Aspirin at 120: Retiring, recombining, or repurposing?

Abstract During the past 20 years, we have witnessed the following trends in aspirin usage: (i) a “dropping” trend, characterized by the early discontinuation of low‐dose aspirin from dual antiplatelet therapy or triple antithrombotic therapy (oral anticoagulation plus dual antiplatelet therapy in patients with atrial fibrillation) following an acute coronary syndrome or after percutaneous coronary intervention; (ii) a “combinatorial” trend, featuring the addition of a lower dose of a P2Y12 inhibitor or direct oral anticoagulant drug to low‐dose aspirin for the long‐term treatment of stable patients with atherosclerotic cardiovascular disease; and (iii) a “repurposing” trend, characterized by growing interest in the oncologic community to assess the chemopreventive effect of aspirin against certain types of cancers (particularly of the gastrointestinal tract), both as primary prevention and adjuvant therapy. The aim of this review is to present the mechanistic rationale underlying these trends, discuss the design and findings of trials testing novel treatments or new therapeutic applications of aspirin, and report on the ISTH Congress results on this topic.

oral anticoagulation [OAC]) 4 plus DAPT in patients with atrial fibrillation (AF) following an acute coronary syndrome (ACS) or after percutaneous coronary intervention (PCI); (ii) a "combinatorial" trend, featuring the addition of a lower dose of a P2Y 12 inhibitor or direct oral anticoagulant (DOAC) to low-dose aspirin for the long-term treatment of stable patients with atherosclerotic cardiovascular disease (ASCVD) 5,6 ; and (iii) a "repurposing" trend, characterized by growing interest in the oncologic community to assess the chemopreventive effect of aspirin against certain cancers, particularly of the gastrointestinal (GI) tract, as both primary prevention and adjuvant therapy. 7,8 The aim of this review is to present the mechanistic rationale underlying these trends and discuss the design and main findings of trials testing novel treatment regimens or exploring new therapeutic applications of aspirin.

| Rationale for dropping aspirin from DAPT and TAT
To start with the dropping trend, this was nicely articulated by a group of interventional cardiologists in a review article 9 questioning the role of aspirin in secondary prevention and asking the provocative question: Is less more? A major argument supporting their reasoning was a controversial finding suggesting that P2Y 12 inhibitors may inhibit thromboxane A 2 (TXA 2 ) production to the same extent as aspirin, making the use of aspirin redundant when combined with an effective P2Y 12 blocker. 10 This finding was reported in a letter to the editor published in 2010 and was based on a short-term study in healthy volunteers. 10 The authors measured the effects of a week's treatment with low-dose aspirin or standard-dose clopidogrel on thromboxane metabolite excretion in 16 healthy volunteers and found a similar 60% reduction in this noninvasive index of platelet activation. 10 They concluded that "If P2Y 12 antagonists alone can inhibit platelet TXA 2 activation pathways, this may explain why addition of aspirin to P2Y 12 antagonists is not necessarily associated with any improvement in efficacy." 10 Accordingly, the "less-is-more" concept was proposed in an effort to mitigate the bleeding liability of DAPT while preserving antithrombotic efficacy, achieved through the concomitant inhibition of multiple platelet activation pathways, thereby trying to optimize net clinical benefit. 9 However, the mechanistic rationale underlying this concept was subsequently questioned by the contradictory findings of Scavone et al, 11 who reported that inherited deficiency or pharmacologic inhibition of platelet P2Y 12 does not affect the platelet capacity to synthesize TXA 2 .

