Ingo Ahrens

New oral anticoagulant drugs in cardiovascular disease

Oral anticoagulation has been limited to vitamin K antagonists (VKAs) for over 60 years. VKAs are effective and recommended for the prevention of venous and arterial thromboembolism in cardiovascular disease, but their pharmacodynamics are difficult to predict and the highly variable interindividual and intraindividual response to treatment accounts for the need of continuous monitoring. This prompted the intensive exploration of numerous substances within the last decade in an attempt to meet the shortcomings of current oral anticoagulation with VKAs. The development and clinical investigation of two novel groups of oral anticoagulants targeting central factors of the coagulation system either factor Xa or thrombin (factor IIa) has now reached the daily clinical practice with the approval of the oral direct thrombin inhibitor dabigatran etexilate and the oral direct factor Xa inhibitor rivaroxaban. Ongoing clinical trials are investigating these substances and other novel oral anticoagulants with similar mechanisms of action in patients with atrial fibrillation and acute coronary syndromes. This review article discusses the clinical evaluation and pharmacological properties of novel oral anticoagulants in late and earlier stages of clinical development, thereby providing a critical analysis and an outlook on the future of oral anticoagulation in cardiovascular disease.

Dissociation of Pentameric to Monomeric C-Reactive Protein on Activated Platelets Localizes Inflammation to Atherosclerotic Plaques

C-reactive protein (CRP) is a predictor of cardiovascular risk. It circulates as a pentamer (pentameric CRP) in plasma. The in vivo existence of monomeric (m)CRP has been postulated, but its function and source are not clear. We show that mCRP is deposited in human aortic and carotid atherosclerotic plaques but not in healthy vessels. pCRP is found neither in healthy nor in diseased vessels. As source of mCRP, we identify a mechanism of dissociation of pCRP to mCRP. We report that activated platelets, which play a central role in cardiovascular events, mediate this dissociation via lysophosphatidylcholine, which is present on activated but not resting platelets. Furthermore, the dissociation of pCRP to mCRP can also be mediated by apoptotic monocytic THP-1 and Jurkat T cells. The functional consequence is the unmasking of proinflammatory effects of CRP as demonstrated in experimental settings that are pathophysiologically relevant for atherogenesis: compared to pCRP, mCRP induces enhanced monocyte chemotaxis; monocyte activation, as determined by conformational change of integrin Mac-1; generation of reactive oxygen species; and monocyte adhesion under static and physiological flow conditions. In conclusion, we demonstrate mCRP generation via pCRP dissociation on activated platelets and H2O2-treated apoptotic THP-1 and Jurkat T cells, thereby identifying a mechanism of localized unmasking of the proinflammatory properties of CRP. This novel mechanism provides a potential link between the established cardiovascular risk marker, circulating pCRP, and localized platelet-mediated inflammatory and proatherogenic effects.

CD40 Ligand Mediates Inflammation Independently of CD40 by Interaction With Mac-1

Background— Strong evidence supports a role for CD40 ligand (CD40L) as marker and mediator of inflammatory diseases such as atherosclerosis. Despite extensive characterization of CD40, the classic receptor of CD40L, its role in immune defense against inflammatory diseases remains uncertain. The present study aimed to characterize the contribution of CD40 signaling to atherogenesis. Methods and Results— Surprisingly, mice deficient in both CD40 and the low-density lipoprotein receptor did not develop smaller lesions in the aortic arch, root, and thoracoabdominal aorta compared with mice deficient only in the low-density lipoprotein receptor that consumed an atherogenic diet for 8 and 16 weeks. By flow cytometry, radioactive binding assays, and immunoprecipitation, we demonstrate that CD40L interacts with the integrin Mac-1, which results in Mac-1–dependent adhesion and migration of inflammatory cells as well as myeloperoxidase release in vitro. Furthermore, mice deficient in CD40L show significantly reduced thioglycolate-elicited invasion of inflammatory cells into the peritoneal cavity compared with mice deficient in CD40 and wild-type controls. Inhibition of Mac-1 in low-density lipoprotein receptor–deficient mice attenuates lesion development and reduces lesional macrophage accumulation. Conclusions— These observations identify the interaction of CD40L and Mac-1 as an alternative pathway for CD40L-mediated inflammation. This novel mechanism expands understanding of inflammatory signaling during atherogenesis.

