Current developments in antithrombotic therapy: the role of antithrombin agents

Hirudin, the specific thrombin inhibitor from medicinal leeches, is now produced by recombinant technology. r-Hirudin and to a lesser extent polyethyleneglycol-coupled hirudin (PEG-hirudin) have been used in many clinical trials. Hirudin has been shown to be more effective than low molecular weight heparin in the prevention of deep venous thrombosis after total hip replacement. In (acute) coronary syndromes hirudin as well as a synthetic hirudin analogue bivalirudin have been studied in large clinical trials. Higher doses of hirudin were associated with an increased risk of bleeding. A large-scale study with bivalirudin in patients with acute MI (the HERO-2 trial) has not shown a reduction in mortality but a 30% reduction of reinfarction within 96 h. Hirudin and argatroban have been successfully used in patients with heparin-induced thrombocytopenia type II. Several orally active thrombin inhibitors are being developed. The combined use of subcutaneous and oral administration of melagatran in patients with hip or knee replacement has led to promising results. It is likely that in the future thrombin inhibitors will replace other forms of anticoagulation in a number of indications.     

Delayed cutaneous hypersensitivity reactions to hirudin.

Hirudin is one of the new synthetic antithrombin agents, which is most commonly used in patients with type II heparin-induced thrombocytopenia and in patients with hypersensitivity reactions to unfractionated heparin as well as low-molecular-weight heparins. Hirudin is comparable to heparin as an antithrombotic agent and also has been studied as a primary treatment in patients who experienced acute myocardial infarctions. We describe a patient with a history of type II heparin-induced thrombocytopenia who was placed on intravenous hirudin therapy. After extravasation of the intravenous hirudin site, the patient developed a delayed hypersensitivity reaction that histologically showed an epithelioid granulomatous infiltrate. Although rare reports of hypersensitivity reactions to hirudin have been published, these reactions have not been well characterized and the histopathologic changes have not been described.     

Recombinant Hirudins

Hirudin is a specific thrombin inhibitor which acts independent of antithrombin III and is able to bind to fluid phase and clot bound thrombin. It has become available for clinical use by production with recombinant techniques just recently. Hirudin produces a relatively stable level of anticoagulation when compared with heparin. However, hirudin is not as effective as heparin in blocking thrombin generation, but is more effective than heparin in inhibiting thrombin activity. In patients with unstable angina, hirudin was more effective than heparin in preventing ischaemic events in the acute phase. This clinical benefit was no longer significant after 30 days in the Global Use of Strategies to open Occluded coronary arteries in acute coronary syndromes (GUSTO)-IIb study. Recombinant hirudins as adjunct to thrombolysis with alteplase do not significantly improve early patency, but decrease the rate of reocclusions. However, as an adjunct to streptokinase, hirudin improves the speed of reperfusion. The therapeutic range of hirudin in combination with thrombolysis is very narrow, as demonstrated by the increased rate of intracerebral bleedings in 3 clinical thrombolysis trials with higher doses of hirudin. Subsequent trials with markedly lower doses of hirudin did not show any increase in bleeding complications or any significant clinical benefit of hirudin compared to heparin as adjunct to thrombolysis. In a large trial comparing heparin and hirudin, patients with unstable angina scheduled for percutaneous transluminal coronary angioplasty (PTCA) had a similar event-free survival rate at 7 months, but hirudin reduced the incidence of early cardiac events. There is some evidence from large studies that hirudin is more effective than unfractionated and low molecular weight heparin in preventing deep vein thrombosis in patients who have had hip surgery. Furthermore, in a small trial in patients with established vein thrombosis, hirudin compared to heparin reduced the rate of new pulmonary embolism and the extension of venous thrombosis. Hirudin seems to be the treatment of choice for patients with heparin induced thrombocytopenia type II. It reduces the incidence of thromboembolic complications in this high risk patient group. To date, the considerably higher costs of recombinant hirudins compared to heparin does not justify their routine use, and it is appropriate to limit their use to individual cases where conventional thrombin inhibitors are contraindicated or much less effective.     

