A novel mathematical model of protein-bound uremic toxin kinetics during hemodialysis

Novel extracorporeal treatment for severe neonatal jaundice: a mathematical modelling study of allo-hemodialysis

Severe Neonatal Jaundice (SNJ) causes long-term neurocognitive impairment, cerebral palsy, auditory neuropathy, deafness, or death. We developed a mathematical model for allo-hemodialysis as a potential blood purification method for the treatment of SNJ in term or near-term infants. With allo-hemodialysis (allo-HD), the neonate's blood flows through hollow fibers of a miniature 0.075 m2 hemodialyzer, while the blood of a healthy adult ("buddy") flows counter-currently through the dialysate compartment. We simulated the kinetics of unconjugated bilirubin in allo-hemodialysis with neonate blood flow rates of 12.5 and 15 mL/min (for a 2.5 kg and 3.5 kg neonate, respectively), and 30 mL/min for the buddy. Bilirubin production rates in neonate and buddy were set to 6 and 3 mg/kg/day, respectively. Buddy bilirubin conjugation rate was calculated to obtain normal steady-state bilirubin levels. Albumin levels were set to 1.1, 2.1, 3.1 g/dL for the neonate and 3.3 g/dL for the buddy. Model simulations suggest that a 6-h allo-hemodialysis session could reduce neonatal bilirubin levels by > 35% and that this modality would be particularly effective with low neonatal serum albumin levels. Due to the high bilirubin conjugation capacity of an adult's healthy liver and the larger distribution volume, the buddy's bilirubin level increases only transiently during allo-hemodialysis. Our modelling suggests that a single allo-hemodialysis session may lower neonatal unconjugated bilirubin levels effectively. If corroborated in ex-vivo, animal, and clinical studies, this bilirubin reduction could lower the risks associated with SNJ, especially kernicterus, and possibly avoiding the morbidity associated with exchange transfusions.

Allo-Hemodialysis, a Novel Dialytic Treatment Option for Patients with Kidney Failure: Outcomes of Mathematical Modelling, Prototyping, and Ex Vivo Testing

It has been estimated that in 2010, over two million patients with end-stage kidney disease may have faced premature death due to a lack of access to affordable renal replacement therapy, mostly dialysis. To address this shortfall in dialytic kidney replacement therapy, we propose a novel, cost-effective, and low-complexity hemodialysis method called allo-hemodialysis (alloHD). With alloHD, instead of conventional hemodialysis, the blood of a patient with kidney failure flows through the dialyzer's dialysate compartment counter-currently to the blood of a healthy subject (referred to as a "buddy") flowing through the blood compartment. Along the concentration and hydrostatic pressure gradients, uremic solutes and excess fluid are transferred from the patient to the buddy and subsequently excreted by the healthy kidneys of the buddy. We developed a mathematical model of alloHD to systematically explore dialysis adequacy in terms of weekly standard urea Kt/V. We showed that in the case of an anuric child (20 kg), four 4 h alloHD sessions are sufficient to attain a weekly standard Kt/V of >2.0. In the case of an anuric adult patient (70 kg), six 4 h alloHD sessions are necessary. As a next step, we designed and built an alloHD machine prototype that comprises off-the-shelf components. We then used this prototype to perform ex vivo experiments to investigate the transport of solutes, including urea, creatinine, and protein-bound uremic retention products, and to quantitate the accuracy and precision of the machine's ultrafiltration control. These experiments showed that alloHD performed as expected, encouraging future in vivo studies in animals with and without kidney failure.

