Thrombin generation and cell-dependent hypercoagulability in sickle cell disease

Essentials Sickle cell disease is increasingly being recognized as a chronic hypercoagulable state. Thrombin generation is elevated in the whole blood, but not the plasma of sickle cell patients. Whole blood thrombin generation inversely correlates to erythrocyte phosphatidylserine exposure. Acquired protein S deficiency is likely explained by binding of protein S to sickle red cells. Click to hear Dr Hillery discuss coagulation and vascular pathologies in mouse models of sickle cell disease.

SUMMARY

Introduction Sickle cell disease (SCD) is a hypercoagulable state with chronic activation of coagulation and an increased incidence of thromboembolic events. However, although plasma pre-thrombotic markers such as thrombin-anithrombin complexes and D-dimer are elevated, there is no consensus on whether global assays of thrombin generation in plasma are abnormal in patients with SCD. Based on our recent observation that normal red blood cells (RBCs) contribute to thrombin generation in whole blood, we hypothesized that the cellular components in blood (notably phosphatidylserine-expressing erythrocytes) contribute to enhanced thrombin generation in SCD. Methods Whole blood and plasma thrombin generation assays were performed on blood samples from 25 SCD patients in a non-crisis 'steady state' and 25 healthy race-matched controls. Results Whole blood thrombin generation was significantly elevated in SCD, whereas plasma thrombin generation was paradoxically reduced compared with controls. Surprisingly, whole blood and plasma thrombin generation were both negatively correlated with phosphatidylserine exposure on RBCs. Plasma thrombin generation in the presence of exogenous activated protein C or soluble thrombomodulin revealed deficiencies in the protein C/S anticoagulant pathway in SCD. These global changes were associated with significantly lower plasma protein S activity in SCD that correlated inversely with RBC phosphatidylserine exposure. Conclusion Increased RBC phosphatidylserine exposure in SCD is associated with acquired protein S deficiency. In addition, these data suggest a cellular contribution to thrombin generation in SCD (other than RBC phosphatidylserine exposure) that explains the elevated thrombin generation in whole blood.

Purified poloxamer 188 for treatment of acute vaso-occlusive crisis of sickle cell disease: A randomized controlled trial

CONTEXT

Sickle cell disease (SCD) can cause severe painful episodes that are often thought to be caused by vaso-occlusion. The current therapy for these uncomplicated painful episodes includes hydration, oxygen, and analgesics. Purified poloxamer 188 may increase tissue oxygenation and thereby reduce inflammation, pain, and the overall duration of such painful episodes in patients with SCD.

OBJECTIVE

To compare the duration of painful episodes in patients with SCD treated with purified poloxamer 188 to that of similar episodes experienced by patients who receive a placebo.

DESIGN AND SETTING

Randomized, double-blind, placebo-controlled, intention-to-treat trial conducted between March 1998 and October 1999 in 40 medical centers in the United States.

PARTICIPANTS

Two hundred fifty-five patients with SCD (aged 9-53 years) who had a painful episode sufficiently severe to require hospitalization and narcotic analgesics.

INTERVENTION

Patients were randomly assigned to receive an intravenous infusion of purified poloxamer 188, 100 mg/kg for 1 hour followed by 30 mg/kg per hour for 47 hours (n = 127), or a matching volume of saline placebo (n = 128).

MAIN OUTCOME MEASURE

Duration of the painful episode, from randomization to crisis resolution.

RESULTS

Mean (SD) duration of the painful episodes was 141 (42) hours in the placebo group compared with 133 (41) hours in those treated with purified poloxamer 188, a 9-hour reduction (P =.04). Subset analyses indicated an even more pronounced purified poloxamer 188 effect in children aged 15 years or younger (21 hours; P =.01) and in patients who were receiving hydroxyurea (16 hours; P =.02). Finally, the proportion of patients achieving crisis resolution was increased by purified poloxamer 188 (65/126 [52%] vs 45/123 [37%]; P =.02). Similar results were observed in children aged 15 years or younger (22/37 [60%] vs 10/36 [28%]; P =.009) and in patients who were also receiving hydroxyurea (12/26 [46%] vs 4/28 [14%]; P =.02).

