p-Cresol: a toxin revealing many neglected but relevant aspects of uraemic toxicity

Raymond Vanholder, Rita De Smet and Gerrit Lesaffer

Nephrology Unit, Department of Intenal Medicine, University Hospital, Gent, Belgium

Correspondence and offprint requests to: R. Vanholder, Nephrology Unit, Department of Intenal Medicine, University Hospital, De Pintelaan 185, B-9000 Gent, Belgium.

Introduction

p-Cresol (4-methylphenol), a 108.1 Da volatile low-molecular-weight compound, is a member of the large family of the phenoles. It is a partially lipophilic moiety which strongly binds to plasma protein (close to 100%) under normal conditions. p-Cresol is metabolized through conjugation, mainly sulphation and glucuronization [1,2], but removal of the unconjugated p-cresol is, at least in part, via the urine [3]. Therefore it is not surprising that this compound, together with several other phenoles, is retained when the kidneys fail [4,5].

Generation

P-Cresol is an end-product of protein breakdown, and an increase of the nutritional protein load in healthy individuals results in enhanced generation and urinary excretion [3]. The serum p-cresol concentration in uraemic patients can be decreased by changing to a low-protein diet [6]. p-Cresol is one of the metabolites of the amino acid tyrosine, and to a certain extent also of phenylalanine, which are converted to 4-hydroxyphenylacetic acid by intestinal bacteria, before being decarboxylated to p-cresol (putrefaction) [7]. The main contributing bacteria are aerobes (mainly enterobacteria), but to a certain extent also anaerobes play a role (mainly Clostridium perfringens) [8]. In uraemia, modifications in the intestinal flora result in the specific overgrowth of bacteria that are specific p-cresol producers [8]. The administration of antibiotics reduces urinary excretion of p-cresol [9], as a result of the liquidation of the producing bacteria [9].

p-Cresol in the intestine of uraemic rats can be adsorbed by the administration of the oral sorbant AST-120, resulting in a decrease of serum p-cresol [10]. Gastric acid suppression by omeprazole promotes protein malabsorption and fermentation, resulting in an increase of p-cresol production and excretion [11].

Environmental factors might also contribute. The liver cytochrome P450 metabolizes toluene to benzyl alcohol, but also to o-cresol and p-cresol [12,13]. Toluene is not only used industrially, but it is also the most widely abusively inhaled solvent. Furthermore, p-cresol is a metabolite of menthofuran [1416], one of the metabolites of R-(+)-pulegone [17], which is found in extracts from the plants Mentha pulegium and Hedeoma pulegioides, commonly known as pennyroyal oil and pennyroyal tea [18]. These extracts are popular as unconventional herbal therapeutic agents and are applied as abortiva, diaphoretics, emmenagogues, and psychedelic drugs. Pennyroyal oil is extensively used for its pleasant mint-like smell in the flavouring industry. The toxicity of pennyroyal oil and menthofuran is well known [19]. Another compound used in traditional medicine, especially in Japan, which is a precursor of p-cresol is wood tar creosote [2].

Because these compounds are freely available, they might contribute to the p-cresol burden of the uraemic patients, but there is no exact knowledge about the frequency and the intensity of this problem.

Uraemic plasma concentrations

Most data regarding uraemic plasma p-cresol go back to the early seventies. At that time, mean plasma concentrations up to 108±53 µmol/l (highest registered value 248 µmol/l) have been reported [6].

Recently, the authors developed a sensitive method to determine total and free p-cresol [5,20]. The free concentration might be of pathophysiological relevance, since the free fraction probably exerts biological activity, i.e. toxicity, in analogy with many drugs [21]. Using the above-mentioned methodology, it appeared that total p-cresol increased progressively during the development of chronic renal failure, whereas protein binding decreased. In healthy controls, virtually no free p-cresol could be found; in contrast, in haemodialysed patients, protein binding was only ±90%. In haemodialysed patients a total concentration of 89±49 µmol/l (highest registered value 177 µmol/l) was found [5]. Free p-cresol averaged 11±9 µmol/l, with as highest registered value 25 µmol/l. The lack of difference with the earlier reported data suggests that the recent improvements of dialysis adequacy did not have much impact on p-cresol removal.

