Endogenous Coproporphyrin I and III are Altered in Multidrug Resistance-Associated Protein 2-Deficient (TR) Rats.

Recent studies have focused on coproporphyrin (CP)-I and CP-III (CPs) as endogenous biomarkers for organic anion transporting polypeptides (OATPs). Previous data showed that CPs are also substrates of multidrug resistance-associated protein (MRP/Mrp) 2 and 3. This study was designed to examine the impact of loss of Mrp2 function on the routes of excretion of endogenous CPs in wild-type (WT) Wistar compared to Mrp2-deficient TR rats. To exclude possible confounding effects of rat Oatps, the transport of CPs was investigated in Oatp-overexpressing HeLa cells. Results indicated that CPs are substrates of rodent Oatp1b2, and that CP-III is a substrate of Oatp2b1. Quantitative targeted absolute proteomic (QTAP) analysis revealed no differences in Oatps, but an expected significant increase in Mrp3 protein levels in TR compared to WT rat livers. CP-I and CP-III concentrations measured by LC-MS/MS were elevated in TR compared to WT rat liver, while CP-I and CP-III estimated biliary clearance was decreased 75- and 840-fold in TR compared to WT rats, respectively. CP-III concentrations were decreased 14-fold in the feces of TR compared to WT rats, but differences in CP-I were not significant. In summary, the disposition of CPs was markedly altered by loss of Mrp2 and increased Mrp3 function as measured in TR rats.