br Discussion The initial reduction and the subsequent incre

Discussion The initial reduction and the subsequent increase in permeability proposes a regulatory process to control bleeding. However, following repeated injury, or during long-term presence of the inflammatory stimuli, these mechanisms may become defective and even detrimental. The expression of TF is a hallmark of chronic inflammatory conditions but TF-fVIIa complex is not always fully active and can exist in an encrypted or partially active format [[47], [48], [49], [50], [51], [52]]. In our experiments, either recombinant TF, or TF contained within microvesicles derived from a cell line did not affect the endothelial permeability. Intriguingly, the inclusion of fVIIa with the TF also was not capable of eliciting similar responses to fXa. Previously we showed that TF-containing microvesicles from MDA-MB-231 squalene epoxidase begin to be assimilated within the endothelial cell membrane within 30 min of incubation [55]. To ensure sufficient intervals cell permeability was analysed at 60 and 120 min following induction. This finding suggest that the presence of TF per se is not sufficient to induce such alterations in permeability which in turn may escalate into thrombus formation [56,57]. Moreover, this observation questions the role of plasma seepage arising from endothelial permeability, as the main cause of distal thromboembolism, for example in cancer patients who have detectable amounts of circulating TF-containing microvesicles. However, the possibility of the involvement of a ternary complex TF-fVIIa-fXa is not dismissed and it is feasible that once sufficient fXa is generated by the coagulation mechanism, the induction of permeability may be enhanced by TF-fVIIa-fXa complex formation. Regardless, the use of anti-fXa agents such as direct oral anticoagulants may be effective in limiting the vascular permeability and oedema formation.
Authors' contributions
Acknowledgements
Anticoagulants played a key role in the prevention and treatment of thromboembolic diseases, such as venous thromboembolism (VTE), atrial fibrillation (AF), and acute coronary syndrome (ACS). Factor Xa (FXa) is well known to play a pivotal role in coagulation reactions, and FXa inhibitors as a promising drug candidate attracted more and more attention of anticoagulant drug developers., , , In the past decades, natural products and its derivatives have long been the great source for drug development, and more than fifty percent of new drugs were derived from natural products with promising therapeutic properties, including anticoagulation. Therefore, natural products should be the foremost in research efforts to develop new FXa inhibitors. Plants of the genus (Compositae) are annual herbs, widely distributed in China. Their aerial parts have long been used as a traditional Chinese medicine to treat hypertension, rheumatic arthritis, malaria, and snakebites., Previous pharmacological studies indicated that diterpenoids separated from showed good activities in anti-inflammatory, immunosuppressive, anti-obesity, as well as antithrombosis capabilities., , , , , , , , Among them, kirenol () and darutigenol () are the main bioactive -pimarane diterpenoids separated from . Strobane diterpenoids are an unusual class of 6/6/7 tricarbocyclic skeleton diterpenoid. There are only five naturally occurring strobane diterpenoids reported in the literature, three strobane diterpenoids were separated and identified from the cortex oleoresin of eastern white pine ( L.),, , and the other two compounds, strobols A() and B() (Chart 1), were isolated in our pre-work from the aerial parts of s pubescens Makino. However, there are no significant pharmacological activity screening were reported up to now. To find more derivatives of kirenol and darutigenol with good activities, the structure modification of these two compounds were undertaken led to the synthesis of two -norstrobane diterpenoids strobols C and D. The structures of strobols C and D resembled to those of strobols A and B. The synthesis of these two compounds could not only certify the rationality of the biosynthesis pathway for strobols A and B, but also enrich the types of strobane diterpenoids for pharmacological activity studies. All compounds were evaluated for their inhibition on factor Xa. Compounds , , and showed significant inhibitory activities with IC values of 1067 ± 164.5, 81.22 ± 11.48, 1023 ± 89.38 nM, respectively. The synthesis of these compounds and their inhibition on factor Xa (FXa) are presented in this paper.