Thermodynamic considerations in solid adsorption of bound solutes for patient support in liver failure

Increasing the Clearance of Protein-Bound Solutes by Recirculating Dialysate through Activated Carbon

Key Points

Conventional hemodialysis provides limited clearance of uremic solutes that bind to plasma proteins. No studies have yet tested whether increasing the clearance of bound solutes provides clinical benefit. Practical means to increase the dialytic clearance of bound solutes are required to perform such studies.

Background

Conventional hemodialysis provides limited clearance of uremic solutes that bind to plasma proteins. However, no studies have tested whether increasing the clearance of bound solutes provides clinical benefit. Practical means to increase the dialytic clearance of bound solutes are required to perform such studies.

Methods

Artificial plasma was dialyzed using two dialysis systems in series. In the first recirculating system, a fixed small volume of dialysate flowed rapidly through an activated carbon block before passing through two large dialyzers. In a second conventional system, a lower flow of fresh dialysate was passed through a single dialyzer. Chemical measurements tested the ability of the recirculating system to increase the clearance of selected solutes. Mathematical modeling predicted the dependence of solute clearances on the extent to which solutes were taken up by the carbon block and were bound to plasma proteins.

Results

By itself, the conventional system provided clearances of the tightly bound solutes p-cresol sulfate and indoxyl sulfate of only 18±10 and 19±11 ml/min, respectively (mean±SD). Because these solutes were effectively adsorbed by the carbon block, the recirculating system by itself provided p-cresol sulfate and indoxyl sulfate clearances of 45±11 and 53±16 ml/min. It further raised their clearances to 54±12 and 61±17 ml/min when operating in series with the conventional system (P < 0.002 versus conventional clearance both solutes). Modeling predicted that the recirculating system would increase the clearances of bound solute even if their uptake by the carbon block was incomplete.

Conclusions

When added to a conventional dialysis system, a recirculating system using a carbon block sorbent, a single pump, and standard dialyzers can greatly increase the clearance of protein-bound uremic solutes.

Targeting Albumin Binding Function as a Therapy Goal in Liver Failure: Development of a Novel Adsorbent for Albumin Dialysis

Liver failure results in impaired hepatic detoxification combined with diminished albumin synthesis and is associated with secondary organ failure. The accumulation of liver toxins has shown to saturate albumin binding sites. This was previously demonstrated by an in vitro test for albumin binding capacity (ABiC) that has shown to inversely correlate with the established MELD (Model for End-Stage Liver Disease) score. In this study, we introduced a new adsorbent material for albumin dialysis treatments that improves albumin binding capacity. The new charcoal adsorbent was developed by an evolutionary test schedule. Batch testing of charcoals was performed as steady-state experiments. The charcoal reflecting the highest increase in albumin binding capacity was then introduced to kinetic models: Perfusion tests were designed to evaluate adsorption capacity and kinetics for liver failure marker toxins. A dynamic recirculation model for liver failure was used for upscaling and comparison against conventional MARS adsorbents as the gold standard in an albumin dialysis setting. Batch tests revealed that powdered activated Hepalbin charcoal displayed the highest ABiC score. Hepalbin charcoal also demonstrated higher adsorptive capacity and kinetics for liver failure marker toxins as determined by perfusion tests. These findings translated to tests of upscaled adsorbents in a dynamic model for liver failure: upscaled Hepalbin adsorbent removes bile acids, direct bilirubin and indirect bilirubin significantly better than MARS adsorbents and significantly increases ABiC. The novel adsorbent Hepalbin offers a significant improvement over both MARS adsorbents concerning liver failure marker toxin removal and ABiC improvement.

Uraemic toxins and new methods to control their accumulation: game changers for the concept of dialysis adequacy

The current concept of an adequate dialysis based only on the dialysis process itself is rather limited. We now have considerable knowledge of uraemic toxicity and improved tools for limiting uraemic toxin accumulation. It is time to make use of these. A broader concept of adequacy that focusses on uraemic toxicity is required. As discussed in the present review, adequacy could be achieved by many different methods in combination with, or instead of, dialysis. These include preservation of renal function, dietary intake, reducing uraemic toxin generation rate and intestinal absorption, isolated ultrafiltration and extracorporeal adsorption of key uraemic toxins. A better measure of the quality of dialysis treatment would quantify the uraemic state in the patient using levels of a panel of key uraemic toxins. Treatment would focus on controlling uraemic toxicity while reducing harm or inconvenience to the patient. Delivering more dialysis might not be the best way to achieve this.

