Trypsin induced destabilization of liposomes composed of dioleoylphosphatidylethanolamine and glycophorin

Destabilization of liposomes composed of phosphatidylethanolamine (PE) and purified glycophorin of human erythrocytes was studied with the release of an entrapped fluorescent dye, calcein. Proteolytic cleavage of liposomes by trypsin induced a rapid increase of turbidity and the leakage of calcein from the liposomes. Kinetic experiments indicated that the destabilization was a second order reaction, i.e. it required liposome collision. Using N-(7-nitro-2,1,3-benzoxadiazol-4-yl) PE as a fluorescent probe for the formation of hexagonal phase of PE, tryptic digestion of the liposomes resulted in a higher tendency of the PE bilayer to transform into the hexagonal phase. We propose that hexagonal (or inverted micellar) structures are involved in the trypsin induced liposome destabilization.     

Biodistribution and anticancer activity of a new vinca alkaloid encapsulated into long-circulating liposomes

S12363 is a potent therapeutic agent with a strong in vitro activity against a variety of tumor types but also a high in vivo toxicity. Loading of this drug into long-circulating liposomes is expected to enhance its therapeutic index. Pharmacokinetics of liposomal S12363 showed that circulating S12363 was entrapped into liposomes until 24 hours after intravenous injection in mice. The liposomal formulation significantly increased the plasma concentration, half-life, and AUC and decreased the plasma clearance rates and volume of distribution of S12363. Liposome extravasation was evaluated with two tumor models by both microscopic analysis and liposome radiolabeling. Liposome accumulation was much more important in the case of B16 melanoma, compared to H460 tumor, with both inoculated subcutaneously and with comparable size. H460 tumor was also inoculated into the lung. The tumor localization did not influence liposome accumulation into the tissue. The liposomal formulation injected into mice bearing B16 melanoma allowed a 10-fold accumulation of S12363 into the tumor interstitium, as compared to the solution. Bioluminescence data, supported by the survival curves of the animals, showed that S12363-liposomes were able to significantly restrict B16 melanoma progression and increase mice survival.     

Interactions of serum proteins with small unilamellar liposomes composed of dioleoylphosphatidylethanolamine and oleic acid: high-density lipoprotein, apolipoprotein A1, and amphipathic peptides stabilize liposomes

Small unilamellar liposomes composed to dioleoylphosphatidylethanolamine (DOPE) and oleic acid (OA) are stabilized by incubation with normal human serum or plasma [Liu, D., & Huang, L. (1989) Biochemistry 28, 7700-7707]. The present report describes a systematic study of interactions of purified serum proteins and lipoproteins with these liposomes. Albumin destabilized liposomes by extracting OA from the liposomes, whereas immunoglobulins and lipoproteins (HDL, LDL, and VLDL) had no effect. However, HDL and, to some extent, VLDL showed a rapid stabilization activity against the lytic effect of albumin. HDL added together with or shortly after the addition of albumin completely abolished the liposome leakage and aggregation effects induced by albumin. SDS-PAGE analysis of the HDL-stabilized liposomes revealed that apolipoprotein A1 was associated with liposomes. Purified apolipoprotein A1, but not a lipid mixture resembling the lipid composition of HDL, showed comparable liposome stabilization activity as HDL. Furthermore, synthetic peptides resembling the amphipathic helices found in apolipoprotein A1 also showed strong liposome stabilization activity. Peptides which were able to form amphipathic helixes of a wedge shape were more effective stabilizers than those which could not. These data indicate that HDL plays a major role in human serum or plasma for the liposome stabilization activity. HDL exerts its activity probably by the interactions of the amphipathic helices of apolipoprotein A1 with the hydrophobic voids found on the outer surface of the highly curved, small liposomes.     

