Presence Of Unfractionated Heparin Research Articles

Unfractionated heparin: optimizing laboratory monitoring and reducing unwanted interference in everyday hemostasis test practice.

Unfractionated heparin (UFH) serves as a commonly used anticoagulant. It is widely utilized for a variety of reasons, including to 1) anticoagulate patients and help treat and / or prevent thrombosis, 2) maintain patency in artificial blood flow circuits, and 3) anticoagulate blood samples collected for laboratory testing (typically for biochemical assays or blood gas analysis). As such, the presence of UFH is nearly ubiquitous in a hospital setting. Therefore, in laboratory practice, UFH may be present in samples intended for monitoring patients on UFH therapy or intended for biochemical tests, or it may interfere with other (hemostasis) laboratory tests. The aim of this manuscript is to review the role of UFH from the perspective of optimizing laboratory testing to monitor UFH therapy and to avoid or overcome unwanted interference with other laboratory tests.

Monitoring of Anticoagulant Activity of Dabigatran and Rivaroxaban in the Presence of Heparins.

The routine monitoring of direct oral anticoagulants (DOACs) may be considered in patients with renal impairment, patients who are heavily obese, or patients requiring elective surgery. Using the heparin-binding copolymer (HBC) and polybrene, we aimed to develop a solution for monitoring the anticoagulant activity of DOACs in human plasma in the interfering presence of unfractionated heparin (UFH) and enoxaparin. The thrombin time (TT) and anti-factor Xa activity were monitored in pooled plasma from healthy volunteers. In these tests, plasma with dabigatran or rivaroxaban was mixed with UFH or enoxaparin and then incubated with HBC or polybrene, respectively. HBC and polybrene neutralized heparins and enabled monitoring of anticoagulant activity of dabigatran in the TT test. Both agents allowed for accurate measurement of anti-factor Xa activity in the plasma containing rivaroxaban and heparins in the concentration range reached in patients’ blood. Here, we present diagnostic tools that may improve the control of anticoagulation by eliminating the contamination of blood samples with heparins and enabling the monitoring of DOACs’ activity.

Successful treatment of thrombotic thrombocytopenia with cerebral sinus venous thrombosis following Ad26.COV2.S vaccination

Successful treatment of thrombotic thrombocytopenia with cerebral sinus venous thrombosis following Ad26.COV2.S vaccination

Thrombodynamics, a new global coagulation test: Measurement of heparin efficiency

The actual coagulation status may be reliably measured using only highly sensitive global functional tests; however, they are not numerous and all of them have disadvantages. Thrombodynamics (TD), a novel global coagulation test, is sensitive to hypo- and hypercoagulable states. The main properties of this test were investigated, and its capabilities for hemostasis analysis were verified through pharmacodynamic monitoring of the most widely used anticoagulants, heparins. The anticoagulant effects in the plasma of donors (n = 20) and patients after hip replacement (n = 20) spiked with unfractionated heparin or enoxaparin were measured in vitro to eliminate the influence of pharmacokinetic factors. Sensitivity for heparins was compared for activated partial thromboplastin time, thrombin generation tests and TD. TD was shown to reliably characterize the pharmacodynamics of any heparin in the entire range of its prophylactic and therapeutic concentrations. Inter-individual variability for the anticoagulant action of heparins was also calculated using the TD data. This variability did not differ between the investigated groups and did not exceed 12% and 20% for the stationary clot growth rate in the presence of unfractionated heparin and enoxaparin, respectively. That finding was in accordance with the values determined earlier using the thrombin generation test. The study results showed that TD has advantages over the other global methods of coagulation analysis. These advantages are good standardization, high reproducibility, independence of the parameter values from patient age and gender, and a narrower parameter distribution in a normal population. These results indicate that TD is a promising universal assessment method that improves the quality of hemostasis analysis because it more reliably detects deviations from the parameters' reference values.

Inhibition of plasmin generation in plasma by heparin, low molecular weight heparin, and a covalent antithrombin-heparin complex.

: The clinical limitations of unfractionated heparin (UFH) and low molecular weight heparin (LMWH) led to the development of an antithrombin-heparin covalent complex (ATH), which displays superior anticoagulant abilities compared with UFH. A recent study investigating its interaction with fibrinolysis showed that ATH inhibited free and fibrin bound plasmin and decreased plasmin generation on fibrin clots. These studies were conducted using purified components and did not elucidate the interaction of ATH with plasmin in the presence of its natural inhibitors α2-antiplasmin (α2-AP) and α2-macroglobulin (α2-M). The aim of this study was to determine the effects of ATH, UFH, and LMWH on plasmin generation in plasma, under more physiological conditions. Plasmin generation in plasma in the absence and presence of anticoagulants was initiated by tissue plasminogen activator and soluble fibrin fragments, and plasmin and plasmin-α2-M complexes generated over time were quantified chromogenically. Generation of plasmin-α2-AP complexes and consumption of plasminogen were quantified by ELISA. Plasmin generation was decreased in the presence of UFH and ATH, whereas LMWH had no effect. Neither plasminogen consumption nor generation of plasmin-α2-AP complexes were affected by UFH or ATH. However, plasmin-α2-M complexes were slightly reduced by ATH suggesting that ATH may be able to compete with α2-M for plasmin. Plasmin generation may be mildly inhibited by heparin-based anticoagulants; however, heparin-catalyzed antithrombin activity is not a major inhibitor of plasmin, as compared to its natural inhibitors α2-AP and α2-M. This adds to our understanding of ATH mechanisms of action and aids in its development for clinical use.

