UREMIC TOXINSUREMIC TOXINS

Sanjeev V NairSanjeev V Nair

PG, Dept of PG, Dept of NephrologyNephrology

IntroductionIntroduction

Uremia: The illness that would remain if the Uremia: The illness that would remain if the extracellular volume and inorganic ion extracellular volume and inorganic ion concentrations were kept normal and the concentrations were kept normal and the known renal synthetic products were replaced known renal synthetic products were replaced in pts without kidneysin pts without kidneys

Uremia is d/t the accumulation of the organic Uremia is d/t the accumulation of the organic waste products that are normally cleared by waste products that are normally cleared by the kidneysthe kidneys..

Symptoms & Signs of UremiaSymptoms & Signs of Uremia

Neural & muscularNeural & muscular FatigueFatigue Loss of concentration Loss of concentration

ranging to coma & ranging to coma & seizuresseizures

Sleep disturbancesSleep disturbances Anorexia & NauseaAnorexia & Nausea Dimunition in taste and Dimunition in taste and

smellsmell CrampsCramps Restless legsRestless legs Peripheral neuropathyPeripheral neuropathy Reduced muscle Reduced muscle

membrane potentialmembrane potential

Endocrine and MetabolicEndocrine and Metabolic Amenorrhoea and sexual Amenorrhoea and sexual

dysfuntiondysfuntion Reduced body temperatureReduced body temperature Reduced resting energy Reduced resting energy

expenditureexpenditure Insulin resistanceInsulin resistanceOtherOther SerositsSerosits ItchingItching HiccupsHiccups Granulocyte and Granulocyte and

Lymphocyte dysfunctionLymphocyte dysfunction Platelet dysfunctionPlatelet dysfunction Shortened erythrocyte Shortened erythrocyte

lifespanlifespan Albumin oxidationAlbumin oxidation

Bergstrom’s criteriaBergstrom’s criteria

A substance with a known chemical structureA substance with a known chemical structure Its plasma &/or tissue concentrations should be Its plasma &/or tissue concentrations should be

higher in uremic pts than in normal ptshigher in uremic pts than in normal pts The high concentrations should be related to specific The high concentrations should be related to specific

uremic symptoms that are ameliorated when the uremic symptoms that are ameliorated when the concentration is reducedconcentration is reduced

The effects observed in uremic pts should be The effects observed in uremic pts should be replicated by raising the solute concentrations to replicated by raising the solute concentrations to uremic levels in normal people, experimental animals uremic levels in normal people, experimental animals or in vitro systemsor in vitro systems

Classification of Uremic toxinsClassification of Uremic toxins

Subdivided into three major groups based Subdivided into three major groups based upon their chemical and physical upon their chemical and physical characteristics: characteristics:

o Small, water-soluble, non-protein-bound Small, water-soluble, non-protein-bound compoundscompounds

o Small, lipid-soluble and/or protein-bound Small, lipid-soluble and/or protein-bound compounds compounds

o Larger so-called middle-moleculesLarger so-called middle-molecules

Small water soluble non-Small water soluble non-protein bound compoundsprotein bound compounds

UreaUrea GuanidinesGuanidines CreatinineCreatinine PhosphorusPhosphorus OxalatesOxalates HH+ + ionsions PolyaminesPolyamines

UreaUrea

Quantitatively the most important Quantitatively the most important solute excretedsolute excreted

But, not the most important But, not the most important contributor to uremiacontributor to uremia

Marker of dialysis adequacyMarker of dialysis adequacy Toxin? Vs Marker of renal injury?Toxin? Vs Marker of renal injury?

Urea: Toxic effectsUrea: Toxic effects Inhibits Na-K-2Cl cotransport in human Inhibits Na-K-2Cl cotransport in human

erythrocytes, as well as a number of cell erythrocytes, as well as a number of cell volume-sensitive cellular transport volume-sensitive cellular transport pathways. pathways.

Urea inhibits macrophage-inducible nitric Urea inhibits macrophage-inducible nitric oxide synthesis at the post-transcriptional oxide synthesis at the post-transcriptional level. level.

Dialysis disequilibrium syndromeDialysis disequilibrium syndrome Urea is a precursor of some of the Urea is a precursor of some of the

guanidines. guanidines. Protein carbamoylation: d/t isocyanateProtein carbamoylation: d/t isocyanate

GuanidinesGuanidines Breakdown product of arginineBreakdown product of arginine Creatinine: breakdown product of GAA. Creatinine: breakdown product of GAA.

Probably not toxic. Creatinine metabolites: Probably not toxic. Creatinine metabolites: Creatol & methylguanidine. Exhibit the Creatol & methylguanidine. Exhibit the highest concentrations relative to normal.highest concentrations relative to normal.

