Postpartum hemorrhage (PPH) is the leading cause of maternal death worldwide. The World Maternal Antifibrinolytic trial showed that antifibrinolytic tranexamic acid (TXA) reduces PPH deaths. Maternal anemia increases the risk of PPH. The World Maternal Antifibrinolytic-2 trial is now assessing whether TXA can prevent PPH in women with anemia. Low red blood cell (RBC) counts promote fibrinolysis by altering fibrin structure and plasminogen activation. We explored interactions between RBCs and TXA in inhibiting fibrinolysis. We used global fibrinolytic assays (ball sedimentation and viscoelasticity) to monitor the lysis of fibrin containing plasminogen and tissue-type plasminogen activator. We applied a fluorogenic kinetic assay to measure plasmin generation in fibrin clots and scanning electron microscopy to study fibrin structure. According to parallel-line bioassay analysis of the fibrin lysis-time data, the antifibrinolytic potency of 4-128 μM TXA was increased in the presence of 10% to 40% (v/v) RBCs. Global fibrinolysis assays showed that the joint effect of RBCs and TXA was about 15% larger than the sum of their individual effects in the inhibition of fibrinolysis. In plasminogen activation, TXA added the same increment of inhibition to the effect of RBCs at any cell count in the fibrin clot. Regarding fibrin structure, TXA thickened fibrin fibers, which impaired plasminogen activation, whereas RBCs promoted fine fibers that were more resistant to plasmin. The antifibrinolytic potency of TXA is enhanced in fibrin formed in the presence of RBCs through inhibition of plasminogen activation and fibrin lysis, which correlates with modifications of fibrin structures.

Background: Tranexamic acid (TXA) is a synthetic lysine analog widely used as an antifibrinolytic agent. Large randomized trials have demonstrated life-saving benefits when TXA is administered early in acute hemorrhage, but results regarding prophylactic administration have been conflicting, and several trials have not shown improved clinical outcomes. The mechanisms underlying this discrepancy remain incompletely understood. Objectives: To investigate the molecular and structural mechanisms that determine TXA efficacy in purified fibrin clots under conditions mimicking therapeutic versus prophylactic administration. Methods: We examined fibrinolysis induced by tissue plasminogen activator (tPA) in vitro using confocal microscopy, viscoelastic testing (ClotPro), turbidimetry, and plasmin generation assays at physiologically and therapeutically relevant concentrations of plasminogen and TXA. Scanning electron microscopy (SEM) was employed to assess fibrin structure. Results: When TXA was incorporated into fibrin clots before the addition of tPA, physiological plasminogen concentrations (2.5 µM) reversed the antifibrinolytic effect, resulting in paradoxical acceleration of lysis. By contrast, when clotting and fibrinolysis occurred simultaneously in the presence of TXA and tPA, TXA consistently prolonged lysis time irrespective of plasminogen concentration. SEM demonstrated that TXA, even at concentrations as low as 16 µM, doubled the top-quartile values of the fibrin fiber diameter, altering susceptibility to plasmin-mediated degradation without accelerating plasminogen activation. Conclusions: TXA efficacy is determined not only by dose but also by timing and the plasminogen availability in the clot microenvironment. These findings provide mechanistic insight into the failure of prophylactic TXA administration and highlight the importance of context in optimizing its clinical use.