| Trials dropping aspirin from DAPT
A major test of the "less-is-more" concept was provided by the GLOBAL-LEADERS study. 12 This was a randomized controlled trial (RCT) of 16 000 patients with either ACS or stable coronary artery disease (CAD) undergoing PCI, in whom this hypothesis was tested to demonstrate superiority of an experimental treatment strategy dropping aspirin after the first month of DAPT, and continuing with ticagrelor monotherapy for the next 23 months, over the reference treatment strategy of DAPT for the first 12 months, followed by aspirin monotherapy for the second year. 12 The primary end point for superior effectiveness of the experimental strategy was the cumulative 2-year composite of all-cause mortality and new Q-wave myocardial infarction (MI). 12 At 2 years, there was no statistically significant difference in the rate of the primary end point nor in allcause mortality. As one would expect from two equally effective antiplatelet agents, the cumulative incidence of Bleeding Academic Research Consortium 3 or 5 bleeding in the two arms was not significantly different, and their Kaplan-Meier curves during the second year (when one arm was assigned to receive low-dose aspirin alone and the other ticagrelor alone) were largely superimposable. 12 F I G U R E 1 One hundred twenty years of aspirin-inspired research and development. The figure schematically summarizes the three phases of aspirin development: (A) as an analgesic, antipyretic, and anti-inflammatory agent; (B) as an antiplatelet drug; and (C) as a chemopreventive agent. Aspirin has inspired research throughout its 120-year life, by providing a tool for mechanistic understanding and a template for new drug development. DAPT, dual antiplatelet therapy; GI, gastrointestinal; NSAID, nonsteroidal anti-inflammatory drug; R&D, research and development

| Trials dropping aspirin from TAT
A second aspect of the "less-is-more" paradigm is represented by the withholding of aspirin among patients with chronic coronary syndromes or ACS and AF who require OAC therapy and may undergo PCI with TAT. 17  numerically lower in the aspirin versus placebo comparison. 24 As acknowledged by the AUGUSTUS Investigators, "avoiding aspirin resulted in a 47% lower risk of bleeding than using aspirin and in a nonsignificantly higher incidence of coronary ischemic events. This finding suggests that the price for a significantly lower incidence of bleeding events without aspirin may be a modestly higher risk of coronary ischemic events." 24 Given the uncertain prognostic significance of the varying bleeding definitions used in these trials, it is questionable that paying the price of a higher risk of coronary ischemic events would be in the best interest of the patients.
As could be easily predicted from established pathophysiologic and pharmacologic knowledge, these four trials consistently showed reduced bleeding outcomes when comparing OAC plus a P2Y 12 inhibitor versus OAC plus DAPT. [21][22][23][24] However, given comparable efficacy and safety of P2Y 12 inhibitors (both clopidogrel and ticagrelor) and low-dose aspirin, 25 the clinically relevant question is not "what happens if I drop aspirin and look at bleeding," but rather "what is the difference in efficacy and safety between an aspirin-free versus a P2Y 12 inhibitor-free regimen versus DAPT?" Clearly, none of the four RCTs was designed to address this more important question that would have required an additional arm and a much larger sample size. From a mechanistic point of view, the assumption of trials testing the hypothesis that omitting aspirin from the antithrombotic regimen would be associated with less bleeding and no increase in SVEs, implies that platelet TXA 2 biosynthesis continues to have an important role in primary hemostasis but is no longer relevant to atherothrombosis. The biological plausibility of this assumption seems highly questionable. While a recent meta-analysis of the four trials concluded that "strategies omitting aspirin caused less bleeding, including intracranial bleeding, without significant difference in MACE, compared with strategies including aspirin," 26 this conclusion is biased by the major design flaws outlined above.

| Rationale for combining aspirin with other antithrombotic agents
In the early trials of aspirin for prevention of death, MI, and stroke in high-risk patients, aspirin alone (at any dose) achieved 23% proportional SVE reduction versus placebo. 27 In particular, aspirin approximately halved the risk of SVE in patients with unstable angina in several placebo-controlled trials carried out in the 1980s and early 1990s. 27 However, in more recent trials, the rate of SVE in aspirintreated patients following ACS was still >10% at 12 months following the acute event. 28 In aspirin-treated patients with stable ASCVD (previous MI or stroke), in spite of a proportional SVE reduction of approximately 25% versus placebo, the recurrence rate of SVE is approximately 3% per year. 6 Since aspirin targets only one mechanism of platelet activation, 29 combining drugs that target different platelet activation pathways has been hypothesized to produce additive benefit, possibly overcoming the increase in bleeding complications, which is intrinsically associated with an intensified antiplatelet regimen. Moreover, since arterial thrombosis involves primary (platelets, von Willebrand factor) as well as secondary (coagulation cascade) haemostasis, 30 another tested strategy consists of combining platelet inhibition with low-dose aspirin together with the inhibition of coagulation factor(s).

