Anticoagulant and platelet aggregating activities of heparin

Heparin-Induced Thrombocytopenia: Prevalence in a Large Cohort of Patients and Confirmed Role of PF4/Heparin Complex as the Main Antigen for Antibodies

We performed a retrospective study on the prevalence of heparin-induced thrombocytopenia (HIT) in 233 patients receiving hog mucosa heparin therapy. Of these, 82 patients received s.c. calcium heparin, 130 patient received unfractionated (UF) i.v. heparin, and 21 patients received low molecular weight heparins (LMWH). An additional four patients, referred to our consultation and diagnosed by us as having clinically active type II HIT (HIT-II) were also studied. The mean platelet count of the 233 patients receiving heparin showed a significant decrease after 2 days of heparin treatment and a following significant increase 6 days later (basal: 257 ± 147 x 109 platelets/L; day 2: 239 ± 122, p < 0.0002; day 6: 286 ± 119, p < 0.004). Of the 212 patients receiving UF heparin, 13 (6%) fulfilled the criteria for HIT-II: seven of these had received i.v. heparin (mean daily dose 26,600 ± 4,082 IU ± SD) and six had received s.c. heparin (mean daily dose 21,428:t 6,900 IU). Their mean basal platelet count was 226 ± 100 SD × 109 platelets/L and the nadir during heparin treatment was 78 ± 39 x 10 9 platelets/L. Thrombotic complications occurred in four (30.7%) of the 13 patients with HIT-II. Since the immunological mechanism has been demonstrated for HIT-II and since platelet factor 4 (PF4) was identified as the co-factor for the binding of heparin-related antibodies, we set up our own enzyme-linked immunosorbent assay (ELISA) for testing antibodies against PF4/heparin complex bound through electrostatic bridges to the solid phase. The highest binding capacity of HIT-related IgG to the multimolecular complex was obtained at 20 μg/ml for PF4 and 3 μg/ml for heparin, corresponding to 250 ng of PF4 and 42 ng of heparin in each microtiter well. Such binding was inhibited in a dose-dependent manner by increasing amounts of heparin, protamine hydrochloride, and a monoclonal antibody anti-human PF4 clone 1OB2. We observed that HIT-related antibodies bound also to PF4/LMWH complexes but the optimal PF4/glycosaminoglycan ratio appeared more critical for LMWH (enoxaparin, fraxiparin, and pamaparin) than for UF heparin. Sera from eight patients with HIT-II were tested by PF4/heparin ELISA; six of these had IgG against the complex PF4/heparin and three also had IgM. The persistence of HIT-related antibodies was investigated in three patients: in one such antibodies were still detectable 3 years after the acute episode, while in the other two, they disappeared after 6 months and 1 year, respectively. Key Words: Heparin-related anti body—Platelet factor 4 (PF4)—Heparin—Low molecular weight heparin—Thrombocytopenia—Thrombosis.

Heparin-induced thrombocytopenia: historical review

Heparin-induced thrombocytopenia (HIT) and heparin-induced thrombosis (HITT), recognized as clinical entities only in the past 30 years, have become a major concern for all who work in the field of thromboembolism, its prevention, and treatment. Studies of the 1930 to 1970 era were primarily devoted to the interaction of heparin with platelets in vivo with no knowledge that such interaction could lead to serious morbidity and mortality in as many as 3-5% of patients receiving the drug. When this was recognized in the 1970s, concerns arose regarding recognition and diagnoses. Clinicians, laboratory investigators, and drug companies entered into many research collaborations. The advent of low molecular weight heparin (LMWH) and heparinoids led to studies of their interaction with the IgG antibody that was identified as the mechanism for heparin-induced thrombocytopenia of the immune type. Newer, more sensitive test systems were developed and applied to studies comparing the new low LMWHs and heparinoids. These studies showed a lower incidence of immune-mediated thrombocytopenia with LMWH. The specificity of the antibody for platelet factor 4-heparin complexes led to the development of new tests in the early 1990s. This was soon followed by experiments with antithrombin drugs such as hirudin and argatroban as replacements for heparin in the HIT-HITT syndrome. Shorter courses of heparin therapy and the use of LMWH in place of heparin had become more standard practice in the 1990s.

