Associate editor: M. Belvisi
Mechanisms of aspirin resistance

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Abstract

Aspirin is integral to the secondary prevention of cardiovascular disease and acts to impair the development of platelet-mediated atherothromboembolic events by irreversible inhibition of platelet cyclooxygenase-1 (COX-1). Inhibition of this enzyme prevents the synthesis of the potent pro-aggregatory prostanoid thromboxane A2. A large number of patients continue to experience atherothromboembolic events despite aspirin therapy, so-called ‘aspirin treatment failure’, and this is multifactorial in aetiology. Approximately 10% however do not respond appropriately to aspirin in a phenomenon known as ‘aspirin resistance’, which is defined by various laboratory techniques. In this review we discuss the reasons for aspirin resistance in a systematic manner, starting from prescription of the drug and ending at the level of the platelet. Poor medication adherence has been shown to be a cause of apparent aspirin resistance, and may in fact be the largest contributory factor. Also important is high platelet turnover due to underlying inflammatory processes, such as atherosclerosis and its complications, leading to faster regeneration of platelets, and hence of COX-1, at a rate that diminishes the efficacy of once daily dosing. Recent developments include the identification of platelet glycoprotein IIIa as a potential biomarker (as well as possible underlying mechanism) for aspirin resistance and the discovery of an anion efflux pump that expels intracellular aspirin from platelets. The absolute as well as relative contributions of such factors to the phenomenon of aspirin resistance are the subject of continuing research.

Introduction

Aspirin is the most widely used drug in the world. Developed in the 19th century and subsequently marketed as a panacea for common ailments (Dreser, 1899), during the course of the 20th century its anti-inflammatory effects were recognised and eventually shown to be due to inhibition of cyclooxygenase resulting in impaired prostaglandin synthesis (Ferreira et al., 1971, Vane, 1971). Its cardioprotective effect was not established, however, until the 1980s, when it was shown to confer an important survival advantage post-myocardial infarction (ISIS-2 Collaborative Group, 1988). More generally, it has since been shown to be highly effective in the secondary prevention of cardiovascular disease (Patrono et al., 2005).

Aspirin is a non-selective and irreversible cyclooxygenase (COX) inhibitor, and prevents the production of thromboxane A2 (TXA2) in platelets by acetylating a serine residue at position 529 of the COX-1 isoform (Roth and Majerus, 1975, Roth et al., 1975). TXA2 is a metabolite of arachidonic acid (AA), with the rate limiting step in its synthesis being that catalysed by COX-1 (Hamberg et al., 1975) (Fig. 1). Once synthesised and released by platelets in response to stimuli, TXA2 binds to its G-protein coupled receptor (the TP receptor) leading to activation of phospholipase C and hence platelet aggregation (Nakahata, 2008). TXA2 is an amplifying signal for other agonists, and so inhibition of COX-1 modulates multiple pathways of platelet activation (FitzGerald, 1991).

Nevertheless, aspirin only prevents approximately 25% of coronary events and ischaemic strokes when used in secondary prevention (Baigent et al., 2009). Much of this can be accounted for by the heterogeneous aetiology of these diseases, especially in the case of ischaemic stroke, where mechanisms may be non-atherothromboembolic and non-platelet mediated (Williams et al., 2011). However, it is estimated that between 5 and 60% of patients on aspirin therapy for secondary prevention do not respond appropriately to aspirin, a heterogeneous phenomenon which has come to be known as aspirin resistance (Hankey & Eikelboom, 2006).

The term ‘aspirin resistance’ has not been universally accepted to describe the failure of aspirin to prevent atherothromboembolic events, as the majority of the mechanisms described below do not fit the classical model of drug resistance. Specifically, the pharmacological target COX-1 often remains sensitive to aspirin whilst its ability to suppress systemic levels of TXA2 and platelet reactivity may be attenuated by a wide range of other factors. Alternative terms such as ‘aspirin non-response’ and ‘aspirin treatment failure’ are therefore favoured by some, but are vulnerable to equivalent semantic criticism. For the duration of this article we shall use the term ‘aspirin resistance’ to describe laboratory-derived findings and ‘aspirin treatment failure’ to describe the occurrence of clinical events in patients on aspirin.

