Reduction-sensitive liposomes from a multifunctional lipid conjugate and natural phospholipids: reduction and release kinetics and cellular uptake.

The development of targeted and triggerable delivery systems is of high relevance for anticancer therapies. We report here on reduction-sensitive liposomes composed of a novel multifunctional lipidlike conjugate, containing a disulfide bond and a biotin moiety, and natural phospholipids. The incorporation of the disulfide conjugate into vesicles and the kinetics of their reduction were studied using dansyl-labeled conjugate 1 in using the dansyl fluorescence environmental sensitivity and the Förster resonance energy transfer from dansyl to rhodamine-labeled phospholipids. Cleavage of the disulfide bridge (e.g., by tris(2-carboxyethyl)phosphine (TCEP), dithiothreitol (DTT), l-cysteine, or glutathione (GSH)) removed the hydrophilic headgroup of the conjugate and thus changed the membrane organization leading to the release of entrapped molecules. Upon nonspecific uptake of vesicles by macrophages, calcein release from reduction-sensitive liposomes consisting of the disulfide conjugate and phospholipids was more efficient than from reduction-insensitive liposomes composed only of phospholipids. The binding of streptavidin to the conjugates did not interfere with either the subsequent reduction of the disulfide bond of the conjugate or the release of entrapped molecules. Breast cancer cell line BT-474, overexpressing the HER2 receptor, showed a high uptake of the reduction-sensitive doxorubicin-loaded liposomes functionalized with the biotin-tagged anti-HER2 antibody. The release of the entrapped cargo inside the cells was observed, implying the potential of using our system for active targeting and delivery.

Aging of oil-in-water emulsions: the role of the oil.

Controlling stability and aging of emulsions is important from commercial and scientific perspectives. Achieving such control comes through gaining an understanding of the relationship between emulsion constituents and microstructure and how these influence the kinetics and mechanism of destabilisation. We present here an investigation determining the rate of destabilisation as a function of time for a series of water/n-alkane/Triton X-100 oil-in-water emulsions. The time dependence of the emulsions was investigated using static light scattering, PFG-NMR and measurement of gross phase separation. By changing the chain length of the oil from hexane to tetradecane, an almost five orders of magnitude variation in emulsion lifetime could be achieved, while maintaining most of the other chemical and physical characteristics of the emulsions. Further, we show that while Ostwald ripening is the dominant destabilisation mechanism, two distinct regimes are evident. Initially, we observed an enhanced Ostwald ripening regime due to the presence of oil-swollen micelles in the aqueous continuum, that is a depletion flocculation mechanism is followed. The presence of oil-swollen micelles was confirmed using PFG-NMR. The micelles aid the gross oil transport between the discrete oil domains. Upon phase separation the oil-swollen micelles are predominantly removed from the emulsion along with the excess water resulting in a concomitant reduction in the ripening rate, producing the more general Ostwald ripening cubic dependence of droplet radius as a function of time for the lower molecular weight oils. The oils with higher molecular weight (decane and above), however, were observed to switch over to destabilisation via creaming. PFG-NMR was shown to be a powerful technique to fully probe emulsion microstructure as a function of time with droplet size and spacing being directly obtained from the data.

Effect of oil on emulsion characteristics: manipulating the interfacial domain.

In this paper, two ternary systems (water, Triton X-100, and octane or tetradecane) were investigated using freeze-fracture transmission electron microscopy, rheology, laser diffraction particle sizing, and pulse field gradient NMR (PFG-NMR). Oil-in-water dispersed droplet emulsions were prepared for Triton X-100 concentrations of 8-12 wt % while maintaining a surfactant-to-oil weight ratio of 1:5. The stability of the emulsions significantly increased with both the surfactant concentration and the chain length of the oil component. The PFG-NMR results could be explained as a superposition of three different types of diffusion: restricted diffusion of the oil in the droplets and free and restricted diffusion of the droplets themselves. The PFG-NMR results were correlated with the electron microscopy images and the particle-sizing data. Moreover, to gain a greater understanding of the role of the oil-surfactant interactions, in particular, the present investigations were placed in context with an earlier publication where toluene was used as the oil with the same emulsifier. The change from the aromatic oil, which is a better solvent for the surfactant, to an alkane-based oil dramatically changed the characteristics of the interfacial domain. On one hand, the concentration range for the formation of emulsions and the variety of microstructures realized were severely restricted and reduced when using the alkanes as compared with toluene. On the other hand, the interfacial film was much more stable leading to an extremely reduced rate of droplet coalescence.

Structural changes of poly(butadiene)-poly(ethyleneoxide) diblock-copolymer micelles induced by a cationic surfactant: scattering and cryogenic transmission electron microscopy studies.

Micelles of the diblock copolymer poly(butadiene)-poly(ethyleneoxide) (B40-b-EO62) and mixed micelles of this polymer with the cationic surfactant dodecyltrimethylammonium bromide (C12TAB) were investigated using static and dynamic light scattering and small-angle neutron scattering. It is shown that the surfactant induces a major structural change from large mainly rodlike aggregates to smaller spherical mixed micelles. The rodlike assemblies found in the absence of surfactant have a contour length L of ca. 500 nm and a diameter d approximately 30 nm. The spherical mixed micelles obtained upon addition of C12TAB possess a hydrodynamic radius of 15 nm and still contain several polymer molecules. The results of the scattering experiments are consistent with observations of the aggregates by cryogenic transmission electron microscopy.