Potential usage of thermosensitive liposomes for macromolecule delivery

Thermosensitive nanoplatforms for photothermal release of cargo from liposomes under intracellular temperature monitoring

Cargo release from thermosensitive liposomes by a near infrared laser was investigated under intracellular temperature monitoring.

Liposomal Vasoactive Intestinal Peptide

Liposomes have been investigated as drug carriers since first discovered in the 1960s. However, the first-generation, so-called classic liposomes found relatively limited therapeutic utility. Nonetheless, the advent in the 1980s of the second-generation sterically stabilized liposomes (SSL) that evade uptake by the host's reticuloendothelial system greatly enhanced their utility as drug carriers because of their prolonged circulation half-life and passive targeting to injured and cancerous tissues. Over the past decade, our work focused on exploiting the bioactivity of vasoactive intestinal peptide (VIP), a ubiquitous 28-amino acid, amphipathic and pleiotropic mammalian neuropeptide, as a drug. To this end, the peptide expresses distinct and unique innate bioactivity that could be harnessed to treat several human diseases that represent unmet medical needs, such as pulmonary hypertension, stroke, Alzheimer's disease, sepsis, female sexual arousal dysfunction, acute lung injury, and arthritis. Unfortunately, the bioactive effects of VIP last only a few minutes due to its rapid degradation and inactivation by enzymes, catalytic antibodies, and spontaneous hydrolysis in biological fluids. Hence, our goal was to develop and test stable, long-acting formulations of VIP using both classic and SSL as platform technologies. We found that spontaneous association of VIP with phospholipid bilayers leads to a transition in the conformation of the peptide from random coil in an aqueous environment to alpha-helix, the preferred conformation for ligand-receptor interactions, in the presence of lipids. This process, in turn, protects VIP from degradation and inactivation and amplifies its bioactivity in vivo. Importantly, we discovered that the film rehydration and extrusion technique is the most suitable to passively load VIP onto SSL at room temperature and yields the most consistent results. Collectively, these attributes indicate that VIP on SSL represents a suitable formulation that could be tested in human disease.

On-off transport through a thermoresponsive hydrogel composite membrane

A novel thermoresponsive composite membrane is described through which the permeation of molecules is dependent on the temperature of the milieu. The thermoresponsive composite membranes utilise a thermosensitive crosslinked polymeric hydrogel, which possesses a volume phase transition temperature (VPTT). The hydrogel was polymerised in situ within the pores of a sintered glass filter, through which significant permeation of molecules only occurred at temperatures above the VPTT of the hydrogel. It was found that the permeation of molecules through the thermoresponsive composite membranes could be modulated by changing the environmental temperature.

Potential usage of thermosensitive liposomes for site-specific delivery of cytokines

Long-circulating liposomes reside long in the bloodstream, and transient swelling during phase transition of liposomes with hyper-osmotic internal aqueous phase causes release of macromolecules. Here we examined the applicability of long-circulating thermosensitive liposomes for delivery of tumor necrosis factor (TNF) by the heating of a local tumor-growing site after injection of TNF-loaded liposomes into tumor-bearing mice. Glucuronate modified thermosensitive liposomes with internal solution of two-fold higher osmotic pressure and sized through 200 nm-pore, released encapsulated [(131)I] human serum albumin at 42 degrees C in vitro and showed long-circulating character in vivo. Cytotoxic action of TNF encapsulated in long-circulating thermosensitive-liposomes (LCTS-liposomes) against L929 fibrosarcoma cells was enhanced at 42 degrees C in vitro. Furthermore, the tumor growth tended to be inhibited more by hyperthermia of mice bearing Meth A sarcoma than without heating after injection of TNF encapsulated in LCTS-liposomes. These results suggest that the cytokine can be released at the tumor site from the circulating CLTS-liposomes.

