Photostability profiles of minoxidil solutions

Photostability profiles of minoxidil solutions exposed to fluorescent light, under the influence of several parameters like concentration, pH, buffer species, photostabilizers, container type, solvent system, and skin penetration enhancers were studied. The assay of minoxidil was done spectrophotometrically at 230 nm and 290 nm, using double beam UV-visible spectrophotometer. The results show that photodegradation of minoxidil solutions follows first order rate. Furthermore, minoxidil solution (4.5 micrograms/ml) was found to be more stable in phosphate buffer as compared to acetate buffer. Amber glass offered equivalent photoprotection as aluminum foil-covered glass. Minoxidil was found to be most stable in a solvent system of water or 25% v/v PEG300 and least stable in 25% v/v propylene glycol. Of the penetration enhancers studied dimethyl sulfoxide (DMSO) caused the least photodecomposition of minoxidil. The order of photoprotection of the five potential photostabilizers were as follows: sodium thiosulfate > EDTA > sodium benzoate approximately thiourea > sodium metabisulfite. Varying the concentration of sodium thiosulfate did not affect the minoxidil photostability. The study also indicates that minoxidil solution (4.5 micrograms/ml) is most photostable in pH 8 phosphate buffer. An increase in ionic strength had a photostabilizing effect until an optimum strength of 0.17, then further increase caused the rate of photodegradation to remain unchanged.