On the mechanism of action of recombinant activated factor VII administered to patients with severe thrombocytopenia and life-threatening haemorrhage: focus on prothrombin activation

We report the ex vivo effect of recombinant activated factor VII (rFVIIa) on prothrombin activation after whole blood clotting. Two patients with severe thrombocytopenia and life-threatening haemorrhage were successfully managed using a single dose of rFVIIa (90 microg/kg). Western blotting using antiprothrombin antibody showed that rFVIIa did not induce thrombin generation in citrated platelet-poor plasma. Patient sera showed significantly impaired prothrombin activation before and after rFVIIa administration. rFVIIa administration shortened the prothrombin time, activated partial thromboplastin time and Ivy bleeding time, and normalized the clot retraction. These data indicate that rFVIIa accelerated thrombin generation without significant increase of generated thrombin.

Tissue factor pathway inhibitor reduces experimental lung metastasis of B16 melanoma

The importance of tissue factor (TF) in tumor biology has been highlighted by studies suggesting its involvement in cell signaling, metastasis and angiogenesis. Since many animal studies have shown that anticoagulant therapy can reduce experimental metastasis, we studied whether the natural inhibitor of TF-mediated blood coagulation, Tissue Factor Pathway Inhibitor (TFPI), might be similarly effective. Using a murine experimental model, we found that intravenous injection of recombinant murine TFPI immediately before introduction of tumor cells reduced metastasis by 83% (P < 0.001). B16 murine melanoma cells stably transfected with a TFPI expression vector exhibited reduced lung seeding following intravenous injection by 81% (P < 0.001) compared with controls. No difference in primary tumor growth was observed between TFPI+ and control cells. Mice receiving intravenous somatic gene transfer of sense TFPI expression vector developed 78% fewer lung nodules than controls (P < 0.05). We conclude that TFPI has significant anti-metastatic activity in this experimental model.

Heparin-induced thrombocytopenia: pathophysiology and management

Immune-mediated heparin-induced thrombocytopenia (HIT) is a well-defined syndrome. Clinical criteria (thrombocytopenia, resistance to heparin anticoagulation, or thromboses during heparin therapy) are defined, and serologic diagnostic tests are available. Earlier recognition of HIT syndrome has allowed for significant advances in therapy, leading to marked reductions in morbidity and mortality from HIT syndrome. This review addresses the epidemiology, pathobiology, and management of HIT syndrome

Pathogenesis of heparin-induced thrombocytopenia and thrombosis

Heparin-induced thrombocytopenia and thrombosis (HIT/T) is a common immune-mediated disorder often manifested by life-threatening thrombosis. There is increasing evidence to indicate that HIT/T is caused by antibodies to complexes between platelet factor 4 (PF4) and heparin that activate platelets, monocytes and vascular endothelium leading to the generation of thrombin. Advances in defining the immunological basis of HIT/T have yielded insights into the antigenic determinants, antibody-antigen interactions and effector responses that contribute to its pathogenesis. However, these studies also reveal that anti-PF4/heparin antibodies develop far more commonly than clinically overt disease, raising questions as to serologic and other factors that predispose to clinical thrombocytopenia and thrombosis. An improved understanding of the natural history of HIT/T and the introduction of alternative anticoagulants have led to a somewhat improved clinical outcome. The recent development of a monoclonal anti-heparin/PF4 antibody and the establishment of a murine model of HIT/T may help to better define the pathogenesis and management of this common autoimmune disorder.

Concentrates containing factor IX could improve haemostasis under conditions of thrombocytopenia: studies in an in vitro model

BACKGROUND AND OBJECTIVES

We explored the effect on haemostasis of different factor IX (FIX) concentrates under thrombocytopenic conditions using an in vitro perfusion technique.

MATERIALS AND METHODS

A moderate experimental thrombocytopenia (25 000-30 000 platelets/microl) was induced by means of a filtration procedure in blood anticoagulated with low-molecular-weight heparin. The effects of three different FIX concentrates - a prothrombin complex concentrate (PCC), an intermediate-purity concentrate (FIX/X), and a high-purity concentrate (HPFIX) - on platelet deposition and fibrin formation on subendothelium were assessed at two different shear rates (600/second and 1200/second). Activation of the coagulation system was monitored through assessment of prothrombin activation fragment 1 + 2 (F1 + 2).