Improvement of temperature-sensitivity of poly(N-isopropylacrylamide)-modified liposomes

As a novel temperature-sensitive drug delivery system, we have designed liposomes coated with poly(N-isopropylacrylamide), which exhibits a coil-globule transition at 32-35 degrees C. In a previous study [ K. Kono, H. Hayashi, T. Takagishi, J. Control. Release 30 (1994) 69-75], it was shown that the release of contents from the polymer-coated liposomes is enhanced above the transition temperature of the polymer but the release is not controlled completely by the conformational change of the polymer. In this study, to improve temperature-sensitivity of the poly(N-isopropylacrylamide)-modified liposomes, the influence of lipid composition on the temperature-sensitivity of the liposomes was investigated. A copolymer of N-isopropylacrylamide and N, N-didodecylacrylamide was synthesized by free radical copolymerization. While the copolymer was insoluble in water, it exhibited a coil-globule transition around 28 degrees C when incorporated into an egg yolk phosphatidylcholine membrane. The copolymer-modified egg yolk phosphatidylcholine/dioleoylphosphatidylethanolamine liposomes were prepared by sonication or reverse phase evaporation. Release of calcein from the copolymer-modified liposomes was very slow below the transition temperature of the copolymer, whereas the release was enhanced above the transition temperature. Incorporation of dioleoylphosphatidylethanolamine into liposome membranes enhanced release greatly above the transition temperature of the copolymer. Fluorometric measurements using 8-anilino-1-naphthalenesulfonate and 1,6-diphenyl-1,3,5-hexatriene suggested that the copolymer decreases membrane fluidity of the liposomes near and above the transition temperature of the copolymer and that interaction between the copolymer and the liposome membranes is enhanced by inclusion of dioleoylphosphatidylethanolamine.

Affinity of antioxidative polyphenols for lipid bilayers evaluated with a liposome system

We developed a method to measure the amounts of antioxidative polyphenols and ubiquinones incorporated into the liquid bilayers of liposomes to estimate their affinities for cell membranes. Results were expressed in terms of an "affinity factor", calculated by division of the amount of compound incorporated by the amount added to the liposomal solution. The results reflected dose-dependence of the biological activities of the compound found in earlier in vitro experiments with mammalian and bacterial cells.

Characterization of biotinylated liposomes sensitive to temperature and pH: new tools for anti-cancer drug delivery

We describe a liposome formulation characterized by sensitivity to concurrent and small temperature and pH changes. Liposome permeability was assessed by monitoring the release of entrapped carboxyfluorescein (CF), using a fluorescence dequenching technique. The thermotropic behavior of the liposomes was investigated by differential scanning calorimetry. After 2 h at 37 degrees C in fetal calf serum, liposomes composed of a mixture of dipalmitoyl phosphatidylcholine/cholesterol/GM1 ganglioside/biotinoyl-dipalmitoyl phosphatidylethanolamine (100:20:6:0.25 molar ratio) released 8% CF at pH 7.4 and 12% CF at pH 6.7. At 41 degrees C the leakage was 72% at pH 7.4 and almost complete (99%) at pH 6.7. The pH and temperature sensitivity, with maximal release when the two circumstances occurred simultaneously, was confirmed by entrapping calcein or [14C]glucose. The reasons for the bilayer sensitivity and the conditions for in vivo drug delivery are discussed.

Temperature-controlled release property of phospholipid vesicles bearing a thermo-sensitive polymer

As a novel temperature-sensitive liposome, dioleoylphosphatidylethanolamine vesicles bearing poly(N-isopropylacrylamide), which shows a lower critical solution temperature (LCST) near 32 degrees C, were designed. Poly(N-isopropylacrylamide) having long alkyl chains which are anchors to the lipid membranes was prepared by radical copolymerization of N-isopropylacrylamide and octadecyl acrylate using azobisisobutyronitrile as the initiator. The copolymer obtained revealed the LCST at about 30 degrees C in an aqueous solution. Dioleoylphosphatidylethanolamine vesicles coated with the copolymer was prepared and release property of the copolymer-coated vesicles was investigated. While release of calcein encapsulated in the copolymer-coated vesicles was limited below 30 degrees C, the release was drastically enhanced between 30 and 35 degrees C. Complete release from the vesicles was achieved within several seconds at 40 degrees C. This temperature-controlled release property of the vesicles can be attributable to stabilization and destabilization of the vesicle membranes induced by the copolymer fixed on the vesicles below and above the LCST, respectively. Moreover, the fluorometric measurement using dioleoyl-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)phosphatidylethan ola mine suggested that the extensive release of calcein observed above the LCST is resulted from the bilayer to HII phase transition of the vesicle membranes. Since LCST of the copolymer is controllable, these vesicles might have potential usefulness as a drug delivery system with high temperature-sensitivity.