asn dialysis advisory group · practically, if one puts a teabag in hot water without doing...

ASN DIALYSIS CURRICULUM

ASN DIALYSIS ADVISORY GROUP

This image cannot currently be displayed.

AN UPDATE ON UREMIC TOXICITY: Part 1

Raymond Vanholder, M.D., Ph.D.Nephrology Section, Department of Internal MedicineGhent, Belgium

2


Disclosures

• Dr. Vanholder’s dialysis unit receives unrestricted research grants from Fresenius Medical Care, Baxter Health Care, Gambro, Bellco and Nipro

3

BASIC PRINCIPLES OF HEMODIALYSIS AND ITS IMPACT ON SOLUTE REMOVAL

4

Presenter

Presentation Notes

Before starting the summary of current data on uremic toxicity and how this problem can be handled we will describe schematically how dialysis methods as we apply them today affect the removal of three types of molecules: small water soluble compounds (prototype: urea – easy to remove with any type of dialysis); protein bound compounds (prototype: indoxyl sulfate – difficult to remove with any type of dialysis); and the larger middle molecules (prototype: β2-microglobulin – can be removed only by membranes with large enough pores, the so-called high-flux membranes).


DIFFUSION VS. CONVECTIONDIFFUSION

CONVECTION

5

Presenter

Presentation Notes

Diffusion is the basic principle of hemodialysis; hereby, solutes move through a semi-permeable membrane from the area with the highest concentration (in dialysis for most molecules the blood side) to the area with the lowest concentration (for most molecules the dialysate side). Diffusion only allows the transmembrane shift of molecules with a diameter that is smaller than the diameter of the pores of the semi-permeable membrane and is most suited for removing small water soluble compounds. Convection is the basic principle of hemofiltration; hereby, a large volume of plasma water is pushed through the semipermeable membrane and with this volume also molecules in that plasma are dragged to the other side (from the Latin convehere – to drag with you). The volume that is filtered out of the plasma is replaced by an equivoluminous volume of substitution fluid to avoid volume depletion. Practically, if one puts a teabag in hot water without doing anything else, the brown color that escapes from the bag does so by diffusion. However, if one starts dragging the bag through the water, this is on the basis of convection. Generally the latter procedure allows more brown color to escape. In hemodiafiltration, diffusion and convection are combined. As for convection in general large pores are needed, the method is suited very well for removing larger molecules


IMPACT OF LOW-FLUX DIALYSIS ON DIFFERENT TYPES OF UREMIC TOXINS

6

Presenter

Presentation Notes

This slide illustrates the impact of low-flux hemodialysis (membrane with small pore size, used in a diffusive mode) on removal of the three types of uremic toxins: small water soluble compounds (small open circles); protein bound componds (full circles, either free or unbound if they are on their own, or bound to albumin, if they are linked to the large ovals; middle molecules (large open circles). Only the small water soluble compounds cross easily the membrane (arrows crossing the membrane); also some of the protein bound componds pass the membrane if they are free (unbound). The protein bound compounds which are bound to albumin however do not cross the membrane and are reflected, in the same way as the middle molecules (broken arrows).


IMPACT OF HIGH-FLUX DIALYSIS ON DIFFERENT TYPES OF UREMIC TOXINS

7

Presenter

Presentation Notes

This slide illustrates the impact of high-flux hemodialysis (membrane with large pore size, used in a diffusive mode) on removal of the three types of uremic toxins. Not only the small water soluble compounds and the free protein bound compounds, but also the middle molecules cross the membrane (arrows crossing the membrane). The protein bound compounds which are bound to albumin however do still not cross the barrier and are reflected (broken arrows).


IMPACT OF HEMODIAFILTRATION ON DIFFERENT TYPES OF UREMIC TOXINS

8

Presenter

Presentation Notes

This slide illustrates the impact of hemo(dia)filtration (membrane with large pore size, used in a convective mode) on removal of the three types of uremic toxins. The middle molecules are removed even more efficiently than with high-flux hemodialysis (arrows crossing the membrane). Also the protein bound compounds which are bound to their binding protein are now removed more efficiently, probably by a mix of convection and, in case of large enough pores, transmembrane removal of albumin. Small water soluble compounds roughly are removed in the same way as with the two other strategies.