RESULTS

Fibrin deposition increased after addition of FIX concentrates, but only showed a significant increase in experiments performed after incubation of PCC at the lower shear rate (600/second) (64.25 +/- 9.61% vs. control 31.22 +/- 8.02%; P < 0.05). Addition of FIX concentrates caused a small increase in the percentage of platelet deposition and area of those aggregates. These differences reached levels of statistical significance in the presence of FIX/X and HPFIX in experiments performed at a shear rate of 600/second. F1 + 2 baseline values in anticoagulated thrombocytopenic blood were 1.15 +/- 0.13 nm and reached levels of 2.49 +/- 0.24 and 3.60 +/- 0.33 nm at shear rates of 600 and 1200/second, respectively. Increments in F1 + 2 observed after addition of different FIX concentrates always remained in the previous ranges.

CONCLUSIONS

Data from the present study provide experimental support favouring the concept that FIX concentrates containing other activated factors could improve haemostasis under conditions of moderate thrombocytopenia.

Controversies in the treatment of pediatric immune thrombocytopenias

Although pediatric immune thrombocytopenia (ITP) is common, there is no consensus on the optimal approach to therapy. Childhood ITP differs in its clinical course and trigger from adult immune thrombocytopenic purpura. There appear to be two clinical phenotypes among children with ITP: children with polyclonal autoantibody production triggered by an external exposure such as infection, and children with coexistent immune deficiency or dysregulation on a congenital or acquired basis. Treatment implications exist for each group. The first may be best managed by observation and conservative measures; for the latter, treatment could include normal intravenous gammaglobulin concentrate, anti-blood group D antigen (anti-Rh factor), or steroids. Early recognition of the thrombocytopenic child with immune dysfunction versus the normal child with a polyclonal response to a particular environmental antigen will result in better prognosis for both by selecting the appropriate therapy and minimizing long-term side effects.

HIV-related thrombocytopenia

Chronic thrombocytopenia is a common hematologic disorder in patients infected with the human immunodeficiency virus (HIV). Although often asymptomatic, the thrombocytopenia may be associated with a variety of bleeding abnormalities. The underlying pathophysiology includes accelerated peripheral platelet destruction and decreased ('ineffective') production of platelets from the infected megakaryocytes. HIV-related thrombocytopenia (HIV-TP) responds to antiretroviral therapy. Most studies have evaluated the use of AZT (zidovudine) and have shown increased platelet production. Combination therapy (HAART) also resulted in sustained platelet increases. When antiretroviral agents fail to improve the platelet count or cannot be used, other therapies, similar to those used in 'classic' immune thrombocytopenia (ITP), can be employed, including steroids, intravenous immunoglobulin (i.v.intravenous anti-D or splenectomy. Anti-D treatment offers advantages for HIV-TP because the duration of effect appears to be significantly longer than the response duration after i.v. therapy (initial results of our open-arm study were confirmed by our randomized trial). Of note, follow-up of heavily treated patients showed no acceleration of CD4 decline and no change in plasma viral load measurements. Splenectomy has been used to treat HIV-positive patients with refractory thrombocytopenia. Although it is effective therapy, there are concerns about infections and selection of appropriate candidates. Other treatment modalities, such as interferon, vincristine, danazol, low-dose splenic irradiation and staphylococcal protein A immunoadsorption have shown limited success in HIV-TP. Alternatively, thrombocytopenia in HIV-infected patients may be treated with pharmacological hyperstimulation of megakaryocytopoiesis (administration of PEG-rHuMGDF or TPO). Latest evidence indicates that the chemokine receptor CXCR4 (coreceptor for the cellular entry of lymphotropic HIV strains) is expressed on megakaryocytes; as a result, the development of chemokine receptor antagonists may modify the course of the disease.

Treatment of heparin-induced thrombocytopenia

OBJECTIVE

To describe heparin-induced thrombocytopenia (HIT or HIT-2), an immune-mediated adverse reaction to heparin or low-molecular-weight heparin. Available treatment options and considerations in developing a therapy approach are discussed.

DATA SOURCES

A search of the National Library of Medicine (1992-June 2001) was done to identify pertinent literature. Additional references were reviewed from selected articles.

STUDY SELECTION

Articles related to laboratory recognition and treatment options of HIT, including the use of agents in selected clinical conditions, were reviewed and included.

