Role of ABCG2 in Secretion into Milk of the Anti-Inflammatory Flunixin and Its Main Metabolite: In Vitro-In Vivo Correlation in Mice and Cows

Flunixin meglumine is a nonsteroidal anti-inflammatory drug (NSAID) widely used in veterinary medicine. It is indicated to treat inflammatory processes, pain, and pyrexia in farm animals. In addition, it is one of the few NSAIDs approved for use in dairy cows, and consequently gives rise to concern regarding its milk residues. The ABCG2 efflux transporter is induced during lactation in the mammary gland and plays an important role in the secretion of different compounds into milk. Previous reports have demonstrated that bovine ABCG2 Y581S polymorphism increases fluoroquinolone levels in cow milk. However, the implication of this transporter in the secretion into milk of anti-inflammatory drugs has not yet been studied. The objective of this work was to study the role of ABCG2 in the secretion into milk of flunixin and its main metabolite, 5-hydroxyflunixin, using Abcg2((-/-)) mice, and to investigate the implication of the Y581S polymorphism in the secretion of these compounds into cow milk. Correlation with the in vitro situation was assessed by in vitro transport assays using Madin-Darby canine kidney II cells overexpressing murine and the two variants of the bovine transporter. Our results show that flunixin and 5-hydroxyflunixin are transported by ABCG2 and that this protein is responsible for their secretion into milk. Moreover, the Y581S polymorphism increases flunixin concentration into cow milk, but it does not affect milk secretion of 5-hydroxyflunixin. This result correlates with the differences in the in vitro transport of flunixin between the two bovine variants. These findings are relevant to the therapeutics of anti-inflammatory drugs.