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The aim of this study was to investigate the effect of quercetin on P-glycoprotein (P-gp) transport ability in vivo.Genotype data were available from a total of 165 health volunteers. An open, randomized, two-period crossover clinical trial was performed in eighteen subjects with different MDR1 3435 C/T genotypes. All subjects took 500mg quercetin or placebo daily from 1st to 13th day or from 43st to 55th day, and 100mg talinolol was given at the 14th or 56th day. The washout period is 28 days.In this study, we found the values of area under the curve (AUC)0-48 h, AUC0-∞ and Cmax of talinolol in all subjects significantly decreased (6496.6±2389.9 vs 7809.5±2386.8 ng.h/ml, P=0.04), (8414.7±344.8 vs 10478.2±4195.4 ng.h/ml, P=0.03), (412.9±132.6 vs 543.3±97.9 ng.h/ml, P=0.01) after administration of quercetin, respectively. There were no significant differences in tmax and t1/2 of talinolol. The results also showed AUC0-48 h (5598.6±2202.1 vs 8229.4±1491.7 ng.h/ml, P=0.02) and AUC0-∞ (7110.0±3437.0 vs 12681.2±4828.2 ng.h/ml, P=0.01) of talinolol to be significantly decreased in MDR1 3435 TT individuals administered of quercetin. The Cmax of talinolol in MDR1 3435 TT (382.4±149.1 vs 584.9±115.2 ng/ml, P=0.04) and MDR1 3435 CT (383.5±104.9 vs 554.6±80.6 ng/ml, P=0.01) individuals significantly decreased after the administration of quercetin.Quercetin significantly induced the activity of P-gp and this induced effect was more obvious in MDR1 3435 TT individuals.