Abstract
Organic anion transporting polypeptides (OATP) 1B1 and 1B3 are expressed in liver cells and are involved in drug uptake in the liver. OATP1B activity varies due to polymorphisms and is decreased by OATP1B inhibitors. Variability of OATP1B activity impacts the pharmacokinetics of OATP1B substrate drugs through drug-drug interactions. Lately, coproporphyrin-I (CP-I) and -III (CP-III) have been featured as quantitative endogenous biomarkers for evaluating the activity of OATP1B. CP-III has been reported to be transported not only by OATP1B but also by OATP2B1 in vitro. We have established a highly sensitive assay for the simultaneous measurement of CP-I and CP-III using a small volume of human plasma. The sample was pretreated by solid-phase extraction and analyzed by ultra-performance liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (UPLC-QTOF/MS). This method that uses 100 μL of plasma met the acceptance criteria of the US Food and Drug Administration guidance for bioanalytical method validation, and the lower limit of quantification was 0.01 ng/mL for both coproporphyrins. The clinical application of the method was evaluated by measuring plasma CP-I and CP-III concentrations in healthy volunteers and rheumatoid arthritis (RA) patients. The measured concentrations were within the calibration range (0.01-50 ng/mL). Using this novel method to measure plasma concentrations of CP-I and CP-III may contribute to the evaluation of the activities of OATP1B1, OATP1B3, and OATB2B1 in healthy individuals and patients with various clinical conditions including RA.