The failure of orally administered glycoprotein IIb/IIIa inhibitors to prevent recurrent cardiac events

Extracellular matrix in vascular homeostasis and disease

The extracellular matrix is an essential component and constitutes a dynamic microenvironment of the vessel wall with an indispensable role in vascular homeostasis and disease. From early development through to ageing, the vascular extracellular matrix undergoes various biochemical and biomechanical alterations in response to diverse environmental cues and exerts precise regulatory control over vessel remodelling. Advances in novel technologies that enable the comprehensive evaluation of extracellular matrix components and cell-matrix interactions have led to the emergence of therapeutic strategies that specifically target this fine-tuned network. In this Review, we explore various aspects of extracellular matrix biology in vascular development, disorders and ageing, emphasizing the effect of the extracellular matrix on disease initiation and progression. Additionally, we provide an overview of the potential therapeutic implications of targeting the extracellular matrix microenvironment in vascular diseases.

Advance in peptide-based drug development: delivery platforms, therapeutics and vaccines

The successful approval of peptide-based drugs can be attributed to a collaborative effort across multiple disciplines. The integration of novel drug design and synthesis techniques, display library technology, delivery systems, bioengineering advancements, and artificial intelligence have significantly expedited the development of groundbreaking peptide-based drugs, effectively addressing the obstacles associated with their character, such as the rapid clearance and degradation, necessitating subcutaneous injection leading to increasing patient discomfort, and ultimately advancing translational research efforts. Peptides are presently employed in the management and diagnosis of a diverse array of medical conditions, such as diabetes mellitus, weight loss, oncology, and rare diseases, and are additionally garnering interest in facilitating targeted drug delivery platforms and the advancement of peptide-based vaccines. This paper provides an overview of the present market and clinical trial progress of peptide-based therapeutics, delivery platforms, and vaccines. It examines the key areas of research in peptide-based drug development through a literature analysis and emphasizes the structural modification principles of peptide-based drugs, as well as the recent advancements in screening, design, and delivery technologies. The accelerated advancement in the development of novel peptide-based therapeutics, including peptide-drug complexes, new peptide-based vaccines, and innovative peptide-based diagnostic reagents, has the potential to promote the era of precise customization of disease therapeutic schedule.

Eptifibatide, an Older Therapeutic Peptide with New Indications: From Clinical Pharmacology to Everyday Clinical Practice

Therapeutic peptides are oligomers or short polymers of amino acids used for various medical purposes. Peptide-based treatments have evolved considerably due to new technologies, stimulating new research interests. They have been shown to be beneficial in a variety of therapeutic applications, notably in the treatment of cardiovascular disorders such as acute coronary syndrome (ACS). ACS is characterized by coronary artery wall damage and consequent formation of an intraluminal thrombus obstructing one or more coronary arteries, leading to unstable angina, non-ST elevated myocardial infarction, and ST-elevated myocardial infarction. One of the promising peptide drugs in the treatment of these pathologies is eptifibatide, a synthetic heptapeptide derived from rattlesnake venom. Eptifibatide is a glycoprotein IIb/IIIa inhibitor that blocks different pathways in platelet activation and aggregation. In this narrative review, we summarized the current evidence on the mechanism of action, clinical pharmacology, and applications of eptifibatide in cardiology. Additionally, we illustrated its possible broader usage with new indications, including ischemic stroke, carotid stenting, intracranial aneurysm stenting, and septic shock. Further research is, however, required to fully evaluate the role of eptifibatide in these pathologies, independently and in comparison to other medications.

Protease-activated receptor 4 activity promotes platelet granule release and platelet-leukocyte interactions