| With an anticoagulant
Early attempts adding an oral anticoagulant to low-dose aspirin used warfarin (vitamin K antagonist [VKA]) in patients with MI.
The full-dose warfarin and low-dose aspirin combination was associated with a reduction in death, nonfatal MI, or thromboembolic stroke, as compared to aspirin alone. 31,32 However, a significant increase in major bleeding was consistently observed across different studies. Moreover, major drawbacks of warfarin are the need for international normalized ratio monitoring, clinically relevant drugdrug interactions (DDIs), and low drug adherence. 33 The development of DOACs, especially those targeting factor Xa (FXa), allowed

| With a P2Y 12 inhibitor
The combination of ADP receptor (P2Y 12 ) blockade by clopidogrel, prasugrel, or ticagrelor, with low-dose aspirin has been successfully tested in clinical settings considered suitable for intensifying platelet inhibition, that is, in patients following ACS with or without coronary revascularization. 28 43 suggesting the achievement of a ceiling benefit/harm balance with DAPT ( Figure 2).
In patients with acute mild to moderate, noncardioembolic stroke or transient ischemic attack, early P2Y 12 blockade by clopidogrel 44,45 or ticagrelor 46 in addition to aspirin has been consistently associated with a significant reduction of ischemic stroke recurrence within the first 3 months in three different trials, with a favorable benefit/harm TA B L E 2 Benefit/risk ratio in recent randomized controlled trials of antithrombotic therapy for primary and secondary prevention While combining platelet COX-1 inhibition with low-intensity FXa inhibition appears to achieve a proportionally larger benefit in preventing SVE in stable patients with ASCVD than the combination of platelet COX-1 and P2Y 12 inhibition, with a comparable increase in bleeding risk, the actual superiority of one approach versus the other would require a very large randomized comparison between the two in order to be reliably assessed. Administration black-box warning on untoward CV effects (Table 3).

| In cancer prevention and treatment
The observation of lower rates of CRC in populations exposed to regular use of aspirin and other NSAIDs triggered interest in assessing whether long-term administration of these drugs may be chemopreventive against CRC and other cancers. 7,8,50,51 The race began some 20 years ago, with simultaneous initiation of placebo-controlled

F I G U R E 3
The platelet contribution to colorectal cancer development: early versus late phases. In the first stages of intestinal tumorigenesis, platelets may play a key role, since they are activated in response to mucosal injury, and release various soluble mediators, for example, thromboxane A 2 (TXA 2 ), prostaglandin E 2 (PGE 2 ), and various growth factors, that may contribute to the induction of several signaling pathways associated with a phenotypic switch of the stromal cells. In this scenario, the activation of stromal cells, in turn, can lead to abnormal expression of cyclooxygenase (COX)-2 in epithelial cells. These molecular events lead to enhanced biosynthesis of the protumorigenic prostanoid, PGE 2 , which is generated mainly by COX-1 in the normal mucosa and by COX-1/COX-2 and COX-2 in the adenoma and adenocarcinoma, respectively. In addition, in the late phase of tumorigenesis, tumor cells may enter into the circulation and interact with platelets. The adhesion of platelets to tumor cells leads to platelet activation and their release of mediators that induce phenotypic changes in tumor cells thereby facilitating their extravasation and colonization of distal organs. The anticancer effect of low-dose aspirin may also be explained by the fact that the drug, in addition to platelet COX-1, is able to acetylate and inhibit COX-