Biodistribution study of doxorubicin encapsulated in liposomes: influence of peptide coating and lipid composition

In this paper we describe the biodistribution of doxorubicin (DXR) encapsulated in three different types of liposomes. Common composition was hydrogenated phosphatidylcholine (HPC)/phosphatidylglycerol (PG) cholesterol (Chol)/X, X being either 10% N-glutaryl phosphatidylethanolamine (NGPE), 10% NGPE + 6% distearoyl-phosphatidylethanolamine-polyethyleneglycol 2000 (DSPE-PEG), or 10% NGPE + 6% DSPE-PEG-COOH. These series of vesicles were coated with an active or an inactive sequence of laminin (laminin receptors, integrins, are overexpressed in tumor cells). Single doses of these preparations were injected, i.v., into healthy mice. For biodistribution experiments, mice were sacrificed at three different time-points post-treatment. Doxorubicin and doxorubicinol (DXOH) levels were determined in plasma, heart, lung, kidney, spleen, and liver using HPLC with daunorubicin (DNR) as internal standard. The results obtained indicate that compositions containing DSPE-PEG have the longest half-lives in plasma, as was to be expected according to the data in the literature. However, the presence of the peptides on the surface of liposomes reduces concentration values in this tissue. Distribution in other organs reveals high differences, among the liposomal samples studied, depending mainly on the presence of active or inactive peptide on the surface of vesicles. Liposomes coated with the laminin active sequence show lower accumulation in studied tissues than free DXR. This indicates that heart toxicity, associated to DXR treatments, could be diminished, and open promising perspectives for its future study in tumor-bearing animals.     

Small, but not large, unilamellar liposomes composed of dioleoylphosphatidylethanolamine and oleic acid can be stabilized by human plasma

Small unilamellar liposomes, composed of dioleoylphosphatidylethanolamine (DOPE) and oleic acid (OA), prepared by sonication, were incubated in the presence of human plasma at 37 degrees C. The release of entrapped calcein after 8-h incubation was about 15% in plasma, compared with about 70% in phosphate-buffered saline under the same conditions. In contrast, dioleoylphosphatidylcholine (DOPC)/OA liposomes under the same conditions release about 70% in plasma and only 10% in PBS. Total release of calcein from the DOPE/OA liposomes was observed in a PBS solution containing bovine serum albumin, and the release was completely blocked by preincubation of the liposomes with plasma. These results indicate that the unstable DOPE/OA liposomes are stabilized by incubation with plasma. The stabilization process was very fast, being completed within 1 min. Only relatively small liposomes (d less than or equal to 200 nm) were completely stabilized by plasma; larger liposomes were progressively less stabilizable. SDS-polyacrylamide gel electrophoresis of liposomes which had been incubated with plasma and then washed indicated that several proteins were tightly associated with liposomes. Using liposomes containing [14C]OA, it was found that about 70% of the original OA was extracted after 1-h incubation with human plasma at 37 degrees C. Thin-layer chromatographic analysis of the plasma-treated liposomes showed the presence of the plasma lipids in the liposomes. These results suggest that liposomes composed of DOPE/OA are stabilized by protein and/or lipid components from human plasma and that the composition of the liposomes is altered. The mechanism of stabilization is discussed in terms of the surface pressure of small vesicles with a high degree of curvature.(ABSTRACT TRUNCATED AT 250 WORDS)     

Biodistribution and clearance of intra-articular liposomes in a large animal model using a radiographic marker

The intra-articular (IA) route of administration in treating arthritis has potential for targeting drug delivery to affected tissues, thereby minimising the attendant side-effects of systemically administered drugs. The ultra-structure of the synovium however facilitates rapid drug efflux from the joint; effectively the IA route is equivalent to other non-IV parenteral routes with regards absorption and redistribution into the systemic circulation. The aim of this study was to extend the drug residence time within the knee joint by using a liposome formulation. DPPC-based liposomes were prepared with the radio contrast agent iohexol as a drug marker. 8 sheep had their right knees injected IA with iohexol liposomes and the contralateral joints with either free iohexol or empty liposomes. Joints were radiographed at multiple time points up to 16 days post-injection. Iohexol-mediated radiopacity was quantified by densitometer. Sheep were sacrificed at the end of the study for microscopy of synovial tissues. Good visualization of iohexol-mediated radiopacity with fine anatomical definition was possible throughout the experiment. Also evident on the films was extra-articular radiopacity with liposomes tracking along muscle facial planes. Cellular and tissue localization with light microscopy was possible through use of frozen sections and because of the large liposome size. Residence of encapsulated iohexol within the knee joint was greatly prolonged. Liposomal iohexol declined bi-exponentially with a terminal elimination half-life of 134 hours. In contrast, free iohexol was undetectable at 3 hours post-injection.     

