Potential for synthetic phospholipids as partial platelet substitutes

Characterisation of a novel thrombomodulin c.1487delC,p.(Pro496Argfs*10) variant and evaluation of therapeutic strategies to manage the rare bleeding phenotype

INTRODUCTION

A novel variant in the thrombomodulin (TM) gene, c.1487delC,p.(Pro496Argfs*10), referred to as Pro496Argfs*10, was identified in a family with an unexplained bleeding disorder. The Pro496Argfs*10 variant results in loss of the transmembrane and intracellular segments of TM and is associated with an increase in soluble TM (sTM) in the plasma. The aim of this study was to characterise the effect of elevated sTM on thrombin generation (TG) and fibrinolysis, and to evaluate therapeutic strategies to manage the patients.

METHODS

Plasma samples were obtained from two patients carrying the variant. TG was triggered using 5 pM tissue factor and measured using the Calibrated Automated Thrombogram. A turbidity clot lysis assay was used to monitor fibrinolysis. TM antigen was quantified by ELISA.

RESULTS

Patients with the Pro496Argfs*10 variant had significantly elevated plasma sTM compared to controls (372.6 vs. 6.0 ng/ml). TG potential was significantly lower in patients but was restored by inhibition of activated protein C (APC) or addition of activated Factor VII (FVIIa) or platelet concentrates. In vitro experiments suggested that activated prothrombin complex concentrates (APCC) posed a risk of thrombosis. The time to 50% lysis was significantly prolonged in patients compared to controls, 69.7 vs. 42.3 min. Clot lysis time was shortened by inhibition of activated thrombin activatable fibrinolysis inhibitor (TAFIa).

CONCLUSIONS

Our data demonstrate that increased sTM enhances APC generation and reduces TG. Simultaneously, the rate of fibrinolysis is delayed due to increased TAFI activation by sTM. Treatment with platelet or FVIIa concentrates may be beneficial to manage this rare bleeding disorder.

Direct interaction of fibrinogen with lipid microparticles modulates clotting kinetics and clot structure

Extensive studies revealed the role of blood lipid microparticles (liposomes, microvesicles) in activation of coagulation cascade. The direct interaction of fibrinogen/fibrin with lipid surfaces and its consequence for hemostasis received much less attention. We observed pronounced changes in both clot morphology and kinetics of fibrin clotting in the presence of artificial liposomes. The evidence was obtained that lipid microparticles per se present a diffusion barrier to the three-dimensional fibril assembling and pose spatial restrictions for fiber elongation. On the other hand, fibrinogen adsorption results in its high local concentration on liposome surface that accelerates fibrin polymerization. Adsorption induces Fg secondary structure alterations which may contribute to the abnormal clot morphology. In dependence on lipid composition and size of microparticles, the interplay of all the outlined mechanisms determines functionally important changes of clot morphology. The obtained results contribute to the knowledge of clotting mechanisms in the presence of artificial and natural lipid microparticles.

Alternatives to standard blood transfusion: availability and promise

Largely due to concerns over safety, a wide variety of alternatives to the conventional blood bank products of red cells, platelet concentrates, plasma and fractionated plasma products are under development. This review attempts to survey the alternative therapies that are being developed, whether they provide viable solutions and what impact they might have on transfusion practice.