Frequency of anti-heparin-PF4 complex antibodies (HIT antibodies) in uremic patients on chronic intermittent hemodialysis

The aim of this study was to determine the frequency of heparin/platelet factor (PF) 4 complex antibodies in 305 uremic patients treated with chronic intermittent hemodialysis using unfractionated heparin or low-molecular-weight heparin for 3 months. Heparin-induced thrombocytopenia (HIT) antibodies were detected by ELISA in 7 patients (2.3%) who had no history of HIT. Two patients abruptly developed HIT associated with the formation of clots in the extracorporeal circuit after they were found to be carrying HIT antibodies. These patients were suspected to have a similar trigger: an increased dose of recombinant human erythropoietin (rHuEPO). The drug might induce parallel changes in hematocrit (Ht) levels and platelet counts until the onset of HIT. After the onset of HIT, a parallel phenomenon between Ht and platelet counts was not found because of the thrombocytopenia due to HIT. Although HIT onset has been reported during the initial phase of dialysis sessions, there have been few reports on the onset of HIT in uremic patients on dialysis with long-term heparin anticoagulation. In this study, HIT was observed in 2 uremic patients on chronic dialysis with intermittent use of heparin. In some patients on chronic intermittent dialysis carrying HIT antibodies, HIT may occur following rHuEPO treatment. The presence of HIT should be borne in mind in chronic dialysis patients carrying HIT antibodies for 3 months or more.     

Local hirudin application — an aid in preventing occlusion of an arteriovenous fistula in dialysis patients?

Summary In this pilot study we tested the effect of an ointment containing hirudin in a high concentration [20,000 ATU (antithrombin units)/100 g ointment] in ten patients on maintenance haemodialysis (six men, four women; mean age: 51 years; mean duration of dialysis treatment: 20.4 months). The duration of hirudin application was 4.5 months, the observation period in the total group was 43 months. During the total dialysis period of 204 months — before hirudin application — in these ten patients a shunt thrombectomy was necessary 18 times because of shunt occlusion. A shunt-thrombosis occurred in none of the ten patients during the period of application of the hirudincontaining ointment.Abkürzungen ATU antithrombin units     

Heparin-Induced Thrombocytopenia

Recombinant hirudins have a definite role in the treatment of patients with heparin-induced thrombocytopenia (HIT). The most important adverse effects are haemorrhages and the induction of antihirudin antibodies. Major haemorrhages were not significantly increased in patients with HIT compared with a historical control group, but prospective data comparing hirudin and heparinoids such as danaparoid are lacking. The definition of the optimal method for monitoring and the availability of an antidote for hirudin would probably increase safety with this drug. To date, haemofiltration using high-flux filter systems is the only way to remove an overdosage of hirudin from the circulation. In patients with renal impairment requiring hirudin treatment, it therefore seems safer to start with a low dose that is subsequently adjusted according to the activated partial prothromboplastin time or ecarin clotting time. Even in special circumstances, such as cardiopulmonary bypass or dialysis, hirudins can be applied successfully if care is taken to monitor their effects meticulously. There are many other indications in which hirudins have shown feasibility (e.g. acute coronary syndromes) but available data preclude definite conclusions.     

