Discoloration and chemical stability of entacapone in concomitant simple suspensions with magnesium oxide

Abstract Background In clinical practice, entacapone and magnesium oxide (MgO) tablets are administered via enteral tubes using a simple suspension method. Simple co-suspensions of entacapone and MgO gradually turned deep red. However, there is still no report about the cause of the discoloration. In this study, the chemical stability of entacapone in co-suspension with MgO was investigated, and the cause of discoloration was elucidated. Passability of the drug through the feeding tube was also evaluated. Methods An entacapone tablet was suspended, with or without an MgO tablet using a simple suspension method. The supernatants of the suspensions were analyzed using high-performance liquid chromatography to examine the chemical stability of entacapone and the passage of the drug through an enteral tube. The supernatants were analyzed using ultraviolet-visible spectrophotometry. Aqueous solutions of entacapone were prepared at various pH values and analyzed using ultraviolet-visible spectrophotometry and 1 H nuclear magnetic resonance spectroscopy. Results High-performance liquid chromatography of simple suspensions with or without MgO showed only one entacapone peak, and the recovery of entacapone from the suspensions was quantitative, even after passing through an enteral tube. Using aqueous solutions of authentic entacapone, the colors and the ultraviolet-visible spectra were found to be dependent on the pH values. Conclusions Entacapone is stable in simple suspensions with or without MgO, and the cause of discoloration by the co-suspension is plausibly not the degradation of entacapone, but the pH-dependent formation of the divalent anion. Discoloration of the co-suspensions has no effect on the dose in tube administration.