CONCLUSIONS

HIT is a rare but potentially severe adverse reaction to heparin that was, until recently, poorly understood and had limited treatment options. Recent advances describing the recognition and clinical manifestations of immune-mediated HIT, including recently available antithrombotic treatment options, have dramatically changed outcomes for patients having this syndrome.

Effective hemostasis with rFVIIa treatment in two patients with severe thrombocytopenia and life-threatening hemorrhage

We report two patients with severe thrombocytopenia and life-threatening bleeding that were successfully managed with recombinant activated factor VII (rFVIIa). The first was a 75-year-old male with Waldenström's macroglobulinemia. During a therapeutic course with fludarabine, he developed severe autoimmune thrombocytopenia resistant to conventional treatment, followed by persistent uncontrollable nasal bleeding. Platelet transfusions failed to increase the platelet count and control the hemorrhage. When hemoglobin levels fell below 8.5 g/dL and the patient's clinical condition got much worse, a single dose of 4.8 mg rFVIIa (90 microg/kg) was given as an i.v. bolus. Ten minutes after the rFVIIa injection, nasal bleeding stopped, the patient's clinical condition progressively improved, and splenectomy could be carried out uneventfully 2 days later. The second patient, a 52-year-old female, was under treatment for pre-B lymphoblastic leukemia. She developed severe thrombocytopenia, secondary to chemotherapy, complicated by massive gastrointestinal bleeding. Despite intensive treatment with platelet transfusions, hemorrhage continued and her condition deteriorated rapidly. She was then given an i.v. bolus injection of 4.8 mg rFVIIa, which resulted in cessation of hemorrhage and dramatic improvement of her clinical status. No adverse effects from the treatment with rFVIIa were observed. In conclusion, rFVIIa appears to be an attractive alternative for controlling hemorrhage in patients with severe thrombocytopenia, especially when platelet transfusions are unavailable or ineffective.

Comparison of platelet count recovery with use of vincristine and prednisone or prednisone alone for treatment for severe immune-mediated thrombocytopenia in dogs

OBJECTIVE

To evaluate the effect of prednisone alone, compared with a combination of prednisone and vincristine, on platelet counts in bleeding dogs with severe primary immune-mediated thrombocytopenia (IMT).

DESIGN

Prospective case study.

ANIMALS

24 dogs with severe primary IMT PROCEDURE: All dogs received immunosuppressive doses of prednisone (1.5 to 2 mg/kg [0.7 to 0.9 mg/lb] of body weight, PO, q 12 h). In addition, 12 dogs received a single dose of vincristine (0.02 mg/kg [0.01 mg/lb], IV). Platelet count, transfusion requirement, and outcome were monitored. A response was defined as an increase in platelet count to > or = 40,000/microl. Dogs in the prednisone group that failed to respond received 1 dose of vincristine on day 7.

RESULTS

Dogs that received prednisone and vincristine had a significantly faster increase in platelet count to > or = 40,000 platelets/microl than dogs that received prednisone alone (mean +/- SD, 4.9 +/- 1.1 vs 6.8 +/- 4.5 days, respectively). A similarly rapid response was observed in dogs that received vincristine on day 7 after treatment with prednisone alone failed. Furthermore, duration of hospitalization was reduced in the vincristine group, compared with the prednisone group (5.4 +/- 0.3 vs 7.3 +/- 0.5 days, respectively). No adverse effects attributable to vincristine were observed in any dog.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of combined vincristine and prednisone is associated with more rapid increase in platelet numbers and shortened duration of hospitalization in dogs with IMT, compared with use of prednisone alone. Early use of vincristine seems warranted in dogs with severe primary IMT.

Argatroban

Antithrombotic and antiplatelet therapies are the cornerstones of management of cardiovascular disorders today. Due to the safety and efficacy limitations of the classic antithrombotic, unfractionated heparin, considerable effort has been directed at developing novel anticoagulants. Direct thrombin inhibitors as a class of drugs offer inhibition of clot-bound as well as fluid-phase thrombin and a more predictable anticoagulant response. Specifically, argatroban, a synthetic small molecule direct thrombin inhibitor, selectively inhibits the catalytic site of thrombin in a reversible manner. Overall, argatroban's short half-life, ease of monitoring with an activated partial thromboplastin time, and safety in renal failure patients make this drug the preferable mode therapy for prevention of thrombosis in heparin-induced thrombocytopenia. The role of adjunctive argatroban therapy in acute coronary syndromes and during percutaneous coronary intervention is currently being studied.

