John F. Patzer

First Clinical Use of a Novel Bioartificial Liver Support System (BLSS)

The first clinical use of the Excorp Medical Bioartificial Liver Support System (BLSS) in support of a 41‐year‐old African‐American female with fulminant hepatic failure is described. The BLSS is currently in a Phase I/II safety evaluation at the University of Pittsburgh/UPMC System. Inclusion criteria for the study are patients with acute liver failure, any etiology, presenting with encephalopathy deteriorating beyond Parsons Grade 2. The BLSS consists of a blood pump; a heat exchanger to control blood temperature; an oxygenator to control oxygenation and pH; a bioreactor; and associated pressure and flow alarm systems. Patient liver support is provided by 70–100 g of porcine liver cells housed in the hollow fiber bioreactor. The patient exhibited transient hypotension and thrombocytopenia at initiation of perfusion. The only unanticipated safety event was a lowering of patient glucose level at the onset of perfusion with the BLSS that was treatable with intravenous glucose administration. Moderate changes in blood biochemistries pre‐ and post perfusion are indicative of liver support being provided by the BLSS. While the initial experience with the BLSS is encouraging, completion of the Phase I/II study is required in order to more fully understand the safety aspects of the BLSS.

Safety observations in phase I clinical evaluation of the Excorp Medical Bioartificial Liver Support System after the first four patients.

A Phase I clinical safety evaluation of the Excorp Medical, Inc, Bioartificial Liver Support System (BLSS) is in progress. Inclusion criteria are patients with acute liver failure of any etiology, presenting with encephalopathy deteriorating beyond Parson’s Grade 2. The BLSS consists of a blood pump, heat exchanger to control blood temperature, oxygenator to control oxygenation and pH, bioreactor, and associated pressure and flow alarm systems. Patient liver support is provided by 70–100 g of porcine liver cells housed in the hollow fiber bioreactor. A single support period evaluation consists of 12 hour extracorporeal perfusion with the BLSS sandwiched between 12 hours of pre (baseline) and 12 hours of post support monitoring. Blood chemistries and hematologies are obtained every 6 hours during monitoring periods and every 4 hours during perfusion. Physiologic parameters are monitored continuously. The patient may receive a second treatment at the discretion of the clinical physician. Preliminary evaluation of safety considerations after enrollment of the first four patients (F, 41, acetaminophen induced, two support periods; M, 50, Wilson’s disease, one support period; F, 53, acute alcoholic hepatitis, two support periods; F, 24, chemotherapy induced, one support period) is presented. All patients tolerated the extracorporeal perfusion well. All patients presented with hypoglycemia at the start of perfusion, treatable by IV dextrose. Transient hypotension at the start of perfusion responded to an IV fluid bolus. Only the second patient required heparin anticoagulation. No serious or unexpected adverse events were noted. Moderate biochemical response to support was noted in all patients. Completion of the Phase I safety evaluation is required to fully characterize the safety of the BLSS.

Noninvasive monitoring of cerebral perfusion pressure in patients with acute liver failure using transcranial doppler ultrasonography

Elevated intracranial pressure (ICP) leads to loss of cerebral perfusion, cerebral herniation, and irreversible brain damage in patients with acute liver failure (ALF). Conventional techniques for monitoring ICP can be complicated by hemorrhage and infection. Transcranial doppler ultrasonography (TCD) is a noninvasive device which can continuously measure cerebral blood flow velocity, producing a velocity‐time waveform that indirectly monitors changes in cerebral hemodynamics, including ICP. The primary goal of this study was to determine whether TCD waveform features could be used to differentiate ALF patients with respect to ICP or, equally important, cerebral perfusion pressure (CPP) levels. A retrospective study of 16 ALF subjects with simultaneous TCD, ICP, and CPP measurements yielded a total of 209 coupled ICP‐CPP‐TCD observations. The TCD waveforms were digitally scanned and seven points corresponding to a simplified linear waveform were identified. TCD waveform features including velocity, pulsatility index, resistive index, fraction of the cycle in systole, slopes, and angles associated with changes in the slope in each region, were calculated from the simplified waveform data. Paired ICP‐TCD observations were divided into three groups (ICP < 20 mmHg, n = 102; 20 ≤ ICP < 30 mmHg, n = 74; and ICP ≥ 30 mmHg, n = 33). Paired CPP‐TCD observations were also divided into three groups (CPP ≥ 80 mmHg, n = 42; 80 > CPP ≥ 60 mmHg, n = 111; and CPP < 60 mmHg, n = 56). Stepwise linear discriminant analysis was used to identify TCD waveform features that discriminate between ICP groups and CPP groups. Four primary features were found to discriminate between ICP groups: the blood velocity at the start of the Windkessel effect, the slope of the Windkessel upstroke, the angle between the end systolic downstroke and start diastolic upstroke, and the fraction of time spent in systole. Likewise, 4 features were found to discriminate between the CPP groups: the slope of the Windkessel upstroke, the slope of the Windkessel downstroke, the slope of the diastolic downstroke, and the angle between the end systolic downstroke and start diastolic upstroke. The TCD waveform captures the cerebral hemodynamic state and can be used to predict dynamic changes in ICP or CPP in patients with ALF. The mean TCD waveforms for corresponding, correctly classified ICP and CPP groups are remarkably similar. However, this approach to predicting intracranial hypertension and CPP needs to be further refined and developed before clinical application is feasible. Liver Transpl 14:1048–1057, 2008.