In addition to a lack of consensus on nomenclature, there is no accord regarding the classification of aspirin resistance beyond the distinction between clinical and laboratory resistance (Bhatt & Topol, 2003). Laboratory resistance is best described as a failure of aspirin to prevent the production of platelet COX-1-derived TXA2, and is generally assessed either by the measurement of metabolites (e.g. serum thromboxane B2; TXB2) or by platelet function testing. Assays used for the detection of laboratory resistance are to varying degrees subject to inherent biases in sensitivity and/or specificity, resulting in poor agreement (Lordkipanidze et al., 2007), and thus the use of a single assay for defining aspirin resistance within a population is unlikely to produce truly meaningful results. A number of approaches have been suggested to aid in the diagnosis and classification of aspirin resistance, but these either lack clinical utility or do not facilitate reliable sub-categorisation of potential mechanisms (Weber et al., 2002, Pulcinelli and Riondino, 2006).

Before considering the mechanisms of aspirin resistance, it is helpful to briefly review its pharmacokinetic and pharmacodynamic profiles. After oral administration, aspirin crosses the mucosal lining of the stomach and upper small intestine in its lipophilic state with a bioavailability of 40–50% (Fig. 2) (Pedersen & FitzGerald, 1984). It undergoes substantial pre-systemic hydrolysis to salicylic acid by plasma and endothelial esterases before entering the systemic circulation (Harris & Riegelman, 1969), and demonstrates a plasma half-life of approximately 15–20 min across the range of treatment doses (Costello & Green, 1982). The portal circulation is the primary location at which platelet COX-1 inhibition occurs, as demonstrated by a fall in serum TXB2 levels being detectable prior to the detection of aspirin within the systemic circulation (Pedersen & FitzGerald, 1984), and due to approximately 50% of absorbed aspirin being conjugated during first-pass hepatic metabolism (Rowland et al., 1972).

Once daily dosing has been found to achieve adequate inhibition of platelet COX-1 in aspirin-sensitive healthy individuals and those with cardiovascular disease (Patrignani et al., 1982, Antithrombotic Trialists' Collaboration, 2002). This is perhaps surprising given the normal platelet survival time of 10 days that should theoretically result in approximately 10% of circulating platelets being aspirin-naïve prior to each daily dose (Dale, 1997). However, the ability of aspirin to inhibit COX-1 in megakaryocytes (Demers et al., 1980) is likely to result in platelets entering the circulation already COX-1-inhibited, as proto-platelets in the final stages of development are unlikely to be able to regenerate COX-1 prior to release (van Pampus et al., 1993).

The extent to which platelet COX-1 and hence TXA2 synthesis must be inhibited before platelet aggregation is impaired remains the subject of debate. The observation that the capacity of platelets to generate TXA2 in vitro greatly exceeds the systemic production of TXA2 measured in vivo suggested that substantial inhibition of platelet COX-1 would still allow full TXA2 mediated aggregation (Patrono et al., 1986). Further work measuring in vivo urinary metabolites of TXA2 and ex vivo assays of platelet reactivity led to the development of the ‘95% hypothesis’, which states that >95% inhibition of COX-1-derived TXA2 is necessary to inhibit aggregation (Hennekens et al., 2004). More recent work, however, using a wide range of agonist concentrations, has revealed that for the TXA2-dependent agonists AA and low-dose collagen, a linear relationship exists between inhibition of platelet TXA2 generation and suppression of TXA2-mediated platelet aggregation (Armstrong et al., 2008). These findings are more consistent with the observation that aspirin resistance is generally not an ‘all or nothing’ phenomenon, but rather lies along a spectrum of varying aspirin response.

In this review, we will systematically consider the mechanisms responsible for aspirin resistance (Fig. 3) and examine the implications for clinical practice.