Energetics of a hexagonal-lamellar-hexagonal-phase transition sequence in dioleoylphosphatidylethanolamine membranes.

The phase diagram of DOPE/water dispersions was investigated by NMR and X-ray diffraction in the water concentration range from 2 to 20 water molecules per lipid and in the temperature range from -5 to +50 degrees C. At temperatures above 22 degrees C, the dispersions form an inverse (HII) phase at all water concentrations. Below 25 degrees C, an HII phase occurs at high water concentrations, an L alpha phase is formed at intermediate water concentrations, and finally the system switches back to an HII phase at low water concentrations. The enthalpy of the L alpha-HII-phase transition is +0.3 kcal/mol as measured by differential scanning calorimetry. Using 31P and 2H NMR and X-ray diffraction, we measured the trapped water volumes in HII and L alpha phases as a function of osmotic pressure. The change of the HII-phase free energy as a function of hydration was calculated by integrating the osmotic pressure vs trapped water volume curve. The phase diagram calculated on the basis of the known enthalpy of transition and the osmotic pressure vs water volume curves is in good agreement with the measured one. The HII-L alpha-HII double-phase transition at temperatures below 22 degrees C can be shown to be a consequence of (i) the greater degree of hydration of the HII phase in excess water and (ii) the relative sensitivities with which the lamellar and hexagonal phases dehydrate with increasing osmotic pressure. These results demonstrate the usefulness of osmotic stress measurements to understand lipid-phase diagrams.     

The relipidation of delipidated Na,K-ATPase. An analysis of complex formation with dioleoylphosphatidylcholine and with dioleoylphosphatidylethanolamine.

Enzymatically inactive, delipidated Na,K-ATPase from dogfish rectal glands was titrated with dioleoylphosphatidylcholine and with dioleoylphosphatidylethanolamine. The process of relipidation has the following characteristic properties. Enzymatic activities reappear independently of each other: first the phosphatase, then the ATPase. The properties of the phosphatase regenerated depend on the ratio of lipid/protein used; the ATPase seems to be independent of this ratio. The simplest model that is consistent with the above results and with the shapes of the titration curves, has the following requirements. Firstly, the enzyme is composed of two subunits that, as far as lipid binding is concerned, are identical and independent of each other. Secondly, lipid adds onto the enzyme as preformed clumps of 25 molecules of phosphatidylcholine or 18 molecules of phosphatidylethanolamine. Thirdly, each subunit binds two clumps of lipid, and binding shows positive cooperativity. Fourthly, when either subunit becomes saturated with lipid, the enzyme exhibits one form of phosphatase. Fifthly, when both subunits are saturated with lipid, the enzyme exhibits a second form of phosphatase and ATPase. The data and their analysis according to this model lead to the suggestion that Na,K-ATPase is a functional dimer, the interaction between subunits being influenced by the Na+ and K+ concentrations in the medium: K+ favouring the functional independence of the subunits and Na+ favouring their functional interaction.     

Effect of unsaturation on the chain order of phosphatidylcholines in a dioleoylphosphatidylethanolamine matrix.

The properties of phosphatidylcholines (PCs) having a perdeuterated stearic acid, 18:0d35, in the sn-1 position and the fatty acid 18:0, 18:1 omega 9, 18:2 omega 6, 18:3 omega 3, 20:4 omega 6, 20:5 omega 3, or 22:6 omega 3 at the sn-2 position were investigated in a matrix of dioleoylphosphatidylethanolamine (DOPE) by 2H and 31P NMR spectroscopy. At a mole ratio of DOPE/PC = 5:1, the lipids form liquid crystalline lamellar phases below 40 degrees C and coexisting lamellar, inverse hexagonal (Hll), and cubic phases at higher temperatures. The sn-1 chain of the PCs in a DOPE matrix is appreciably more ordered than in pure PCs, corresponding to an increase in the hydrophobic bilayer thickness of approximately 1 A. Distearoylphosphatidylcholine in the DOPE matrix has a higher sn-1 chain order than the unsaturated PCs. We observed distinct differences in the lipid order of upper and lower sections of the hydrocarbon chains caused by changes of temperature, unsaturation, headgroups, and ethanol. Unsaturation lowers chain order, mostly in the lower third of the hydrocarbon chains. By contrast, the increase in chain order caused by the DOPE matrix and the decrease in order with increasing temperature have a constant magnitude for the upper two-thirds of the chain and are smaller for the lower third. Addition of 2 M ethanol reduced order parameters, in effect reversing the increase in chain order caused by the DOPE matrix.     