RNA Aptamer Against FXa Synergizes with FXa Catalytic Site Inhibitors to Effectively and Reversibly Anticoagulate Blood in an Ex Vivo Oxygenator Circuit

Despite notable disadvantages, unfractionated heparin (UFH) remains the standard anticoagulant for clinical procedures requiring potent and reversible anticoagulation such as cardiopulmonary bypass (CPB). Limitations of UFH that contribute to patient morbidity in these settings include the fact that it permits thrombin generation and can cause the antibody-mediated syndrome Heparin-induced Thrombocytopenia (HIT), in addition to independent toxicities associated with its reversal agent, protamine.11F7t is an anticoagulant RNA aptamer which inhibits FXa but unfortunately achieves less intense anticoagulation than UFH. The latter is also true for clinical FXa catalytic site inhibitors such as rivaroxaban, apixaban, or edoxoban. However, 11F7t does not inhibit FXa’s catalytic activity but instead binds a FXa exosite to impede FVa binding and thus prothrombinase assembly. Owing to these different mechanisms, we previously reported that 11F7t can potently synergize with a FXa catalytic site inhibitor to prevent clot formation for >180 minutes (min) in whole blood thromboelastography (TEG) assays, thereby replicating the effect of UFH (5U/mL).Here we sought to determine whether combinations of 11F7t plus a FXa catalytic site inhibitor can also prevent clotting as effectively as UFH in flowing blood within an ex vivo CPB membrane oxygenator circuit. In addition, we investigated whether efficient and simultaneous reversal of both anticoagulants could also be achieved post-circulation using desGla-Xa-S195A, which is a Gla-domainless catalytically inactive recombinant FXa mutant analogous to Andexanet Alfa, a therapeutic currently in clinical trials as an antidote for several FXa inhibitors. We also quantified levels of thrombin generation during circulation based on Prothrombin fragment 1+2 (F1+2) measurement. Finally, we investigated whether purified IgG obtained from three HIT patients could induce platelet aggregation in the presence of 11F7t, as occurs with UFH.Human whole blood anticoagulated with either (A) UFH (5U/ml), or a combination of 11F7t (2µM) plus either (B) rivaroxaban (2µM), (C) apixaban (2µM), or (D) edoxaban (2µM) was circulated within a miniature oxygenator circuit at 33°C for 120 min at a 50 mL/min flow rate. While anticoagulation with either 11F7t or each of the FXa catalytic site inhibitors alone failed to maintain circuit blood fluidity, strategies (A) through (D) each prevented visible clot formation for 120 min and achieved therapeutic anticoagulation levels (>400 sec) as measured by the Activated Clotting Time (ACT). In addition, post-circulation scanning electron micrographs of the oxygenator membranes were similar for all four strategies and revealed minimal fibrinous and cellular debris. Successful normalization of the ACT was achieved upon administration of desGla-Xa-S195A (2µM) for strategies (B) through (D), similarly to that observed for UFH reversal by protamine. In addition, elevation of F1+2 levels post-circulation was significantly higher with UFH compared to each of the 11F7t plus FXa catalytic site inhibitor strategies (B-D). Finally, the HIT patient-derived purified IgG only induced platelet aggregation in the presence of UFH but not aptamer 11F7t (strategies B-D).We have shown that the anticoagulant synergy achieved by combining aptamer 11F7t with a FXa catalytic site inhibitor can prevent blood clotting within an ex vivo oxygenator circuit as effectively as UFH, and may be additionally advantageous in limiting thrombin generation. Moreover, administration of desGla-Xa-S195A or a similar inactive FXa decoy like Andexanet Alfa may enable simultaneous reversal of both anticoagulants. This dual anticoagulant strategy may provide a useful alternative to UFH in clinical settings like CPB that necessitate both potent and reversible anticoagulation, and may be especially valuable for patients with a prior history of HIT. DisclosuresArepally:Biokit: Patents & Royalties. Camire:Pfizer: Consultancy, Patents & Royalties, Research Funding; Novo Nordisk: Research Funding; Bayer: Consultancy; sparK: Membership on an entity’s Board of Directors or advisory committees, Patents & Royalties.

Oxidized antithrombin is a dual inhibitor of coagulation and angiogenesis: Importance of low heparin affinity

Endogenous proteins that promote vascular endothelial cell based inhibition of angiogenesis are an attractive option for antitumor therapy. Inactive cleaved and latent conformations of antithrombin (AT) are antiangiogenic, but not its native form which is an inhibitor of proteases involved in blood coagulation. Unlike native, the cleaved and latent conformations are reactive center loop inserted conformations which binds heparin with very low affinity. We use a sulfoxy modified AT to assess the role of reactive center loop insertion and heparin affinity in antiangiogenic function. Chorioallantoic membrane assay (CAM) shows that antiangiogenic activity of latent and oxidized AT are better than thalidomide, a potent antiangiogenic drug. Wound healing experiments suggest that latent and oxidized conformations can influence endothelial cell migration. Latent and cleaved conformations of AT shows an increase in α-helical content in the presence of unfractionated heparin, but not the oxidized AT. Unlike the loop inserted polymer, cleaved and latent conformations, oxidized AT has factor Xa inhibitory activity indicating that loop insertion is not necessary for antiangiogenic role. The results of our study establish that active conformation of AT can become antiangiogenic while maintaining its anticoagulant activity possibly through chelation of low affinity heparin in the vicinity of endothelial cell.

