Dose-finding, safety, and tolerability study of an oral platelet glycoprotein IIb/IIIa inhibitor, lotrafiban, in patients with coronary or cerebral atherosclerotic disease

Overcoming challenges in developing small molecule inhibitors for GPVI and CLEC-2

GPVI and CLEC-2 have emerged as promising targets for long-term prevention of both arterial thrombosis and thrombo-inflammation with a decreased bleeding risk relative to current drugs. However, while there are potent blocking antibodies of both receptors, their protein nature comes with decreased bioavailability, making formulation for oral medication challenging. Small molecules are able to overcome these limitations, but there are many challenges in developing antagonists of nanomolar potency, which is necessary when considering the structural features that underlie the interaction of CLEC-2 and GPVI with their protein ligands. In this review, we describe current small-molecule inhibitors for both receptors and strategies to overcome such limitations, including considerations when it comes to in silico drug design and the importance of complex compound library selection.

The progress in the research of antiplatelet agents (1995-2017)

Antiplatelet therapy displays a critical role in the treatment and prevention of antithrombotic disorders. Many new antiplatelet agents have been developed following the emergence of various clinical limitations of classical antiplatelet drugs. This review covers mainly the recent advances in the development of P2Y₁₂ antagonists and GPIIb/IIIa antagonists. Meanwhile, it summarizes promising approaches to new platelet surface receptors such as prostanoid EP3 receptor, thromboxane A₂ prostanoid receptor, protease-activated receptors, GPIb-IX-V receptor and P-selectin. In addition, PI3K_β, a critical protein at the inside signaling pathway of platelet activation is also mentioned as an important antiplatelet target. Moreover, the development of respective drug candidates is discussed in detail.

Oral antiplatelet therapy for acute ischaemic stroke

BACKGROUND

In people with acute ischaemic stroke, platelets become activated and can cause blood clots to form and block an artery in the brain, resulting in damage to part of the brain. Such damage gives rise to the symptoms of stroke. Antiplatelet therapy might reduce the volume of brain damaged by ischaemia and also reduce the risk of early recurrent ischaemic stroke, thereby reducing the risk of early death and improving long-term outcomes in survivors. However, antiplatelet therapy might also increase the risk of fatal or disabling intracranial haemorrhage.

OBJECTIVES

To assess the efficacy and safety of immediate oral antiplatelet therapy (that is started as soon as possible and no later than two weeks after stroke onset) in people with acute presumed ischaemic stroke.

SEARCH METHODS

We searched the Cochrane Stroke Group Trials Register (last searched 16 October 2013), the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library Issue 4, 2013), MEDLINE (June 1998 to May 2013), and EMBASE (June 1998 to May 2013). In 1998, for a previous version of this review, we searched the register of the Antiplatelet Trialists' Collaboration, MedStrategy and contacted relevant drug companies.

SELECTION CRITERIA

Randomised trials comparing oral antiplatelet therapy (started within 14 days of the stroke) with control in people with definite or presumed ischaemic stroke.

DATA COLLECTION AND ANALYSIS

Two review authors independently applied the inclusion criteria and assessed trial quality. For the included trials, they extracted and cross-checked the data.

MAIN RESULTS

We included eight trials involving 41,483 participants. No new trials have been added since the last update.Two trials testing aspirin 160 mg to 300 mg once daily, started within 48 hours of onset, contributed 98% of the data. The risk of bias was low. The maximum follow-up was six months. With treatment, there was a significant decrease in death or dependency at the end of follow-up (odds ratio (OR) 0.95, 95% confidence interval (CI) 0.91 to 0.99). For every 1000 people treated with aspirin, 13 people would avoid death or dependency (number needed to treat 79). Antiplatelet therapy was associated with a small but definite excess of symptomatic intracranial haemorrhages, but this small hazard was significantly outnumbered by the benefit, the reduction in recurrent ischaemic stroke and pulmonary embolus.

