipodisks as carriers for poorly soluble drugs, as well as for certain protein/peptide drugs. For example, the use of lipodisks for the delivery of peptides with antitumoral and antimicrobial effects is studied. We have found that peptides bound to the lipodisks are very effectively protected against enzymatic degradation. Further, formulation in lipodisks allows for a slow and extended release of the peptides, which in turn can prolong the therapeutic effect.

Amphiphilic α-helical peptides bind preferentially to the rim of the lipodisk.

The planar structure and good stability of the lipodisks opens up for their use also as model membranes in the study of drug-membrane interaction. Very promising results have recently been obtained in studies where liposomes were replaced by lipodisks in drug partition studies. Due to their structural similarity with biological membranes phospholipid liposomes have been extensively used as model membranes and during the last ten years several liposome-based methods for the determination of drug partitioning have been developed and tested. The self-closed and often multilamellar nature of conventional liposomes may, however, complicate the evaluation of the experimental data. Further, since phospholipid liposomes do not represent thermodynamically stable but merely kinetically trapped structures they tend to aggregate and fuse with time. By substituting the liposomes with lipodisks both these problems may be avoided.