Nitroxide radicals. Controlled release from and transport through biomimetic and hollow fibre membranes.

Stable nitroxide radicals have found wide applications in chemistry and biology and they have some potential applications in medicine due to their antioxidant properties. Nitrocellulose filters impregnated with lipid-like substances are used as an imitation of biomembranes and could be used as a controlled drug release vehicle, while experiments with hollow fibres can be useful in the modelling of a drug delivery via blood vessels. This paper describes mechanisms of the nitroxide transport in four different model systems, i.e. a) exit of nitroxide into aqueous solution from porous nitrocellulose filters, impregnated with organic solvents, b) transport of nitroxides through the impregnated membrane from one into another aqueous solution, c) transport of nitroxides from bulk phase of organic solvents through the impregnated membrane into aqueous phase with ascorbic acid, and d) transport of nitroxides from liquid organic phase into aqueous solution through porous hollow fibres. The results are analysed in terms of mass transfer resistance of a membrane, organic and aqueous phase, based on nitroxide diffusion and distribution coefficients. Ascorbic acid reduced nitroxides in water and enhanced the rate of their transfer due to the decrease of transport resistance of unstirred aqueous layers. It is demonstrated that in the case of biomembranes the rate limiting step could be the transport through unstirred aqueous layers and membrane/water interface.