Metabolism of salicylate during chronic aspirin therapy

Pharmacokinetics of aspirin: evaluating shortcomings in the literature

INTRODUCTION

Aspirin is known for its therapeutic benefits in preventing strokes and relieving pain. However, it is toxic to some individuals, and the biological mechanisms causing toxicity are unknown. Limited literature is available on the role of glycine conjugation as the principal pathway in aspirin detoxification. Previous studies have quantified this two-step enzyme reaction as a singular enzymatic process. Consequently, the individual contributions of these enzymes to the kinetics remain unclear.

AREAS COVERED

This review summarized the available information on the pharmacokinetics and detoxification of aspirin by the glycine conjugation pathway. Literature searches were conducted using Google Scholar and the academic journal databases accessible through the North-West University Library. Furthermore, the factors affecting interindividual variation in aspirin metabolism and what is known regarding aspirin toxicity were discussed.

EXPERT OPINION

The greatest drawback in understanding the pharmacokinetics of aspirin is the limited information available on the substrate preference of the xenobiotic ligase (ACSM) responsible for activating salicylate to salicyl-CoA. Furthermore, previous pharmacokinetic studies did not consider the contribution of other substrates from the diet or genetic variants, to the detoxification rate of glycine conjugation. Impaired glycine conjugation might contribute to adverse health effects seen in Reye's syndrome and cancer.

P2RY12-Inhibitors Reduce Cancer-Associated Thrombosis and Tumor Growth in Pancreatic Cancers

Platelet function can be modified by cancer cells to support tumor growth, causing alterations in the delicate hemostatic equilibrium. Cancer-cell and platelet interactions are one of the main pillars of Trousseau’s syndrome: a paraneoplastic syndrome with recurring and migrating episodes of thrombophlebitis. Altogether, this leads to a four-fold risk of thrombotic events in cancer patients, which in turn, portend a poor prognosis. We previously demonstrated that anti-P2RY12 drugs inhibit cancer-associated-thrombosis and formation of tumor metastasis in pancreatic cancer models. Here, we aimed to (1) compare the effects of aspirin and clopidogrel on pancreatic cancer prevention, (2) characterize the effects of clopidogrel (platelet P2RY12 inhibitor) on cancer-associated thrombosis and cancer growth in vivo, (3) determine the effect of P2RY12 across different digestive-tract cancers in vitro, and (4) analyze the expression pattern of P2RY12 in two different cancer types affecting the digestive system. Clopidogrel treatment resulted in better survival rates with smaller primary tumors and less metastasis than aspirin treatment. Clopidogrel was also more effective than aspirin at dissolving spontaneous endogenous thrombi in our orthotopic advanced cancer mouse model. P2RY12 expression gives pancreatic adenocarcinomas proliferative advantages. In conclusion, we propose the hypothesis that clopidogrel should be further studied to target and prevent Trousseau’s syndrome; as well as diminish cancer growth and spread. However, more studies are required to determine the implicated pathways and effects of these drugs on cancer development.

Efficacy and safety of low dose aspirin and magnesium sulfate in the treatment of pregnancy induced hypertension: A protocol for systematic review and meta-analysis

BACKGROUND

Magnesium sulfate combined with low-dose aspirin can significantly reduce adverse reactions and effectively lower blood pressure in patients with pregnancy induced hypertension, but the overall efficacy and safety of the combination of drugs are not clear. The purpose of this study was to evaluate the efficacy and safety of magnesium sulfate combined with low-dose aspirin in the treatment of pregnancy induced hypertension.

METHODS

Randomized controlled trials focusing on the administration of magnesium sulfate combined with low-dose aspirin for pregnancy induced hypertension were searched from PubMed, EMbase, Cochrane Library, Web of Science, China National Knowledge Infrastructure (CNKI), WanFang, and the Chongqing VIP Chinese Science and Technology Periodical Database. Two researchers independently screened titles, abstracts, and full texts, and extracted relevant data. The RevMan 5.3 software and Stata 14 software were used for statistical analysis.

RESULTS

The effect and safety of magnesium sulfate combined with low-dose aspirin in the treatment of pregnancy induced hypertension were assessed by summarizing the related randomized controlled trials.

