The objective of this study is to confirm the feasibility of using a panel of endogenous substrates/metabolites as a robust biomarker of OCTs and OATs by conducting a controlled, comprehensive clinical drug-drug interaction study in healthy adult volunteers. Metformin and furosemide will be used as probe drugs for OCTs and OATs, respectively; cimetidine and probenecid will be used as corresponding inhibitors. Results from this study will validate this novel approach, which will be extended to children by collaborators at Children's Mercy Hospital in Kansas City, MO.

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.

Inclusion Criteria:

Are from 18-65 years old and healthy

• Are not taking any medications (prescription and non-prescription) or dietary/herbal supplements that can interfere with your ability to eliminate the study drugs from your body
• Are willing to stop taking dietary/herbal supplements and citrus juices for several weeks
• Are willing to stop consuming caffeinated beverages or other caffeine-containing products the evening before and the morning of the first day of each study arm
• Are willing to stop drinking alcoholic beverages for at least 1 day prior to any study day and during the study day
• Are willing to use an acceptable method of birth control that does not include oral contraceptive pills or patches (such as abstinence, copper IUD, condom) throughout your participation in the study and for at least 3 weeks after you last take the study drugs
• Have the time to participate

Exclusion Criteria:

• Are under 18 or over 65 years old
• Smoke/vape/chew tobacco products
• Use cannabis products, including marijuana, hemp, and other THC- and CBDcontaining products• Are taking medications or dietary/herbal supplements that can interfere with your ability to eliminate the study drugs from your body
• Have a chronic illness such as (but not limited to) kidney disease, liver disease, diabetes mellitus, high blood pressure, coronary artery disease, chronic obstructive pulmonary disease, cancer, or HIV/AIDS
• Have a hematologic (blood) disorder
• Have a history of drug or alcohol addiction or major psychiatric illness
• Have a history of allergy to metformin, cimetidine, furosemide, or probenecid
• Are pregnant, nursing, or plan to become pregnant within 3 weeks after participation

10 months