UREMIC SOLUTE KINETICS HAS A MAJOR IMPACT ON THEIR REMOVAL

FIGURE: Two-compartment kinetic model. V1: plasmatic volume, V2: extraplasmatic volume, C1: plasmatic concentration, C2: extraplasmatic concentration, MTdialyser: mass transfer in the dialyser, K21: intercompartment clearance, G: solute generation.

K21

Eloot et al, NDT, 27:4021-4029; 20129

Presenter

Presentation Notes

To explain the impact of different dialysis timeframes on solute removal, we need to explain compartmental concentration pattern during dialysis. Almost all uremic retention solutes are distributed over at least two compartments: one is the plasmatic volume which is in direct contact with the dialyzer. The other is the extraplasmatic volume, which is in contact with the plasma but not with the dialyzer. At the start of dialysis we may assume that concentration of solutes in both compartments is the same. Mass transfer (MT) refers to the total amount of solute removed from the plasma to the dialysate. K21 refers to the shift of solute from the extraplasmatic to the plasmatic compartment.




K21

Eloot et al, NDT, 27:4021-4029; 201210

Presenter

Presentation Notes

From the moment dialysis starts, the dialyzer starts purifying the plasmatic volume, which results in mass transfer (MT) into the dialysate.



K21


Eloot et al, NDT, 27:4021-4029; 201211

Presenter

Presentation Notes

From the moment solute concentration decreases in the plasma, also diffusion of solutes from the extraplasmatic to plasmatic volume starts. This process is symbolized by K21 (clearance from compartment 2 (extraplasmatic) to compartment 1 (plasmatic)).



K21


Eloot et al, NDT, 27:4021-4029; 201212

Presenter

Presentation Notes

However, the shift of solutes from extraplasmatic to plasmatic is slower than the one from plasma to the dialysate. As a consequence concentration in the plasma decreases more quickly than the extraplasmatic concentration, but a lower plasmatic concentration also means less solute available for diffusion and thus lower mass transfer and efficiency of dialysis. This phenomenon also gives rise to rebound after dialysis, as after the end of dialysis, solute continues to move from extraplasmatic to intraplasmatic. Thus solute concentration after dialysis rises very quickly so that part of the removal effect is lost. The shorter dialysis is, the more pronounced this compartment disbalance and rebound will be.


3x4h _QB300 6x2h _QB300 3x8h _QB200

C2

C1

C2

C1

C2

C1

0

2

4

6

8

10

12

14

16

18

20

0 240 480 720 960 1200 1440 1680 1920 2160 2400 2640 2880

B2M

con

cent

ratio

n (m

g/L)

time (min)

STEEP INITIAL DECLINE

SLOWER SUBSEQUENT DECLINE

REBOUND

13

Presenter

Presentation Notes

In this last theoretical slide, before starting the proper discussion of uremic toxins, we illustrate the effect of different dialysis timeframes on the concentration in the plasmatic and the extraplasmatic compartments of molecules with a multicompartmental distribution. The thick lines illustrate the plasmatic compartment of a theoretical molecules distrivuted over two compartments: this implies a fast decline that slows down during the session and a rebound afterwards. The thin line illustrtaes the concentration in the extraplasmatic compartment. A situation is described whereby we always start from standard alternate day (3 to 4 hrs) regime and always from the same concentration, and the blue lines illustrate concentrations if this regime is maintained. There is a steep decline of plasma concentration during the initial phase of dialysis, but due to a loss of mass transfer, this effect is attenuated later during dialysis. Dialysis is followed by a marked rebound, after which concentration continues to rise gradually till immediately before the next dialysis it reaches more or less the same level as before the first dialysis. Concentration in the extraplasmatic compartment is markedly lagging behind that in the plasma which hampers mass transfer. The next illustration is that of low efficiency extended alternate day dialysis (8 hours, blood flow 200 mL/min) (green lines). The initial decline in concentration in plasma is less steep but more consistent and is continued for a longer period. Removal as a consequence is more important and there is less rebound at the end of dialysis. The concentration rise till the next dialysis is less important than with the shorter regime. Removal from the extraplasmatic compartment is also more efficient. The last illustration is focusing on daily short (2 hr) dialysis (red lines). Now dialysis stops near the end of the steep phase of the decline in plasma concentration. There is less time in between dialyses for concentration to rise again, and due to the fast succession of sessions, concentration pre-dialysis gradually declines. Although extraplasmatic concentration is lagging behind, again due to the succession of sessions, a gradual decline of extraplasmatic concentration also takes place. Of note, the illustration only focuses on an evolution over 48 hours. Real equilibrium with alternative regimes is reached only after 1 to 2 weeks, at even lower concentrations than in this figures, and with daily and extended alternatiev day dialysis ending at virtually same levels.


