The role of cavitation in liposome formation

Liposome size is a vital parameter of many quantitative biophysical studies. Sonication, or exposure to ultrasound, is used widely to manufacture artificial liposomes, yet little is known about the mechanism by which liposomes are affected by ultrasound. Cavitation, or the oscillation of small gas bubbles in a pressure-varying field, has been shown to be responsible for many biophysical effects of ultrasound on cells. In this study, we correlate the presence and type of cavitation with a decrease in liposome size. Aqueous lipid suspensions surrounding a hydrophone were exposed to various intensities of ultrasound and hydrostatic pressures before measuring their size distribution with dynamic light scattering. As expected, increasing ultrasound intensity at atmospheric pressure decreased the average liposome diameter. The presence of collapse cavitation was manifested in the acoustic spectrum at high ultrasonic intensities. Increasing hydrostatic pressure was shown to inhibit the presence of collapse cavitation. Collapse cavitation, however, did not correlate with decreases in liposome size, as changes in size still occurred when collapse cavitation was inhibited either by lowering ultrasound intensity or by increasing static pressure. We propose a mechanism whereby stable cavitation, another type of cavitation present in sound fields, causes fluid shearing of liposomes and reduction of liposome size. A mathematical model was developed based on the Rayleigh-Plesset equation of bubble dynamics and principles of acoustic microstreaming to estimate the shear field magnitude around an oscillating bubble. This model predicts the ultrasound intensities and pressures needed to create shear fields sufficient to cause liposome size change, and correlates well with our experimental data.     

Monoclonal antibody mediated intracellular targeting of tallysomycin S(10b)

The potency of tallysomycin S(10b) (TLM S(10b)) an analogue bleomycin was enhanced by up to 875-fold when it was conjugated to the internalizing antibody BR96. Attachment to the antibody is achieved via a Cathepsin B cleavable linker. The enhancement in potency is believed to be a result of cellular uptake of the conjugate upon antigen binding followed by rapid release of the drug inside the lysosome. This method provides a novel approach for increasing the potency and therapeutic index of nominally moderately-active cytotoxic agents.     

Heat-activated liposome targeting to streptavidin-coated surfaces

There is a great need of improved anticancer drugs and corresponding drug carriers. In particular, liposomal drug carriers with heat-activated release and targeting functions are being developed for combined hyperthermia and chemotherapy treatments of tumors. The aim of this study is to demonstrate the heat-activation of liposome targeting to biotinylated surfaces, in model experiments where streptavidin is used as a pretargeting protein. The design of the heat-activated liposomes is based on liposomes assembled in an asymmetric structure and with a defined phase transition temperature. Asymmetry between the inside and the outside of the liposome membrane was generated through the enzymatic action of phospholipase D, where lipid head groups in the outer membrane leaflet, i.e. exposed to the enzyme, were hydrolyzed. The enzymatically treated and purified liposomes did not bind to streptavidin-modified surfaces. When activation heat was applied, starting from 22 °C, binding of the liposomes occurred once the temperature approached 33 ± 0.5 °C. Moreover, it was observed that the asymmetric structure remained stable for at least 2 weeks. These results show the potential of asymmetric liposomes for the targeted binding to cell membranes in response to (external) temperature stimulus. By using pretargeting proteins, this approach can be further developed for personalized medicine, where tumor-specific antibodies can be selected for the conjugation of pretargeting agents.     