Section snippets

Adherence

There is a well-established inverse relationship between the prescribed number of medication doses per day and medication adherence, and this is especially pertinent in relation to aspirin therapy as one of the multiple medications routinely prescribed post-myocardial infarction (Claxton et al., 2001). In patients on aspirin for secondary prevention, the results of platelet function assay were found unsurprisingly to correlate with medication adherence, with most of those initially identified

Drug interactions: Proton pump inhibitors

The major side effect from aspirin therapy is upper gastrointestinal bleeding from a combination of impaired platelet aggregation and a reduction in the production of protective gastric prostanoids. Doses as low as 10 mg reduce gastric prostaglandin levels by approximately 40% (Cryer & Feldman, 1999) and lead to gastric and duodenal erosions (Hart et al., 2010). The extent of the associated morbidity and mortality was recently quantified in a meta-analysis of nine randomised controlled trails

Alternative aspirin preparations

An alternative strategy to prevent gastrointestinal irritation and haemorrhage is to minimise gastric absorption of aspirin and thus attenuate any local prostanoid inhibition. To this end, preparations of aspirin with an acid-resistant coating (enteric-coated aspirin) were developed. As with co-prescription of PPIs, the potential problems of decreased absorption as a product of ionisation and gastrointestinal hydrolysis by esterases in the small intestine arise once again.

The pharmacokinetics

Esterase-mediated metabolism of aspirin

Acetylsalicylic acid may undergo hydrolysis to salicylate by esterases in the gastrointestinal tract prior to absorption (Builder et al., 1977). No correlation between gastrointestinal esterase activity and aspirin bioavailability has been documented, although a prolonged absorption time due to PPI therapy or use of enteric-coated aspirin is likely to enhance this process.

Within the systemic circulation, aspirin undergoes inactivation by two pathways; pH-dependent autolysis and enzymic

Anion efflux pump

The oral bioavailability and intracellular concentration of certain drugs are modulated by unidirectional channels which pump them into or out of cells. For example, the absorption of clopidogrel is modulated by the expression of the P-glycoprotein (P-gp) efflux transporter within intestinal epithelial cells which, when upregulated by the presence of the 3435T/T genetic variant, leads to impaired absorption (Taubert et al., 2006) and is associated with an increase in cardiovascular mortality (

Drug interactions: Non-steroidal anti-inflammatory drugs

The antiplatelet action of aspirin can be attenuated by compounds that block its ability to acetylate the COX-1 binding site. Non-steroidal anti-inflammatory drugs (NSAIDs) bind to COX with isoform selectivity that varies considerably within this extensive drug class, achieving 70–90% of COX inhibition at conventional analgesic doses (Schroeder et al., 2006). The longer half-life and stronger initial COX binding affinity of most NSAIDs when compared to aspirin produces a competitive interaction

Cyclooxygenase-1 polymorphisms

Attempts to identify a genetic factor responsible for aspirin resistance have been motivated by the dual aims of identifying a unifying mechanism for the phenomenon and developing a genetic test without the inherent unreliability of current platelet function assays. COX-1 is highly polymorphic, with 10 natural variations identified as the result of single nucleotide substitutions (UniProt Consortium, 2012). The most widely studied polymorphism is the C50T, which is in complete linkage

High platelet turnover

The regeneration of platelet COX-1 increases the capacity for the circulating pool of platelets to generate TXA2 and hence aggregate. This process is continually occurring and, at the normal platelet lifespan of 10 days (Dale, 1997), once daily dosing of aspirin is sufficient to maintain adequate inhibition of platelet TXA2 (Patrignani et al., 1982). In states of high platelet turnover, this may not necessarily be the case.

High platelet turnover can result from numerous aetiologies that can be

Platelet cyclooxgenase-2

Aside from COX-1, which is constitutively expressed in most tissues and governs normal prostanoid production, the inducible COX isoform COX-2 primarily contributes to prostanoid production at times of stress (i.e. inflammation). There is a 63% sequence homology between the isoforms (Vane et al., 1998), with COX-2 able to accept a wider range of fatty acid substrates than COX-1 which is largely limited to utilising arachidonic acid (AA) (Otto & Smith, 1995). The difference in sequence homology

Non-platelet thromboxane A2 production

Platelets are not the sole source of TXA2 within the circulation, as demonstrated by the observation that in healthy subjects approximately 30% of urinary metabolites of TXA2 derive from sources that are not inhibited by aspirin therapy (Catella & FitzGerald, 1987). Monocytes synthesise TXA2 in response to stimuli (Orlandi et al., 1994) in a predominantly COX-1 mediated process which is poorly sensitive to the inhibitory effect of aspirin (Penglis et al., 2000). Similarly COX-1 is

Tachyphylaxis: an adaptation to aspirin therapy?