Summary

• In vitro effect

• In vivo effect

• Removal of small molecules

• Analytical data on adequacy

• Clinical outcomes

• [Removal of protein-bound and middle molecules covered in Uremic Toxins: part 2]

Vanholder R, et al. Review on uremic toxins: classification, concentration, and interindividual variability. Kidney Int , 63: 1934–1943; 2003.Duranton F, et al. Normal and pathologic concentrations of uremic toxins. J Am Soc Neph ,23:1558-1270; 2012.

14

Presenter

Presentation Notes

This is the summary of the content for the rest of this slide-show. We will for several classes of compounds discuss studies on their in vitro and in vivo effects, and on their removal (analytical data on adequacy as well as clinical outcomes). We will do this for the three major classes of uremic solutes: small water soluble compounds (prototype urea); protein bound compounds (prototype indoxyl sulfate); and the larger, so-called middle molecules (prototype β2-microglobulin). For each of these groups, some specific examples will be discussed. The above classification was at first described in the encyclopedic review of known uremic solutes, published in KI in 2003 (Vanholder et al), with and update in JASN in 2012 (Duranton et al).


Urea

15

Presenter

Presentation Notes

We first will discuss urea, because it is a classical marker of uremic toxicity and removal by dialysis, but also because some exciting new data were generated.


Effect of increasing dialysate urea

This figure was published in Mayo Clin Proc, 47, Johnson et al., Effects of urea loading in patients with far-advanced renal failure, 21-29. Copyright Elsevier (1972).

Presenter

Presentation Notes

Urea generally is considered as a relatively enert molecule on the biological level. In this classical 40-year-old study from the Mayo clinic, the authors added urea to the dialysate at a level exceeding the maxima reached pre-dialysis by a factor 2, and dialyzed a group of patients for 3 consecutive months. Above in orange one can see the urea concentrations, that went up and then stabilized at a high level, until after three months again dialysis was performed without urea. Creatinine (green) in contrast did not change. The authors meticulously followed during this three months of high urea dialysis uremic symptoms (table below) and scored them along their intensity as 0, 1, 2 or 3. It is clear that there is no relation between uremic symptoms and urea concentration, suggesting no marked biological activity of this molecule.


UREA DISRUPTS INTESTINAL WALL PROTECTIVE BARRIER

Vaziri et al, Am J Nephrol, 37: 1-6; 2013

Figure 1Bar graphs depicting the TER (transepithelialelectrical resistance) in intestinal epithelial T84 cell monolayers incubated for 24 h in regular media and those incubated in media containing 42 or 72 mg/dl urea. *** p < 0.001.

Presenter

Presentation Notes

However, some recent data seem to suggest that urea, at concentrations as occurring in uremia, nevertheless exerts some biological activity. In this study, urea was added to intestinal cell monolayers and the transepithelial electrical resistance was assessed as a measure of the efficacy of the intestinal wall barrier. A decrease of resistance corresponds to a loss of barrier function, and the latter in its turn corresponds to an increase in permeability for intestinal contaminants like lipopolysaccharide (endotoxin). This study shows a dose responsive loss of barrier function in the presence of urea. As this mechanism could be at the origing of more transfer of lipopolysaccharide into the body, this is a potentially pro-inflammatory component.


UREA INDUCES INSULIN RESISTANCE

D’Apolito et al, J Clin Invest, 120: 203-213; 2010

Urea causes decreased insulin sensitivity in differentiated 3T3L1 adipocytes. (A) Effect of urea on insulin-stimulated glucose uptake in differentiated 3T3L1 cells.

