Improving renal outcomes in chronic anemia: Learning from paroxysmal nocturnal hemoglobinuria
Authors: Antonio Guasch
Journal: American Journal of Hematology (2010)

More than 100 years have passed since the original description of William Gull (1866), further characterized by the German physician Paul Strübing in 1882, of patients with episodic hemolysis and dark discoloration of the urine in the absence of red cells on urinary microscopy. As an astute physician, Strübing noticed that the dark color in the urine was more apparent in the first voided morning specimen. He hypothesized that erythrocytes of patients with paroxysmal nocturnal hemoglobinuria were defective, and were more susceptible to effects of the accumulation of carbon dioxide and lactic acid that he thought occurred during sleep hours because of a general slowing of the circulation; this resulted in hemolysis and the typical dark red morning urine. A century later, we know that paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hematopoietic disorder that arises from a somatic mutation of the phosphotidylinositol glycan class A (PIGA) gene in hematopoietic stem cells. This gene is involved in the synthesis of certain anchoring proteins (glycosyl phosphatidylinositol-anchored proteins) on the plasma membrane. Consequently, the stem cell(s) and its progeny (erythrocytes, white blood cells, and platelets) lack two surface regulatory proteins, CD55 and CD59, involved in the activation of complement, making PNH erythrocytes more susceptible to both intravascular and extravascular hemolysis. The clinical presentation of PNH is characterized by Coombs negative hemolysis, venous thrombosis, often affecting uncommon sites like hepatic or mesenteric veins, and marrow failure. Some patients may experience constitutional symptoms (fatigue, malaise), but nocturnal hemoglobinuria occurs in only 25% of cases. Still, other patients may experience dysphagia, odinophagia, abdominal pain, and, in males, impotence. Although spontaneous remissions may occur in up to 15% of individuals, the clinical course is characterized by a chronic process with episodes of hemolysis, thromboembolic complication, and cytopenias. In one study of 80 patients that were given only supportive treatment, the mean survival was 10 years [1]. Bone marrow transplantation is the only curative treatment for PNH, but is associated with significant morbidity and mortality. More recently, eculizumab, a humanized monoclonal antibody against C5 that inhibits the terminal complement activation (membrane attack complex) and other proinflammatory mediators (C5a) has been approved by the Food and Drug Administration for use in PNH. Eculizumab is highly effective in reducing intravascular hemolysis, decreases the need for transfusions, and reduces the risk of thrombosis. Since it does not restore normal erythrocytes, it must be given on a long-term basis. The kidneys are thought to be commonly affected in PNH, although the reported series have focused on selected patients and/or postmortem findings. Clark et al. conducted a long-term study of 21 PNH patients followed at Vanderbilt University between 1957 and 1980 who underwent intravenous pyelography (IVP) and renal arteriograms, urinary concentration testing, and creatinine clearance determinations. Ten of the 17 tested patients had evidence of cortical infarcts by IVP, 10 of 11 tested patients were unable to concentrate the urine above a specific gravity greater than 1.016 (normal individuals should be able to achieve a specific gravity of >1.025 after water deprivation), and 13 of 19 patients tested had reduced creatinine clearances, six of them had values of less than 60 ml/min. Intermittent proteinuria was also seen in most patients [2]. More recently, magnetic resonance imaging (MRI) studies in PNH patients have revealed hemosiderosis in the renal cortex, with a reversal of the cortico-medullary intensity ration in T1 weighted images [3]. In the current issue of this journal, Hillmen et al describe the short and long-term effects of treatment with the complement inhibitor eculizumab on renal function in patients with paroxysmal nocturnal hemoglobinuria. The authors extracted renal outcomes data from three independent studies in PNH: a pilot, Phase 2 study [4], and two Phase 3 studies (TRIUMPH [5] and SHEPHERD [6] studies), along with a subsequent extension study [7]. In total, renal functional data was available in 195 adult patients followed for up to 18 months. The primary end point of these studies was the degree of hemolysis as assessed by LDH levels, with secondary end points of functional assessment of chronic illness and certain measurements of quality of life. None of the trials measured renal outcomes as an end point. However, as part of the trials, serum creatinine and a urinalysis were measured at baseline, 6, 12, and 18 months. This provided the investigators with additional data to quantify renal function. Glomerular filtration rate (GFR) was estimated from the modification of renal disease study (MDRD) equation (input values are serum creatinine, age, gender, and ethnicity) and renal damage was assessed from the urinalysis (presence of proteinuria, or imaging when available). Patients were grouped according to the level of eGFR, using a well-accepted classification of the stages of chronic kidney disease: Stages 1–5 [8]. The major findings of the study are that significant renal disease occurs in 65% of adult individuals with PNH enrolled in the study, with advanced kidney failure (Stages 3–5 chronic kidney disease (CKD)) being present in 20% of individuals. At 6 months, treatment with eculizumab was associated with improvement in the stage of CKD in 29% of patients