Albumin dialysis in artificial liver support systems: open-loop or closed-loop dialysis mode?

In artificial liver support systems, the open-loop albumin dialysis mode (OLM) is usually used to remove protein-bound toxins from the blood of patients with liver failure. However, there is still interest in the closed-loop albumin dialysis mode (CLM) because this mode may enable not only the regeneration and reuse of albumin but also the miniaturization of artificial liver systems. In this article, we compared the two modes under a fixed amount of albumin in dialysate experimentally and theoretically. The results show that according to the detoxification efficiency in the 3 hour dialysis for removing albumin-bound bilirubin, CLM is better than OLM. The usage efficiency of albumin in CLM is also higher. Moreover, the advantage of CLM is more significant when the concentration of bilirubin in blood is lower. Under a given amount of albumin in dialysate, if the concentration of bilirubin in blood is high, one may further increase the performance of CLM by means of increasing the flow rate of the albumin dialysate or using the highly concentrated albumin dialysate.

Proteomic analysis of polypeptides captured from blood during extracorporeal albumin dialysis in patients with cholestasis and resistant pruritus

Albumin dialysis using the molecular adsorbent recirculating system (MARS) is a new therapeutic approach for liver diseases. To gain insight into the mechanisms involved in albumin dialysis, we analyzed the peptides and proteins absorbed into the MARS strong anion exchange (SAX) cartridges as a result of the treatment of patients with cholestasis and resistant pruritus. Proteins extracted from the SAX MARS cartridges after patient treatment were digested with two enzymes. The resulting peptides were analyzed by multidimensional liquid chromatography coupled to tandem mass spectrometry. We identified over 1,500 peptide sequences corresponding to 144 proteins. In addition to the proteins that are present in control albumin-derived samples, this collection includes 60 proteins that were specific to samples obtained after patient treatment. Five of these proteins (neutrophil defensin 1 [HNP-1], secreted Ly-6/uPAR-related protein 1 [SLURP1], serum amyloid A, fibrinogen alpha chain and pancreatic prohormone) were confirmed to be removed by the dialysis procedure using targeted selected-reaction monitoring MS/MS. Furthermore, capture of HNP-1 and SLURP1 was also validated by Western blot. Interestingly, further analyses of SLURP1 in serum indicated that this protein was 3-fold higher in cholestatic patients than in controls. Proteins captured by MARS share certain structural and biological characteristics, and some of them have important biological functions. Therefore, their removal could be related either to therapeutic or possible adverse effects associated with albumin dialysis.

Bioartificial liver systems: why, what, whither?

Acute liver disease is a life-threatening condition for which liver transplantation is the only recognized effective therapy. While etiology varies considerably, the clinical course of acute liver failure is common among the etiologies: encephalopathy progressing toward coma and multiple organ failure. Detoxification processes, such as molecular adsorbent recirculating system (MARS®) and Prometheus, have had limited success in altering blood chemistries positively in clinical evaluations, but have not been shown to be clinically effective with regard to patient survival or other clinical outcomes in any Phase III prospective, randomized trial. Bioartificial liver systems, which use liver cells (hepatocytes) to provide metabolic support as well as detoxification, have shown promising results in early clinical evaluations, but again have not demonstrated clinical significance in any Phase III prospective, randomized trial. Cell transplantation therapy has had limited success but is not practicable for wide use owing to a lack of cells (whole-organ transplantation has priority). New approaches in regenerative medicine for treatment of liver disease need to be directed toward providing a functional cell source, expandable in large quantities, for use in various applications. To this end, a novel bioreactor design is described that closely mimics the native liver cell environment and is easily scaled from microscopic (<1 ml cells) to clinical (∼600 ml cells) size, while maintaining the same local cell environment throughout the bioreactor. The bioreactor is used for study of primary liver cell isolates, liver-derived cell lines and stem/progenitor cells.