Effect of phospholipid structure on stability and survival times of liposomes in circulation

Applications and latest research progress of liposomes in the treatment of ocular diseases

The special structure of eyes and the existence of various physiological barriers make ocular drug delivery one of the most difficult problems in the pharmaceutical field. Considering the problems of patient compliance, local administration remains the preferred method of drug administration in the anterior part of eyes. However, local administration suffers from poor bioavailability, need for frequent administration, and systemic toxicity. Administration in the posterior part of the eye is more difficult, and intravitreal injection is often used. But intravitreal injection faces the problems of poor patient compliance and likely side effects after multiple injections. The development of nanocarrier technology provides an effective way to solve these problems. Among them, liposomes, as the most widely used carrier in clinical application, have the characteristics of amphiphilic nanostructure, easy surface modification, extended release time, good biocompatibility, etc. The liposomes are expected to overcome obstacles and effectively deliver drugs to the target site to improve ocular drug bioavailability. This review summarized the various controllable properties of liposomes for ocular delivery as well as the application and research progress of liposomes in various ocular diseases. In addition, we summarized the physiological barriers and routes of administration contained in eyes, as well as the prospects of liposomes in the treatment of ocular diseases.

Liposomal Drug Product Development and Quality: Current US Experience and Perspective

Research in the area of liposomes has grown substantially in the past few decades. Liposomes are lipid bilayer structures that can incorporate drug substances to modify the drug's pharmacokinetic profile thereby improving drug delivery. The agency has received over 400 liposomal drug product submissions (excluding combination therapies), and there are currently eight approved liposomal drug products on the US market. In order to identify the pain points in development and manufacturing of liposomal drug products, a retrospective analysis was performed from a quality perspective on submissions for new and generic liposomal drug products. General analysis on liposomal drug product submissions was also performed. Results indicated that 96% of the submissions were Investigational New Drug (IND) applications, 3% were New Drug Applications (NDAs), and the remaining 1% was Abbreviated New Drug Applications (ANDAs). Doxorubicin hydrochloride was the most commonly used drug substance incorporated into the liposomes (31%). The majority of the liposomal products were administered via intravenous route (84%) with cancer (various types) being the most common indication (63%). From a quality perspective, major challenges during the development of liposomal drug products included identification and (appropriate) characterization of critical quality attributes of liposomal drug products and suitable control strategies during product development. By focusing on these areas, a faster and more efficient development of liposomal drug products may be achieved. Additionally, in this way, the drug review process for such products can be streamlined.

Mechanisms of neurosteroid interactions with GABA(A) receptors

Neuroactive steroids have some of their most potent actions by augmenting the function of GABA(A) receptors. Endogenous steroid actions on GABA(A) receptors may underlie important effects on mood and behavior. Exogenous neuroactive steroids have potential as anesthetics, anticonvulsants, and neuroprotectants. We have taken multiple approaches to understand more completely the interaction of neuroactive steroids with GABA(A) receptors. We have developed many novel steroid analogues in this effort. Recent work has resulted in synthesis of new enantiomer analogue pairs, novel ligands that probe various properties of the steroid pharmacophore, fluorescent neuroactive steroid analogues, and photoaffinity labels. Using these tools, combined with receptor binding and electrophysiological assays, we have begun to untangle the complexity of steroid actions at this important class of ligand-gated ion channel.

Effects of natural and enantiomeric cholesterol on the thermotropic phase behavior and structure of egg sphingomyelin bilayer membranes

Phospholipids, sphingolipids, and sterols are the major lipid components of the plasma membranes of eukaryotic cells. Because these three lipid classes occur naturally as enantiomerically pure compounds, enantiospecific lipid-lipid and lipid-sterol interactions could in principle occur in the lipid bilayers of eukaryotic plasma membranes. Although previous biophysical studies of phospholipid and phospholipid-sterol model membrane systems have consistently failed to observe such enantiomerically selective interactions, a recent monolayer study of the interactions of natural and enantiomeric cholesterol with egg sphingomyelin has apparently revealed the existence of enantiospecific sterol-sphingolipid interactions. To determine whether enantiospecific sterol-sphingolipid interactions also occur in more biologically relevant lipid-bilayer systems, differential scanning calorimetric, x-ray diffraction, and neutral buoyant-density measurements were utilized to study the effects of natural and enantiomeric cholesterol on the thermotropic phase behavior and structure of egg sphingomyelin bilayers. The calorimetry experiments show that the natural and enantiomeric cholesterol have essentially identical effects on the temperature, enthalpy, and cooperativity of the gel/liquid-crystalline phase transition of egg sphingomyelin bilayers within the limits of experimental error. As well, the x-ray diffraction and neutral buoyancy experiments indicate that bilayers formed from mixtures of natural or enantiomeric cholesterol and egg sphingomyelin have, within experimental uncertainty, the same structure and mass density. We thus conclude that significant enantioselective cholesterol-sphingolipid interactions do not occur in this lipid-bilayer model membrane system.