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 ( approximately 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.

Advances in bioartificial liver assist devices.

Abstract: Rapid advances in development of bioartificial liver assist devices (BLADs) are exciting clinical interest in the application of BLAD technology for support of patients with acute liver failure. Four devices (Circe Biomedical HepatAssist®, Vitagen ELAD™, Gerlach BELS, and Excorp Medical BLSS) that rely on hepatocytes cultured in hollow‐fiber membrane technology are currently in various stages of clinical evaluation. Several alternative approaches for culture and perfusion of hepatocytes have been evaluated in preclinical, large animal models of liver failure, or at a laboratory scale. Engineering design issues with respect to xenotransplantation, BLAD perfusion, hepatocyte functionality and culture maintenance, and ultimate distribution of a BLAD to a clinical site are delineated.

Bound solute dialysis

We used the thermodynamic principles governing bound solute dialysis, commonly referred to as “albumin dialysis” or “sorbent dialysis” and practiced clinically with the Molecular Adsorbent Recirculating System (MARS) and Biologic-DT approaches, respectively, to develop a comprehensive understanding of the process. Dimensionless parameters emerging from the thermodynamic analysis that govern bound solute dialysis are as follows: (1) &lgr;, the binding power of the solute binding moiety; (2) &kgr;, the dialyzer mass transfer/blood flow rate ratio; (3) &agr;, the dialysate/blood flow rate ratio; (4) &bgr;, the dialysate/blood binding moiety concentration ratio, and (5) &psgr;, the solute/binding moiety concentration ratio in the blood. Results from a mathematical model of countercurrent bound solute dialysis for &phgr; = 0.9 indicate that for a given binding moiety (fixed &lgr;), the most important parameter for achieving high removal rates is the dialyzer mass transfer ratio for free (unbound) solute. The results also show solute removal approaching an asymptote with increasing &bgr; that is dependent on &kgr; and independent of &agr;. More importantly, results indicate that once a dialysis membrane is chosen, solute removal is virtually independent of blood flow rate, dialysate flow rate, and amount of binding moiety in the dialysate, provided the amount is greater than approximately 90% of that required to reach the asymptote. Experimental observations over a range of blood flow rates (100–400 ml/minute), dialysate flow rates (50–400 ml/minute), and dialysate/blood albumin concentration ratios (&bgr; = 0–0.3) corroborate the model predictions and indicate that < 4 g/L albumin in the dialysate solution is required for effective bound solute dialysis. The experimental results also show evidence of enhanced mass transfer once the dialysis membrane pore structure surface saturates with albumin.

Principles of Bound Solute Dialysis

Abstract:  Toxins that bind to albumin in the bloodstream and are associated with progressing liver failure have proven refractory to removal by conventional hemodialysis. Such toxins can, however, be removed by adding a binder to the dialysate that serves to capture the toxin as it is dialyzed across the membrane. Several approaches based upon this concept are in various stages of clinical evaluation. The thermodynamic basis common to these approaches has been used to develop an engineering description of ‘bound solute dialysis’ which has further been used to define the clinical expectations and limitations of the approach. Three dimensionless, independently controllable, operating parameters emerged from this analysis (i): κ, the dialyzer mass transfer/blood flow rate ratio (clinical range: 0.5–2.5); (ii) α, the dialysate/blood flow rate ratio (clinical range: 0.1–2.0); and (iii) β, the dialysate/blood binder concentration ratio (clinical range: 0.02–5.0). In the absence of binder in the dialysate, bound toxin removal is sensitive to κ and α, with greater removal associated with greater κ and/or α. Bound toxin removal, however, is dependent primarily upon κ and independent of α and β once a small amount of binder, β > 0.02, is added to the dialysate. The improvement in bound toxin removal over conventional hemodialysis is dependent upon how tightly the toxin binds albumin ranging from a 6‐fold increase for a relatively tightly bound solute such as unconjugated bilirubin, to 1.5‐fold increase for a less tightly bound drug such as warfarin at 24 h perfusion time. Clinically, bound solute dialysis can be practiced in single‐pass mode with as little as 1–2 g albumin/L dialysate. Because of the constraints imposed by the thermodynamic nature of the process, intervention should be made as early in the disease progression as feasible.

