Humanized mouse model of thrombosis is predictive of the clinical efficacy of antiplatelet agents

Characterization of Biomarkers of Hemostasis and Bleeding-Related Outcomes in Children With Cirrhosis

OBJECTIVES

We aimed to evaluate differences in laboratory tests, bleeding, transfusions, and thrombosis between (1) children without and with cirrhosis and (2) children and adults with cirrhosis, and to correlate thromboelastography (TEG) parameters with biomarkers of hemostasis, bleeding, and transfusions in children and adults with cirrhosis.

METHODS

This single-center, retrospective study included 20 children without cirrhosis, 40 children with cirrhosis, and 40 adults with cirrhosis who underwent a liver transplant (LT). We collected demographic data, preoperative laboratory values, and intraoperative TEG parameters. Biomarkers of hemostasis just prior to the start of LT surgery were analyzed including international normalized ratio (INR), platelet, fibrinogen level, R time, K time, alpha angle (α), and maximum amplitude (MA). We also collected outcome data including blood loss, transfusion requirements, and thrombosis.

RESULTS

A significantly higher proportion of children with cirrhosis had abnormal PT ( P = 0.001), platelet ( P = 0.001), K time ( P = 0.02), and MA ( P = 0.05) compared to children without cirrhosis. The incidences of thrombosis, bleeding events, blood loss or PRBC transfusion were not significantly different between these 2 groups. A significantly higher proportion of adults with cirrhosis had abnormal R time ( P = 0.01) and alpha angle ( P = 0.01) than children with cirrhosis.

CONCLUSIONS

Children with cirrhosis had defects in fibrinogen and platelets compared to children without cirrhosis at time of LT; however, these abnormalities did not translate into higher rates of bleeding in the former. Adults with cirrhosis had more defects in clotting factors compared to children with cirrhosis.

Direct chemical reprogramming of human cord blood erythroblasts to induced megakaryocytes that produce platelets

Reprogramming somatic cells into megakaryocytes (MKs) would provide a promising source of platelets. However, using a pharmacological approach to generate human MKs from somatic cells remains an unmet challenge. Here, we report that a combination of four small molecules (4M) successfully converted human cord blood erythroblasts (EBs) into induced MKs (iMKs). The iMKs could produce proplatelets and release functional platelets, functionally resembling natural MKs. Reprogramming trajectory analysis revealed an efficient cell fate conversion of EBs into iMKs by 4M via the intermediate state of bipotent precursors. 4M induced chromatin remodeling and drove the transition of transcription factor (TF) regulatory network from key erythroid TFs to essential TFs for megakaryopoiesis, including FLI1 and MEIS1. These results demonstrate that the chemical reprogramming of cord blood EBs into iMKs provides a simple and efficient approach to generate MKs and platelets for clinical applications.

A sensitive and adaptable method to measure platelet-fibrin clot contraction kinetics

Background

Platelet-fibrin clot contraction is critical for wound closure and maintenance of vessel patency, yet a molecular understanding of the process has lagged because of a lack of flexible quantitative assay systems capable of assaying multiple samples simultaneously.

Objectives

We devised a sensitive and inexpensive method to assess clot contraction kinetics under multiple conditions.

Methods

Clot contraction was measured using time-lapse digital photography, automated image processing with customized software, and detailed kinetic analysis using available commercial programs.

Results

Our system was responsive to alterations in platelet counts and calcium, fibrinogen, and thrombin concentrations, and our analysis detected and defined three phases of platelet-fibrin clot formation: initiation, contraction, and stabilization. Lag time, average contraction velocity, contraction extent, and area under the curve were readily calculated from the data. Using pharmacological agents (blebbistatin and eptifibatide), we confirmed the importance of myosin IIA and the interactions of integrin αIIbβ3-fibrinogen/fibrin in clot contraction. As further proof of our system's utility, we showed how 2-deoxyglucose affects contraction, demonstrating the importance of platelet bioenergetics, specifically glycolysis.

Conclusions

Our system is an adaptable platform for assessing the effects of multiple conditions and interventions on clot contraction kinetics in a regular laboratory setting, using readily available materials. The automated image processing software we developed will be made freely available for noncommercial uses. This assay system can be used to directly compare and define the effects of different treatments or genetic manipulations on platelet function and should provide a robust tool for future hemostasis/thrombosis research and therapeutic development.

