Platelet biology and response to antiplatelet therapy in women: implications for the development and use of antiplatelet pharmacotherapies for cardiovascular disease

Women are underrepresented in cardiovascular studies, even as their preponderance in the aging population steadily increases. Although concerns have been raised about the differential benefit of antiplatelet medications for women, the propensity for increased bleeding among women has also been recognized. A better understanding of the factors contributing to the observed sex-related differences in platelet biology is warranted. These factors include differences in the frequency and expression of genetic polymorphisms affecting platelet responsiveness to agonists (with and without antiplatelet therapies), which might be obtained through population-based studies and in large controlled clinical trials; inflammatory marker levels and their influence on atherothrombotic risk, and the role of specific hormones in mediating platelet activation and function. Knowledge gained about these mechanistic factors might inform the development of sex-specific antithrombotic treatment regimens that confer optimized safety and efficacy.

Genetic regulation of platelet receptor expression and function: application in clinical practice and drug development

Understanding genetic contributions to platelet function could have profound clinical ramifications for personalizing platelet-directed pharmacotherapy, by providing insight into the risks and possible benefits associated with specific genotypes. This article represents an integrated summary of presentations related to genetic regulation of platelet receptor expression and function given at the Fifth Annual Platelet Colloquium in January 2010. It is supplemented with additional highlights from the literature covering (1) approaches to determining and evidence for the associations of genetic variants with platelet hypo- and hyperresponsive phenotypes, (2) the ramifications of these polymorphisms with regard to clinical responses to antiplatelet therapies, and (3) the role of platelet function/genetic testing in guiding antiplatelet therapy.

Bridging the gap between clinical trials of antiplatelet therapies and applications among elderly patients

Although patients aged > or =75 years represent nearly 40% of all those hospitalized with acute coronary syndromes, their enrollment in trials of therapeutic interventions has been relatively modest. Thus, scarce information exists to guide clinicians in decision-making and assessing projections of safety and efficacy for antiplatelet agents. The pathobiology of aging, including age-related changes in vascular repair and integrity, applies to patient management and offers a platform for investigation. Because older patients receive excess dosing of antithrombotic agents much more often than their younger counterparts do, initial steps toward optimized care include attention to indications, dosing, and duration of treatment. This review, representing a summary of information presented at the Fourth Annual Platelet Colloquium held in Washington, DC, in January 2009 and supplemented with recent clinical trial results, underscores an increasingly narrow safety index for antiplatelet agents in the elderly and the all-important balance of safety and efficacy--a dynamic continuum that remains paramount in quality of care. Considerations for future trial designs, registries, and analyses of existing data are highlighted to better guide clinicians toward the optimal management of this rapidly growing, high-risk group.

Platelet functions beyond hemostasis

Although their central role is in the prevention of bleeding, platelets probably contribute to diverse processes that extend beyond hemostasis and thrombosis. For example, platelets can recruit leukocytes and progenitor cells to sites of vascular injury and inflammation; they release proinflammatory and anti-inflammatory and angiogenic factors and microparticles into the circulation; and they spur thrombin generation. Data from animal models suggest that these functions may contribute to atherosclerosis, sepsis, hepatitis, vascular restenosis, acute lung injury, and transplant rejection. This article represents an integrated summary of presentations given at the Fourth Annual Platelet Colloquium in January 2009. The process of and factors mediating platelet-platelet and platelet-leukocyte interactions in inflammatory and immune responses are discussed, with the roles of P-selectin, chemokines and Src family kinases being highlighted. Also discussed are specific disorders characterized by local or systemic platelet activation, including coronary artery restenosis after percutaneous intervention, alloantibody-mediated transplant rejection, wound healing, and heparin-induced thrombocytopenia.

G-protein-coupled receptors as signaling targets for antiplatelet therapy

Platelet G protein-coupled receptors (GPCRs) initiate and reinforce platelet activation and thrombus formation. The clinical utility of antagonists of the P2Y(12) receptor for ADP suggests that other GPCRs and their intracellular signaling pathways may represent viable targets for novel antiplatelet agents. For example, thrombin stimulation of platelets is mediated by 2 protease-activated receptors (PARs), PAR-1 and PAR-4. Signaling downstream of PAR-1 or PAR-4 activates phospholipase C and protein kinase C and causes autoamplification by production of thromboxane A(2), release of ADP, and generation of more thrombin. In addition to ADP receptors, thrombin and thromboxane A(2) receptors and their downstream effectors-including phosphoinositol-3 kinase, Rap1b, talin, and kindlin-are promising targets for new antiplatelet agents. The mechanistic rationale and available clinical data for drugs targeting disruption of these signaling pathways are discussed. The identification and development of new agents directed against specific platelet signaling pathways may offer an advantage in preventing thrombotic events while minimizing bleeding risk.

Collective responsibility and the practice of medicine

In the following essay, the theoretical apparatus for distinguishing various types of collectivities (aggregates and conglomerates) is described. This is followed by a consideration of how responsibility ascriptions to different types of collectivities are to be understood vis à vis those to individual group members. It is suggested that the "medical profession" (distinctly different from the "medical team" and the "hospital corporation") is an aggregate collectivity. That is, the "medical profession" consists of the "sum" of the identities of its membership, which can be shown to entail that if the "profession" is held responsible for something, each of its members is responsible, in some way, for it. This is to suggest that the "medical profession" is not a shield that hides individual medical practitioners from responsibility for the general state of health care. Quite the contrary. The use of the name of the aggregate in such a responsibility ascription puts each and every one of them "on call."