In CAPD-patients, there was a trend towards lower values (62±20 µmol/l, highest registered value 84 µmol/l). Because of the low number of patients, this observation needs to be extended to larger patient populations.

Toxicity

p-Cresol has previously been reported to affect several biochemical, biological and physiological functions: (i) it diminishes the oxygen uptake of rat cerebral cortex slices [22]; (ii) it increases the free active drug concentration of warfarin and diazepam [23]; (iii) it has been related to growth retardation in the weanling pig [9]; (iv) it alters cell membrane permeability, at least in bacteria [24]; (v) it induces LDH leakage from rat liver slices [25]; (vi) it induces susceptibility to auditive epileptic crises [26]; and (vii) it blocks cell K+ channels [27].

In our own in-vitro study of a panel of 18 known uraemic retention solutes, p-cresol inhibited polymorphonuclear response to activation. It was the only compound to have such an effect at concentrations currently reported in end-stage renal disease [28]. The effect was dose dependent. Conjugation to p-cresylsulphate reduced the lipophilicity of p-cresol and at the same time neutralized the inhibition of the polymorphonuclears [28].

In another in-vitro study, hepatocyte aluminum uptake was increased dose dependently in the presence of p-cresol [29], which was associated with a decrease in cell growth and an increase of AST (aspartate aminotransferase) release in the culture medium. p-Cresol was also shown to inhibit the release of platelet-activating factor by rat peritoneal macrophages [30].

Dialytic removal

Virtually all currently known methods for extracorporeal solute removal fail to eliminate substantial amounts of p-cresol. Dialytic therapies that eliminate over 70% of urea and creatinine over a 4-h session, hardly remove 30% of p-cresol, probably because of multicompartmental behaviour and strong protein binding. High-flux dialysis removes no more p-cresol than conventional dialysis [unpublished observation].

In addition, there is no correlation between the removal of the current markers urea and creatinine, and that of p-cresol [unpublished observation]. A similar discrepancy has been observed for various other protein-bound toxins with an inhibitory effect on biological functions [unpublished observation].

Lower p-cresol levels are found with CAPD than with haemodialysis [5], but it is unknown whether this is attributable to a more substantial (continuous) removal, to higher residual renal function, or to differences in intestinal flora or uptake.

Potential alternative removal methods

In view of the disappointingly low removal by the classical dialysis methods, alternative methods to decrease plasma concentration should be considered. One way could be to reduce the intestinal load of p-cresol. The binding resin AST-120 decreased p-cresol in the uraemic rat [10]. Unfortunately, AST-120 is not available outside Japan. Modification of intestinal flora by antibiotics [9], as well as decreases in intestinal transit [31], diminish overall daily urinary excretion of phenols and/or p-cresol, and may hence affect their serum concentration. A diet with a minimal amount of animal proteins [32], as well as supplementation of Lactobacillus [33], decreased daily urinary p-cresol output. Whether such manoeuvres would remove clinically relevant quantities of p-cresol remains an open question. It is of note that dialysis against albumin-containing dialysate is effective in removing phenol, a compound structurally related to p-cresol [34]. Although such systems are available for clinical use, especially in acute hepatic failure, it will be difficult to obtain cost-effective results on the long run in chronic dialysis patients. Possibly, alternative dialytic approaches such as daily haemodialysis or long, slow haemodialysis, might be effective in removing more p-cresol.

Conclusions

Recently collected knowledge regarding p-cresol illustrates many neglected aspects about uraemic toxicity: (i) some of the uraemic retention solutes with biological impact are lipophilic and/or are strongly protein bound; (ii) these compounds probably follow a complex distribution and removal pattern; (iii) their plasma concentration is lower in CAPD than in haemodialysis, and haemodialytic removal is inadequate and unpredictable from the behaviour of classical water-soluble markers such as urea and creatinine; and (iv) an important contribution to their generation is delivered by the intestinal flora and environmental factors, and altering these processes might influence their serum concentration.