Transfusion of fresh washed platelets does not prevent experimental polymicrobial-induced septic shock in mice

INTRODUCTION

The specific role of platelets during sepsis is not yet fully understood, probably related to the paradox of platelets being potentially beneficial but also deleterious via their thrombotic functions.

OBJECTIVE

To evaluate the impact of thrombocytopenia on septic shock in mice and to investigate whether transfusion of fresh washed platelets, either fully functional or with impaired hemostatic properties, might have beneficial effects.

METHODS

Septic shock was induced by cecal ligation and puncture (CLP). Experimental depletion of circulating platelets was induced with a rat anti-mouse GPIbα monoclonal antibody. Transfusion of either wild-type washed platelets, platelets treated with the antiplatelet drugs acetylsalicylic acid (ASA) and clopidogrel, or GPIIbIIIa-deficient washed platelets treated with ASA and clopidogrel was performed 4 h after CLP surgery.

RESULTS

Depletion of circulating platelets negatively affected septic shock, worsening systemic inflammation, coagulopathy, organ damage, and mortality, raising the question of whether a higher platelet count could be protective. Transfusion of fully functional platelets or platelets with combined treatment with ASA and clopidogrel, with or without additional GPIIbIIIa deficiency, afforded an immediate return of circulating platelet counts to their initial values before surgery. However, transfusion of each of the three types of platelets did not prevent arterial hypotension, inflammatory response, coagulopathy, and organ damage during septic shock.

CONCLUSION

Depletion of circulating platelets negatively affects septic shock, while transfusion of washed platelets has no significant beneficial effect in mice.

Intravital Assessment of Blood Platelet Function. A Review of the Methodological Approaches with Examples of Studies of Selected Aspects of Blood Platelet Function

Platelet biology owes to intravital studies not only a better understanding of platelets’ role in primary hemostasis but also findings that platelets are important factors in inflammation and atherosclerosis. Researchers who enter the field of intravital platelet studies may be confused by the heterogeneity of experimental protocols utilized. On the one hand, there are a variety of stimuli used to activate platelet response, and on the other hand there are several approaches to measure the outcome of the activation. A number of possible combinations of activation factors with measurement approaches result in the aforementioned heterogeneity. The aim of this review is to present the most often used protocols in a systematic way depending on the stimulus used to activate platelets. By providing examples of studies performed with each of the protocols, we attempt to explain why a particular combination of stimuli and measurement method was applied to study a given aspect of platelet biology.

Structure-guided design of pure orthosteric inhibitors of αIIbβ3 that prevent thrombosis but preserve hemostasis

A prevailing dogma is that inhibition of vascular thrombosis by antagonizing platelet integrin αIIbβ3 cannot be achieved without compromising hemostasis, thus causing serious bleeding and increased morbidity and mortality. It is speculated that these adverse outcomes result from drug-induced activating conformational changes in αIIbβ3 but direct proof is lacking. Here, we report the structure-guided design of peptide Hr10 and a modified form of the partial agonist drug tirofiban that act as "pure" antagonists of αIIbβ3, i.e., they no longer induce the conformational changes in αIIbβ3. Both agents inhibit human platelet aggregation but preserve clot retraction. Hr10 and modified tirofiban are as effective as partial agonist drugs in inhibiting vascular thrombosis in humanized mice, but neither causes serious bleeding, establishing a causal link between partial agonism and impaired hemostasis. Pure orthosteric inhibitors of αIIbβ3 may thus provide safer alternatives for human therapy, and valuable tools to probe structure-activity relationships in integrins.

The mode of anesthesia influences outcome in mouse models of arterial thrombosis

BACKGROUND

Arterial thrombosis models are important for preclinical evaluation of antithrombotics but how anesthetic protocol can influence experimental results is not studied.

OBJECTIVES

We studied how three most commonly used rodent anesthetics affect the induction of thrombosis and thrombus resolution with antiplatelet agent integrilin (Eptifibatide).

METHODS

The Folts, electrolytic, and FeCl3 models of carotid artery thrombosis were evaluated. The extent of blood flow reduction required to elicit cyclic flow reductions (CFR) was examined in the Folts model. The occlusion time and stability following electrolytic or FeCl3 injury was assessed. The efficacy of Eptifibatide was studied in each cohort and clot composition following FeCl3 application was assessed histologically.