AUTHORS' CONCLUSIONS

Antiplatelet therapy with aspirin 160 mg to 300 mg daily, given orally (or by nasogastric tube or per rectum in people who cannot swallow) and started within 48 hours of onset of presumed ischaemic stroke, reduced the risk of early recurrent ischaemic stroke without a major risk of early haemorrhagic complications; long-term outcomes were improved.

Medical treatment of patients with intracranial atherosclerotic disease

The medical treatment of patients with symptomatic intracranial atherosclerotic disease (ICAD) is directed toward reducing the risk of new ischemic events. The overall strategy is divided into: (1) prevention of occurrence of intraluminal thrombus, with or without embolism; (2) plaque stabilization and regression; and (3) management of atherogenic risk factors. In patients with ICAD, short-term and long-term anticoagulation (compared with aspirin) has not shown to be beneficial. The current guidelines recommend that aspirin monotherapy, the combination of aspirin and extended release dipyridamole, and clopidogrel monotherapy (rather than oral anticoagulants) are all acceptable options in patients with noncardioembolic ischemic stroke and transient ischemic attack. The findings of another pilot trial suggest that symptomatic ICAD is a dynamic lesion and cilostazol may prevent its progression. Overall, the subgroup analysis from randomized trials, provide evidence about benefit of aggressive atherogenic risk factor management among patients with ICAD. Current guidelines recommend statin therapy with intensive lipid-lowering effects for patients with atherosclerotic ischemic stroke or transient ischemic attack with or without known coronary artery disease to reduce the risk of stroke and cardiovascular events.

Adhesion molecules: potential therapeutic and diagnostic implications

The role of cell adhesion molecules (CAMs) and extracellular matrix (ECM) proteins in various pathological processes, including angiogenesis, thrombosis, inflammation, apoptosis, cell migration, and proliferation is well documented. These processes can lead to both acute and chronic disease states such as ocular diseases, metastasis, unstable angina, myocardial infarction, stroke, osteoporosis, a wide range of inflammatory diseases, vascular remodeling, and neurodegenerative disorders. A key success in this field was identification of the role of platelet glycoprotein (GP)IIb/IIIa in the prevention and diagnosis of various thromboembolic disorders. The use of soluble adhesion molecules as potential diagnostic markers for acute and chronic leukocyte, platelet, and endothelial cell insult is becoming increasingly common. The development of various therapeutic and diagnostic candidates based on the key role of CAMs, with special emphasis on integrins in various diseases, as well as the structure-function aspects of cell adhesion and signaling of the different CAMs and ECM are highlighted.

Antiplatelet therapy for acute ischaemic stroke

BACKGROUND

In patients with acute ischaemic stroke, platelets become activated. Antiplatelet therapy might reduce the volume of brain damaged by ischaemia and reduce the risk of early recurrent ischaemic stroke. This might reduce the risk of early death and improve long-term outcome in survivors. However, antiplatelet therapy might also increase the risk of fatal or disabling intracranial haemorrhage.

OBJECTIVES

To assess the efficacy and safety of antiplatelet therapy in acute ischaemic stroke.

SEARCH STRATEGY

We searched the Cochrane Stroke Group Trials Register (last searched June 2007), the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library Issue 2, 2007), MEDLINE (June 1998 to May 2007), and EMBASE (June 1998 to May 2007). In 1998, for a previous version of this review, we searched the register of the Antiplatelet Trialists' Collaboration, MedStrategy and contacted relevant drug companies.

SELECTION CRITERIA

Randomised trials comparing antiplatelet therapy (started within 14 days of the stroke) with control in patients with definite or presumed ischaemic stroke.

DATA COLLECTION AND ANALYSIS

Two review authors independently applied the inclusion criteria and assessed trial quality, and for the included trials, extracted and cross-checked the data.

