The gut-kidney axis: indoxyl sulfate, p-cresyl sulfate and CKD progression.

The fundamental insights into uraemic toxicity have evolved little since publication of the classical monograph by Cushny nearly a century ago [1]. Organic metabolites are still thought to substantially contribute to uremia (albeit urea might not be the culprit), yet evidence unequivocally demonstrating toxicity of any single uraemic constituent is lacking [2]. To advance research in uraemic toxicity, the European Toxin work group (EUTox) developed a classification of uraemic blood constituents according to characteristics that affect their removal during dialysis [3]. Besides small water-soluble molecules (e.g. urea and creatinine) and peptides/proteins (e.g. β2-microglobulin), they identified a group of solutes that circulate in equilibrium between free solute versus bound to carrier proteins. Tight protein binding severely limits solute clearances by dialysis [4]. Intriguingly, a substantial number of these so-called proteinbound uraemic retention solutes originate from protein fermentation in the large intestine, including p-cresyl sulfate and indoxyl sulfate [5]. Recently, several groups demonstrated direct associations between p-cresol, mainly reflecting p-cresyl sulfate, and overall mortality and cardiovascular disease in end-stage renal disease [6,7] and in chronic kidney disease (CKD) [8,9]. Likewise, direct associations between indoxyl sulfate and overall mortality and cardiovascular disease were reported [10]. While indoxyl sulfate and p-cresyl sulfate are frequently thought of as independent uraemic retention solutes, they share common ground. First, as mentioned before, p-cresyl sulfate and indoxyl sulfate both originate from bacterial protein fermentation in the large intestine. Colonic microbiota degrade tryptophan to indole. Further hydroxylation results in 3-hydroxy-indole, the majority of which is sulfonated to indoxyl sulfate. In parallel, fermentation of tyrosine results in p-cresol and ultimately p-cresyl sulfate [11]. Recently, we reported on sulfate conjugation of p-cresol in CKD [12,13]. Second, most p-cresyl sulfate and indoxyl sulfate circulates noncovalently bound to albumin and competes for the same albumin-binding sites (Sudlow site II) [14] (Figure 1). Indoxyl sulfate and p-cresyl sulfate are interchangeable marker molecules to study behaviour of protein-bound solutes during dialysis [15]. In this issue of Nephrology Dialysis Transplantation, Wu et al. demonstrate that serum indoxyl sulfate is associated with progression of CKD, confirming previous findings. Niwa et al. f irst advanced the hypothesis that accumulation of indoxyl sulfate accelerates glomerular sclerosis and progression of kidney disease [16,17]. Animal and small-scale human studies on CKD patients suggested retardation of CKD progression by adsorption of indole in the large intestine [18,19]. Intriguingly, Wu et al. equally demonstrate that p-cresyl sulfate is associated with CKD progression. Does this indicate that indoxyl sulfate and p-cresyl sulfate be considered equally valid markers for CKD progression? This illustrates one of the key problems we are faced with when investigating uremia. One of the hallmarks of uraemic retention solutes is that they all move more or less in the same direction. When glomerular filtration rate falls, concentrations of the uraemic retention solutes we measure, and most likely a host of solutes that we are not aware of, all rise. Indeed, in the current study, Wu et al. observed a moderate correlation between indoxyl sulfate and estimated glomerular filtration rate (eGFR) (r −0.72, P < 0.001), between p-cresyl sulfate and eGFR (r −0.64, P < 0.001) and between indoxyl sulfate and p-cresyl sulfate (r 0.66, P < 0.001). From a statistical point of view, if nominally related measures actually quantify the same phenomenon, then they are redundant, i.e. collinear. This might lead us to conclude that indoxyl sulfate and p-cresyl sulfate are plain markers of kidney function. The strongpoint of the study by Wu et al. is that they went to great length to correct for residual confounding, including by correction for related protein-bound uraemic retention solutes. They thus demonstrate that, while indoxyl sulfate is independently associated with CKD progression, this association is lost after correction for p-