A Forum for Improving Drug Safety

COMMENT: The mutant in vivo mouse model or the so-called “knock out” (ko) mouse in which a single gene is deleted or disrupted is increasingly used for the exploration of drug interactions. In the case of the OATP transporters, the so-called SLCO or SLC21 subfamily, the mouse has a single hepatic SLCO transporter, the SLC01B2 tranporter, in place of the 2 human hepatic transporters, SLC01B1 and SLCO1B3 which reside on the basolateral membrane of liver cells (the portal vein side) and influx many common drugs including rifampin and pravastatin.

In this article, details are given of how this “ko” mouse was utilized to test these 2 known SLCO substrates. Conjugated bilirubin was modestly increased, but no jaundice was seen in the ko mice suggesting that other liver transporters take up the slack of importing conjugated bilirubin when SLCO1B2 is lacking. The plasma AUC of rifampin in the KO mouse was 1.7 times that of the wild-type and the plasma clearance of rifampin was reduced. Pravastatin was similarly affected. The authors make it clear that mice SLCOs are different enough from humans that an exact comparison remains unjustifiable, but this in vivo animal model is useful for substrate, inhibitor and inducer analysis as well as drug interaction testing.

ABSTRACT:Mol Pharmacol. 2008 Apr 15 [Epub ahead of print] Targeted Disruption of Murine Organic Anion-Transporting Polypeptide 1b2 (oatp1b2/Slco1b2) Significantly Alters Disposition of Prototypical Drug Substrates Pravastatin and Rifampin. Zaher H, Meyer Zu Schwabedissen HE, Tirona RG, Cox ML, Obert LA, Agrawal N, Palandra J, Stock JL, Kim RB, Ware JA.

Organic anion transporting polypeptide (OATP) 1B1 and 1B3 are widely acknowledged as important and rate-limiting to the hepatic uptake of many drugs in clinical use. Accordingly, to better understand the in vivo relevance of OATP1B transporters, targeted disruption of murine Slco1b2 gene was carried out. Interestingly,Slco1b2(-/-) mice were fertile, developed normally, and did not exhibit any overt phenotypic abnormalities. We confirmed the loss of Oatp1b2 expression in liver using real-time PCR, Western Blot analysis and immunohistochemistry. Oatp1a4 and Oatp2b1, but not Oatp1a1 expression was greater in female Slco1b2(-/-) mice, but expression of other non-oatp transporters did not significantly differ between wildtype and Slco1b2(-/-) males. Total bilirubin level was elevated by 2-fold in the Slco1b2(-/-) mice despite that liver enzymes ALT and AST were normal. Pharmacological characterization was carried out using two prototypical substrates of human OATP1B1 and 1B3, rifampin and pravastatin. After a single intravenous dose of rifampin (1mg/kg), a 1.7-fold increase in plasma AUC was observed while the liver-to-plasma ratio was reduced by 5-fold, and nearly 8- fold when assessed at steady -state conditions after 24 hours of continuous subcutaneous (SC) infusion in Slco1b2(-/-) mice.

Similarly, continuous SC-infusion at low dose rate (8 microg/hr) or high dose rate (32 microg/hr) pravastatin resulted in a 4-fold lower liver-plasma ratio in the in Slco1b2(-/-) mice. This is the first report of altered drug disposition profile in the Slco1b2 knockout mice and suggests the utility of this model for understanding the in vivo role of hepatic OATP transporters in drug disposition.