Conformation-Specific Blockade of the Integrin GPIIb/IIIa: A Novel Antiplatelet Strategy That Selectively Targets Activated Platelets

Platelet activation causes conformational changes of integrin GPIIb/IIIa (&agr;IIb&bgr;3), resulting in the exposure of its ligand-binding pocket. This provides the unique possibility to design agents that specifically block activated platelets only. We used phage display of single-chain antibody (scFv) libraries in combination with several rounds of depletion/selection to obtain human scFvs that bind specifically to the activated conformation of GPIIb/IIIa. Functional evaluation of these scFv clones revealed that fibrinogen binding to human platelets and platelet aggregation can be effectively inhibited by activation-specific scFvs. In contrast to clinically used GPIIb/IIIa blockers, which are all conformation unspecific, activation-specific GPIIb/IIIa blockers do not induce conformational changes in GPIIb/IIIa or outside-in signaling, as evaluated by ligand-induced binding-site (LIBS) exposure in flow cytometry or P-selectin expression in immunofluorescence microscopy, respectively. In contrast to the conformation-unspecific blocker abciximab, activation-specific scFvs permit cell adhesion and spreading on immobilized fibrinogen, which is mediated by nonactivated GPIIb/IIIa. Mutagenesis studies and computer modeling indicate that exclusive binding of activation-specific scFv is mediated by RXD motifs in the heavy-chain complementary-determining region (CDR) 3 of the antibodies, which in comparison with other antibodies forms an exceptionally extended loop. In vivo experiments in a ferric-chloride thrombosis model of the mouse carotid artery demonstrate similar antithrombotic potency of activation-specific scFv, when compared with the conformation-unspecific blockers tirofiban and eptifibatide. However, in contrast to tirofiban and eptifibatide, bleeding times are not prolonged with the activation-specific scFvs, suggesting lower bleeding risks. In conclusion, activation-specific GPIIb/IIIa blockade via human single-chain antibodies represents a promising novel strategy for antiplatelet therapy.

High-Mobility Group Box Protein 1 Neutralization Reduces Development of Diet-Induced Atherosclerosis in Apolipoprotein E―Deficient Mice

Objective—High-mobility group box protein 1 (HMGB1) is a DNA-binding protein and cytokine highly expressed in atherosclerotic lesions, but its pathophysiological role in atherosclerosis is unknown. We investigated its role in the development of atherosclerosis in ApoE−/− mice. Methods and Results—Apolipoprotein E–deficient (ApoE−/−) mice fed a high-fat diet were administered a monoclonal anti-HMGB1 neutralizing antibody, and the effects on lesion size, immune cell accumulation, and proinflammatory mediators were assessed using Oil Red O, immunohistochemistry, and real-time polymerase chain reaction. As with human atherosclerotic lesions, lesions in ApoE−/− mice expressed HMGB1. Treatment with the neutralizing antibody attenuated atherosclerosis by 55%. Macrophage accumulation was reduced by 43%, and vascular cell adhesion molecule-1 and monocyte chemoattractant protein-1 expression was attenuated by 48% and 72%, respectively. CD11c+ dendritic cells were reduced by 65%, and the mature (CD83+) population was reduced by 60%. Treatment also reduced CD4+ cells by nearly 50%. mRNAs in lesions encoding tumor necrosis factor-&agr; and interleukin-1&bgr; tended to be reduced. Mechanistically, HMGB1 stimulated macrophage migration in vitro and in vivo; in vivo, it markedly augmented the accumulation of F4/80+Gr-1(Ly-6C)+ macrophages and also increased F4/80+CD11b+ macrophage numbers. Conclusion—HMGB1 exerts proatherogenic effects augmenting lesion development by stimulating macrophage migration, modulating proinflammatory mediators, and encouraging the accumulation of immune and smooth muscle cells.

