Spontaneous Formation of Ultrasmall Unilamellar Vesicles in Mixtures of an Amphiphilic Drug and a Phospholipid.

We have observed ultrasmall unilamellar vesicles, with diameters of less than 20 nm, in mixtures of the tricyclic antidepressant drug amitriptyline hydrochloride (AMT) and the unsaturated zwitterionic phospholipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) in physiological saline solution. The size and shape of spontaneously formed self-assembled aggregates have been characterized using complementary techniques, i.e., small-angle neutron and X-ray scattering (SANS and SAXS) and cryo-transmission electron microscopy (cryo-TEM). We observe rodlike mixed micelles in more concentrated samples that grow considerably in length upon dilution, and a transition from micelles to vesicles is observed as the concentration approaches the critical micelle concentration of AMT. Unlike the micelles, the spontaneously formed vesicles decrease in size with each step of dilution, and ultrasmall unilamellar vesicles, with diameters as small as about 15 nm, were observed at the lowest concentrations. The spontaneously formed ultrasmall unilamellar vesicles maintain their size for as long we have investigated them (i.e., several months). To the best of our knowledge, such small vesicles have never before been reported to form spontaneously in a biocompatible phospholipid-based system. Most interestingly, the size of the vesicles was observed to be strongly dependent on the chemical structure of the phospholipid, and in mixtures of AMT and the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), the vesicles were observed to be considerably larger in size. The self-assembly behavior in the phospholipid-drug surfactant system in many ways resembles the formation of equilibrium micelles and vesicles in mixed anionic/cationic surfactant systems.

Investigation of the enhanced ability of bile salt surfactants to solubilize phospholipid bilayers and form mixed micelles.

The self-assembly in mixtures of the anionic bile salt surfactant sodium deoxycholate (NaDC) and the zwitterionic phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) in physiological saline solution has been investigated using light scattering, small-angle X-ray scattering and cryo-transmission electron microscopy. Rather small tri-axial ellipsoidal NaDC-DMPC mixed micelles form at a high content of bile salt in the mixture, which increase in size as an increasing amount of DMPC is incorporated into the micelles. Eventually, the micelles begin to grow substantially in length to form long wormlike micelles. At higher mole fractions of DMPC, the samples become turbid and cryo-TEM measurements reveal the existence of large perforated vesicles (stomatosomes), coexisting with geometrically open disks. To our knowledge, stomatosomes have not been observed before for any bile salt-phospholipid system. Mixed micelles are found to be the sole aggregate structure in a very wide regime of bile salt-phospholipid compositions, i.e. up to about 77 mol% phospholipid in the micelles. This is much higher than the corresponding value of 25 mol% observed for the conventional surfactant hexadecyltrimethylammonium bromide (CTAB) mixed with DMPC in the same solvent. The enhanced ability of bile salt surfactants to solubilize phospholipid bilayers and form mixed micelles is rationalized using bending elasticity theory. From our theoretical analysis, we are able to conclude that amphiphilic molecules rank in the following order of increasing spontaneous curvature: phospholipids < conventional surfactants < bile salts. The bending rigidity of the different amphiphilic molecules increases according to the following sequence: bile salts < conventional surfactants < phospholipids.