This study aimed to investigate the effects of calcitriol on endogenous biomarkers taurine and pyridoxic acid (PDA) associated with the organic anion transporters Oat1 and Oat1/3, respectively, and compare these changes with those observed for the clinical substrate, methotrexate (MTX). Male rats were administered intraperitoneal (i.p.) injections of either vehicle (maize oil) or calcitriol (2.56 nmol/kg/day) for 4 consecutive days. Plasma, urine, and tissue samples were analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Calcitriol markedly increased plasma taurine levels, decreased its urinary excretion, and reduced taurine concentrations in most tissues. In contrast, PDA exhibited only a moderate increase in plasma levels, with no significant change in urinary excretion, but a notable increase in kidney concentrations. Additional probenecid inhibition studies supported the Oat1-mediated modulation. Intravenous (i.v.) pharmacokinetic studies of taurine (10 mg/kg) revealed altered plasma exposure, clearance, and tissue distribution following calcitriol and probenecid inhibition. In addition, calcitriol significantly affected MTX pharmacokinetics, reinforcing its effect on Oat1/3 function. Taurine induced more significant changes than PDA, indicating its greater sensitivity as an endogenous biomarker for Oat1 activity. These findings highlight the modulatory effects of calcitriol on Oat-mediated transport and demonstrate the utility of taurine and PDA as translational biomarkers for investigating transporter-mediated drug-drug interactions in drug development.
Vo et al. (Tue,) studied this question.