肝细胞靶向pH敏脂质体介导的硫代反义寡核苷酸对HCV5‘NCR基因表达的抑制作用

反义寡核苷酸是一种阴离子大分子物质,细胞生物利用度较低且易被细胞溶酶体酶降解,为了增强反义药物在病变靶细胞内的有效浓度,根据受体介导的内吞作用原理,针对肝细胞专一性表达的去唾液酸糖蛋白受体,设计及制备了一种肝靶向性脂质体,这种脂质体同时具有pH敏性.采用竟争抑制实验及鸡红细胞溶血实验分析了其肝细胞靶向性及pH敏性;应用肝靶向pH敏脂质体作为药物运载工具,介导反义寡核苷酸HCV363作用于转基因细胞HepG2.9706细胞,通过荧光素酶活性检测,观察了硫代反义寡核苷酸对HCV 5′NCR调控功能的抑制活性.结果显示,不同浓度半乳糖溶液对5%18-gal脂质体有一定的抑制作用,浓度超过20mmol/L时,达到饱和,最大抑制率为38%;溶血实验显示脂质体与红细胞膜融合作用有显著的pH值依赖性,pH＜6时,血红素释放量明显增加;肝靶向pH敏性脂质体介导的HCV363对HepG2.9706细胞中HCV 5′NCR调控基因具有显著的剂量依赖性抑制作用,浓度为1.0umol/L时,抑制率达86%.综上,所制备的脂质体具有一定的肝细胞靶向性及显著的pH敏感性,这种脂质体能够增强HCV特异性硫代反义寡核苷酸的细胞内抑制活性,这为针对肝炎病毒的反义寡核苷酸的体内活性评价提供了有用的转运体系.     

Selective targeting and photocoagulation of ocular angiogenesis mediated by a phage-derived human antibody fragment.

Molecules that selectively target and occlude new blood vessels would be useful for diagnosis and treatment of pathologies associated with angiogenesis. We show that a phage-derived human antibody fragment (L19) with high affinity for the ED-B domain of fibronectin, a marker of angiogenesis, selectively localizes to newly formed blood vessels in a rabbit model of ocular angiogenesis. The L19 antibody, chemically coupled to a photosensitizer and irradiated with red light, mediates complete and selective occlusion of ocular neovasculature and promotes apoptosis of the corresponding endothelial cells. These results demonstrate that new ocular blood vessels can be distinguished immunochemically from preexisting ones and suggest that the targeted delivery of photosensitizers may be effective in treating angiogenesis-related pathologies.     

Pros and cons of the liposome platform in cancer drug targeting

Coating of liposomes with polyethylene-glycol (PEG) by incorporation in the liposome bilayer of PEG-derivatized lipids results in inhibition of liposome uptake by the reticulo-endothelial system and significant prolongation of liposome residence time in the blood stream. Parallel developments in drug loading technology have improved the efficiency and stability of drug entrapment in liposomes, particularly with regard to cationic amphiphiles such as anthracyclines. An example of this new generation of liposomes is a formulation of pegylated liposomal doxorubicin known as Doxil or Caelyx, whose clinical pharmacokinetic profile is characterized by slow plasma clearance and small volume of distribution. A hallmark of these long-circulating liposomal drug carriers is their enhanced accumulation in tumors. The mechanism underlying this passive targeting effect is the phenomenon known as enhanced permeability and retention (EPR) which has been described in a broad variety of experimental tumor types. Further to the passive targeting effect, the liposome drug delivery platform offers the possibility of grafting tumor-specific ligands on the liposome membrane for active targeting to tumor cells, and potentially intracellular drug delivery. The pros and cons of the liposome platform in cancer targeting are discussed vis-à-vis nontargeted drugs, using as an example a liposome drug delivery system targeted to the folate receptor.     

The remarkable multivalency of the Hsp70 chaperones

Hsp70 proteins are key to maintaining intracellular protein homeostasis. To carry out this task, they employ a large number of cochaperones and adapter proteins. Here, we review what is known about the interaction between the chaperones and partners, with a strong slant toward structural biology. Hsp70s in general, and Hsc70 (HSPA8) in particular, display an amazing array of interfaces with their protein cofactors. We also review the known interactions between Hsp70s with lipids and with active compounds that may become leads toward Hsp70 modulation for treatment of a variety of diseases.     

The role of surface charge and hydrophilic groups on liposome clearance in vivo.

