Abstract

Although the metabolism and disposition of diclofenac (DF) has been studied extensively, information regarding the plasma levels of diclofenac acyl-β-D-glucuronide (DF-AG), a major metabolite, in human subjects is limited. Therefore, DF-AG concentrations were determined in plasma (acidified blood derived) of six healthy volunteers following a single oral DF dose (50 mg). Levels of DF-AG in plasma were high, as reflected by a DF-AG/DF ratio of 0.62 ± 0.21 (Cmax mean ± SD) and 0.84 ± 0.21 (area under the concentration-time curve mean ± SD). Both DF and DF-AG were also studied as substrates of different human drug transporters in vitro. DF was identified as a substrate of organic anion transporter 2 only (OAT2, Km = 46.8 μM). In contrast, DF-AG was identified as a substrate of numerous OATs (Km = 8.6, 112, 207, 244 μM; OAT2, OAT3, OAT4, and OAT1, respectively), two organic anion-transporting polypeptides (OATP1B1, Km = 34 μM; OATP2B1, Km = 105 μM), breast cancer resistance protein (Km = 152 μM), and two multidrug resistance proteins (MRP2, Km = 145 μM; MRP3, Km = 196 μM). It is concluded that the disposition of DF-AG, once formed, can be mediated by various candidate transporters known to be expressed in the kidney (basolateral, OAT1, OAT2, OAT3; apical MRP2, BCRP and OAT4) and liver (canalicular, MRP2 and BCRP; basolateral, OATP1B1, OATP2B1, OAT2 and MRP3). DF-AG is unstable in plasma and undergoes conversion to parent DF. Therefore, caution is warranted when assessing renal and hepatic transporter-mediated drug-drug interactions with DF and DF-AG.