Associate editor: M. BelvisiMechanisms of aspirin resistance
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.
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