Inhibition of TXA2 biosynthesis and of platelet aggregation have been demonstrated to occur consistently in healthy individuals receiving short courses of aspirin (FitzGerald et al., 1983, Nuotto et al., 1983). Long-term aspirin therapy in patients in whom aspirin sensitivity had previously been established has, however, demonstrated a progressive decline in platelet aggregation in response to multiple agonists (Helgason et al., 1994, Pulcinelli et al., 2004). At first glance this finding might

Discussion

Multiple potential mechanisms may underlie the phenomenon of aspirin resistance, as reviewed here, but poor adherence, concurrent NSAID use and regeneration of COX-1 are likely to contribute to the majority of cases. In fact, such is the impact of medication non-adherence that some authors advocate investing resource into highlighting the impact of non-adherence rather than routine platelet function testing to identify those patients who may be truly aspirin resistant (Shantsila & Lip, 2008).

Conflict of interest statement

The authors declare that there are no conflicts of interest.

References (141)

  • A.M. Gori et al.

    Incidence and clinical impact of dual nonresponsiveness to aspirin and clopidogrel in patients with drug-eluting stents

    J Am Coll Cardiol

    (2008)
  • P. Gresner et al.

    Increased blood plasma hydrolysis of acetylsalicylic acid in type 2 diabetic patients: a role of plasma esterases

    Biochim Biophys Acta

    (2006)
  • E.L. Grove et al.

    Effect of platelet turnover on whole blood platelet aggregation in patients with coronary artery disease

    J Thromb Haemost

    (2011)
  • G.J. Hankey et al.

    Aspirin resistance

    Lancet

    (2006)
  • P.A. Harris et al.

    Influence of the route of administration on the area under the plasma concentration-time curve

    J Pharm Sci

    (1969)
  • G. Jedlitschky et al.

    The nucleotide transporter MRP4 (ABCC4) is highly expressed in human platelets and present in dense granules, indicating a role in mediator storage

    Blood

    (2004)
  • J. Karha et al.

    Lack of effect of enteric coating on aspirin-induced inhibition of platelet aggregation in healthy volunteers

    Am Heart J

    (May 2006)
  • D. Kearney et al.

    Optimal suppression of thromboxane A(2) formation by aspirin during percutaneous transluminal coronary angioplasty: no additional effect of a selective cyclooxygenase-2 inhibitor

    J Am Coll Cardiol

    (2004)
  • T.M. MacDonald et al.

    Effect of ibuprofen on cardioprotective effect of aspirin

    Lancet

    (2003)
  • T. Mattiello et al.

    Aspirin extrusion from human platelets through multidrug resistance protein-4-mediated transport: evidence of a reduced drug action in patients after coronary artery bypass grafting

    J Am Coll Cardiol

    (2011)
  • J.L. Mega et al.

    Genetic variants in ABCB1 and CYP2C19 and cardiovascular outcomes after treatment with clopidogrel and prasugrel in the TRITON-TIMI 38 trial: a pharmacogenetic analysis

    Lancet

    (2010)
  • N. Nakahata

    Thromboxane A2: physiology/pathophysiology, cellular signal transduction and pharmacology

    Pharmacol Ther

    (2008)
  • M. Orlandi et al.

    Thromboxane A2 synthase activity in platelet free human monocytes

    Biochim Biophys Acta

    (1994)
  • J.C. Otto et al.

    Prostaglandin endoperoxide synthases-1 and -2

    J Lipid Mediat Cell Signal

    (1995)
  • A.C. Papp et al.

    Production of eicosanoids by deendothelialized rabbit aorta: interaction between platelets and vascular wall in the synthesis of prostacyclin

    Thromb Res

    (1986)
  • S. Pascale et al.

    Aspirin-insensitive thromboxane biosynthesis in essential thrombocythemia is explained by accelerated renewal of the drug target

    Blood

    (2012)
  • F.M. Pulcinelli et al.