The effect of negative surface charge and hydrophilic groups on liposome clearance from blood was investigated in mice using liposome-entrapped 67gallium-deferoxamine as a label. The presence of negatively-charged lipids may retard or accelerate liposome clearance. Physicochemical features contributing to optimal retardation of liposome clearance include a hydrophilic carbohydrate moiety and a sterically hindered negatively-charged group. The relevance of the negative charge steric effect is suggested by the finding that phosphatidylinositol phosphate (PIP) and trisialoganglioside (GT1) are less effective than phosphatidylinositol (PI) and monosialoganglioside (GM1), respectively, in retarding liposome clearance. The need for negative charge in addition to the carbohydrate group for optimal effect on retardation of clearance is indicated by the observation that asialoganglioside (AGM1) is less effective than GM1 in this respect. The negative charge effect is observed with liposome bilayers having both low and high temperature phase-transitions. Increasing the molar fraction of negatively-charged lipid (hydrogenated PI derived from soya) from 23 to 41% resulted in a dramatic acceleration of liposome clearance. The clearance-accelerating effect of the high negative charge was specifically directed to the liver with selective reduction of spleen uptake. Increasing liposome size also had an accelerating effect on clearance but in this case it was accompanied by a non-specific concomitant increase of both liver and spleen uptake.     

In vivo gene targeting of IL-3 into immature hematopoietic cells through CD117 receptor mediated antibody gene delivery

BACKGROUND: Targeted gene transfection remains a crucial issue to permit the real development of genetic therapy. As such, in vivo targeted transfection of specific subsets of hematopoietic stem cells might help to sustain hematopoietic recovery from bone marrow aplasia by providing local production of growth factors. METHODS: Balb/C mice were injected intravenously, with an anti-mouse c-kit (CD117) monoclonal antibody chemically coupled to a human IL-3 gene-containing plasmid DNA. Mice were sacrificed for tissue analyses at various days after injection of the conjugates. RESULTS: By ELISA, the production of human IL-3 was evidenced in the sera of animals 5 days after treatment. Cytofluorometric analysis after in vivo transfection of a reporter gene eGFP demonstrated transfection of CD117+/Sca1+ hematopoietic immature cells. By PCR analysis of genomic DNA and RNA using primer specific pIL3 sequences, presence and expression of the human IL-3-transgene were detected in the bone marrow up to 10 days in transfected mice but not in control animals. CONCLUSIONS: These data clearly indicate that antibody-mediated endocytosis gene transfer allows the expression of the IL-3 transgene into hematopoietic immature cells, in vivo. While availability of marketed recombinant growth factors is restricted, this targeting strategy should permit delivery of therapeutic genes to tissues of interest through systemic delivery. In particular, the ability to specifically target growth factor expression into repopulating hematopoietic stem cells may create new opportunities for the treatment of primary or radiation-induced marrow failures.     

The role of iron in oxygen radical mediated lipid peroxidation

The role of iron in the peroxidation of polyunsaturated fatty acids is reviewed, especially with respect to the involvement of oxygen radicals. The hydroxyl radical can be generated by a superoxide-driven Haber-Weiss reaction or by Fenton's reaction; and the hydroxyl radical can initiate lipid peroxidation. However, lipid peroxidation is frequently insensitive to hydroxyl radical scavengers or superoxide dismutase. We propose that the hydroxyl radical may not be involved in the peroxidation of membrane lipids, but instead lipid peroxidation requires both Fe2+ and Fe3+. The inability of superoxide dismutase to affect lipid peroxidation can be explained by the fact that the direct reduction of iron can occur, exemplified by rat liver microsomal NADPH-dependent lipid peroxidation. Catalase can be stimulatory, inhibitory or without affect because H2O2 may oxidize some Fe2+ to form the required Fe3+, or, alternatively, excess H2O2 may inhibit by excessive oxidation of the Fe2+. In an analogous manner reductants can form the initiating complex by reduction of Fe3+, but complete reduction would inhibit lipid peroxidation. All of these redox reactions would be influenced by iron chelation.