CONCLUSION

This article provides theoretical support for the clinical application of magnesium sulfate combined with low-dose aspirin in the treatment of pregnancy induced hypertension.

OSF REGISTRATION NUMBER

DOI 10.17605/OSF.IO/SZFGB.

Correlation between long-term aspirin use and F-fluorodeoxyglucose uptake in colorectal cancer measured by PET/CT

PURPOSE

The aim of this study was to evaluate the relationship between long-term aspirin use with pretreatment 18 Fluorodeoxyglucose (FDG) uptake of primary lesions of Colorectal cancer (CRC) and evaluate their clinical significance.

MATERIALS AND METHODS

We enrolled 84 patients with CRC who underwent 18F-FDG PET/CT scanning before surgery between 1st July 2008 and 1st March 2013 and followed up until 1st March 2014. Maximum standardized uptake value (SUVmax) of the primary tumor was measured by 18F-FDG PET/CT. The history of aspirin taken and other clinicopathogical factors were also obtained and their relationships were examined by Mann-Whitney or χ2 tests. Progression-free survival (PFS) was determined by standard Kaplan-Meier survival analysis. Cox proportional hazards regression was performed to determine whether history of aspirin taken, pretreatment SUVmax, age, gender, TNM stage, tumor sizes and differentiation influenced outcomes.

RESULTS

CRC Patients with long-term history of aspirin use had lower SUVmax of primary lesions than control group (9.74±2.62 vs. 13.91±6.18) and showed a trend towards improved PFS after curative surgery. However, pretreatment of SUVmax showed no prognostic value in patients with CRC.

CONCLUSIONS

Long-term aspirin use is associated with lower pretreatment SUVmax of CRC and is a promising prognostic factor for predicting PFS in patients with CRC.

Aspirin may modify tumor microenvironment via antiplatelet effect

High-quality evidence suggests that aspirin is a promising agent for cancer prevention and treatment. Direct inhibition of cyclooxygenase-2 (COX-2) pathway is generally thought to be the main mechanism by which aspirin inhibits cancer development. However, either pharmacological properties of aspirin or recent results of epidemiologic studies do not support that mechanism. To address this inconsistency, we hypothesize that antiplatelet effect of aspirin via inhibition of COX-1 may be one of potential mechanisms to inhibit carcinogenesis. Aberrant platelet activation will lead to promote hostility of tumor microenvironment by releasing an abundant array of angiogenesis regulators. Given the outstanding ability of antiplatelet, aspirin may restore balance of pro- and anti-angiogenic factors released from platelet to "normalize" tumor vasculature and shape tumor microenvironment to some extent, which will not only diminish tumor aggressiveness and progression, but also enhance the sensitivity to therapeutic treatment. Thus, targeting the platelet activation leading to alter tumor microenvironment may provide a novel way to tumor therapy.

Mode of action of aspirin as a chemopreventive agent

Aspirin taken for several years at doses of at least 75 mg daily reduced long-term incidence and mortality due to colorectal cancer. The finding of aspirin benefit at low-doses given once daily, used for cardioprevention, locates the antiplatelet effect of aspirin at the center of its antitumor efficacy. In fact, at low-doses, aspirin acts mainly by an irreversible inactivation of platelet cyclooxygenase (COX)-1 in the presystemic circulation, which translates into a long-lasting inhibition of platelet function. Given the short half-life of aspirin in the human circulation(approximately 20 min) and the capacity of nucleated cells to resynthesize the acetylated COX-isozyme(s), it seems unlikely that a nucleated cell could be the target of aspirin chemoprevention. These findings convincingly suggest that colorectal cancer and atherothrombosis may share a common mechanism of disease, i.e. platelet activation in response to epithelial(in tumorigenesis) and endothelial(in tumorigenesis and atherothrombosis) injury. Activated platelets may also enhance the metastatic potential of cancer cells (through a direct interaction and/or the release of soluble mediators or exosomes) at least in part by inducing the overexpression of COX-2. COX-independent mechanisms of aspirin, such as the inhibition of NF-kB signaling and Wnt/β-catenin signaling and the acetylation of extra-COX proteins, have been suggested to play a role in its chemopreventive effects. However, their relevance remains to be demonstrated in vivo at clinical doses.

