# Indoxyl Sulfate in the Gut–Kidney Axis: Pathophysiology and Clinical Significance in CKD-Associated Colorectal Cancer

## TL;DR

This paper explores how indoxyl sulfate in the gut-kidney axis contributes to colorectal cancer in patients with chronic kidney disease and suggests new diagnostic and therapeutic approaches.

## Contribution

The paper introduces a two-compartment model of indoxyl sulfate and proposes novel diagnostic and therapeutic strategies for CKD-associated CRC.

## Key findings

- CKD-induced dysbiosis leads to increased hepatic synthesis of indoxyl sulfate, which promotes CRC via AhR and Akt signaling.
- A two-compartment model suggests systemic and localized indole hotspots contribute to CRC carcinogenesis.
- A novel diagnostic panel combining serum and urinary indoxyl sulfate is proposed to guide therapy.

## Abstract

Chronic Kidney Disease (CKD) and Colorectal Cancer (CRC) share a profound epidemiological link, supported by Mendelian randomization studies suggesting causality. This review articulates a refined Gut–Kidney Axis, focusing on the pathophysiology of indole-derived uremic toxins. CKD-induced dysbiosis drives hepatic synthesis and systemic accumulation of indoxyl sulfate, which is proposed to promote carcinogenesis via Aryl Hydrocarbon Receptor (AhR) and Akt signaling, ultimately upregulating c-Myc and EGFR. We propose a two-compartment model: while systemic indoxyl sulfate reflects the total gut indole pool (mainly from planktonic bacteria), adherent bacteria like Fusobacterium nucleatum may create high-concentration indole hotspots within the tumor microenvironment. Clinically, we advocate for protein-independent DNA methylation biomarkers (SEPT9, SDC2) to avoid renal confounding. Furthermore, we propose a novel diagnostic panel integrating serum indoxyl sulfate (systemic load) and urinary indoxyl sulfate (gut production) to guide therapy. Therapeutically, targeting upstream drivers (AhR/Akt) may bypass resistance to anti-EGFR therapies in KRAS-mutated tumors. We also discuss the repurposing of the oral adsorbent AST-120 and emerging bacteriophage therapies as strategies to disrupt this oncogenic axis. This review offers a comprehensive framework for stratified management of CKD-associated CRC.