References

  1. Niwa T, Maeda K, Ohki T, Saito A, Kobayashi K. A gas chromatographic-mass spectrometric analysis for phenols in uremic serum. Clin Chim Acta 1981; 110: 51–57[ISI][Medline]
  2. Ogata N, Matsushima N, Shibata T. Pharmacokinetics of wood creosote: glucuronic acid and sulfate conjugation of phenolic compounds. Pharamacology 1995; 51: 195–204
  3. Geypens B, Claus D, Evenepoel P et al. Influence of dietary protein supplements on the formation of bacterial metabolites in the colon. Gut 1997; 41: 70–76[Abstract/Free Full Text]
  4. Niwa T. Phenol and p-cresol accumulated in uremic serum measured by HPLC with fluorescence detection. Clin Chem 1993; 39: 108–111[Abstract/Free Full Text]
  5. De Smet R, David F, Sandra P et al. A sensitive HPLC method for the quantification of free and total p-cresol in patients with chronic renal failure. Clin Chim Acta 1998; 278: 1–21[ISI][Medline]
  6. Wengle B, Hellström K. Volatile phenols in serum of uraemic patients. Clin Sci 1972; 43: 493–498[ISI][Medline]
  7. Curtius HC, Mettler M, Ettlinger L. Study of the intestinal tyrosine metabolism using stable isotopes and gas chromatography–mass spectrometry. J Chromatogr 1976; 126: 569–580[Medline]
  8. Hida M, Aiba Y, Sawamura S, Suzuki N, Satoh T, Koga Y. Inhibition of the accumulation of uremic toxins in the blood and their precursors in the feces after oral administration of Lebenin®, a lactic acid bacteria preparation, to uremic patients undergoing hemodialysis. Nephron 1996; 74: 349–355[ISI][Medline]
  9. Yokoyama MT, Tabori C, Miller ER, Hogberg MG. The effects of antibiotics in the weanling pig diet on growth and the excretion of volatile phenolic and aromatic bacterial metabolites. Am J Clin Nutr 1982; 35: 1417–1424[Abstract]
  10. Niwa T, Ise M, Miyazaki T, Maeda K. Suppressive effect of an oral sorbent on the accumulation of p-cresol in the serum of experimental uremic rats. Nephron 1993; 65: 82–87[ISI][Medline]
  11. Evenepoel P, Claus D, Geypens B et al. Evidence for impaired assimilation and increased colonic fermentation of protein, related to gastric acid suppression therapy. Aliment Pharmacol Ther 1998; 12: 1011–1019[ISI][Medline]
  12. Hanioka H, Hamamura M, Kakino K et al. Dog liver microsomal P450 enzyme-mediated toluene biotransformation. Xenobiotica 1995; 25: 1207–1217[ISI][Medline]
  13. Sequeira DJ, Eyer CS, Cawley GF, Nick TG, Backes WL. Ethylbenzene-mediated induction of cytochrome P450 isoenzymes in male and female rats. Biochem Pharmacol 1992; 44: 1171–1182[ISI][Medline]
  14. Madyastha KM, Raj CP. Metabolic fate of menthofuran in rats. Novel oxidative pathways. Drug Metab Dispos 1992; 20: 295–301[Abstract]
  15. Madyastha KM, Raj CP. Evidence for the formation of a known toxin, p-cresol, from menthofuran. Biochem Biophys Res Commun 1991; 177: 440–446[ISI][Medline]
  16. Thomassen D, Slattery JT, Nelson SD. Menthofuran-dependent and independent aspects of pulegone hepatotoxicity: roles of glutathione. J Pharmacol Exp Ther 1990; 253: 567–572[Abstract]
  17. Gordon WP, Huitric AC, Seth CL, MacClanahan RH, Nelson SD. The metabolism of the abortifacient terpene, (R)-(+)-pulegone, to a proximate toxin, menthofuran. Drug Metab Dispos 1987; 15: 589–594[Abstract]
