Artifactual formylation of the secondary amine of duloxetine hydrochloride by acetonitrile in the presence of titanium dioxide: Implications for HPLC method development

Duloxetine hydrochloride, a secondary amine containing pharmaceutical, currently marketed as Cymbalta™, is shown to undergo N-formylation as an artifact of sample preparation prior to HPLC analysis for impurities. The reaction was discovered as a result of an investigation into variability in the levels of N-formyl duloxetine observed upon HPLC analysis. The reaction is catalyzed by sonication and/or light in the presence of titanium dioxide and is proposed to occur via a radical-initiated mechanism. The mechanism is supported by controlled sample preparation studies with deuterium-labeled acetonitrile and LC/MS studies that showed incorporation of one deuterium into N-formyl duloxetine. This discovery is broadly relevant because sonication is commonly used to aid dissolution of pharmaceuticals in acetonitrile for HPLC analysis, titanium dioxide is a commonly used excipient, the amount of light found in modern analytical laboratories is sufficient to cause the reaction to occur, and secondary amines are present in the structures of many pharmaceuticals. The artifactual reaction was effectively eliminated by changing the sample solvent to methanol and replacing sonication with shaking to aid sample dissolution.