Argatroban in HIT type II and acute coronary syndrome

Prior to initiation of the ARG-911 and ARG-915 clinical trials, there was no optimal replacement for heparin anticoagulation in patients with heparin-induced thrombocytopenia (HIT) type II. These prospective, historical controlled studies were designed to determine the usefulness of argatroban, a direct thrombin inhibitor (DTI) that is not immunogenic and does not interact with heparin antibody, in answering this clinical need. Clinical outcomes (37-day period) for 568 argatroban-treated and 193 control patients demonstrated significantly reduced risks of the primary efficacy composite endpoint (all-cause death, all-cause amputation, new thrombosis) and the secondary endpoints (death due to thrombosis, new thrombosis) with argatroban. Argatroban patients also experienced a more rapid recovery of platelet count. Bleeding events were similar among both groups. It was concluded that argatroban anticoagulation, compared with historical controls, improves clinical outcomes without increasing bleeding risk in patients having HIT with or without thrombosis. Argatroban has since been approved in the US for both prophylaxis and treatment of thrombosis in patients with HIT. Argatroban has been used in percutaneous coronary interventions in patients with and without HIT, for peripheral vascular procedures in both large and small vessels in HIT patients, and as an adjunct to thrombolytic therapy for the treatment of AMI. Treatment success rates and the same or less bleeding was demonstrated with argatroban compared to heparin controls. These pilot studies suggest that argatroban will provide reliable anticoagulation during interventional procedures. A consistent safety profile of argatroban has been demonstrated in all studies to date. The main attributes of argatroban are its rapid onset of action, fast reversibility of its anticoagulant effect, inhibition of clot-bound thrombin, easily monitored by the aPTT and ACT and no dosage adjustment in renal-impaired individuals. These properties make argatroban a predictable and useful anticoagulant for HIT and non-HIT patients.     

Plasma calcium in haemodialysis patients: total calcium or ionized calcium? Should we systematically provide a value of total corrected calcium on our protocols?

Ionized calcium is the only physiologically active form of calcium. Because of the variation of albumin, pH and haemoconcentration observed during haemodialysis session in patients with chronic renal failure, measure of total calcium does not reflect the real variation of ionized calcium. However, many formulae to correct total calcium by albumin have been proposed but none of them has been validated in dialysis patients. At present time, computing progress permit laboratory to systematically provide a value of corrected total calcium on protocols but is it really indicated? Our results showed that any of those formulae allows obtaining a value of total calcium that possesses a significant critical difference in relation to total calcium. Thus, correction formulae must be abandoned in aid of ionized calcium in haemodialysis patients.     

The effect of maintenance dialysis on lymphocyte function. II. Peritoneal dialysis.

Lymphocyte function, as assessed by PHA-stimulated DNA synthesis, was studied in chronic renal failure patients maintained by peritoneal dialysis. In contrast to patients on haemodialysis, peritoneal dialysis patients had normal lymphocyte responsiveness pre-dialysis. There was no deterioration in the post-peritoneal dialysis plasma's ability to support control lymphocyte responsiveness as occurred after haemodialysis. Plasma dilution and reconstitution studies indicated that haemodialysis depleted the patients' plasma of nutrients and also added toxins, but peritoneal dialysis had neither effect.     

Dimethylarginines in chronic renal failure

BACKGROUND: Nitric oxide (NO) is a potent chemical mediator involved in many functions. In vivo production of NO is thought to be regulated by endogenous analogues of L-arginine: asymmetric dimethylarginine (ADMA). AIM: To examine the effect of renal function and dialysis on the serum concentrations of ADMA and symmetric dimethylarginine (SDMA). METHODS: Blood samples were obtained from nine healthy subjects, patients with renal failure before (n = 17) and after haemodialysis (n = 9), nine patients on chronic ambulatory peritoneal dialysis (CAPD), and 13 patients with chronic renal failure on conservative treatment. Serum samples were extracted using a solid phase cation exchange column and the extracts were analysed by high performance liquid chromatography (HPLC). RESULTS: Serum concentrations of ADMA in patients with renal failure (mean, 1.04 micromol/litre; SD, 0.17) were significantly higher than those of controls (mean, 0.61 micromol/litre; SD, 0.13). Haemodialysis significantly decreased the serum concentration by 36% (before dialysis: mean 0.99 (SD, 0.25) micromol/litre; after dialysis: mean, 0.63 (SD, 0.15) micromol/litre). Serum SDMA concentrations were higher in patients with renal failure, and haemodialysis decreased the concentration by 60%. There was no difference in serum arginine concentrations between the groups. CONCLUSION: Serum concentrations of ADMA are increased in renal failure and haemodialysis reduces the concentration.     