Saliva versus plasma pharmacokinetics: theory and application of a salivary excretion classification system

The aims of this work were to study pharmacokinetics of randomly selected drugs in plasma and saliva samples in healthy human volunteers, and to introduce a Salivary Excretion Classification System. Saliva and plasma samples were collected for 3-5 half-life values of sitagliptin, cinacalcet, metformin, montelukast, tolterodine, hydrochlorothiazide (HCT), lornoxicam, azithromycin, diacerhein, rosuvastatin, cloxacillin, losartan and tamsulosin after oral dosing. Saliva and plasma pharmacokinetic parameters were calculated by noncompartmental analysis using the Kinetica program. Effective intestinal permeability (Peff) values were estimated by the Nelder-Mead algorithm of the Parameter Estimation module using the SimCYP program. Peff values were optimized to predict the actual average plasma profile of each drug. All other physicochemical factors were kept constant during the minimization processes. Sitagliptin, cinacalcet, metformin, tolterodine, HCT, azithromycin, rosuvastatin and cloxacillin had salivary excretion with correlation coefficients of 0.59-0.99 between saliva and plasma concentrations. On the other hand, montelukast, lornoxicam, diacerhein, losartan and tamsulosin showed no salivary excretion. Estimated Peff ranged 0.16-44.16 × 10(-4) cm/s, while reported fraction unbound to plasma proteins (fu) ranged 0.01-0.99 for the drugs under investigation. Saliva/plasma concentrations ratios ranged 0.11-13.4, in agreement with drug protein binding and permeability. A Salivary Excretion Classification System (SECS) was suggested based on drug high (H)/low (L) permeability and high (H)/low (L) fraction unbound to plasma proteins, which classifies drugs into 4 classes. Drugs that fall into class I (H/H), II (L/H) or III (H/L) are subjected to salivary excretion, while those falling into class IV (L/L) are not. Additional data from literature was also analyzed, and all results were in agreement with the suggested SECS. Moreover, a polynomial relationship with correlation coefficient of 0.99 is obtained between S* and C*, where S* and C* are saliva and concentration dimensionless numbers respectively. The proposed Salivary Excretion Classification System (SECS) can be used as a guide for drug salivary excretion. Future work is planned to test these initial findings, and demonstrate SECS robustness across a range of carefully selected (based on physicochemical properties) drugs that fall into classes I, II or III.

Drug structure-transport relationships

Malcolm Rowland has greatly facilitated an understanding of drug structure–pharmacokinetic relationships using a physiological perspective. His view points, covering a wide range of activities, have impacted on my own work and on my appreciation and understanding of our science. This overview summarises some of our parallel activities, beginning with Malcolm’s work on the pH control of amphetamine excretion, his work on the disposition of aspirin and on the application of clearance concepts in describing the disposition of lidocaine. Malcolm also spent a considerable amount of time developing principles that define solute structure and transport/pharmacokinetic relationships using in situ organ studies, which he then extended to involve the whole body. Together, we developed a physiological approach to studying hepatic clearance, introducing the convection–dispersion model in which there was a spread in blood transit times through the liver accompanied by permeation into hepatocytes and removal by metabolism or excretion into the bile. With a range of colleagues, we then further developed the model and applied it to various organs in the body. One of Malcolm’s special interests was in being able to apply this knowledge, together with an understanding of physiological differences in scaling up pharmacokinetics from animals to man. The description of his many other activities, such as the development of clearance concepts, application of pharmacokinetics to the clinical situation and using pharmacokinetics to develop new compounds and delivery systems, has been left to others.