Effect of Membrane Permeance and System Parameters on the Removal of Protein-Bound Uremic Toxins in Hemodialysis

Inadequate clearance of protein-bound uremic toxins (PBUTs) during dialysis is associated with morbidities in chronic kidney disease patients. The development of high-permeance membranes made from materials such as graphene raises the question whether they could enable the design of dialyzers with improved PBUT clearance. Here, we develop device-level and multi-compartment (body) system-level models that account for PBUT-albumin binding (specifically indoxyl sulfate and p-cresyl sulfate) and diffusive and convective transport of toxins to investigate how the overall membrane permeance (or area) and system parameters including flow rates and ultrafiltration affect PBUT clearance in hemodialysis. Our simulation results indicate that, in contrast to urea clearance, PBUT clearance in current dialyzers is mass-transfer limited: Assuming that the membrane resistance is dominant, raising PBUT permeance from 3 × 10-6 to 10-5 m s-1 (or equivalently, 3.3 × increase in membrane area from ~ 2 to ~ 6 m2) increases PBUT removal by 48% (from 22 to 33%, i.e., ~ 0.15 to ~ 0.22 g per session), whereas increasing dialysate flow rates or adding adsorptive species have no substantial impact on PBUT removal unless permeance is above ~ 10-5 m s-1. Our results guide the future development of membranes, dialyzers, and operational parameters that could enhance PBUT clearance and improve patient outcomes.

Can the response to dialysis treatment be predicted by using patient-specific modeling of fluid and solute exchanges? A multicentric evaluation

BACKGROUND

Parametric multipool kinetic models were used to describe the intradialytic trends of electrolytes, breakdown products, and body fluids volumes during hemodialysis. Therapy customization can be achieved by the identification of parameters, allowing patient-specific modulation of mass and fluid balance across dialyzer, capillary, and cell membranes. This study wants to evaluate the possibility to use this approach to predict the patient's intradialytic response.

METHODS

6 sessions of 68 patients (DialysIS© project) were considered. Data from the first three sessions were used to train the model, identifying the patient-specific parameters, that, together with the treatment settings and the patient's data at the session start, could be used for predicting the patient's specific time course of solutes and fluids along the sessions. Na+ , K+ , Cl- , Ca2+ , HCO3 - , and urea plasmatic concentrations and hematic volume deviations from clinical data were evaluated.

RESULTS

nRMSE predictive error is on average equal to 4.76% when describing the training sessions, and only increases by 0.97 percentage points on average in independent sessions of the same patient.

CONCLUSIONS

The proposed predictive approach represents a first step in the development of tools to support the clinician in tailoring the patient's prescription.

Increased clearance of indoxyl sulphate in renal failure rats with the addition of water-soluble poly-β-cyclodextrin to the dialysate

AIM

Indoxyl sulphate (IS), a protein-bound uremic toxin that dramatically increases in the sera of patients with end-stage renal disease (ESRD), is poorly removed by conventional haemodialysis (HD). The purpose of this study was to explore whether the addition of water-soluble sorbent poly-β-cyclodextrins (PCDs) to dialysate can increase the clearance of IS in uremic rats in vivo.

METHODS

Male SD rats (450-550 g, n = 18) with nephrectomy plus IS injection (10-mg/kg) were randomly divided into three groups: 1. The HD group (n = 6): conventional HD for 4 h; 2. the 2% PCD group: 2% PCD added to the dialysate, HD for 4 h; and 3. the 4% PCD group: 4% PCD added to the dialysate, HD for 4 h. The serum IS levels in model rats were similar to those of ESRD patients. A stable and safe rat HD treatment mode was established by adjusting the vascular access, blood flow rate, dialysate flow rate, dialysis pipe, dialysate configuration, temperature, treatment environment, and other aspects.

RESULTS

Our study found that adding 2% PCD to dialysate significantly improved the clearance of IS approximately twofold compared with conventional HD. Further increasing the PCD concentration to 4% did not increase IS clearance.

CONCLUSION

Therefore, our study showed that adding water-soluble sorbent PCDs to dialysate significantly improved the clearance of IS in uremic rats in vivo.

