Lipid based formulations for topical application: characterization and delivery of active substances
Doctoral thesis, 2007
In this thesis, the interaction between lipids, solvents and active substances in lipid based formulations have been studied and related to formulation structure or release behavior on human skin or a model thereof. For the different investigations, isotropic equilibrium lipid phases, i.e. bicontinuous cubic and sponge phases or water-in-oil microemulsions, were used.
Bicontinuous liquid crystalline cubic phases formed of the polar lipids monoolein (MO) and phytantriol (PT) were used to deliver δ-aminolevulinic acid (ALA) and its methylester (m-ALA) in nude BALB/c mice in vivo and the fluorescent probe sulphorhodamine B (SRB) in full-thickness human skin in vitro. Fluorescence imaging and spectrophotometric measurements revealed that ALA and m-ALA delivered from the cubic phases induced an intense and homogenous fluorescence in mice. By using two-photon microscopy (TPM), it was revealed that the lipid cubic formulations penetrated into micro-fissures of the skin, thus, were acting as reservoirs for the active substance (SRB), which may explain the results in the former study.
A detailed phase diagram was made for the propylene glycol (PG)-PT-water system to reveal the effect caused by the lipid- as well as water-miscible solvent, i.e. PG. The results showed that lower amounts of PG could be incorporated in the cubic phase, but larger amounts did not induce a sponge phase, which is possible in the corresponding MO system. MO contains an ester bond that is susceptible to hydrolysis and therefore the PT system is a good alternative when high and low pH is present.
A characterization of the bicontinuous MO sponge phase with up to 16% (w/w) of m-ALA was performed with polarized light, small-angle X-ray diffraction (SAXD) and NMR diffusometry (NMRD). Additions over 4% m-ALA, transformed the isotropic sponge phase into an anisotropic phase. Isotropic phases were re-formed with small additions of water to the anisotropic samples. All methods used indicated that a bicontinuous structure was present in the investigated samples. The sponge phase was also shown to be able to conduct electrical current and was therefore used in iontophoretic experiments. The MO sponge phase with 16% m-ALA showed in combination with iontophoresis slightly lower fluxes than the clinically used references, while results from passive diffusion were similar to the references.
The in vitro transport of benzophenone-3 (BZ-3) from a medium chain triglyceride (MCT) based microemulsion (ME) over silicone and human epidermal membrane was also investigated as the polarity of the hydrophobic domain was changed. The polarity change was done by an exchange of the slightly polar lipid MCT to the non-polar lipid isohexadecane. Passive diffusion experiments performed over the silicone membrane revealed an unexpected minimum in release rate as the polarity was changed into less polar. NMRD and SAXD were used to characterize the microstructures of the MEs, and revealed the presence of loosely held organizations in some of the investigated samples. However, a minimum in release was not found when using the human epidermal membrane instead of the silicone membrane.
bicontinuous cubic phase
bicontinuous sponge phase
medium chain triglycerides