Case report: Kasabach-Merritt syndrome: a review of the therapeutic options and a case report of successful treatment with radiotherapy and interferon alpha

We describe the successful treatment of a neonate with Kasabach-Merritt syndrome who received local irradiation and interferon alpha therapy after failure of corticosteroid treatment. A male neonate, born after an uneventful pregnancy, had a huge haemangioma involving the upper right cervical region as well as severe thrombocytopenia. He was treated with corticosteroids, interferon alpha and radiotherapy. Prednisolone therapy (5 mg kg(-1) day(-1)) was started at 41 days of age. No therapeutic effect was observed after 2 weeks. At this time the tumour size had increased dramatically, platelet counts had decreased progressively and coagulation abnormalities had developed. Because corticosteroid therapy had been ineffective and the child was in a life-threatening condition, irradiation was delivered up to a total dose of 9.5 Gy in five fractions. Simultaneously, prednisolone therapy was slowly decreased and interferon alpha therapy (3 million U m(-2) day(-1)) was started and continued for 6 weeks. After irradiation with 9.5 Gy and beginning interferon alpha therapy, the tumour decreased in size and coagulation parameters normalized within 4 weeks. 6 months later, platelet counts and coagulation parameters were still normal. The tumour had further decreased in size. No acute severe side effects were observed. Radiation therapy combined with interferon alpha treatment is an alternative treatment modality when high dose corticoid steroid therapy has been ineffective in patients with Kasabach-Merritt syndrome, despite the risks of growth delay and secondary malignancy. In children showing no response to corticosteroids, radiotherapy and/or interferon alpha should be considered in Kasabach-Merritt syndrome.

Rapid recovery of platelet count following administration of liposome-encapsulated clodronate in a mouse model of immune thrombocytopenia

Immune thrombocytopenic purpura (ITP) is a haematological disorder characterized by increased platelet consumption. The destruction of platelets is mediated by the reticulo-endothelial system (RES), particularly by splenic and hepatic macrophages. Previously, we demonstrated in a mouse model of thrombocytopenia that the depletion of these cells by liposome-encapsulated clodronate (LIP-CLOD) induces the recovery of the platelet count. We now report that LIP-CLOD is capable of reversing the thrombocytopenia with minimal effects on both, functional RES integrity and platelet functionality. Our data indicate that thrombocytopenic mice treated with low doses of LIP-CLOD/body weight increase the platelet count to haemostatically safe values within 18 h of treatment. The predictable bleeding time was significantly decreased in these mice, suggesting that the circulating platelets have enhanced haemostatic capacity. Platelet functionality measured through the ADP-induced fibrinogen-binding assay showed normal platelet activation after treatment. Regarding immunological competence, mice treated with LIP-CLOD showed similar antibody titres against sheep red blood cells. However, antibody-dependent cell-mediated cytotoxicity carried out by splenocytes was reduced. All these data demonstrate that LIP-CLOD deserves consideration as a potential therapeutic approach in thrombocytopenic states in which the rapid increase of platelet count is the primary goal.

Targeted approaches for the treatment of thrombocytopenia

Molecular targeting of novel therapies has the promise of inducing very specific biologic effects. In clinical hematology and oncology, molecular targeting of specific cell surface receptors with erythropoietin, G-CSF, or GM-CSF has been used to stimulate erythropoiesis and granulopoiesis, respectively. Although anemia and neutropenia can be corrected with targeted therapy, safe and effective treatment of thrombocytopenia remains an unmet medical need. While platelet transfusions still represent the standard of care for severe thrombocytopenia, there are several negative aspects associated with their use, including issues of availability, transient effectiveness, costs, adverse effects, negative perception by patients, and infection considerations. Despite extensive investigations of cytokines which act primarily on primitive levels of hematopoiesis, pharmacologic interventions to date have failed to elevate platelet counts in a reliable, highly effective, and well-tolerated fashion. Recombinant human interleukin-11 has been approved by the U.S. Food and Drug Administration for the treatment of chemotherapy-induced thrombocytopenia but has only modest efficacy and significant side effects. The identification of c-Mpl as the thrombopoietin receptor has opened new avenues for the therapeutic manipulation of thrombopoiesis. The development of specific c-Mpl ligands, including recombinant human thrombopoietin (rHuTPO), has allowed investigators to target this receptor for the treatment of chemotherapy-induced thrombocytopenia and other medical disorders characterized by extremely low platelet counts. As a potent stimulator of platelet production, rHuTPO has the potential to reduce the need for platelet transfusions and their attendant complications.