The effects of heparin coating of oxygenator fibers on platelet adhesion and protein adsorption

Platelet adhesion on the cardiopulmonary bypass oxygenator membrane is associated with impaired hemostasis. We investigated the effects of heparin coating of the oxygenator membrane on protein adsorption and platelet adhesion on the surface. Noncoated and heparin-coated polypropylene membranes were incubated in whole blood with small- (1 U/mL) or large-dose (5 U/mL) heparin as an anticoagulant for 3 h at 37 degrees C. The amount of platelets adhering on each fiber was assessed by using enzyme immunoassays using monoclonal antibodies directed against CD42b (GP Ib) and CD61 (GP IIb/IIIa). Platelet activation was assessed by measuring plasma guanosine monophosphate 140 levels. The amount and composition of the adsorbed proteins on the surface were analyzed by using a bicinchoninic acid protein assay and by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting technique. The heparin coating of the fibers significantly reduced platelet adhesion on the surface. However, platelet activation was reduced by heparin coating only with small-dose heparinization. The adsorption of platelet adhesive proteins such as fibrinogen and von Willebrand factor was not altered, whereas that of fibronectin was increased by heparin coating. We conclude that heparin coating of the oxygenator fibers can decrease platelet adhesion without affecting adsorption of major adhesive proteins. Surface heparin coating is associated with an increased fibronectin adsorption on the fibers.

IMPLICATIONS

Heparin coating can reduce platelet adhesion and activation in the presence of small-dose heparinization, potentially reducing the inflammatory response and activation of thrombosis and fibrinolysis.

Characterization of factors affecting the stability of frozen heparinized plasma

The use of heparin rather than citrate as primary anticoagulant has been shown to significantly improve the initial activity, stability and recovery of factor VIII:C from human plasma, cryoprecipitates or factor VIII concentrates if the plasma was initially frozen at -80 degrees C and subsequently stored at this temperature. If frozen and stored at progressively warmer temperatures however, increasing amounts of insoluble protein aggregates, termed storage precipitates (SPs), were recovered in the thawed plasma and cryoprecipitate fractions. Plasma recovery by centrifugation at 7,000 g for 7 min [Method I (MI)], 2 x 10 min (MII) or 15 min (MIII) had little effect on SP formation after 1 month at any storage temperature. After 4 months at -20 degrees C, more SP was recovered from MIII plasma whereas at -40 degrees C, more SP was recovered from MI plasma. Also, the preparation method had little or no effect on factor VIII:C activity at equivalent storage times or temperatures. A trend towards improved factor VIII recoveries was noted at lower freezing and storage temperatures however. SP formation was associated with reduced fibrinogen levels in the recovered plasma without loss of antithrombin-III or increased fibrinopeptide-A. Western blots showed polymerization of A alpha or gamma-chains of fibrinogen. SP formation was reduced or eliminated with factor XIII inhibitors, antibody to the active factor XIII a subunit or adjustment of heparinized plasma to 5-10 mM sodium citrate before initial freezing and storage. Although plasma factor VIII:C recoveries were only slightly affected at these citrate concentrations under most conditions, its recovery in cryoprecipitates was substantially improved owing to the reduction or absence of SPs.

Protective role of heparin on in vitro functional aortic response in Watanabe heritable hyperlipidemic rabbits

The effects of prolonged in vivo heparin treatment upon vasomotor responses and content of cholesterol and energy related compounds were studied in isolated thoracic and abdominal aortas from Watanabe heritable hyperlipidemic (WHHL) rabbits. Unfractionated heparin was administered subcutaneously (2 mg/kg twice a day) to 3-month-old WHHL rabbits for a period of 6 months. A group of WHHL rabbits was treated with saline solution and considered as control. Aortic cholesterol infiltration and serum cholesterol were not significantly decreased by the prolonged heparin treatment. In heparin-treated WHHL rabbits, the in vitro aortic endothelium-dependent relaxation produced by acetylcholine or calcimycin (A 23187) was greater than in saline-treated WHHL group. ATP-induced aorta relaxation (endothelium-dependent and endothelium-independent) did not vary significantly in the two groups of WHHL rabbits, even after mechanical removal of endothelium. Also the noradrenaline-induced aorta contraction did not vary between the two groups of WHHL rabbits. No significant variation in energy-related compounds (except for ADP) was found in the aortic arch. These results suggest that heparin produces a protective effect on aortic tissue by acting mainly at endothelial level.

