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The interaction of circulating lipoproteins with platelets can increase platelet reactivity, promoting platelet activation, aggregation, and thrombus formation.[1] This can explain why patients with hypercholesterolemia, especially those with familial hypercholesterolemia, and increased levels of low-density lipoprotein (LDL-C) have heightened platelet reactivity.[1] However, the mechanism(s) explaining these observations are not completely elucidated. Among the prevailing hypotheses, it has been postulated that the LDL-C and oxidized LDL-C modulate platelet membrane lipid composition and interact directly via multiple platelet receptors (e.g., CD36 and lectin-like oxidized LDL receptor 1), resulting in hyper-reactivity, morphologic changes, and increased aggregation.[1]

Statins have been associated with a reduction in platelet reactivity both in the absence and presence of antiplatelet agents, including aspirin and clopidogrel.[1] These effects have been associated with LDL-C level reduction and pleiotropic effects, including modulation of inflammation.[1] The platelet lipidome also appears to be altered in patients with coronary artery disease (CAD) on statin treatment and upregulated lipids, mainly characteristic triglycerides. In contrast, downregulated lipids mostly compromise glycerophospholipids, which may play a role in the pathophysiology of CAD.[2] [3]

The discovery of the role of the proprotein convertase subtilisin/kexin type 9 (PCSK9) in LDL-C metabolism and subsequent development of PCSK9 inhibitors has led to marked reduction of LDL-C with a concomitant decrease in cardiovascular events.[4] Whether these effects are related only to LDL-C reduction or off-target properties, such as a reduction in platelet reactivity, has been debated.[5] Preclinical studies have suggested that elevated levels of circulating PCSK9 are associated with increased platelet reactivity and thrombus formation, potentially by binding to platelet CD36 receptors.[6] [7] Moreover, in a nonrandomized study of patients undergoing percutaneous coronary intervention (PCI) treated with potent oral P2Y12 inhibitors (i.e., ticagrelor or prasugrel), elevated circulating levels of PCSK9 were associated with increased platelet reactivity and thrombotic events.[8]

Functionally synergistic interactions between antiplatelets and PCSK9 inhibitor lipid-lowering drugs may reduce the incidence of atherothrombotic vascular events, as recently reviewed by Schrör et al.[9] Indeed, several randomized studies have been designed to assess the effects of PCSK9 inhibitors on coronary plaque morphology and platelet reactivity.[10] [11] [12] [13] [14] [15] In particular, PACMAN-AMI was a double-blind, placebo-controlled, randomized clinical trial that enrolled patients undergoing PCI for acute myocardial infarction (MI). Patients (n = 300) were randomized to biweekly subcutaneous alirocumab (i.e., antibody-based PCSK9 inhibitor) or placebo, initiated less than 24 hours after PCI, in addition to high-intensity statin therapy. At 52 weeks, adding subcutaneous biweekly alirocumab to high-intensity statin therapy resulted in significantly greater coronary plaque regression in non-infarct-related arteries compared with placebo.[12]

In this issue of Thrombosis and Haemostasis, the PACMAN-AMI investigators report the effects of alirocumab on platelet function in patients with acute MI who had undergone successful PCI.[16] This was a prespecified, powered, pharmacodynamic (PD) substudy of the patients enrolled in Bern University Hospital who were on potent P2Y12 inhibitors. PD assessments were performed at baseline, 24 hours, 4-, and 52-week. The primary endpoint was P2Y12 reaction units (PRU) at 4 weeks, as assessed by VerifyNow. Secondary endpoints included aspirin reaction units (ARU) assessed by VerifyNow, platelet-derived noncoding ribonucleic acid (RNAs, microRNAs, and YRNAs), high-sensitivity C-reactive protein (hs-CRP), and a comprehensive lipid panel.

The study population (n = 139) was composed mostly of men (84.9%) with a median age of 58 years, presenting mainly with ST-segment elevation MI (51.8%). The prevalence of conventional cardiovascular risk factors was low, and none of the patients had a prior history of MI, PCI, or coronary artery bypass grafting surgery. Only a small fraction of patients were on aspirin (4.3%) or statins (12.9%); none were oral P2Y12 inhibitors. At baseline, no differences existed in any of the platelet function tests or assessed biomarkers. Post-discharge, all were prescribed statins, with 98.5% on low-dose aspirin (100 mg). At 4 weeks, 90.6% of the patients were on ticagrelor. Compared to placebo, alirocumab was associated with a significant reduction in LDL-C (24.0 ± 16.2 vs. 77.6 ± 26.7; p < 0.001), triglycerides, lipoprotein (a), and apolipoprotein B, without difference in hs-CRP. There were no significant differences in levels of PRU, primary endpoint, between patients allocated to alirocumab and placebo (12.5 [27.0] vs. 19.0 [30.0]; p = 0.260), ARU levels (383.3 [96.0] vs. 385.0 [75.0]; p = 0.890), nor in any of the evaluated circulating microRNAs and YRNAs. The results at 52 weeks were consistent with those at 4 weeks, with a significant reduction in LDL-C levels (24.9 ± 28.5 vs. 76.6 ± 27.3; p < 0.001) without significant differences in PRU (25.0 [37.0] vs. 34.0 [59.0]; p =0.07).

