Jerome Connor

Biodistribution of pH-sensitive immunoliposomes

Liposomes composed of either dioleoylphosphatidylethanolamine and oleic acid (pH-sensitive) or dioleoylphosphatidylcholine and oleic acid (pH-insensitive) were injected into C3H and Balb/c mice in order to determine the tissue distribution of both the lipid and the aqueous content. The lipid component was monitored by use of [3H]cholestanyl ether and the aqueous content was monitored by use of encapsulated 125I-tyraminyl-inulin. The pH-insensitive liposomes injected into both types of mice were rapidly cleared from the blood stream followed by accumulation primarily in the liver, followed by the spleen. The presence of a monoclonal antibody on the liposome surface caused a slight acceleration in liver accumulation, though generally gave the same profile as the antibody-free liposomes. pH-sensitive liposomes were leaky upon exposure to the mouse plasma following injection. The lipid component, though, displayed a large amount (e.g., 50-70% in C3H mice) of accumulation in the lung for up to 6 h, followed by a subsequent appearance in the liver and spleen. The presence of monoclonal antibody had no effect on the tissue distribution profile. These results indicate that the pH-sensitive liposomes, although ineffective as an aqueous drug delivery agent, may be effective as a means of delivering lipophilic drugs to the lung.

Proton and divalent cations induce synergistic but mechanistically different destabilizations of pH-sensitive liposomes composed of dioleoyl phosphatidylethanolamine and oleic acid

Protons and divalent cations show synergistic effects on the destabilization of liposomes composed of unsaturated phosphatidylethanolamine and oleic acid (Düzgünes et al., Biochemistry (1985) 24, 3091). We have extended these observations and investigated the effects of Ca2+ and Mg2+ on the proton-induced destabilization of dioleoyl phosphatidylethanolamine/oleic acid (DOPE/OA) (4:1 molar ratio) liposomes. Temperature-induced aggregation was measured by 90 degrees light scattering. Lipid mixing was used to monitor vesicle destabilization and freeze-fracture electron microscopy was used to examine the structures formed from DOPE/OA vesicles in the presence of Ca2+ and/or protons. Both Mg2+ and Ca2+ shift the pH required for 50% lipid mixing to higher values. Temperature-induced vesicle aggregation occurs at lower temperatures in the presence of divalent cations and/or protons, indicating that intervesicular repulsions are decreased. Freeze-fracture electron micrographs show that the structures formed from DOPE/OA in the presence of Ca2+ differ significantly from those found in the presence of protons. In general, protons induce the formation of hexagonal phase, while the presence of Ca2+ leads to the formation of extensive regions of lamellar sheets with numerous lipidic particles. The synergistic effect of divalent cations and proton may be important for the maximal biological activity of DOPE/OA liposomes.

[8] pH-Sensitive immunoliposomes

Publisher Summary This chapter discusses the use of pH sensitive immunoliposomes. In an effort to design immunoliposomes that can also become fusion active when exposed to a mildly acidic environment, one have developed the pH-sensitive immunoliposomes. The design of the antibody-free, pH sensitive liposomes has been described in detail. Preparation of the pH-sensitive immunoliposomes is described in two steps in this chapter. The procedures for fatty acylation of antibody are discussed first. Incorporation of the fatty acyl antibody into the pH-sensitive liposomes is then described. Characterization of the pH-sensitive immunoliposomes and their interaction with target cells is also presented. In this case, liposome contents are delivered to the lysosomes of the target cells. Whether the contents are degraded or inactivated by the lysosomal hydrolases and whether the contents are released intact from the lysosomes, depends on the nature of the entrapped molecule. In any event, cytoplasmic delivery by pH-sensitive immunoliposomes has greatly increased their carder potential.

Acid-Induced Fusion of Liposomes

Liposomes have become an important model system for studying the phenomenon of membrane fusion (Blumenthal, 1985; Gregoriadis, 1984). The simplicity of form of artificial lipid membranes makes them an effective tool for elucidating the actual mechanism of fusion between opposing membranes. The information derived from the study of model liposome fusion is A. crucial contribution to the understanding of biologically relevant fusion activities, including endocytosis, exocytosis, and viral infection.