The ability of low-frequency ultrasound (LFUS) to release encapsulated drugs from sterically stabilized liposomes in a controlled manner was demonstrated. Three liposomal formulations having identical lipid bilayer compositions and a similar size ( approximately 100 nm) but differing in their encapsulated drugs and methods of drug loading have been tested. Two of the drugs, doxorubicin and methylpredinisolone hemisuccinate, were remote loaded by transmembrane gradients (ammonium sulfate and calcium acetate, respectively). The third drug, cisplatin, was loaded passively into the liposomes. For all three formulations, a short exposure to LFUS (<3 min) released nearly 80% of the drug. The magnitude of drug release was a function of LFUS amplitude and actual exposure time, irrespective of whether irradiation was pulsed or continuous. Furthermore, no change in liposome size distribution or in the chemical properties of the lipids or of the released drugs occurred due to exposure to LFUS. Based on our results, we propose that the mechanism of release is a transient introduction of porelike defects in the liposome membrane, which occurs only during exposure to LFUS, after which the membrane reseals. This explains the observed uptake of the membrane-impermeable fluorophore pyranine from the extraliposomal medium during exposure to LFUS. The implications of these findings for clinical applications of controlled drug release from liposomes are discussed.
Objective. The use of glucocorticoids (GCs) in rheumatoid arthritis is limited by side effects related to unfavorable pharmacokinetics and biodistribution. Liposomal GC formulations have been studied since the 1970s in an attempt to overcome this obstacle, but none has entered clinical use. We undertook this study to determine whether a novel approach could overcome the limitations that have thus far prevented the clinical use of these formulations: low drug:lipid ratio, low encapsulation efficiency, and lack of controlled release.Methods. We used ϳ80-nm sterically stabilized (pegylated) nanoliposomes (NSSLs), which were remoteloaded with an amphipathic weak acid GC (such as methyl prednisolone hemisuccinate) utilizing an intraliposome (aqueous compartment)-high/extraliposome (bulk medium)-low transmembrane calcium acetate gradient. This unique method actually "traps" the GC in the liposomal aqueous phase as a calcium-GC precipitate.Results. Our liposome formulation exhibited high encapsulation efficiency (94%) and a high drug:lipid mole ratio (0.41) and demonstrated controlled release of the encapsulated GC during systemic circulation and in inflamed paws in rats with adjuvant-induced arthritis. In addition, both in arthritic rats and in a Beagle dog, we showed the pharmacokinetic advantage of using liposomes as GC carriers. Finally, we demonstrated the superior therapeutic efficacy of our liposome formulation over that of free GCs in arthritic rats, both in early and in peak disease stages.Conclusion. Amphipathic weak acid GCs remoteloaded into ϳ80-nm NSSLs overcome past limitations of liposomal GC formulations. The unique loading method, which also leads to controlled release, improves the therapeutic effect both systemically and locally. Such a development has great potential for improving GC therapy.
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