Eculizumab Reduces Pulmonary Hypertension through Inhibition of Hemolysis-Associated Nitric Oxide Consumption in Patients with Paroxysmal Nocturnal Hemoglobinuria

The depletion of nitric oxide (NO) by cell-free plasma hemoglobin and arginase during intravascular hemolysis has been implicated in the dysregulation of vasomotor tone and the enhancement of procoagulant and prothrombotic activities. Pulmonary hypertension (PHT), an emerging common complication of hereditary hemolytic anemias, has been mechanistically and epidemiologically linked to intravascular hemolysis and NO depletion. Paroxysmal nocturnal hemoglobinuria (PNH) is a disease characterized by chronic and brisk hemolysis, as well as elevated cell-free plasma hemoglobin. We have previously reported that approximately 50% of PNH patients have PHT as measured by doppler echocardiograpy. Further, the level of N-terminal pro-B-type natriuretic peptide (NT-proBNP) has been generally demonstrated to be a sensitive and specific measure of right-sided cardiac stress due to PHT and has more recently been shown to be a strong predictor of pulmonary hypertension (PHT) and mortality in patients with hemolytic anemias (defined as NT-proBNP ≥ 160 pg/ml). Eculizumab, a terminal complement inhibitor, has been demonstrated to significantly and rapidly reduce hemolysis, thereby providing beneficial effects on regulation of smooth muscle tone and thrombosis in patients with PNH. To evaluate the efficacy of eculizumab in the regulation of cell-free plasma hemoglobin levels, nitric oxide depletion, and subsequent cardiovascular morbidities in patients with PNH. Levels of hemoglobinemia, arginase and nitric oxide depletion were assessed in 73 evaluable eculizumab- and placebo-treated PNH in the phase III randomized, placebo-controlled trial (TRIUMPH). In addition, levels of N-terminal pro-B-type natriuretic peptide (NT-proBNP) were assessed as a measure of PHT, and systolic and diastolic systemic arterial pressures were examined in eculizumab- and placebo-treated patients. At baseline, levels of lactate dehydrogenase (LDH), cell-free plasma hemoglobin, arginase 1 and arginase 1 enzyme activity were highly elevated compared to normal values. Levels of hemolysis and NO consumption were shown to be much greater in PNH (more than 6- and 10-fold, respectively) than in patients with other hemolytic diseases. There were substantial correlations between cell-free plasma hemoglobin levels and both LDH (R = 0.5094) and plasma consumption of nitric oxide (NO) (R = 0.9529). Strong correlations between arginase 1 and both cell-free plasma hemoglobin (R = 0.9367) and arginase 1 enzyme activity (R = 0.9081) were also demonstrated. Following eculizumab therapy, measures of hemolysis were significantly reduced from baseline, including LDH (2200 ± 158 to 327 ± 68 U/L) and cell-free plasma hemoglobin (98.8 ± 23.24 to 15.2 ± 5.05 mg/dL), while levels in placebo-treated patients remained unchanged; a concomitant reduction in NO consumption was also observed (see Figure). In addition, at baseline, 46.6% (34/73) of PNH patients in the TRIUMPH study had levels of NT-proBNP ≥ 160 pg/ml, indicating PHT in these patients. Eculizumab-treated patients showed a 50% reduction in the incidence of PHT over the course of the 26-week treatment period from 52.5% to 26.3%, while PHT did not change with placebo (39.4% to 43.8%; P<0.001). Additionally, eculizumab significantly improved dyspnea (EORTC-QLQ-C30) compared to placebo (Effect size 0.69; P=0.0002). Similarly, eculizumab treatment was associated with a concomitant decrease in systemic arterial blood pressure as compared to patients receiving placebo. To summarize, in patients with hemolytic anemia, the depletion of NO is associated with the development of cardiovascular morbidities, including PHT. The current data demonstrate that intravascular hemolysis in untreated patients with PNH is associated with high levels of plasma ferrous oxyhemoglobin, which stoichiometrically catabolizes NO, and high levels of plasma arginase 1, which catabolizes arginine – all leading to significant depletion of NO. We show a high prevalence of PHT in untreated patients with PNH as indicated by levels of NT-proBNP. Further, the data demonstrate that in a placebo-controlled study, the anti-hemolytic effect of chronic eculizumab treatment significantly increases nitric oxide bioavailability and reduces pulmonary hypertension.

Figure.

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Effect of eculizumab on cell-free plasma hemoglobin and NO consumption

Figure.

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Effect of eculizumab on cell-free plasma hemoglobin and NO consumption

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