Quantification of fumarate and investigation of endogenous and exogenous fumarate stability in rat plasma by LC–MS/MS

Background: Fumaric acid is a commonly used excipient in pharmaceutical products. It is not known if its presence may lead to fluctuation of endogenous fumarate levels. An LC–MS/MS method was developed and validated to quantify fumarate in support of a toxicokinetics study. Results: Stability evaluation showed that endogenous fumarate was stable for 6 h at room temperature, while exogenously added fumaric acid was converted to malate within 1 h due to the presence of fumarase. Citric acid, a fumarase inhibitor, prevented the conversion of added fumaric acid in rat plasma. Conclusion: The method was validated in citric acid stabilized rat plasma using a surrogate matrix approach. A discrepancy in stability was observed between endogenous fumarate and exogenously added fumaric acid.