Recent advances in liposomal drug-delivery systems

Cytosolic drug delivery using pH- and light-sensitive liposomes

A growing body of literature describes the development and applications of novel targeting and/or contents release triggering schemes to improve the therapeutic index of drugs encapsulated within liposomes. This review focuses on literature appearing between January 1995-December 1997 that report 1) antibody and receptor-mediated targeting approaches for improving drug localization and 2) acid, enzymatic, thermal or photochemical triggering processes that destabilize membranes and improve drug bioavailability via cytoplasmic delivery of liposomal contents.

The resistance of retroviral vectors produced from human cells to serum inactivation in vivo and in vitro is primate species dependent

The ability to deliver genes as therapeutics requires an understanding of the vector pharmacokinetics similar to that required for conventional drugs. A first question is the half-life of the vector in the bloodstream. Retroviral vectors produced in certain human cell lines differ from vectors produced in nonhuman cell lines in being substantially resistant to inactivation in vitro by human serum complement (F. L. Cosset, Y. Takeuchi, J. L. Battini, R. A. Weiss, and M. K. Collins, J. Virol. 69:7430-7436, 1995). Thus, use of human packaging cell lines (PCL) may produce vectors with longer half-lives, resulting in more-efficacious in vivo gene therapy. However, survival of human PCL-produced vectors in vivo following systemic administration has not been explored. In this investigation, the half-lives of retroviral vectors packaged by either canine D17 or human HT1080 PCL were measured in the bloodstreams of macaques and chimpanzees. Human PCL-produced vectors exhibited significantly higher concentrations of circulating biologically active vector at the earliest time points measured (>1, 000-fold in chimpanzees), as well as substantially extended half-lives, compared to canine PCL-produced vectors. In addition, the circulation half-life of human PCL-produced vector was longer in chimpanzees than in macaques. This was consistent with in vitro findings which demonstrated that primate serum inactivation of vector produced from human PCL increased with increasing phylogenetic distance from humans. These results establish that in vivo retroviral vector half-life correlates with in vitro resistance to complement. Furthermore, these findings should influence the choice of animal models used to evaluate retroviral-vector-based therapies.

Liposomes, micelles and microemulsions as new delivery systems for cytotoxic alkaloids

This review describes the design and performance of specialized delivery systems, such as liposomes, micellar solutions and microemulsions, for the administration of cytotoxic alkaloids. Special attention is directed towards three types of compound, Vinca, Camptotheca and Taxus alkaloids, which have been previously indicated as of promise as antitumour agents but which still present serious drawbacks. In this respect, this review analyses different delivery strategies that are able to substantially improve the therapeutic applicability of such antitumour drugs.

Influence of lipopolymer concentration on liposome degradation and blood clearance

It is well known, that a prolonged liposome circulation time can be achieved by incorporation of lipopolymers into the lipid membrane thereby reducing interactions with destabilizing factors in the blood stream, e.g. phagocytic cells and lipoproteins. However, very little is known about the enzymatic degradation of steric hindered liposomes introduced into body fluids. In this study, the blood clearance and the PLA2 catalyzed degradation of unilamellar dipalmitoylphosphatidylcholine (DPPC) liposomes incorporated with increasing amounts of dipalmitoylphosphatidylethanolamine-polyethyleneglycol (DPPE-PEG), was investigated. The results demonstrated an increase in PLA2 activity for increasing amounts of lipopolymer in the lipid membrane, while the liposome blood clearance was prolonged by incorporation of DPPE-PEG into the liposomes. Hence, these results suggest that it may be possible for long circulating liposomes to obtain a site specific liposome degradation and release of drug substance in tissue with high levels of PLA2.

