Effect of cholesterol on the uptake and intracellular degradation of liposomes by liver and spleen; a combined biochemical and gamma-ray perturbed angular correlation study

Molecular Rotational Correlation Times and Nanoviscosity Determined by 111m Cd Perturbed Angular Correlation (PAC) of γ-rays Spectroscopy

The nanoviscosity experienced by molecules in solution may be determined through measurement of the molecular rotational correlation time, τc , for example, by fluorescence and NMR spectroscopy. With this work, we apply PAC spectroscopy to determine the rate of rotational diffusion, λ=1/τc , of a de novo designed protein, TRIL12AL16C, in solutions with viscosities, ξ, from 1.7 to 88 mPa⋅s. TRIL12AL16C was selected as molecular probe because it exhibits minimal effects due to intramolecular dynamics and static line broadening, allowing for exclusive elucidation of molecular rotational diffusion. Diffusion rates determined by PAC data agree well with literature data from fluorescence and NMR spectroscopy, and scales linearly with 1/ξ in agreement with the Stokes-Einstein-Debye model. PAC experiments require only trace amounts (∼1011 ) of probe nuclei and can be conducted over a broad range of sample temperatures and pressures. Moreover, most materials are relatively transparent to γ-rays. Thus, PAC spectroscopy could find applications under circumstances where conventional techniques cannot be applied, spanning from the physics of liquids to in-vivo biochemistry.

Immunological and Toxicological Considerations for the Design of Liposomes

Liposomes hold great potential as gene and drug delivery vehicles due to their biocompatibility and modular properties, coupled with the major advantage of attenuating the risk of systemic toxicity from the encapsulated therapeutic agent. Decades of research have been dedicated to studying and optimizing liposomal formulations for a variety of medical applications, ranging from cancer therapeutics to analgesics. Some effort has also been made to elucidate the toxicities and immune responses that these drug formulations may elicit. Notably, intravenously injected liposomes can interact with plasma proteins, leading to opsonization, thereby altering the healthy cells they come into contact with during circulation and removal. Additionally, due to the pharmacokinetics of liposomes in circulation, drugs can end up sequestered in organs of the mononuclear phagocyte system, affecting liver and spleen function. Importantly, liposomal agents can also stimulate or suppress the immune system depending on their physiochemical properties, such as size, lipid composition, pegylation, and surface charge. Despite the surge in the clinical use of liposomal agents since 1995, there are still several drawbacks that limit their range of applications. This review presents a focused analysis of these limitations, with an emphasis on toxicity to healthy tissues and unfavorable immune responses, to shed light on key considerations that should be factored into the design and clinical use of liposomal formulations.

Equivalence-by-design: targeting in vivo drug delivery profile

In the United States (U.S.), drug products are considered therapeutically equivalent if they meet regulatory criteria of pharmaceutical equivalence and bioequivalence. These requirements can be traced back to 1977 when the U.S. Food and Drug Administration (FDA) published the regulations on bioavailability and bioequivalence. Over the years, to keep up with the advancement in science and technology, the FDA has been constantly updating the regulatory approaches to assessing and ensuring equivalence. In view of the recent growth in novel pharmaceutical dosage forms and delivery systems, this paper examines the current framework for documentation of therapeutic equivalence and explores the opportunities of further advancing equivalence methods for complex drug products. It is proposed that equivalence may be established by matching the in vivo drug delivery profile (iDDP) between drug products in comparison. This can be achieved by characterizing the iDDP of the reference formulation with application of an equivalence-by-design approach for pharmaceutical development. Critical variables can be identified to serve as in vitro markers or biomarkers for mapping the desired drug delivery profile in vivo. A multidisciplinary approach may be necessary to develop these markers for characterization of iDDPs.