Heparin-induced thrombocytopenia with thrombotic sequelae: a review

Heparin-induced thrombocytopenia (HIT) occurs in 1-5% of patients treated with heparin. The pathogenesis involves the formation of antibodies to heparin-platelet factor 4 complexes, and the major clinical sequelae are thrombotic. Diagnosis is based on a combination of clinical and laboratory data. Treatment consists of stopping heparin, but, insofar as the risk of thrombosis remains high, treatment by alternative antithrombotic agents is indicated. Most clinical experience has been with danaparoid sodium and hirudin. The use of low-molecular-weight heparins (LMWH) in subsequent HIT episodes has been described, but is not recommended, especially with the introduction of new agents, such as oral thrombin inhibitors and pentasaccharides, which are hoped to reduce the use of heparins and the occurrence of HIT.     

Urea distribution in renal failure

An assessment of intracellular urea removed during haemodialysis has been made from urea extraction and plasma urea estimations. An apparent wide variation in the movement of intracellular urea in patients with acute renal failure from obstetric and traumatic causes and with chronic renal failure is reported.A method for the estimation of red cell water urea is presented. In two patients with chronic renal failure the red cell urea level was much higher than would have been expected from the plasma urea level before dialysis. In two obstetric patients there was no such discrepancy.The conclusion is drawn that research should be directed to variations of intracellular metabolism in renal failure before a more rational approach can be made to its management.     

Leachability of a new plasticizer tri-(2-ethylhexyl)-trimellitate from haemodialysis tubing

In two groups of patients on chronic haemodialysis treatment, the common PVC-DEHP blood tubing was replaced with tubing containing tri-(2-ethylhexyl)-trimellate (TOTM) as plasticizer. The aim of the present study was to measure the amount of TOTM and/or its metabolites (TAE s) in plasma, resulting from TOTM that might leach from the dialysis tubes. The arterial levels of TAE s at the start of the dialysis sessions were monitored by Selected Ion Monitoring (SIM) analysis once a week for a period of 42 days. A gradual decrease in TAE was observed during the first 21 days of the haemodialysis treatment with the PVC-TOTM tubes. After three weeks, the TAE levels remained constant until the end of the study. On day 120 of the haemodialysis treatment, the plasma TAE concentrations from the inflow and outflow tubes of the dialyzer were monitored during a single haemodialysis session at different times after starting dialysis. At the beginning of the dialysis session, the mean concentration of TAE was 55.81 +/- 14.98 ng/ml (mean +/- standard error), while at the end the levels were 73.34 +/- 17.05 ng/ml. There were no significant differences between venous and arterial sampling points in the trimellitic ester concentrations. Less TOTM is apparently leached from haemodialysis than DEHP. TOTM can be recommended as an alternative plasticizer to DEHP, but its possible toxicity in humans should be investigated before it can be used routinely.     

Neurohumoral responses to a single haemodialysis in chronic renal patients

The effect of volume reduction on vasoactive substances and their role in estimating dry weight in haemodialysis patients was studied. Plasma atrial natriuretic peptide (ANP), catecholamines, antidiuretic hormone, renin activity and serum aldosterone were measured in 12 patients before and after bicarbonate haemodialysis. Haemodynamical changes were registered and cardiac function and diameter of the inferior vena cava were measured by echocardiography before and after dialysis. Plasma concentration of ANP was significantly reduced by haemodialysis from 209 +/- 51 to 69 +/- 13 pg mL(-1) (n = 12, P < 0.05), whereas concentrations of the other hormones were unchanged. The change in the concentration of ANP did not have significant correlation with weight reduction. The concentration of ANP correlated positively with the diameter of the inferior vena cava (r = 0.70, P < 0.05) after dialysis, but not before dialysis. The concentration of ANP before or after haemodialysis or its change during dialysis did not correlate with any other biochemical parameter. The results show that plasma ANP level is decreased after volume reduction in patients with chronic renal failure, whereas other hormonal systems are unresponsive. However, plasma concentration of ANP seems to have no role in estimating dry weight in chronic haemodialysis patients.     