MAIN RESULTS

Twelve trials involving 43,041 participants were included. Two trials testing aspirin 160 mg to 300 mg once daily started within 48 hours of onset contributed 94% of the data. The maximum follow up was six months. With treatment, there was a significant decrease in death or dependency at the end of follow up (odds ratio 0.95, 95% confidence interval 0.91 to 0.99). For every 1000 patients treated with aspirin, 13 patients will avoid death or dependency (number needed to treat to benefit: 79). Antiplatelet therapy was associated with a small but definite excess of symptomatic intracranial haemorrhages, but this was more than offset by the reduction of recurrent ischaemic strokes and pulmonary embolus.

AUTHORS' CONCLUSIONS

Antiplatelet therapy with aspirin 160 mg to 300 mg daily, given orally (or by nasogastric tube or per rectum in patients who cannot swallow), and started within 48 hours of onset of presumed ischaemic stroke reduces the risk of early recurrent ischaemic stroke without a major risk of early haemorrhagic complications and improves long-term outcome.

Future innovations in anti-platelet therapies

Platelets have long been recognized to be of central importance in haemostasis, but their participation in pathological conditions such as thrombosis, atherosclerosis and inflammation is now also well established. The platelet has therefore become a key target in therapies to combat cardiovascular disease. Anti-platelet therapies are used widely, but current approaches lack efficacy in a proportion of patients, and are associated with side effects including problem bleeding. In the last decade, substantial progress has been made in understanding the regulation of platelet function, including the characterization of new ligands, platelet-specific receptors and cell signalling pathways. It is anticipated this progress will impact positively on the future innovations towards more effective and safer anti-platelet agents. In this review, the mechanisms of platelet regulation and current anti-platelet therapies are introduced, and strong, and some more speculative, potential candidate target molecules for future anti-platelet drug development are discussed.

Association of the antagonism of von Willebrand factor but not fibrinogen by platelet alphaIIbbeta3 antagonists with prolongation of bleeding time

BACKGROUND

The alphaIIbbeta3 antagonists inhibit platelet aggregation and are used as antithrombotic agents for cardiothrombotic disease. The present study investigates the correlation of inhibition of fibrinogen and von Willebrand factor (VWF) binding by alphaIIbbeta3 antagonists with the inhibition of platelet aggregation and prolongation of bleeding time (BT).

METHODS

Inhibition of fibrinogen and VWF binding were assessed in a purified alphaIIbbeta3-binding assay. As an in vitro cell-based assay, platelet aggregation and VWF-mediated adhesion studies were performed using human platelets. In vivo effects on BT were measured using a template device in dogs at the same time as an ex vivo aggregation study was performed.

RESULTS

In vitro studies demonstrated that the antiaggregatory effects of alphaIIbbeta3 antagonists correlate with their inhibition of fibrinogen binding, but not VWF. Interestingly, the effects of alphaIIbbeta3 antagonists on BT could be differentiated from the inhibition of platelet aggregation. Furthermore, this differentiation was strongly correlated with the different inhibitory potencies between fibrinogen and VWF binding, as well as that between VWF-mediated adhesion and aggregation.

CONCLUSIONS

Our study provides novel evidence showing that the inhibitory effect of alphaIIbbeta3 antagonists on VWF, but not fibrinogen binding, correlates with their ability to prolong BT.

Relationship of the implantable cardioverter defibrillator and chronic resynchronization therapy: the perfect marriage?

BACKGROUND

The two major modes of death in the patient with a reduced ejection fraction (EF) are death due to heart failure and death due to lethal arrhythmia, essentially the two sides of the same coin. Over the last 20 years, two therapies-cardiac resynchronization therapy (CRT) and the implantable cardioverter defibrillator (ICD)-have been developed and tested in clinical trials. They are now, in conjunction with appropriate medical therapy, the mainstays of therapy for these two commonly encountered clinical problems.