Physical properties and pharmacological activity in vitro and in vivo of optimised liposomes prepared from a new cancerostatic alkylphospholipid

Liposomes from octadecyl-(1,1-dimethyl-4-piperidino-4-yl)-phosphate (OPP), a new alkylphospholipid derivative with an improved cancerostatic activity, were prepared for the first time and the activity in vitro and in vivo was characterised. The formation of liposomes (MLV, SUV and LUVET) differing in cholesterol content, charge, and sterical stabilisation is possible without serious problems, despite the lysolipid-like structure of the OPP. Liposomes with a low amount of cholesterol and with PEG2000DSPE-coating were the most stable OPP liposomes, both in buffer and in serum. The cytotoxicity of micellar or liposomal OPP against breast cancer cell lines in vitro was in the range of 20-60 microM. The cytotoxicity of the liposomal formulation was inversely related to the content of cholesterol, whereas the sterical stabilisation and/or the incorporation of a positive charge had only a very moderate modulating effect on the inhibition of cell proliferation. The strongest antitumour effect on the xenotransplanted breast cancer MT-3 in vivo was obtained with sterically stabilised OPP liposomes with low CH content. The beneficial therapeutic effect of these liposomes was accompanied by better tolerance and a significant inhibition of haemolysis compared to micellar OPP.

Serum-mediated recognition of liposomes by phagocytic cells of the reticuloendothelial system - The concept of tissue specificity

The mechanisms responsible for the clearance and recognition of intravenously injected liposomes by phagocytic cells of the reticuloendothelial system are presently not well understood. Here, we address a multiplicity of physicochemical and physiopathological factors which influence the clearance kinetics and tissue distribution of liposomes administered into the circulation. Emphasis is particularly drawn towards the concept of liposome-blood protein interaction and its role in liposome recognition by various phagocytes of the reticuloendothelial system.

Effect of sterical stabilization on macrophage uptake in vitro and on thickness of the fixed aqueous layer of liposomes made from alkylphosphocholines

A serious problem using liposomes for therapeutic purposes is the fast removal from blood circulation by components of the reticuloendothelial system (RES) most likely after opsonization of the vesicles. This study was performed to quantify the reduction in macrophage uptake in vitro of sterically stabilized liposomes (PEG-liposomes) prepared from hexadecylphosphocholine, cholesterol and poly(ethylene glycol2000) distearoylphosphoethanolamine (PEG2000DSPE) for the first time. The uptake was determined using HPC-liposomes of different defined size (125, 250 and 1000 nm) without and with sterical stabilization by incorporating 5 mol% of PEG2000DSPE. HPTS was used as fluorescence marker allowing the discrimination between general uptake and the part of liposomes internalized into the low pH-compartment (Daleke, L.D., Hong, K. and Papahadjopoulos. D. (1990) Biochim. Biophys. Acta 1024, 352-366). Liposomal uptake by J774 mouse macrophage-like cells was time-dependent. Both the uptake and internalization were clearly reduced for PEG-liposomes compared to plain liposomes. Sterical stabilization reduced the general uptake of liposomes in vitro by more than 50% and the internalization by about 50-60%. PEG-liposomes additionally showed a delay in internalization into the macrophages during the first 6 h. Size of used liposomes had only a minor influence on liposomal uptake but highest concentration of lipid was found for large multilammelar vesicles (MLV). The fixed aqueous layer thickness (FALT) was determined by zeta potential measurements of plain and sterically stabilised HPC-liposomes (100 nm) in solutions of different ion concentrations. The calculation of the thickness was based on the linear correlation between ln zeta (zeta-potential) and kappa (Debye Hückel-Parameter). FALT was calculated and found to be for plain HPC-liposomes 0.83 +/- 0.17 nm and for PEG-HPC-liposomes 3.57 +/- 0.17 nm. Exchange of the HPC by an alkylphospholipid with different head group has no or only minor effect (PEG-OPP-liposomes 3.44 +/- 0.31 nm). Thus the reduced uptake of HPC-LUVET correlates with an increased thickness of the fixed aqueous layer around these liposomes and could support the hypothesis that the thickness is an important property responsible for preventing opsonization and resulting finally in a reduced macrophage uptake.