The Effects of Thrombocytopenia on Clotting Profile in Whole Blood As Determined By Thromboelastography When Anticoagulant Is Present at Therapeutic or Prophylactic Concentration

Background: Anticoagulant therapy for the treatment of venous thromboembolism in patients with concomitant thrombocytopenia has been based on anecdotal evidence. The platelet (PLT) threshold at which anticoagulant therapy should be withheld is still controversial. A PLT count of 50 × 109/L was recommended to be the threshold in the past, but newer reviews have lowered the threshold to 30 × 109/L. We previously used thromboelastography (TEG) to study clotting in plasma reconstituted with autologous PLT. Since red cells also play a significant role in hemostasis and coagulation, we hereby developed a TEG model with whole blood (WB) in order to better mimic in vivo conditions to evaluate the clot formation in thrombocytopenic blood.Objective: Using TEG to monitor clotting in whole blood samples containing unfractionated heparin (UFH) or dalteparin, we evaluated the differences in clotting profile when PLT in the samples were reduced to thrombocytopenic range.Methods: Whole blood was collected from healthy volunteers in syringes containing citrate phosphate dextrose adenine (CDPA-1, pH=5.5) and 30 μg/L corn trypsin inhibitor. Magnetic CD 61 antibody chromatography was used to deplete PLT in the blood to a count of ≤ 15 × 109/L. Platelet-depleted whole blood (PDWB) was then mixed with untouched blood from the same donor to obtain the predefined PLT counts. Clotting was initiated in the TEG cups with 10 mM CaCl2 and tissue factor (TF) in the presence of either UFH (0.3 U/mL or 0.1 U/mL) or dalteparin (1 IU/mL or 0.3 IU/mL). Due to the mechanistic differences between UFH and dalteparin, we optimized the amount of TF to maximize the sensitivity of TEG assay for individual anticoagulants; thus, 2.25 pM and 2.05 pM were used for UFH and dalteparin experiments, respectively. However, the same amount of TF was used to evaluate the clotting with the same anticoagulant at both therapeutic and prophylactic concentrations. Clotting was monitored using a Haemoscope TEG at 37 ºC for a maximum of 3 hr or until maximum amplitude (MA) had been achieved. Three parameters of clotting profile including R, MA and area under the curve within the first 15 min of clotting (AUC15) were used for further analysis. A p-value < 0.05 was considered statistically significant.Results: All3 parameters showed significant compromise of clotting when PLT decreased from 150 × 109/L to < 15 × 109/L in the presence of UFH or dalteparin at therapeutic range. When these anticoagulants were reduced to prophylactic concentration, the clotting was also significantly moderated, but to a lesser extent, comparing samples with PLT at 150 × 109/L and those with PLT < 15 × 109/L. These are in accordance with the bleeding tendency in vivo.At 30 × 109/L, the newer recommended PLT threshold at which anticoagulant should be withheld in thrombocytopenic patients, the clotting parameters did not show any significant difference as compared to those at the traditional threshold of 50 × 109/L when UFH and dalteparin were at therapeutic concentrations. Similarly, when UFH was reduced to a prophylactic concentration, we detected no significant difference in the clotting profile between 50 × 109 PLT/L and 30 × 109 PLT/L. In contrast, in samples with dalteparin at a prophylactic concentration, MA was significantly lower at 30 × 109 PLT/L when compared with that at 50 × 109 PLT/L although R and AUC15 had no statistical difference. Additionally, samples of PDWB containing either anticoagulant at prophylactic concentration had better clot formation than those samples of 50 × 109 PLT/L containing UFH or dalteparin at therapeutic concentration.Conclusion: The TEG profile of WB clotting in this in vitro model simulates bleeding tendency observed clinically. In the presence of UFH or dalteparin at therapeutic concentration, there was no statistical difference in the TEG parameters comparing thrombocytopenic blood with 50 × 109 PLT/L and 30 × 109 PLT/L, supporting the latter as the new threshold to hold anticoagulant in thrombocytopenic patients. In addition, instead of holding all anticoagulants in severe thrombocytopenic patients with PLT < 30 × 109/L, administering UFH or dalteparin at prophylactic doses may offer a safe alternative, as both imped clotting in TEG even less than those at therapeutic concentration with thrombocytopenic blood at 50 × 109 PLT/L. [Display omitted] [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Relieving Inhibition of Prothrombinase By TFPIα: a Procoagulant Activity of Unfractionated, Low Molecular Weight, and Nonanticoagulant Heparins

Introduction: Prothrombinase, the complex of factor Xa (FXa) and factor Va (FVa), is inhibited by tissue factor pathway inhibitor (TFPI)α during the initiation of coagulation (Wood JP et al, PNAS 2013). Efficient inhibition of thrombin generation by prothrombinase requires an interaction between the TFPIα basic C-terminus and an acidic region of the FVa B-domain. This acidic region is present in FXa-activated FVa and FVa released from activated platelets, but is rapidly removed by thrombin. Thus, prothrombinase inhibition only occurs during the initiation phase of thrombin generation. As the exosite interaction is charge-dependent, large negatively charged molecules, including unfractionated heparin (UFH), block it, prevent prothrombinase inhibition, and promote thrombin generation. Studies using the negatively charged molecule polyphosphate have suggested a size requirement for blocking this TFPIα activity (Smith SA et al, Blood2010). A similar size-dependence may exist with heparins and could have clinical implications, as currently-used heparins range from long (unfractionated heparin; UFH) to medium (low molecular weight heparins; LMWHs) to short (the antithrombin-binding pentasaccharide fondaparinux). Studies were performed to assess the ability of the LMWHs enoxaparin and dalteparin, fondaparinux, and the nonanticoagulant heparin 2-O, 3-O desulfated heparin (ODSH) to block TFPIα and promote thrombin generation through this mechanism.Methods: TFPIα inhibition of thrombin generation by prothrombinase, assembled with a form of FVa containing the acidic region of the B domain, was measured in the absence or presence of UFH, enoxaparin, dalteparin, fondaparinux, and ODSH. The effect of these compounds on the direct inhibition of FXa by TFPIα was measured using a FXa chromogenic substrate. The effect of these compounds on thrombin generation in plasma was measured by calibrated automated thrombography using human plasma immunodepleted of antithrombin III and heparin cofactor II (AT3/HCII-depleted plasma).Results: TFPIα inhibited prothrombinase activity (IC50 = 6.8 nM), and UFH blocked this inhibition (IC50 = 12.5 nM or 14.9 nM at 0.5 or 1 U/mL, respectively). Enoxaparin (0.8 U/mL; IC50 = 30.3 nM) and dalteparin (1 U/mL; IC50 = 29.7 nM) appeared to be more effective at reversing TFPIα inhibition. The reason for this apparent enhanced effect of LMWHs compared to UFH is not clear, as UFH and the LMWHs similarly enhanced the direct inhibition of FXa by TFPIα, and the differential activity was also observed when heparins were normalized to saccharide concentration. The same pattern was observed when measuring thrombin generation in AT3/HCII-depleted plasma, with LMWHs being more procoagulant than UFH. Consistent with TFPIα inhibition being charge-dependent, ODSH promoted thrombin generation similarly to LMWHs in both purified systems and AT3/HCII-depleted plasma. In contrast, clinical doses of fondaparinux had no effect in any assay. In a purified system, ~1000 times the clinical dose of fondaparinux was required to promote thrombin generation.Conclusion: There is a size-dependence for blocking TFPIα inhibition of prothrombinase using heparins, as the pentasaccharide has no effect. However, both LMWHs and UFH are sufficiently long to express this procoagulant activity at therapeutic doses. In addition, the nonanticoagulant heparin ODSH blocks prothrombinase inhibition by TFPIα. This procoagulant activity is likely most clinically relevant under conditions of antithrombin deficiency, which may result from sepsis, liver failure, or administration of L-asparaginase. Under any of these conditions, UFH, LMWHs, and ODSH may have unanticipated procoagulant activity mediated by blocking TFPIα. DisclosuresCamire:Pfizer: Consultancy, Patents & Royalties, Research Funding. Mast:Novo Nordisk: Research Funding.