    Inhibition of platelet aggregation by aspirin progressively decreases in long-term treated patients

    J Am Coll Cardiol

    (2004)
  • A.B. Adamopoulos et al.

    Do proton pump inhibitors attenuate the effect of aspirin on platelet aggregation? A randomized crossover study

    J Cardiovasc Pharmacol

    (2009)
  • B. Aktas et al.

    Dipyridamole enhances NO/cGMP-mediated vasodilator-stimulated phosphoprotein phosphorylation and signaling in human platelets: in vitro and in vivo/ex vivo studies

    Stroke

    (2003)
  • T. Andersson et al.

    Evaluation of the pharmacodynamics of acetylsalicylic acid 81 mg with or without esomeprazole 20 mg in healthy volunteers

    Am J Cardiovasc Drugs

    (2012)
  • K. Anger et al.

    Platelet hyporesponsiveness to acetylsalicylic acid can be transferred by plasma in humans

    Pharmacology

    (2010)
  • Collaborative meta-analysis of randomised trials of antiplatelet therapy for prevention of death, myocardial infarction, and stroke in high risk patients

    BMJ

    (2002)
  • C. Baigent et al.

    Aspirin in the primary and secondary prevention of vascular disease: collaborative meta-analysis of individual participant data from randomised trials

    Lancet

    (2009)
  • O. Belton et al.

    Cyclooxygenase-1 and -2-dependent prostacyclin formation in patients with atherosclerosis

    Circulation

    (2000)
  • D.L. Bhatt et al.

    Clopidogrel with or without omeprazole in coronary artery disease

    N Engl J Med

    (2010)
  • D.L. Bhatt et al.

    Scientific and therapeutic advances in antiplatelet therapy

    Nat Rev Drug Discov

    (2003)
  • G.G. Biondi-Zoccai et al.

    A systematic review and meta-analysis on the hazards of discontinuing or not adhering to aspirin among 50,279 patients at risk for coronary artery disease

    Eur Heart J

    (2006)
  • D. Capodanno et al.

    Pharmacodynamic effects of different aspirin dosing regimens in type 2 diabetes mellitus patients with coronary artery disease

    Circ Cardiovasc Interv

    (2011)
  • F. Catella-Lawson et al.

    Cyclooxygenase inhibitors and the antiplatelet effects of aspirin

    N Engl J Med

    (2001)
  • T. Chakroun et al.

    The cyclooxygenase-1 C50T polymorphism is not associated with aspirin responsiveness status in stable coronary artery disease in Tunisian patients

    Genet Test Mol Biomarkers

    (2011)
  • N. Clappers et al.

    The C50T polymorphism of the cyclooxygenase-1 gene and the risk of thrombotic events during low-dose therapy with acetyl salicylic acid

    Thromb Haemost

    (2008)
  • J.P. Collet et al.

    Bedside monitoring to adjust antiplatelet therapy for coronary stenting

    N Engl J Med

    (2012)
  • P.B. Costello et al.

    Aspirin survival in human blood modulated by the concentration of erythrocytes

    Arthritis Rheum

    (1982)
  • D. Cox et al.

    Effect of enteric coating on antiplatelet activity of low-dose aspirin in healthy volunteers

    Stroke

    (2006)
  • G.L. Dale

    Platelet kinetics

    Curr Opin Hematol

    (1997)
  • L.M. Demers et al.

    The effects of aspirin on megakaryocyte prostaglandin production

    Proc Soc Exp Biol Med

    (1980)
  • J.G. Dillinger et al.

    Biological efficacy of twice daily aspirin in type 2 diabetic patients with coronary artery disease

    Am Heart J

    (2012)
  • H. Dreser

    Pharmakologisches uber Aspirin (Acetylsalicylsaure)

    Pflugers Arch

    (1899)
  • J.W. Eikelboom et al.

    Incomplete inhibition of thromboxane biosynthesis by acetylsalicylic acid: determinants and effect on cardiovascular risk

    Circulation

    (2008)
  • J.W. Eikelboom et al.

    Aspirin-resistant thromboxane biosynthesis and the risk of myocardial infarction, stroke, or cardiovascular death in patients at high risk for cardiovascular events

    Circulation

    (2002)
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