Guanidosuccinicacid (GSA): platelet Guanidosuccinicacid (GSA): platelet dysfunctiondysfunction

GSA & guanidinopropionic acid inhibit GSA & guanidinopropionic acid inhibit neutrophil superoxide productionneutrophil superoxide production

Guanidino compounds exert both pro- and Guanidino compounds exert both pro- and anti-inflammatory effects upon leukocytes.anti-inflammatory effects upon leukocytes.

GuanidinesGuanidines GSA, GSA, γγ-guanidinobutyric acid, -guanidinobutyric acid,

methylguanidine, homoarginine & creatine methylguanidine, homoarginine & creatine induce seizures after systemic and/or induce seizures after systemic and/or cerebroventricular administration in cerebroventricular administration in animals.animals.

A mixture of guanidino compounds A mixture of guanidino compounds suppresses the natural killer cell response. suppresses the natural killer cell response.

Guanidines cause structural damage to Guanidines cause structural damage to proteins by deamidation/isomerization, proteins by deamidation/isomerization, which in turn reduces the binding of which in turn reduces the binding of homocysteinehomocysteine

GuanidinesGuanidines Asymmetric dimethylarginine (ADMA), which is Asymmetric dimethylarginine (ADMA), which is

significantly increased in ESRD: inhibitory significantly increased in ESRD: inhibitory effects on NO synthesis.effects on NO synthesis.

In the brain, ADMA causes vasoconstriction and In the brain, ADMA causes vasoconstriction and inhibition of acetylcholine-induced inhibition of acetylcholine-induced vasorelaxation.vasorelaxation.

It has also been implicated in the development of It has also been implicated in the development of hypertension and adverse cardiovascular events.hypertension and adverse cardiovascular events.

Administration of ADMA to healthy volunteers Administration of ADMA to healthy volunteers resulted in an increase of vascular resistance resulted in an increase of vascular resistance and blood pressure in a dose-related manner. It and blood pressure in a dose-related manner. It was also demonstrated that ADMA increased was also demonstrated that ADMA increased vascular stiffness and decreased cerebral vascular stiffness and decreased cerebral perfusion in healthy subjectsperfusion in healthy subjects

The increase in symmetric The increase in symmetric dimethylarginine (SDMA): linked to an dimethylarginine (SDMA): linked to an increase in reactive oxygen production increase in reactive oxygen production and an inhibition of NO-synthesis. and an inhibition of NO-synthesis.

In addition, methylguanidine also shows In addition, methylguanidine also shows a modest inhibitory activity on cytokine- a modest inhibitory activity on cytokine- and endotoxin-inducible NO-synthesis and endotoxin-inducible NO-synthesis

Removal: restricted, large volumes of Removal: restricted, large volumes of distribution & different intradialytic distribution & different intradialytic behaviourbehaviour

PhosphorusPhosphorus High concentrations: clearly related to High concentrations: clearly related to

pruritus and hyperparathyroidismpruritus and hyperparathyroidism Phosphorus excess also inhibits 1 Phosphorus excess also inhibits 1 αα--

hydroxylase and hence the production hydroxylase and hence the production of calcitriol, the active vitamin D of calcitriol, the active vitamin D metabolitemetabolite

Phosphorus retention also alters Phosphorus retention also alters polyamine metabolism by causing a polyamine metabolism by causing a decrease in intestinal dysfunction and decrease in intestinal dysfunction and proliferation of intestinal villi proliferation of intestinal villi

Strategies for control. Strategies for control. HD: unpredictable; reboundHD: unpredictable; rebound

Polyamines: may contribute to Polyamines: may contribute to anorexia, vomiting, and adverse anorexia, vomiting, and adverse central nervous system effects central nervous system effects

OxalateOxalate Metabolic acidosisMetabolic acidosis

Protein bound Protein bound compoundscompounds

PhenolsPhenols IndolesIndoles HomocysteineHomocysteine Furanpropanoic acidFuranpropanoic acid MyoinositolMyoinositol

Phenols Phenols

Derived from AAs tyrosine and Derived from AAs tyrosine and phenylalanine.phenylalanine.

Hippurate: glycine conjugate of Hippurate: glycine conjugate of benzoate derived from veg food. benzoate derived from veg food. Endogenously from phenylalanine. Endogenously from phenylalanine. Not toxic.Not toxic.

