Pharmacokinetics and brain distribution studies of 6-hydroxykynurenic acid and its structural modified compounds.

OBJECTIVES: 6-Hydroxykynurenic acid (6-HKA) is an organic acid component in extracts of Ginkgo biloba leaves and acts as a major contributor to neurorestorative effects, while its oral bioavailability was low. Therefore, using prodrug method to improve the bioavailability and brain content of 6-HKA is significant. METHODS: Three structural modified compounds of 6-HKA were synthesized, and ultra performance liquid chromatography-tandem mass spectrometry methods for quantification of these structural modified compounds in rat plasma and rat brain homogenate were established and comprehensively validated. The methods were effectively applied to investigate the effects of structural modification on apparent permeability coefficients in cells, the pharmacokinetics and the brain distribution in rats. KEY FINDINGS: The results illustrated that esterification can greatly improve the apparent permeability coefficient and bioavailability of 6-HKA. Comparing with direct oral administration of 6-HKA, the bioavailability of isopropyl ester was greatly improved (from 3.96 ± 1.45% to 41.8 ± 15.3%), and the contents of 6-HKA in rat brains (49.7 ± 9.2 ng/g brain) were significantly higher after oral administration. CONCLUSIONS: The bioavailability and the brain content of 6-HKA can be improved by the prodrug method. Among three structural modified compounds, isopropyl-esterified 6-HKA was the most promising treatment.

The characterisation of the in vitro metabolism and transport of 6-hydroxykynurenic acid, an important constituent of Ginkgo biloba extracts.

6-Hydroxykynurenic acid (6-HKA) is a nitrogen-containing phenolic acid compound in Ginkgo biloba leaves. The pharmacological activities of 6-HKA have been reported and shown that 6-HKA has the potential to become a therapeutic drug and may play an important role in the treatment of nervous system diseases. However, there are few studies on the drug metabolism and transport of 6-HKA. The aim of this study is to investigate the in vitro metabolism of 6-HKA and its interaction with multiple important drug transporters.The in vitro metabolism experiments in the present study demonstrate that 6-HKA might not undergo phase-I or phase-II metabolism in hepatic microsomes/S9 of rats. In addition, some drug transporters, including OAT1/3, OCT2, MDR1, OATP1B1, MATE1/2K and OCTN2, were investigated. The cellular uptake assays indicate that 6-HKA exhibits inhibition to the transport of classical substrates mediated by OAT3, OCT2, MATE2K and OCTN2 but has no significant effect on the transport of substrates mediated by MDR1, OAT1, OATP1B1 or MATE1. Further investigation of cellular accumulation assays shows that 6-HKA might be the substrate of OAT3, but not OCT2 or OCTN2. The bidirectional transport study suggests that 6-HKA is not a substrate of MDR1.The information about the in vitro metabolism of 6-HKA and the interaction between 6-HKA and some transporters will help us to better understand the pharmacokinetic properties of 6-HKA and provide reference for its pharmacodynamics, DDIs and drug-food interactions studies.