METHOD AND RESULTS

Both of these therapies were conceived and patented by two Baltimore cardiologists, Michel Mirowski and Morton Mower (Table I). The path to everyday acceptance of both therapies was remarkably similar. The concept and early success of both devices was accomplished but the proof of concept depended on a series of carefully designed randomized clinical trials that showed that both the CRT and ICD devices saved lives in the low EF population, especially when used together. These trials overcame substantial skepticism on behalf of elements of the cardiology and electrophysiology establishment.

CONCLUSION

We are now at a crossroads in the further extension of either therapy. The majority of the indications for either device alone or in combination are established. In the next few years, assuming the continued commitment on the part of regulatory agencies to fully embrace evidence-based medicine, we will see indications extended but only by the careful clinical trials that became the bedrock of their initial acceptance.

A perspective on the toxicological mechanisms possibly contributing to the failure of oral glycoprotein IIb/IIIa antagonists in the clinic

In clinical trials in patients with acute or unstable coronary syndromes and/or undergoing percutaneous coronary intervention, oral glycoprotein (GP) IIb/IIIa antagonists did not show therapeutic benefit over aspirin during long-term administration. Moreover, high-dose oral administration of these agents was associated with greater fatality risk compared with that of lower doses. This article postulates that continuous exposure of the GP IIb/IIIa receptor (integrin alpha(IIb)beta(3)) to these agents may result in some form of resistance or activation of other biological systems. These toxicological mechanisms may help explain some factors that could potentially contribute to the failure of these agents in clinical trials. Several hypotheses are presented: (i) modulation of platelet response because of long-term exposure to GP IIb/IIIa antagonists; (ii) role of related integrins and associated proteins to compensate for the loss of platelet activity because of dysfunctional GP IIb/IIIa receptors occupied by inhibitors; (iii) effects of the GP IIb/IIIa antagonists on other cellular systems such as the caspase and procaspase enzymes in apoptosis and possibly the ryanodine receptor involved in sarcoplasmic reticulum calcium release. These toxicological mechanisms could potentially limit the utility of these oral agents in long-term administration.

Antiplatelet therapy: in search of the 'magic bullet'

The central importance of platelets in the development of arterial thrombosis and cardiovascular disease is well established. No other single cell type is responsible for as much morbidity and mortality as the platelet and, as a consequence, it represents a major target for therapeutic intervention. The growing awareness of the importance of platelets is reflected in the increasing number of patients receiving antiplatelet therapy, a trend that is likely to continue in the future. There are, however, significant drawbacks with existing therapies, including issues related to limited efficacy and safety. The discovery of a 'magic bullet' that selectively targets pathological thrombus formation without undermining haemostasis remains elusive, although recent progress in unravelling the molecular events regulating thrombosis has provided promising new avenues to solve this long-standing problem.

Oral platelet glycoprotein IIb/IIIa receptor inhibitors--part II

Although the hypothesis of benefit from prolonged oral IIb/IIIa inhibition was appealing, the large Phase III trials have uniformly shown there was no improvement in outcome. In addition, there was an increased mortality seen in patients treated with the oral IIb/IIIa inhibitor. This latter finding is not adequately explained, but is likely a multifactorial problem of this strategy of platelet inhibition. The trials found that, even with no improvement in efficacy, there was increased bleeding, meaning that for chronic therapy with IIb/IIIa inhibition there does not appear to be a therapeutic window. Accordingly, chronic oral IIb/IIIa inhibition appears to have been well tested but has not worked. Fortunately, there are several other oral antiplatelet agents available that have shown beneficial results, including clopidogrel. In addition, other newer classes of antiplatelet agents are in earlier stages of development. Thus, agents targeted more "upstream" in platelet activation pathways may offer a more tolerable and efficacious approach to long-term antiplatelet therapy.

Antiplatelet therapy for acute ischaemic stroke

BACKGROUND

In patients with acute ischaemic stroke, platelets become activated. Antiplatelet therapy might reduce the volume of brain damaged by ischaemia and reduce the risk of early recurrent ischaemic stroke. This might reduce the risk of early death and improve long-term outcome in survivors. However, antiplatelet therapy might also increase the risk of fatal or disabling intracranial haemorrhage.

