Acute kidney injury (AKI) is a common clinical syndrome characterized by an abrupt deterioration in kidney function resulting in abnormalities in volume-regulatory, metabolic-regulatory, excretory, and endocrine functions. A recent study from the Nationwide Inpatient Sample estimated the incidence of AKI to be 288 per 100,000 US population in 20021; the incidence of AKI requiring dialysis was estimated to be 27 per 100,000 population. International epidemiology studies have estimated rates of AKI requiring dialysis to be 45 per 100,000 in England, 20 per 100,000 in Scotland, and 8 per 100,000 in Australia. The incidence of ARF has risen over the past two decades and continues to do so.1,2
Despite decades of improvements in the provision of renal replacement therapy, the morbidity and mortality associated with AKI in the intensive care setting (ICU) remains extremely high. Between 20% and 60% of all patients diagnosed with AKI require dialytic support. Of those requiring dialysis who survive, 12%–33% require long-term renal replacement therapy. The mortality in ARF that requires dialysis is generally reported to be >50%.3–5 For example, the overall in-hospital mortality rate in the Beginning and Ending Supportive Therapy for the Kidney (BEST Kidney) cohort study was 60%.4 As with the Program to Improve Care in Acute Renal Disease (PICARD), mortality varied widely across centers. Among countries contributing more than 100 patients to the cohort, in-hospital mortality ranged from 51% to 77%.5
Much of the morbidity and mortality described earlier in the text is the consequence of systemic cellular damage that results from immune dysregulation. Indeed, the cause of death in patients with AKI is usually the development of a systemic inflammatory response syndrome (SIRS).6 Conventional renal replacement therapies treat volume overload, uremia, acidosis, and electrolyte derangements but has no direct effect on the immune dysregulation that frequently attends AKI.7 Inflammatory cascades initiated by endothelial dysfunction in SIRS are further dysregulated in the setting of AKI, as suggested by recent data that demonstrate that the levels of the proinflammatory cytokines interleukin (IL)-6 and IL-8 in the plasma predict mortality in patients with AKI.8,9 Furthermore, strategies that modulate the inflammatory response provide significant beneficial effects in experimental AKI.10
Recently, data published by the RAD Investigator Group revealed improved patient survival in the setting of AKI in the ICU for those patients who were treated with a selective cytopheretic device (SCD).11 In a post hoc, retrospective analysis of this prospective, randomized, controlled trial, the investigators found that patients treated with the SCD had a mortality rate of 50% if treated with heparin versus 25% if treated with citrate (n = 12 for each treatment arm) at 28 days and 75 versus 33% at 90 days (χ2 < 0.05). The subgroups were comparable with respect to Sequential Organ Failure Assessment (SOFA) scores, organ failure number, and incidence of sepsis. It was hypothesized that the observed improvement in mortality was mediated by the stabilizing effect that the SCD had on leukocyte activation by the ionized calcium (Cai) environment conferred by citrate in the blood circuit.12,13 This hypothesis was further tested in a septic shock porcine model. The SCD was evaluated by administering Escherichia coli into the peritoneum of pigs. This septic shock model demonstrated that the SCD treatment lowers neutrophil activity (serum myeloperoxidase and CD11b cell-surface expression), diminish neutrophil tissue invasion, decrease systemic capillary leak, preserve cardiac output and mean arterial pressure, and prolong survival time.14
The pilot study described herein is the first prospective evaluation of the SCD in humans. This is a prospective, single-arm, single-center study designed to evaluate the safety and efficacy of SCD treatment on clinical outcomes in AKI requiring continuous renal replacement therapy (CRRT) in the ICU.
Study Design and Objectives
This is a prospective, nonrandomized, interventional study designed to evaluate the effect of treatment with the SCD on in-hospital mortality in the acute renal failure population being treated with continuous venovenous hemofiltration (CVVH) with regional citrate anticoagulation. All subjects received standard intensive care treatment for patients undergoing CVVH in addition to the SCD treatment. CVVH treatment was delivered by standard modes, i.e., 2.5 L/h replacement fluid dose predilution.
The SCD Device
The SCD (Cytopherx, Inc., Ann Arbor, MI) comprised a blood tubing set and a proprietary membrane cartridge with the following characteristics: polysulfone fibers with external surface area 1.4 m2 in four subjects and 2.0 m2 in five subjects, MWCO 50,000, and blood compartment 130 ml. The SCD cartridge, in the presence of citrate anticoagulant, acts to sequester and inhibit activated leukocytes. The device is connected in series to a standard CRRT device. Blood from the CRRT circuit is diverted after the CRRT hemofilter through the extra capillary space (ECS) of the SCD. Blood circulates through this space, and it is returned to the patient through the venous return line of the CRRT circuit. Regional citrate anticoagulation is used for the entire CRRT and SCD blood circuits (Figure 1).
