TRANSMEMBRANE GRADIENT DRIVEN PHASE-TRANSITIONS WITHIN VESICLES - LESSONS FOR DRUG-DELIVERY

Phase transitions in closed vesicles, i.e., microenvironments defined by the size of the vesicle, its contents, and permeability of its memb rane are becoming increasingly important in several scientific discipl ines including catalysis, growth of small crystals, cell function stud ies, and drug delivery, The membrane composed from lipid bilayer is in general impermeable to ions and larger hydrophilic ions, Ion transpor t can be regulated by ionophores while permeation of neutral and weakl y hydrophobic molecules can be controlled by concentration gradients, Some weak acids or bases, however, can be transported through the memb rane due to various gradients, such as electrical, ionic (pH) or speci fic salt (chemical potential) gradients. Upon permeation of appropriat e species and reaction with the encapsulated species precipitation may occur in the vesicle interior. Alternatively, these molecules can als o associate with the leaflets of the bilayer according to the transmem brane potential. Efficient liposomal therapeutics require high drug to lipid ratios and drug molecules should have, especially when associat ed with long circulating liposomes, low leakage rates, In this article we present very efficient encapsulation of two drugs via their intral iposomal precipitation, characterize the state of encapsulated drug wi thin the liposome and try to fit the experimental data with a recently developed theoretical model. Nice agreement between a model which is based on chemical potential equilibration of membrane permeable specie s with experimental data was observed. The high loading efficiencies, however, are only necessary but not sufficient condition for effective therapies. If adequate drug retention within liposomes, especially in the case of long-circulating ones, is not achieved, the therapeutic i ndex decreases substantially. Anticancer drug doxorubicin precipitates in the liposome interior in a form of gel with low solubility product and practically does not leak out in blood circulation in the scale o f days. With an antibiotic, ciprofloxacin, the high loading efficacy a nd test tube stability is not reproduced in in vitro plasma leakage as says and in vivo. We believe that the reasons are higher solubility pr oduct of precipitated drug in the liposome, larger fraction of neutral molecules due closer pK values of the drug with the pH conditions in the solutions and high membrane permeability of this molecule, High re solution cryoEM shows that encapsulated anticancer agent doxorubicin i s precipitated in the form of bundles of parallel fibers while antibio tic ciprofloxacin shows globular precipitate, Doxorubicin gelation als o causes the change of vesicle shape.