Ex Situ Inhibition of Hepatic Uptake and Efflux Significantly Changes Metabolism: Hepatic Enzyme-Transporter Interplay

Abstract

The disposition of digoxin and the influence of the organic anion transporting polypeptide (Oatp)2 inhibitor rifampicin and the P-glycoprotein (P-gp) inhibitor quinidine on its hepatic disposition were examined in the isolated perfused rat liver. Livers from groups of rats were perfused in a recirculatory manner after a bolus dose of digoxin (10 g), a dual substrate for Oatp2 and P-gp as well as CYP3A. Perfusions of digoxin were also examined in groups of rats in the presence of the inhibitors: rifampicin (100 M) or quinidine (10 M). In all experiments, perfusate samples were collected for 60 min. Digoxin and its primary metabolite were determined in perfusate and liver by liquid chromatography/mass spectrometry. The area under the curve (AUC) from 0 to 60 min was determined. The AUC Ϯ S.D. of digoxin was increased from control (3880 Ϯ 210 nM⅐min) by rifampicin (5200 Ϯ 240 nM⅐min; p Ͻ 0.01) and decreased by quinidine (3220 Ϯ 340 nM⅐min; P Ͻ 0.05). It is concluded that rifampicin limits the hepatic entrance of digoxin and reduced the hepatic exposure of digoxin to CYP3A by inhibiting the basolateral Oatp2 uptake transport, whereas quinidine increased the hepatic exposure of digoxin to CYP3A by inhibiting the canalicular P-gp transport. These data emphasize the importance of uptake and efflux transporters on hepatic drug metabolism.

Characterization of Digoxin Uptake in Sandwich-Cultured Human Hepatocytes

“…In contrast to the results (for humans) reported here, which show digoxin is not a substrate of hepatic OATPs, it has been shown that digoxin is a substrate of hepatic Oatp1a4 in rats (Noé et al, 1997), although there is no ortholog of Oatp1a4 in humans. In addition, amiodarone, quinidine, and rifampicin have been shown to inhibit uptake of digoxin in rat hepatocytes (Lambert et al, 1989;Hedman and Meijer, 1998;Olinga et al, 2001;Kodawara et al, 2002;Lam and Benet, 2004;Lau et al, 2004). None of these compounds inhibited digoxin uptake in SCHH in the current study.…”

Discovering Plausible Mechanistic Details of Hepatic Drug Interactions

“…S5. Those profiles were strikingly similar to corresponding referent quinidine and rifampicin profiles (Lau et al, 2004) without either having undergone METABOLISM. That similarity indicates that rifampicin and quinidine metabolism was very modest.…”

“…Hunt et al (2006) and Yan et al (2008a,b) describe an in silico liver composed of similarly constructed LOBULES. Because perfusate was recycled in (Lau et al, 2004), the data did not require that level of detail, so we designed, validated, and used a simpler in silico liver. The rationale for doing so is expanded on in the supplemental material.…”

“…The mechanisms are relatively well understood (Lau et al, 2004). Digoxin, a P-glycoprotein (P-gp) substrate (de Lannoy and Silverman, 1992), is transported basally in hepatocytes by organic anion-transporting polypeptide (Oatp) 2 (slc21a5) (Noé et al, 1997).…”

“…Spaces were designated to represent bile and perfusate. Three different mobile objects carried information identifying each as representing digoxin, rifampicin, or quinidine (Lau et al, 2004). RISL components used that information to distinguish between them.…”