Effect of antacid and ascorbic acid on serum salicylate concentration

The problems and pitfalls of NSAID therapy in the elderly (Part II)

The elderly are most susceptible to pharmacokinetic drug interactions between various NSAIDs and anticoagulants, sulphonylurea hypoglycaemic agents, certain anticonvulsants, methotrexate, digoxin, aminoglycosides and lithium. Pharmacodynamic interactions between some NSAIDs and antihypertensive drugs, anticoagulants, sulphonylurea agents and other NSAIDs are also potentially significant in the elderly. Despite the finding that mean therapeutic responses of large groups of patients have been generally equivalent for the wide range of NSAIDs studied thus far, it is also apparent that marked variability exists in the response of individual patients to different NSAIDs. Subsequent dosage increments may predispose 'nonresponders' and some less sensitive 'responders' to toxicity from NSAIDs. This interindividual variability in response to NSAIDs may be contributed to by the differing physicochemical properties of NSAIDs, physician prescribing habits and patient expectations, variations in NSAID pharmacokinetics, and the differing effects of NSAIDs other than their common ability to inhibit prostaglandin synthesis. The principles for drug prescribing in the elderly are no different from those that should be applied to the prescribing of medication in any patient. The clinician should strive to make a diagnosis and should avoid treating symptoms in isolation. Critical assessment of the indication for prescribing NSAID therapy must include consideration of the available effective and safe alternatives. If an NSAID is commenced the lowest effective dose should be the desired goal, but after an appropriate trial it is acceptable clinical practice to employ an alternative NSAID. There is no justification for combination NSAID therapy. The progress of each patient must be carefully monitored, particularly during the first few months of treatment, while periodic review of the ongoing need for the NSAID is essential.

Drug interactions with non steroidal anti-inflammatory drugs (NSAIDs)

Drug interactions occur when the pharmacologic profile of one drug is altered by the administration of another drug. These interactions may be due to changes in absorption, distribution, metabolism or excretion. NSAIDs are associated with drug interactions but only a proportion are clinically relevant. Many are due to displacement of a drug from its plasma protein binding sites by NSAIDs which are tightly protein-bound. They may not occur with all NSAIDs but might be selective: most NSAIDs do not have clinically important interactions with oral hypoglycemic agents whereas phenylbutazone, azaprozone & aspirin prolong their half-life. Similarly phenylbutazone and azaprozone prolong coumadin's half-life. Lithium clearance may be decreased by indomethacin, piroxicam, phenylbutazone and diclofenac. Methotrexate (MTX) may be displaced from its binding protein sites by NSAIDs. This is generally not clinically relevant with low doses of MTX as utilized in rheumatoid arthritis patients with normal renal function. NSAIDs also may reduce renal blood flow, tubular excretion of drugs & renal prostaglandin production and may attenuate the effect of anti-hypertensive drugs. Renal failure & hyperkalemia have been reported in patients receiving triamterene & indomethacin. The clinician should be aware of important drug-drug interactions prior to prescribing NSAIDs. Continued scrutiny of these effects are indicated to increase the safety profile.

Interaction of salicylate and corticosteroids in man

Corticosteroids have been reported to decrease the plasma concentrations of salicylate when salicylates have been administered chronically at high doses. In the present study, two single, oral doses of sodium salicylate, 10 mg kg-1 were administered to six adult subjects with a variety of inflammatory conditions both before and during treatment with daily oral doses of prednisone (12-60 mg). Concomitant prednisone therapy did not increase the whole body clearance of single doses of salicylate in these subjects (0.0275 +/- 0.08 l kg-1 h-1 before prednisone; 0.0247 +/- 0.03 l kg-1 h-1 during prednisone therapy; P greater than 0.05). These results indicate that corticosteroids do not alter the clearance of single doses of salicylate in man.

The basis for variability of response to anti-rheumatic drugs

The reasons for variability of response to anti-rheumatic drugs are myriad. All the factors that contribute to kinetic variability, for example, contribute to differences in response between individuals. Thus, differences in drug formulation, protein binding, drug metabolism and excretion, all contribute to variable responses. Further, factors which contribute to differential clinical response/toxicity must be considered. Here, age, gender, genetic background, weight, concomitant diseases and numerous environmental factors come into play. Among the environmental factors are such diverse elements as smoking, activity and diet. Finally our ability to measure change, be it in response or toxicity, is limited, introducing apparent variability (as much as real variability) into the equation. While we cannot, at present, delineate the contribution of each factor to individual variability, it is hoped that systematic, persistent effort will help us understand and then control these elements, leading to improved ability to individualize therapy and decrease the variability of response to anti-rheumatic drugs.