Using a Human Circulation Mathematical Model to Simulate the Effects of Hemodialysis and Therapeutic Hypothermia

Background: We developed a hemodynamic mathematical model of human circulation coupled to a virtual hemodialyzer. The model was used to explore mechanisms underlying our clinical observations involving hemodialysis. Methods: The model consists of whole body human circulation, baroreflex feedback control, and a hemodialyzer. Four model populations encompassing baseline, dialysed, therapeutic hypothermia treated, and simultaneous dialysed with hypothermia were generated. In all populations atrial fibrillation and renal failure as co-morbidities, and exercise as a treatment were simulated. Clinically relevant measurables were used to quantify the effects of each in silico experiment. Sensitivity analysis was used to uncover the most relevant parameters. Results: Relative to baseline, the modelled dialysis increased the population mean diastolic blood pressure by 5%, large vessel wall shear stress by 6%, and heart rate by 20%. Therapeutic hypothermia increased systolic blood pressure by 3%, reduced large vessel shear stress by 15%, and did not affect heart rate. Therapeutic hypothermia reduced wall shear stress by 15% in the aorta and 6% in the kidneys, suggesting a potential anti-inflammatory benefit. Therapeutic hypothermia reduced cardiac output under atrial fibrillation by 12% and under renal failure by 20%. Therapeutic hypothermia and exercise did not affect dialyser function, but increased water removal by approximately 40%. Conclusions: This study illuminates some mechanisms of the action of therapeutic hypothermia. It also suggests clinical measurables that may be used as surrogates to diagnose underlying diseases such as atrial fibrillation.

Determinants of Hemodialysis Performance: Modeling Fluid and Solute Transport in Hollow-Fiber Dialyzers

Hemodialysis constitutes the lifeline of patients with end stage renal disease, yet the parameters that affect hemodialyzer performance remain incompletely understood. We developed a computational model of mass transfer and solute transport in a hollow-fiber dialyzer to gain greater insight into the determinant factors. The model predicts fluid velocity, pressure, and solute concentration profiles for given geometric characteristics, membrane transport properties, and inlet conditions. We examined the impact of transport and structural parameters on uremic solute clearance by varying parameter values within the constraints of standard clinical practice. The model was validated by comparison with published experimental data. Our results suggest solute clearance can be significantly altered by changes in blood and dialysate flow rates, fiber radius and length, and net ultrafiltration rate. Our model further suggests that the main determinant of the clearance of unreactive solutes is their diffusive permeability. The clearance of protein-bound toxins is also strongly determined by blood hematocrit and plasma protein concentrations. Results from this model may serve to optimize hemodialyzer operating conditions in clinical practice to achieve better clearance of pathogenic uremic solutes.

Blood flow rate: An independent risk factor of mortality in Chinese hemodialysis patients

BACKGROUND

Studies suggested the association between blood flow rate (BFR) and mortality might be beyond dialysis adequacy. This study aimed to explore if BFR is an independent predictor of clinical outcomes in Chinese hemodialysis (HD) patients.

METHODS

This study included data from patients in China Dialysis Outcomes and Practice Patterns Study (DOPPS) Phase 5. Patients with a record of BFR were included, and demographic data, comorbidities, hospitalization, and death records were collected. Associations between BFR and all-cause mortality and hospitalization were analyzed using Cox regression models.

RESULTS

One thousand four hundred twelve (98.9%) patients were included. Most patients were with BFR < 300 ml/min. After full adjustment, each 10-ml/min increase of BFR was associated with a 6.4% decrease in all-cause mortality risk (HR: 0.936, 95% CI: 0.880-0.996) but not first hospitalization (HR: 0.987, 95% CI: 0.949-1.027). The impact of BFR on mortality may be more prominent in patients who were male gender, nondiabetic, albumin < 4.0 g/dl, and hemoglobin ≥ 9.0 g/dl.

CONCLUSION

Increased BFR is independently associated with a lower risk of all-cause mortality within the range of BFR 200-300 ml/min. And this effect is more pronounced in patients who were male gender, nondiabetic, albumin < 4.0 g/dl, and hemoglobin ≥ 9.0 g/dl.