RESULTS

Isoflurane and ketamine-xylazine (ket-x) elicited higher basal blood flow velocities. For reliable CFR in the Folts model, a higher degree of blood flow reduction was required under ket-x and isoflurane. For the FeCl3 and electrolytic models, injury severity had to be increased in mice under ket-x anesthesia to achieve rapid occlusion. FeCl3-injured artery sections from ket-x and isoflurane-treated mice showed vessel dilatation and clots that were more fibrin/red-cell rich compared to pentobarbitone. Integrilin led to cycle abolishment for all three Folts-injury cohorts but for the electrolytic model a 2.5-fold higher dose was required to restore blood flow under pentobarbitone. Integrilin after FeCl3 arterial injury was partially ineffective in isoflurane-treated mice.

CONCLUSIONS

Anesthesia impacts rodent carotid artery occlusion experiments and alters integrilin efficacy. It is important to consider anesthetic protocols in animal experiments involving pharmacological agents for treatment of atherothrombosis.

Epidermal growth factor-like repeats of SCUBE1 derived from platelets are critical for thrombus formation

Aims

SCUBE1 [signal peptide-CUB-epidermal growth factor (EGF) domain-containing protein 1], expressed in endothelial cells (ECs) and platelets, exists in soluble or membrane forms. We previously showed that soluble SCUBE1 is a biomarker for platelet activation and also an active participant of thrombosis. However, whether the adhesive module of its EGF-like repeats is essential and the specific contribution of SCUBE1 synthesized in ECs or platelets to thrombosis in vivo remain unclear.

Methods and results

We generated new mutant (Δ2) mice lacking the entire EGF-like repeats to evaluate the module’s functional importance during thrombogenesis in vivo. The Δ2 platelet-rich plasma showed markedly impaired platelet aggregation induced by agonists including adenosine diphosphate, collagen, the thrombin agonist PAR-4 peptide and the thromboxane A2 analogue U46619. Consistently, genetic ablation of the EGF-like repeats diminished arterial thrombosis and protected Δ2 mice against lethal thromboembolism. On flow chamber assay, whole blood isolated from Δ2 or wild-type (WT) mice pre-treated with blocking antibodies against the EGF-like repeats showed a significant decrease in platelet deposition and thrombus formation on collagen-coated surfaces under arterial shear rates. Moreover, we created animals expressing SCUBE1 only in ECs (S1-EC) or platelets (S1-PLT) by reciprocal bone-marrow transplantation between WT and Δ2 mice. The time of carotid arterial thrombosis induced by ferric chloride was normal in S1-PLT chimeric mice but much prolonged in S1-EC animals.

Conclusions

We demonstrate that platelet-derived SCUBE1 plays a critical role in arterial thrombosis via its adhesive EGF-like repeats in vivo and suggest targeting these adhesive motifs of SCUBE1 for potential anti-thrombotic strategy.

P2Y12 Receptor Function and Response to Cangrelor in Neonates With Cyanotic Congenital Heart Disease

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Platelets from neonatal patients with cyanotic congenital heart disease have a nearly identical response to adenosine diphosphate activation and P2Y₁₂ receptor blockade with cangrelor as their adult counterparts.

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Integrating high-throughput technologies with unique biological platforms can provide considerable insight into the potential clinical use of antiplatelet agents for neonatal and pediatric patients at risk for thrombosis.

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Cangrelor may prove to be an effective antithrombotic drug with pharmacological properties well suited for use in the immediate post-operative period for neonates palliated with systemic-to-pulmonary artery shunts.

Shunt thrombosis remains a major cause of morbidity and mortality, especially during the initial palliation for single-ventricle physiology. The authors present evidence that the P2Y₁₂ inhibitor cangrelor may fill a therapeutic void in thromboprophylaxis. They base this theory on results showing that platelets from neonatal patients with cyanotic congenital heart disease have a robust response to adenosine diphosphate and are amenable to P2Y₁₂ inhibition with cangrelor. Unique to this study was their ability to establish drug efficacy in an avatar mouse model that permits the in vivo evaluation of human platelet–mediated thrombus formation illustrating that this P2Y₁₂ inhibitor yields the intended biological response.