Salicylate measurement: clinical usefulness and methodology

The salicylates are the most commonly used analgesic, antipyretic, and anti-inflammatory drugs. They are available in hundreds of preparations, many of which are over-the-counter medications. The easy access to large quantities of the drug and the widespread perception that the drug is harmless have contributed to salicylate intoxication becoming a serious and common problem, particularly among the pediatric and geriatric populations. Salicylate is still the major drug for the treatment of rheumatic diseases. The use of salicylate in high doses for the management of these patients requires close monitoring of serum salicylate levels because of the large interindividual variation in dose-serum level relationships and the narrowness of the therapeutic range. Thus, both for the management of patients intoxicated with salicylate and patients who are on high-dose salicylate therapy, the measurement of serum salicylate levels is an important clinical laboratory service. Recent research on the inhibitory effect of aspirin on platelet aggregation has led to the prophylactic use of aspirin in low doses as an antithrombotic drug. This new therapeutic use of aspirin can be aided by monitoring low serum levels of salicylate and perhaps aspirin itself. This article reviews the current state of the knowledge of the pharmacokinetics and clinical toxicology of salicylate, the clinical usefulness of salicylate measurement by the clinical laboratory, and recent development in the analytical technology for salicylate analysis.

Interactions of analgesics with other drugs

Antipyretic analgesics, such as salicylates, acetaminophen, and pyrazolones, are often given concomitantly with a variety of other drugs. Drug interactions that occur at the receptors are known as pharmacodynamic interactions; alterations in absorption (bioavailability), distribution (plasma protein-binding), and elimination (renal excretion, hepatic metabolism) are termed pharmacokinetic interactions. For example, antacids and food both delay the absorption of analgesics. Highly protein-bound drugs (such as phenylbutazone, phenytoin, or warfarin) can compete with the common binding sites of salicylates. Hepatic elimination of salicylates can be influenced by drugs such as beta-blockers and cimetidine. Clinically important interactions involving salicylates, acetaminophen, and other antipyretic analgesics are discussed.

Urinary pH and plasma levels of salicylate after administration of different buffered acetylsalicylic acid formulations

In a controlled cross-over study comprising eight healthy subjects of effervescent acetylsalicylic acid (ASA) and an experimental ASA formulation were compared with unbuffered ASA and placebo concerning effects on the urinary pH within a dosage interval after 2 days' medication with 3 g ASA daily. The effects on the urinary pH were related to the morning plasma salicylate concentrations observed. Both the buffered formulations significantly increased the median pH of the period studied compared to unbuffered ASA, the effervescent by 1.5 units and the experimental by 0.6 units. Unbuffered ASA significantly decreased the median pH compared to placebo. Those subjects with the most acidic urine during placebo treatment showed the most pronounced pH elevations due to effervescent ASA. The plasma salicylate concentration was significantly lower with the effervescent formulation compared with unbuffered ASA, but there was no statistical difference between the experimental tablet and unbuffered ASA. The variable effects on the urinary pH and the plasma salicylate concentrations induced by the two buffered preparations are explained by the different absorbabilities of the buffering agents included. The results presented are consistent with recommendations not to use bicarbonate-containing ASA formulations continuously when high plasma levels are desirable.

Pharmacokinetics and pharmacodynamics of drug delivery

The complex pharmacokinetics that promote transit of a drug from ingestion to elimination are characterized by variability. The rate and degree to which a drug undergoes absorption, distribution, biotransformation and excretion are a result of both endogenous and exogenous factors, some of which can be predicted, many of which cannot. One of the greatest problems associated with conventional drug formulations has been the dose-related appearance of side-effects that often accompany the initial surge of drug release. Drugs with short half-lives or narrow therapeutic ratios may alternately produce over-medication and under-medication in a patient. Innovations in drug form design have resulted in a new precision-release formulation that permits zero-order steady state drug release. Sodium indomethacin trihydrate ('Osmosin') is the first drug to be contained within this new precision-release formulation which approaches the objective of delivering drugs at a therapeutically optimal rate.