Clinical pharmacology of anticancer agents in relation to formulations and administration routes

In the past years, alternative administration routes and pharmaceutical formulations of anticancer agents have been investigated in order to improve conventional chemotherapy treatment. The impact of these adjustments on the pharmacokinetics and pharmacodynamics is discussed. A review of the literature shows many examples of alternative administration forms of anticancer agents with improved pharmacokinetics. Local administration routes have been investigated in order to reduce the systemic toxicity and to enhance the local efficacy of conventional chemotherapy. Oral administration of anticancer agents is preferred by patients for its convenience and its potential for outpatient treatment. In addition, oral administration facilitates a prolonged exposure to the cytotoxic agent. However, poor bioavailability and substantial interpatient variability are noted as limitations for oral chemotherapy. Increased tumour selectivity can also be achieved by the use of specific pharmaceutical formulations, such as liposomes and macromolecular drug conjugates. The composition of these formulations often determine the pharmacokinetic behaviour of the formulated drug. In conclusion, several alternative administration forms of anticancer agents have been designed in the past years, with the potential for improvement of conventional chemotherapy, however, more extensive clinical evaluation of these novel strategies is warranted to prove their real clinical value.

Stabilized plasmid-lipid particles: construction and characterization

A detergent dialysis procedure is described which allows encapsulation of plasmid DNA within a lipid envelope, where the resulting particle is stabilized in aqueous media by the presence of a poly(ethyleneglycol) (PEG) coating. These 'stabilized plasmid-lipid particles' (SPLP) exhibit an average size of 70 nm in diameter, contain one plasmid per particle and fully protect the encapsulated plasmid from digestion by serum nucleases and E. coli DNase I. Encapsulation is a sensitive function of cationic lipid content, with maximum entrapment observed at dioleoyldimethylammonium chloride (DODAC) contents of 5 to 10 mol%. The formulation process results in plasmid-trapping efficiencies of up to 70% and permits inclusion of 'fusigenic' lipids such as dioleoylphosphatidylethanolamine (DOPE). The in vitro transfection capabilities of SPLP are demonstrated to be strongly dependent on the length of the acyl chain contained in the ceramide group used to anchor the PEG polymer to the surface of the SPLP. Shorter acyl chain lengths result in a PEG coating which can dissociate from the SPLP surface, transforming the SPLP from a stable particle to a transfection-competent entity. It is suggested that SPLP may have utility as systemic gene delivery systems for gene therapy protocols.

Short- vs. long-circulating magnetoliposomes as bone marrow-seeking MR contrast agents

We evaluated the relaxation enhancement and biodistribution of short- vs. long-circulating magnetoliposomes as a new contrast agent for magnetic resonance (MR) imaging of bone marrow. Magnetoliposomes with (ML-PEG) and without (ML) incorporation of polyethylene glycol (PEG, Mw 2000) were prepared, measuring 40 nm in diameter with 1-6 iron oxide crystals/vesicle. PEGylation selectively enhanced the T2 relaxivity of magnetoliposomes by 10% to 15%, with R1 and R2 values of 3 and 240 s(-1)/mM at 1.5 T and 37 degrees C. ML (n = 6) and ML-PEG (n = 6) preparations were administered IV into young (6-8 weeks old) and adult (>1 year old) Sprague-Dawley rats at 100 pmol Fe/kg. PEGylation increased blood half-life (P<0.05 for t > 30 minutes), following a biexponential clearance with a long half-life of 53.2+/-13.2 minutes. The clearance of ML was monoexponential, with a half-life 7.4+/-0.4 minutes. MR imaging revealed a pronounced uptake in bone marrow, including the iliac bone, femur, tibia, and upper and lower vertebrae. The bone marrow uptake of ML-PEG was comparable to that of ML, with both reaching a plateau within 30 minutes following injection. Fast spin-echo T2-weighted imaging was found to provide optimal contrast enhancement and allowed a clear depiction of red to yellow marrow conversion due to normal aging. While the use of magnetoliposomes can provide the added benefit of therapeutic drug or gene delivery, further investigation is warranted to assess their usefulness in differentiating normal vs. abnormal marrow conditions.