  18. Nelson SD. Mechanisms of the formation and disposition of reactive metabolites that can cause acute liver injury. Drug Metab Rev 1995; 27: 147–177[ISI][Medline]
  19. Anderson IB, Mullen WH, Meeker JE et al. Pennyroyal toxicity: measurement of toxic metabolite levels in two cases and review of the literature. Ann Intern Med 1996; 124: 726–734[Abstract/Free Full Text]
  20. De Smet R, Glorieux G, Hsu C, Vanholder R. P-cresol and uric acid: two old uremic toxins revisited. Kidney Int 1997; 52 [Suppl 62]: S8–11[ISI]
  21. Nowak I, Shaw LM. Mycophenolic acid binding to human serum albumin: characterization and relation to pharmacodynamics. Clin Chem 1995; 41: 1011–1017[Abstract/Free Full Text]
  22. Lascelles PT, Taylor WH. The effect upon tissue respiration in vitro of metabolites which accumulate in uraemic coma. Clin Sci 1966; 31: 403–413[ISI][Medline]
  23. MacNamara PJ, Lalka D, Gibaldi M. Endogenous accumulation products and serum protein binding in uremia. J Lab Clin Med 1981; 98: 730–740[ISI][Medline]
  24. Keweloh H, Diefenbach R, Rehm HJ. Increase of phenol tolerance of Escherichia coli by alterations of the fatty acid composition of the membrane lipids. Arch Microbiol 1991; 157: 49–53[ISI][Medline]
  25. Thompson DC, Perera K, Fisher R, Brendel K. Cresol isomers: comparison of toxic potency in rat liver slices. Toxicol Appl Pharmacol 1994; 125: 51–58[ISI][Medline]
  26. Yehuda S, Carasso RL, Mostofsky DI. Essential fatty acid preparation (SR-3) raises the seizure threshold in rats. Eur J Pharmacol 1994; 254: 193–198[ISI][Medline]
  27. Elliott AA, Elliott JR. Voltage-dependent inhibition of RCK1 K+ channels by phenol, p-cresol, and benzyl alcohol. Mol Pharmacol 1997; 51: 475–483[Abstract/Free Full Text]
  28. Vanholder R, De Smet R, Waterloos MA et al. Mechanisms of uremic inhibition of phagocyte reactive species production: characterization of the role of p-cresol. Kidney Int 1995; 47: 510–517[ISI][Medline]
  29. Abreo K, Sella M, Gautreaux S et al. P-cresol, a uremic compound, enhances the uptake of aluminum in hepatocytes. J Am Soc Nephrol 1997; 8: 935–942[Abstract]
  30. Wratten ML, Tetta C, De Smet R et al. Uremic ultrafiltrate inhibits platelet-activating factor synthesis Blood Purif (in press)
  31. Cummings JH, Hill MJ, Bone ES, Branch WJ, Jenkins DJA. The effect of meat protein and dietary fiber on colonic function and metabolism. II. Bacterial metabolites in feces and urine. Am J Clin Nutr 1979; 32: 2094–2101[ISI][Medline]
  32. Ling WH, Hänninen O. Shifting from a conventional diet to an uncooked vegan diet reversably alters fecal hydrolytic activities in humans. J Nutr 1992; 122: 924–930[ISI][Medline]
  33. Ling WH, Korpela R, Mykkänen H, Salminen S, Hänninen O. Lactobacillus strain GG supplementation decreases colonic hydrolytic and reductive enzyme activities in healthy female adults. J Nutr 1994; 124: 18–23[ISI][Medline]
  34. Stange J, Ramlow W, Mitzner S, Schmidt R, Klinkmann H. Dialysis against a recycled albumin solution enables the removal of albumin-bound toxins. Artif Organs 1993; 17: 809–813[ISI][Medline]