The antithrombotic profile of aspirin. Aspirin resistance, or simply failure? (2004)

As pointed out by Patrono [38], recurrent vascular events despite the chronic use of aspirin should be defined as treatment failure instead aspirin [unless we attribute treatment failure to aspirin resistance]. Aspirin is a poorly defined term. It can imply a clinical inability of aspirin to protect individuals from arterial thrombotic events; or laboratory indications of the failure of aspirin to inhibit platelet activity, mainly platelet aggregation; or a close-to-normal urinary concentration of thromboxane metabolites. Possible mechanisms of aspirin were detailed by Gaetano and Cerletti [39] and were summarized by Cambria-Kiely and Gandhi [40]. They include: 1. Bioavailability of aspirin; 2. Platelet function; 3. Polymorphisms; 4. Platelet interactions with other blood cells and cell-derived products; 5. Several other factors i.e. stimulation of platelet aggregation by cigarette smoking; ASA resistant platelet aggregability by increased levels of norepinephrine, as seen during excessive exercise or periods of mental stress; biosynthesis of F[2]-isoprostane 8-iso-prostaglandin [PGF2 alpha], a bioactive product of arachidonic acid peroxidation; and increased platelet sensitivity to collagen.

The urinary concentrations of the metabolite 11-dehydrothromboxane B2 indicate the level of TXA2 generation. Eikelboom et al [24] indicated that in aspirin-treated patients, urinary concentrations of 11-dehydrothromboxane B2 predict the future risk of myocardial infarction or cardiovascular death. These authors also support the view that failure to suppress thromboxane generation defines aspirin [24]. This hypothesis assumes a direct association between the rise of urinary 11-dehydrothromboxane B2 levels and increment of vascular events [myocardial infarction, stroke and cardiovascular death].

Poor platelet responsiveness to aspirin was defined by Friend et al. [26] as aggregation of ≥ 50% of platelets using the PFA-100 device. Gum et al [19] defined aspirin on the basis of the platelet aggregation assay: aggregation of ≥ 70% with 10 μM ADP, and of ≥ 20% with 0.5 mg/ml arachidonic acid, constituted aspirin resistance. They also defined aspirin semiresponders as meeting one but not both of these criteria. There seems to be no correlation between the results obtained by aggregometry and by the PFA-100 device, as showed by Gum et al [19]: of the 18 patients who were aspirin resistant by aggregation, only 4 were aspirin resistant by PFA-100.

Anti-aggregatory treatment with ASA was considered by Tarjan et al. [17] to be ineffective if typical aggregation curves were obtained above the following final inducer concentrations: ADP: > 5 μM, epinephrine: > 5 μM, arachidonic acid: > 250 μM, collagen: > 2 μg/ml. Compliance by subjects was proven by HPLC determination of urinary metabolites of ASA, performed immediately after admission [17].

Sane et al. [25] considered patients to be aspirin non-responsive when 4 of the following 5 parameters were observed: collagen-induced aggregation >70%; adenosine diphosphate-induced aggregation >60%, whole blood aggregation >18 ohms; expression of active GP IIb/IIIa >220 log mean fluorescence intensity units; and P-selectin positivity >8%. When the PFA-100 device was used, aspirin was defined in terms of a normal collagen and/or epinephrine closure time [< or = 193 seconds] [19].

Weber et al. [41] proposed to classify aspirin into three categories. Type 1 [pharmacokinetic type] entails the inhibition of platelet thromboxane formation in vitro but not in vivo. Type 2 [pharmacodynamic type] is characterized by the inability of aspirin to inhibit platelet thromboxane formation both in vivo and in vitro. Type 3 [pseudoresistance] involves thromboxane-independent platelet activation.

According to Koksch et al. [42] aspirin involves, besides thromboxane formation, an impaired inhibition of platelet aggregation and an increased expression of P-selectin, a marker of α-granule secretion associated with the progression of atherosclerosis.

Also Weber et al [43] suggested that the inducible isoform of cyclooxygenase in platelets, COX-2, confers aspirin resistance, although this opinion was challenged by Patrignani et al. [44].

Aspirin may be caused by an increased sensitivity of platelets to collagen. A platelet aggregation study specific for collagen dose response may be useful for strict selection of ASA responders for low-dose ASA therapy, and for identifying ASA non-responders for high-dose ASA therapy [45].

Using a collagen/epinephrine coated cartridge in the PFA-100 [R], a prevalence of aspirin of 29.2% was determined by Macchi et al. [46]. These authors support the view that hypersensitivity to adenosine diphosphate could provide a possible explanation for aspirin resistance.

Buchanan and Brister [47] used bleeding time to define responders and non-responders. Aspirin effected a dose-dependent prolongation of bleeding time in 60% of volunteers [ASA responders], which was associated with decreases in platelet TXA2 and 12-hydroxyeicosatetraenoic acid [12-HETE] synthesis and in platelet aggregation and adhesion. However, in volunteers whose bleeding time was not prolonged [ASA non-responders], platelet 12-HETE synthesis and platelet adhesion were unchanged or increased [P < 0.001] despite platelet TXA2 and aggregation being inhibited. Beside the problems of methodology, the dissociation between TXA2 and bleeding time makes this test inadequate for defining non-responsive patients.

Andersen et al. [48] showed that the levels of TXA2 were extremely low in both aspirin responders and non-responders. However, the levels of soluble P-selectin were significantly higher in non-responders than responders.

Resistance to other antiplatelet drugs has also been described. resistance has been documented [49]. Clopidogrel non-responders were defined by an inhibition of ADP [5 and 20 mol/L] induced platelet aggregation that was less than 10% of the baseline value 4 h after clopidogrel 600 mg intake. Semi-responders corresponded to patients with an inhibition of 10 to 29%; responders are patients with an inhibition over 30%. Up to 4.7% of the patients undergoing coronary stenting developed thrombotic stent occlusion, despite intensive clopidogrel treatment; the parallel with aspirin seems striking. However, as there is no standard definition of aspirin resistance, comparison between the results of different studies is difficult.

We support the view that aspirin cannot be defined by the level of serum thromboxane or its urinary metabolites, because these measurements do not correlate with the reduction of inhibition of platelet aggregation in response to multiple stimuli, and also because:

1. Although most of the thromboxane is believed to come from the platelets, there are additional cellular origins: monocytes/macrophages are also a rich source of thromboxane A2 [50].

2. Unlike the platelet, the macrophage is capable of synthesizing new COX-2 after aspirin has inhibited it. COX-2 is the enzyme responsible for most of the metabolism of arachidonic acid in the macrophage, and low dose aspirin is not sufficient to inhibit COX-2 [50] maximally.

3. Macrophages in atheromata may contribute significantly to the pool of thromboxane A2 [51].

4. Aspirin only inhibits monocyte PGHS-2, which is inducible by inflammatory stimuli, transiently at very high concentrations [52].