Will need to remove or alter as re-use fee is $41

Presenter

Presentation Notes

However, some recent data seem to suggest that urea, at concentrations as occurring in uremia, nevertheless exerts some biological activity. In this study, urea was added to adipocytes to study the response to insulin (glucose transport). If insulin is added in absence of urea (yellow bar), glucose transport is standardized at 100%. If next to insulin also urea is added, however, the response is severely blunted (purple bar) and almost identical to experiments in absence of insulin (red and blue bars). When next to insulin mannitol is added in stead of urea to obtain the same molar concentration as with urea, response to insulin is normal (100% - orange bar). The data point to insulin resistance in the presence of urea. Insulin resistance in its turn has been linked to cardio-vascular morbidity and mortality.


TRANSPORT OF UREA AND ANALOGUES VIA ERYTHROCYE CELL WALL

Erythrocyte solute permeability measured by stopped-flow light scattering. (A) Representative curves for the time course of scattered light intensity at 10 °C in response to a 250mMinwardly directed gradient

of urea analogues. (B)Averaged solute permeability coefficients (Ps) for experiments done as in panelA(mean±S.E., n=3).

This figure was published in Biochim Biophys Acta, 1768, Zhao et al., Comparative transport efficiencies of urea analogues through urea transporter UT-B, 1815-1821. Copyright Elsevier (2007).

Presenter

Presentation Notes

Even if urea would, in contrast to what was believed before, exert toxic effects, then still it’s removal remains a not really appropriate marker of the removal of other uremic solutes for the reason as illustrated on this slide: whereas there is almost no resistance for urea at the level of the cell membrane, meaning that the removal of urea from the intracellular compartment is almost as quick as that from the plasma, most other molecules are subject to substantially more resistance. Guanidine, the molecule entirely at the bottom of this slide, is a solute with similar molecular weight and origin as urea, but transcellular resistance is markedly higher resulting in a multcompartmental behavior and hence more difficult removal by dialysis as compared to urea (as also illustrated a bit further in this presentation by in vivo experimental data).


SOLUTE CONCENTRATION CORRELATES WITH RENAL FUNCTION AND PROTEIN INTAKE (NOT Kt/V)

Eloot et al, Plos One, 8:e76838; 2013

Solute Covariates / R² full model Urea nPCR / 0.850Crea nPCR / 0.150Uric acid nPCR/ 0.195beta-2-microglobulin nPCR/ 0.172Hippuric acid nPCR/ 0.187 nPCR / 0.220Indoxyl sulfate nPCR / 0.059p-cresylsulfate nPCR/ 0.268p-cresylglucuronide RRF/ 0.134 nPCR / 0.151Free hippuric acid RRF / 0.206Free indoxyl sulfate RRF / 0.144Free indole acetic acid RRF / 0.166Free p-cresylsulfate nPCR/ 0.189 RRF / 0.257Free p-cresylglucuronide RRF/ 0.135 nPCR/ 0.150

Model: age, gender, nPCR, Kt/V, RRF, diabetes, body weight, vintage

Presenter

Presentation Notes

In this study, various factors with potential impact on uremic solute concentration [age, gender, protein intake (nPCR), residual renal function (RRF), diabetes, body weight and dialysis vintage] were correlated to the pre-dialysis concentration of a large number of retention solutes, but significant correlations were found only for protein intake and residual renal function, not for Kt/V. These data suggest that next to dialysis adequacy, other factors weigh at least as importantly on uremic solute concentration.


ADMA + SDMAGUANIDINO COMPOUNDS

21

Presenter

Presentation Notes

Asymmetric dimethyl arginine (ADMA) and symmetric dimethyl arginine (SDMA) are also small water soluble compounds. They belong to the larger group of guanidino compounds.


ADMA CONCENTRATION IS LINKED TO MORTALITY

ADMA=asymmetric dimethylarginine* Denominator represents number of patients at risk** Adjusted for age and sex

Zoccali et al., Lancet , 358: 2113-2115; 2001 22

Presenter

Presentation Notes

ADMA has repeatedly been associated with negative cardio-vascular outcomes. In this study by Zoccali et al, each rise in ADMA concentration is associated with higher mortality.


