Antiplatelet therapy in secondary stroke prevention

Aspirin resistance in South African Caucasian patients with thrombotic cerebrovascular events

OBJECTIVE

Stroke is the second commonest cause of death in both high and low- and middle-income countries [Lopez AD, Mathers CD, Ezzati M, Jamison DT, Murray CJ. Global and regional burden of disease and risk factors, 2001: systematic analysis of population health data. The Lancet 2006; 367:1747-57]. In South Africa, a population undergoing demographic and epidemiological transition, stroke is the third commonest cause of death [Norman R, Bradshaw D, Schneider M, Pieterse D, Groenewald P. Revised burden of disease estimates for the comparative risk factor assessment, South Africa, 2000. Cape Town: Medical Research Council. 2006]. Although aspirin remains an essential part of stroke prevention, platelet response to therapeutic doses is not uniform. Some patients exhibit aspirin resistance and develop secondary thrombotic events. We decided to determine the prevalence of aspirin resistance and/or platelet hypersensitivity, as determined by platelet aggregometry, in sixty Caucasian patients who have suffered one or more Strokes and/or Transient Ischaemic Attacks (TIAs) as compared with sixty control subjects.

METHODS

Aspirin resistance was determined by platelet aggregation (>20%) to one or more of the four agonists, namely arachidonic acid (1.5 mM), adrenaline (0.05 microg/ml), collagen (0.2 microg/ml) or ADP (0.1x10(-5) M).

RESULTS

Two patients demonstrated "complete aspirin resistance" (non-responder to aspirin) with resistance to arachidonic acid (high concentration) noted. Three patients demonstrated "partial aspirin resistance" (semi-responder to aspirin). One contol subject showed "complete aspirin resistance". There is a 1.67% chance of a control subject being resistant to aspirin in a general South African Caucasian population. A history of prior stroke or transient ischaemic attack was associated with a statistically significant increase in risk of aspirin resistance with an odds ratio of 5.36.

CONCLUSION

These results essentially concur with those of the studied literature in showing an 8% prevalence (statistically significant) of aspirin resistance (complete and partial) in South African Caucasian patients with previous atherothrombotic cerebrovascular events i.e. CVAs and/or TIAs. The current study shows an increased prevalence of aspirin resistance in people who have had prior strokes/TIAs and raises the question whether people who have had these events are somehow predisposed to vascular events or indeed recurrent vascular events. "Aspirin resistant" patients or "poor responders" to aspirin must be considered at heightened risk of atherothrombotic events and laboratory monitoring of antiplatelet therapy may become clinically useful.

Inhibition of glucose uptake in murine cardiomyocyte cell line HL-1 by cardioprotective drugs dilazep and dipyridamole

Inhibition of adenosine reuptake by nucleoside transport inhibitors, such as dipyridamole and dilazep, is proposed to increase extracellular levels of adenosine and thereby potentiate adenosine receptor-dependent pathways that promote cardiovascular health. Thus adenosine can act as a paracrine and/or autocrine hormone, which has been shown to regulate glucose uptake in some cell types. However, the role of adenosine in modulating glucose transport in cardiomyocytes is not clear. Therefore, we investigated whether exogenously applied adenosine or inhibition of adenosine transport by S-(4-nitrobenzyl)-6-thioinosine (NBTI), dipyridamole, or dilazep modulated basal and insulin-stimulated glucose uptake in the murine cardiomyocyte cell line HL-1. HL-1 cell lysates were subjected to SDS-PAGE and immunoblotting to determine which GLUT isoforms are present. Glucose uptake was measured in the presence of dipyridamole (3-300 microM), dilazep (1-100 microM), NBTI (10-500 nM), and adenosine (50-250 microM) or the nonmetabolizable adenosine analog 2-chloro-adenosine (250 microM). Our results demonstrated that HL-1 cells possess GLUT1 and GLUT4, the isoforms typically present in cardiomyocytes. We found no evidence for adenosine-dependent regulation of basal or insulin-stimulated glucose transport in HL-1 cardiomyocytes. However, we did observe a dose-dependent inhibition of glucose transport by dipyridamole (basal, IC(50) = 12.2 microM, insulin stimulated, IC(50) = 13.09 microM) and dilazep (basal, IC(50) = 5.7 microM, insulin stimulated, IC(50) = 19 microM) but not NBTI. Thus our data suggest that dipyridamole and dilazep, which are widely used to specifically inhibit nucleoside transport, have a broader spectrum of transport inhibition than previously described. Moreover, these data may explain previous observations, in which dipyridamole was noted to be proischemic at high doses.