Removal of Protein-Bound Uremic Toxins Using Binding Competitors in Hemodialysis: A Narrative Review

Removal of protein-bound uremic toxins (PBUTs) during conventional dialysis is insufficient. PBUTs are associated with comorbidities and mortality in dialysis patients. Albumin is the primary carrier for PBUTs and only a small free fraction of PBUTs are dialyzable. In the past, we proposed a novel method where a binding competitor is infused upstream of a dialyzer into an extracorporeal circuit. The competitor competes with PBUTs for their binding sites on albumin and increases the free PBUT fraction. Essentially, binding competitor-augmented hemodialysis is a reactive membrane separation technique and is a paradigm shift from conventional dialysis therapies. The proposed method has been tested in silico, ex vivo, and in vivo, and has proven to be very effective in all scenarios. In an ex vivo study and a proof-of-concept clinical study with 18 patients, ibuprofen was used as a binding competitor; however, chronic ibuprofen infusion may affect residual kidney function. Binding competition with free fatty acids significantly improved PBUT removal in pre-clinical rat models. Based on in silico analysis, tryptophan can also be used as a binding competitor; importantly, fatty acids or tryptophan may have salutary effects in HD patients. More chemoinformatics research, pre-clinical, and clinical studies are required to identify ideal binding competitors before routine clinical use.

A model-based analysis of phenytoin and carbamazepine toxicity treatment using binding-competition during hemodialysis

Hemodialysis (HD) has limited efficacy towards treatment of drug toxicity due to strong drug-protein binding. In this work, we propose to infuse a competitor drug into the extracorporeal circuit that increases the free fraction of a toxic drug and thereby increases its dialytic removal. We used a mechanistic model to assess the removal of phenytoin and carbamazepine during HD with or without binding-competition. We simulated dialytic removal of (1) phenytoin, initial concentration 70 mg/L, using 2000 mg aspirin, (2) carbamazepine, initial concentration 35 mg/L, using 800 mg ibuprofen, in a 70 kg patient. The competitor drug was infused at constant rate. For phenytoin (~ 13% free at t = 0), HD brings the patient to therapeutic concentration in 460 min while aspirin infusion reduces that time to 330 min. For carbamazepine (~ 27% free at t = 0), the ibuprofen infusion reduces the HD time to reach therapeutic concentration from 265 to 220 min. Competitor drugs with longer half-life further reduce the HD time. Binding-competition during HD is a potential treatment for drug toxicities for which current recommendations exclude HD due to strong drug-protein binding. We show clinically meaningful reductions in the treatment time necessary to achieve non-toxic concentrations in patients poisoned with these two prescription drugs.

Selective Transport of Protein-Bound Uremic Toxins in Erythrocytes

To better understand the kinetics of protein-bound uremic toxins (PBUTs) during hemodialysis (HD), we investigated the distribution of hippuric acid (HA), indole-3-acetic acid (IAA), indoxyl sulfate (IS), and p-cresyl sulfate (pCS) in erythrocytes of HD patients. Their transport across the erythrocyte membrane was explored in the absence of plasma proteins in vitro in a series of loading and unloading experiments of erythrocytes from healthy subjects and HD patients, respectively. Furthermore, the impact of three inhibitors of active transport proteins in erythrocytes was studied. The four PBUTs accumulated in erythrocytes from HD patients. From loading and unloading experiments, it was found that (i) the rate of transport was dependent on the studied PBUT and increased in the following sequence: HA < IS < pCS < IAA and (ii) the solute partition of intra- to extra-cellular concentrations was uneven at equilibrium. Finally, inhibiting especially Band 3 proteins affected the transport of HA (both in loading and unloading), and of IS and pCS (loading). By exploring erythrocyte transmembrane transport of PBUTs, their kinetics can be better understood, and new strategies to improve their dialytic removal can be developed.

