Higher concentrations of heparin and hirudin are required to inhibit thrombin generation in tissue factor-activated cord plasma than in adult plasma

Differential Contributions of Intrinsic and Extrinsic Pathways to Thrombin Generation in Adult, Maternal and Cord Plasma Samples

Background

Thrombin generation (TG) is a pivotal process in achieving hemostasis. Coagulation profiles during pregnancy and early neonatal period are different from that of normal (non-pregnant) adults. In this ex vivo study, the differences in TG in maternal and cord plasma relative to normal adult plasma were studied.

Methods

Twenty consented pregnant women and ten consented healthy adults were included in the study. Maternal and cord blood samples were collected at the time of delivery. Platelet-poor plasma was isolated for the measurement of TG. In some samples, anti-FIXa aptamer, RB006, or a TFPI inhibitor, BAX499 were added to elucidate the contribution of intrinsic and extrinsic pathway to TG. Additionally, procoagulant and inhibitor levels were measured in maternal and cord plasma, and these values were used to mathematically simulate TG.

Results

Peak TG was increased in maternal plasma (393.6±57.9 nM) compared to adult and cord samples (323.2±38.9 nM and 209.9±29.5 nM, respectively). Inhibitory effects of RB006 on TG were less robust in maternal or cord plasma (52% vs. 12% respectively) than in adult plasma (81%). Likewise the effectiveness of BAX499 as represented by the increase in peak TG was much greater in adult (21%) than in maternal (10%) or cord plasma (12%). Further, BAX499 was more effective in reversing RB006 in adult plasma than in maternal or cord plasma. Ex vivo data were reproducible with the results of the mathematical simulation of TG.

Conclusion

Normal parturient plasma shows a large intrinsic pathway reserve for TG compared to adult and cord plasma, while TG in cord plasma is sustained by extrinsic pathway, and low levels of TFPI and AT.

Effect of rivaroxaban, in contrast to heparin, is similar in neonatal and adult plasma

Neonates have lower levels of clotting factors as well as inhibitors. Effects of heparin in neonatal plasma differ from those in adult plasma, and dosage recommendations cannot be extrapolated from adult trials. Riveroxaban is an oral direct factor Xa inhibitor that can achieve an anticoagulant effect without dependence on anti-thrombin. We performed comparative thrombin generation measurements in neonatal cord and adult plasma with different concentrations of unfractionated heparin and rivaroxaban to evaluate the potential of rivaroxaban in neonatal anticoagulation. The impact of heparin or rivaroxaban on the neonatal and adult hemostatic system was determined measuring calibrated automated thrombin generation and activated partial thromboplastin time in platelet-poor plasma pools of 15 adult samples or 15 neonatal cord samples and addition of seven increasing concentrations of heparin or rivaroxaban, respectively, to the pooled samples. Lag time, time to peak and peak height of thrombin generation in neonatal cord samples were significantly less affected by different heparin concentrations than in adult samples, whereas the impact on reduction of endogenous thrombin potential was higher in neonatal cord samples. The impact of rivaroxaban on thrombin generation parameters showed better comparability between neonatal cord and adult samples. Both anticoagulants showed the same differences in activated partial thromboplastin time between adult and neonatal plasma at each concentration. Rivaroxaban shows a very similar pattern in neonatal cord and adult plasma in suppressing thrombin generation and prolonging activated partial thromboplastin time values, suggesting that dose finding may be easier with rivaroxaban in neonates.

Thrombin generation before and after multicomponent blood collection

BACKGROUND

Apheresis technology has made tremendous progress up to the development of automated blood component collection, which offers increased efficiency in donor blood use, but the concern about the contact of donor blood with artificial surfaces remains. Activation of the hemostatic system is a major issue in this context and is controversial. The aim of this study was to estimate the effect of apheresis on continuous thrombin generation (TG), representing a new tool to examine the overall function of the plasmatic clotting system.

STUDY DESIGN AND METHODS

Twenty-six voluntary blood donors, fulfilling the law requirement for apheresis donation, participated in the study. Two units of platelets (6 x 10(11)) and 1 unit of red cells (250 mL; hematocrit level, 80%) were collected using two types of cell separators (Amicus, Fenwal, Inc.; and Trima Accel, Gambro BCT). Each donor underwent collection on both apheresis systems with at least 8 weeks in between. Samples of blood were collected before, immediately after, and 48 hours after apheresis. TG was measured using a slow fluorogenic substrate by means of calibrated automated thrombography (CAT).

