pH-sensitive liposomes: possible clinical implications.

When pH-sensitive molecules are incorporated into liposomes, drugs can be specifically released from these vesicles by a change of pH in the ambient serum. Liposomes containing the pH-sensitive lipid palmitoyl homocysteine (PHC) were constructed so that the greatest pH differential (6.0 to 7.4) of drug release was obtained near physiological temperature. Such liposomes could be useful clinically if they enable drugs to be targeted to areas of the body in which pH is less than physiological, such as primary tumors and metastases or sites of inflammation and infection.

Liposomes and local hyperthermia: selective delivery of methotrexate to heated tumors.

Liposomes with phase transitions a few degrees above physiological temperature delivered more than four times as much methotrexate to murine tumors heated to 42 degrees C as to unheated control tumors. Most of the accumulated drug appeared to be intracellular and bound to dihydrofolate reductase, the enzyme blocked by methotrexate in its role as an antineoplastic agent.

Design of liposomes for enhanced local release of drugs by hyperthermia.

Liposomes can be designed to release an entrapped drug preferentially at temperatures attainable by mild local hyperthermia. In a test system in vitro, protein synthesis by Escherichia coli is inhibited and killing of the cells is enhanced by heating neomycin-containing liposomes to their phase transition temperature to maximize drug release. In the presence of serum the ratio of release at 44 degrees C to that at 37 degrees C can be made greater than 100:1, suggesting possible applications in the treatment of tumors or local infection.

Selective delivery of liposome-associated cis-dichlorodiammineplatinum(II) by heat and its influence on tumor drug uptake and growth.

In an attempt to optimize the chemotherapeutic treatment of mouse tumor Sarcoma 180, liposomes containing cis-dichlorodiammineplatinum(II) (PDD), having transition temperatures of few degrees higher than the rectal temperature of mice, were used in combination with local hyperthermia. The uptake of radioactive PDD by tumors heated for 1 hr at 42 degrees was almost four-fold greater when the drug was associated in liposomes than if administered as free drug. Uptake of liposome-administered radioactive platinum by liver was twice that obtained with free PDD, whereas its incorporation by the kidney was the same by either method of drug administration. The effect of various combinations of hyperthermia, drug-containing liposomes, and free PDD on tumor growth was also studied. Treatment with liposome-associated PDD plus local heating resulted in a dose-modifying factor of 7 when compared with free drug and no hyperthermia. The dose-modifying factor was 2.5 when PDD liposomes and heat were compared within free drug and heat. Thus, PDD could be specifically released from liposomes by heat and resulted in both a greater drug uptake and a delayed tumor growth following treatment. Potential normal tissue toxicity problems, however, still need to be resolved before clinical application of this combined modality will be possible.

Clinical prospects for liposomes.

The use of liposomes has recently been the subject of considerable attention as a promising and versatile approach to drug delivery. Particularly intriguing is the possibility of targeting liposomes to specific areas of the body such as tumors or sites of inflammation or parasitic invasion for either local accumulation or release of associated drugs. This review focuses mainly on recent in vivo work having clinical potential. An extensive discussion of liposome preparation and entrapment of drugs for controlled release in vivo is also included. The stability of liposomes in biological fluids is a major problem. The mode of administration, either intraperitoneal, subcutaneous, local, oral, or respiratory, is closely related to the life of the liposomes in vivo. Following in vivo administration the lifetime of a liposome is critically dependent on its composition, size, and charge. Liposome toxicity appears to be minimal, but should be considered when administering liposomes to patients. Tissues such as the liver, spleen, and lungs, because of macrophage ingestion of liposomes, become potential sites of drug toxicity. The use of liposomes to deliver antiparasitic drugs in the treatment of malaria and leishmaniasis is promoting; so it is the use of surfactant-carrying liposomes in the treatment of respiratory distress syndrome in premature babies. Recent cancer studies utilizing liposomes both in vivo and in vitro have shown promise. In tumor-bearing animals a liposome drug delivery system has caused a regression, delayed tumor growth, and increased survival time. Although the clinical use of liposomes is only in its infancy, its potential in future therapy appears promising.

A comparison of X-irradiation and ferrous ion-ascorbate on oxidation of phosphatidylglycerols in multilamellar liposomes.

The effect of ferrous ion-ascorbate and X-radiation on multilamellar liposomes, composed of either completely saturated, unsaturated or a mixture of saturated and unsaturated fatty acids, is reported. Lipid composition is shown to be of critical importance in determining the extent to which peroxidation occurs. Liposomes composed of the mixture of saturated and unsaturated fatty acids are peroxidized to a lesser extent by ferrous ion-ascorbate. The reduced peroxidation is apparently the result of an inhibition mechanism shown by the saturated lipid component. In contrast, liposomes composed of mixed lipids do not reduce the level of peroxidation induced by ionizing radiation. These results show that the composition of liposomes plays a role in determining the extent to which peroxidation occurs when a chemical oxidant is employed, but composition is a negligible factor when ionizing radiation is the oxidant.

pH-sensitive liposomes: acid-induced liposome fusion.

Sonicated unilamellar liposomes containing phosphatidylethanolamine and palmitoylhomocysteine fuse rapidly when the medium pH is lowered from 7 to 5. Liposome fusion was demonstrated by (i) mixing of the liposomal lipids as shown by resonance energy transfer, (ii) gel filtration, and (iii) electron microscopy. The pH-sensitive fusion of liposomes was observed only when palmitoylhomocysteine (greater than or equal to 20 mol%) was present in the liposomes. The presence of phosphatidyl-ethanolamine in the liposomes greatly enhanced fusion whereas the presence of phosphatidylcholine inhibited fusion. During fusion of liposomes containing phosphatidylethanolamine and palmitoylhomocysteine (8:2, mol/mol), almost all of the encapsulated calcein was released. Inclusion of cholesterol (40 mol%) in the liposomes substantially decreased leakage without impairing fusion.

Induced drug release from lipid vesicles in serum by pH-change.

Drugs can be released from lipid vesicles by pH-change in calf, horse or human serum when pH-sensitive trigger molecules are incorporated in the vesicle lipid bilayer. The lipid composition is so chosen that the drug release is best performed at 37 degrees C. Specific drug targeting is envisaged to loci of the body with lower than physiological pH, such as primary or metastatic tumors.