Differential vascular endothelial cell toxicity of established and novel BCR-ABL tyrosine kinase inhibitors.

BCR-ABL tyrosine kinase inhibitors (TKIs) have dramatically improved survival in Philadelphia chromosome-positive leukemias. Newer BCR-ABL TKIs provide superior cancer outcomes but with increased risk of acute arterial thrombosis, which further increases in patients with cardiovascular comorbidities and mitigates survival benefits compared to imatinib. Recent studies implicate endothelial cell (EC) damage in this toxicity by unknown mechanisms with few side-by-side comparisons of multiple TKIs and with no available data on endothelial impact of recently approved TKIs or novels TKIs being tested in clinical trials. To characterize BCR-ABL TKI induced EC dysfunction we exposed primary human umbilical vein ECs in 2D and 3D culture to clinically relevant concentrations of seven BCR-ABL TKIs and quantified their impact on EC scratch-wound healing, viability, inflammation, and permeability mechanisms. Dasatinib, ponatinib, and nilotinib, the TKIs associated with thrombosis in patients, all significantly impaired EC wound healing, survival, and proliferation compared to imatinib, but only dasatinib and ponatinib impaired cell migration and only nilotinib enhanced EC necrosis. Dasatinib and ponatinib increased leukocyte adhesion to ECs with upregulation of adhesion molecule expression in ECs (ICAM1, VCAM1, and P-selectin) and leukocytes (PSGL1). Dasatinib increased permeability and impaired cell junctional integrity in human engineered microvessels, consistent with its unique association with pleural effusions. Of the new agents, bafetinib decreased EC viability and increased microvessel permeability while asciminib and radotinib did not impact any EC function tested. In summary, the vasculotoxic TKIs (dasatinib, ponatinib, nilotinib) cause EC toxicity but with mechanistic differences, supporting the potential need for drug-specific vasculoprotective strategies. Asciminib and radotinib do not induce EC toxicity at clinically relevant concentrations suggesting a better safety profile.

Polyphosphate is a novel cofactor for regulation of complement by a serpin, C1 inhibitor.

The complement system plays a key role in innate immunity, inflammation, and coagulation. The system is delicately balanced by negative regulatory mechanisms that modulate the host response to pathogen invasion and injury. The serpin, C1-esterase inhibitor (C1-INH), is the only known plasma inhibitor of C1s, the initiating serine protease of the classical pathway of complement. Like other serpin-protease partners, C1-INH interaction with C1s is accelerated by polyanions such as heparin. Polyphosphate (polyP) is a naturally occurring polyanion with effects on coagulation and complement. We recently found that polyP binds to C1-INH, prompting us to consider whether polyP acts as a cofactor for C1-INH interactions with its target proteases. We show that polyP dampens C1s-mediated activation of the classical pathway in a polymer length- and concentration-dependent manner by accelerating C1-INH neutralization of C1s cleavage of C4 and C2. PolyP significantly increases the rate of interaction between C1s and C1-INH, to an extent comparable to heparin, with an exosite on the serine protease domain of the enzyme playing a major role in this interaction. In a serum-based cell culture system, polyP significantly suppressed C4d deposition on endothelial cells, generated via the classical and lectin pathways. Moreover, polyP and C1-INH colocalize in activated platelets, suggesting that their interactions are physiologically relevant. In summary, like heparin, polyP is a naturally occurring cofactor for the C1s:C1-INH interaction and thus an important regulator of complement activation. The findings may provide novel insights into mechanisms underlying inflammatory diseases and the development of new therapies.

Polyphosphate/platelet factor 4 complexes can mediate heparin-independent platelet activation in heparin-induced thrombocytopenia.