Inhibition of Plasmin Generation in Plasma By Heparin, Low Molecular Weight Heparin and a Covalent Antithrombin-Heparin Complex (ATH)

Introduction: Unfractionated heparin (UFH) and low molecular weight heparin (LMWH) are widely used anticoagulants for thrombosis treatment. However, these anticoagulants have limitations such as increased bleeding, variable dose response, required frequent monitoring, and, in the case of LMWH, inability to inhibit thrombin. This has led to the development of a covalent complex of antithrombin and heparin (ATH), which has been shown to overcome many of these shortcomings. ATH has faster rates of inhibition of many coagulation factors, is able to inhibit clot-bound thrombin, and is a more effective inhibitor of both venous and arterial thrombosis in animal models. Moreover, in a rabbit thrombosis model, ATH has been shown to decrease clot mass and fibrin accretion, while the contrary was observed for UFH. From these observations, it was suggested that ATH may enhance fibrin breakdown and thus led to investigations into the effects of UFH and ATH on fibrinolysis. In vitro studies have shown that UFH enhances antithrombin inhibition of plasmin. In addition, ATH displays a slightly greater inhibition of plasmin generation and activity. Such studies were conducted in purified systems, in the absence of other plasmin inhibitors naturally present in plasma. Therefore, the aim of the present study was to compare the effects of UFH, LMWH, and ATH on plasmin generation in plasma.Methods: At 37°C tissue plasminogen activator (tPA) and soluble fibrin fragments (fib) were added to normal adult pooled platelet poor plasma supplemented with 0.35, 0.7, 1.4, or 2.1 U anti-Xa/ml UFH, LMWH, or ATH, to initiate plasmin generation (8.93nM tPA and 300µg/ml fib). At various time points, subsamples were mixed with excess plasminogen activator inhibitor 1 (PAI-1) (55.12nM) to stop further plasmin generation. The plasmin concentration at each time point was determined using a plasmin-specific chromogenic substrate and a standard curve produced from purified plasmin.Results: Comparisons of mean area under the curve (AUC) for plasmin generation displayed a significant decrease in plasmin generation in the presence of all three anticoagulants at all doses tested (p<0.05). Comparing the anticoagulants at similar doses, plasmin generation was significantly decreased in the presence of ATH (15384.66±1930.23nM/min) compared to LMWH (23892.28±3090.54nM/min) at 0.7 U/ml (p<0.05). At a dose of 1.4 U/ml, there was significantly less plasmin generated, over time, in the presence of UFH (20089.49±3022.1623nM/min) and ATH (19273.86±1805.7323nM/min) when compared to LMWH (24743.18±1265.1023nM/min) (p<0.05). There was no significant difference in plasmin inhibition between UFH and ATH at any of the doses tested.Conclusion: The present study supports previous findings that UFH and ATH can facilitate antithrombin inhibition of plasmin. It is also observed that LMWH catalyzes the inhibition of plasmin by antithrombin but possibly to a lesser extent. These findings suggest that ATH has a similar inhibitory effect on plasmin generation and activity in plasma compared to UFH, despite its overall superior anticoagulant properties. Therefore, previous in vivo observations displaying decrease in clot mass with administration of ATH was due to its enhanced anticoagulant abilities and not fibrinolysis enhancement. These findings add to our understanding of ATH mechanisms of action and aid in its development for clinical use. DisclosuresNo relevant conflicts of interest to declare.

Effects of factor VIII levels on the APTT and anti-Xa activity under a therapeutic dose of heparin.

