Short communication
Influence of combinations of acetylsalicylic acid, acetaminophen, and diclofenac on platelet aggregation

https://doi.org/10.1016/j.ejphar.2008.07.036Get rights and content

Abstract

Acetylsalicylic acid (aspirin) is often given together with other nonsteroidal anti-inflammatory drugs and acetaminophen. The latter have been accused in epidemiologic studies to cause an increased cardiovascular risk. We have, therefore, analysed the influence of various such drug combinations on platelet aggregation in vitro. Citrated blood was incubated with either 25 μg/ml acetaminophen, 0.5 μg/ml aspirin, 0.04 μg/ml diclofenac, or buffer; followed by a second of the above-mentioned solutions. After a 20 min incubation, platelet aggregation was assessed with a platelet function analyser (PFA-100®), which measures the pore closure time (CT) by aggregating platelets. The length of CT reflects the degree of platelet inhibition. Acetaminophen alone did not affect platelet aggregation. Aspirin and diclofenac both increased CT (184 ± 69 s, P < 0.01 and 196 ± 54 s, P < 0.001; control 120 ± 13 s). Combinations of either aspirin and diclofenac, aspirin and acetaminophen, or diclofenac and acetaminophen increased CT further (290 ± 22 s, 281 ± 36 s, 288 ± 25 s, respectively, P < 0.001). The time sequence of drug application was important: when diclofenac or acetaminophen was added before aspirin, platelet aggregation was less inhibited than when given in opposite order, i.e. aspirin prior to diclofenac or acetaminophen. We conclude that acetaminophen by itself does not affect platelet aggregation, but potentiates the antiaggregatory effect of aspirin or diclofenac. Aspirin given before acetaminophen or diclofenac had a more potent antiaggregatory effect than vice versa. These observations may have clinical implications.

Introduction

The inhibition of platelet aggregation is the cornerstone treatment of vasoocclusive events such as myocardial infarction, stroke, or peripheral vascular occlusion (Antithrombotic Trialist's Collaboration, 2002). Most commonly, this is achieved by an inhibition of the membrane-bound cyclooxygenase-1 (COX-1), the enzyme responsible for the production of the important platelet agonist thromboxane A2 from arachidonic acid. Acetylsalicylic acid (aspirin) has been used successfully over decades for this purpose and is routinely given to patients with any arteriosclerotic disease (Patrono et al., 2004). It irreversibly acetylates a serine residue at position 529 of the COX enzyme (Catella-Lawson et al., 2001), inducing an antiaggregatory effect, which persists for the entire lifespan of the platelet. Aspirin is not the only drug acting on platelet-COX; all nonsteroidal anti-inflammatory drugs (NSAID) inhibit COX-induced platelet aggregation to some degree (Catella-Lawson et al., 2001). In contrast to aspirin, this effect is, however, reversible.

More selective COX-2 inhibitors, so called coxibs, had been developed and were about to replace traditional NSAID, when observations of an increased cardiovascular risk forced a withdrawal of rofecoxib from the market (Bombardier et al., 2000, Bresalier et al., 2005). It was hypothesized that coxibs could interfere with the protective antiaggregatory effect of aspirin and the cardiovascular safety of all NSAID became again questioned (Chan et al., 2006, Hippisley-Cox and Coupland, 2005, Kearney et al., 2006, Kurth et al., 2003, MacDonald and Wei, 2003, McGettigan and Henry, 2006). Indeed, frequent consumption of NSAID (Kearney et al., 2006, MacDonald and Wei, 2003) and acetaminophen (Chan et al., 2006) seems to be associated with an increased risk of cardiovascular events. The mechanisms behind these epidemiologic observations have not been elucidated so far.

This possible interference of aspirin and other NSAID is clinically very important, because a large proportion of patients are taking aspirin (for vascular disease) together with other NSAID (for osteoarthritic diseases) or acetaminophen (as an antipyretic or analgesic drug). In the present in vitro study we have tested the antiaggregatory effect of aspirin, acetaminophen, diclofenac, and phosphate buffered saline (PBS; control) either alone or in paired combinations, applying one drug 10 min after the other.

Section snippets

Sampling of human blood

A total of 20 healthy volunteers (aged 25–74, 3 women) gave their informed consent to the study, which had been approved by the institutional ethics committee. They were not taking any drug with a potential effect on platelet aggregation such as aspirin, NSAID, or clopidogrel one week before blood sampling and they had no personal history of bleeding. Women were not taking oral contraceptives. Blood was drawn from an antecubital vein by atraumatic venipuncture with a 21-gauge butterfly cannula

Results

The influence of single drugs and drug combinations on platelet aggregation measured with the PFA-100® is shown in Fig. 1. Acetaminophen 25 μg/l alone (APP followed by PBS) did not influence the PFA-100® closure time compared with control (PBS followed by PBS). The NSAID diclofenac 0.04 μg/ml (DIC followed by PBS) and aspirin 0.5 μg/ml (ASS followed by PBS) prolonged the closure time, indicating a comparable platelet inhibition. An interesting and surprising observation was that the

Discussion

Acetaminophen is a weak inhibitor of COX-2 and even less of COX-1 (Lages and Weiss, 1989). In our study, it did not influence platelet aggregation when given alone in a concentration of 25 μg/ml, which is about twice that reached by 1 g acetaminophen given orally. Munsterhjelm et al. (2005) have observed that PFA-100® closure times are only prolonged when high in vitro concentrations of 80 μg/ml are used. The same group has recently shown that acetaminophen 1 g intravenously exerted a mild

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