Enantiospecificity of cholesterol function in vivo

The importance of the absolute configuration of cholesterol for its function in vivo is unknown. To directly test this question in vivo, we synthesized the enantiomer of cholesterol (ent-cholesterol) and tested its ability to substitute for natural cholesterol (nat-cholesterol) in the growth, viability, and behavior of Caenorhabditis elegans, a cholesterol auxotroph. First-generation animals grown on ent-cholesterol were viable with only mild behavioral defects. However, ent-cholesterol produced 100% lethality/arrest of their second generation progeny. Isotopically labeled ent-cholesterol incorporated into animals, indicating that its lethality was not secondary to cholesterol starvation. When mixed with nat-cholesterol, ent-cholesterol was not inert; rather, it antagonized the activity of nat-cholesterol. These results demonstrate for the first time that the absolute configuration of cholesterol, not just its physical properties, is essential for its functions in vivo.

Probing the role of C-1 ester group in Naja naja phospholipase A2-phospholipid interactions using butanetriol-containing phosphatidylcholine analogues

To understand the role of the ester moiety of the sn-1 acyl chain in phospholipase A2-glycerophospholipid interactions, we introduced an additional methylene residue between the glycerol C1 and C2 carbon atoms of phosphatidylcholines, and then studied the kinetics of hydrolysis and the binding of such butanetriol-containing phospholipids with Naja naja phospholipase A2. Hydrolysis was monitored by using phospholipids containing a NBD-labelled sn-2 acyl chain and binding was ascertained by measuring the protein tryptophan fluorescence. The hydrolysis of butanetriol-containing phospholipids was invariably slower than that of the glycerol-containing phospholipids. In addition, the enzyme binding with the substrate was markedly decreased upon replacing the glycerol residue with the 1,3,4-butanetriol moiety in phosphatidylcholines. These results have been interpreted to suggest that the sn-1 ester group in glycerophospholipids could play an important role in phospholipase A2-phospholipid interactions.

Influence of liposome charge and composition on their interaction with human blood serum proteins

Lipid composition and specially their electrostatic properties, were found to greatly influence the stability of liposomes in human blood serum. The amount and type of serum proteins bound to the liposomes were also clearly influenced by lipid composition and charge of liposomes. A good correlation was found between the amount of serum proteins adsorbed to a given type of liposome and its instability as measured by the release of an encapsulated fluorescent probe. Liposomes that bind the highest amount of protein were the least stable, except for the case of liposomes containing gangliosides, which were fairly stable even at a high amount of bound protein. Liposomes with neutral charge containing phosphatidylcholine were the most stable and bound the lowest amount of protein. Liposomes with positive charge behaved similarly to those with neutral charge. However, the stability of negatively charged liposomes was very dependent on their composition. Those liposomes containing only one class of negatively charged phospholipids bound a great amount of protein and were very unstable. However, those liposomes containing also phosphatidylcholine bound less protein and were more stable. The examination of the electrophoresis patterns of serum proteins bound to the different types of liposomes indicated the presence of specific proteins which correlated with liposome instability.

Phospholipids containing nitrogen- and sulfur-linked chains: kinetics of cholesterol exchange between vesicles

We have examined the kinetics of [14C]cholesterol exchange between unilamellar vesicles formed from the following synthetic glycerophosphatidylcholines: (a) those having acyl (OC(O)R), acylamino (NHC(O)R), carbamoyl (NHC(O)OR), and acylthio (SC(O)R) chains at the sn-2 position, and (b) those having alkyl (OR) and thioalkyl (SR) chains at the sn-1 position. Replacement of the glycerol oxygen atom at the sn-2 position of PC with a NH group did not affect the rate of cholesterol exchange to a significant extent, suggesting that the amide group of sphingomyelin is not primarily responsible for the very slow rate of exchange of cholesterol observed from sphingomyelin vesicles. Replacement of the glycerol oxygen at the sn-2 position of phosphatidylcholine with a sulfur atom caused the rate of spontaneous cholesterol exchange to increase by a factor of 1.6. Substitution of an O-alkyl chain for the acyl chain at the sn-1 position of 2-acylthiophosphatidylcholine or substitution of a thioalkyl chain for the O-alkyl sn-1 chain of 1-alkyl-2-acylaminodeoxyphosphatidylcholine also did not result in a marked difference in cholesterol exchange rate. The data suggest that interactions other than intermolecular hydrogen bonding are involved in determining the rates of intermembrane cholesterol exchange. Significantly, these kinetic studies also lend support to the continued use in model membranes of synthetic sulfur- and nitrogen-substituted phosphatidylcholines, which have been employed to study properties of lipolytic enzymes, since synthetic acylamino- and acylthio-phospholipids form vesicles that give cholesterol exchange rates that closely resemble those found in vesicles prepared with diester-phosphatidylcholines.

