Abstract

We have completed a clinical trial evaluating the safety of N-acetylcysteine (NAC) infusion in adult patients with sickle cell disease (SCD) at disease baseline (NCT01800526). This trial was inspired by the hypothesis that von Willebrand factor (VWF) has an important role in the pathophysiology of vaso-occlusion and hemolysis in SCD and that NAC can reduce signs and symptoms of the disease by reducing the activity of VWF. A secondary hypothesis was that NAC could also act by reversing or preventing some of the oxidative changes associated with SCD.

VWF is stored in the Weibel-Palade bodies of endothelial cells and α-granules of platelets and released from endothelial cells upon activation and mediates the attachment of platelets to the vessel wall, and secondarily, of erythrocytes and leukocytes. We previously showed that VWF levels and adhesive activity in SCD plasma were elevated and that total active VWF correlated with the extent of hemolysis (determined by plasma LDH levels), suggesting that VWF participates in the pathophysiology of SCD (Chen et al, Blood. 2011; 117:3680‐3). In another study, we showed that NAC was able to reduce VWF multimer size and platelet-binding activity ex vivo in human plasma and in vivo in mice (Chen et al, JCI, 2011, 121:522).

In this clinical trial, we enrolled 5 subjects at disease baseline, 4 with SCD and 1 with sickle trait. All subjects received two high-dose NAC infusions: initially 150 mg/kg and 4 weeks later 300 mg/kg. NAC infusion was administered in subjects 1 and 2 as a bolus infusion of half of the dose in the 1sthour and the remaining half in 7 hr. Subjects 3-5 received NAC as constant infusion for 8 hr. Blood was collected immediately before infusion, during infusion (at 1 and 4 hr), immediately after infusion (8 hr), and 24 and 72 hr after infusion. We examined blood for red blood cell (RBC) fragments, dense cells, platelet activation status, platelet-monocyte complexes and the concentrations of reduced and oxidized glutathione. We also examined plasma for VWF concentration and multimer distribution, ADAMTS13 antigen and activity, and evaluated plasma redox status by measuring the concentrations of NAC and cysteine in their reduced, oxidized, and mixed disulfide forms, as well as the concentration of protein-bound cysteine. The small molecule thiols were measured by mass spectrometry.

Results:1) All subjects tolerated NAC infusion well, except that subjects 1 and 2 experienced pruritus during the bolus infusion at the 300 mg/kg dose. We therefore changed the protocol to deliver the drug over 8 hr by continuous infusion for subjects 3 through 5. 2) During NAC infusion, the percentage of dense cells and concentration of RBC fragments decreased rapidly (average decrease: low dose 44%; high dose 31%) and the change persisted up to 72 hr. 3) Platelet activation also decreased, as determined by reduced P-selectin expression (low dose, 7.7 ± 0.7% to 5.0 ± 1.9%; high dose, 17.8 ± 0.07% to 7.3% ± 0.4), PAC-1 binding (evaluates activated αΙΙbβ3), and formation of platelet-monocyte complexes (monocytes with attached platelets: low dose, 52 ± 16% to 27 ± 12%; high dose, 77 ± 12% to 50 ± 17%). 4) The highest molecular weight VWF multimers transiently disappeared during the NAC infusion and recovered by 24 hr. There were no appreciable changes in VWF antigen or ADAMTS13 antigen or activity with NAC infusion. 5) With NAC infusion, the concentrations of Cys and NAC increased in plasma, both total and reduced forms, with a concomitant decrease in protein-bound Cys in the one patient studied (free thiol Cys increased 23 fold with the high dose infusion at 8 hr; protein-bound Cys decreased 87%). Similarly, the concentrations of reduced and oxidized glutathione increased in whole blood.

Summary: NAC infusion in SCD patients at disease baseline appears safe and is well tolerated. NAC infusion decreases RBC fragments, dense cells, and the size of large VWF multimers, and inhibits platelet activation and formation of platelet-monocyte complexes. In addition, NAC infusion increases the total and free-thiol concentrations of cysteine and glutathione in blood, relieving oxidative stress in SCD. Our data suggest that NAC may be of benefit for SCD patients during vaso-occlusive crisis through a variety of mechanisms, which include improvements in red cell parameters, decreased platelet adhesion, reduced adhesive activity of VWF, and protection against oxidative damage.

Disclosures

No relevant conflicts of interest to declare.

Author notes

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