A pharmacokinetic interpretation of increasing concentrations of DEHP in haemodialysed patients

The degree of exposure to DEHP was assessed in 11 patients with chronic renal failure undergoing maintenance haemodialysis. The amount of DEHP leached from the dialyser during a 4-h dialysis session was estimated by monitoring the DEHP blood concentration using a HPLC method. When a patient undergoes a dialysis treatment, the concentration of di-2-ethylhexyl phthalate (DEHP) in venous blood is increased when the blood crosses through the dialysis apparatus. This increase may be explained either because DEHP is not extracted by the dialyser or because DEHP comes from the dialysis bath due to contact of blood against plasticized pipes. To explain the increasing concentration of DEHP during treatment of renal failure using plasticized tubing, we propose a pharmacokinetic compartmental model in order to fit raw data obtained from dialysed patients and to get the amount of DEHP which enters the body by AUC calculations. Results obtained after HPLC analysis show a high degree of interpatient variability in DEHP retained. This amount can reach a toxicity level because of repetitive dialysis treatments over prolonged periods of time. In the coming years, it seems necessary to reconsider the use of DEHP as a plasticizer in medical devices. Highly unacceptable amounts of DEHP leached during the dialysis session could be easily avoided by careful selection of haemodialysis tubing.     

Pharmacology and controlled release of hirudin for cardiovascular disorders

Hirudin is the most potent specific inhibitor of thrombin known. Hirudin was originally isolated from leeches, but it is now also available in synthetic form (recombinant hirudin). The inhibitor is currently undergoing clinical trials as a potential replacement for the extensively used thrombin inhibitor heparin. In this review, the biochemical and pharmacokinetic characteristics of hirudin (native and recombinant) and the efficacy of hirudin in treating and preventing cardiovascular disorders is discussed. The advantages of local controlled delivery of hirudin for treating cardiovascular disorders are then presented. Several implantable polymers applicable for controlled delivery system also are introduced. Finally, the feasibility of controlled delivery of r-hirudin for local therapy of cardiovascular disorders is addressed.     

Lepirudin

Lepirudin is a recombinant hirudin derived from transfected yeast cells. The hirudins are direct thrombin inhibitors which render the thrombin molecule incapable of promoting fibrin formation and catalysing other haemostatic reactions. In initial studies, parenteral lepirudin has shown promising efficacy as an antithrombotic agent. Lepirudin increased or maintained platelet counts at normal baseline values while maintaining adequate anticoagulation in patients with heparin-induced thrombocytopenia (HIT), and has not been associated with the development of immune-mediated thrombocytopenia. Preliminary studies in patients with deep vein thrombosis (DVT) suggest that lepirudin may be more effective than unfractionated heparin (UFH) at preventing pulmonary perfusion defects. In patients with unstable angina pectoris, preliminary data also showed lepirudin to be significantly more effective than UFH according to the combined incidence of cardiovascular mortality, new acute myocardial infarction (AMI) or refractory angina. However, additional studies involving larger patient numbers are necessary before firm conclusions can be made regarding the relative efficacy of lepirudin in these indications. Similarly, promising but limited data on the use of lepirudin during haemodialysis or heart surgery and in patients with disseminated intravascular coagulation (DIC) require further confirmation. Bleeding complications and the possible induction of allergic or anaphylactic reactions are the most serious adverse events associated with lepirudin therapy. Major bleeding complication rates appear to be similar with lepirudin and UFH monotherapy; however, lepirudin may be associated with an increased incidence of minor bleeding including bruising. Initial encouraging results showing an improvement in coronary artery patency with high-dose lepirudin versus UFH as an adjunct to thrombolytic therapy in patients with AMI were subsequently overshadowed by reports of a high incidence of major bleeding events including cerebral haemorrhage among lepirudin recipients. Moreover, at lower doses which did not produce an unacceptably high incidence of haemorrhagic complications, lepirudin appeared to have only a small efficacy advantage over UFH. CONCLUSIONS: Lepirudin has shown promising activity as an antithrombotic agent and may be a suitable substitute anticoagulant for heparin in patients with HIT. The narrow therapeutic window of lepirudin makes it difficult to assess the role of this agent when used as an adjunct to thrombolytic therapy in patients with AMI. However, initial data suggest that lepirudin may be a potentially useful agent in the management of patients with unstable angina, DVT or DIC and in preventing thrombus formation in extracorporeal circuits. Further studies should more fully elucidate the efficacy of lepirudin in these indications.     