Site-specific percutaneous absorption of methyl salicylate and VX in domestic swine

The site specificity of the percutaneous absorption of methyl salicylate (MeS) and the organophosphate nerve agent VX (O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothioate) was examined in anaesthetized domestic swine that were fully instrumented for physiological endpoints. Four different anatomical sites (ear, perineum, inguinal crease and epigastrium) were exposed to the MeS and the serum levels were measured over a 6-h time period. The dose absorbed at the ear region was 11 microg cm(-2) with an initial flux of 0.063 microg cm(-2)min(-1), whereas at the epigastrium region the dose absorbed was 3 microg cm(-2) with an initial flux of 0.025 microg cm(-2)min(-1). For this reason further studies were carried out with VX on the ear and the epigastrium only. In animals treated with agent on the epigastrium, blood cholinesterase (ChE) activity began to drop 90 min after application and continued to decline at a constant rate for the remainder of the experiment to ca. 25% of awake control activity. At this time there were negligible signs of poisoning and the medical prognosis was judged to be good. In contrast, the ChE activity in animals receiving VX on the ear decreased to 25% of awake control values within 45 min and levelled out at 5-6% by 120 min. Clinical signs of VX poisoning paralleled the ChE inhibition, progressing in severity over the duration of the exposure. It was judged that these animals would not survive. The dramatic site dependence of agent absorption leading to vastly different toxicological endpoints demonstrated in this model system has important ramifications for chemical protective suit development, threat assessment, medical countermeasures and contamination control protocols.

Salicylate intoxication in the elderly. Recognition and recommendations on how to prevent it

Aspirin (acetylsalicylic acid) and its salicylate derivatives are effective antipyretic, analgesic, and anti-inflammatory agents that are still very widely used by the elderly despite the advent of newer, potentially safer nonsteroidal anti-inflammatory drugs (NSAIDs). However, none of the new NSAIDs have been proven to be more effective than aspirin or salicylic acid. Chronic salicylate intoxication which is most common in the elderly, may occur with therapeutic doses. Increased toxicity in older patients often appears due to inadvertent overdosage. Dual prescribing or additional use of nonprescription salicylates are some causes of unwitting long term toxicity. According to some studies, systemic clearance of salicylate (mainly by hepatic metabolism) is reduced with age, as is renal elimination. These changes are of increased importance in the elderly using high therapeutic doses of salicylates when metabolism is saturated and more unchanged drug is available for renal excretion. In the face of renal impairment, the risk of toxicity is increased. The diagnosis of acute salicylate intoxication generally does not pose diagnostic problems. Patients often present with a history of intentional overdose, with hyperventilation, fever, and nausea. The diagnosis can be confirmed by measuring serum salicylate concentrations. Chronic intoxication often poses a diagnostic dilemma with atypical presentations mimicking other disease states such as diabetic ketoacidosis, delirium, cerebrovascular accident, myocardial infarction or cardiac failure. The diagnosis of salicylate intoxication should be borne in mind when an older patient presents with recent deterioration in activities of daily living with no known cause. Plasma salicylate concentrations should be measured if salicylate intoxication is suspected, even if there is no documented history of salicylate ingestion. The risk of salicylate nephrotoxicity is also increased with age, and upper gastrointestinal haemorrhage is associated with increased mortality in older age groups. Treatment of acute toxicity consists of prompt recognition of salicylate intoxication, use of activated charcoal, correction of acid-base abnormalities, general supportive measures, and if concentrations are extremely high, dialysis can be effectively used. Chronic toxicity, which can occur even with marginally high salicylate concentrations, is treated with drug withdrawal and supportive therapy. Chronic salicylate toxicity can be averted by prescription of conservative doses of drug, avoidance of concomitant use of different salicylate preparations, and therapeutic monitoring to guide dosage. Renal function should be monitored to detect nephrotoxicity from chronic salicylate therapy. Patients should be regularly screened for evidence of gastrointestinal bleeding.(ABSTRACT TRUNCATED AT 400 WORDS)