In pregnant women, activated partial thromboplastin time (APTT) does not precisely reflect the anticoagulant effect of a therapeutic dose of heparin. However, the measurement of anti-Xa activity can be used to monitor the anticoagulant effect of heparin, since the plasma concentrations of coagulation factors increase in pregnant women. We evaluated the in vitro effects of increased concentrations of fibrinogen and other coagulation factors (FVII, FVIII, and FIX) on the results of assays of APTT and anti-Xa activity in plasma samples with various therapeutic concentrations of unfractionated heparin (UFH). In the presence of UFH, APTT was shortened by increased concentrations of fibrinogen, FVII, or FVIII, and this effect was much stronger when the FVIII concentration was increased. In the plasma samples containing 0.5 or 0.7 U/mL of UFH, the APTT was shortened by approximately half or one-third, respectively, when 6UFVIII/mL was added to the sample. The anti-Xa activity was not influenced by increased concentrations of the coagulation factors. In the present study, we also evaluated the sensitivities to UHF of four APTT reagents, and found a 1.65-fold difference in the sensitivity to UFH among APTT reagents. Our results demonstrate that increased FVIII concentration shortens APTT under therapeutic doses of UFH, and that APTT thus underestimates the anticoagulant effect of UFH in pregnant women, mainly due to the increased FVIII concentration.

The influence of free hemoglobin and bilirubin on heparin monitoring by activated partial thromboplastin time and anti-Xa assay.

Elevated free hemoglobin (Hb) and bilirubinemia complicate extracorporeal membrane oxygenation and could affect unfractionated heparin (UH) therapy monitoring by anti-Xa assay and activated partial thromboplastin time (aPTT). To compare in vitro response of anti-Xa and aPTT assays to UH in samples with artificial hyperbilirubinemia and hyperhemoglobinemia and to estimate if this interference is also observed in vivo in pediatric extracorporeal membrane oxygenation. Measurement of aPTT and anti-Xa activity in plasma spiked with UH and increased concentration of free Hb and/or conjugated bilirubin. All samples with anti-Xa activity, antithrombin, free Hb, and bilirubin determination and infused dose of UH from inpatients on extracorporeal membrane oxygenation were extracted from the clinical patient database and analyzed. Each increment of free Hb by 100 mg/dL significantly shortened aPTT, whereas an increment of bilirubin by 6 mg/dL caused significant prolongation of aPTT and stepwise increase of free Hb and/or bilirubin in plasma decreased anti-Xa activity by 0.03 to 0.05 IU/mL. Extracorporeal membrane oxygenation samples with free Hb 50 mg/dL or greater had significantly lower anti-Xa activity compared with normal ones: 0.33 (0.25-0.42) versus 0.4 (0.31-0.48) IU/mL (P = .01), despite the identical UH infusion and similar antithrombin activity. Moderate increase of free Hb by 59 mg/dL was associated with absolute decrease of anti-Xa activity by 0.07 IU/mL. Activated partial thromboplastin time and anti-Xa assay are affected by elevated level of free Hb and/or bilirubin in the presence of UH, and lower anti-Xa activity is noted in extracorporeal membrane oxygenation patients with elevated free Hb. Severe hemolysis and/or hyperbilirubinemia could compromise UH monitoring based on these assays.

Beta (β)‐antithrombin activity in children and adults: implications for heparin therapy in infants and children

Antithrombin, a hemostatic protein and naturally occurring anticoagulant, is a major thrombin inhibitor. The capacity of antithrombin to inhibit thrombin is known to increase a 1000-fold whilst in the presence of unfractionated heparin. β-antithrombin is an isoform of antithrombin with a high affinity for unfractionated heparin. This study aimed to determine the differences in the anticoagulant activity of the β-antithrombin isoform in children compared with adults. Plasma samples were obtained from 105 healthy individuals from the following age groups: neonates (day 1 and day 3), 28days to 1year, 1-5years, 6-10years, 11-16years and adults. The method utilized to measure the activity of β-antithrombin in plasma is a modified version of the total antithrombin assay routinely used in diagnostic laboratories. The modified version of this assay allows for the specific quantification of the β-antithrombin glycoform anticoagulant activity alone, as the β-antithrombin molecule is activated under a high salt concentration, which in turn does not allow activation of other antithrombin isoforms. This study demonstrated that there are no age-specific differences in the activity of β-antithrombin. However, considering that the total AT activity is significantly reduced in neonates, our results suggest that in this population β-antithrombin activity is a major contributor to the overall activity of AT.

Identification of pro- and anti-proliferative oligosaccharides of heparins

Heparins, unfractionated heparin (UFH) and low molecular weight heparins (LMWHs), are heterogeneous mixtures of anticoagulant and non-anticoagulant oligosaccharides. In addition to their well-known anticoagulant effect, heparins have shown to mediate a wide range of non-anticoagulant effects, including the modulation of cellular growth. However, contradictory results have been reported with regard to their effects on cellular proliferation, with some studies suggesting anti-proliferative while others indicating pro-proliferative effects. This study investigated the proliferation of human colonic epithelial cancer cells in the presence of UFH and LMWHs (enoxaparin and dalteparin). In our experimental setting, all heparins caused a dose-dependent reduction in cellular growth, which correlated well with the induction of cell cycle arrest in the G₁ phase and which was not associated with significant changes in cell viability. The effects on cellular proliferation of 14 different oligosaccharides of enoxaparin obtained through ion-exchange chromatography were also assessed. Surprisingly, only two oligosaccharides showed distinctive anti-proliferative effects while the majority of oligosaccharides actually stimulated proliferation. Interestingly, the smallest oligosaccharide devoid of any anticoagulant activity showed the strongest anti-proliferative effect. Notably, heparins are currently standardised only according to their anticoagulant activity but not based on other non-anticoagulant properties. Our results indicate that slight differences in the composition of heparins' non-anticoagulant oligosaccharides, due to different origins of material and preparation methods, have the potential to cause diverse effects and highlight the need for additional characterisation of non-anticoagulant activities.

