Naturally occurring inorganic platelet polyphosphates and polyphosphate nucleic acid scavengers including RNA and DNA have recently attracted considerable attention as procoagulants and prothrombotic agents.1,2 Polyphosphates promote the activation of the contact activation system, which initiates the intrinsic pathway of blood coagulation. Platelet polyphosphate, a dense granule linear polymer containing 60 to 100 orthophosphate groups, binds factor XII and high-molecular-weight kininogen (HMWK), resulting in reciprocal activation of factor XII and prekallikrein. A plethora of other activities associated with polyphosphates has been reported, including activation of factor VII-activating protease, acceleration of factor XI activation by both thrombin and factor Xa, profibrinolytic activity, and acceleration of the production of the inflammatory mediator bradykinin.
Kaplan-Meier graph showing survival of mice pretreated with saline (n=11) or 20 mg/kg PAMAM G-3 (n=12) followed by injection of intravenous collagen/epinephrine to induce pulmonary embolism.Adapted from Jain S et al. Proc Natl Acad Sci USA. 2012;109:12938-12943.
Kaplan-Meier graph showing survival of mice pretreated with saline (n=11) or 20 mg/kg PAMAM G-3 (n=12) followed by injection of intravenous collagen/epinephrine to induce pulmonary embolism.Adapted from Jain S et al. Proc Natl Acad Sci USA. 2012;109:12938-12943.
The identification and characterization of polyphosphate activity has raised new interest in the ongoing puzzle of contact activation. In the clinical laboratory, the intrinsic pathway is probed using the activated partial thromboplastin time (aPTT), which uses artificial, non-physiologic substances such as glass, kaolin, or ellagic acid to activate the contact system.Human deficiencies of factor XII, prekallikrein, or HMWK produce prolonged aPTTs, yet they are not associated with a bleeding diathesis. If in vitro properties of the contact system represent some sort of clinically irrelevant artifact, then do potent contact activators found in platelets or released from cells play physiologic or pathophysiologic roles?
Jain et al. in the laboratory of Bruce Sullenger at Duke University evaluated the anticoagulant and antithrombotic activity of several polyphosphate-binding polymers, including β-cyclodextrin–containing polycation (CDP), hexadimethrine bromide (HDMBr), polyamidoamine dendrimer, 1,4-diaminobutane core, generation 1(PAMAM G-1), PAMAM G-3, and PAMAM-G5. Polyphosphates with an average chain length of 60 and 130 residues were used as procoagulant activators. All of the polyphosphate-binding polymers neutralized the procoagulant activities of PolyP 60 and PolyP 130. PAMAM G-3 was chosen for further study because of its relative potency and reportedly favorable toxicity profile. PAMAM G-3 neutralized the procoagulant properties of PolyP 60 in human whole blood measured by thromboelastography. Binding studies using isothermal titration calorimetry revealed that PAMAM G-3 binds PolyP 60, PolyP 130, CpG, poly I:C, and plasmid DNA with high affinity, displaying dissociation constants ranging from 0.2 to 10 nM. In a murine carotid artery injury model, PAMAM G-3 neutralized the shortening of the carotid artery occlusion time induced by FeCl3. PAMAM G-3 also decreased mortality of mice in a collagen/epinephrine-induced pulmonary thromboembolism model. In contrast, PAMAM G-3 did not increase blood loss in a murine tail transection model under conditions in which standard heparin significantly increased bleeding.
Based on these observations, Jain et al. concluded that PAMAM G-3 is an antithrombotic agent that does not produce a bleeding diathesis. Hemostasis often is viewed as a yin-and-yang balance between hemorrhage and thrombosis. Past rational strategies to identify the perfect antithrombotic agent have not been entirely successful. For example, tissue plasminogen activator initially was proposed as a potential nonhemorrhagic thrombolytic agent because of its fibrin specificity. However, upon clinical evaluation, bleeding became its dose-limiting feature.3 Only time will tell if the contact activation system represents the ideal antithrombotic target.