Human platelets express two protease-activated receptors (PARs), PAR1 (F2R) and PAR4 (F2RL3), which are activated by a number of serine proteases that are generated during pathological events and cause platelet activation. Recent interest has focused on PAR4 as a therapeutic target, given PAR4 seems to promote experimental thrombosis and procoagulant microparticle formation, without a broadly apparent role in hemostasis. However, it is not yet known whether PAR4 activity plays a role in platelet-leukocyte interactions, which are thought to contribute to both thrombosis and acute or chronic thrombo-inflammatory processes. We sought to determine whether PAR4 activity contributes to granule secretion from activated platelets and platelet-leukocyte interactions. We performed in vitro and ex vivo studies of platelet granule release and platelet-leukocyte interactions in the presence of PAR4 agonists including PAR4 activating peptide, thrombin, cathepsin G, and plasmin in combination with small-molecule PAR4 antagonists. Activation of human platelets with thrombin, cathepsin G, or plasmin potentiated platelet dense granule secretion that was specifically impaired by PAR4 inhibitors. Platelet-leukocyte interactions and platelet P-selectin exposure the following stimulation with PAR4 agonists were also impaired by activated PAR4 inhibition in either a purified system or in whole blood. These results indicate PAR4-specific promotion of platelet granule release and platelet-leukocyte aggregate formation and suggest that pharmacological control of PAR4 activity could potentially attenuate platelet granule release or platelet-leukocyte interaction-mediated pathological processes.

Platelet signalling networks: pathway perturbation demonstrates differential sensitivity of ADP secretion and fibrinogen binding

Platelet signalling responses to single agonists have been identified previously. However, a model of the total platelet signalling network is still lacking. In order to gain insights into this network, we explored the effects of a range of platelet-function inhibitors in two independent assays of platelet function, namely fibrinogen binding and ADP secretion. In this study, we targeted the intracellular signalling molecules targeted intracellular signalling molecules, Syk and PI3K and targeted intracellular signalling molecules, Syk and PI3K, the prostaglandin synthesis enzyme cyclooxygenase, surface receptors for TxA(2) and ADP (P2Y1 and P2Y12) and the integrin cell adhesion molecule, αIIbβ3. We demonstrate that the platelet responses of fibrinogen binding and secretion can be differentially affected by the individual inhibitors permitting the generation of a model delineating novel regulatory links in the platelet signalling network. Importantly, the model illustrates the interconnections among portions that are traditionally studied as separate modules, promoting a more integrated view of the platelet.

The Role of Glycoprotein IIb/IIIa Inhibitors- A Promise Not Kept?

For over one decade Glycoproteins IIb/IIIa inhibitors (GPI) have been administered to prevent coronary artery thrombosis. Initially these agents were used for acute coronary syndromes and subsequently as adjunctive pharmacotherapy for percutaneous coronary interventions (PCIs).

Most benefit of GPI emerged from reduction of ischemic events: mostly non-q-wave myocardial infarctions (NQWMIs) during PCI. However, individual randomized clinical trials could not demonstrate that any of these agents could significantly reduce mortality in any clinical subset of patients. Studies of employing prolonged oral GPI administration resulted in excessive death. The non-homogenous statistically-significant reduction of ischemic endpoints was accompanied by an excess of bleeding, vascular complications, and thrombocytopenia. The clinical and ecomomic burden of major bleeding and thrombocytopenia is substantial. The ACUITY trial has initiate a new debate regarding the efficacy and safety of GPI.

Selective “patient-tailored” use of GPI limited to moderate-high risk PCI patients with low bleeding propensity is suggested. Research of new algorithms emphasizing abbreviated GPI administration, careful access site and bleeding surveillance, in conjunction with lower doses of unfractionated heparin or new and safer anti-thrombins may further enhance patient safety.

A new rapid method to measure human platelet cholesterol: a pilot study

INTRODUCTION

Platelet cholesterol (PC) could be used to assess "tissue" cholesterol of patients with vascular disease. However, the methods available so far to measure PC involve a complex extraction process. We developed a rapid method to measure PC and assessed its correlation with serum total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), LDL-C/HDL-C ratio, triglycerides (TG), and non-HDL-C.

METHODS

We assessed repeatability (20 times, 3 participants) and reproducibility (8 times, 2 participants). A group of 47 healthy participants was studied. Blood was collected to analyze serum TC, LDL-C, HDL-C, and TG. Citrated blood was used to prepare a platelet pellet. A "clear soup" was produced (by disrupting this pellet using freeze-thaw and sonication cycles) and used to measure PC.

RESULTS

Repeatability of PC showed a coefficient of variation (CV) of 4.8%. The reproducibility of PC over a period of 2 months was CV 7.5% and 8.1% (8 measurements for 2 participants). The PC of participants with serum LDL-C >2.6 mmol/L (treatment goal recommended by the National Cholesterol Education Program Adult Treatment Panel III) was 377 ± 120 μmol/10(12) platelets (n = 25). There was a significant correlation (Spearman, correlation coefficient) of PC (n = 25) with serum LDL-C (r(s) = 0.45, P = .02), LDL-C/HDL-C (r(s) = 0.45, P = .02), TG (r(s) = 0.43, P = .03), and non-HDL-C (r(s) = 0.53, P = .007).