The First Report Of Differences In Fibrin Clot Nanostructure and Response To Tissue Plasminogen Activator (TPA) and Unfractionated Heparin (UFH) In Infants Compared To Adults

Introduction Age specific differences in the function and concentration of haemostatic proteins are well established and are thought to contribute to the lower incidence of thromboembolism in infants and children compared to adults. We hypothesized that there are age-specific differences in the fibrin clot nanostructure and that these differences impact on fibrinolytic and anticoagulant treatment. Aim To determine the differences in fibrin clot nanostructure and response to tissue Plasminogen Activator (tPA) and Unfractionated Heparin (UFH) in infants compared to adults. Materials and Methods Four plasma samples from infants (0.86±0.16 years) and four samples from adults (32.04±15.80 years) were investigated. Thrombin generation (TG) was performed on paper discs in a microplate with 300mmol/L rhodamine substrate (P2Rho), 4mmol/L phospholipids, 5x10-9 mmol/L tissue factor and 16.7 mmol/L CaCl2; with and without additional 50ng/ml tPA, and 0.01IU/ml UFH. Fluorescence signal was recorded with a Fluoroskan Ascent fluorometer, with samples tested in triplicate. The resulting fibrin clots were fixed with 2.5% glutaraldehyde; washed with PBS and fixed with osmium tetroxide OsO4. The clots were dehydrated in ethanol and hexamethyldisilazane and were placed on carbon adhesive tabs. Each clot was coated with gold and studied using Scanning Electron Microscopy (SEM) on a Phenom Pro desktop SEM. The age-specific differences in fibrin fibre thickness and pore size were quantified using the Fibermetric software, with 36 representative images for infants and the same number for adults at each baseline, tPA and UFH, at magnification 10.000, counting 500 fibres per image. Results We observed significant differences in fibrin clot nanostructure between infants and adults (Fig 1). Results are expressed as median and range. Fibrin fibers were thicker in infants compared to adults (343.3nm, 61.2nm to 997.1nm vs 352.3nm, 26.1nm to 997.5nm, p&lt;0.001). In addition, the pore size was larger in infants (0.057pm2, 0.0041pm2 to 19.97pm2 vs 0.052pm2, 0.001pm2 to 15.24pm2, p&lt;0.001). In the presence of tPA in the adult sample, the fibrin fibers were thicker (352.3nm, 26.1nm to 997.5nm vs 355.8nm, 70.0nm to 999.6nm, p&lt;0.05) and pore sizes larger (0.052pm2, 0.001pm2 to 15.24pm2 vs 0.049pm2, 0.0041pm2 to 20.26pm2, p&lt;0.001) compared to baseline. Strikingly, in children, tPA did not induce changes in the fibrin fiber thickness (343.3nm, 61.2nm to 997.1nm vs 343.0nm, 65.6nm to 998.8nm, p&gt;0.05) and pore size (0.057pm2, 0.0041pm2 to 19.97pm2 vs 0.050pm2, 0.0041pm2 to 19.34pm2, p&gt;0.05). In the presence of UFH in adults fibrin fibers were thicker (352.3nm, 26.1nm to 997.5nm vs 339.0, 66.9nm to 996.4nm, p&lt;0.001) and pore sizes larger (0.052pm2, 0.001pm2 to 15.24pm2 vs 0.0540pm2, 0.0041pm2 to 21.87pm2, p&lt;0.001) compared to baseline. In infants, in the presence of UFH, fibrin fibers were thicker (343.3nm, 61.2nm to 997.1nm vs 369.5nm, 109.1nm to 998.4nm, p&lt;0.001), and pore sizes larger (0.057pm2, 0.0041pm2 to 19.97pm2 vs 0.055pm2, 0.0041pm2 to 24.25pm2, p&lt;0.05). Discussion and Conclusion Our study suggests that the fibrin clots in infants are weaker compared to those of adults and is consistent with the lower TG in infants. This is further supported by published evidence that clots produced with low thrombin concentrations are composed of thick fibrin fibres, whereas those generated with high thrombin concentrations are composed of a tight network of thin fibres. The effect of both tPA and UFH on fibrin clots nanostructure in adults is consistent with published literature and is investigated in children for the first time. The fact that tPA did not have an effect on the fibrin clots of infants is not surprising considering that the concentration of plasminogen is 39%, tPA is 40% and TAFI function is 60% of the adult value; while PAI function is 50% higher compared to adults. UFH had an effect of the fibrin clot nanostructure in both infants and adults, with the effect on pore size being more pronounced in adults. While developmental haemostasis has been understood for many years, this is the first study to provide definitive proof that the end product of coagulation is a different clot structure in children. This further enhances our understanding of why thrombosis are less common in children, and has significant implications for treatment. Disclosures: No relevant conflicts of interest to declare.

Interactions of Heparin and a Covalently Linked Antithrombin Heparin Complex with the Components of the Fibrinolytic Pathway.