Bilayer-stabilizing properties of myelin basic protein in dioleoylphosphatidylethanolamine systems

31P-NMR and X-ray diffraction techniques are used to study the comparative ability of myelin basic protein (MBP) vs. other basic proteins to convert hexagonal (HII) phases to stable lamellar (L alpha) structures. Pure dioleoylphosphatidylethanolamine (DOPE) at pH 9 and 7, and mixtures of DOPE/phosphatidylserine (PS) (95:5 and 80:20% w/w) at pH 7 were employed for this investigation. The polymorphic behavior of the lipid suspensions was evaluated in the presence and absence of several basic proteins (MBP, calf thymus histone, lysozyme, melittin) and the cationic polypeptide, polylysine (PL). Each of the proteins and PL was capable of binding the pure DOPE HII phase at pH 9 but with varying morphological consequences, i.e., lamellar stabilization (MBP, histone, PL), formation of new protein-DOPE HII phases (lysozyme) or lipid disordering/vesiculation (melittin). Reduction to pH 7 resulted in the dissociation of protein from DOPE - with the exception of melittin - and the reformation of a pure lipid HII phase. Additions of PS to DOPE at pH 7 facilitated protein binding, but among the proteins examined, only MBP was capable of converting the lipid suspension into a stable multilamellar form. Differences in the lipid morphology produced by each protein are discussed in terms of protein physicochemical characteristics. In addition, a possible relationship between MBP-lipid interactions and the stability of myelin sheath lipid multilayers is inferred from the significant bilayer-stabilizing capacity of MBP.     

Observation of inverted cubic phase in hydrated dioleoylphosphatidylethanolamine membranes

We report the observation of an inverted cubic phase in aqueous dispersions of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) by small-angle X-ray diffraction. DOPE is a paradigm in the study of nonlamellar phases in biological systems: it exhibits a well-known phase transition from the lamellar (L alpha) to the inverted hexagonal phase (HII) as the temperature is raised. The transition is observed to occur rapidly when a DOPE dispersion is heated from 2 degrees C, where the L alpha phase is stable, to 15 degrees C, where the HII phase is stable. We report on the induction of a crystallographically well-defined cubic lattice that is slowly formed when the lipid dispersion is rapidly cycled between -5 and 15 degrees C hundreds of times. Once formed, the cubic lattice is stable at 4 degrees C for several weeks and exhibits the same remarkable metastability that characterizes other cubic phases in lipid-water systems. X-ray diffraction indicates that the cubic lattice is most consistent with either the Pn3m or Pn3 space group. Tests of lipid purity after induction of the cubic indicate the lipid is at least 98% pure. The cubic lattice can be destroyed and the system reset by cycling the specimen several times between -30 and 2 degrees C. The kinetics of the formation of the cubic are dependent on the thermal history of the sample, overall water concentration, and the extreme temperatures of the cycle.(ABSTRACT TRUNCATED AT 250 WORDS)     

X-ray diffraction study of the polymorphic behavior of N-methylated dioleoylphosphatidylethanolamine

