Combination therapy of non-steroidal anti-inflammatory drugs (NSAIDs) and methotrexate (MTX) sometimes triggers adverse effects, such as liver injury, renal failure, gastrointestinal disorders, and myelosuppression, owing to the reduction of MTX clearance. Previous reports have suggested that NSAIDs inhibit renal MTX uptake via organic anion transporters (OATs) and reduced folate transporter (RFC)-1 and efflux via multidrug resistance-associated proteins (MRPs). Recently, our laboratory found inhibitory effects of NSAIDs-glucuronide (NSAIDs-Glu), a major metabolite of NSAIDs, on MRP-mediated MTX transport as a new site of interaction between MTX and NSAIDs. However, it remains unclear that whether NSAIDsGlu inhibit renal uptake of MTX. Therefore, the present study aimed to evaluate inhibitory effects of several NSAIDs-Glu (diclofenac, R-and S-ibuprofen, R-and S-flurbiprofen, and R-and S-naproxen) on human OAT1 and OAT3-mediated MTX transport. In this study, [ 3 H]MTX uptake was observed by using human OAT1 and OAT3-overexpressing HEK293 cells in the presence or absence of NSAIDs-Glu. All examined NSAIDs-Glu exhibited concentration-dependent inhibitory effects on MTX uptake via OAT1 and OAT3. Our results indicated that NSAIDs-Glu are more potent (5-to 15-fold) inhibitors of OAT3 than OAT1. Moreover, stereoselective inhibitory effects of NSAIDs-Glu on OATs-mediated MTX uptake were not observed, unlike on MRPs-mediated transport. These findings suggest that inhibition of OAT1 and OAT3-mediated renal uptake of MTX by plasma NSAIDs-Glu may be one of the competitive sites underlying complex drug interaction between MTX and NSAIDs.Key words drug interaction; methotrexate; non-steroidal anti-inflammatory drug; glucuronide; organic anion transporter Although methotrexate (MTX) is often co-administered with non-steroidal anti-inflammatory drugs (NSAIDs) to patients suffering from rheumatoid arthritis and neoplastic diseases, several NSAIDs, such as ketoprofen, indomethacin, naproxen, and diclofenac, reduce MTX clearance. Thus, combined administration of MTX and NSAIDs can result in elevated plasma MTX concentrations and severe adverse effects, such as liver injury, renal failure, gastrointestinal disorders, and myelosuppression. [1][2][3][4][5] In fact, the MTX prescribers' information states that combined use of MTX and NSAIDs may increase plasma MTX concentrations and adverse effects.MTX is primarily excreted in urine via glomerular filtration and active tubular secretion in its unchanged form.6) At a clinical plasma concentration of MTX (0.2-10 µM), approximately 25% of renal clearance is excreted by glomerular filtration and the remaining 75% is excreted by active tubular secretion. 7)In the tubular secretory process, organic anion transporters (OAT) 1 and 3 and reduced folate carrier (RFC)-1 transport MTX from the blood across the basolateral membrane. [8][9][10][11] Multidrug resistance-related proteins (MRP) 2 and 4 and breast cancer-resistant protein (BCRP) transport MTX across the apical membrane into the urine w...

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