P-cresol: binds avidly to albumin → P-cresol: binds avidly to albumin → used as a marker for clearance of used as a marker for clearance of protein bound susbstances in RRTs protein bound susbstances in RRTs

P-cresol vs p-cresylsulfate & p-P-cresol vs p-cresylsulfate & p-cresylglucuronatecresylglucuronate

P-cresylsulfate stimulated baseline P-cresylsulfate stimulated baseline leukocyte activity, thereby pointing leukocyte activity, thereby pointing to a pro-inflammatory effectto a pro-inflammatory effect

P-cresol essentially inhibits activated P-cresol essentially inhibits activated leukocyte function.leukocyte function.

Clearance: by conventional HD Clearance: by conventional HD hampered.hampered.

HomocysteineHomocysteine

AA produced by the demethylation of AA produced by the demethylation of methionine. Levels 2-4x in CKDmethionine. Levels 2-4x in CKD

S-adenosyl homocysteine (AdoHcy): S-adenosyl homocysteine (AdoHcy): competes with S-adenosyl-methionine competes with S-adenosyl-methionine (AdoMet) and inhibits (AdoMet) and inhibits methyltransferasesmethyltransferases

Hyperhomocysteinemia also disturbs Hyperhomocysteinemia also disturbs epigenetic control of gene expression by epigenetic control of gene expression by inducing macromolecule inducing macromolecule hypomethylationhypomethylation

Moderate hyperhomocysteinemia is an Moderate hyperhomocysteinemia is an independent risk factor for independent risk factor for cardiovascular disease in the general cardiovascular disease in the general population & is a prevalent population & is a prevalent cardiovascular risk factor in patients cardiovascular risk factor in patients with ESRDwith ESRD

Hcy increases the proliferation of Hcy increases the proliferation of vascular smooth muscle cellsvascular smooth muscle cells

Also disrupts several vessel wall-Also disrupts several vessel wall-related anticoagulant functions, related anticoagulant functions, resulting in enhanced resulting in enhanced thrombogenicity.thrombogenicity.

IndolesIndoles Metabolite of tryptophanMetabolite of tryptophan Indoxyl sulfate (Indican): toxic to renal tubular Indoxyl sulfate (Indican): toxic to renal tubular

cells and ↑sing levels accelerate loss of remnant cells and ↑sing levels accelerate loss of remnant nephrons.nephrons.

Also plays a role by inhibiting endothelial cell Also plays a role by inhibiting endothelial cell proliferation and repair, while it induces a proliferation and repair, while it induces a significant production of free radical species in significant production of free radical species in endothelial cells.endothelial cells.

A role in aortic calcification and elements of A role in aortic calcification and elements of bone dysfunction, such as osteoblast dysfunction bone dysfunction, such as osteoblast dysfunction and downregulation of pathways of parathyroid and downregulation of pathways of parathyroid hormone gene expression and low turn-over hormone gene expression and low turn-over bone diseasebone disease

Reduction: High flux HD, Bifidobacteria.Reduction: High flux HD, Bifidobacteria.

Aliphatic aminesAliphatic amines

MMA,DMA,TMA: >10x ↑se in levels MMA,DMA,TMA: >10x ↑se in levels in ESRDin ESRD

Probably metabolic products of Probably metabolic products of choline and trimethylamineoxide.choline and trimethylamineoxide.

Toxicities may include: neural-motor Toxicities may include: neural-motor symptoms, hemolysis & inhibition of symptoms, hemolysis & inhibition of lysosomal function. Poorly dialyzedlysosomal function. Poorly dialyzed

MMA→anorexia*MMA→anorexia* TMA: Uremic fetorTMA: Uremic fetor

Furanpropanoic acidFuranpropanoic acid

3-Carboxy-4-methyl-5-propyl-2-3-Carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF)furanpropionic acid (CMPF)

A major inhibitor of drug protein A major inhibitor of drug protein bindingbinding

A correlation between neurologic A correlation between neurologic abnormalities and the plasma abnormalities and the plasma concentration of CMPFconcentration of CMPF

Clearance by HD virtually nil. PD Clearance by HD virtually nil. PD achieves better clearance.achieves better clearance.

PolyolsPolyols

Myoinositol: normally oxidized by Myoinositol: normally oxidized by kidneykidney

Reflects impaired degradationReflects impaired degradation ? Uremic polyneuropathy? Uremic polyneuropathy Manniltol, sorbitol, arabitol, Manniltol, sorbitol, arabitol,

erythritol.erythritol.

