Chemical and physical characterization of remotely loaded bupivacaine liposomes: comparison between large multivesicular vesicles and small unilamellar vesicles

Large multivesicular liposomes (LMVV) remotely loaded with bupivacaine (Bupisome) were previously demonstrated to be a stable, long-acting local anesthetic. We demonstrate that this is not the case for small unilamellar vesicles (SUV) of the same lipid composition also remotely loaded with bupivacaine. We show that the trapped volume in LMVV is 21-fold higher and the drug-to-lipid mole ratio is 10-fold higher than in SUV. Cryo-transmission electron micrographs and differential interference contrast microscopy show that there are no bupivacaine crystals inside LMVV and SUV. The thermotropic characterization studied by DSC demonstrates that the drug interacts with the liposome membrane, which, together with the above results on the drug-to-lipid ratio, explains the small in vitro drug release from the SUV and large (but <100%) release from the LMVV after 24 h at 37 °C. The absence of analgesia in mice treated locally with SUV loaded with bupivacaine compared with prolonged analgesia from LMVV correlates well with the in vitro results. The study indicates that in LMVV and SUV, part of the bupivacaine is associated with the liposomal membrane, which is poorly available for analgesia. The membrane fraction is very high in SUV and much smaller in LMVV. The much larger trapped volume of the LMVV explains the higher drug availability and better analgesia of LMVV.