Factors influencing uptake and retention of amino-containing drugs in large unilamellar vesicles exhibiting transmembrane pH gradients

The level of uptake and retention of amino-containing drugs in large unilamellar vesicles (LUVs) following uptake in response to a transmembrane pH gradient (DeltapH) can vary dramatically depending on the drug. For example, the anticancer drugs doxorubicin and epirubicin can be readily retained, whereas the anticancer drug vincristine and the antibiotic ciprofloxacin tend to leak out rapidly. In this investigation, we examine the influence of the hydrophobicity of the entrapped amines (that induce the DeltapH) and the anionic lipid content of the LUV on drug retention. It is shown that entrapment of increasingly hydrophobic monoamines (methylamine to amylamine) all lead to an induced DeltapH of 3 units and essentially complete drug uptake under the conditions employed, but do not lead to improved retention of vincristine and ciprofloxacin. However, significantly improved retention could be achieved by substitution of the anionic lipid distearoylphosphatidylglycerol (DSPG) for distearoylphosphatidylcholine (DSPC) in the LUV bilayer. Further, it is shown that if the induced DeltapH is reduced to 1.4 units (driven by entrapped diamine) nearly 100% accumulation of doxorubicin and epirubicin could be achieved, whereas only 25% loading for vincristine and ciprofloxacin was observed. Taken together these results provide methodology for improving (weak base) drug retention in liposomes and indicate that drugs that can partition into the lipid bilayer exhibit improved uptake and retention characteristics.

Effects of PEG-lipids on permeability of phosphatidylcholine/cholesterol liposomes in buffer and in human serum

The permeability of liposomal membranes was studied as a function of the amount of incorporated PEG-lipid. The fluorescent dyes ethidium, propidium and 5(6)-carboxy fluorescein were used as markers for measurements of spontaneous leakage. The results show that addition of up to 8 mol% of PEG(2000)-DSPE into liposomal membranes of DSPC/Cho and EPC/Cho reduces the permeability of carboxyfluorescein in buffer solution. In contrast, the leakage of the more amphiphilic dye ethidium was not to any measurable extent affected by PEG-lipid inclusion. Another important difference was that ethidum leakage showed a clear dependence on temperature whereas leakage of carboxyfluorescein from pegylated liposomes did not. We conclude that the mechanisms by which the two dyes permeate the liposomal bilayer are qualitatively different. Both ethidium and carboxyfluorescein did interact with human serum components in a way that made measurements in serum unreliable. The more hydrophilic ethidium analogue propidium was shown not to interact with human serum components to any detectable extent. This made propidium suitable for permeability determinations in human serum. It was found that liposomes composed of pure EPC or EPC with 5 mol% DSPE-PEG, displayed a dramatic increase in permeability when subjected to a medium composed of 20% human serum in buffer. Addition of 40 mol% cholesterol to the EPC bilayers reduced the observed release rate in human serum substantially, whereas no stabilizing effect was observed upon PEG-lipid inclusion.

Ionophore-mediated uptake of ciprofloxacin and vincristine into large unilamellar vesicles exhibiting transmembrane ion gradients

A new method, based on the ion-translocating properties of the ionophores nigericin and A23187, is described for loading large unilamellar vesicles (LUVs) with the drugs vincristine and ciprofloxacin. LUVs composed of distearoylphosphatidylcholine/cholesterol (DSPC/Chol) (55:45 mol/mol) or sphingomyelin (SPM)/Chol (55:45 mol/mol) exhibiting a transmembrane salt gradient (for example, internal solution 300 mM MnSO4 or K2SO4; external solution 300 mM sucrose) are incubated in the presence of drug and, for experiments involving divalent cations, the chelator EDTA. The addition of ionophore couples the outward movement of the entrapped cation to the inward movement of protons, thus acidifying the vesicle interior. External drugs that are weak bases can be taken up in response to this induced transmembrane pH gradient. It is shown that both nigericin and A23187 facilitate the rapid uptake of vincristine and ciprofloxacin, with entrapment levels approaching 100% and excellent retention in vitro. Following drug loading, the ionophores can be removed by gel exclusion chromatography, dialysis, or treatment with biobeads. In vitro leakage assays (addition of 50% mouse serum) and in vivo pharmacokinetic studies (in mice) reveal that the A23187/Mn2+ system exhibits superior drug retention over the nigericin/K+ system, and compares favorably with vesicles loaded by the standard DeltapH or amine methods. The unique features of this methodology and possible benefits are discussed.

