| Rebaudioside A(RA), a diterpene glycoside found in stevia extracts, has been used frequently as a natural sweetener in beverages as well as a functional food component. After oral ingestion, RA is hydrolyzed by microflora to steviol, the aglycone, in the gastrointestinal tract. Steviol is then absorbed into the circulation and subject to glucuronidation and renal excretion. In humans and rats, the major circulating metabolite of RA is steviol acyl glucuronide(SVAG). In the present study, the potential interaction between SVAG and selected drugs was investigated in vitro and in vivo.Pharmacokinetic studies in rats revealed that the formation of SVAG reached a maximum approximately 6hr after oral administration. In the liver, the SVAG concentration was more than 20-times higher than that in plasma. Rats treated with antibiotics showed decreased activities of beta-glucosidase and beta-glucuronidase in the gastrointestinal tract. Consequently, the plasma Cmax and AUC of SVAG were reduced by more than 3-fold. In contrast, the antibiotic treatment did not alter the liver to plasma ratio of SVAG.Because recent reports have shown that acyl glucuronides of gemfibrozil and clopidogrel are potent time-dependent inhibitors of CYP2C8 activity, the inhibitory effect of steviol acyl glucuronide(SVAG) on major cytochrome P450 enzymes was investigated. Results showed that SVAG did not inhibit the activities of major CYP enzymes such as CYP1A2, 2C9, 2C19, 2D6 and 3A4, except CYP2C8. Further investigations demonstrated that SVAG was a reversible but not a time-dependent inhibitor of CYP2C8-mediated paclitaxel 6α-hydroxylation. SVAG was also capable of inhibiting CYP2C8-mediated repaglinide 3’-hydroxylation in human liver microsomes and recombinant human CYP2C8, with Ki values of 15.8 μM and 11.6 μM, respectively. In contrast, SVAG did not exhibit inhibitory effect on CYP2C8 activity in rat liver microsomes. In addition, co-administration of rebaudioside A with repaglinide in rats did not lead to AUC and Cmax changes of repaglinide. Although mathematic prediction using a simplified mechanistic model revealed a moderate interaction potential between repaglinide and SVAG, cautions should be given to patients with hypoglycemia if repaglinide and rebaudioside A are used in combination for the blood sugar control.The kidney is the main organ for excretion of drugs and xenobiotics. Organic anion transporters(OATs) are located in the proximal tubular basolateral membranes and play important roles in the uptake of drugs and xenobiotics from the blood. Steviol acyl glucuronide(SVAG) was excreted primarily by kidney in humans and our previous studies showed that the SVAG was a substrate of OAT3. Using stably transfected HEK293 cells with hOAT3, studies showed that the uptake of SVAG was inhibited by probenecid with an IC50 value of 4.95 μM. In kidney slices, the active uptake of SVAG followed a weak Michalis-Menten kinetics with a Km of 368.1 μM. The SVAG uptake would be suppressed by inhibitors of human OATs including probenecid and estrone-3-sulfate. When RA was co-administered with 20 mg/kg probenecid, pharmacokinetic data showed a 2.15- and 1.71- flod increase in the Cmax and the AUC values of steviol acyl glucuronide.In conclusion, the intestinal microflora could play a role in the formation of SVAG and the organic anion transporter 3(OAT3) might play a vital role in SVAG kidney uptake and renal clearance that could be affected by strong inhibitors. Furthermore, SVAG showed moderate and reversible inhibitory effect on CYP2C8 activity. Taken together, cautions should be given when RA is combined with OAT inhibitors such as probenecid or in patients with kidney diseases when repaglinide and RA are used in combination for blood sugar control. |
