Gut microbiome-related tryptophan metabolites, particularly indole-3-acrylic acid, indole-3-propionic acid, kynurenic acid, xanthurenic acid, and 3-hydroxyanthranilic acid, inhibit OAT1 and OAT3 transporters in vitro and increase systemic exposure of furosemide by 1.3- to 2.9-fold in rats.
Key Findings
Results
Five gut microbiome-related tryptophan metabolites inhibited OAT1 activity by up to 83.7% in vitro.
The metabolites tested were indole-3-acrylic acid (IA), indole-3-propionic acid (IPA), kynurenic acid (KA), xanthurenic acid (XA), and 3-hydroxyanthranilic acid (HAA).
IC50 values for OAT1 ranged from 5.41 to 121 μM across these five metabolites.
Assays were conducted using transporter-overexpressing cell lines.
The remaining 7 of the 12 tryptophan metabolites tested showed minimal effects on OAT1.
Results
The same five tryptophan metabolites inhibited OAT3 activity, with IC50 values ranging from 0.31 to 9.50 μM.
OAT3 inhibition was observed for IA, IPA, KA, XA, and HAA.
IC50 values for OAT3 (0.31 to 9.50 μM) were notably lower than those for OAT1 (5.41 to 121 μM), suggesting greater potency at OAT3.
Inhibition reached up to 83.7% across both OAT1 and OAT3 assays.
Assays were performed using transporter-overexpressing cell lines.
Results
Molecular docking analysis provided qualitative support for potential interactions of the tryptophan metabolites with OAT1.
Molecular docking was used to assess binding interactions at the OAT1 transporter.
The analysis was described as providing 'qualitative support' for the interactions observed in vitro.
The paper does not report specific docking scores for individual metabolites in the abstract.
Molecular docking was used as a complementary method to the in vitro transporter assays.
Results
Coadministration of the five inhibitory tryptophan metabolites significantly increased systemic exposure of furosemide by 1.3- to 2.9-fold in rats.
Furosemide was used as a representative OAT1/OAT3 substrate in in vivo pharmacokinetic studies.
Studies were conducted in rats.
Systemic exposure increases ranged from 1.3-fold to 2.9-fold depending on the specific metabolite coadministered.
Changes in renal excretion of furosemide accompanied the increases in systemic exposure.
These results were described as 'proof-of-concept rat pharmacokinetic data.'
Results
Most of the 12 tryptophan metabolites tested showed minimal effects on six other major drug transporters.
Transporters assessed beyond OAT1 and OAT3 included OATP1B1, OATP1B3, OCT2, MATE1, MDR1, and BCRP.
The lack of significant inhibition of these transporters suggests selectivity of the active metabolites toward OAT1 and OAT3.
Both hepatic uptake transporters (OATP1B1/1B3) and efflux transporters (MDR1, BCRP) were minimally affected.
The renal cation transporter OCT2 and its associated efflux transporter MATE1 were also largely unaffected.
Background
The gut microbiome modulates tryptophan metabolism through both direct generation of indole derivatives and indirect modulation of the host-driven kynurenine pathway.
Indole derivatives such as IA and IPA arise directly from microbial tryptophan catabolism.
Kynurenine pathway metabolites such as KA, XA, and HAA are host-derived but are modulated indirectly by microbial metabolites.
A total of 12 gut microbiome-related tryptophan metabolites were examined in this study.
This mechanistic framing provides the rationale for investigating these metabolites as potential modulators of drug disposition.
Conclusions
The authors conclude that further studies are required to determine whether the transporter-related effects of tryptophan metabolites are clinically relevant in humans.
The in vivo data were generated in rats and described as 'proof-of-concept.'
The paper notes that 'further studies are required to determine whether these transporter-related effects are clinically relevant in humans.'
The study provides mechanistic in vitro evidence and rat pharmacokinetic data but does not include human data.
Clinical relevance would depend on whether circulating concentrations of these metabolites in humans reach inhibitory levels relative to the reported IC50 values.
What This Means
This research suggests that certain natural byproducts of gut bacteria may interfere with the body's ability to transport and eliminate drugs. The gut microbiome breaks down the amino acid tryptophan into various chemical compounds, and this study tested 12 of those compounds against several major drug transporter proteins. Five of them—indole-3-acrylic acid, indole-3-propionic acid, kynurenic acid, xanthurenic acid, and 3-hydroxyanthranilic acid—were found to significantly block two kidney transporters called OAT1 and OAT3, which are responsible for removing many common drugs from the body through urine.
When these five metabolites were given alongside furosemide, a widely used diuretic (water pill) that relies on OAT1 and OAT3 for elimination, the amount of furosemide in the bloodstream of rats increased by 1.3- to 2.9-fold compared to furosemide given alone. This happened because the metabolites partially blocked the kidney transporters, slowing the drug's removal. Importantly, these compounds had little to no effect on six other major drug transporters, suggesting their inhibitory action is relatively specific to OAT1 and OAT3.
This research suggests that the composition of a person's gut microbiome could influence how their body processes certain medications by altering the levels of these tryptophan-derived compounds. If people with different gut microbiome profiles produce varying amounts of these metabolites, it could help explain why some individuals respond differently to drugs cleared by OAT1 and OAT3. However, the study was conducted in cell lines and rats, and the authors emphasize that additional research is needed to determine whether these effects are meaningful in humans.
Kang M, Lee K, Kim M, Jeong H, Chae Y. (2026). Gut microbiome-related tryptophan metabolites modulate drug transporters, with prominent effects on OAT1 and OAT3.. Toxicology and applied pharmacology. https://doi.org/10.1016/j.taap.2026.117868