Liposomes as drug delivery system: a strategic approach for the treatment of HIV infection

As the number of individuals infected with human immunodeficiency virus (HIV) is growing dramatically throughout the world, it is important to develop strategies to improve the treatment of this deadly disease. It is now well established that macrophages play a central role in HIV pathogenesis, acting as reservoirs for dissemination of virus throughout the immune system. As liposomes are naturally taken up by cells of the mononuclear phagocytic system, liposome-based therapy represents a convenient approach to improve the delivery of anti-HIV agents into infected cells improving thereby the efficacy of drugs and reducing their adverse side-effects. A more specific targeting of HIV-infected cells could also be obtained by using liposomes bearing surface attached-antibodies. This review details the applications of liposomes as drug carriers for the treatment of AIDS. It also gives an overlook of the different strategies that could be explored to control the progression of the disease in infected individuals.

A new look at lipid-membrane structure in relation to drug research

Lipid-bilayer membranes are key objects in drug research in relation to (i) interaction of drugs with membrane-bound receptors, (ii) drug targeting, penetration, and permeation of cell membranes, and (iii) use of liposomes in micro-encapsulation technologies for drug delivery. Rational design of new drugs and drug-delivery systems therefore requires insight into the physical properties of lipid-bilayer membranes. This mini-review provides a perspective on the current view of lipid-bilayer structure and dynamics based on information obtained from a variety of recent experimental and theoretical studies. Special attention is paid to trans-bilayer structure, lateral molecular organization of the lipid bilayer, lipid-mediated protein assembly, and lipid-bilayer permeability. It is argued that lipids play a major role in lipid membrane-organization and functionality.

Increase in phospholipase A2 activity towards lipopolymer-containing liposomes

Phospholipase A2 (PLA2)-catalyzed hydrolysis of dipalmitoylphosphatidylcholine (DPPC) liposomes incorporated with submicellar concentrations of polyethyleneoxide covalently attached to dipalmitoylphosphatidylethanolamine (DPPE-PEG2000) has been studied in the gel-to-fluid transition region of the host DPPC lipid bilayer matrix. By means of fluorescence and light-scattering measurements, the characteristic PLA2 lag time has been determined as a function of lipopolymer concentration and temperature. The degree of lipid hydrolysis was followed using radioactive labeled lipids. Differential scanning calorimetry has been applied to characterize the thermodynamic phase behavior of the lipopolymer-containing liposomes. A remarkable lipopolymer concentration-dependent decrease in the lag time was observed over broad temperature ranges. The radioactive measurements demonstrate an increase in catalytic activity for increasing amounts of lipopolymers in the bilayer. Hence, the lipopolymers act as a promoter of PLA2 lipid hydrolysis resulting in a degradation of the bilayer structure and a concomitant destabilization of the liposomes. This behavior is in contrast to the generally observed protective and stabilization effect in biological fluids exerted by lipopolymers in polymer-grafted liposomes. It is proposed that the enhanced activity of the small water soluble and interfacially active enzyme may involve a non-uniform distribution of the lipopolymers in the lipid matrix due to a coupling between local lipid bilayer curvature and composition of the non-bilayer-preferring lipopolymers.

