Blood flow enhancement in skin or oral mucosa after the topical application of liposome entrapped rubifacient as measured by EPR oximetry in vivo: the influence of size and liposome composition.

Many studies performed in the last decade showed significantly higher absorption rates and a greater local pharmacological effect for drugs applied to the skin entrapped in liposomes as compared to conventional topical formulation [1]. It is also well established that liposome bilayer properties play an important role in transport. But there is a lack of methods by which it is possible to directly measure the response of the organism to the topically applied drug in vivo. One such method is electron paramagnetic resonance (EPR) oximetry in vivo. In this study, EPR oxymetry was used to measure the difference in the response of the body to a vasodilatator drug, which was applied to the skin or oral mucosa entrapped in different types of liposomes. By EPR oximetry, the partial pressure of oxygen (pO 2) in tissues in vivo is measured. For this purpose, a paramagnetic probe has to be introduced into the tissue and its EPR spectra measured. This method is based on the fact that molecular oxygen is paramagnetic and due to its interaction with the paramagnetic probe, it broadens the EPR spectra line-width [2]. In our experiment, two different types of liposomes were used: Hydrogenated soy lecithin (Emulmetic 320) cholesterol (70:30 wt%, total weight 48 mg/ml) (HSL), which, according to in vitro EPR measurements, facilitates transport into the skin in the first 30 min after application. The other one had the same composition but instead of hydrogenated SL, natural soy lecithin (phospholipon 80) (NSL) was used, which, according to in vitro studies, does not facilitate transport in such a short time [3]. MLV and LUVET obtained by extrusion of MLV through polycarbonate filters with a pore diameter from 800 to 100 nm were used. A vasodilator benzyl nicotinate (BN, 0.83 wt%) was entrapped into the liposomes as a lipophilic drug that can serve as a marker by which the difference in the response of the organism due to different formulations can be followed. Liposomes were mixed with hydrogel hydroxyethylcellulose. As a paramagnetic probe, lithium phthalocyanine was introduced into the skin of anaesthetised mice or the oral mucosa of rats, and its EPR spectra were measured over time after the topical application of the liposomal preparation with the entrapped vasodilator BN, and after repeated application of the preparations. The measurements in vivo were performed with a low frequency EPR spectrometer (1.1 GHz) using surface coil detectors.