MIDDLE MOLECULESMIDDLE MOLECULES

The ‘middle molecule hypothesis’- The ‘middle molecule hypothesis’- based on dialysis modalities and based on dialysis modalities and clearanceclearance

*300-2000D?*300-2000D? Never tested. ? ToxicNever tested. ? Toxic Better clearance with Hemofiltration Better clearance with Hemofiltration

than HDthan HD

ββ2-microglobulin2-microglobulin MW approx 12,000 DMW approx 12,000 D Resp for Dialysis-related amyloidResp for Dialysis-related amyloid Also enhances monocytic migration and Also enhances monocytic migration and

cytokine secretion, suggesting that foci cytokine secretion, suggesting that foci containing AGE-ß2M may initiate containing AGE-ß2M may initiate inflammatory response, leading to inflammatory response, leading to bone/joint destructionbone/joint destruction

ß2M levels correlated with mortality, with ß2M levels correlated with mortality, with each 10mg/L increase in predialysis level each 10mg/L increase in predialysis level being associated with an 11% increase for being associated with an 11% increase for all-cause mortalityall-cause mortality

Poor clearance with PD alone; Improved Poor clearance with PD alone; Improved clearances with use of High flux dialyzers.clearances with use of High flux dialyzers.

Parathyroid hormoneParathyroid hormone MW approx 9000 DMW approx 9000 D

PTH- Masry hypothesisPTH- Masry hypothesisBrainBrain Abnormal EEGAbnormal EEG

Peripheral nervePeripheral nerve ↑↑sed motor nerve sed motor nerve conduction velocitiesconduction velocities

HeartHeart ↓↓sed cardiac indexsed cardiac index

↓↓sed LVEFsed LVEF

PancreasPancreas Impaired insulin secretionImpaired insulin secretion

PMNLsPMNLs Impaired phagocytosisImpaired phagocytosis

ItchingItching

Tissue necrosisTissue necrosis

Soft tissue calcificationSoft tissue calcification

Bone resorptionBone resorption

Sexual functn Sexual functn (impotence)(impotence)

PTHPTH

↑↑sed PTH levels in the uremic state sed PTH levels in the uremic state are usually a/w ↓Vit D levels,↓sed are usually a/w ↓Vit D levels,↓sed Ca levels & ↑sed PO4 levels.Ca levels & ↑sed PO4 levels.

• OsteodystrophyOsteodystrophy• Cardiovascular complications Cardiovascular complications

including calcificationsincluding calcifications• Alterations in immune systemAlterations in immune system

Advanced glycosylation end Advanced glycosylation end productsproducts

MW 2000-6000 DMW 2000-6000 D Uremia → a status of increased carbonyl Uremia → a status of increased carbonyl

stress, resulting from increased oxidation or stress, resulting from increased oxidation or decreased detoxification of these carbonyl decreased detoxification of these carbonyl compounds.compounds.

AGEs AGEs • cause an inflammatory reaction in cause an inflammatory reaction in

monocytes by the induction of IL-6, TNF-monocytes by the induction of IL-6, TNF-αα & IF-& IF-γγ..

• modify ß2Mmodify ß2M• react with and chemically inactivate NO react with and chemically inactivate NO

AGEsAGEs

Doubtful role as a uremic toxinDoubtful role as a uremic toxin Removal improved with dialyzers of Removal improved with dialyzers of

large pore size; protein leaking large pore size; protein leaking dialyzers vs non-protein leaking dialyzers vs non-protein leaking dialyzersdialyzers

Other middle moleculesOther middle molecules

Leptin: 16 kD plasma protein that Leptin: 16 kD plasma protein that suppresses appetitesuppresses appetite

Dinucleoside polyphosphates: Dinucleoside polyphosphates: • Diadenosine polyphosphates induce Diadenosine polyphosphates induce

proliferation of smooth muscle cells proliferation of smooth muscle cells • Uridine adenosine tetraphosphate is a Uridine adenosine tetraphosphate is a

strong vasoconstrictor, which is released strong vasoconstrictor, which is released by endothelinby endothelin

Clearance strategiesClearance strategies

Dialysis: non-specific; removal of Dialysis: non-specific; removal of lipophilic compounds inadeqautelipophilic compounds inadeqaute

High flux dialyzers; ?HEMO StudyHigh flux dialyzers; ?HEMO Study Hemodiafiltration: ↑sed middle Hemodiafiltration: ↑sed middle

molecule clearance. ? Survival molecule clearance. ? Survival advantageadvantage

Duration of dialysisDuration of dialysis Preservation of residual renal functionPreservation of residual renal function ↓↓sed dietary intakesed dietary intake

EUtox Working GroupEUtox Working Group

Launched in 1999 by the ESAOLaunched in 1999 by the ESAO Composed of leading researchers Composed of leading researchers

and industry scientists.and industry scientists. Work in the area of identification Work in the area of identification

and characterization of uremic and characterization of uremic toxins and in the knowledge of their toxins and in the knowledge of their pathophysiologic importancepathophysiologic importance