Triple Anti-platelet Therapy

Role of platelets in acute coronary syndromes (ACS) is well established. Anti-platelet agents are standard of care for the prevention of ACS. However, due to the high risk of bleeding, anti-platelet agents except aspirin are not indicated for the prevention of ACS in primary prevention population as the risk of an ACS event is low. However, individuals with established CAD are at an increased risk of subsequent events and aspirin is indicated for such patients. In addition, those who has had a stent placed, usually get dual anti-platelet therapy (aspirin + either clopidogrel, prasugrel, or ticagrelor). One would imagine that in a very high-risk population, inhibition of an additional pathway may provide additional benefit. However, the TRACER trial, found that addition of vorapaxar, an oral protease-activated-receptor 1 (PAR 1) antagonist that inhibits thrombin-induced platelet activation, had no additional benefit in patient with acute coronary syndrome. In stead, addition of vorapaxar to the standard dual anti-platelet regiment was associated with increase risk of major bleeding. This study, suggested that perhaps too much of platelet inhibition may not be beneficial for ACS prevention but increases risk of bleeding.

More recently, a meta-analysis found that adding cilostazol, another anti-platelet agent that acts by inhibiting phosphodiestrase, to standard dual anti-platelet therapy was associated with 36% reduction in major adverse cardiac events (MACE; odds ratio (OR) = 0.64; 95% confidence interval (CI) = 0.51-0.81, P < .01), a 40% reduction (OR = 0.60, 95% CI = 0.44-0.80; P < .01) in target vessel revascularization (TVR), a 44% reduction (OR = 0.56, 95% CI = 0.34-0.91; P = .02) in target lesion revascularization (TLR) and a 47%/44% reduction in in-segment/in-stent restenosis (P < .01) and lower in-segment/in-stent late loss (P < .01). The effect sizes are large showing that the addition of cilostazol is very effective in reducing events. Cilostazol also inhibits smooth muscle contraction resulting in peripheral arterial dilatation. It is possible that the beneficial effect may be due to a combination of these two effects.

Thrombocytopenia in vWD type 2B

Von Willebrand factor (vWF) is a chaperone protein for coagulation factor VIII and is essential for the recruitment of platelets to the growing thrombus under conditions of high shear stress usually present in the arterial system. Deficiency (quantitative or qualitative) of vWF is associated with bleeding tendency, clinically known as von Willebrand disease (vWD).   Type 2 vWD is due to functional defect in vWF and type 2B is associated with gain-of-function mutations in the exon 28 of vWF gene resulting in an increase in the affinity of VWF for platelets. The region encoded by exon 28 binds to the platelet vWF receptor, glycoprotein Iba (GpIba). Patient with type 2B vWD present with bleeding and moderate to severe thrombocytopenia as well as a decreased in the high molecular weight vWF multimers. Thrombocytopenia is associated with the presence of giant platelets and spontaneous platelet aggregates. The molecular mechanism underlying the thrombocytopenia are unclear.

GpIba is present on the surface of megakaryocytes as well on platelets. Thus it is possible that interaction of mutated vWF from patients with type 2B vWD with megakaryocytes results in decreased platelet formation and release of giant platelets. In fact, Nurden et al showed that this may be the case. They showed that culture of megakaryocytes from controls performed with or without purified vWF had a positive influence on platelet production with specific inhibition by an antibody blocking vWF binding to GpIba . Megakaryocytes cultured with vWF from patients with type 2B vWD showed disorganized demarcation membrane system and abnormal granule distribution when examined under electron microscopy. The platelets produced from such megakaryocytes had abnormalities similar to those found in patients with vWD type 2B. This impaired megakaryocytopoiesis could not only explain the occurrence of giant platelets, but also contribute to a lower platelet count in VWD type 2B patients.

In addition to defects in platelet production, there may also be defects in platelet utilization, that is increased uptake of platelets (with attached vWF) by the monocyte-macrophage system of the body. Casari et al showed that this is also the case in a series of experiments reported here. They found that vWD type 2B platelets have a shorter circulatory half-life than wild-type (wt) platelets, which could contribute to the lower platelet counts in vWD type 2B mice. Further analysis revealed that VWF type 2B is present at the surface of platelets of thrombocytopenic
vWD type 2B mice, and that these vWF/platelet complexes were taken up efficiently by macrophages in liver and spleen. Thus, they provide direct evidence that part of the thrombocytopenia in vWD type 2B can be explained by an increased clearance of VWF/platelet
complexes.

Flow Cytometry Analysis in R

Here are some really nice lectures on this topic.