A simple and sensitive spectrofluorimetric method has been developed and validated for determination of oseltamivir phosphate (OSP). The proposed method is based on condensation reaction of the primary amino group of OSP with ninhydrin and phenylacetaldehyde in buffered medium (pH 6.5). The formed yellow fluorescent product exhibits excitation and emission maxima at 390 and 460 nm, respectively. The selectivity improvement of our proposed method is based on the water insolubility of the oseltamivir carboxylic acid (OSC) the active metabolite of OSP, which contains the same primary amino group as OSP but cannot, condensed with ninhydrin and phenylacetaldehyde reagents. The different experimental parameters affecting the formation and stability of the reaction product were carefully studied and optimized. The fluorescence intensity concentration plot is rectilinear in the range of 2-15 μg ml-1 with detection and quantitation limits of 0.32 and 0.98 μg ml-1, respectively. The proposed method was successfully applied for determination of OSP in commercial capsules, suspension and spiked human plasma with good percentage recovery. In addition, the developed procedure was extended to study the stability of OSP under different stress conditions; including acid and alkali hydrolysis, oxidation, photolysis, and thermal degradation. Furthermore, the kinetic of alkaline and acidic degradation of the cited drug were investigated. The apparent first order degradation rate constants were 0.258 and 0.318 K h-1 with half times of 2.68 and 2.17 h, for acidic and alkaline degradation, respectively.
Keywords: Drug stability testing; Human plasma; Ninhydrin; Phenylacetaldehyde; Selectivity, oseltamivir phosphate; Spectrofluorimetric determination.