Nonlinear pharmacokinetics: clinical Implications

Nonlinear pharmacokinetics (in other words, time or dose dependences in pharmacokinetic parameters) can arise from factors associated with absorption, first-pass metabolism, binding, excretion and biotransformation. Nonlinearities in absorption and bioavailability can cause increases in drug concentrations that are disproportionately high or low relative to the change in dose. One of the more important sources of nonlinearity is the partial saturation of presystemic metabolism exhibited by such drugs as verapamil, propranolol and hydralazine. In such cases, circulating drug concentrations are sensitive not only to dose size but also to rate of absorption: slower absorption may decrease the overall systemic availability. The binding of drugs to plasma constituents, blood cells and extravascular tissue may exhibit concentration dependence. This can cause pharmacokinetic parameters based on total blood or serum drug concentrations to be concentration-dependent. Often, in these cases, parameters based on free drug concentration appear linear. An important consideration in regard to concentration-dependent serum binding is the difficulty in relating total concentration to a usual therapeutic range if free concentration is a better indicator of drug effect. Measurement of free concentration is needed in these cases, particularly if the intersubject variability in binding is high. An example of this behaviour is valproic acid. Partial saturation of elimination pathways can result in the well known behaviour typical of Michaelis-Menten pharmacokinetics. Small changes in dosing rate can make much larger differences in steady-state concentration. The time to achieve a given fraction of steady-state becomes longer as the dosing rate approaches the maximum elimination rate. Alcohol and phenytoin are examples of drugs that exhibit such behaviour. The sensitivity of steady-state concentration and cumulation rate to changes in dosing rate are both influenced by the magnitude of parallel first-order elimination pathways: even a first-order pathway that is only 1 to 2% of maximum clearance (which occurs at very low concentration) can be an important determinant of steady-state concentration and cumulation rate when concentrations are high. Theophylline and salicylate have significant parallel first-order elimination pathways as well as saturable pathways. Autoinduction causes an increase in clearance with long term administration. In some cases, dosage adjustment must be made to compensate for the increase, and the possibility that the degree of induction may be dose- or concentration-dependent must be kept in mind. Carbamazepine is a major example of autoinduction. It is fortunate that only a few of the many hundreds of drugs in use exhibit nonlinear behaviour that leads to clinical implications.(ABSTRACT TRUNCATED AT 400 WORDS)

Adverse reactions and interactions with aspirin. Considerations in the treatment of the elderly patient

Aspirin (acetylsalicylic acid) is probably the most frequently used medication, and its use is generally uneventful. However, aspirin is also noted for numerous side effects and drug interactions that can complicate the course of therapy. The elderly, especially those with complicated medical histories, are more prone to the adverse effects of salicylates and may develop gastrointestinal tract bleeding, renal insufficiency, asthma and CNS toxicity. In the clinical situation, important drug interactions can occur with concurrent use of anticoagulants, sulphonylureas, diuretics, methotrexate and antacids. In long term aspirin therapy, enteric-coated or nonacetylated forms of aspirin are associated with fewer side effects and may be better tolerated. Monitoring of therapy (especially in the higher risk patient), with frequent assessments of the clinical state and measurements of serum creatinine, electrolytes and salicylate concentrations, may diminish the likelihood of toxicity.

Salicylate pharmacokinetics in patients with rheumatoid arthritis

1. The pharmacokinetics of salicylic acid (SA) and its major metabolite salicyluric acid (SU) were studied in nine patients with rheumatoid arthritis following a 900 mg oral dose of acetylsalicylic acid and during 6 weeks of chronic administration of enteric coated aspirin (3,900 mg day). Response to therapy was also monitored. 2. The various pharmacokinetic parameters determined in the study were similar to those observed in other single dose salicylate studies amongst healthy volunteers but were not predictive of salicylate concentration in the chronic dose study. 3. Plasma concentrations of SA (total and unbound) were found to decline significantly over the 6 weeks and plasma SU concentrations increased. 4. During the chronic dosing study, there was a significant increase in the Vmax (total and unbound) for the formation of SU, whilst the Km and SU clearance remained constant. Also, the elimination rate constant (k) for salicylate was not significantly affected. 5. Therapeutic response to salicylate therapy was not significantly affected by the decline in SA concentrations.

