Liposomal bupivacaine. Extended duration nerve blockade using large unilamellar vesicles that exhibit a proton gradient.

BACKGROUND: There is a clinical requirement for longer-acting local anesthetics, particularly for the management of post-operative and chronic pain. In this regard, liposomes have been suggested to represent a potentially useful vehicle for sustained drug release after local administration. In the current study, the authors used a transmembrane pH gradient to efficiently encapsulate bupivacaine within large unilamellar vesicles. They report on the kinetics of drug uptake and release and the duration of nerve blockade. METHODS: The rate and extent of bupivacaine uptake into large unilamellar vesicles that exhibit a pH gradient (interior acidic) were determined and compared to drug association with control liposomes that did not exhibit a proton gradient. In subsequent studies, researchers examined the kinetics of bupivacaine release from these liposome systems in vitro. Using the guinea pig cutaneous wheal model, the rate of clearance of the liposome carrier was monitored after intradermal administration, using a radiolabelled lipid marker, and the duration of nerve blockade produced by free and liposomal bupivacaine was compared. RESULTS: Bupivacaine was rapidly and efficiently accumulated within liposomes that exhibited a pH gradient (interior acidic) with trapping efficiencies of 64-82% of total drug, depending on the initial bupivacaine:phospholipid ratio. Little uptake was seen, however, for control vesicles that did not exhibit a transmembrane proton gradient. Using an in vitro model of drug clearance, liposomally encapsulated bupivacaine was found to be slowly released for a longer period of time compared with either the free drug or bupivacaine associated with control (no pH gradient liposomes). In the guinea pig cutaneous wheal model, more than 85% of the liposomal carrier was found to remain at the site of administration for 2 days. The sustained drug release afforded by liposomes that exhibited a pH gradient resulted in an increase in the duration of nerve blockade of as much as threefold compared with either the free drug or bupivacaine in the presence of control (no pH gradient) liposomes. Recovery of half maximal response (R2.5) after administration of 0.75% free bupivacaine, for example, was approximately 2 h, whereas the same dose of bupivacaine in pH gradient liposomes exhibited a R2.5 of approximately 6.5 h. CONCLUSIONS: Large unilamellar vesicles that exhibit a pH gradient can efficiently encapsulate bupivacaine and subsequently provide a sustained-release system that greatly increases the duration of neural blockade.

Partitioning and antioxidant action of the water-soluble antioxidant, Trolox, between the aqueous and lipid phases of phosphatidylcholine membranes: 14C tracer and product studies.

The water-soluble antioxidant, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic (Trolox), (4-14C)-labelled, was used to trace its location in the aqueous and lipid phases of liposomes. Trolox was found to partition 20 to 25% into the lipid phase of multilamellar (MLV) and 38-46% into the lipid phase of unilamellar (ULV) egg lecithin liposomes. Trolox and its oxidation products partition readily (40%) into the lipid phase of dilinoleoylphosphatidylcholine (DLPC) MLV liposomes during inhibited peroxidation, thermally initiated by azo-bis(2,4-dimethylvaleronitrile) (ADVN). The time-course of the consumption of Trolox during peroxidation of DLPC liposomes, initiated by ADVN, was followed by separation and analyses of [4-14C]Trolox and its oxidation products. Such studies showed that the consumption of Trolox followed the profile of the inhibition of oxygen uptake. This indicates that Trolox can be used in quantitative studies of membrane peroxidation; for example, to measure the rate of chain initiation (Ri). The product distribution of hydroperoxides, the 9- and 13-cis,trans (c,t) and trans,trans (t,t) isomers, formed during inhibited peroxidation of linoleate, in DLPC and methyl linoleate in dimyristoyl PC (DMPC) liposomes was determined by HPLC of the derived hydroxy methyl esters. The c,t/t,t (kinetic/thermodynamic) ratios were related to the antioxidant activity of the inhibitors. Both Trolox and alpha-tocopherol (vitamin E) gave relatively high initial c,t/t,t ratios (6.6 and 7.1) during inhibited peroxidation of DLPC, initiated by water-soluble azo-bis(2-amidinopropane.HCl) (ABAP). High initial c,t/t,t ratios (6.2) were also observed for alpha-tocopherol-inhibited peroxidation of DLPC liposomes, initiated by lipid-soluble ADVN. On the other hand, the combination of Trolox with ADVN-initiated peroxidation of DLPC or of methyl linoleate in DMPC gave relatively low initial c,t/t,t ratios of 3.5 and 1.3. These results are interpreted in terms of the relative hydrogen atom donating ability of the antioxidants and the homogeneity of the system used. The 9/13 ratios of hydroperoxides were constant (0.9 to 1.0) in all experiments and did not give evidence for preferential trapping by Trolox of peroxyls at the 9-position.