The prevalent DNA modification in higher organisms is the methylation of cytosine to 5-methylcytosine (5mC), which is partially converted to 5-hydroxymethylcytosine (5hmC) by the Tet (ten eleven translocation) family of dioxygenases. Despite their importance in epigenetic regulation, it is unclear how these cytosine modifications are reversed. Here, we demonstrate that 5mC and 5hmC in DNA are oxidized to 5-carboxylcytosine (5caC) by Tet dioxygenases in vitro and in cultured cells. 5caC is specifically recognized and excised by thymine-DNA glycosylase (TDG). Depletion of TDG in mouse embyronic stem cells leads to accumulation of 5caC to a readily detectable level. These data suggest that oxidation of 5mC by Tet proteins followed by TDG-mediated base excision of 5caC constitutes a pathway for active DNA demethylation.

Background and Purpose Flavonols and terpene lactones are putatively responsible for the properties of Ginkgo biloba leaf extracts that relate to prevention and treatment of cardiovascular disease and cerebral insufficiency. Here, we characterized rat systemic and cerebral exposure to these ginkgo compounds after dosing, as well as the compounds’ pharmacokinetics. Experimental Approach Rats received single or multiple doses of ShuXueNing injection (prepared from GBE50 for intravenous administration) or GBE50 (a standardized extract of G. biloba leaves for oral administration). Brain delivery of the ginkgo compounds was assessed with microdialysis. Various rat samples were analysed using liquid chromatography/mass spectrometry. Key Results Slow terminal elimination features of the flavonols counterbalanced the influence of poor oral bioavailability on their systemic exposure levels, which also resulted in significant accumulation of the compounds in plasma during the subchronic treatment with ShuXueNing injection and GBE50. Unlike the flavonols, the terpene lactones had poor enterohepatic circulation due to their rapid renal excretion and unknown metabolism. The flavonol glycosides occurred as major forms in plasma after dosing with ShuXueNing injection, while the flavonol aglycone conjugates were predominant in plasma after dosing with GBE50. Cerebral exposure was negligible for the flavonols and low for the terpene lactones. Conclusion and Implications Unlike the significant systemic exposure levels, the levels of cerebral exposure to the flavonols and terpene lactones are low. The elimination kinetic differences between the two classes of ginkgo compounds influence their relative systemic exposure levels. The information gained is relevant to linking ginkgo administration to the medicinal effects.

BACKGROUND AND PURPOSEGinsenosides are bioactive saponins derived from Panax notoginseng roots (Sanqi) and ginseng. Here, the molecular mechanisms governing differential pharmacokinetics of 20(S)-protopanaxatriol-type ginsenoside Rg1, ginsenoside Re and notoginsenoside R1 and 20(S)-protopanaxadiol-type ginsenosides Rb1, Rc and Rd were elucidated. EXPERIMENTAL APPROACHInteractions of ginsenosides with human and rat hepatobiliary transporters were characterized at the cellular and vesicular levels. A rifampin-based inhibition study in rats evaluated the in vivo role of organic anion-transporting polypeptide (Oatp)1b2. Plasma protein binding was assessed by equilibrium dialysis. Drug-drug interaction indices were calculated to estimate potential for clinically relevant ginsenoside-mediated interactions due to inhibition of human OATP1Bs. KEY RESULTSAll the ginsenosides were bound to human OATP1B3 and rat Oatp1b2 but only the 20(S)-protopanaxatriol-type ginsenosides were transported. Human multidrug resistance-associated protein (MRP)2/breast cancer resistance protein (BCRP)/bile salt export pump (BSEP)/multidrug resistance protein-1 and rat Mrp2/Bcrp/Bsep also mediated the transport of the 20(S)-protopanaxatriol-type ginsenosides. Glomerular-filtration-based renal excretion of the 20(S)-protopanaxatriol-type ginsenosides was greater than that of the 20(S)-protopanaxadiol-type counterparts due to differences in plasma protein binding. Rifampin-impaired hepatobiliary excretion of the 20(S)-protopanaxatriol-type ginsenosides was effectively compensated by the renal excretion in rats. The 20(S)-protopanaxadiol-type ginsenosides were potent inhibitors of OATP1B3. CONCLUSION AND IMPLICATIONSDifferences in hepatobiliary and in renal excretory clearances caused markedly different systemic exposure and different elimination kinetics between the two types of ginsenosides. Caution should be exercised with the long-circulating 20(S)-protopanaxadiol-type ginsenosides as they could induce hepatobiliary herb-drug interactions, particularly when patients receive long-term therapies with high-dose i.v. Sanqi or ginseng extracts.