Transport Advances in Disposable Bioreactors for Liver Tissue Engineering

Acute liver failure (ALF) is a devastating diagnosis with an overall survival of approximately 60%. Liver transplantation is the therapy of choice for ALF patients but is limited by the scarce availability of donor organs. The prognosis of ALF patients may improve if essential liver functions are restored during liver failure by means of auxiliary methods because liver tissue has the capability to regenerate and heal. Bioartificial liver (BAL) approaches use liver tissue or cells to provide ALF patients with liver-specific metabolism and synthesis products necessary to relieve some of the symptoms and to promote liver tissue regeneration. The most promising BAL treatments are based on the culture of tissue engineered (TE) liver constructs, with mature liver cells or cells that may differentiate into hepatocytes to perform liver-specific functions, in disposable continuous-flow bioreactors. In fact, adult hepatocytes perform all essential liver functions. Clinical evaluations of the proposed BALs show that they are safe but have not clearly proven the efficacy of treatment as compared to standard supportive treatments. Ambiguous clinical results, the time loss of cellular activity during treatment, and the presence of a necrotic core in the cell compartment of many bioreactors suggest that improvement of transport of nutrients, and metabolic wastes and products to or from the cells in the bioreactor is critical for the development of therapeutically effective BALs. In this chapter, advanced strategies that have been proposed over to improve mass transport in the bioreactors at the core of a BAL for the treatment of ALF patients are reviewed.

Thermodynamic Considerations in Solid Adsorption of Bound Solutes for Patient Support in Liver Failure

New detoxification modes of treatment for liver failure that use solid adsorbents to remove toxins bound to albumin in the patient bloodstream are entering clinical evaluations, frequently in head-to-head competition. While generally effective in reducing toxin concentration beyond that obtainable by conventional dialysis procedures, the solid adsorbent processes are largely the result of heuristic development. Understanding the principles and limitations inherent in competitive toxin binding, albumin versus solid adsorbent, will enhance the design process and, possibly, improve detoxification performance. An equilibrium thermodynamic analysis is presented for both the molecular adsorbent recirculating system (MARS) and fractionated plasma separation, adsorption, and dialysis system (Prometheus), two advanced systems with distinctly different operating modes but with similar equilibrium limitations. The Prometheus analysis also applies to two newer approaches: sorbent suspension reactor and microsphere-based detoxification system. Primary results from the thermodynamic analysis are that: (i) the solute-albumin binding constant is of minor importance to equilibrium once it exceeds about 10(5) L/mol; (ii) the Prometheus approach requires larger solid adsorbent columns than calculated by adsorbent solute capacity alone; and (iii) the albumin-containing recycle stream in the MARS approach is a major reservoir of removed toxin. A survey of published results indicates that MARS is operating under mass transfer control dictated by solute-albumin equilibrium in the recycle stream, and Prometheus is approaching equilibrium limits under current clinical protocols.

Slow continuous ultrafiltration with bound solute dialysis.

Bound solute dialysis (BSD), often referred to as “albumin dialysis” (practiced clinically as the molecular adsorbents recirculating system, MARS, or single-pass albumin dialysis, SPAD) or “sorbent dialysis” (practiced clinically as the charcoal-based Biologic-DT), is based upon the thermodynamic principle that the driving force for solute mass transfer across a dialysis membrane is the difference in free solute concentration across the membrane. The clinically relevant practice of slow continuous ultrafiltration (SCUF) for maintenance of patients with liver failure is analyzed in conjunction with BSD. The primary dimensionless operating parameters that describe SCUF-BSD include (1) β, the dialysate/blood binder concentration ratio; (2) &kgr;, the dialyzer mass transfer/blood flow rate ratio; (3) α, the dialysate/blood flow rate ratio; and, (4) γ, the ultrafiltration/blood flow rate ratio. Results from mathematical modeling of solute removal during a single pass through a dialyzer and solute removal from a one-compartment model indicate that solute removal is remarkably insensitive to γ. Solute removal approaches an asymptote (improvement in theoretical clearance over that obtainable with no binder in the dialysate) with increasing β that is dependent on &kgr; and independent of α. The amount of binder required to approach the asymptote decreases with increasing solute-binder equilibrium constant, i.e., more strongly bound solutes require less binder in the dialysate. The results of experimental observations over a range of blood flow rates, 100 to 180 mL/min, dialysate flow rates, 600 to 2150 mL/h, ultrafiltration rates, 0 to 220 mL/h, and dialysate/blood albumin concentration ratios, β = 0.01 to 0.04, were independently predicted remarkably well by the one-compartment model (with no adjustable parameters) based on BSD principles.