A Novel Beneficial Role for Macitentan in Patients at High Thrombosis Risk: Attenuation of Platelet-Mediated Derangements of Blood Coagulation in Eisenmenger Syndrome

Introduction

Eisenmenger syndrome is a rare manifestation of congenital cardiac disease with pulmonary hypertension. Potentially dangerous thrombotic and haemorrhagic complications frequently occur in patients with ES and raise significant clinical dilemmas. Underlying mechanisms are unknown but are thought to be linked to the underlying pulmonary vascular disease. Endothelin-1 (ET-1) signalling in the pulmonary vasculature is implicated in the pathogenesis of ES and Macitentan (a novel dual ET-1 receptor antagonist) has been shown to improve clinical outcomes. However the effect of Macitentan on reversing abnormal blood coagulation is unknown.

We aimed to characterise derangements in physiological procoagulant and anticoagulant pathways in ES and to assess the effect of Macitentan therapy in modulating haemostasis in this disorder. We hypothesized that an enhanced understanding of pro-and anticoagulant mechanisms in these patients has the potential to improve risk stratification and targeting of antithrombotic therapy to patients at high risk of thrombosis.

Methods

Blood was collected from subjects with ES and age/gender-matched healthy controls into citrated tubes, some containing corn trypsin inhibitor (CTI, a contact pathway inhibitor; final concentration 50µg/mL).

Parameters of plasma thrombin generation in platelet-rich and platelet-poor plasma were assessed by calibrated automated thrombography (CAT) in the presence or absence of exogenous tissue factor (TF), anionic phospholipid (PL) and activated protein C (APC).

Results

Platelet-dependent blood coagulation measured by CAT was enhanced in ES platelet-rich plasma (PRP) relative to healthy volunteers despite lower platelet counts: peak thrombin generation (TG) was significantly higher in ES subjects v controls in PRP (230.6±10.9 vs 188.3±13.1 nM; p=0.025) and lagtime to TG (6.1±0.3 vs 7.8±0.5 mins, p=0.005), time to peak TG (11.5±0.5 vs 15.2±0.6 mins, p=0.0003) and TG velocity index (43.9±3.7 vs 26.3±2.4 nM/min, p=0.001) were significantly accelerated. No significant differences in TG parameters in PPP were observed between groups suggesting a critical mechanistic role for platelet activity in supporting hypercoagulability in ES. TG in PRP was also found to be significantly enhanced in ES in the presence of CTI, suggesting that in vitro or in vivocontact pathway activation does not contribute to the enhanced coagulation activation observed in this disorder.

The anticoagulant activity of APC is PL dependent and in healthy PPP incubated with exogenous PL (as a substitute for platelet PL), TG was potently inhibited. Conversely, in healthy PRP (in the absence of exogenous PL), APC activity was attenuated. Remarkably, this effect was not observed among the ES subjects, where APC anticoagulant activity was significantly increased in PRP, with significantly lower residual peak plasma thrombin generation and endogenous thrombin potential relative to that observed in healthy volunteer PRP. APC anticoagulant activity in PPP (both in the presence and absence of exogenous PL) was similar between groups. These findings suggest that platelets also modulate the activity of the APC pathway in ES, but in a manner which could promote bleeding.

Among the 7 subjects who commenced macitentan therapy, at 6 months peak TG in PRP was significantly lower (234.5±9.8 vs 204.6±14.1 nM, p=0.002) and the enhanced sensitivity to APC in PRP was also attenuated. No significant difference in any parameter of TG was observed in PPP following macitentan therapy.

Conclusion

Platelet activity appears to contribute significantly to hypercoagulability in ES but also appears to modulate the activity of endogenous anticoagulant pathways reflecting the clinical phenotype where bleeding and thrombotic risks frequently co-exist. Our findings suggest that targeted pharmacological agents such as the dual endothelin-1 antagonists (which crucially do not confer a bleeding risk) may attenuate platelet-mediated derangements in blood coagulation. Given the significant clinical dilemmas posed by the presence of competing thrombotic and haemorrhagic tendencies in ES, these finding are likely to be of significant translational relevance.