Spectrophotometric monitoring of nimesulide photodegradation by a combined hard-soft multivariate curve resolution-alternative least square method.

Nimesulide, a Cox-2 inhibitor anti-inflammatory drug, is a light sensitive compound and its biological activity is decreased upon photodegradation. The photodegradation kinetic of nimesulide was investigated spectrophotometrically using multivariate curve resolution analysis to overcome spectral overlapping of reactant and degradation products. The absorbance spectra of the nimesulide methanolic solution, exposed to daily sunlight, were recorded at different times. Three absorbing chemical species, coexisted in the reaction system, were detected by application of factor analysis to the absorbance data matrix. The soft-modeling multivariate curve resolution-alternative least square (MCR-ALS) analysis of the evolutionary absorbance data revealed that nimesulide undergoes photodegradation through a two-step consecutive manner, where both steps obey first-order kinetic. By application of the kinetic hard model constraint to the MCR-ALS analysis, an excellent agreement was obtained between the fitted concentration profiles and those obtained by soft method. The first-order rate constants of the first and second degradation products were calculated as 0.052 (+/-0.007) and 0.009 (+/-0.001)h(-1), respectively. Finally, the pure spectra of the resolved chemical species were calculated, where that of nimesulide was the same as that obtained experimentally.