RESULTS

CAT data changed only slightly, and no significant changes were seen before, immediately after, and 48 hours after apheresis (p > 0.05). The variables did not differ significantly between the two different apheresis systems (p > 0.05).

CONCLUSION

Using a CAT-based technique, no change in variables of continuous TG were observed, suggesting that multicomponent blood collection did not lead to severe alterations in the hemostatic system of the donors.

Pilot dose-finding and safety study of bivalirudin in infants <6 months of age with thrombosis

BACKGROUND

Thrombosis is not uncommon in children with serious medical conditions. Unfractionated heparin, the most commonly used anticoagulant in the acute management of thrombosis, has significant pharmacologic limitations, especially in infants. Newer anticoagulants have improved properties relative to heparin, and this may enhance the outcome in children.

OBJECTIVE

To determine dosing, and to assess the safety and efficacy of bivairudin for infants with thrombosis.

METHODS

Infants <6 months old were chosen for this pilot study as they may most benefit from a direct thrombin inhibitor because of their physiologically low antithrombin levels. This was an open label, dose-finding and safety study. Patients received one of three bolus doses and one of two initial infusion doses with subsequent dosing adjusted utilizing the activated partial thromboplastin time. Safety was assessed by specific bleeding endpoints. Efficacy was determined by reassessing the initial imaging study at 48-72 h and by measurement of molecular markers of thrombin generation.

RESULTS

Sixteen patients completed the study. All three bolus doses resulted in therapeutic anticoagulation, as did both initial infusion doses. A dose-response effect was noted for the continuous infusion but not the bolus dosing. Two patients met the study criteria for major bleeding, both with gross hematuria, which resolved with a reduction in the bivalirudin infusion rate. In terms of efficacy, 37.5% of patients had complete or partial resolution of their thrombosis by 48-72 h. There was a significant decrease in all three molecular markers of thrombin generation.

CONCLUSION

This study demonstrates the potential utility of bivalirudin in the pediatric population.

Thrombin generation in mesalazine refractory ulcerative colitis and the influence of low molecular weight heparin

BACKGROUND

In ulcerative colitis (UC), a state of hypercoagulation has frequently been observed. Low molecular weight heparin (LMWH) has shown beneficial effects as an adjuvant treatment of steroid refractory UC in open trials. We assessed potential therapeutic effects of the LMWH reviparin in hospitalised patients with mesalazine refractory UC, as well as its influence on haemostasis factors.

METHODS

Twenty-nine patients with mild-to-moderately active UC were included in a double-blind placebo controlled trial. All patients had a flare-up of disease under mesalazine treatment. Reviparin (Clivarin) 3,436 IU anti-Xa/0.6 ml or placebo s.c. was added, and self-administered twice daily for 8 weeks. Patients were monitored for possible adverse events and changes in clinical symptoms. Endoscopical, histological, biochemical and haemostasis parameters were analysed.

RESULTS

Tolerability and compliance were excellent and no serious adverse events occurred. No significant differences were observed on the clinical, endoscopical and histological outcome, as compared to placebo. A high intrinsic and extrinsic thrombin potential was found before LMWH therapy. However, the significant reduction in the thrombin generation by LMWH was not related to the reduction in disease activity.

CONCLUSION

The LMWH reviparine reduces thrombin generation in patients with mild-to-moderately active, mesalazine refractory UC, but is not associated with a reduction in disease activity.

Comparison of the anticoagulant effect of a direct thrombin inhibitor and a low molecular weight heparin in an acquired antithrombin deficiency in children with acute lymphoblastic leukaemia treated with L-asparaginase: an in vitro study

Thrombosis occurs in 37% of children with acute lymphoblastic leukaemia (ALL) and is related to an L-asparaginase-induced acquired antithrombin (AT) deficiency. The incidence dictates the need for anticoagulant prophylaxis. Direct thrombin inhibitors (DTI) are independent of AT for effect and may thus have advantages in this population. The objective of this study was to determine the interaction of an AT deficiency with the anticoagulant effects of a DTI and a low molecular weight heparin (LMWH). Plasma samples from children with ALL were pooled (mean AT 0.53 U/ml). LMWH 0.3 and 0.7 U/ml or melagatran 0.3 and 0.5 micromol/l were added to the pools, then divided and AT was added back to one aliquot. In additional experiments, AT was added to AT immuno-depleted plasma. Endogenous thrombin generation capacity (ETGC) was assessed by the continuous method. In plasma with LMWH, there was a 66-88% decrease in ETGC in AT-normalised samples compared with neat. Conversely, no significant difference in ETGC with or without AT added for melagatran was seen. Experiments with AT-depleted plasma showed no effect of AT level on anticoagulant activity of DTI, but a significant relationship for LMWH. By contrast to LMWH, DTI provides a consistent anticoagulant response independent of AT levels in children with AT deficiency.