OBJECTIVES

The aim of this review is to assess the efficacy and safety of antiplatelet therapy in acute ischaemic stroke.

SEARCH STRATEGY

We searched the Cochrane Stroke Group Trials Register (last searched August 2002), the Cochrane Controlled Trials Register (CCTR) (Cochrane Library Issue 1 2002), MEDLINE (June 1998-October 2001), and EMBASE (June 1998-February 2002). In 1998, for previous versions of this review, we searched the register of the Antiplatelet Trialists Collaboration, MedStrategy and contacted relevant drug companies.

SELECTION CRITERIA

Randomised trials comparing antiplatelet therapy (started within 14 days of the stroke) with control in patients with definite or presumed ischaemic stroke.

DATA COLLECTION AND ANALYSIS

Two reviewers independently applied the inclusion criteria and assessed trial quality, and for the included trials, extracted and cross-checked the data.

MAIN RESULTS

Nine trials involving 41,399 patients were included. Two trials testing aspirin 160 to 300 mg once daily started within 48 hours of onset contributed 98% of the data. The maximum follow-up was six months. With treatment, there was a significant decrease in death or dependency at the end of follow-up (OR = 0.94; 95% CI 0.91 to 0.98). In absolute terms, 13 more patients were alive and independent at the end of follow-up for every 1000 patients treated. Furthermore, treatment increased the odds of making a complete recovery from the stroke (OR = 1.06; 95% CI 1.01 to 1.11). In absolute terms, 10 more patients made a complete recovery for every 1000 patients treated. Antiplatelet therapy was associated with a small but definite excess of 2 symptomatic intracranial haemorrhages for every 1000 patients treated, but this was more than offset by a reduction of 7 recurrent ischaemic strokes and about one pulmonary embolus for every 1000 patients treated.

REVIEWER'S CONCLUSIONS

Antiplatelet therapy with aspirin 160 to 300 mg daily, given orally (or per rectum in patients who cannot swallow), and started within 48 hours of onset of presumed ischaemic stroke reduces the risk of early recurrent ischaemic stroke without a major risk of early haemorrhagic complications and improves long-term outcome.

Oral glycoprotein IIb/IIa antagonists for unstable angina--is there still a chance for the oral substances?

The intravenous glycoprotein IIb/IIIa antagonists abciximab, tirofiban and eptifibatide are well accepted for the therapy of patients with unstable angina and/or as concomitant medication during coronary interventions. Despite the fact that these drugs are not used in all patients presenting with unstable angina during coronary interventions, the scientific evidence is overwhelming including the substantial reduction in mortality 3 years after utilisation of abciximab for coronary interventions in patients with unstable angina. In addition to these two indications, intravenous glycoprotein IIb/IIIa antagonists are currently being investigated for use in patients undergoing carotid artery interventions, peripheral arterial interventions and stroke, as well as adjunct therapy in patients undergoing fibrinolytic therapy during acute myocardial infarction. In contrast, the large trials being performed in patients with unstable angina and following coronary interventions using oral glycoprotein IIb/IIIa antagonists have been very disappointing. There were only minor therapeutic effects detectable, resulting in a slight reduction in ischemic cardiac events in some investigations, however, in all studies, there was a slight trend towards an increased mortality in the glycoprotein IIb/IIIa receptor-antagonist-treated group of patients. In meta-analysis, an approximately 35% relative increase in mortality has been calculated for patients being treated long term with the oral glycoprotein antagonists. The reason for this therapeutic failure is still unknown, however, the limited bioavailability of these drugs, together with our still very limited knowledge about the regulation of the platelet fibrinogen receptor, may be partially responsible for this therapeutic failure. Other compounds with improved pharmacokinetic properties are currently in clinical development.