Liposomal phosphatidylserine inhibits tumor cytotoxicity of liver macrophages induced by muramyl dipeptide and lipopolysaccharide

Liposomes can very efficiently deliver immunomodulators to macrophages so as to induce tumor cytotoxicity. Liposomes most widely used for that purpose contain negatively charged lipids, in particular phosphatidylserine (PS), to enhance liposome uptake by the macrophages. We investigated the effect of three negatively charged liposomal lipids on the in vitro activation of liver macrophages to tumor cytotoxicity by muramyl dipeptide (MDP) and lipopolysaccharide (LPS). Both MDP- and LPS-induced tumor cytotoxicity towards murine colon adenocarcinoma cells were strongly inhibited by PS-containing liposomes. Under comparable conditions phosphatidylglycerol (DPPG)-containing or dicetyl phosphate (DCP)-containing liposomes did not inhibit or only marginally inhibited the induction of tumor cytotoxicity. We did not observe PS-mediated inhibition of tumor cell toxicity when the exposure of the macrophages to PS-liposomes was limited to the 4-h activation period prior to addition of the tumor target cells, suggesting that the inhibitory effect is accomplished at the level of the later stages of the activation process. Previously, we showed that macrophages which are activated to tumor cytotoxicity during a 24-h incubation with MDP become refractory to a second activation with MDP. Now we observed that simultaneous incubation with PS-containing liposomes partially prevents this refractoriness, which is also compatible with an interfering action of PS at a relatively late stage in the activation process. We conclude that PS, despite its reported stimulatory effect on liposome uptake by macrophages, can seriously antagonize the effectiveness of immunomodulating agents acting on macrophages. This bears relevance to the use of PS-containing liposomes as a vehicle for such agents. The results are discussed in perspective of earlier reported pharmacological effects of PS and its metabolites.

Altered tissue-specific opsonic activities and opsono-recognition of liposomes in tumour-bearing rats

Reticuloendothelial phagocytic and serum opsonic activity was evaluated at terminal stages of tumour growth in rats transplanted subcutaneously with chondrosarcoma in an attempt to evaluate the role of opsonic protein(s) in governing liposome recognition and clearance by the macrophage system. The liver of the tumour-bearing animals manifested a decline in the uptake of multilamellar vesicles composed of egg phosphatidylcholine: cholesterol: dicetyl phosphate (mole ratio 7:2:1) from the blood when compared to healthy animals. In contrast, an increase in splenic clearance of liposomes was encountered in tumour-bearing rats. Studies with isolated liver non-parenchymal cells suggested that liposome recognition in both health and at terminal stages of cancer growth is influenced by a serum opsonin, which can be precipitated by 35-50% ammonium sulphate, as well as the concentration of calcium levels in serum. Serum of healthy animals equally enhanced liposome recognition by the hepatic macrophages of both normal and tumour-bearing rats. In contrast, both cell populations manifested poor liposome recognition in the presence of serum pooled from tumour-bearing animals and the results were comparable to the corresponding liposome-cell interaction in the absence of serum. The opsonic activity of serum derived from tumour-bearing rats could be demonstrated either by prior dialysis of serum against de-ionized water or by addition of EGTA. Liver phagocytes of healthy animals recognized more liposome in the presence of dialysed or EGTA-chelated tumour-serum than that of liver cells derived from tumour transplanted rats. A significant increase in serum calcium concentration was found in all tumour-bearing rats. When the concentration of calcium in the serum of normal animals was increased to the level that is encountered in tumour-bearing rats, a sharp drop in liposome recognition by liver phagocytes was observed. This drop in opsonic activity was not related to changes in the ionic strength of serum. The ammonium sulphate precipitated opsonin was also calcium-sensitive and its opsonic activity was abolished in the presence of calcium. Studies with isolated splenic phagocytes suggested that an increase in the opsonic activity of serum, but not the elevated calcium level, was responsible for hyperphagocytosis of liposomes by the splenic phagocytes of tumour-transplanted animals. The opsonic molecule which enhanced liposome recognition by liver non-parenchymal cells failed to enhance liposome clearance by the splenic phagocytes. These findings suggest that the alteration in macrophage clearance of liposomes during the terminal growth of cancer may be mediated in part by changes in the opsonic capacity of serum.

