Mechanisms of Adverse Drug Reactions

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Chapter: Pharmacovigilance: Mechanisms of Adverse Drug Reactions

An adverse drug reaction (ADR) may be defined as ‘an appreciably harmful or unpleasant reaction, resulting from an intervention related to the use of a medicinal product.


Mechanisms of Adverse Drug Reactions

INTRODUCTION

An adverse drug reaction (ADR) may be defined as ‘an appreciably harmful or unpleasant reaction, resulting from an intervention related to the use of a medicinal product, which predicts hazard from future administration and warrants prevention or specific treatment, or alteration of the dosage regimen, or with-drawal of the product’ (Edwards and Aronson, 2000). This has to be contrasted with the term adverse drug event, which refers to untoward occurrences follow-ing drug exposure but not necessarily caused by the medicine (Asscher, Parr and Whitmarsh, 1995). This chapter focuses on ADRs rather than adverse drug events.

Although the drug discovery process has been revolutionised by new techniques such as combinatorial chemistry and high-throughput screening, drug safety assessment lags well behind and is still reliant on many of same technologies that have been used for several decades. By the time a drug is marketed, only about 1500–3000 patients may have been exposed to the drug (Asscher, Parr and Whitmarsh, 1995; Rawlins, 1995). Thus, only those adverse reactions occurring at a frequency of greater than 1 in 500–1000 will have been identified at the time of licensing. Assessment of ADRs therefore is likely to represent an important aspect of drug therapy for many years to come, and indeed, with the development of new biotechnology compounds, it is likely that the pattern of these reactions will change. Furthermore, using gene and protein screening technologies, many new targets will be discovered. As new drugs are devel-oped to modulate the function of these targets, it is very unlikely that we will fully understand the biol-ogy of the new target molecule(s), and this will lead to unforeseen adverse reactions. For example, adverse effects such as the exacerbation of multiple sclero-sis, systemic lupus erythematosus (SLE) and blood dyscrasias that have been reported with anti-tumour necrosis factor (anti-TNF) therapies (Furst et al., 2000; Sharief and Hentges, 1991) or cardiovascular events with cyclo-oxygenase-II (COX-II) inhibitors (Fries et al., 2006) may not have been reasonably expected given the known pharmacology of these therapies.

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