Fetal and Neonatal Thrombophilia

Thrombophilia of the fetus and neonate may contribute to higher prevalence of perinatal thrombosis. Due to the potential interaction between thrombophilic risk factors of the neonate and maternal thrombophilia and placental vasculopathy, we recommend thrombophilia assessment be performed in any child and in the mother in case of perinatal thrombosis. Further attention and larger prospective studies are required to establish the role of thrombophilic risk factors in the pathogenesis of any other perinatal complications.

Novel method of expression and purification of hirudin based on pBAD TOPO, pTYB12 vectors and gene synthesis

To express recombinant hirudins in Escherichia coli cells, a fragment of chemically synthesized DNA was used, containing codons for the individual amino acids preferred by the host cells. Gene synthesis was based on the design of two DNA fragments, so-called mega primers H1 and H2 with a complementary fragment, and their incubation with Taq polymerase. The gene obtained in this fashion was multiplied using the PCR, and then expressed in E. coli cells with the use of TOPO vectors pBAD and pTYB12. Using this method, hirudins were obtained in the amount of 17 mg/l E. coli strain, with the activity of 17 antithrombin units (ATU)/mg protein. The method can be considered as an easy and inexpensive route to small protein synthesis.

Thrombin generation in factor VIII-depleted neonatal plasma: nearly normal because of physiologically low antithrombin and tissue factor pathway inhibitor

BACKGROUND

Bleeding in hemophilic neonates has a low incidence. A possible explanation for this could be the peculiarities of the neonatal hemostatic system, especially low levels of the inhibitors tissue factor pathway inhibitor (TFPI) and antithrombin (AT).

OBJECTIVE

We investigated the influence of an elevation of these inhibitors to adult levels on the thrombin generation (TG) in normal neonatal plasma and factor (F) VIII-depleted neonatal plasma by means of incubation with anti-FVIII-antibodies.

PATIENTS/METHODS

TG was measured after activation with low amounts of tissue factor (TF) by using Calibrated Automated Thrombography.

RESULTS

TG in FVIII-depleted neonatal plasma was nearly as high as in normal neonatal plasma. TG decreased after elevation of AT in both neonatal plasmas. After elevation of TFPI TG decreased much more in FVIII-depleted neonatal plasma than in normal neonatal plasma. After elevation of both inhibitors their synergistic effect led to a stronger decrease of TG in FVIII-depleted neonatal plasma. TG measured in plasma of one hemophilic newborn showed the same pattern as in FVIII-depleted neonatal plasma.

CONCLUSION

Our observation provides a biochemical basis for the rare bleeding in hemophilic neonates and shows the important role of the natural inhibitors in the hemostatic system of hemophilic patients.

An evaluation of the procoagulant action of recombinant activated factor VII in cord whole blood versus adult whole blood using thromboelastography

Recombinant activated factor VII (rFVIIa) has been reported to be effective in adult patients in various clinical situations and might be beneficial in neonates with bleeding tendency. In the present study we compared the procoagulant action of increasing amounts of rFVIIa in both cord whole blood and adult whole blood with respect to changes in the values of the clotting time, clot formation time, and maximum clot firmness by means of thromboelastography. Thromboelastography allows evaluation of the effects of rFVIIa on haemostasis in whole blood. When increasing amounts of rFVIIa were added in vitro to whole blood samples, significant decreases in the values of the clotting time and clot formation time and a significant increase in the maximum clot firmness were observed. Cord whole blood was significantly more sensitive to rFVIIa addition than adult whole blood, an effect probably attributable to the low anticoagulant capacity of the neonatal plasma. Maximum clot firmness values were significantly lower in cord whole blood than in adult whole blood, an effect mainly attributable to the hypofunctional state of neonatal platelets. Since cord whole blood exerted a significantly higher sensitivity to addition of rFVIIa, we speculate that lower doses of rFVIIa might be required to treat neonates with bleeding tendency compared with the adult rFVIIa administration strategy.