Organic anion transporting polypeptides (OATPs in humans, Oatps in other species) are sodium-independent transmembrane proteins that mediate cellular uptake of a broad range of endogenous and exogenous compounds. Polymorphisms affecting the uptake capacity of OATP1B1 (the most studied of the human OATPs) are associated with life-threatening toxicity upon irinotecan treatment, due to increased plasma concentrations of its cytotoxic metabolite, SN-38 (1). Here, we use the recently generated Oatp1a/1b knockout mouse model (Slco1a/1b-/-
, (2)) to investigate the role of Oatp1a/1b in the disposition of irinotecan and SN-38 in vivo
. Upon intravenous administration of 10 mg/kg irinotecan to male wild-type and Slco1a/1b-/-
mice, plasma levels of both irinotecan and SN-38 were significantly increased in the Oatp1a/1b knockout mice, leading to ~2- and 3-fold increased AUCs respectively. Despite the increased plasma exposure, the liver concentrations were not significantly different between the two strains of mice, although the liver concentrations tended to be slightly lower in the Oatp1a/1b knockout mice. However, an impaired liver uptake of irinotecan and SN-38 was evident in the significantly lower liver-to-plasma ratios in the absence of Oatp1a/1b. We also investigated if irinotecan and/or SN-38 distribute extensively to erythrocytes in vitro
, and what is the role of Oatp1a/1b in this process. We added irinotecan or SN-38 to fresh isolated blood from mice, and then determined the amount recovered in plasma and erythrocytes. Unexpectedly, upon addition of irinotecan to blood isolated from Oatp1a/1b knockout mice, there was a much higher conversion rate of irinotecan to SN-38 in comparison with wild-type blood. Irinotecan is converted to SN-38 by carboxyesterases (CES) occurring in plasma and liver. We subsequently demonstrated that liver mRNA levels of Ces3
, the gene encoding one of the enzymes responsible for the conversion of irinotecan to SN-38, were ~600-fold higher in the Slco1a/1b-/-
mice. To circumvent this confounder, we administered SN-38 directly intravenously (1 mg/kg) and evaluated the biliary output of SN-38 in 10 min bile fractions collected in gall bladder cannulated-mice. Absence of Oatp1a/1b resulted in a significantly decreased cumulative biliary excretion of SN-38 starting within 20 min after administration. Based on this data, we conclude that the Oatp1a/1b knockout model is not a suitable model to investigate the disposition of irinotecan (because of the Ces3 upregulation), but there is some evidence that murine Oatp1a/1b uptake transporters have an impact on the disposition of SN-38 after direct administration of this metabolite.
1. Takane H, Kawamoto K, Sasaki T, Moriki K, Moriki K, Kitano H, et al. Life-threatening toxicities in a patient with UGT1A1*6/*28 and SLCO1B1*15/*15 genotypes after irinotecan-based chemotherapy. Cancer Chemother Pharmacol 2009 May;63(6):1165-1169.
2. van de Steeg E, Wagenaar E, van der Kruijssen CM, Burggraaff JE, de Waart DR, Elferink RP, et al. Organic anion transporting polypeptide 1a/1b-knockout mice provide insights into hepatic handling of bilirubin, bile acids, and drugs. J Clin Invest 2010 Aug 2;120(8):2942-2952.