The PACMAN-AMI investigators should be commended for this new piece of evidence on the lipid–platelet interplay. Strengths of the study include the high-quality data derived from prespecified and powered substudy of a double-blind, placebo-controlled randomized trial, including platelet function testing and platelet-derived noncoding RNAs. Prior evidence suggests that the effects of lipid-lowering therapies on platelet reactivity are enhanced in patients with a baseline high platelet reactivity (HPR, PRU > 208) and without concomitant antiplatelet therapy.[1] The PACMAN-AMI substudy included patients treated with potent oral P2Y12 inhibitors and high-intensity statin (rosuvastatin 20 mg/qd). Thus, most patients exhibited low platelet reactivity (LPR, i.e., PRU < 85) at baseline and 24 hours. Conversely, a prior study assessing the role of PCSK9 inhibitors on platelet reactivity excluded patients with baseline LPR as these patients already exhibit a high degree of platelet inhibition.[13] [17] [18] A likely explanation for these findings is the design and selection criteria of the main PACMAN-AMI trial, which was aimed at studying patients with acute MI and coronary plaque morphological changes.[12] Overall, from a PD perspective, the lack of a significant effect of alirocumab compared to placebo on platelet reactivity in PACMAN-AMI was not surprising. Although the design of the study might not favor the ascertainment of the drug effect, it should be highlighted that the investigators correctly applied the recommendations of contemporary clinical guidelines.

Two findings warrant further insights in the context of prior evidence ([Table 1]). First, when the PACMAN-AMI primary endpoint data are compared to a prior study, the effect size of PCSK9 inhibition on platelet reactivity is smaller. Franchi et al reported that in patients with atherosclerotic cardiovascular diseases (ACVDs) with on-clopidogrel HPR (i.e., PRU > 208), evolocumab led to a mean PRU reduction of 22, and those with NPR (i.e., PRU between 85 and 208) to a mean PRU reduction of 17. In PACMAN-AMI, where most patients achieved LPR using potent P2Y12 inhibition, PCSK9 inhibition led to a mean PRU reduction of 16. Additionally, Franchi et al found enhanced reduction in platelet reactivity among patients with high baseline PCSK9 levels. Overall, these data suggest that patients with HPR and elevated circulating levels of PCSK9 are those in whom PCSK9 inhibition could lead to a significant reduction in platelet reactivity. Conversely, those with NPR or LPR and low PCSK9 levels may experience only a modest change. Second, there was no correlation between the reduction in LDL-C levels and PRU in either the Franchi et al study or PACMAN-AMI. This observation is inconsistent with prior evidence suggesting an interaction of circulating LDL-C and oxidized LDL-C with platelet surface receptors. Rather than excluding an interplay between LDL-C and platelet reactivity, this finding reflects the poor understanding of these complex biological pathways and a disconnect between preclinical and randomized trial findings.

Table 1 Comparison of randomized trials assessing the role of PCSK9 inhibitors on platelet function

PACMAN-AMI[16]

Franchi et al[13]

EVOPACS[15]

EVACS I & II[14]

Population

ACS post successful PCI (n = 139)

ACVD[a] on clopidogrel treatment, not on LDL-C target despite GDMT (n = 84)

ACS patients not on LDL-C target (n = 260)

ACS patients (n = 46)

On-treatment platelet reactivity

LPR

HPR and NPR

–

–

Antiplatelet therapy

Aspirin: 100%

Prasugrel: 9.4%

Ticagrelor: 90.6%

Aspirin: 82.1%

Clopidogrel: 100%

Aspirin: 96.2%

Clopidogrel: 9.2%

Prasugrel: 10.8%

Ticagrelor: 72.7%

Aspirin: 100%

Clopidogrel: 35.0%

Ticagrelor: 61.0%

PCSK9 inhibitor

Alirocumab, 150 mg biweekly

Evolocumab 420 mg

Evolocumab, 420 mg monthly

Evolocumab, 420 mg monthly

Statins

100%[b]

93%[b]

100%[b]

98%

Methods

Double-blind placebo-controlled RCT

Double-blind placebo-controlled RCT

Double-blind placebo-controlled RCT

Double-blind placebo-controlled RCT

Primary endpoint

Difference in PRU at 4 weeks

Difference in PRU at 4 weeks in each cohort

ΔADP-induced platelet aggregation at 8 weeks

Difference in circulating PF4 at 4 weeks

Platelet function tests

VerifyNow (PRU and ARU),

platelet-derived noncoding RNAs

VerifyNow (PRU),

LTA (ADP 20 and 5 µM), TEG

Multiplate analyzer (ADP)