vs. only 17% in the placebo group, and this effect persisted up to 18 months. This effect was more pronounced in patients with lesser degrees of renal insufficiency (Stages 1–2 CKD): at 18 months, 67% of patients with Stages 1–2 had improvement in the stage of CKD, vs. 23% of patients with Stages 3–5; there were no difference in response rates according to age. Importantly, 20 patients with baseline CKD Stage 2, normalized all renal functional parameters (proteinuria and eGFR) at 18 months, and 25% of patients with baseline proteinuria, had resolution of the proteinuria by 18 months. The study is important for two reasons. First, it allowed the authors to quantitate the level of renal insufficiency and/or degree of renal involvement in a clinical hematological trial that was not particularly geared towards renal outcomes. The authors wisely used simple tools that are available in most clinical settings (serum creatinine, urinalysis and some clinical demographic patient characteristics) to define the degree of damage in that particular population. Second, using a categorical approach to the degree of chronic kidney disease, may allow us to detect clinically important outcomes that may not be apparent when kidney function is measured as a continuous variable with a very large spread of the distribution of estimated values (i.e., eGFR ranging from 15 to more than 100 ml/min). For instance, a similar reduction in eGFR of 10 ml/min, may have different implications depending on the stage of CKD: it could represent a transition from Stage 1 to 2 (of limited clinical implications), but may signal a transition from Stage 2 to Stage 3 CKD. We know that Stage 3 CKD is associated with the development of renal osteodystrophy, secondary hyperparathyroidism, and other abnormalities of calcium and phosphorus metabolism that could impact subsequent cardiovascular outcomes. It is important to point out several limitations of the study that may affect the overall conclusions. First, the studies were not designed to assess renal function as their primary goal. Thus, it is unlikely that the original randomization strategies corrected for variables that could affect renal outcomes like degree of proteinuria, the use of medications that affect renal function (i.e. cyclosporine or non-steroidal anti-inflammatory drug (NSAID)s, used by 15% of patient at baseline) or even certain comorbidities like vascular disease, hypertension, diabetes, etc. Second, the classification of stages of CKD was designed as an epidemiological tool to determine prevalence of kidney disease and to guide clinical management of patients with CKD, but has not been validated as a measure of clinical outcomes. This is compounded by a lack of precision of the MDRD equation to estimate GFR especially above 60 ml/min. In one study [9], the precision of eGFR as compared to measured GFR (inulin clearance) at levels >60 ml/min/1.73 m2 was very poor (1SD = 21 ml/min/1.73 m2). In another study, including 2,095 patients, the MDRD equation misclassified about 30% of the population, with values ranging from 21 to 36% depending on the stage of CKD [10]. Part of the reason of the uncertainty of eGFR at high levels of true, measured GFR is that the formula was derived from a group of patients who had advanced kidney disease (Stages 3–5), and not from patients with near normal levels of GFR. In addition, intrinsic limitations of serum creatinine as a marker of GFR are carried over in the estimation equations. For instance, the estimated GFR computed for a 56-year-old white female with hypothetical serial serum creatinine values of 0.7, 0.8, and 0.9 mg/dl are 92, 79, and 69 ml/min/1.73 m2, respectively. In a 56-year-old African-American male, with serial serum creatinine values of 1.0 and 1.1 mg/dl, the respective eGFRs are 100 and 89 ml/min/1.73 m2. Thus, small changes in serum creatinine of dubious clinical significance, correspond to large changes in eGFR, and result in changes in CKD-stage patient assignment, but they may just reflect normal variability in day to day serum creatinine. For these reasons, the National Kidney Disease Education Program [11] has recommended that clinical laboratories do not report a numerical value when the estimated GFR is >60 ml/min/1.73 m2, but they need to indicate that the GFR exceeds 60 ml/min/1.73 m2. Another potential limitation of the pooled analysis, is that the presence of kidney damage was based on dipstick positive proteinuria and not using quantitative measurements. It is known that the colorimetric urinalysis method is very susceptible to interference by other colors, pigments, or even medications, providing false positives. Conversely, the dipstick urinalysis for protein may be negative even in the presence of clinical proteinuria depending on urine dilution factors (i.e., state of hydration of the patient). It is preferable that proteinuria be assessed by chemical determination of protein or albumin levels using random spot samples (i.e., spot protein/creatinine ratio or albumin/creatinine ratio). The latter are more sensitive, less prone to false positives and less affected by dilution factors. They are better markers to establish if renal damage has resolved. Notwithstanding these limitations, the study provides important information on the extent of kidney involvement in PNH and supports the hypothesis that preventing hemolysis in patients with hemolytic anemia preserves renal function and may be associated with improved renal outcomes. This model, with perhaps some modifications, will probably be used in the future in other hematological studies where renal outcomes are an important consideration. As such, the study is an important step in the right direction.