Abstract 2211 Introduction:Unfractionated heparin (UFH) is used for the prophylaxis and treatment of thromboembolic diseases. UFH catalyzes inhibition by antithrombin (AT) of the serine proteases in the coagulation cascade. Additionally, UFH has been shown to interact with components of the fibrinolytic pathway in vitro. However UFH has several limitations which impact its utility as a therapeutic agent. Our lab has developed a novel covalent antithrombin-heparin complex (ATH) which inhibits most serine proteases of the coagulation pathway significantly faster when compared to non covalent mixtures of AT and UFH. However, the interactions of ATH with the components of the fibrinolytic pathway have not been studied before. Thus, the present study investigates possible serpin-heparin interactions of AT + UFH vs ATH within the fibrinolytic pathway. Methods:Discontinuous second order rate constant assays under pseudo-first order conditions were carried out to obtain second order rate constant (k2) values for the inhibition of plasmin by AT+UFH versus ATH. Briefly, at specific time intervals 20 nM plasmin was inhibited by 200 nM AT + 0–5000 nM UFH or by 200 nM ATH in the presence of 2.5 mM Ca2+. Reactions were neutralized by the simultaneous addition of a solution containing polybrene and plasmin substrate S-2366™ in buffer. Residual plasmin activity was measured and the final k2 values calculated. For experiments involving tPA, wells containing 40nM tPA and increasing concentrations of AT, UFH or ATH, at mole ratios ranging from 0 to 20:1, were incubated for 15 min. Reactions with tPA were neutralized by simultaneous addition of a solution containing either polybrene and tPA substrate, S-2288™ in buffer, (ATH and UFH) or only the substrate S-2288™ in buffer (AT). Enzyme activity was then determined by measuring rate of substrate cleavage (Vmax). Results:When plasmin was inhibited by AT in the absence of UFH, k2 values of 2.82×105 +/− 4.46×104 M−1 min−1 were observed. The k2 values increased with addition of successively higher concentrations of UFH up to a plateau with maximal k2 of 5.74×106 +/− 2.78×105 M−1 min−1 at a UFH concentration of 3000nM. For inhibition of plasmin by ATH, k2 values of 6.39 × 106 +/− 5.88 × 105 M−1 min−1 were observed. Inhibition of plasmin by ATH was not significantly different when compared to the highest k2 values obtained with UFH. (p=0.36) No statistically significant difference in tPA enzyme activity was observed when Vmax values for tPA alone were compared with those in the presence of AT, UFH or ATH. (p=0.932, p=0.085, p=0.31 respectively) Significance:The characteristic shape of the curve obtained from the k2vs. UFH plot suggests that the mechanism responsible for inhibition of plasmin by AT+UFH involves conformational activation of the serpin. The k2 values in this study for inhibition of plasmin by both AT+UFH and ATH were three orders of magnitude lower than k2 values for inhibition of thrombin or factor Xa. Furthermore these results suggest that tPA is not inhibited by AT + UFH or ATH, and is not influenced by the presence of UFH alone. Cumulatively, this indicates that the fibrinolytic pathway is minimally impacted by AT + UFH or ATH, allowing maximal antithrombotic potential to be achieved during anticoagulation. Overall, the favourable anticoagulant properties of ATH combined with the findings of this study strengthens the utility of the covalent conjugate over conventional UFH for the treatment of thromboembolic disorders. Disclosures:No relevant conflicts of interest to declare.

Interaction of Protein C Inhibitor with the Type II Transmembrane Serine Protease Enteropeptidase

The serine protease inhibitor protein C inhibitor (PCI) is expressed in many human tissues and exhibits broad protease reactivity. PCI binds glycosaminoglycans and certain phospholipids, which modulate its inhibitory activity. Enteropeptidase (EP) is a type II transmembrane serine protease mainly found on the brush border membrane of epithelial cells in the duodenum, where it activates trypsinogen to initiate the digestion of food proteins. Some active EP is also present in duodenal fluid and has been made responsible for causing pancreatitis in case of duodeno-pancreatic reflux. Together with its substrate trypsinogen, EP is furthermore present in the epidermis and in some cancer cells. In this report, we show that PCI inhibited EP with an apparent 2nd order rate constant of 4.48×104 M−1 s−1. Low molecular weight (LMWH) and unfractionated heparin (UFH) slightly reduced the inhibitory effect of PCI. The SI (stoichiometry of inhibition) value for the inhibition of EP by PCI was 10.8 in the absence and 17.9 in the presence of UFH (10 U/ml). By inhibiting trypsin, chymotrypsin, and additionally EP, PCI might play a role in the protection of the pancreas from autodigestion. Furthermore the interaction of PCI with EP may influence the regulation of epithelial differentiation.

Comparison of Seven Generic Enoxaparins with Lovenox® on In Vitro Cross-Reactivity with Antibodies From Heparin Induced Thrombocytopenia.

AbstractAbstract 1105 Introduction:Heparin induced thrombocytopenia (HIT) is the major complication of heparin treatment but is less frequent in patients treated with low molecular weight heparins (LMWHs) than with unfractionated heparin (UFH). Cross immunological reactions of LMWHs with HIT antibodies from patients treated with UFH is a common finding. The immunological profile is among the criteria for similarity of a generic LMWHs. The cross reactivity of generic LMWHs with HIT antibodies has yet to be compared with the innovator product. Aim:To compare the profile of platelet aggregation induced by HIT antibodies in the presence of Lovenoxâ or seven generic enoxaparins. Methods:Platelet activation by HIT antibodies in the presence of UFH (Héparine Choay, France), branded enoxaparin (Lovenox®) and 7 generics (Novex®, Enoxa®, Dilutol®, Versa®, Cutenox®, Loparin®, Fibrinox®) was assessed by Multiple Electrode Aggregometry (MEA, Multiplate, Dynabyte, Germany) according to previously published assay1. Briefly, whole blood (340 μl) from 5 individual healthy donors was incubated with pooled platelet poor plasma (200 μl) from well characterised HIT positive patients and 40 μl of UFH or Lovenox® or generic enoxaparins yielding a final concentration of 1 anti-Xa IU/ml. Platelet aggregation was recorded over 15 min and the area under the curve (AUC, AU*min) was determined. Results:UFH and branded enoxaparin showed high cross reactivity with HIT antibodies in 4 out of 5 experiments. All of the 7 generic enoxaparins gave also a high positive cross-reactivity but the intensity of platelet aggregation varied. The magnitude of the response was classified as follows: Versa® > Enoxa® > Lovenox® > Novex® > Fibrinox® > Cutenox® > Loparin® > Dilutol®. In one experiment Lovenox® showed border-line cross-reactivity with HIT antibodies. Among generic enoxaparins Versa® showed the same extent of cross reactivity as that of Lovenox®. The other generic enoxaparins did not cross react with HIT antibodies. Conclusions:This is the first study that provides key data on the comparison of in vitro cross reactivity with HIT antibodies of generic enoxaparins versus Lovenox®. In the presence of a cross-reactivity of the originator enoxaparin with heparin-PF4-IgG immune complex, there is also a cross-reactivity of the 7 studied generic enoxaparins. The intensity of cross-reactivity is an additional criterion for comparison of generic LMWHs and the originator one. The differences on the intensity of cross reactivity follow the same pattern in the case of border line cross reactivity of the originator enoxaparin.