CONCLUSION

This technique of measuring PC has the advantage of being reproducible, fast, and simpler than previous methods. Thus, it may be useful for multiple sampling when investigating changes in PC in hypercholesterolemic patients. More extensive evaluation is necessary.

Cell-targeting and cell-penetrating peptides for delivery of therapeutic and imaging agents

This review will discuss the basic concepts concerning the use of cell-targeting peptides (CTPs) and cell-penetrating peptides (CPPs) in the context of nanocarrier technology. It deals with the discovery and subsequent evolution of CTPs and CPPs, issues concerning their interactions with cells and their biodistribution in vivo, and their potential advantages and disadvantages as delivery agents. The article also briefly discusses several specific examples of the use of CTPs or CPPs to assist in the delivery of nanoparticles, liposomes, and other nanocarriers.

Ligand switching in cell-permeable peptides: manipulation of the alpha-integrin signature motif

A synthetic cell-permeable peptide corresponding to the highly conserved alpha-integrin signature motif, Palmityl-K(989)VGFFKR(995) (Pal-FF), induces integrin activation and aggregation in human platelets. Systematic replacement of the F(992)-F(993) with amino acids of greater or lesser hydrophobicity to create Pal-KVGxxKR peptides demonstrate that hydrophobic amino acids (isoleucine, phenylalanine, tyrosine, tryptophan) are essential for agonist potency. In marked contrast, substitution with small and/or hydrophilic amino acids (glycine, alanine, serine) causes a switch in the biological activity resulting in inhibition of platelet aggregation, adhesion, ADP secretion, and thromboxane synthesis. These substituted, hydrophilic peptides are not true pharmacological antagonists, as they actively induce a phosphotyrosine signaling cascade in platelets. Singly substituted peptides (Pal-AF and Pal-FA) cause preferential retention of pro- or anti-thrombotic properties, respectively. Because the alpha-integrin signature motif is an established docking site for a number of diverse cytoplasmic proteins, we conclude that eliminating critical protein-protein interactions mediated through the hydrophobic amino acids, especially F(993), favors an anti-thrombotic pathway in platelets. Agents derived from the inhibitory peptides described in this study may represent a new therapeutic strategy for anti-platelet or anti-integrin drug development.

Safety and tolerability of SCH 530348 in patients undergoing non-urgent percutaneous coronary intervention: a randomised, double-blind, placebo-controlled phase II study

BACKGROUND

An antithrombotic drug is needed that safely reduces cardiovascular events in patients undergoing percutaneous coronary intervention (PCI). We therefore assessed the tolerability and safety of SCH 530348-an oral platelet protease-activated receptor-1 antagonist.

METHODS

We randomly assigned patients aged 45 years or older and undergoing non-urgent PCI or coronary angiography with planned PCI to an oral loading dose of SCH 530348 (10 mg, 20 mg, or 40 mg) or matching placebo in a 3:1 ratio in a multicentre international study. Those in the SCH 530348 group who subsequently underwent PCI (primary PCI cohort) continued taking an oral maintenance dose (0.5 mg, 1.0 mg, or 2.5 mg per day), and patients in the placebo group continued placebo for 60 days. The primary endpoint was the incidence of clinically significant major or minor bleeding according to the thrombolysis in myocardial infarction (TIMI) scale. Both investigators and patients were unaware of treatment allocation. Analysis was by intention to treat. This study is registered with ClinicalTrials.gov, number NCT00132912.

FINDINGS

257 patients were assigned to placebo and 773 to SCH 530348. The primary endpoint occurred in 2 (2%) of 129, 3 (3%) of 120, and 7 (4%) of 173 patients, respectively, in the SCH 530348 10 mg, 20 mg, and 40 mg groups compared with 5 (3%) of 151 patients in the placebo group (p=0.5786). TIMI major plus minor bleeding occurred in 3 (2%) of 136, 5 (4%) of 139, and 4 (3%) of 138 patients given SCH 530348 0.5 mg, 1.0 mg, and 2.5 mg once per day, respectively (p=0.7561).