Promegapoietin-1a, an engineered chimeric IL-3 and Mpl-L receptor agonist, stimulates hematopoietic recovery in conventional and abbreviated schedules following radiation-induced myelosuppression in nonhuman primates

Promegapoietin-1a (PMP-1a), a multifunctional agonist for the human interleukin 3 and Mpl receptors, was evaluated for its ability to stimulate hematopoietic reconstitution in nonhuman primates following severe radiation-induced myelosuppression. Animals were total body x-irradiated (250 kVp) to 600 cGy total midline tissue dose. PMP-1a was administered s.c. in several protocols: A) daily (50 microg/kg) for 18 days; B) nine doses (5 microg/kg) every other day for 3 weeks; C) a single high dose (100 microg/kg) at 20 hours, or D) a single high dose (100 microg/kg) at 1 hour following TBI. The irradiation controls received 0.1% autologous serum for 18 consecutive days. Hematopoietic recovery was assessed by bone marrow clonogenic activity, peripheral blood cell nadirs, duration of cytopenias, time to recovery to cellular thresholds, and requirements for clinical support. PMP-1a, irrespective of administration schedule, significantly improved all platelet-related parameters: thrombocytopenia was eliminated, the severity of platelet nadirs was significantly improved, and recovery of platelet counts to > or =20,000/miccrol was significantly reduced in all PMP-1a-treated cohorts. As a consequence, all PMP-1a-treated cohorts were transfusion-independent. Neutrophil regeneration was augmented in all treatment schedules. Additionally, all PMP-1a-treated cohorts showed an improvement in red blood cell nadir and recovery. PMP-1a in conventional or abbreviated schedules induced significant thrombopoietic regeneration relative to the control cohort, whereas significant improvement in neutrophil recovery was schedule-dependent in radiation-myelosuppressed nonhuman primates.

Bivalirudin: a direct thrombin inhibitor

BACKGROUND

Studies of the anticoagulant effects of hirudin, which is derived from the saliva of the leech Hirudo medicinalis, led to the development of compounds that can directly inhibit thrombin activity without the need for additional cofactors. One of these is the direct thrombin inhibitor bivalirudin, which has recently been approved by the US Food and Drug Administration for use as an anticoagulant in patients with unstable angina undergoing percutaneous transluminal coronary angioplasty.

OBJECTIVE

This is a review of the pharmacologic properties, efficacy, tolerability, and potential cost-effectiveness of bivalirudin in the treatment of ischemic coronary syndromes.

METHODS

Articles were identified by searches of MEDLINE (1966-September 2001), International Pharmaceutical Abstracts (1970-September 2001), and the Iowa Drug Information Service (1966-September 2001) using the terms bivalirudin and Hirulog. The reference lists of retrieved articles were also reviewed for relevant articles.

RESULTS

Bivalirudin is a synthetic polypeptide that directly inhibits thrombin by binding simultaneously to its active catalytic site and its substrate recognition site. After intravenous administration, peak plasma concentrations occur in 2 minutes. In patients given a 1.0-mg/kg bolus followed by a 2.5-mg/kg per hour infusion, a median activated clotting time of 346 seconds is achieved with little interpatient or intrapatient variability. Clearance of bivalirudin occurs through a combination of renal elimination and proteolytic cleavage, and doses may need to be decreased in the presence of renal dysfunction. In patients undergoing percutaneous coronary interventions, bivalirudin has been associated with equivalent efficacy but lower bleeding rates (P < 0.001) than unfractionated heparin (UFH). Data from the Hirulog Early Reperfusion/Occlusion-2 study suggest no reduction in mortality with bivalirudin compared with heparin when either is added to aspirin and streptokinase in patients with acute myocardial infarction, despite a lower reinfarction rate (P < 0.001). Experience with bivalirudin in patients with unstable angina and heparin-induced thrombocytopenia (HIT), as well as in patients receiving glycoprotein IIb/IIIla inhibitors, is limited. The differences in bleeding rates between bivalirudin and heparin in published clinical trials probably reflect differences in levels of anticoagulation achieved in comparator groups.

CONCLUSIONS

Given its high cost, bivalirudin should be reserved for use as an alternative to UFH, primarily in patients with HIT, until clinical trials have more clearly demonstrated its benefits in terms of efficacy or safety.