The hypotonic trisodium citrate (4%, 136 mmol/L) was infused into prefilter at a fixed ratio of 3.5–4.5 mmol/L to the blood flow by means of a three-way stopcock at the junction of the catheter. The calcium-free replacement fluid delivered at the rate of 2.5 L/h was composed of 121 mmol/L sodium, 3.2 mmol/L potassium, 0.75 mmol/L magnesium, 109 mmol/L chloride, 7 mmol/L bicarbonate, and 250 mg/dl of dextrose. The calcium chloride solution (5%, 340 mmol/L calcium) was infused into the venous return by means of a three-way stopcock at the junction of the catheter. We replaced the calcium chloride at 1.0 mmol/L of blood flow as an initial dose. Systemic Cai was checked repeatedly, and accordingly, calcium infusion rate was adjusted to maintain the level of calcium in circuit between 0.2 and 0.4 mmol/L and ionized systemic calcium at normal range (1.0–1.2 mmol/L).
Adult male and female (nonpregnant) patients, aged 18–80 years, requiring either CVVH or continuous veno-venous hemodialysis (CVVHD) for the treatment of acute renal failure secondary to acute tubular necrosis (ATN) were enrolled. Acute tubular necrosis is defined as acute renal failure occurring in a setting of acute ischemic or nephrotoxic injury and oliguria (<20 ml/hr) for >24 hours or an increase in serum creatinine >2 mg/dl (>1.5 mg/dl in women) over a period of <4 days (Table 1).
In this exploratory clinical study, three domains of interest are designated, and within each of those domains, both key endpoints and exploratory endpoints were prespecified as noted below:
- Clinical efficacy: The primary clinical efficacy endpoint is in-hospital all cause mortality. In addition, urine output was assessed as a surrogate marker for renal recovery. Both of these clinical parameters were compared with data from historical controls from the PICARD dataset matched for age and SOFA score.5
- Patient safety: The domain of patient safety and tolerability was assessed based on the occurrence of adverse events (all, related, unexpected serious, and unexpected serious related), as well as with leukocyte and platelet levels.
- Device integrity and device performance: Several parameters of device integrity and performance were monitored including evidence of leakage, cracking, clotting, and hemolysis.
In-hospital mortality data were compared between the SCD-treated patients and the case-matched historical control using the Fisher exact test. Age and SOFA scores were compared between the latter groups using the paired t-test. Multiple regression analysis was performed with mortality as the dependant variable and the following independent variables in the regression equation: age, SOFA score, average change in urine output over the first 7 days during or after treatment, and treatment modality (SCD vs. CVVH historical cohort). Urine output and change in urine output between the SCD and controls were compared using ANOVA analysis. All other variables reported are analyzed using descriptive statistics only.
The patients enrolled in the trial did not differ from the case-matched controls with respect to age and SOFA score (Table 2). The mortality for the case-matched controls was 77.78%, whereas the observed mortality in the SCD treatment group was 22.22% (p = 0.027). Multiple regression analysis identified treatment with SCD as the only significant variable affecting mortality among age, SOFA score, average change in urine output over the first 7 days during or after treatment, and treatment modality (SCD vs. CVVH historical cohort, β = −0.5728; p = 0.0222) (Table 3).
Mean total urine output in the nine subjects receiving SCD treatment increased from a baseline of approximately 500 ml/d to more than 1,500/d by day 7 of treatment (Figure 2). Although there was no statistically significant difference in the change in urine output over the treatment time between the SCD and control group, mean urine output increased over time in the SCD-treated group and diminished over time in the PICARD case-matched control group (Figure 3).
In the nine subjects analyzed on SCD treatment, no neutropenic events were reported. Mean white blood cell (WBC) counts remained normal throughout treatment, with a mild decline noted on initiation of therapy that was shown to rebound by day 7 (Figure 4).
In the nine subjects analyzed on SCD treatment, no bleeding events were reported. Average platelet counts remained in the functional range (>50,000) throughout treatment, with a mild decline noted on initiation of therapy that was shown to plateau by day 4 to an average platelet count of 75,000 (Figure 5).
No serious adverse events were reported (Table 4). No adverse events attributable to the SCD therapy per se were noted throughout the treatment period. Adverse events noted included hypercalcemia (8), hypocalcemia (1), hypophosphatemia (2), hypernatremia (1), and thrombocytopenia (1), with the last patient requiring a platelet transfusion on a decline in platelet count below 20,000.
