Rivaroxaban Reduces Arterial Thrombosis by Inhibition of FXa-Driven Platelet Activation via Protease Activated Receptor-1

RATIONALE

A reduced rate of myocardial infarction has been reported in patients with atrial fibrillation treated with FXa (factor Xa) inhibitors including rivaroxaban compared with vitamin K antagonists. At the same time, low-dose rivaroxaban has been shown to reduce mortality and atherothrombotic events in patients with coronary artery disease. Yet, the mechanisms underlying this reduction remain unknown.

OBJECTIVE

In this study, we hypothesized that rivaroxaban's antithrombotic potential is linked to a hitherto unknown rivaroxaban effect that impacts on platelet reactivity and arterial thrombosis.

METHODS AND RESULTS

In this study, we identified FXa as potent, direct agonist of the PAR-1 (protease-activated receptor 1), leading to platelet activation and thrombus formation, which can be inhibited by rivaroxaban. We found that rivaroxaban reduced arterial thrombus stability in a mouse model of arterial thrombosis using intravital microscopy. For in vitro studies, atrial fibrillation patients on permanent rivaroxaban treatment for stroke prevention, respective controls, and patients with new-onset atrial fibrillation before and after first intake of rivaroxaban (time series analysis) were recruited. Platelet aggregation responses, as well as thrombus formation under arterial flow conditions on collagen and atherosclerotic plaque material, were attenuated by rivaroxaban. We show that rivaroxaban's antiplatelet effect is plasma dependent but independent of thrombin and rivaroxaban's anticoagulatory capacity.

CONCLUSIONS

Here, we identified FXa as potent platelet agonist that acts through PAR-1. Therefore, rivaroxaban exerts an antiplatelet effect that together with its well-known potent anticoagulatory capacity might lead to reduced frequency of atherothrombotic events and improved outcome in patients.

Role of RXRβ in platelet function and arterial thrombosis

OBJECTIVE

Retinoid X receptors (RXR) are a family of nuclear receptors that play critical roles in the regulation of numerous fundamental biological processes including cell proliferation, differentiation, and death. Earlier studies suggested that treatment with RXR agonists attenuates platelet activation in all adults (male and femal) and mice; however, the underlying molecular mechanisms have remained insufficiently understood. To elaborate further on this issue, we characterized megakaryocyte and platelet-specific RXR knockout mice to study platelet function in vitro and arterial thrombosis in vivo.

APPROACH AND RESULTS

First, we identified RXRβ as the dominant RXR receptor in mouse platelets, prompting us to generate a megakaryocyte and platelet-specific PF4Cre ;RXRβflox/flox mouse. Second, we studied activation, spreading, and aggregation of platelets from C57Bl/6 wild-type mice (WT), PF4Cre+ ;RXRβflox/flox mice, and PF4Cre- ;RXRβflox/flox littermate controls in the presence or absence of RXR ligands, that is, 9-cis-retinoic acid (9cRA) and methoprene acid (MA). We found that in vitro treatment with RXR ligands attenuates spreading and aggregation of platelets and increases proplatelet particle formation from megakaryocytes (MK). However, these effects are also observed in RXRβ-deficient platelets and MKs and are thus independent of RXRβ. Third, we investigated arterial thrombus formation in an iron chloride (FeCl3)-induced vascular injury model in vivo, which is also not affected by the absence of RXRβ in platelets.

CONCLUSIONS

Absence of the most abundant RXR receptor in mouse platelets, RXRβ, does not affect platelet function in vitro and thrombus formation in vivo. Furthermore, RXR agonists' mediated effects on platelet function are independent of RXRβ expression. Hence, our data do not support a significant contribution of RXRβ to arterial thrombosis in mice.

[Important metabolic parameters in the peripartum period as affected by peridural anesthesia]

In a clinical study we compared two groups of healthy patients at term: - 10 patients received no analgesics or very small doses of pethidine (control group) during the course of labour, - 17 patients were given CO2-bupivacaine via epidural catheter because they asked for it and because their obstetricians prescribed it (CEDA-group). After giving the same infusion solution of 120 ml/h 5% half-isotonic fructose to all the parturients, the following biochemical parameters were measured at the beginning and at the end of the first stage of labour, at delivery, and two hours later: Blood gases and acid-base status, blood sugar, lactate, betahydroxybutyric acid, ACTH, cortisol, hematocrit, electrolytes, and serum osmolality. The above mentioned parameters, except electrolytes and serum osmolality, were also determined in umbilical-cord blood immediately after delivery. In the labour ward, infants were observed and their capillary blood gases, acid-bases status, and blood sugar were measured 30, 60, and 120 minutes after birth. Lactate, betahydroxybutyric acid, ACTH, and cortisol levels rose significantly until delivery in both of the groups; significant differences between the two groups could be seen in blood gases, blood sugar, and ACTH levels. In the umbilical cord there were only significant differences in blood sugar. In summary it can be concluded that although labour pain can be controlled by epidural analgesia, the stress of labour is only influenced by different analgesic methods to a certain degree.

Untersuchungen zum Verhalten wichtiger metabolischer Parameter in der Peripartalperiode unter dem Einfluß der Periduralanästhesie

In einer klinischen Studie wurden zwei Gruppen von kreislauf- und stoffwechselgesunden Patientinnen am Geburtstermin verglichen. 10 Patientinnen erhielten zur Schmerzbekämpfung über den Geburts-verlauf keine Analgetika Oder Minidosen von Pethidin (Kontroll-gruppe), 17 Patientinnen auf eigenen Wunsch und Ordination des Geburtshelfers eine kontinuierliche PDA mit fraktionierten Dosen von Bupivacain-CCh (PDA-Gruppe). Bei gleichem Infusionsregime, 120 ml/h einer 5 % halbisotonen Laevuloselösung, wurden zu Beginn der Eröffnungsperiode, der Austreibungsperiode, zum Geburtszeitpunkt und 2h postpartal bei der Mutter folgende biochemische Parameter bestimmt: Blutgase und Säuren-Basen-Status, Blutzucker, Laktat, Betahydroxybuttersäure, ACTH, Cortisol, Hk, Elektrolyte und Osmolalität. Unmittelbar postpartal wurden die oben angegebenen Werte, außer Elektrolyten und Osmolalität, im kindlichen Nabelschnurblut untersucht. 30, 60 und 120 min post partum diente die kapilläre Blutabnahme von Blutgasen, Säuren-Basen-Status und Blutzucker beim Neugeborenen der Beurteilung des postpartalen Adaptationszustandes. In beiden Patientengruppen stiegen Laktat, Betahydroxybuttersäure, ACTH und Cortisol bis zur Geburt signifikant an, signifikante Unterschiede zwischen beiden Gruppen dagegen konnten nur bei den Blutgasen, den Blutzucker- und ACTH-Werten errechnet werden. In der kindlichen Nabelschnur waren nur beim Blutzucker signifikante Unterschiede festzustellen. Als Fazit kann man sagen, daß man den Müttern in der langen Eröffnungsperiode den Geburtsschmerz wirksam nehmen, den Geburtsstreß aber, dokumentiert durch die oben aufgeführten Stoffwechsel-parameter, offensichtlich nur bis zu einem gewissen Grad.