ADMA HAS IN VOLUNTEERS A MARKED HEMODYNAMIC EFFECT

Effect of 0.10 mg ADMA {middle dot} kg-1 {middle dot} min-1 on cardiac output (A) and systemic vascular resistance (SVR) B) in 7 healthy volunteers

Kielstein et al., Circulation, 109: 172-177; 200423

Presenter

Presentation Notes

This is very likely related to the cardio-vascular impact of ADMA. ADMA is an inhibitor of Nitric Oxide (NO) synthase, which as such is a protector of the endothelium and its function. Inhibition of NO-synthase is damaging the endothelium. The hemodynamic impact of ADMA is clearly demonstrated in this study on healthy volunteers by Kielstein et al, in which ADMA was infused intravenously to obtain plasma concentrations comparable with those observed in the uremic status. During infusion of ADMA, Cardiac Output decreased and Systemic Vascular Resistance (SVR) increased. This effect was maintained for more than 2 hours after ADMA infusion had been stopped.


SDMA INDUCES IN VITRO CYTOKINE GENERATION

Schepers et al, CJASN, 6: 2374-2383; 2011

Presenter

Presentation Notes

Although both ADMA and SDMA are structurally comparable with only minimal steric differences, SDMA, in contrast to ADMA, until recently was considered as biologically inert (non-toxic). However, when in in vitro studies, the impact of SDMA at concentrations like observed in uremia on intracellular monocyte cytokine generation is studied, a rise was observed for both TNFα and Il-6, suggesting a pro-inflammatory impact.


Schepers et al, CJASN, 6: 2374-2383; 2011

Variables associated with the serum levels of SDMA and ADMA by linear regression

SDMA IS MORE STRONGLY CORRELATED WITH CYTOKINE CONCENTRATION THAN ADMA

25

Presenter

Presentation Notes

In a clinical study, this in vitro suggestion of a pro-inflammatory potential for SDMA is corroborated by showing a strong correlation of SDMA concentration with both TNF-α and IL-6. Of note, the correlation for ADMA was much weaker.


MOST GUANIDINES ARE DISTRIBUTED OVER A LARGER COMPARTMENT THAN UREA

*: p<0.05; V: distribution volume (L); Eff Rem: effective removal (%);

Eloot et al., KI, 67: 1566-1575; 200526

Presenter

Presentation Notes

Thus the question arises, in how far urea, which like the guanidines is a small water soluble compound with the same metabolic origin, has the same compartmental distribution and kinetics. To check this question , in this analysis, kinetics of guanidino compounds were compared to those of urea, based on data collected in patients during hemodialysis. It appeared somewhat surprisingly that the guanidino compounds showed a kinetic behavior that was entirely different from that of urea. Distribution volume for all guanidino compounds in this table is markedly larger than that of urea, resulting in a lower effective removal. Distribution volume is up to 3 times higher resulting in a decrease of effective removal by up to 45% (guanidino acetic acid). These are however calculated values based on kinetic modeling.


A SOLUTION TO THIS PROBLEM IS MODIFYING THE TIMEFRAME OF DIALYSIS

Eloot et al., NDT, 24: 2225-2232; 2009

* P<0.05, compared to reference dialysis

27

Presenter

Presentation Notes

How could removal during dialysis be improved? In this study by Eloot et al, several mathematical simulations were made, assessing which modifications in dialysis timing might increase removal, compared to a reference 4 hour dialysis three times weekly at a blood flow (QB) of 300 ml/min. In the upper part of the table, compounds with a relatively small distribution volume are illustrated (urea and guanidino succinic acid); below are substances with a larger distribution volume (creatinine and methylguanidine). For the compounds with a large distribution volume, extending dialysis time (3 times 8 hours at 150 ml/min) seems more beneficial, whereas for the solutes with small distribution volume increasing frequency (6 times 2 hours at 300 ml/min) seems more advantageous. Overall, more frequent and longer dialysis at the same time is the most profitable solution (6 times 8 hours at 200 ml/min). These data support the importance to optimize removal by allowing a flexible dialysis time regime, as can be obtained in home hemodialysis programs.


See Uremic Toxins: part 2 for discussion of protein-bound and middle molecules

28


Conclusions

• Adequacy of removal of uremic solutes is hampered by characteristicsof dialyzers and dialysis and by the multicompartmental distribution of most uremic toxins

• Removal can be enhanced by opening pore size and addingconvection, but also by applying extended or frequent dialysis

• Urea, our current marker, of dialysis adequacy has long been considered to be inert but recent data may suggest a biological (toxic) effect

• ADMA and SDMA are guanidines with proven toxic effects

• Kinetics of urea are not representative for that of other water solublecompounds, like the guanidines

29

asn dialysis advisory group · practically, if one puts a teabag in hot water without doing...

Documents