In Vitro Demonstration of Direct Platelet-Related Effects of PCSK9 Enzyme

The pivotal role of platelets during arterial ischemic events, such as myocardial infarction (MI), stroke, and cardiovascular (CV) death, and the significant role of potent antiplatelet therapy in reducing these events have been well defined. An independent association between plasma PCSK9 levels, elevated platelet reactivity, and ischemic outcomes in patients with acute coronary syndromes (ACS) has been demonstrated. The association of plasma PCSK9 levels with coronary artery disease (CAD) risk, independent of low-density lipoprotein cholesterol (LDL-C) levels, has been suggested. Based on a meta-analysis of trials of medium-high risk cardiovascular disease (CVD) patients, the investigators have demonstrated a significant mortality and MI reduction with PCSK9 antibodies and suggested a relevant ant ischemic/antiplatelet role of PCSK9 antibodies. These observations support the hypothesis that the anti-ischemic effects of PCSK9 inhibitor therapy are related to its pleiotropic effects on platelets.

In vitro, animal, and human studies have suggested the direct effects of PCSK9 and PCSK9 inhibitors on platelets. PCSK9 indirectly activates platelets through elevated LDL-C and oxLDL levels. oxLDL can activate platelets by binding to the scavenger receptors CD36 and LOX-1. In addition, a direct relationship between PCSK9 and platelet activation through the scavenger receptors CD36 and Lectin-like oxidized LDL receptor (LOX)-1 has been suggested. Incubation of human recombinant PCSK9 protein with platelet-rich plasma collected from healthy subjects significantly enhanced activated GPIIb/IIIa receptor expression and platelet aggregation induced by suboptimal concentrations of epinephrine. In in vitro experiments, human recombinant PCSK9 exposure enhanced LOX-1 expression in human vascular endothelial cells and smooth muscle cells. Similarly, LOX-1 KO mice showed lower PCSK9 levels and PCSK9-KO mice showed much lower LOX-1 levels, indicating a reciprocal relationship between PCSK9 and LOX-1. A similar reciprocal relationship between PCSK9 levels and LOX-1 and CD36 expression has not been demonstrated in human platelets.

In vitro immunoblotting experiments demonstrated the presence of PCSK9 in platelets obtained from patients with CAD, and flow cytometry experiments demonstrated that collagen-related peptide (CP) significantly enhanced the surface expression of PCSK9 on platelets. Furthermore, PCSK9 antibodies significantly inhibited ADP- and CP-induced platelet aggregation and platelet-dependent thrombus formation on the immobilized collagen surface under high shear rate (1000s-1). Binding of PCSK9 to CD36 triggers platelet activation through activation of nicotinamide adenine dinucleotide phosphate oxidase (Nox)2, release of reactive oxygen species (ROS), and subsequent activation of GPIIb/IIIa receptor and p-selectin release. In addition, Nox2 induces the generation of 8-Iso-prostaglandin F2α and thromboxane (Tx) A2 through the activation f cytosolic phospholipase A2 and finally results in the activation of the GP-IIb/IIIa receptor and p-selectin release. The relationship between plasma PCSK9, urinary 11-dehydro-TxB2, and CV events in patients with atrial fibrillation has been demonstrated. Furthermore, washed platelets resuspended in plasma from patients with familial hypercholesterolemia after PCSK9 inhibitor treatment induced lower platelet aggregation and soluble Nox2-derived peptide (sNox2-dp) release than those obtained using plasma before PCSK9 inhibitor treatment in an in vitro study. This reduction was reversed by the addition of oxLDL. Furthermore, oxLDL-induced platelet aggregation was reduced by inhibiting CD36, LOX-1, and Nox2. In the same study, 6-month treatment with PCSK9 inhibitors was shown to reduce serum LDL-C, oxLDL, serum TxB2, sNox2-dp, and PCSK9 levels.

These preliminary observations suggest that:

* CV benefits associated with PCSK9 inhibitor therapy may be in part related to its pleiotropic effects on platelets. * In vitro, animal, and clinical studies suggested direct effects of PCSK9 on platelets. * PCSK9 influence platelet function through CD36/LOX-1 receptor mediated Nox2 activity.

However, the potential direct platelet effects of PCSK9 on platelets have not been demonstrated in a comprehensive study involving a diverse population of healthy volunteers and subjects with cardiovascular risk factors. Inclusion of at least 3 CV risk factors will increase the chance of finding subjects with an elevated platelet reactivity phenotype and demonstration of enhanced platelet-related effect of PCSK9. The presence of these risk factors has been traditionally attributed to increased prevalence of CVD and associated risk.

In contemporary CV clinical trials, a very low percentage (\<10%) of African Americans are represented despite a more pronounced CVD risk. Furthermore, it has been demonstrated that PCSK9 levels are significantly higher in African Americans vs Caucasians (104 ± 29 vs 95 ± 30 ng/mL, respectively; P = 0.020). At Sinai Hospital of Baltimore, the investigators have reported higher rates of post-PCI ischemic events and greater mortality in African Americans than in Whites. Rare loss-of-function (LoF) mutations of PCSK9 have been associated with low levels of LDL-C (28% lower in Blacks and 15% in Whites) and with an even more impressive reduction in the risk of coronary artery disease (CAD) (88% and 47%, respectively). PCSK9 LoF variants were associated with 35 mg/dL and 13 mg/dL lower LDL-C levels in African Americans and Caucasians, respectively, and a 49% and 18% lower risk for coronary heart disease in African Americans and Caucasians, respectively. In a study of 10,196 US African American adults, PCSK9 LoF variants were associated with lower concentrations of lipoprotein (Lpa) and OxPL-apoB. These data indicate a direct relationship between PCSK9 and the risk of CAD, and the importance of lowering PCSK9 levels in reducing CAD risk among African Americans.

The knowledge gained by our exploratory mechanistic study will help us to understand the direct effects of PCSK9 and PCSK9 inhibitor on platelets and facilitate the clinical application of PCSK9 inhibitors in a broad range of patients with cardiovascular risk.

• Difference in ADP-induced platelet aggregation (%) with PCSK9 enzyme alone and with addition of PCSK9 antibody [Срок оценки: Throughout study completion, an average of about 1 year]
• Difference in collagen related peptide-induced platelet aggregation (%) with PCSK9 enzyme alone and with addition of PCSK9 antibody [Срок оценки: Throughout study completion, an average of about 1 year]