Heparin-induced thrombocytopenia (HIT) is a thrombotic disorder initiated by antibodies to complexes between platelet factor 4 (PF4) and heparin. The risk of recurrent thromboembolism persists after heparin is cleared and platelet activation leading to release of PF4 has dissipated. We asked whether antigenic complexes between polyphosphates and PF4 released from activated platelets might intensify or sustain the prothrombotic phenotype of HIT. PF4 forms stable, ultralarge complexes with polyphosphates of various sizes, including those released from platelets, which are recognized by the HIT-like monoclonal KKO, an immunoglobulin G2bκ monoclonal heparin/PF4 binding antibody, and by human HIT antibodies. KKO helps to protect PF4/polyphosphate complexes from degradation by phosphatases. Complement is activated when HIT antibodies bind to PF4/polyphosphate complexes and PF4 reverses the inhibition of complement by polyphosphates. Polyphosphates and PF4 are stored primarily in separate granules in resting platelets, but they colocalize when the cells are activated. Platelets activated by subaggregating doses of thrombin receptor activating peptide release polyphosphates and PF4, which form antigenic complexes that allow KKO to further activate platelets in the absence of heparin and exogenous PF4. These studies suggest that thrombin- or immune complex-mediated release of endogenous antigenic PF4/polyphosphate complexes from platelets may augment the prothrombotic risk of HIT and perpetuate the risk of thrombosis after heparin has been discontinued.

FXIa and platelet polyphosphate as therapeutic targets during human blood clotting on collagen/tissue factor surfaces under flow.

Factor XIIa (FXIIa) and factor XIa (FXIa) contribute to thrombosis in animal models, whereas platelet-derived polyphosphate (polyP) may potentiate contact or thrombin-feedback pathways. The significance of these mediators in human blood under thrombotic flow conditions on tissue factor (TF) -bearing surfaces remains inadequately resolved. Human blood (corn trypsin inhibitor treated [4 µg/mL]) was tested by microfluidic assay for clotting on collagen/TF at TF surface concentration ([TF]wall) from ∼0.1 to 2 molecules per µm(2). Anti-FXI antibodies (14E11 and O1A6) or polyP-binding protein (PPXbd) were used to block FXIIa-dependent FXI activation, FXIa-dependent factor IX (FIX) activation, or platelet-derived polyP, respectively. Fibrin formation was sensitive to 14E11 at 0 to 0.1 molecules per µm(2) and sensitive to O1A6 at 0 to 0.2 molecules per µm(2). However, neither antibody reduced fibrin generation at ∼2 molecules per µm(2) when the extrinsic pathway became dominant. Interestingly, PPXbd reduced fibrin generation at low [TF]wall (0.1 molecules per µm(2)) but not at zero or high [TF]wall, suggesting a role for polyP distinct from FXIIa activation and requiring low extrinsic pathway participation. Regardless of [TF]wall, PPXbd enhanced fibrin sensitivity to tissue plasminogen activator and promoted clot retraction during fibrinolysis concomitant with an observed PPXbd-mediated reduction of fibrin fiber diameter. This is the first detection of endogenous polyP function in human blood under thrombotic flow conditions. When triggered by low [TF]wall, thrombosis may be druggable by contact pathway inhibition, although thrombolytic susceptibility may benefit from polyP antagonism regardless of [TF]wall.

How it all starts: Initiation of the clotting cascade.

The plasma coagulation system in mammalian blood consists of a cascade of enzyme activation events in which serine proteases activate the proteins (proenzymes and procofactors) in the next step of the cascade via limited proteolysis. The ultimate outcome is the polymerization of fibrin and the activation of platelets, leading to a blood clot. This process is protective, as it prevents excessive blood loss following injury (normal hemostasis). Unfortunately, the blood clotting system can also lead to unwanted blood clots inside blood vessels (pathologic thrombosis), which is a leading cause of disability and death in the developed world. There are two main mechanisms for triggering the blood clotting, termed the tissue factor pathway and the contact pathway. Only one of these pathways (the tissue factor pathway) functions in normal hemostasis. Both pathways, however, are thought to contribute to thrombosis. An emerging concept is that the contact pathway functions in host pathogen defenses. This review focuses on how the initiation phase of the blood clotting cascade is regulated in both pathways, with a discussion of the contributions of these pathways to hemostasis versus thrombosis.

Affinity-based design of a synthetic universal reversal agent for heparin anticoagulants.