Renal replacement therapy has traditionally focused on fluid and electrolyte regulation, hemodynamic stability, and toxin removal. The kidney, in addition to its excretory role, however, has a less recognized immunoregulatory function. The mammalian renal proximal tubule cells are antigen-presenting cells15 that have costimulatory molecules16 and process a variety of inflammatory cytokines.15,17 The roles of the renal tubule cells in glutathione metabolism,18 regulation of vitamin D, and production and catabolism of multiple cytokines19 are critical to immunoregulation to maintain tissue integrity and host defense under stress conditions.20
The cause of death in patients with AKI is usually the development of immune dysregulation leading to SIRS.6 In this setting, endothelial dysfunction is both caused by and promotes an inflammatory chemical cascade that exacerbates AKI. Recent human studies have demonstrated that the levels of the proinflammatory cytokines IL-6 and IL-8 in the plasma predict mortality in patients with AKI.8,9 Strategies that modulate the inflammatory response provide significant beneficial effects in experimental AKI.10
Inflammation is traditionally viewed as a physiological reaction to tissue injury, and leukocytes, as mediators of inflammation, contribute to the inflammatory response by the secretion of cytotoxic proteins, by phagocytotic activity, and by targeted attack of foreign antigens.21 Although leukocyte accumulation in tissues is critical for the initial response to injury, the persistent, unmodulated presence of leukocytes in tissues also contributes to a wide variety of diseases, including but not limited to atherosclerotic coronary and peripheral vascular disease, multiple sclerosis, vasculitis, and SIRS. Many therapeutic targets have been identified in an attempt to inhibit cytokine release and/or function or immune cells function, including G-protein-coupled receptors (GPCR),22 selectins,23,24 and integrins.25 These interventions have had limited clinical success thus far, despite very promising in vitro and in vivo laboratory data.21–25 Unlike the therapeutic targets described earlier in the text, the SCD functions to sequester and deactivate leukocytes before their accumulation in the peripheral tissues and organs.
In this nonrandomized, single-center pilot study, the safety and efficacy of treatment with a SCD were evaluated in patients with a conventional CVVH circuit. SCD treatment significantly reduced all-cause in-hospital mortality in ICU patients with AKI compared with case-matched controls from a national dataset (PICARD). This improved survival was demonstrated to be independent of age and SOFA score. Treatment with the SCD was well tolerated, without significant effects on hematological parameters, including WBC and platelet counts, and with an adverse event profile expected for a seriously ill population in the ICU with AKI. The blood flow patency of the SCD was comparable with single-cartridge CRRT modalities.
The mechanism of action of the SCD is currently being investigated. This SCD was developed with a proprietary design process using polysulfone hollow fibers. The SCD cartridge is a standard polysulfone hemofiltration cartridge (F40 in the first five patients and F50 in the last five patients; Fresenius AG, Bad Homburg, Germany). The proprietary circuit26 was designed to diminish the shear stress (SS) of blood along the hollow fibers, from greater than arterial SS (>30 dyn/cm2) in the first cartridge to levels below venular SS (<1 dyn/cm2) in the SCD cartridge, to provide an environment for leukocyte adherence.27 Recently published animal data suggest that selective binding of activated leukocytes by the outer aspect of the polysulfone hemodialyzer membrane occurs.13 The leukocytes are subsequently deactivated by the low-Cai environment,11 which is mediated by the presence of citrate in the extracapillary space of the SCD. This testable mechanism explains some of the clinical observations, including the absence of neutropenia, the increase in urine output, and the improved mortality described earlier in the text.
Oudemans-van Straaten et al.28 recently observed that citrate seems to improve patient and kidney survival and seems to confer a specific benefit in severe organ failure and sepsis. Although this finding has not been confirmed by other investigators and remains highly controversial,29 an argument can be made that the survival benefit found in this pilot study may comes from the use of citrate alone (at the least partially). Although we acknowledge that this is a possibility, this important question will be definitively addressed in a subsequent pivotal randomized controlled trial that will compare the effect of the SCD on mortality to standard CVVH with citrate.
This pilot study has several additional limitations, including its small size, nonrandomized design, and single-center population. Nevertheless, despite the small number of patients, the mortality difference between the SCD-treated patients and those treated with conventional CRRT in a case-matched control is statistically significant and is consistent with a prior retrospective analysis of a multicenter, randomized controlled trial.10 The change in urine output, although not statistically significant due to the wide confidence intervals in the historical control group, is suggestive; the mean urine output in the control group decreased over time, whereas that in the SCD group showed a sustained increase with duration of treatment.
The SCD represents a novel therapeutic approach to alter the acute inflammatory response seen in AKI requiring renal replacement therapy in the ICU. As such, this approach represents a paradigm shift in addressing the primary pathophysiology that leads to the poor outcomes of this disorder. Further evaluation of the safety and efficacy of the device is warranted, and multicenter investigations are beginning in the United States.
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