Dose- and shear rate-dependent effects of heparin on thrombogenesis induced by rabbit aorta subendothelium exposed to flowing human blood

The effect of heparin on thrombogenesis induced by the subendothelium of rabbit aorta was investigated in 24 healthy volunteers after intravenous injection of different doses (0, 1000, 2500, and 5000 IU). By using an ex vivo perfusion chamber system, the interaction between flowing blood and exposed subendothelium was measured at low (50 s-1), intermediate (650 s-1), and high (2600 s-1) wall shear rates. The low shear rate simulated blood flow in venous, the intermediate shear rate in arterial, and the high shear rate in small or stenosed arterial vessels. Deposition of fibrin, platelets, and platelet thrombi on vascular subendothelium (SE) was quantified by morphometrical and immunological techniques. Fibrin deposition prevailed at low shear rates and was only minimal at high shear rates (30 +/- 1% vs. 1 +/- 0.4% coverage of SE with fibrin, means +/- SEM, p less than 0.001). In contrast, the interaction of platelets with SE was more intense at high compared to low shear rates, as indicated by higher platelet adhesion (54 +/- 5% vs. 4 +/- 1% coverage of SE with platelets, p less than 0.001) and platelet thrombus volumes (4.8 +/- 1.3 vs. 0.5 +/- 0.1 microns 3/microns 2, p less than 0.001). Fibrin deposition on SE was inhibited by heparin in a dose-dependent manner and was abolished after high doses. In addition, high doses of heparin reduced the height and volume of platelet thrombi at low and intermediate wall shear rates, but no effect was found at the high shear rate. Our data show that heparin inhibits the formation of both fibrin and platelet thrombi on vascular subendothelium. The lack of effect of heparin on platelet thrombus formation at high shear rates indicates that thrombin modulates the growth rate and/or stability of platelet thrombi at low and intermediate shear rates, whereas additional factors may control platelet thrombus growth and stability at high shear conditions.

Heparin-induced platelet aggregation. II. Dose/response relationships for two low molecular weight heparin fractions (CY 216 and CY 222)

We have previously shown that unmodified heparin (bovine lung or porcine mucosal) and a low molecular weight heparin fraction, PK 10169, cause platelet aggregation in a dose and molecular weight-dependent manner. In this report, we show that two other low molecular weight heparin fractions, CY 216 and CY 222, also cause platelet aggregation in a dose and molecular weight-dependent manner. Utilizing heparin and defined fractions of CY 216 and CY 222 separated on the basis of molecular weight, we determined dose/response (D/R) relationships for each of these agents and their individual fractions. In comparison to an unmodified porcine mucosal heparin, CY 216 yielded a D/R curve that was shifted down and to the right, indicating that this agent is less potent in causing platelet aggregation. The D/R curve for CY 222, which has a lower molecular weight that CY 216, was shifted further down and to the right, indicating that it was less potent than CY 216. The D/R curves obtained with the fractions of CY 216 and CY 222 demonstrate that as the molecular weight of the fractions decrease, they become progressively less potent in causing platelet aggregation. Fractions with molecular weights of less than approximately 3,000 daltons are essentially without activity in causing platelet aggregation. Platelet aggregation studies with CY 216 and CY 222 fractions separated on the basis of affinity for antithrombin III (AT III) indicate that the platelet aggregating activity of these agents may not be related to their affinity for AT III. However, these latter results are not conclusive and need to be expanded.

Heparin-induced thrombocytopenia

There are two types of heparin-induced thrombocytopenia. Type I is more common, has an early onset, and is mild, transient, and benign. Type I is due to direct heparin-induced platelet aggregation and is rarely associated with thromboembolic sequela. Type II is infrequent, has a late onset, and is more severe. Type II is due to an immune-mediated platelet aggregation caused by IgG and IgM that becomes bound to platelets. In Type II, the antibody titers decline over several months; however, early reexposure can result in a catastrophic secondary immune response. Frequently, Type II is associated with life- or limb-threatening thromboembolic complications (white clots), including stroke.

