Antiplatelet activity of β‐blockers: new light on existing data (2014)

β-Adrenergic receptor blockers have been used successfully for decades for the prevention of cardiovascular events in the setting of arterial hypertension and coronary artery disease. The biological plausibility of this protective activity is supported by the reduction in blood pressure and intrinsic adrenoreceptor-mediated cardiac effects, only indirectly related to thrombogenesis. Limited information is available concerning additional mechanisms that may contribute directly to the protective activity of individual β-blockers, or of the class as a whole, towards atherothrombosis. A flare of scientific investigation accompanied the development of β-blockers, aimed at defining their potential as antiplatelet agents. In order to investigate the effects of individual β-blockers on platelet function, light transmission aggregometry (LTA) was mostly used, either to explore in vitro the effects of the tested compounds up to very high concentrations and ex vivo, in treated patients and healthy subjects. A series of small, but adequately designed clinical trials was generated in a time span of three decades. The systematic review and meta-analysis performed by Bonten et al. 1. now summarizes the sparse, inconsistent information, including clinical studies in which LTA was used to examine platelet function as a surrogate end-point of an antithrombotic activity of different β-blockers. Converging evidence suggests that specific compounds actually inhibit ex vivo platelet aggregation, and that antiplatelet activity may not be regarded as a class effect. A substantial heterogeneity exists among β-blockers concerning their selectivity for the adrenergic receptors and the presence of specific biochemical characteristics, of relevance for the pharmacokinetic and pharmacodynamic properties, dictating a preferential use in the treatment of arterial hypertension, ischaemic heart disease, heart failure or noncardiovascular diseases. This may be the case also for the antiaggegatory activity observed with β-blockers. Of the tested compounds, particularly lipophilic and nonselective β-blockers display this activity 1. Explanation of a clinical finding, however, necessitates preliminary mechanistic investigation. According to this proposition, several in vitro studies have shown that the antiplatelet activity of some β-blockers is independent of occupancy of β-adrenergic receptors, which are scarcely expressed on the platelet surface and do not transduce proaggregatory signals 2. Propranolol in vitro, at concentrations similar to those achieved in vivo, raises the threshold dose for the platelet aggregatory response to adenosine diphosphate (ADP). At higher concentrations, a broarder spectrum of activities was identified; platelet adhesion, aggregation induced by different agonists and the release reaction were inhibited 3. The effects of β-blockers on human platelets have been related to their liposolubility; for example, carvedilol, which is more lipophilic than propranolol, possibly by interacting with membrane macromolecules, produces a dose-dependent, adrenergic receptor-independent, antiplatelet effect 4, 5. Carvedilol has additional antioxidant and vasodilator properties that may contribute to its efficacy in the treatment of heart failure 6. However, neither the biochemical mechanisms nor the signalling pathways leading to inhibition of platelet activation have been investigated in detail. Recently, nebivolol was found in vitro to exert antiaggregatory effects, mediated by the release of nitric oxide in platelets, which translates into a blunted platelet-dependent thrombogenesis in experimental models 6, 7. Some critical points should be considered concerning the methodology of meta-analyses dealing with studies of small sample size and/or using surrogate end-points. The overall quality (or risk of bias) of a meta-analysis is directly proportional to the quality of the primary studies 8. Distortion deriving from biased primary studies increases exponentially if surrogate end-points are used that are not well standardized, particularly if technically demanding and of limited use. Thus, the common criticism, applicable to all meta-analyses, i.e. ‘to combine apples with oranges’, could be considerably worsened when analysing platelet aggregation, in this specific case, or endothelial function, in another plausible scenario. When LTA is used as a quantitative index of platelet response, standardized stimuli, careful consideration of the assay conditions and appreciation of methodological variability are required, along with qualitative inspection of the aggregation curve, in order to define the response to stimuli, as in the case of ADP 9. A direct comparison of different studies is not feasible when threshold doses of agonists rather than the amplitude of the aggregation curve is used to quantify the platelet response to an agonist, defining an inhibitory effect that could be missed if high doses of the agonist are used. Difficulties are also generated by the absence of a predefined value of platelet aggregation that defines a meaningful biological effect. This is, for instance, the case for aspirin and nonsteroidal anti-inflammatory drugs, for which platelet aggregation is ‘inappropriately sensitive’ as an index of platelet function when compared with the generation of thromboxane A2, the biomarker of platelet cyclo-oxygenase activity 10. Light transmission aggregometry is an ex vivo analysis of platelet function that may not be adequate to detect transient effects that occur in vivo, such as those depending on the release of nitric oxide in the case of nebivolol 7. Other tests, useful to quantify platelet activation in vivo, have been developed. Soluble P-selectin, which is shed from activated platelets, has been used extensively to assess the pathophysiology of atherothrombosis, but only in a limited number of studies investigating the effects of antiplatelet agents and β-blockers 11. Finally, functional tests or biomarkers of in vivo platelet activation need to be strictly related to the biochemical effect of the studied drug, as is the measurement of thromboxane A2 with aspirin 10. Platelet functionality may be altered as a consequence of the clinical characteristics of the studied subjects, and this may represent a confounder limiting the identification of pharmacodynamic effects when nonspecific tests are applied. Disregarding technical problems related to LTA, ‘traditional bias’ associated with meta-analyses, especially selection bias, should be pursued, even more than in traditional clinically oriented meta-analyses. Measures of heterogeneity should be reported clearly and sensitivity analysis deepened. To improve the reliability of meta-analyses, grading scales to measure the quality of primary studies are increasingly used 12. Unfortunately, most of these scales have been created to suit clinical trials in comparison with observational studies and refer to randomization, blindness etc. In the specific case of meta-analyses of studies using surrogate end-points, a grading scale should ideally be created also taking into account the specific devices and procedures used. In the absence of large-scale clinical trials addressing the prevention of cardiovascular events with different β-blockers, the meta-analysis performed by Bonten et al. sheds light on the elusive antiplatelet activity of β-blockers using strict criteria in study selection and definition of the parameters of platelet function analysis 1. Although not the ideal test, LTA is suitable for a preliminary investigation of β-blockers with potential antiplatelet activity when either healthy subjects or patients were considered. To overcome methodological imitations, data concerning platelet aggregation were normalized and grouped according to the clinical conditions of the studied subjects, duration of exposure, platelet agonists and the protocols used in the analysis of aggregation 1. This careful inspection of existing data now defines a previously unappreciated antiplatelet activity of β-blockers and raises interest in the underlying mechanisms and its clinical implications. Extensive evidence of cardiovascular protection with β-blockers has been obtained from large-scale randomized clinical trials and meta-analyses. However, limited information is available from head-to-head comparisons of new and old compounds concerning major cardiovascular events. Heterogeneity among individual compounds appears evident. The evidence of an antiplatelet activity endowed in some β-blockers needs to be substantiated by further clinical investigation. All authors have completed the Unified Competing Interest form at http://www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: no support from any organization for the submitted work for the submitted work; no financial relationships with any organizations that might have an interest in the submitted work in the previous 3 years; no other relationships or activities that could appear to have influenced the submitted work.