The effect of blood serum on the size and stability of phospholipid liposomes

Liposomes have been prepared by sonication (SUV) and reverse phase evaporation (REV) from dipalmitoylphosphatidylcholine (DPPC) and its mixtures with phosphatidylinositol (PI), stearylamine and cholesterol. The effect of rat and human blood serum on the liposomes has been investigated by measurement of the particle size in the serum-liposome mixtures by photon correlation spectroscopy in the range of serum protein concentration up to approx. 25 mg ml-1. At low serum protein concentrations the measured particle sizes exceed those calculated from the known sizes and concentrations of liposomes and serum particles in the mixtures: a result consistent with serum-induced aggregation of the liposomes, but the aggregates dissociate at higher serum protein concentration. The effect of serum on the release of encapsulated [14C]glucose from REV liposomes has been investigated over a range of serum protein concentration by gel filtration. At low serum concentration a proportion of the liposomes remain intact but as the serum concentration is increased the size of the liposomes decreases with concomitant release of encapsulated glucose. At high serum concentrations (approx. 24 mg protein per ml) the larger liposomes in the distribution are disrupted and some of the liposomal lipid becomes associated with serum protein. The results are discussed with reference to the effect of blood on the uptake of liposomes by rat liver.

Carbamyl phosphatidylcholine--cholesterol interactions in unilamellar vesicles

Small unilamellar vesicles formed from 1-palmitoyl-2-O-(N-(heptadec-8-cis-enyl)carbamyl)-sn-glycero-3-pho sphocholine (CMPC) or 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in the presence of varying amounts of cholesterol have been studied using fluorescence polarization and NMR (1H and 13C) techniques. The fluorescence polarization and 1H-NMR data clearly indicate that the phospholipid packing order in CMPC bilayers is significantly greater than that in the POPC bilayers. The 13C-NMR chemical shift measurements show that this difference between the two phospholipids possibly arises due to the intramolecular hydrogen-bond formation between the -NH and the phosphate residues in the CMPC molecule. It is further shown that unlike POPC, the CMPC packing order is not much affected by including cholesterol in the phospholipid bilayers. These results demonstrate that introduction of one -NH residue adjacent to the C-2 carbonyl carbon in the POPC molecule could make its structure more ordered in the vesicles bilayer, and also would alter its interactions with cholesterol.

In vivo stability of ester- and ether-linked phospholipid-containing liposomes as measured by perturbed angular correlation spectroscopy

To evaluate liposome formulations for use as intracellular sustained-release drug depots, we have compared the uptake and degradation in rat liver and spleen of liposomes of various compositions, containing as their bulk phospholipid an ether-linked phospholipid or one of several ester-linked phospholipids, by perturbed angular correlation spectroscopy. Multilamellar and small unilamellar vesicles (MLVs and SUVs), composed of egg phosphatidylcholine, sphingomyelin, distearoyl phosphatidylcholine (DSPC), dipalmitoyl phosphatidylcholine (DPPC) or its analog dihexadecylglycerophosphorylcholine (DHPC), and cholesterol plus phosphatidylserine, and containing 111In complexed to nitrilotriacetic acid, were injected intravenously in rats. Recovery of 111In-labeled liposomes in blood, liver, and spleen was assessed at specific time points after injection and the percentage of liposomes still intact in liver and spleen was determined by measurement of the time-integrated angular perturbation factor [G22(infinity)] of the 111In label. We found that MLVs but not SUVs, having DHPC as their bulk phospholipid, showed an increased resistance against lysosomal degradation as compared to other phospholipid-containing liposomes. The use of diacyl phospholipids with a high gel/liquid-crystalline phase-transition temperature, such as DPPC and DSPC, also retarded degradation of MLV, but not of SUV in the dose range tested, while the rate of uptake of these liposomes by the liver was lower.