Protein Z Dependent Protease Inhibitor Inhibition of Factor XIa Is Accelerated by Unfractionated Heparin in the Presence and Absence of Protein Z: Further Evidence of the Potential Significance of XIa Inhibitory Activity.

Protein Z dependent protease inhibitor (ZPI) is a human plasma serpin that inhibits factors Xa and XIa in the coagulation pathway. Kinetic studies have shown that ZPI is an efficient inhibitor of factor Xa. This interaction is accelerated by protein Z in the presence of calcium and phospholipid. The inhibition of factor XIa is by an alternative mechanism that does not require phospholipid but is influenced by both heparin and protein Z; heparin increases inhibition whereas protein Z /calcium delays inhibition. The latter maybe physiologically relevant to XIa inhibitory activity as ZPI and protein Z circulate in complex in plasma, with ZPI in slight excess. The kinetics of ZPI/XIa/heparin interactions have not been fully characterised. In this study we report the kinetic analysis of the interaction between factor XIa and recombinant ZPI expressed in Escherichia coli, in the presence of unfractionated heparin, pentasaccharide and protein Z.Results: Recombinant ZPI (rZPI) had characteristics similar to native protein as previously described; serpin-protease complexes were non-covalent, and reactive centre loop proteolytic cleavage of ZPI was noted during this reaction. On SDS gel rZPI had a molecular weight of 50kDa. Circular dichroism of rZPI showed an unfolding transition on thermal denaturation typical of a serpin in native conformation with a Tm of 58.8°C. In contrast loop cleaved rZPI had increased thermal stability as expected for a serpin. A discontinuous method was used to measure the rate of inhibition of target proteases by rZPI; the first order rate constant was measured over time under pseudo first order conditions, and was used to derive the second order rate constant (ka) as shown in Table 1.Table 1: Second order rate constants (ka)Xa (Ms−1)XIa (Ms−1)ZPI1.9 x 1039.0 x 104ZPI + protein Z*6.6 x 1051.8 x 104ZPI + unfractionated heparin (1IU/mL)-1.0 x 106ZPI + pentasaccharide (500nM)-1.0 x 105ZPI + protein Z* + unfractionated heparin (1IU/mL)-6.0 x 105* And calcium/phospholipid vesiclesIn the presence of calcium and phospholipid vesicles the ka for Xa in the absence and presence of protein Z was similar to previous reports. The ka for ZPI inhibition of XIa increased 11 fold in the presence of unfractionated heparin. Heparin titration curves suggest that a template mechanism is likely; the heparin pentasaccharide had no impact on inhibition rate consistent with this observation. It has previously been reported that protein Z impedes the ZPI inhibition of XIa; our results support this with a 5-fold reduction in ka. However the addition of heparin to ZPI in the presence of Protein Z leads to a significant 33-fold increase in inhibitory activity with a rate similar to the inhibitory activity of ZPI against Xa in the presence of protein Z.Conclusions: There are several physiological inhibitors of factor XIa. In previous reports ZPI inhibition only increased 2-fold in the presence of heparin. Our results confirm that heparin increases the ZPI inhibition of XIa but with a significantly higher increase than previously reported. The increase in activity is even more marked in the presence of protein Z. Although protein Z alone inhibits ZPI activity against XIa the combination with heparin achieves a 33 fold increase in activity. Of all the serpin inhibitors of XIa, antithrombin is considered a major inhibitor in the presence of heparin. Our results show that the ka for FXIa inhibition in the presence of ZPI, protein Z and heparin is more than 20 fold higher than the ka reported for the antithrombin/heparin inhibition of XIa (2-3x104Ms−1). We have previously reported a higher incidence of venous thrombosis in patients with nonsense mutations in the ZPI gene suggesting the clinical significance of this protein. The suggestion that the inhibition of XIa maybe more relevant than that of Xa has been raised in recent murine work where ZPI knockouts have a more severe phenotype than protein Z knockouts. ZPI inhibition of XIa in the presence of endogenous heparins maybe an important contributor to its physiological significance.

Identification of Surface Epitopes of Human Coagulation Factor Va That Are Important for Interaction with Activated Protein C and Heparin

Inactivation of factor Va (FVa) by activated protein C (APC) is a key reaction in the down-regulation of thrombin formation. FVa inactivation by APC is correlated with a loss of FXa cofactor activity as a result of three proteolytic cleavages in the FVa heavy chain at Arg306, Arg506, and Arg679. Recently, we have shown that heparin specifically inhibits the APC-mediated cleavage at Arg506 and stimulates cleavage at Arg306. Three-dimensional molecular models of APC docked at the Arg306 and Arg506 cleavage sites in FVa have identified several FVa amino acids that may be important for FVa inactivation by APC in the absence and presence of heparin. Mutagenesis of Lys320, Arg321, and Arg400 to Ala resulted in an increased inactivation rate by APC at Arg306, which indicates the importance of these residues in the FVa-APC interaction. No heparin-mediated stimulation of Arg306 cleavage was observed for these mutants, and stimulation by protein S was similar to that of wild type FVa. With this, we have now demonstrated that a cluster of basic residues in FVa comprising Lys320, Arg321, and Arg400 is required for the heparin-mediated stimulation of cleavage at Arg306 by APC. Furthermore, mutations that were introduced near the Arg506 cleavage site had a significant but modest effect on the rate of APC-catalyzed FVa inactivation, suggesting an extended interaction surface between the FVa Arg506 site and APC.