Pharmacokinetic Drug-Drug Interaction Study to Identify Biomarkers of Kidney Transporters

The kidneys are major organs responsible for the excretion of both endogenous and exogenous compounds, the latter including drugs and other xenobiotics. Excretion occurs via passive or active processes, the latter involving transporters such as organic cation transporters (OCTs) and organic anion transporters (OATs). Inhibition of these transporters, coupled with the large interindividual variability in transporter expression, can lead to toxic accumulation of compounds/xenobiotics cleared primarily by this route. During drug discovery and development, if in vitro evidence suggests renal transporters mediate excretion of a new chemical entity, the Food and Drug Administration recommends conducting a controlled clinical study to evaluate potential risks. These time-consuming and expensive clinical studies routinely involve adult participants and known substrates of renal transporters. However, such studies are not always feasible in children due to the enhanced potential for toxicities. This limitation led to the hypothesis that endogenous substrates could be used as surrogates, or biomarkers, of individual renal transporter function.

Endogenous OCT substrates, such as 1-methyladenosine (m1A) and 1-methylnicotinamide (MNA), as well as OATs, such as homovanillic acid (HVA) and pyridoxic acid (PDA), are promising biomarkers of renal transporters in adults. However, using one or few such endogenous substrates can be misleading due to factors other than variability in specific renal transporter function. We propose to address this knowledge gap by using a panel of endogenous substrates/metabolites that recently has been identified as a robust biomarker of rodent Octs and Oats. Validation of these substrates/metabolites as biomarkers of OCTs and OATs in humans, both adults and children, will aid in the development of physiologically-based pharmacokinetic models that can be used to predict renal transporter-mediated xenobiotic excretion, drug-drug interactions, and toxicity in children.