Analysis of commercially available generic formulations of fluoxetine HCl revealed the presence of lactose as the most common excipient. We show that such formulations are inherently less stable than formulations with starch as the diluent due to the Maillard reaction between the drug, a secondary amine hydrochloride, and lactose. The Amadori rearrangement product was isolated and characterized; the characterization was aided by reduction with sodium borohydride and subsequent characterization of this reduced adduct. The lactose-fluoxetine HCl reaction was examined in aqueous ethanol and in the solid state, in which factors such as water content, lubricant concentration, and temperature were found to influence the degradation. N-Formylfluoxetine was identified as a major product of this Maillard reaction and it is proposed that N-formyl compounds be used as markers for this drug-excipient interaction since they are easy to prepare synthetically. Many characteristic volatile products of the Maillard reaction have been identified by GC/MS, including furaldehyde, maltol, and 2,3-dihydro-3,5-dihydroxy-6-methyl-4 H-pyran-4-one. Close similarity between the degradation products of simple mixtures and formulated generic products was found; however, at least one product decomposed at a rate nearly 10 times that predicted from the simple models. Maillard products have also been identified in unstressed capsules. The main conclusion is that drugs which are secondary amines (not just primary amines as sometimes reported) undergo the Maillard reaction with lactose under pharmaceutically relevant conditions. This finding should be considered during the selection of excipients and stability protocols for drugs which are secondary amines or their salts, just as it currently is for primary amines.
Fluoxetine HCl was prepared by seven different synthetic routes, all previously reported. The major impurities in each route were identified by GC/MS, HPLC/MS, and gradient HPLC analysis. Impurities were classified as being derived from impurities in 4-chlorobenzotrifluoride, those arising during the SNAr reaction of this compound and 3-methylamino-1-phenylpropanol, and those arising during the synthesis of this alcohol. Fifteen impurities belonging to the latter two categories were identified, and their structures were confirmed by synthesis of authentic material for most of the compounds. It was found that a variety of analytical tools was needed for complete characterization of the impurity profile of fluoxetine HCl and that purification of the intermediate and recrystallization of the drug itself are highly effective in minimizing the levels of the impurities.
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