INTERPRETATION

Oral SCH 530348 was generally well tolerated and did not cause increased TIMI bleeding, even when administered concomitantly with aspirin and clopidogrel. Further testing in phase III trials to accurately define the safety and efficacy of SCH 530348 is warranted.

Compartmentalization regulates the interaction between the platelet integrin alpha IIb beta 3 and ICln

The volume-regulating protein, ICln, interacts with the conserved KxGFFKR alpha-integrin signature motif. ICln is an abundant protein (4455 +/- 650 molecules/platelet) found exclusively in the soluble cytosolic fraction of unactivated platelets. In contrast, its binding partner, the platelet integrin alpha(IIb)beta(3), is present in detergent-insoluble fractions associated with membrane and cytoskeleton subcellular localizations. This study investigated factors that regulate the interaction of ICln with alpha(IIb)beta(3) during platelet activation. His-tagged recombinant ICln bound equally to purified alpha(IIb)beta(3) and to integrin from resting or activated platelets. Binding was not affected by direct integrin activation with Mn(++) or by inhibitors of integrin occupancy (abciximab, RGD). However, the capacity for interaction between integrin and recombinant ICln was slowly downregulated following prolonged platelet activation for >300 s. In parallel, ICln redistributed to membrane and cytoskeletal platelet subcellular fractions. The time-course of this redistribution preceded the downregulation of integrin binding capacity and suggests that only a short window of opportunity exists for ICln interaction with alpha(IIb)beta(3) to occur. Thus, although ICln has the inherent capacity to bind to alpha(IIb)beta(3) regardless of its activation state, it can only do so following platelet activation. Activation-dependent subcellular redistribution of ICln represents a novel, temporally-regulated mechanism for control of integrin function in platelets.

Glycoprotein IIb/IIIa inhibitors: questioning indications and treatment algorithms

Glycoprotein inhibitors (GPI) are viewed as beneficial adjunctive pharmacotherapy agents for percutaneous coronary interventions (PCIs). The major benefit of GPI is derived from the reduction of ischemic events (mostly non-Q-wave myocardial infarctions) during PCI. There is no single randomized clinical trial demonstrating that any of these agents significantly reduces mortality in any clinical subset of patients. Studies of sustained oral GPI resulted in excessive death and myocardial infarctions. Reduction of ischemic end points was counteracted by excessive bleeding, vascular complications, and thrombocytopenia. These complications bear considerable medical and economic impact. The Acute Catheterization and Early Intervention Triage Strategy trial demonstrated that GPI, when added to heparin, enoxaparine, or bivalirudin, do not reduce mortality or ischemic events but significantly increase bleeding complications. Major bleeding resulted in threefold mortality at 1 year. In view of available data, the use of GPI should be limited to moderate-risk to high-risk PCI patients with low bleeding propensity. Protocols of abbreviated GPI administration and careful bleeding surveillance, in conjunction with lower doses of unfractionated heparin or new and possibly safer antithrombins, can potentially improve patient safety.

Heterotricyclic himbacine analogs as potent, orally active thrombin receptor (protease activated receptor-1) antagonists

Pursuing our earlier efforts in the himbacine-based thrombin receptor antagonist area, we have synthesized a series of compounds that incorporate heteroatoms in the C-ring of the tricyclic motif. This effort has resulted in the identification of several potent heterocyclic analogs with excellent affinity for the thrombin receptor. Several of these compounds demonstrated robust inhibition of platelet aggregation in an ex vivo model in cynomolgus monkeys following oral administration. A detailed profile of 28b, a benchmark compound in this series, with a Ki of 4.3 nM, is presented.

Impact of allocation concealment on conclusions drawn from meta-analyses of randomized trials

BACKGROUND

Randomized trials without reported adequate allocation concealment have been shown to overestimate the benefit of experimental interventions. We investigated the robustness of conclusions drawn from meta-analyses to exclusion of such trials.

MATERIAL

Random sample of 38 reviews from The Cochrane Library 2003, issue 2 and 32 other reviews from PubMed accessed in 2002. Eligible reviews presented a binary effect estimate from a meta-analysis of randomized controlled trials as the first statistically significant result that supported a conclusion in favour of one of the interventions.