Structural and metabolic consequences of liposome-lipoprotein interactions

The two major proposed uses for liposomes, i.e., drug delivery and mobilization of peripheral deposits of cholesterol, each impose requirements and restrictions on liposomal structure, particularly as it affects interactions with lipoproteins. This chapter focuses on the role of lipoproteins and apolipoproteins in (1) disrupting membrane structure and causing the leakage of liposomal contents by inducing disc formation and (2) marking liposomes for whole-particle uptake by receptors involved in lipoprotein metabolism. Control of membrane stability and whole-particle half-life can be achieved by several strategies, such as membrane stiffening, shielding the membrane surface, and increasing the dose or predosing with "empty" liposomes. The rationales and applicabilities of these strategies are discussed in the contexts of liposomes as drug delivery vehicles and as antiatherogenic particles. Directions for further basic and applied research are also presented.

Controlling liposome blood clearance by surface-grafted polymers

The incorporation of polymer-lipid conjugates, initially using PEG and subsequently other selected flexible, hydrophilic polymers, into lipid bilayers gives rise to sterically stabilized liposomes that exhibit reduced blood clearance and concomitant changes in tissue distribution largely because of reduced, but not eliminated, phagocytic uptake. Changes in tissue distribution includes 'passive' targeting localization into sites of tumors, infection, inflammation characterized by presence of a 'leaky' vasculature which represent useful applications for drug delivery. The polymer forms a surface coating which has been characterized by physical measurements and it appears to function through steric inhibition of the protein binding and cellular interactions leading to phagocytic uptake. The current understanding of the physical and biological properties are reviewed. Ongoing work in the field involves interests to increase complexity such as addition of (1) selective targeting ligands by chemical conjugation to the exterior surface of the polymer coating, (2) capabilities for intracellular release of encapsulated agents into the cytoplasm, and (3) both simultaneously.

Recent advances in liposome technologies and their applications for systemic gene delivery

The recent clinical successes experienced by liposomal drug delivery systems stem from the ability to produce well-defined liposomes that can be composed of a wide variety of lipids, have high drug-trapping efficiencies and have a narrow size distribution, averaging less than 100 nm in diameter. Agents that prolong the circulation lifetime of liposomes, enhance the delivery of liposomal drugs to specific target cells, or enhance the ability of liposomes to deliver drugs intracellularly can be incorporated to further increase the therapeutic activity. The physical and chemical requirements for optimum liposome drug delivery systems will likely apply to lipid-based gene delivery systems. As a result, the development of liposomal delivery systems for systemic gene delivery should follow similar strategies.

Use of circular dichroism spectroscopy in determining the conformation of a monoclonal antibody prior to its incorporation in an immunoliposome

Attachment of antibodies to liposomes endows target specificity to liposomes for a certain cell or organ that express the targeted antigenic determinant. These so-called immunoliposomes hold high promise as targeted drug carriers. One approach of immunoliposome preparation involves conjugating antibodies to hydrophobic anchors (e.g. fatty acids or phospholipid molecules) for incorporation into the liposome membrane. Often, these conjugation reactions are harsh and may result in undesirable chemical and structural changes in the antibody molecule. This necessitates confirmation of the target specificity of the derivatized antibody prior to its incorporation into the liposome. Our approach to this problem is to utilize circular dichroism spectroscopy, which can detect subtle structural differences in proteins with high reproducibility and accuracy in relatively short period of time. In addition, circular dichroism is a non-destructive technique. In this study, we demonstrate the ability of circular dichroism to confirm the conformation of a model antibody, HYB-241, conjugated to N-glutarylphosphatidylethanolamine, prior to its mixing with dioleoylphosphatidylethanolamine/dioleoylphosphatidic acid to form a target-sensitive immunoliposome.