The polymorphic phase behavior of aqueous dispersions of dioleoylphosphatidylethanolamine (DOPE) and its N-methylated analogues, DOPE-Me, DOPE-Me2, and DOPC, has been investigated by X-ray diffraction. In the fully hydrated lamellar (L alpha) phase at 2 degrees C, the major structural difference is a large increase in the interlamellar water width from DOPE to DOPE-Me, with minor increases with successive methylation. Consistent with earlier reports, inverted hexagonal (HII) phases are observed upon heating at 5-10 degrees C in DOPE and at 65-75 degrees C in DOPE-Me and are not observed to at least 85 degrees C in DOPE-Me2 or DOPC. In DOPE, the L alpha-HII transition is facile and is characterized by a relatively narrow temperature range of coexistence of L alpha and HII domains, each with long-range order. DOPE-Me exhibits complex nonequilibrium behavior below the occurrence of the HII phase: Upon heating, the L alpha lattice spontaneously disorders on a time scale of days; on cooling from the HII phase, the disorder rises on a time scale of minutes. It is shown that, in copious water, the disordered state transforms very slowly into phases with cubic symmetry. This process is assisted by the generation of small amounts of lipid degradation products. The relative magnitudes of the monolayer spontaneous radius of curvature, R0 [Kirk, G. L., Gruner, S. M., & Stein, D. L. (1984) Biochemistry 23, 1093; Gruner, S. M. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 3665], are inferred from the HII lattice spacings vs temperature and are shown to increase with increasing methylation. The relative magnitudes of R0 are categorized as small for DOPE, intermediate for DOPE-Me, and large for DOPC. It is suggested, and examples are used to illustrate, that small R0 lipid systems exhibit facile, low-temperature L alpha-HII transitions, intermediate R0 systems exhibit complex nonequilibrium transition behavior and are likely to form cubic phases, and large R0 systems are stable as L alpha phases. The relationship between the cubic phases and minimal periodic surfaces is discussed. It is suggested that minimal periodic surfaces represent geometries in which near constant, intermediate R0 values can be obtained concomitantly with monolayers of near constant thickness, thereby leading to equilibrium cubic phases. Thus, the relative magnitude of the spontaneous radius of curvature may be used to predict mesomorphic behavior.(ABSTRACT TRUNCATED AT 400 WORDS)     

Synthesis and characterization of NVOC-DOPE, a caged photoactivatable derivative of dioleoylphosphatidylethanolamine

A caged, photocleavable derivative of dioleoylphosphatidylethanolamine (DOPE) called NVOC-DOPE was prepared by reaction of DOPE with 6-nitroveratryloxycarbonyl chloride. In contrast to egg phosphatidylethanolamine (EPE), NVOC-DOPE or its 1:1 mixture with EPE forms liposomes at both pH 7.4 and 5.0. Photolysis (lambda > 300 nm) of aqueous liposomal dispersions of NVOC-DOPE at pH 9.0, 7.4, or 5.0 results in complete conversion to DOPE and subsequent release of entrapped calcein dye. The temporal and spatial control associated with the photorelease technique suggests that NVOC-DOPE can be used to study a range of important dynamic membrane processes such as membrane fusion and the action of membrane-associated enzymes.     

Photoinduced destabilization of liposomes.

The stability of two-component liposomes composed of the polymerizable 1,2-bis-[10-(2',4'-hexadienoyloxy)decanoyl]-sn-glycero-3-phosphati dylcholine (SorbPC) and either a phosphatidylethanolamine (PE) or a phosphatidylcholine (PC) were examined via fluorescence leakage assays. Ultraviolet light exposure of SorbPC-containing liposomes forms poly-SorbPC, which phase separates from the remaining monomeric lipids. If the nonpolymerizable lipids are PE's, then the photoinduced polymerization destabilizes the liposome with loss of aqueous contents. The permeability of the control dioleoylPC/SorbPC membranes was not affected by photopolymerization of SorbPC. The photodestabilization of dioleoylPE/SorbPC (3:1) liposomes required the presence of oligolamellar liposomes. NMR spectroscopy of extended bilayers of dioleoylPE/SorbPC (3:1) showed that the photopolymerization lowers the temperature for the appearance of 31P NMR signals due to the formation of isotropically symmetric lipid structures. These observations suggest the following model for the photoinduced destabilization of liposomes composed of PE/SorbPC; photopolymerization induced phase separation with the formation of enriched domains of PE, which allows the close approach of apposed regions of enriched PE lamellae and permits the formation of an isotropically symmetric structure between the lamellae. The formation of such an interlamellar attachment (ILA) between the lamellae of an oligolamellar liposome provides a permeability pathway for the light-stimulated leakage of entrapped water-soluble reagents.     

