Comparison of in vitro metabolism and cytotoxicity of capsaicin and dihydrocapsaicin

• We compare in vitro metabolism and cytotoxicity of capsaicin and dihydrocapsaicin.
• Novel in vitro metabolites were identified for both capsaicinoids by LC–MS/MS.
• Cytotoxic responses were more pronounced for dihydrocapsaicin than capsaicin.

Capsaicin and dihydrocapsaicin are the major active components in pepper spray products, which are widely used for law enforcement and self-protection. The use of pepper sprays, due to their irreversible and other health effects has been under a strong debate. In this study, we compared metabolism and cytotoxicity of capsaicin and dihydrocapsaicin using human and pig liver cell fractions and human lung carcinoma cell line (A549) in vitro. Metabolites were screened and identified by liquid chromatography–tandem mass spectrometry (LC–MS/MS). Using liver cell fractions, a novel aliphatic hydroxylated metabolite (m/z 322) was detected to dihydrocapsaicin but no structure was found corresponding to capsaicin. Instead, a novel phase I metabolite of capsaicin, corresponding to the structure of aliphatic demethylation and dehydrogenation (m/z 294) was identified. In addition, two novel conjugates, glycine conjugates (m/z 363 and m/z 365) and bi-glutathione (GSH) conjugates (m/z 902 and m/z 904), were identified for both capsaicin and dihydrocapsaicin. The medium of the exposed A549 cells contained ω-hydroxylated (m/z 322) and alkyl dehydrogenated (m/z 304) forms, as well as a glycine conjugate of capsaicin. As to dihydrocapsaicin, an alkyl dehydrogenated (m/z 306) form, a novel alkyl hydroxylated form, and a novel glycine conjugate were found. In A549 cells, dihydrocapsaicin evoked vacuolization and decreased cell viability more efficiently than capsaicin. Furthermore, both compounds induced p53 protein and G1 phase cell cycle arrest. Usefulness of the found metabolites as biomarkers for capsaicinoid exposures will need further investigations with additional toxicity endpoints.