CONCLUSIONS

A decrease in the duration of painful episodes and an increase in the proportion of patients who achieved resolution of the symptoms were observed when the purified poloxamer 188-treated patients were compared with the patients receiving placebo. However, the difference between these groups was significant but relatively small. In subgroup analysis, a more significant effect on both parameters was observed in children and in patients who were receiving concomitant hydroxyurea. It is important to confirm both of these observations in further prospective trials.

Bone marrow necrosis in sickle cell disease: a description of three cases and a review of the literature

Bone marrow necrosis (BMN) ranges from a localized to a widespread generalized process. Most often seen in patients with leukemia and other malignant conditions, generalized BMN has also been observed in patients with sickle cell disease (SCD), where it is almost certainly a consequence of blood vessel occlusion. Activation of the clotting system seems to play a role in this clinical setting. Systemic fat embolism and acute multi-organ failure syndrome can also complicate BMN in patients with SCD. We describe here 3 cases of BMN associated with SCD. Each patient exhibited an unusually severe vaso-occlusive crisis accompanied by persistent fever, a high level of serum lactate dehydrogenase, leukoerythroblastosis, and large numbers of nucleated red cells. Despite such suggestive clinical features, diagnosis of BMN still requires a bone marrow biopsy. Particularly in patients with SCD, the early institution of transfusion therapy can be life-saving. The ominous prognosis ascribed to generalized BMN seems to reflect the poor outcome of such underlying conditions as leukemia; however, the prognosis of generalized BMN is not so poor in association with SCD and other nonmalignant states.

Renal cell carcinoma

The overall incidence of renal cell carcinoma is rising, for reasons not fully explained by increased abdominal imaging. Risk factors associated with renal cell carcinoma include hypertension, smoking, increased body mass index, and diet. There is an inverse association of renal cell carcinoma risk with consumption of a variety of carotenes. In addition, increased red meat intake has been associated with increased risk. Partial nephrectomy may be as effective as radical nephrectomy as treatment for localized disease, and radiosurgery may be as effective as surgical resection in the management of brain metastases. Immunotherapy remains the mainstay for systemic treatment, with response rates between 5% and 20%. Survival in renal cell carcinoma is related to pathologic stage, nuclear grade, microscopic vascular invasion, DNA content, nuclear morphometry, and histologic pattern. In addition, patients with deletion (8p)/-8, +12, and +20 appear to have a worse prognosis.

Renal abnormalities in sickle cell disease

Sickle cell anemia and the related hemoglobinopathies are associated with a large spectrum of renal abnormalities. The patients have impaired urinary concentrating ability, defects in urinary acidification and potassium excretion, and supranormal proximal tubular function. The latter is manifest by increased secretion of creatinine and by reabsorption of phosphorus and beta(2)-microglobulin. Young patients with sickle cell disease (SCD) have supranormal renal hemodynamics with elevations in both effective renal plasma flow (ERPF) and glomerular filtration rate (GFR). These parameters decrease with age as well as following the administration of prostaglandin inhibitors. Proteinuria, a common finding in adults with sickle cell disease, may progress to the nephrotic syndrome. Proteinuria, hypertension, and increasing anemia predict end-stage renal disease (ESRD). While ESRD can be managed by dialysis and/or renal transplantation, there may be an increased rate of complications in renal transplant recipients with SCD. Hematuria is seen in individuals with all of the SCDs as well as with sickle cell trait. In most cases the etiology of the hematuria turns out to be benign. However, there does appear to be an increased association between SCD and renal medullary carcinoma. Therefore, those SCD patients who present with hematuria should initially undergo a thorough evaluation in order to exclude this aggressive neoplasm. Papillary necrosis may occur due to medullary ischemia and infarction. Erythropoietin levels are usually lower than expected for their degree of anemia and decrease further as renal function deteriorates. An abnormal balance of renal prostaglandins may be responsible for some of the changes in sickle cell nephropathy. Acute renal failure is a component of the acute multiorgan failure syndrome (MOFS). Finally, progression of sickle cell nephropathy to ESRD may be slowed by adequate control of hypertension and proteinuria. However, the prevention of the renal complications of SCD will require a cure for this genetic disorder.