Removal of Protein-Bound Uremic Toxins during Hemodialysis Using a Binding Competitor

BACKGROUND AND OBJECTIVES

Current hemodialysis techniques fail to efficiently remove the protein-bound uremic toxins p-cresyl sulfate and indoxyl sulfate due to their high degree of albumin binding. Ibuprofen, which shares the same primary albumin binding site with p-cresyl sulfate and indoxyl sulfate, can be infused during hemodialysis to displace these toxins, thereby augmenting their removal.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We infused 800 mg ibuprofen into the arterial bloodline between minutes 21 and 40 of a conventional 4-hour high-flux hemodialysis treatment. We measured arterial, venous, and dialysate outlet concentrations of indoxyl sulfate, p-cresyl sulfate, tryptophan, ibuprofen, urea, and creatinine before, during, and after the ibuprofen infusion. We report clearances of p-cresyl sulfate and indoxyl sulfate before and during ibuprofen infusion and dialysate concentrations of protein-bound uremic toxins normalized to each patient's average preinfusion concentrations.

RESULTS

We studied 18 patients on maintenance hemodialysis: age 36±11 years old, ten women, and mean vintage of 37±37 months. Compared with during the preinfusion period, the median (interquartile range) clearances of indoxyl sulfate and p-cresyl sulfate increased during ibuprofen infusion from 6.0 (6.5) to 20.2 (27.1) ml/min and from 4.4 (6.7) to 14.9 (27.1) ml/min (each P<0.001), respectively. Relative median (interquartile range) protein-bound uremic toxin dialysate outlet levels increased from preinfusion 1.0 (reference) to 2.4 (1.2) for indoxyl sulfate and to 2.4 (1.0) for p-cresyl sulfate (each P<0.001). Although median serum post- and predialyzer levels in the preinfusion period were similar, infusion led to a marked drop in serum postdialyzer levels for both indoxyl sulfate and p-cresyl sulfate (-1.0 and -0.3 mg/dl, respectively; each P<0.001). The removal of the nonprotein-bound solutes creatinine and urea was not increased by the ibuprofen infusion.

CONCLUSIONS

Infusion of ibuprofen into the arterial bloodline during hemodialysis significantly increases the dialytic removal of indoxyl sulfate and p-cresyl sulfate and thereby, leads to greater reduction in their serum levels.

In silico comparison of protein-bound uremic toxin removal by hemodialysis, hemodiafiltration, membrane adsorption, and binding competition

Protein-bound uremic toxins (PBUTs) are poorly removed during hemodialysis (HD) due to their low free (dialyzable) plasma concentration. We compared PBUT removal between HD, hemodiafiltration (HDF), membrane adsorption, and PBUT displacement in HD. The latter involves infusing a binding competitor pre-dialyzer, which competes with PBUTs for their albumin binding sites and increases their free fraction. We used a mathematical model of PBUT/displacer kinetics in dialysis comprising a three-compartment patient model, an arterial/venous tube segment model, and a dialyzer model. Compared to HD, improvements in removal of prototypical PBUTs indoxyl sulfate (initial concentration 100 µM, 7% free) and p-cresyl sulfate (150 µM, 5% free) were: 5.5% and 6.4%, respectively, for pre-dilution HDF with 20 L replacement fluid; 8.1% and 9.1% for post-dilution HDF 20 L; 15.6% and 18.3% for pre-dilution HDF 60 L; 19.4% and 22.2% for complete membrane adsorption; 35.0% and 41.9% for displacement with tryptophan (2000 mg in 500 mL saline); 26.7% and 32.4% for displacement with ibuprofen (800 mg in 200 mL saline). Prolonged (one-month) use of tryptophan reduces the IS and pCS time-averaged concentration by 28.1% and 29.9%, respectively, compared to conventional HD. We conclude that competitive binding can be a pragmatic approach for improving PBUT removal.