Novel Single-Chain Antibody-Targeted Microbubbles for Molecular Ultrasound Imaging of Thrombosis Validation of a Unique Noninvasive Method for Rapid and Sensitive Detection of Thrombi and Monitoring of Success or Failure of Thrombolysis in Mice

Background— Molecular imaging is a fast emerging technology allowing noninvasive detection of vascular pathologies. However, imaging modalities offering high resolution currently do not allow real-time imaging. We hypothesized that contrast-enhanced ultrasound with microbubbles (MBs) selectively targeted to activated platelets would offer high-resolution, real-time molecular imaging of evolving and dissolving arterial thrombi. Methods and Results— Lipid-shell based gas-filled MBs were conjugated to either a single-chain antibody specific for activated glycoprotein IIb/IIIa via binding to a Ligand-Induced Binding Site (LIBS-MBs) or a nonspecific single-chain antibody (control MBs). Successful conjugation was assessed in flow cytometry and immunofluorescence double staining. LIBS-MBs but not control MBs strongly adhered to both immobilized activated platelets and microthrombi under flow. Thrombi induced in carotid arteries of C57Bl6 mice in vivo by ferric chloride injury were then assessed with ultrasound before and 20 minutes after MB injection through the use of gray-scale area intensity measurement. Gray-scale units converted to decibels demonstrated a significant increase after LIBS-MB but not after control MB injection (9.55±1.7 versus 1.46±1.3 dB; P <0.01). Furthermore, after thrombolysis with urokinase, LIBS-MB ultrasound imaging allows monitoring of the reduction of thrombus size ( P <0.001). Conclusion— We demonstrate that glycoprotein IIb/IIIa–targeted MBs specifically bind to activated platelets in vitro and allow real-time molecular imaging of acute arterial thrombosis and monitoring of the success or failure of pharmacological thrombolysis in vivo. # Clinical Perspective {#article-title-50}Background— Molecular imaging is a fast emerging technology allowing noninvasive detection of vascular pathologies. However, imaging modalities offering high resolution currently do not allow real-time imaging. We hypothesized that contrast-enhanced ultrasound with microbubbles (MBs) selectively targeted to activated platelets would offer high-resolution, real-time molecular imaging of evolving and dissolving arterial thrombi. Methods and Results— Lipid-shell based gas-filled MBs were conjugated to either a single-chain antibody specific for activated glycoprotein IIb/IIIa via binding to a Ligand-Induced Binding Site (LIBS-MBs) or a nonspecific single-chain antibody (control MBs). Successful conjugation was assessed in flow cytometry and immunofluorescence double staining. LIBS-MBs but not control MBs strongly adhered to both immobilized activated platelets and microthrombi under flow. Thrombi induced in carotid arteries of C57Bl6 mice in vivo by ferric chloride injury were then assessed with ultrasound before and 20 minutes after MB injection through the use of gray-scale area intensity measurement. Gray-scale units converted to decibels demonstrated a significant increase after LIBS-MB but not after control MB injection (9.55±1.7 versus 1.46±1.3 dB; P<0.01). Furthermore, after thrombolysis with urokinase, LIBS-MB ultrasound imaging allows monitoring of the reduction of thrombus size (P<0.001). Conclusion— We demonstrate that glycoprotein IIb/IIIa–targeted MBs specifically bind to activated platelets in vitro and allow real-time molecular imaging of acute arterial thrombosis and monitoring of the success or failure of pharmacological thrombolysis in vivo.

What do the RE-LY, AVERROES and ROCKET-AF trials tell us for stroke prevention in atrial fibrillation?

What do the RE-LY, AVERROES and ROCKET-AF trials tell us for stroke prevention in atrial fibrillation? -

Targeting Ligand-Induced Binding Sites on GPIIb/IIIa via Single-Chain Antibody Allows Effective Anticoagulation Without Bleeding Time Prolongation