METHODS

We assessed the methods sections of the trials in each included meta-analysis for adequacy of allocation concealment. We replicated each meta-analysis using the authors' methods but included only trials that had adequate allocation concealment. Conclusions were defined as not supported if our result was not statistically significant.

RESULTS

Thirty-four of the 70 meta-analyses contained a mixture of trials with unclear or inadequate concealment as well as trials with adequate allocation concealment. Four meta-analyses only contained trials with adequate concealment, and 32, only trials with unclear or inadequate concealment. When only trials with adequate concealment were included, 48 of 70 conclusions (69%; 95% confidence interval: 56-79%) lost support. The loss of support mainly reflected loss of power (the total number of patients was reduced by 49%) but also a shift in the point estimate towards a less beneficial effect.

CONCLUSION

Two-thirds of conclusions in favour of one of the interventions were no longer supported if only trials with adequate allocation concealment were included.

AlphaIIbbeta3 priming and clustering by orally active and intravenous integrin antagonists

BACKGROUND

Drugs that block platelet-platelet and platelet-fibrin interactions via the alpha(IIb)beta(3) (glycoprotein IIb/IIIa) receptor are used daily in patients undergoing percutaneous coronary interventions. Along with expected increases in spontaneous bleeding, clinical trials have revealed a surprising increase in thrombosis when these drugs are used without other anticoagulants. A better understanding of their mechanisms can minimize these risks.

OBJECTIVES

This study tested the hypothesis that interventions designed to block fibrinogen binding inevitably leave the alpha(IIb)beta(3) receptor in an activated state. It compared the effects on platelet function and alpha(IIb)beta(3) conformation of the orally active compounds orbofiban and roxifiban, the i.v. agents eptifibatide and tirofiban, and echistatin, an arginine-glycine-aspartate (RGD) disintegrin.

METHODS

The integrin antagonist concentrations required to saturate platelets and to block platelet-platelet and platelet-fibrin interactions were determined by flow cytometry, aggregometry, and clot-based adhesion assays, respectively. Analytical ultracentrifugation measured each antagonist's effects on the solution structure of alpha(IIb)beta(3). Fluorescence anisotropy provided equilibrium and kinetic data for integrin:antagonist interactions.

RESULTS

Both orally active drugs bound more tightly and inhibited platelet aggregation and adhesion to fibrin more effectively than echistatin. Analytical ultracentrifugation yielded this order for perturbing alpha(IIb)beta(3) conformation (priming) and promoting oligomerization (clustering): echistatin > eptifibatide > orbofiban > tirofiban > roxifiban. Roxifiban was also most effective at disrupting the rapidly forming/slowly dissociating alpha(IIb)beta(3):echistatin complex.

CONCLUSIONS

Our results suggest that the same molecular mechanisms that enable glycoprotein IIb/IIIa inhibitors to bind tightly to the alpha(IIb)beta(3) receptor and block fibrinogen binding contribute to their ability to perturb the resting integrin's conformation, thus limiting the safety and efficacy of both oral and i.v. integrin antagonists.

A Mechanistic Model for Paradoxical Platelet Activation by Ligand-Mimetic α IIb β 3 (GPIIb/IIIa) Antagonists

Objective— Integrins are attractive therapeutic targets. Inhibition of integrin α IIb β 3 effectively blocks platelet aggregation. However, limitations with intravenous α IIb β 3 antagonists and failure of oral α IIb β 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 α IIb β 3 antagonists and define three requirements: (1) Induction of ligand-bound conformation of α IIb β 3 , (2) receptor clustering, (3) prestimulation of platelets. Conformational change is inducible by clinically used ligand-mimetic α IIb β 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 α IIb β 3 antagonists results in platelet activation as detected by P-selectin, CD63, and CD40L expression as well as by measuring Ca 2+ -signaling. Blockade of the ADP receptor P2Y 12 by AR-C69931MX and clopidogrel inhibits α IIb β 3 antagonist-induced platelet activation.

Conclusion— These findings can explain limitations of ligand-mimetic anti-α IIb β 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.