| A S PIRIN IN THE 2 02 0 IS TH CONG RE SS REP ORT: FUTURE CHALLENG E S
Over 100 abstracts presented at the ISTH 2020 Congress dealt with aspirin either in the title or in the text, in a context of future ways of "repurposing" and "recombining" rather than "retiring." Within the repurposing setting, aspirin appears to be the reference antithrombotic drug in the primary CV prevention of rare acquired or inborn diseases of red blood cells, such as polycythemia vera 69 and thalassemia, 70 respectively. Even in young people with thalassemia, aspirin may protect their brain from any type of ischemia (arterial, embolic, micro-and macrovascular, silent and overt). 71 The still-ongoing coronavirus disease 2019 (COVID-19) pandemics has renewed the attention on the well-known liaison between infection and haemostasis. Consistently, platelet and endothelial activation documented in patients with COVID-19 72 reinforces the rationale for two ongoing trials testing the efficacy of aspirin in mitigating the prothrombotic state, reducing hospitalization and fatal complications of this disease. 73,74 Aspirin, and possibly other antiplatelet drugs, may face relevant pharmacologic challenges in the near future, which should be addressed to maintain an optimal antithrombotic protection. While in the early trials of aspirin (for either primary or secondary prevention) body size was within the normallow weight range, 19 obesity will increasingly characterize the majority of patients with CV disease in the near future. Obesity by itself is a condition that may affect aspirin pharmacology, 75 possibly through higher degradation rate by plasma esterases. 76 Moreover, increased platelet turnover has been associated with a reduced responsiveness to standard once-daily low-dose aspirin in primary 77 or transiently acquired 78 platelet disorders. A high platelet regeneration rate seems also to modify the pharmacodynamics of the P2Y 12 inhibitor clopidogrel in patients on DAPT. 79 Interestingly, BRAFV600E mutated mice develop hepatocarcinogenesis similarly to humans with the same mutation, with an associated increase in thrombopoietin, megakaryopoiesis, and platelet deposition in hepatic sinusoids. 80 These mice were protected from hepatic carcinogenesis by low-dose aspirin in association with a parallel reduction of platelet deposition in liver sinusoids. 80 Within the recombining setting, Wong et al 81 showed that aspirin combined with a novel factor XIa (FXIa) inhibitor did not increase bleeding time in animal models. Since FXIa inhibitors are in phase II/ III clinical trials for venous thromboembolism, the association with aspirin may also be of relevance to ASCVD.

| CON CLUS IONS
Despite being 120 years old, aspirin continues to be considered a cornerstone of antithrombotic therapy in ASCVD. It is, in fact, the only antiplatelet agent with current recommendations for its use throughout the CV risk continuum, from primary prevention in people with high-risk diabetes mellitus to treatment of ACS (Table 4). 48 82 We have critically reviewed a current trend for considering aspirin-free antithrombotic regimens in patients undergoing PCI, and conclude that there is no valid mechanistic rationale nor convincing evidence from RCTs for withdrawing low-dose aspirin rather than a P2Y 12 inhibitor from DAPT or TAT in this setting. Under clinical circumstances in which DAPT (or TAT) is the standard of care, studies comparing an aspirin-free regimen to DAPT are as flawed as those comparing clopidogrel-free regiment to DAPT, and their results cannot provide unbiased guidance to clinical practice. 84,85 Furthermore, the results of the GLOBAL-LEADERS trial do not support the "less-is-more" paradigm. 9 The combined use of low-dose aspirin with a low-dose FXa inhibitor 6 or with an effective P2Y 12 inhibitor represents a rational approach to reduce residual CV risk of patients with ASCVD, in light of the multifactorial nature of atherothrombosis. 16 However, we should be aware of the exponential rise in NNT values to obtain an additional reduction in SVE, in view of the relatively low annual event rate and moderate effect size of these interventions in stable patients.
The evidence supporting a chemopreventive effect of low-dose aspirin against CRC 3,7,8 was considered sufficiently strong to convince the USPSTF to issue a new recommendation for its use in primary CVD prevention with the additional long-term benefit of CRC prevention 63 and to stimulate several oncology networks to initiate new aspirin trials in the adjuvant setting. 64 Finally, we believe that the role of aspirin in the treatment of OA should be reassessed in light of new scientific evidence and regulatory constraints on the use of currently available NSAIDs. 67,68

AUTH O R CO NTR I B UTI O N S
CP and BR reviewed the literature and wrote the manuscript.

ACK N OWLED G M ENTS
The authors' research on aspirin and cancer is supported by a grant from Cancer Research UK, the AsCaP Project.

R EL ATI O N S H I P D I SCLOS U R E
BR has received consultant and speaker fees from Bayer AG, Novartis, and MedScape. CP reports receiving consultant and speaker fees from Acticor Biotech, Amgen, Bayer, GlaxoSmithKline, Eli Lilly, Tremeau, and Zambon; he chairs the Scientific Advisory Board of the International Aspirin Foundation.