Objective—Therapeutic anticoagulation is widely used, but limitations in efficacy and bleeding complications cause an ongoing search for new agents. However, with new agents developed it seems to be an inherent problem that increased efficiency is accompanied by an increase in bleeding complications. We investigate whether targeting of anticoagulants to activated platelets provides a means to overcome this association of potency and bleeding. Methods and Results—Ligand-induced binding sites (LIBS) on fibrinogen/fibrin-binding GPIIb/IIIa represent an abundant clot-specific target. We cloned an anti-LIBS single-chain antibody (scFvanti-LIBS) and genetically fused it with a potent, direct factor Xa (fXa) inhibitor, tick anticoagulant peptide (TAP). Specific antibody binding of fusion molecule scFvanti-LIBS-TAP was proven in flow cytometry; anti-fXa activity was demonstrated in chromogenic assays. In vivo anticoagulative efficiency was determined by Doppler-flow in a ferric chloride–induced carotid artery thrombosis model in mice. ScFvanti-LIBS-TAP prolonged occlusion time comparable to enoxaparine, recombinant TAP, and nontargeted mutant-scFv-TAP. ScFvanti-LIBS-TAP revealed antithrombotic effects at low doses at which the nontargeted mutant-scFv-TAP failed. In contrast to the other anticoagulants tested, bleeding times were not prolonged by scFvanti-LIBS-TAP. Conclusions—The novel clot-targeting approach of anticoagulants via single-chain antibody directed against a LIBS-epitope on GPIIb/IIIa promises effective anticoagulation with reduced bleeding risk.

A Mechanistic Model for Paradoxical Platelet Activation by Ligand-Mimetic αIIbβ3 (GPIIb/IIIa) Antagonists

Objective—Integrins are attractive therapeutic targets. Inhibition of integrin &agr;IIb&bgr;3 effectively blocks platelet aggregation. However, limitations with intravenous &agr;IIb&bgr;3 antagonists and failure of oral &agr;IIb&bgr;3 antagonists prompted doubts on the current concept of ligand-mimetic integrin blockade. Methods and Results—Evaluating P-selectin expression on platelets by flow cytometry, we report a mechanism of paradoxical platelet activation by ligand-mimetic &agr;IIb&bgr;3 antagonists and define three requirements: (1) Induction of ligand-bound conformation of &agr;IIb&bgr;3, (2) receptor clustering, (3) prestimulation of platelets. Conformational change is inducible by clinically used ligand-mimetic &agr;IIb&bgr;3 antagonists, RGD-peptides, and anti-LIBS antibodies. In a mechanistic experimental model, clustering is achieved by crosslinking integrins via antibodies, and preactivation is induced by low-dose ADP. Finally, we demonstrate that platelet adhesion on collagen represents an in vivo correlate of platelet prestimulation and receptor clustering, in which the presence of ligand-mimetic &agr;IIb&bgr;3 antagonists results in platelet activation as detected by P-selectin, CD63, and CD40L expression as well as by measuring Ca2+-signaling. Blockade of the ADP receptor P2Y12 by AR-C69931MX and clopidogrel inhibits &agr;IIb&bgr;3 antagonist-induced platelet activation. Conclusion—These findings can explain limitations of ligand-mimetic anti-&agr;IIb&bgr;3 therapy. They describe potential benefits of concomitant ADP receptor blockade and support a shift in drug development from ligand-mimetic toward allosteric or activation-specific integrin antagonists.

Functionalized Magnetic Resonance Contrast Agent Selectively Binds to Glycoprotein IIb/IIIa on Activated Human Platelets under Flow Conditions and Is Detectable at Clinically Relevant Field Strengths

Recent progress in molecular magnetic resonance imaging (MRI) provides the opportunity to image cells and cellular receptors using microparticles of iron oxide (MPIOs). However, imaging targets on vessel walls remains challenging owing to the quantity of contrast agents delivered to areas of interest under shear stress conditions. We evaluated ex vivo binding characteristics of a functional MRI contrast agent to ligand-induced binding sites (LIBSs) on activated glycoprotein IIb/IIIa receptors of human platelets, which were lining rupture-prone atherosclerotic plaques and could therefore facilitate detection of platelet-mediated pathology in atherothrombotic disease. MPIOs were conjugated to anti-LIBS single-chain antibodies (LIBS-MPIO) or control antibodies (control MPIO). Ex vivo binding to human platelet-rich clots in a dose-dependent manner was confirmed on a 3 T clinical MRI scanner and by histology (p < .05 for LIBS-MPIO vs control MPIO). By using a flow chamber setup, significant binding of LIBS-MPIO to a platelet matrix was observed under venous and arterial flow conditions, but not for control MPIO (p < .001). A newly generated MRI contrast agent detects activated human platelets at clinically relevant magnetic field strengths and binds to platelets under venous and arterial flow conditions, conveying high payloads of contrast to specific molecular targets. This may provide the opportunity to identify vulnerable, rupture-prone atherosclerotic plaques via noninvasive MRI.