Partitioning of a homologous series of alkyl p-aminobenzoates in dipalmitoylphosphatidylcholine liposomes: effect of liposome type

Liposomes in tissue engineering and regenerative medicine

Liposomes are vesicular structures made of lipids that are formed in aqueous solutions. Structurally, they resemble the lipid membrane of living cells. Therefore, they have been widely investigated, since the 1960s, as models to study the cell membrane, and as carriers for protection and/or delivery of bioactive agents. They have been used in different areas of research including vaccines, imaging, applications in cosmetics and tissue engineering. Tissue engineering is defined as a strategy for promoting the regeneration of tissues for the human body. This strategy may involve the coordinated application of defined cell types with structured biomaterial scaffolds to produce living structures. To create a new tissue, based on this strategy, a controlled stimulation of cultured cells is needed, through a systematic combination of bioactive agents and mechanical signals. In this review, we highlight the potential role of liposomes as a platform for the sustained and local delivery of bioactive agents for tissue engineering and regenerative medicine approaches.

Quantifying molecular partition into model systems of biomembranes: an emphasis on optical spectroscopic methods

Optical spectroscopies have been intensively used to determine partition coefficients by a plethora of methodologies. The present review is intended to give detailed and useful information for the determination of partition coefficients and addresses several relevant aspects, namely: (i) definition and calculation of the partition coefficient between aqueous and lipidic phases; (ii) partition coefficients vs. "binding" formalisms; (iii) advantages of spectroscopic methodologies over separation techniques; (iv) formalisms for various experimental approaches based on UV-Vis absorption or fluorescence parameters (fluorescence intensity, lifetime, anisotropy and quenching); (v) experimental hints, artifacts and model limitations; and (vi) a brief survey of nonoptical techniques.

Effect of liposome type and membrane fluidity on drug–membrane partitioning analyzed by immobilized liposome chromatography

Immobilized liposome chromatography (ILC) has been proven to be a useful method for the study or rapid screening of drug-membrane interactions. To obtain an adequate liposomal membrane phase for ILC, unilamellar liposomes were immobilized in gel beads by avidin-biotin binding. The retardation of 15 basic drugs on the liposome column could be converted to membrane partitioning coefficients, K(LM). The effects of small or large unilamellar liposomes and multilamellar liposomes on the drug-membrane partitioning were compared. The K(LM) values for both small and large liposomes were similar, but higher than those for the multilamellar liposomes. The basic drugs showed stronger partitioning into negatively charged liposomes than into either neutral liposomes or positively charged liposomes. The membrane fluidity of the immobilized liposomes was modulated by incorporating cholesterol into the liposomal membranes, by changing the acyl chain length and degree of unsaturation of the phospholipids, and by changing the temperature for ILC runs. Our data show that K(LM) obtained using ILC correlated well with those reported by batch studies using free liposomes. It is concluded that negatively charged or cholesterol-containing large unilamellar liposomes are suitable models for the ILC analysis of drug-membrane interactions.

Cubic phases for studies of drug partition into lipid bilayers

Drug partition into lipid bilayers in a cubic liquid-crystalline phase was investigated. Glyceryl monooleate was used to form the lipid bilayer in a reversed bicontinuous cubic liquid-crystalline phase. The reason for using the cubic phase is that it may coexist with an external aqueous phase, and that the phase boundary (cubic phase/aqueous bulk) is well-defined due to the stiffness of the cubic phase. This makes the cubic phase a potential candidate for high throughput screening (HTS) of the lipophilicity and the dissociation constant (if any) of drug compounds. Clomethiazole (CMZ), lidocaine, prilocaine and 4-phenylbutylamine (4-PBA) were chosen as model drug compounds. It was shown that it is possible to determine a pH-dependent apparent partition coefficient, Kbl/w, of a drug compound using a lipid bilayer expressed as a cubic liquid-crystalline structure. Good agreement was found when the resulting Kbl/w vs. pH curves for CMZ, lidocaine and prilocaine were fitted to a mathematical expression. This included the bilayer/water partition coefficient for the unionised and ionised drug respectively and the pKa of the drug. The effect of different experimental conditions; such as amount of cubic phase, temperature, agitation, sample preparation and interfacial area between the cubic phase and the aqueous bulk on the partition kinetics were investigated as well. The studies reveal that the time needed to reach partition equilibrium was, as expected, substantially reduced (from days to hours) by decreasing the amount of cubic phase, increasing the interfacial area between the cubic phase and the aqueous phase, and increasing the temperature and the agitation of the sample. It was also shown that the bilayer affinity of 4-PBA was increased when a zwitterionic lipid (i.e. dioleoyl phosphatidylcholine, DOPC) was incorporated in the bilayer.

Avidin-biotin immobilization of unilamellar liposomes in gel beads for chromatographic analysis of drug-membrane partitioning

To construct a homogeneous lipid membrane chromatographic phase, biotinylated unilamellar liposomes of small and large sizes (SUVs and LUVs, respectively) were immobilized in avidin- or streptavidin-derived gel beads in amounts up to 55 micromol phospholipid/ml gel bed at yields above 50%. The immobilized liposomes exhibited excellent stability due to avidin-biotin multiple-site binding. The trapped volume and size distribution of the immobilized liposomes (0.33-0.42 microl/micromol lipid and 20-30 nm diameter for SUVs, 1.7-1.9 microl/micromol lipid and 80-120 nm for LUVs) indicated the unilamellarity and integrity of the immobilized liposomes. Partitioning of 15 pharmaceutical drugs into the bilayers of LUVs immobilized in different gel matrices correlated very well, as shown by chromatographic drug retention analysis. The partitioning of several beta-blockers into the immobilized LUVs showed a close correlation with their partitioning, reported in the literature, into free liposomes. The avidin-biotin-immobilized unilamellar liposomes can thus be used for chromatographic analysis and screening of solute-membrane interactions.

Factors affecting microencapsulation of drugs in liposomes

Liposomes have been used as carriers for drugs, toxins, enzymes, proteins/peptides and other bioactive materials there are several liposomal formulations that are being investigated in preclinical and clinical trials. Achieving high encapsulation as well as retention of the encapsulated drug is very important in developing liposomes as drug carriers. A high drug-to-lipid ratio is likely to reduce the cost of formulations and also the risk of lipid-induced toxicity following their injection. Comparison of the encapsulation efficiency of the drug in liposomes with the therapeutic dose indicates whether, in principle, liposomes can be used as a delivery system for that drug. The optimization of the liposomal encapsulation of a drug is usually based on trial and error, rather than on a thorough investigation of the factors affecting it. To obtain optimum encapsulation of a drug into a liposomal preparation, parameters influencing both the liposome and the drug need to be carefully considered during the early stages of development. In this review, factors that affect encapsulation of drugs in liposomes such as liposome size and type, charge on the liposome surface, bilayer rigidity, method of preparation, remote loading, addition of ion pairing, and complexing agents and characteristics of the drug to be encapsulated are discussed.