Introduction
Aspirin use is recommended by the United States Preventive Services Task
Force to prevent cardiovascular disease (CVD) in selected, high risk
patients.[1] It is well known that aspirin inhibits platelet
cyclooxygenase 1 (COX1)[2]; however, aspirin has additional effects
on platelets, which are not fully explained by its effects on COX-1. For
example, 81mg/day aspirin is sufficient to fully suppress platelet
COX1[3] yet higher doses cause additional reductions in platelet
function as assessed by non-COX1 dependent platelet function assays such
as adenosine diphosphate (ADP) and collagen induced platelet
aggregation.[4] In addition, despite consistent platelet COX-1
inhibition, non-COX1 dependent measures of platelet function wane over
time[5] suggesting some adaptation of platelets to chronic aspirin
exposure. Residual platelet function measured using non-COX1 dependent
platelet function assays are risk markers for future cardiovascular
events during aspirin therapy.[6, 7] Last after decades of
uncertainty around the appropriate aspirin dose for cardiovascular risk
reduction[8] only recently has there been randomized trial data
comparing the effectiveness and safety of low- vs. high-dose aspirin.
This trial did not identify major differences in efficacy or safety
between aspirin doses; however, nearly 50% of patients crossed over to
the alternative dose with 3 weeks of randomization. [9] Therefore,
gaps in our knowledge around one of the most prescribed medicines
worldwide remain.
Although anucleate, it is well known that platelets are a rich source of
messenger RNA (mRNA). Platelets are invested with mRNA from
megakaryocytes during thrombopoiesis in a regulated manner[10], can
uptake transcripts from other vascular cells[11], and are also a
major source of transcripts found in plasma extracellular vesicles
[12]. Genetic and gene expression studies have confirmed that
variable platelet mRNA levels lead to variable levels of platelet
function.[13-15] We have previously shown that a blood-based,
“aspirin response signature” of transcripts primarily of platelet
origin correlate with platelet function on aspirin and predict future
cardiovascular events in patients taking aspirin.[16] These data
support the concept that the biologic basis for variable platelet
function lies, at least in part, in variable platelet gene expression.
We[17] and others[18] have shown that aspirin (or its stable
metabolite salicylate) can alter expression of selected genes in
megakaryocytes as well as in circulating platelets in patients exposed
to aspirin. However, these prior studies were limited in scope as they
did not systematically characterize aspirin’s effects on platelet gene
expression.
Based on these knowledge gaps, we pursued the hypothesis that
variability in the platelet function response to aspirin is associated
with aspirin induced changes in the platelet transcriptome. We tested
this hypothesis using an open label, randomized crossover study of low-
vs. high-dose aspirin in human volunteers with longitudinal COX-1 direct
and indirect assays of platelet function, drug concentrations, and RNA
profiling at each time point.