Heparin-based anticoagulant drugs have been widely used for the prevention of blood clotting during surgical procedures and for the treatment of thromboembolic events. However, bleeding risks associated with these anticoagulants demand continuous monitoring and neutralization with suitable antidotes. Protamine, the only clinically approved antidote to heparin, has shown adverse effects and ineffectiveness against low-molecular weight heparins and fondaparinux, a heparin-related medication. Alternative approaches based on cationic molecules and recombinant proteins have several drawbacks including limited efficacy, toxicity, immunogenicity, and high cost. Thus, there is an unmet clinical need for safer, rapid, predictable, and cost-effective anticoagulant-reversal agents for all clinically used heparins. We report a design strategy for a fully synthetic dendritic polymer-based universal heparin reversal agent (UHRA) that makes use of multivalent presentation of branched cationic heparin binding groups (HBGs). Optimization of the UHRA design was aided by isothermal titration calorimetry studies, biocompatibility evaluation, and heparin neutralization analysis. By controlling the scaffold's molecular weight, the nature of the protective shell, and the presentation of HBGs on the polymer scaffold, we arrived at lead UHRA molecules that completely neutralized the activity of all clinically used heparins. The optimized UHRA molecules demonstrated superior efficacy and safety profiles and mitigated heparin-induced bleeding in animal models. This new polymer therapeutic may benefit patients undergoing high-risk surgical procedures and has potential for the treatment of anticoagulant-related bleeding problems.

Nontoxic polyphosphate inhibitors reduce thrombosis while sparing hemostasis.

Polyphosphate (polyP) is secreted by activated platelets and has been shown to contribute to thrombosis, suggesting that it could be a novel antithrombotic target. Previously reported polyP inhibitors based on polycationic substances, such as polyethylenimine, polyamidoamine dendrimers, and polymyxin B, although they attenuate thrombosis, all have significant toxicity in vivo, likely due to the presence of multiple primary amines responsible for their polyP binding ability. In this study, we examined a novel class of nontoxic polycationic compounds initially designed as universal heparin reversal agents (UHRAs) to determine their ability to block polyP procoagulant activity and also to determine their utility as antithrombotic treatments. Several UHRA compounds strongly inhibited polyP procoagulant activity in vitro, and 4 were selected for further examination in mouse models of thrombosis and hemostasis. Compounds UHRA-9 and UHRA-10 significantly reduced arterial thrombosis in mice. In mouse tail bleeding tests, administration of UHRA-9 or UHRA-10 was associated with significantly less bleeding compared with therapeutically equivalent doses of heparin. Thus, these compounds offer a new platform for developing novel antithrombotic agents that target procoagulant anionic polymers such as polyP with reduced toxicity and bleeding side effects.

Inhibition of polyphosphate as a novel strategy for preventing thrombosis and inflammation.

Inorganic polyphosphates are linear polymers of orthophosphate that modulate blood clotting and inflammation. Polyphosphate accumulates in infectious microorganisms and is secreted by activated platelets; long-chain polyphosphate in particular is an extremely potent initiator of the contact pathway, a limb of the clotting cascade important for thrombosis but dispensable for hemostasis. Polyphosphate inhibitors therefore might act as novel antithrombotic/anti-inflammatory agents with reduced bleeding side effects. Antipolyphosphate antibodies are unlikely because of polyphosphate's ubiquity and simple structure; and although phosphatases such as alkaline phosphatase can digest polyphosphate, they take time and may degrade other biologically active molecules. We now identify a panel of polyphosphate inhibitors, including cationic proteins, polymers, and small molecules, and report their effectiveness in vitro and in vivo. We also compare their effectiveness against the procoagulant activity of RNA. Polyphosphate inhibitors were antithrombotic in mouse models of venous and arterial thrombosis and blocked the inflammatory effect of polyphosphate injected intradermally in mice. This study provides proof of principle for polyphosphate inhibitors as antithrombotic/anti-inflammatory agents in vitro and in vivo, with a novel mode of action compared with conventional anticoagulants.