Platelets are dispensable for antibody-mediated transfusion-related acute lung injury in the mouse

Essentials Role of platelets in immunological transfusion-related acute lung injury (TRALI) is debated. Immunological TRALI was tested in mice exhibiting severe thrombocytopenia or platelet dysfunction. Platelets are required to prevent lung hemorrhage but not edema formation and respiratory distress. Platelets are dispensable for the initiation and development of TRALI.

SUMMARY

Background Transfusion-related acute lung injury (TRALI) is a serious transfusion-related complication. Previous conflicting studies have indicated that platelets are either crucial or dispensable for TRALI. Objectives To evaluate the role of platelets in major histocompatibility complex (MHC) I-induced-TRALI. Methods Antibody-mediated TRALI was experimentally induced in mice by lipopolysaccharide priming followed by the administration of an anti-MHC I mAb. Results TRALI was tested in the context of severe thrombocytopenia provoked by the administration of diphtheria toxin (DT) in transgenic iDTR mice selectively expressing DT receptor in megakaryocytes. The pathologic responses occurring within the first 10 min following the injection of the anti-MHC I mAb, i.e. the severity of lung edema and the drop in aortic blood oxygenation, were similar in severely thrombocytopenic DT-iDTR and control mice. At later times, mortality was nevertheless increased in DT-iDTR mice, owing to lung hemorrhages. When less severe thrombocytopenia was induced with an antiplatelet mAb, TRALI started and developed similarly as in control mice, but hemorrhages were absent. Furthermore, when platelet functions were defective because of administration of aspirin or clopidogrel, or because of glycoprotein (GP)IIbIIIa deficiency, TRALI still developed but no lung hemorrhages were observed. In contrast, when GPVI was immunodepleted, TRALI still occurred, but was occasionally accompanied by hemorrhages. Conclusions Platelets are dispensable for the initiation and development of MHC I-induced TRALI. Although they do not protect against the disruption of the vascular endothelial cell barrier and the subsequent plasma leakage and edema formation, platelets are essential to prevent more serious damage resulting in hemorrhages in alveoli.

Platelet transfusion reverses bleeding evoked by triple anti-platelet therapy including vorapaxar, a novel platelet thrombin receptor antagonist

Vorapaxar is a novel protease-activated receptor-1 (PAR-1) antagonist recently approved for the reduction of thrombotic cardiovascular events in patients with a history of myocardial infarction or with peripheral arterial disease. Patients who received vorapaxar in addition to standard of care antiplatelet therapy had an increased incidence of major bleeding events compared with placebo. To assess whether platelet transfusion can restore hemostasis in primates on triple antiplatelet therapy, template bleeding times were assessed concurrently in the buccal mucosa, finger pad, and distolateral tail of anesthetized cynomolgus macaques to evaluate bleeding with vorapaxar as either monotherapy or in combination with aspirin or aspirin and clopidogrel. Aspirin (5mg/kg, IV) or vorapaxar (1mg/kg, PO) alone had no significant effect on bleeding times in the three vascular beds examined. A modest (<2-fold) increase in bleeding time was achieved in the three beds with the dual combination of aspirin and vorapaxar. Major increases in bleeding time were achieved in the three beds with the triple combination of aspirin (5mg/kg, IV), vorapaxar (1mg/kg, PO), and clopidogrel (1mg/kg, PO). Transfusion of fresh human platelet rich plasma, but not platelet poor plasma, reversed the increase in bleeding time in the triple therapy group. Transfusion of human platelets may be a viable approach in situations requiring a rapid reversal of platelet function in individuals treated with triple anti-platelet therapy that includes vorapaxar.