Partially N-desulfated heparin as a non-anticoagulant heparin: some physico-chemical and biological properties

A partially N-desulfated preparation of heparin (UFH) obtained by thermally inactivating heparinic acid for 24 hours at 50 degrees C (TIHA) was examined for its physico-chemical and biological properties in vitro and in vivo. TIHA has a molecular weight of 14,700 and 27% remaining N-sulfate groups (UFH = 17,500; 100% N-sulfate groups). TIHA has no anticoagulant activity measurable by conventional amidolytic or clotting tests. However, in a rabbit stasis-induced thrombosis model and using two different thrombogenic stimuli (Feiba and PCC(Konyne)/RVV), TIHA afforded a dose-dependent (1.0-2.5 mg/kg) protection sufficient to impair thrombosis (UFH: fully effective at 0.13 mg/kg). TIHA did not produce any bleeding at supramaximal antithrombotic dosage in a rat tail bleeding and a rabbit ear blood loss model and it did not augment ADP-induced aggregation of platelets. In contrast, a completely N-desulfated derivative of UFH (Inoue and Nagasawa, Carbohydr. Res. 46, 87-95, 1976) also lacking measurable in vitro activity was completely inactive in vivo. The results in this study suggest that TIHA may be considered as a non-anticoagulant heparin still retaining antithrombotic activity and also with lower haemorrhagic effect than UFH.

Heparin-induced platelet aggregation: dose/response relationships for a low molecular weight heparin derivative (PK 10169) and its subfractions

Addition of heparin or heparin derivatives to citrate anticoagulated platelet-rich plasma caused platelet aggregation in a dose-dependent manner. Utilizing heparin, a low molecular weight heparin derivative (PK 10169) and its various subfractions, we determined dose/response relationships for platelet aggregation and found that the ability of these agents to cause platelet aggregation was dependent upon the molecular weight of the individual subfraction used. In comparison to unmodified porcine mucosal heparin, the lower molecular weight derivative (PK 10169) yielded a dose/response curve that was shifted down and to the right, and indicated that this agent was less potent in causing platelet aggregation. In addition, as the molecular weight of PK 10169 subfractions decreased, their dose/response curves were progressively shifted down and to the right. The lowest molecular weight subfraction was essentially without platelet aggregating activity. We also measured the anti IIa and anti Xa activities of these agents and concluded that these activities did not appear to correlate with platelet aggregating activity. Platelet aggregation studies with PK 10169 subfractions of high and low affinity for antithrombin III (AT III) indicated that the platelet aggregating activity of these compounds may not be related to their affinity for AT III, but results were not definitive.

Heparin-binding platelet proteins demonstrated by crossed affinity immunoelectrophoresis

Platelet proteins that interact with heparin were studied using crossed affinity immunoelectrophoresis. Platelet proteins solubilized in Triton X-100 were applied to crossed immunoelectrophoresis against anti-platelet antibodies, and an intermediate gel containing heparin covalently linked to Sepharose 4B was inserted. Six immunoprecipitates were absent or showed an altered position compared to control immunoplates, indicating that the corresponding antigens were bound to the immobilized heparin. These precipitates represented platelet factor 4, thrombospondin, glycoprotein Ib, and three antigens termed G4, 17 and 25. The subcellular location of the heparin-binding proteins was either in the surface membrane (glycoprotein Ib and the antigens 17 and 25), or in the alpha-granules (platelet factor 4, thrombospondin and G4). both forms of platelet factor 4 appearing after crossed immunoelectrophoresis, i.e. a line-form and a peak-form, bound strongly to the heparin. Glycoprotein Ib showed a weak binding whereas its proteolytic split product glycocalicin did not significantly bind to the heparin in the present system. It is concluded that the platelets contain at least six heparin-binding proteins which are present on the cellular surface or are able to be exposed to the extracellular medium after the release-reaction has occurred.

Normal and low molecular weight heparins: interaction with human platelets

Porcine mucosal heparin was chemically depolymerized. The depolymerization was stopped at different steps to obtain two low molecular weight (LMW) heparins with a molecular weight of 10 000 and 6000, respectively. The LMW heparins were tested in vitro for anti-clotting activities and for platelet serotonin release in different systems in comparison with normal heparins, dermatan and heparan sulphate. After addition of various amounts of heparin preparations to washed platelets, no significant release was observed for all tested heparins. On the contrary, different heparins showed an inhibition of serotonin-release induced by collagen in platelet rich plasma, whereas the ADP-induced release was increased. The effect on the platelet release appears related to the molecular weight. In fact, it is significant only for normal heparins whereas it is not for LMW heparins. A good relation was observed, also, between anti-activated factor X activity/antiglobal clotting activity (Xa/APTT) ratio of different heparins and the effect on platelet release.