Induction of tumour necrosis factor-alpha during haemodialysis. Influence of the membrane type.

Some of the secondary clinical effects induced by long-term haemodialysis in patients with end-stage renal failure have been related to an increased production of interleukin-1 (IL-1). We investigated the role of another cytokine which shares a number of biological properties with IL-1, tumour necrosis factor-alpha (TNF-alpha). In long-term haemodialysed patients, we found at the beginning of the dialysis increased plasma TNF-alpha levels and enhanced monocyte capacity to produce TNF-alpha spontaneously ex vivo. Non-haemodialysed uraemic patients also presented increased plasma TNF-alpha levels. During dialysis with cellulose acetate (CA) or polysulphone (PS) membranes, plasma TNF-alpha levels and the spontaneous and lipopolysaccharide-induced production of TNF-alpha by monocytes remained at predialysis levels. In contrast, when cuprophane membranes were used, there was a significant increase in plasma TNF-alpha levels and in both spontaneous (10-fold) and lipopolysaccharide-induced (seven-fold) ex vivo TNF-alpha production by monocytes. These results suggest that monocytes are stimulated during haemodialysis with the poorly biocompatible cuprophane membrane.     

Renal failure and thyroid function tests: the effects of commencing haemodialysis

Thyroid function tests are often noted to be abnormal in patients with chronic renal failure. We investigated thyroid and pituitary function in six patients with renal failure. In order to study the effects of improvement in the uraemic state on these tests, they were repeated after regular haemodialysis was commenced. Serum thyroxine and free thyroxine levels were at the lower limit of the normal range prior to dialysis and both showed a non-significant increase after regular haemodialysis was started. The TSH response to TRH (TRH test) decreased in five of the six patients after dialysis but this did not achieve statistical significance. The abnormalities in thyroid function tests often observed in clinically euthyroid patients with renal failure do not appear to change significantly after the institution of regular dialysis.     

Localisation of aluminium by histochemical and electron probe x-ray microanalytical techniques in bone tissue of cases of renal osteodystrophy.

Histochemical studies and electron probe x-ray microanalysis for aluminium have been performed on 16 samples of undecalcified bone from cases of renal osteodystrophy associated with a syndrome suggestive of dialysis encephalopathy (five cases), age and sex matched controls for these and a group of patients with chronic renal failure (six cases) who have never been on haemodialysis. Aluminium was detected only in the patients with a dialysis encephalopathy-like syndrome. This group had significant histological bone disease the features of which were broadly consistent with the so-called atypical renal osteomalacia which is thought to be due to a metal toxin. Aluminium was demonstrated at the interface between osteoid and mineralised tissue--that is, at the site of the calcification front, where it could interfere with the mineralisation process. In the group of patients who had never been subjected to haemodialysis there was also significant histological bone disease but no evidence of aluminium accumulation. In this group the bone disease was of a more typical pattern of osteomalacic changes coupled with those of hyperparathyroidism.