Exploiting the kinetic interplay between GPIbα-VWF binding interfaces to regulate hemostasis and thrombosis

Platelet-von Willebrand factor (VWF) interactions must be tightly regulated in order to promote effective hemostasis and prevent occlusive thrombus formation. However, it is unclear what role the inherent properties of the bond formed between the platelet receptor glycoprotein Ibα and the A1 domain of VWF play in these processes. Using VWF-A1 knock-in mice with mutations that enhance (I1309V) or disrupt (R1326H) platelet receptor glycoprotein Ibα binding, we now demonstrate that the kinetic interplay between two distinct contact surfaces influences the site and extent to which platelets bind VWF. Incorporation of R1326H mutation into the major site shortened bond lifetime, yielding defects in hemostasis and thrombosis comparable to VWF-deficient animals. Similarly, disrupting this region of contact with an allosteric inhibitor impaired human platelet accrual in damaged arterioles. In contrast, the I1309V mutation near the minor site prolonged bond lifetime, which was essential for the development of a type 2B-like VWD phenotype. However, combining the R1326H and I1309V mutations normalized both bond kinetics and the hemostatic and thrombotic properties of VWF. These findings broaden our understanding of mechanisms governing platelet-VWF interactions in health and disease, and underscore the importance of combined biophysical and genetic approaches in identifying potential therapeutic avenues for treating bleeding and thrombotic disorders.

RUC-4: a novel αIIbβ3 antagonist for prehospital therapy of myocardial infarction

OBJECTIVE

Treatment of myocardial infarction within the first 1 to 2 hours with a thrombolytic agent, percutaneous coronary intervention, or an αIIbβ3 antagonist decreases mortality and the later development of heart failure. We previously reported on a novel small molecule αIIbβ3 antagonist, RUC-2, that has a unique mechanism of action. We have now developed a more potent and more soluble congener of RUC-2, RUC-4, designed to be easily administered intramuscularly by autoinjector to facilitate its use in the prehospital setting. Here, we report the properties of RUC-4 and the antiplatelet and antithrombotic effects of RUC-2 and RUC-4 in animal models.

APPROACH AND RESULTS

RUC-4 was ≈ 20% more potent than RUC-2 in inhibiting human ADP-induced platelet aggregation and much more soluble in aqueous solutions (60-80 mg/mL). It shared RUC-2's specificity for αIIbβ3 versus αVβ3, did not prime the receptor to bind fibrinogen, or induce changes in β3 identified by a conformation-specific monoclonal antibody. Both RUC-2 and RUC-4 prevented FeCl3-induced thrombotic occlusion of the carotid artery in mice and decreased microvascular thrombi in response to laser injury produced by human platelets infused into transgenic mice containing a mutated von Willebrand factor that reacts with human but not mouse platelets. Intramuscular injection of RUC-4 in nonhuman primates at 1.9 and 3.85 mg/kg led to complete inhibition of platelet aggregation within 15 minutes, with dose-dependent return of platelet aggregation after 4.5 to 24 hours.

CONCLUSIONS

RUC-4 has favorable biochemical, pharmacokinetic, pharmacodynamic, antithrombotic, and solubility properties as a prehospital therapy of myocardial infarction, but the possibility of increased bleeding with therapeutic doses remains to be evaluated.

Microfluidic thrombosis under multiple shear rates and antiplatelet therapy doses

The mainstay of treatment for thrombosis, the formation of occlusive platelet aggregates that often lead to heart attack and stroke, is antiplatelet therapy. Antiplatelet therapy dosing and resistance are poorly understood, leading to potential incorrect and ineffective dosing. Shear rate is also suspected to play a major role in thrombosis, but instrumentation to measure its influence has been limited by flow conditions, agonist use, and non-systematic and/or non-quantitative studies. In this work we measured occlusion times and thrombus detachment for a range of initial shear rates (500, 1500, 4000, and 10000 s(-1)) and therapy concentrations (0-2.4 µM for eptifibatide, 0-2 mM for acetyl-salicylic acid (ASA), 3.5-40 Units/L for heparin) using a microfluidic device. We also measured complete blood counts (CBC) and platelet activity using whole blood impedance aggregometry. Effects of shear rate and dose were analyzed using general linear models, logistic regressions, and Cox proportional hazards models. Shear rates have significant effects on thrombosis/dose-response curves for all tested therapies. ASA has little effect on high shear occlusion times, even at very high doses (up to 20 times the recommended dose). Under ASA therapy, thrombi formed at high shear rates were 4 times more prone to detachment compared to those formed under control conditions. Eptifibatide reduced occlusion when controlling for shear rate and its efficacy increased with dose concentration. In contrast, the hazard of occlusion from ASA was several orders of magnitude higher than that of eptifibatide. Our results show similar dose efficacy to our low shear measurements using whole blood aggregometry. This quantitative and statistically validated study of the effects of a wide range of shear rate and antiplatelet therapy doses on occlusive thrombosis contributes to more accurate understanding of thrombosis and to models for optimizing patient treatment.