Oral glycoprotein IIb/IIIa antagonists in coronary artery disease

Despite the efficacy of intravenous glycoprotein IIb/IIIa inhibition in patients undergoing percutaneous coronary intervention and those presenting with acute coronary syndromes, the application of oral glycoprotein IIb/IIIa inhibition to the chronic management of coronary artery disease has not met with the same success. To explain these results, factors related to dosing, and inadequate inhibition or activation of platelet pro-coagulant activity have been recently suggested. However, although the disparity between intravenous and oral glycoprotein IIb/IIIa experience remains largely enigmatic, the discordant effect on ischemic endpoints observed within the phase III oral glycoprotein IIb/IIIa inhibitor trials potentially implicates a mechanism unrelated to platelet function.

Oral glycoprotein IIb/IIIa inhibitors: why don't they work?

Despite their promise as orally active potent inhibitors of platelet aggregation, the oral platelet glycoprotein IIb/IIIa inhibitors have failed to provide a reduction in late ischemic events. In fact, with five large-scale randomized trials now complete, including over 42 000 patients, these agents have been associated with a surprising, yet consistent, excess in mortality. Peculiarly, this fatality risk has occurred in the absence of a commensurate increase in other ischemic end-points. While these findings have curtailed the further clinical development of this class of potent platelet inhibitors, the obvious dissociation between platelet suppression and adverse outcome requires further clarification. Multiple putative explanations for this excess in ischemic events with oral glycoprotein IIb/IIIa inhibitors have been proposed, but definitive data implicating a specific mechanism are currently not available. While the lack of concurrent aspirin may account for some of this effect, it is unlikely to fully explain the mortality excess. Potential mechanisms include partial agonist activity leading to increased expression of platelet-leukocyte adhesion molecules, sub-optimal inhibition of platelet aggregation, genetic polymorphisms, especially phospholipase A(2) polymorphism, and promotion of cardiac myocyte apoptosis via activation of caspase 3. Definitive elucidation of these adverse mechanisms will be required if further clinical development of the oral platelet glycoprotein IIb/IIIa inhibitors is to be pursued.

Oral glycoprotein IIb/IIIa antagonists: new insights from the SYMPHONY trial

Platelet aggregation plays a central role in the pathophysiology of acute coronary syndromes, and the platelet glycoprotein IIb/IIIa receptor has been identified as the critical final mediator of this process. Antagonists of this receptor used parenterally during both acute coronary syndromes and percutaneous coronary interventions reduce the likelihood of subsequent major cardiac complications. However, after the treatment period little further benefit accrues. Based on these observations and that of the significant benefit of aspirin in cardiovascular secondary prevention, oral glycoprotein IIb/IIIa receptor antagonists are being evaluated with the goal of extending the benefit of glycoprotein IIb/IIIa inhibition into chronic secondary prevention. This paper will review the results of the SYMPHONY study of one such oral agent, sibrafiban, and the current state of the oral glycoprotein IIb/IIIa inhibitor field.

Oral platelet glycoprotein IIb/IIIa inhibition

Platelet aggregation plays a central role in the pathogenesis of thrombosis and the acute coronary syndromes. When given intravenously, potent selective antagonists of fibrinogen binding to the glycoprotein (GP) IIb/IIIa receptor, the final common pathway for platelet aggregation, have been effective in the treatment of acute coronary syndromes. Their benefit ceases, however, with the end of the infusion. Aspirin reduces the incidence of secondary vascular events by 25% to 30% after an acute coronary syndrome, and clopidogrel provides modest improvement over aspirin. However, both are relatively weak antiplatelet agents that each block only one of many pathways to platelet activation and surface membrane expression of the competent GP IIb/IIIa receptor. With the success of the intravenous GP IIb/IIIa antagonists in the acute setting, recent interest has focused on the potential benefit of oral GP IIb/IIIa antagonists used long-term for secondary prevention. The oral agents tested in phase III studies thus far have not performed up to expectations, however. The following paper reviews these studies and the implications of their results.