Oral platelet glycoprotein IIb/IIIa receptor inhibitors--Part I

With the central importance of antiplatelet therapy in patients with coronary artery disease and the numerous positive trials with glycoprotein (GP) IIb/IIa inhibitors given intravenously, it was hoped that one could extend the benefit of IIb/IIIa inhibition to long-term treatment. Although the hypothesis that prolonged oral IIb/IIIa inhibition was appealing, many issues have been identified in the initial Phase II trials that would limit the usefulness of these compounds. Variability of the level of platelet inhibition was one major culprit that distinguished the oral compounds from intravenous ones. The problems that arose were that increased bleeding has been seen when levels of platelet inhibition are high (e.g., > 90%) and that, conversely, efficacy would likely be limited when levels of platelet inhibition were low. If further development of this class of drugs is undertaken, formal dosing studies would have to establish an oral dosing strategy that achieves appropriately high (80-95% inhibition) and steady levels of inhibition.

Integrin alphaIIbbeta3:ligand interactions are linked to binding-site remodeling

This study tested the hypothesis that high-affinity binding of macromolecular ligands to the alphaIIbbeta3 integrin is tightly coupled to binding-site remodeling, an induced-fit process that shifts a conformational equilibrium from a resting toward an open receptor. Interactions between alphaIIbbeta3 and two model ligands-echistatin, a 6-kDa recombinant protein with an RGD integrin-targeting sequence, and fibrinogen's gamma-module, a 30-kDa recombinant protein with a KQAGDV integrin binding site-were measured by sedimentation velocity, fluorescence anisotropy, and a solid-phase binding assay, and modeled by molecular graphics. Studying echistatin variants (R24A, R24K, D26A, D26E, D27W, D27F), we found that electrostatic contacts with charged residues at the alphaIIb/beta3 interface, rather than nonpolar contacts, perturb the conformation of the resting integrin. Aspartate 26, which interacts with the nearby MIDAS cation, was essential for binding, as D26A and D26E were inactive. In contrast, R24K was fully and R24A partly active, indicating that the positively charged arginine 24 contributes to, but is not required for, integrin recognition. Moreover, we demonstrated that priming--i.e., ectodomain conformational changes and oligomerization induced by incubation at 35 degrees C with the ligand-mimetic peptide cHarGD--promotes complex formation with fibrinogen's gamma-module. We also observed that the gamma-module's flexible carboxy terminus was not required for alphaIIbbeta3 integrin binding. Our studies differentiate priming ligands, which bind to the resting receptor and perturb its conformation, from regulated ligands, where binding-site remodeling must first occur. Echistatin's binding energy is sufficient to rearrange the subunit interface, but regulated ligands like fibrinogen must rely on priming to overcome conformational barriers.

Mapping the Dose–Effect Relationship of Orbofiban from Sparse Data with an Artificial Neural Network

A neural network (NN) pharmacodynamic model was developed that correlates the inhibition of ex vivo platelet aggregation by orbofiban, an oral glycoprotein IIb/IIIa antagonist, with the administered dose and patient characteristics. Data were obtained from a Phase-II dose-finding study of orbofiban in patients presenting with acute coronary syndromes. A back-propagation NN was designed to predict drug effect measured at predose and 4 and 6 h on treatment days 1, 28, and 84 (nine responses/patient). The training set consisted of patients for whom complete response profiles were reported (n = 67), and remaining patients were included in the validation data set (n = 47). The concentration-effect relationship was described additionally using a population direct-effect inhibitory sigmoidal model, and a comparison of the predictive performances of both models was performed. The final NN reasonably described orbofiban pharmacodynamics from sparse data sets without specifying a structural model or drug concentrations. Despite considerable inter-patient variability in response-time profiles, the population model revealed a strong correlation between drug concentration and effect and exhibited greater precision than the NN model. Although the population model showed greater precision, these results suggest that NNs may be useful for individualizing pharmacotherapy when drug concentrations are relatively unpredictable or unavailable.

Simultaneous modeling of abciximab plasma concentrations and ex vivo pharmacodynamics in patients undergoing coronary angioplasty