Cytokines associated with increased erythropoiesis in Sprague-Dawley rats administered a novel hyperglycosylated analog of recombinant human erythropoietin

We previously reported an increased incidence of thrombotic toxicities in Sprague-Dawley rats administered the highest dose level of a hyperglycosylated analog of recombinant human erythropoietin (AMG 114) for 1 month as not solely dependent on high hematocrit (HCT). Thereafter, we identified increased erythropoiesis as a prothrombotic risk factor increased in the AMG 114 high-dose group with thrombotic toxicities, compared to a low-dose group with no toxicities but similar HCT. Here, we identified pleiotropic cytokines as prothrombotic factors associated with AMG 114 dose level. Before a high HCT was achieved, rats in the AMG 114 high, but not the low-dose group, had imbalanced hemostasis (increased von Willebrand factor and prothrombin time, decreased antithrombin III) coexistent with cytokines implicated in thrombosis: monocyte chemotactic protein 1 (MCP-1), MCP-3, tissue inhibitor of metalloproteinases 1, macrophage inhibitory protein-2, oncostatin M, T-cell-specific protein, stem cell factor, vascular endothelial growth factor, and interleukin-11. While no unique pathway to erythropoiesis stimulating agent-related thrombosis was identified, cytokines associated with increased erythropoiesis contributed to a prothrombotic intravascular environment in the AMG 114 high-dose group, but not in lower dose groups with a similar high HCT.

Humanizing thrombi in mice

Antiplatelet therapies form the cornerstone of atherothrombosis prevention, reducing the morbidity and mortality associated with cardiovascular disease. Despite these benefits, there is still an unmet need for more effective and safer pharmacological agents. To expedite this process, biological platforms that better reflect the intravascular environment in humans will be required in order to shorten drug development time, enable better determination of dosing regimes, and aid in the design of clinical studies. This article focuses on a unique genetically modified animal model that predicts the in vivo response of antiplatelet agents in humans more accurately than is currently possible using conventional murine models of thrombosis.

Structure-guided design of a high-affinity platelet integrin αIIbβ3 receptor antagonist that disrupts Mg²⁺ binding to the MIDAS

An integrin found on platelets, α(IIb)β(3) mediates platelet aggregation, and α(IIb)β(3) antagonists are effective antithrombotic agents in the clinic. Ligands bind to integrins in part by coordinating a magnesium ion (Mg(2+)) located in the β subunit metal ion-dependent adhesion site (MIDAS). Drugs patterned on the integrin ligand sequence Arg-Gly-Asp have a basic moiety that binds the α(IIb) subunit and a carboxyl group that coordinates the MIDAS Mg(2+) in the β(3) subunits. They induce conformational changes in the β(3) subunit that may have negative consequences such as exposing previously hidden epitopes and inducing the active conformation of the receptor. We recently reported an inhibitor of α(IIb)β(3) (RUC-1) that binds exclusively to the α(IIb) subunit; here, we report the structure-based design and synthesis of RUC-2, a RUC-1 derivative with a ~100-fold higher affinity. RUC-2 does not induce major conformational changes in β(3) as judged by monoclonal antibody binding, light scattering, gel chromatography, electron microscopy, and a receptor priming assay. X-ray crystallography of the RUC-2-α(IIb)β(3) headpiece complex in 1 mM calcium ion (Ca(2+))/5 mM Mg(2+) at 2.6 Å revealed that RUC-2 binds to α(IIb) the way RUC-1 does, but in addition, it binds to the β(3) MIDAS residue glutamic acid 220, thus displacing Mg(2+) from the MIDAS. When the Mg(2+) concentration was increased to 20 mM, however, Mg(2+) was identified in the MIDAS and RUC-2 was absent. RUC-2's ability to inhibit ligand binding and platelet aggregation was diminished by increasing the Mg(2+) concentration. Thus, RUC-2 inhibits ligand binding by a mechanism different from that of all other α(IIb)β(3) antagonists and may offer advantages as a therapeutic agent.