Preparation and properties of sterically stabilized hexadecylphosphocholine (miltefosine)-liposomes and influence of this modification on macrophage activation

The aim of the study was to investigate for the first time the preparation, physical properties and macrophage activating effect of sterically stabilized liposomes made from hexadecylphosphocholine (HPC, Miltefosine) using different poly(ethylene glycol) lipids for coating. We could demonstrate that it is possible to prepare different liposomal vesicle types (MLV, SUV and LUVET) without any problem and with a high stability in buffer (release of hydrophilic marker was < 5% after half a year) and in plasma (t1/2 up to several days). The preparation method, including size of polycarbon membrane filter used for the preparation of LUVETs had the main influence on vesicle size and size distribution. The addition of a charged lipid like DCP and different amounts of PEG-lipid up to 10% had no effect on size and stability of PEG-LUVETs. A comparison of activating potency of PEG-HPC-vesicles with commonly used HPC-liposomes was performed with mouse peritoneal macrophages. HPC-liposomes induced a clear release of NO and TNF from mouse peritoneal macrophages especially in a synergistical action with LPS. On the contrary the effect of PEG-liposomes was similar to control cells after a combined activation in vivo/in vitro. The reduced interaction of these liposomes with the MPS was also demonstrated by an unchanged carbon ink uptake after treatment of mice (i.p.) with liposomes prepared with and without PEG-lipid. PEG-HPC-liposomes combine the advantages of HPC, liposomes and PEG-coating, resulting in a promising preparation for treatment of mammary cancers.

Plasma protein adsorption patterns on liposomes: establishment of analytical procedure

After intravenous (i.v.) injection, colloidal drug carriers such as liposomes, emulsions, polymeric or solid lipid nanoparticles immediately interact with plasma proteins. The adsorbed plasma protein patterns depend on physico-chemical characteristics of the carriers' surface and are regarded as a key factor for the in vivo behavior of the carriers. The comprehension of the correlation between protein adsorption and in vivo organ distribution can be utilized to obtain drug targeting to different tissues. Carriers with different protein adsorption patterns will interact with different tissue-specific receptors or will be recognized by different macrophage subpopulations. Two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) was applied to determine the protein adsorption patterns on polystyrene particles as model carriers. To transfer this analytical method to i.v. injectable colloidal carriers such as liposomes, a new sample preparation method was developed. The separation of liposomes from plasma after incubation was achieved by gel filtration using a Sepharose 2B column. This technique allowed a mild separation independent from eluent composition and only according to size differences. Possible protein desorption from the liposomes and adsorption onto the gel were minimized by using an eluent with a sufficiently high ionic strength. To estimate the efficiency of separation, the content of liposomes and plasma in each fraction being eluted was determined by ultraviolet (UV) spectroscopy. With this new separation method plasma protein adsorption patterns on liposomes could be analyzed for the first time. The sample preparation by gel filtration seemed to have no influence on liposome stability as far as size distribution is concerned.

Binding and uptake of liposomes containing a poly(ethylene glycol) derivative of cholesterol (stealth liposomes) by the macrophage cell line J774: influence of PEG content and its molecular weight