Biomarkers

LDL-C at 4 weeks

Significant reduction

Significant reduction in HPR and NPR cohorts

Significant reduction

Significant reduction

Primary endpoint met

No

No

No

–

Platelet reactivity[c]

Mean reduction of 16 PRU

HPR: mean reduction of 22 PRU

NPR: mean reduction of 17

PRU

Evolocumab: mean

−5.6 ± 23.5 U

Placebo: mean

−1.7 ± 19.7 U

Evolocumab: median 10.7 (IQR 5.9–12.7) ng/1,000 plt

Placebo: median 13.1

(IQR 10.7–14.5) ng/1,000 plt

Correlation between ΔLDL-C and ΔPRU

No correlation

HPR: no correlation

NPR: no correlation

–

–

Reduction in other lipoproteins

Triglycerides

Lipoprotein (a)

Apolipoprotein B

HRP: none

NPR: triglycerides

–

Apolipoprotein B

Abbreviations: 95% CI, 95% confidence interval; ACS, acute coronary syndrome; ACVD, atherosclerotic cardiovascular disease; ADP, adenosine diphosphate; ARU, aspirin reaction units; GDMT, guideline-directed medical therapy; HPR, high platelet reactivity; IQR, interquartile range; LDL, low-density lipoprotein; LPR, low platelet reactivity; NPR, normal platelet reactivity; PCI, percutaneous coronary intervention; PCSK9, proprotein convertase subtilisin/kexin type 9; PLT; platelet; PRU, P2Y12 reaction units; RCT, randomized controlled trial; RNA, ribonucleic acid; TEG, thromboelastography; U, units.

a ASCVD was defined as a history of ACS, stable or unstable angina, coronary or other arterial revascularization, stroke, transient ischemic attack, or peripheral artery disease presumed to be of atherosclerotic origin.

b High-intensity statin, defined as atorvastatin ≥40 mg/qd or rosuvastatin ≥20 mg/qd.

c If mean reduction was not reported, the results of the investigational drug and placebo are shown. Δ: difference between baseline and follow-up.

Alirocumab was not associated with a reduction in ARU levels compared to placebo at any time. While there are no existing data on the impact of PCSK9 inhibition on ARU, these results are consistent with prior evidence with statins and omega-3 fatty acids, showing no strong evidence of aspirin response modulation.[1] On the other hand, prior observational investigations indicated a correlation between circulating microRNAs and platelet function tests.[19] Of the nine RNAs assessed, no differences were observed between alirocumab and placebo. The role of microRNAs as platelet function biomarkers remains under investigation, and the influence of antiplatelets and lipid-lowering therapies on them is unknown. Ultimately, alirocumab was not associated with a reduction in hs-CRP compared to placebo. Although the association between inflammation and increased platelet reactivity is established, prior data do not support that PCSK9 inhibition can modulate inflammation when combined with statins.[20]

The results of this PACMAN-AMI substudy must be considered in light of two important limitations. First, platelet function testing was only assessed by means of VerifyNow, which is very specific to the P2Y12 receptor signaling pathway but not to global platelet reactivity. Second, the timing of platelet function assessments relative to P2Y12 inhibitor intake was not standardized, potentially impacting measures of platelet reactivity.

PCSK9 inhibition holds a class I recommendation for secondary prevention in patients with ACVD not achieving their LDL-C goal on a maximum tolerated dose of a statin and ezetimibe.[21] The benefits from PCSK9 inhibition seem mostly related to coronary plaque stabilization and regression without a clear clinically relevant effect on platelet reactivity. Areas for further research include the role of PCK9 inhibition in patients without statins, PCSK9 inhibition with small-interfering RNA, and the effect on platelet reactivity of lipid-lowering therapies targeting lipoprotein (a) or triglycerides ([Fig. 1]).

Fig. 1 The lipid–platelet interplay. Simplified biological explanation of the interaction between platelets and circulating lipoproteins, including the potential effect of lipid-lowering therapies. Statins are the only pharmacological group with evidence supporting LDL-C and non-LDL-C-mediated effects on platelet reactivity in patients with and without antiplatelet therapy. *There are no studies assessing the effect of lipoprotein (a) treatments (e.g., olpasiran or pelacarsen) or small-interfering RNA PCSK9 inhibitors (e.g., inclisiran) on platelet reactivity. HDL-C, high-density lipoprotein cholesterol; LDL-C, low-density lipoprotein cholesterol; PCSK9, proprotein convertase subtilisin/kexin type 9; Ox-LDL, oxidate LDL; rHDL; recombinant HDL; RNA, ribonucleic acid.Publication History

Received: 06 February 2024

Accepted: 07 February 2024

Article published online:
19 March 2024

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