Effect of urea, dimethylurea, and tetramethylurea on the phase behavior of dioleoylphosphatidylethanolamine

The phase behavior of dioleoylphosphatidylethanolamine in aqueous solutions of urea, N,N'-dimethylurea (DMU), and N,N,N',N'-tetramethylurea (TMU) has been characterized by synchrotron X-ray diffraction and differential scanning calorimetry. All three solutes stabilize the lamellar liquid-crystalline phase at the expense of lamellar-gel phase and inverted hexagonal phase of the phospholipid when present in concentrations up to 3 M. X-ray diffraction data demonstrated that the repeat spacing of DOPE increased with increasing urea concentration, but decreased as the DMU and TMU concentrations increased. The repeat spacing of DOPE in the liquid-crystal phase dispersed in the three solutes is d(urea)>d(DMU)>d(TMU). The molecular mechanisms underlying these observations are discussed in terms of either membrane Hofmeister effect, where urea acts as a water structure breaker, or a direct insertion effect of the amphiphilic DMU and TMU molecules into the lipid head groups in the interfacial region of the phospholipid bilayer.     

Biodistribution of phosphodiester and phosphorothioate siRNA

Short interfering RNAs (siRNAs) are valuable tools for analyzing protein function in mammalian cell culture. This success has led to high expectations for in vivo and therapeutic applications. However, the pharmacokinetic properties of siRNA are not known. Here we report the biodistribution of a phosphodiester (PO) siRNA duplex and examine the effect of phosphorothioate (PS) linkages. Our findings indicate that biodistribution of siRNA is similar to that for single-stranded antisense oligonucleotides and offer insights for use of siRNA in vivo.     

Biodistribution of photosensitizing agents

• 1.1. The features of neoplasia which predict for drug responsiveness are rapid growth and/or inefficient repair of damage, especially to DNA.
• 2.2. PDT has the advantage of yielding responses regardless of the growth fraction of a tumor, and repair appears to play only a minor role.
• 3.3. While an entirely different spectrum of tumors can be targeted with PDT, the perhaps unavoidable accompaniment is that a new set of rules for efficacy will need to be established.
• 4.4. The selectivity of PDT is based on the need for irradiation which can be directed, along with the short tissue half-life of the cytotoxic product, singlet oxygen. Sensitizers which target specific cellular organelles could promote PDT efficacy, if in vitro data (Woodbum et at., 1992b Photochem. Photobiol. 55, 697–704) can be translated into clinical practice.
• 5.5. It remains to be established whether total drug distribution to neoplastic tissues or concentration in specific sub-cellular sites is the more important factor.
• 6.6. Questions relating to the role of biodistribution as a factor in efficacy of PDT sensitizers of photosensitizers remain to be explored. Just as the political cartographers are grappling with changes in territorial boundaries of known lands, we continue to clarify the rules relating to PDT boundaries. In this regard, it is clearly important for determinants of pharmacokinetics and biodistribution to be evaluated and understood.
• 7.7. Once clinical reports on the “second generation” agents are published, we may get a better picture, although it is not unusual for clinical reports to raise more questions than they answer.
• 8.8. It seems safe to conclude that there is nothing “magic” about HPD, and that additional efficacious photosensitizers for PDT can be produced.
• 9.9. If we find that a wide variety of different structures are useful in the clinic, a likely conclusion is that there are multiple mechanisms of tumor localization. Since the nature of neoplasia is so diverse, this possibility should not be surprising.
• 10.10. In view of the finding, cited above, that liposomes show about the same degree of tumor selectivity as does Photofrin, it may be feasible to target any efficient photosensitizer for neoplastic tissues by development of an appropriate delivery system.

     

Bending, hydration and interstitial energies quantitatively account for the hexagonal-lamellar-hexagonal reentrant phase transition in dioleoylphosphatidylethanolamine.

We have accounted for the unusual structural change wherein dioleoylphosphatidylethanolamine undergoes a hexagonal-lamellar-hexagonal transition sequence as the water content is reduced systematically. We describe the role played by the energies of bending, hydration, voids in hexagonal interstices, and van der Waals interaction in this transition sequence. We have used the X-ray diffraction and osmotic stress experiments on the two phases to derive the structural parameters and all of the force constants defining the energetics of the hexagonal and lamellar phases. We have calculated the chemical potentials of lipid and water in both phases and derived the phase diagram of the lipid with no free, adjustable parameters. The calculated temperature/osmotic stress and temperature/composition diagrams quantitatively agree with experiment. The reentrant transition appears to be driven by a delicate balance between the hydration energy in the lamellar phase and bending energy in the hexagonal phase, whereas the energy of voids in hexagonal interstices defines its energy scale and temperature range. Van der Waals attraction between the bilayers in the lamellar phase does not appear to be important in this transition.