Clinical pharmacokinetics of non-steroidal anti-inflammatory drugs

The number of non-steroidal anti-inflammatory drugs (NSAIDs) available for clinical use has dramatically increased during the last decade. As a general rule, NSAIDs are well absorbed from the gastrointestinal tract, with the exception of aspirin (and possibly diclofenac, tolfenamic acid and fenbufen) which undergoes presystemic hydrolysis to form salicylic acid. Concomitant administration of NSAIDs with food or antacids may in some cases lead to delayed or even reduced absorption. The NSAIDs are highly bound to plasma proteins (mainly albumin), which limits their body distribution to the extracellular spaces. Apparent volumes of distribution of NSAIDs are, therefore, very low and usually less than 0.2 L/kg. The elimination of these drugs depends largely on hepatic biotransformation; renal excretion of unchanged drugs is usually small (less than 5% of the dose). Total body clearance is low and for most NSAIDs is less than 200 ml/min. The effect of age and disease on the disposition of NSAIDs has not been extensively studied. Due to the central role of the liver in the overall elimination of the majority of these compounds, hepatic disease will most likely lead to a significant alteration in their pharmacokinetic behaviour. NSAIDs have been reported to be involved in numerous pharmacokinetic drug interactions. Aspirin decreases the plasma concentrations of many other NSAIDs, although the clinical significance of this is uncertain. Due to the extremely high plasma protein binding of NSAIDs (around 99% in many cases), competition for the same binding sites on plasma proteins may be at least partly responsible for some interactions of NSAIDs with other highly bound drugs; however, another mechanism such as decreased metabolism or decreased urinary elimination is usually involved as well. The most important interactions with NSAIDs are those involving the oral anticoagulants and oral hypoglycaemic agents, though not all NSAIDs have been found to interact with these drugs. In clinical practice, there appear to be no clear-cut guidelines to assist the clinician in the selection of the most appropriate drug for an individual patient. The selection of an anti-inflammatory drug should be based on clinical experience, patient convenience (e.g. once or twice daily dosage schedule), side effects and cost. Since a marked interindividual variability exists in the clinical response to a given NSAID, clinicians prescribing these agents may try several of them sequentially until an adequate response is obtained.