Effect of polyethyleneglycol-phospholipids on aggregate structure in preparations of small unilamellar liposomes

Phospholipids with covalently attached poly(ethylene glycol) (PEG lipids) are commonly used for the preparation of long circulating liposomes. Although it is well known that lipid/PEG-lipid mixed micelles may form above a certain critical concentration of PEG-lipid, little is known about the effects of PEG-lipids on liposome structure and leakage at submicellar concentrations. In this study we have used cryogenic transmission electron microscopy to investigate the effect of PEG(2000)-PE on aggregate structure in preparations of liposomes with different membrane compositions. The results reveal a number of important aggregate structures not documented before. The micrographs show that enclosure of PEG-PE induces the formation of open bilayer discs at concentrations well below those where mixed micelles begin to form. The maximum concentration of PEG-lipid that may be incorporated without alteration of the liposome structure depends on the phospholipid chain length, whereas phospholipid saturation or the presence of cholesterol has little or no effect. The presence of cholesterol does, however, affect the shape of the mixed micelles formed at high concentrations of PEG-lipid. Threadlike micelles form in the absence of cholesterol but adapt a globular shape when cholesterol is present.

Tissue distribution of the cytofectin component of a plasmid-DNA/cationic lipid complex following intravenous administration in mice

Allovectin-7 is a gene therapy agent that consists of plasmid DNA (pDNA) encoding the human HLA-B7 class I and beta2-microglobulin genes (VCL-1005), complexed with the cationic lipid DMRIE Br and DOPE. A tritiated version of the cytofectin component, DMRIE Br, was synthesized by regiospecific isotope incorporation to a very high specific activity. The 3H-labeled DMRIE/DOPE mixture was complexed with VCL-1005 to produce a radiolabeled version of Allovectin-7. The VCL-1005/3H-DMRIE/DOPE complex was administered intravenously to mice, and the tissue distribution of radioactivity was analyzed 24 hr later. Excretion of radioisotope was monitored for 96 hr post dosing. At 24 hr post administration, a tissue distribution for the radioisotope of liver >> spleen > lung >> heart > brain approximately muscle approximately blood was observed. During the 96-hr period post dose, very little administered radioactivity (<17%) was excreted and the majority of the isotope (83%) remained in the animal. This is the first report on the biodistribution of the cytofectin component of a pDNA-cationic lipid complex for which the distribution of the plasmid component has also been reported.

Sterically stabilized pH-sensitive liposomes. Intracellular delivery of aqueous contents and prolonged circulation in vivo

Liposomes that destabilize at mildly acidic pH are efficient tools for delivering water-soluble drugs into the cell cytoplasm. However, their use in vivo is limited because of their rapid uptake from circulation by the reticuloendothelial system. Lipid-anchored polyethylene glycol (PEG-PE) prolongs the circulation time of liposomes by steric stabilization. We have found that addition of PEG-PE to the membrane of pH-sensitive liposomes composed of cholesteryl hemisuccinate (CHEMS) and dioleoylphosphatidylethanolamine (DOPE) confers steric stability to these vesicles. This modification significantly decreases the pH-dependent release of a charged water-soluble fluorophore, calcein, from liposomes suspended in buffer or cell culture medium. However, the ability of such liposomes to release calcein intracellularly, measured by a novel flow cytometry technique involving dual fluorescence labeling, remains unaltered. As expected, the release of calcein from liposomes endocytosed by cells is inhibited upon pretreatment of the cells with NH4Cl, an inhibitor of endosome acidification. The unique properties of these liposomes were also demonstrated in vivo. The distribution kinetics of 111In-containing CHEMS/DOPE/PEG-PE liposomes injected intravenously into rats has pharmacokinetic parameters similar to control, non-pH-sensitive, sterically stabilized CHEMS/distearoylphosphatidylcholine/PEG-PE liposomes. In contrast, regular pH-sensitive liposomes lacking the PEG-PE component are cleared rapidly. Sterically stabilized pH-sensitive liposomes may therefore be useful for the intracellular delivery in vivo of highly negatively charged molecules such as genes, antisense oligonucleotides, and ribozymes for the treatment of various diseases.

Gene therapy for ovarian cancer: development of novel treatment strategies

In the last decade, advances in molecular biology have lead to the development of techniques that permit the manipulation of mammalian cell DNA for diagnostic and therapeutic purposes. Gene therapy has subsequently evolved as a treatment option in patients with malignancies. In this article, we have summarized current strategies in gene therapy for ovarian cancer.