The binding and intake of liposomes containing a different molar content and chain length of a PEG-Chol derivative had been studied in cultured macrophage cell line J774. The decrease in binding and endocytosis of the liposomes containing PEG-Chol is dependent on (i) the PEG chain length, (ii) the molar content of the surfactant, (iii) the liposome concentration in the external medium. The best results in reducing the uptake of liposomes were obtained by a PEG-Chol liposome suspension with a high molar content (25%) which presents a non negligible amount of free PEG-Chol. Moreover, we could show an increase by 2 for binding and by about 5 for endocytosis of filtrated-liposomes containing 25 mol% of 8800PEG-Chol, in the absence of free PEG-Chol in the suspension. Binding and intake of control liposomes was also inhibited in the presence of free PEG-Chol. Fluid phase endocytosis of SRh was inhibited up to 45% of control in the presence of liposomes containing PEG-Chol or free PEG-Chol. Based on the comparison of 4400PEG-Chol with the most commonly used PEG derivative 5000PEG-PE, PEG-Chol is more powerful in terms of reducing their binding and endocytosis by J774 cells. Inhibition of the fluid phase endocytic process is attributed to the binding of PEG-Chol to the cells' plasma membrane inducing a decrease in surface hydrophobicity of the cells, resulting in a marked decrease in the extent of phagocytic ingestion.

Surface-modified liposomes: assessment and characterization for increased stability and prolonged blood circulation

Advances in therapeutic applications of liposomes have been achieved through surface modifications increasing their biological stability: reduced constituent exchange and leakage as well as reduced unwanted uptake by cells of the mononuclear phagocytic system. The recent conclusions obtained from in vivo and in vitro studies are reviewed with an emphasis on evaluating the methods used and thus the kinds of conclusions which can be drawn. A number of issues are raised as to the limitations of the methods employed. Steric stabilization, meaning reduction in particle interactions by a surface steric barrier, has been proposed as a theoretical basis for the results and some of the initial results testing this hypothesis are reviewed here with respect to identification of the extent to which physical properties of the surface coatings correlate with the biological properties. At this time it seems that no one method is ideal so that multiple measures give the best characterization.

Conversion of liposomal 5-fluoro-2'-deoxyuridine and its dipalmitoyl derivative to bile acid conjugates of alpha-fluoro-beta-alanine and their excretion into rat bile

We studied the hepatic processing and biliary excretion of metabolites of the radiolabeled cytostatic agent 5-fluoro,-2'-deoxy[6-3H]uridine (FUdR) and its lipophilic derivative FUdR-dipalmitate incorporated in liposomes. After intracardial injection in rats, free FUdR was cleared from the circulation within minutes. When FUdR or FUdR-dipalmitate was encapsulated in multilamellar vesicles (MLVs) composed of distearoylphosphatidylcholine/dipalmitoylphosphatidylglycerol (DSPC/DPPG/CHOL, 10:1), as expected, the clearance of 3H label was substantially delayed; incorporation of 50 mol% cholesterol in the liposomal bilayer caused a 2-fold further reduction in elimination rate. Incorporation of FUdR-dipalmitate in small unilamellar vesicles (SUV) of similar composition produced a several-fold further decrease in elimination rate: more than 40% of the injected dose was still circulating after 6 h. The plasma concentration of free FUdR after administration of liposomal FUdR-dipalmitate was below the detection limit (5 x 10(-8) M) at any time. Although only about 9% of the administered radioactivity was excreted into the bile within 48 h after injection of [3H]FUdR, a rapid initial excretion rate was observed (4% of the injected dose in the first 2 h). The bile-associated radioactivity was identified mainly as the catabolite alpha-fluoro-beta-alanine (FBAL), conjugated with the three major bile acid species present in rat bile, i.e., muricholic acid, cholic acid and chenodeoxycholic acid in a ratio of 1:3:1. Liposome incorporation of FUdR or FUdR-dipalmitate did not affect the nature of the excretory products but caused a significant decrease in the initial rate at which label appeared in the bile (< 2% in 6 h).

67Gallium-labeled liposomes with prolonged circulation: preparation and potential as nuclear imaging agents

A method is described for 67Ga-labeling liposomes containing a polyethylene glycol coating which exhibit prolonged blood circulation, reduced liver and spleen uptake and accumulation in tumors. Applications as agents for diagnostic imaging and delivery of therapeutic agents are considered. Previous methods were adapted to compensate for the presence of low temperature phase transition phospholipids resulting in consistent loading with low levels of residual unentrapped label.

