Evidence that diclofenac and celecoxib are thyroid hormone receptor beta antagonists
Introduction
Non-steroidal anti-inflammatory drugs (NSAIDS) inhibit cyclooxygenase (COX), the enzymes that are responsible for prostaglandin production [1]. There are two isoforms, COX-1 which is constitutively expressed, and COX-2 which is inducible. NSAIDS are widely used for their analgesic, antipyretic and anti-inflammatory properties however despite their therapeutic effectiveness, their use has been widely scrutinized due to their tendency to produce side effects. Since prostaglandins protect the gastrointestinal tract and are important in platelet aggregation, NSAID reduction of prostanoid production increases the risk of gastrointestinal ulceration and bleeds. Due to the toxic effects of NSAIDs such as diclofenac on gastrointestinal mucosa, COX-2 selective drugs such as celecoxib were developed. Clinical trials revealed the side effects of both pan- and COX-1 sparing NSAIDs led to gastrointestinal damage and cardiovascular complications including myocardial infarction [2], [3].
There are currently two conflicting models that explain the cardiovascular side effects of NSAIDs. The first model put forward by Cheng et al. states that under normal physiological conditions endothelial COX-2 drives the production of prostacyclins whilst platelet COX-1 drives the production of thromboxanes [4]. The model predicts that a balance between pro-thrombotic and antithrombotic state exist under normal physiological conditions. However, when an NSAID which inhibits COX-2 in endothelial cells is introduced, the balance is disrupted and a pro-thrombotic state develops [4].
Recent evidence has emerged that provides evidence that COX-2 is not expressed in endothelial cells [5], [6], but is highly expressed in the renal medulla [7], indicating a need for a new model for what causes NSAID induced side effects to be developed. Loss or inhibition of COX-2 in mice and man leads to an increase in the production of endogenous eNOS inhibitor, asymmetric dimethyl arginine (ADMA) which suggests that specific pathways are altered by COX-2 inhibition [7].
While much debate about the side effects of NSAIDs has concentrated on the direct effects of NSAIDs on COX activity, we investigated the indirect side effects of celecoxib and diclofenac using computational chemistry methods. In silico modelling indicated a potential for both drugs to associate with thyroid hormone receptor β (TRβ), and further analysis using in vitro methods indicate that both celecoxib and diclofenac possess TRβ antagonistic properties. This nuclear receptor is of great interest, with clear relationships between hypothyroidism associated with increased heart muscle stiffness and an increased risk of myocardial infarction [8].
Section snippets
In silico methods
Open Virtual ToxLab .5,21 [9] was used to predict toxic potential by predicting binding affinities to 10 off-target nuclear receptors, 4 cytochrome P450 enzymes, a transcription factor and a potassium ion channel and forecast endocrine and metabolic disruption, some aspects of carcinogenicity and cardiotoxicity. The default values of the software for the predictions of toxic potentials for diclofenac and celecoxib were used as described previously [9].
The Pharmmapper, freely available web
In silico modelling
Using VirtualTox screening programme, structures for diclofenac and celecoxib were assessed for the potential binding to a series of target protein known to be correlated with the side effects, and a normalized toxicity potential was calculated (Table 1). The results suggest that both drugs can potentially bind all nuclear receptors, albeit with various affinities. Both drugs exhibited no affinity with CYP enzymes, arylhydrocarbon (AhR), and human Ether-à-go-go-Related Gene (hERG K). The
Discussion
Our data is the first to demonstrate that T3 induces vasodilation of rat mesenteric arteries, which can be significantly reduced by the TRβ antagonist MLS. Moreover, diclofenac and celecoxib have the capacity to bind to and antagonise TRβ and TRα receptors.
Thyroid hormones have been shown to act directly on rat aortic artery smooth muscle cells [20], rat skeletal muscle resistance arteries [21] and rat mesenteric arteries [22] to induce vasodilation in a short time frame, indicating a
Disclosure
The authors declare that there is no conflict of interest regarding the publication of this article.
Funding
The conduct of the research and preparation of the article was funded by the University of Hertfordshire, UK.
Acknowledgements
Authors thank to Biograf3R for the licence to use Open Virtual ToxLab software. We also thank Open Eye Scientific Software, Inc., for the free academic licence of the Open Eye Toolkits. We also thank Schrodinger Ltd. for their support in the final stages of the manuscript revision by providing the licence for their software.
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