Abstract

Transporter gene knockout rats are practically advantageous over murine models for pharmacokinetic and excretion studies, but their phenotypic characterization is lacking. At present, relevant aspects of pharmacokinetics, metabolism, distribution, and excretion of transporter probes [P-glycoprotein (P-gp): loperamide and paclitaxel; breast cancer resistance protein (Bcrp): sulfasalazine; and multidrug resistance-associated protein 2 (Mrp2): carboxydichlorofluorescein] were studied systematically across SAGE P-gp, Bcrp, and Mrp2 knockout rats. In Mdr1a knockout rats, loperamide and paclitaxel oral bioavailability was 2- and 4-fold increased, respectively, whereas clearance was significantly reduced (40–42%), consistent with the expected 10- to 20-fold reduction in paclitaxel excretion. N-Desmethyl-loperamide pharmacokinetics were not altered in any of the three knockouts after oral loperamide. In rats lacking P-gp, paclitaxel brain partitioning was significantly increased (4-fold). This finding is consistent with observations of loperamide central nervous system opioid pharmacology in Mdr1a knockout rats. Sulfasalazine oral bioavailability was markedly increased 21-fold in Bcrp knockouts and, as expected, was also 2- to 3-fold higher in P-gp and Mrp2 knockout rats. The sulfapyridine metabolite/parent ratio was decreased 10-fold in rats lacking Bcrp after oral, but not intravenous, sulfasalazine administration. Carboxydichlorofluorescein biliary excretion was obliterated in Mrp2 knockout rats, resulting in 25% decreased systemic clearance and 35% increased half-life. In contrast, carboxydichlorofluorescein renal clearance was not impaired in the absence of Mrp2, Bcrp, or P-gp. In conclusion, SAGE Mdr1a, Bcrp, and Mrp2 knockout rats generally demonstrated the expected phenotypes with respect to alterations in pharmacokinetics of relevant probe substrates; therefore, these knockout rats can be used as an alternative to murine models whenever a larger species is practically advantageous or more relevant to the drug discovery/development program.