An integrated structural pharmacokinetic/pharmacodynamic (PK/PD) model was developed for the glycoprotein IIb/IIIa antagonist abciximab administered to patients undergoing percutaneous transluminal coronary angioplasty. PK/PD data, in the form of plasma abciximab concentrations and ex vivo platelet aggregation in the presence of 20 microM adenosine diphosphate, were obtained from two previously conducted clinical studies. Study 1 consisted of patients who were given abciximab as a single intravenous injection of 0.25 mg/kg (n = 32). Patients in study 2 received an identical bolus dose, followed by a 36-h infusion at 0.125 microg/kg/min (n = 15). The PK component of the final model included drug-receptor binding, nonspecific distribution, and linear systemic clearance, whereas the PD module assumed that ex vivo dynamics were controlled by free plasma drug concentration. Mean PK/PD data from both studies were fitted simultaneously using nonlinear regression. PK profiles from both studies show rapidly decreasing plasma abciximab concentrations at early time points, but with extended terminal disposition phases. The maximum effect (Emax = 83.6%) was achieved rapidly and gradually returned to baseline values, although inhibition could be measured long after abciximab concentrations dropped below the detection limit. The final model well described the resulting PK/PD profiles and allowed for parameter estimation with relatively small coefficients of variation. Simulations were conducted to assess predicted receptor-occupancy and effects of selected parameters on PK/PD profiles. Models such as the one developed in this study demonstrate how drug binding to pharmacological targets may influence the PK of certain drugs and also provide a suitable paradigm for defining the PK/PD relationships of similar compounds.

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.

Prothrombotic and antithrombotic pathways in acute coronary syndromes

The acute coronary syndromes arise from procoagulant changes in complex plaques, which trigger both platelet activation and coagulation pathways. These 2 pathways intersect at a number of points that form positive-feedback loops to sustain and accelerate thrombus formation. In normal hemostasis and with a healthy endothelium, intravascular thrombosis is prevented, and vascular patency is protected by the fibrinolytic system and a number of antithrombotic factors, such as antithrombin, thrombomodulin, and tissue factor pathway inhibitor. However, atherosclerosis is characterized by a hypercoagulable state, and the fibrinolytic balance is skewed toward occlusive thrombus formation at critical sites on vulnerable plaques. This review focuses on cellular and humoral mechanisms and the antithrombotic strategies that are important during the acute phase of an ischemic coronary syndrome, both in patients managed conservatively and in patients scheduled for an interventional procedure. These strategies include fibrinolytic therapy, antiplatelet therapies (aspirin, clopidogrel, glycoprotein IIb/IIIa receptor inhibitors), and low-molecular-weight heparin.

Role of antiplatelet drugs in the prevention of cardiovascular events

Antiplatelet drugs have an established place in the prevention of vascular events in a variety of clinical conditions, such as myocardial infarction, stroke and cardiovascular death. Both European and American guidelines recommend the use of antiplatelet drugs in patients with established coronary heart disease and other atherosclerotic disease. In high-risk patients, such as those with post-acute myocardial infarction (AMI), ischaemic stroke or transient ischaemic attack, and in patients with stable or unstable angina, peripheral arterial occlusive disease or atrial fibrillation, antiplatelet treatment may reduce the risk of a serious cardiovascular event by approximately 25%, including reduction of non-fatal myocardial infarction by 1/6, non-fatal stroke by 1/4 and cardiovascular death by 1/6. Some data indicate that antiplatelet drugs may also have a role in primary prevention. In people who are aged over 65 years, or have hypertension, hypercholesterolaemia, diabetes, obesity or familial history of myocardial infarction at young age, aspirin may reduce both cardiovascular deaths and total cardiovascular events. Aspirin has been studied and used most extensively. It may exert its beneficial effect not only by acting on platelets, but also by other mechanisms, such as preventing thromboxane A2 (TXA2)-induced vasoconstriction or reducing inflammation. Indeed, experimental data show that low-dose aspirin may suppress vascular inflammation and thereby increase the stability of atherosclerotic plaque. Moreover, in human studies, aspirin seems to be most effective in those with elevated C-reactive protein levels. Vascular events, however, do occur despite aspirin administration. This may be due to platelet activation by pathways not blocked by aspirin, intake of drugs that interfere with aspirin effect or aspirin resistance. In the CAPRIE (Clopidogrel vs. Aspirin in Patients at Risk of Ischaemic Events) study, long-term clopidogrel administered to patients with atherosclerotic vascular disease was more effective than aspirin in reducing the combined risk of ischaemic stroke, myocardial infarction or vascular death. In the setting of coronary stenting, a double regimen including aspirin and ticlopidine or clopidogrel has proved more effective in the prevention of in-stent thrombosis than aspirin alone. Chronic oral administration of the inhibitors of platelet membrane receptor GP IIb/IIIa has been largely disappointing.