Tumor accumulation of novel RES-avoiding liposomes

For passive targeting of liposomes to tumor tissues, we earlier developed reticuloendothelial system (RES)-avoiding liposomes modified with a uronic acid derivative, palmityl-D-glucuronide (PGlcUA) (Namba, Y., Sakakibara, T., Masada, M., Ito, F. and Oku, N. (1990) Chem. Pharm. Bull. 38, 1663-1666). In this present study, we examined the blood clearance and biodistribution of PGlcUA-liposomes (dipalmitoylphosphatidylcholine/cholesterol/PGlcUA = 40:40:20 as a molar ratio) in normal and tumor-bearing mice. Liposomes containing dipalmitoylphosphatidylglycerol (DPPG) instead of PGlcUA was also examined as a control. When [3H]inulin-encapsulated PGlcUA-liposomes and DPPG-liposomes were intravenously injected into normal mice, approx. 50% of the 3H radioactivity was recovered from the liver, the bulk of RES, at 12 h after administration of DPPG-liposomes, while only approx. 20% of it was found there when PGlcUA-liposomes were administered. Radioactivity remaining in the plasma at 12 h after injection was 5-fold higher when PGlcUA-liposomes were injected than when DPPG-liposomes were used. Biodistribution of liposomes in tumor-bearing mice was also examined. Mice were inoculated with 10(7) S180 cells into the hind leg. After 1 week, liposomes were injected. Radioactivity of [3H]inulin originally encapsulated in the PGlcUA-liposomes accumulated in the tumor to an extent 3-4-fold higher than that of the marker in DPPG-liposomes. Liver/tumor ratio of the radioactivity was 12 for DPPG-liposomes and only 2 for PGlcUA-liposomes. This latter value is the lowest of various liposome formulations ever reported.

Lateral interactions of pig apolipoprotein A-1 with egg yolk phosphatidylcholine and with cholesterol in mixed monolayers at the triolein-saline interface

Interfacial tensions of egg yolk phosphatidylcholine (PC) and cholesterol monolayers adsorbed at the triolein-saline interface were measured in the presence and absence of pig apolipoprotein A-1 (apoA-1) in the saline phase. In the absence of apoA-1, the adsorptions of PC and cholesterol at the interface from the triolein phase are cooperative, showing large lateral attractive interactions between the PC molecules and the cholesterol molecules in the monolayer. In the presence of apoA-1, the PC adsorption is anti-cooperative, indicating strong lateral attractive interactions between the PC and the apoA-1 molecules, i.e., apparently, repulsive lateral interactions between the PC molecules. On the other hand, lateral interactions of very low magnitude are observed between the cholesterol and apoA-1 molecules in the monolayer. Values of the lateral interaction energy are evaluated from the adsorption data by the Defay-Prigogine-Flory theory of monolayers. The large difference in lateral interaction energy with apoA-1 between PC and cholesterol in a mixed monolayer is discussed in connection with current problems in lipoprotein catabolism: reverse cholesterol transport, alterations in affinity of lipid particles to apoA-1, and formation of high-density lipoproteins and abnormal lipoproteins.

Specific accumulation of cholesterol-rich liposomes in the inflammatory tissue of rats with adjuvant arthritis

High performance liquid chromatography has shown that after intravenous injection cholesterol-poor liposomes (100 nm) are unstable and their phospholipid is redistributed. Under identical conditions cholesterol-rich liposomes remain structurally intact within the circulation. When injected intravenously cholesterol-rich liposomes accumulate within the inflamed paws of rats with adjuvant induced arthritis to the same extent as cholesterol-poor liposomes. Uptake in inflamed tissue of three cholesterol-rich liposome preparations was always significantly greater than the uptake noted in normal tissue. The degree of accumulation in inflamed tissue was found to depend on the size of the liposome, with the greatest uptake, 7% of the injected dose, achieved by the smallest vesicle (100 nm). These results indicate that intact liposomes accumulate at inflamed joint tissue sites. Therefore the passive targeting of anti-inflammatory drugs encapsulated within these liposomes could be contemplated.