The effect of concurrent aspirin upon plasma concentrations of tenoxicam

1. The effect of chronic, high-dose aspirin therapy upon the disposition of a single dose and multiple doses of tenoxicam was examined in normal volunteers. 2. Aspirin caused a 24% drop in the t1/2 (P less than 0.005), a 49% rise in the volume of distribution (P less than 0.0003) and a 98% increase in the clearance (P less than 0.0001) of tenoxicam after a single dose of the tenoxicam. 3. Steady-state concentrations of tenoxicam decreased significantly from 10.4 +/- 1.5 to 4.5 +/- 1.6 micrograms ml-1 in the presence of chronic, high-dose aspirin treatment. 4. Tenoxicam percentage free measured in plasma from a normal volunteer was 0.56 +/- 0.05% over the tenoxicam concentration range 1-20 micrograms ml-1 and rose to 1.24 +/- 0.07% in the presence of aspirin 150 micrograms ml-1. 5. The effect of aspirin upon the disposition of tenoxicam was consistent with a competitive protein binding interaction.

Changes in salicylate serum concentration and metabolism during chronic dosing in normal volunteers

Steady-state serum salicylic acid (SA) concentrations and the formation rates of salicyluric acid (SU), salicylphenolic glucuronide (SPG), salicylacyl glucuronide (SAG), and gentistic acid (GA), and the excretion rate of unchanged SA were determined in three normal subjects following the administration of a single oral dose of acetylsalicylic acid (ASA) 37 mg kg-1 and during multiple dosing with ASA 56 mg kg-1 day-1. Steady-state SA concentrations fell 23 per cent during the 4-week study period (mean +/- SD: 239 +/- 35 to 183 +/- 23 micrograms ml-1; p less than 0.05). Absorption of drug did not decrease during this time. Significant increases in the Vmax for SU and SPG formation occurred between baseline and during multiple dosing. Mean Vmax SU increased from 0.93 to 1.68 mg kg-1 h-1 (p less than 0.05) and mean Vmax SPG increased from 0.09 to 0.19 mg kg-1 h-1 (p less than 0.05). First-order rate constants for the formation of SAG and GA and for the excretion of unchanged SA did not change significantly between baseline and multiple dosing. The mechanism for the decline in CpssSA with time during high-dose SA therapy is most likely due to the induction of the formation rates of the major metabolites of SA, namely SU and SPG.

Influence of age, gender, and obesity on salicylate kinetics following single doses of aspirin

Salicylate kinetics following single, 650-mg intravenous and oral doses of aspirin were evaluated in humans in 2 studies. Complete conversion of aspirin to salicylate was assumed. The first study involved 25 young (25-40 years) and 21 elderly (66-89 years) healthy male and female volunteers. Mean salicylate clearance was lower in elderly females compared with that in young females; however, the difference between young men and elderly men was not significant. Salicylate free fraction in plasma increased significantly with age in men and women. After correction for free fraction, unbound mean clearance was reduced in elderly men compared with young men, and in elderly women compared with young women. Peak plasma salicylate concentrations after taking oral aspirin were not significantly influenced by age, and systemic availability of salicylate in all groups was complete. The second study compared 20 obese subjects (mean weight 113 kg) with 20 normal weight controls (mean weight 67 kg) matched for age, sex, height, and smoking habits. Small differences between obese and control groups were observed in total salicylate volume of distribution (Vd), unbound Vd, and mean clearance of total or unbound salicylate. Following normalization for total weight, however, values of total Vd and mean clearance were significantly smaller in obese subjects than in normal weight subjects. Rate and completeness of salicylate absorption were not influenced by obesity when aspirin was ingested, although peak levels were lower in obese subjects. If applied to multiple doses, the reduced unbound clearance of salicylate in the elderly would imply increased accumulation unless doses are appropriately adjusted downward. During long-term therapy, salicylate dosage for obese individuals should not be adjusted upward in proportion to total weight.

Clinical pharmacokinetics of the salicylates

The use of salicylates in rheumatic diseases has been established for over 100 years. The more recent recognition of their modification of platelet and endothelial cell function has lead to their use in other areas of medicine. Aspirin (acetylsalicylic acid) is still the most commonly used salicylate. After oral administration as an aqueous solution aspirin is rapidly absorbed at the low pH of the stomach millieu. Less rapid absorption is observed with other formulations due to the rate limiting step of tablet disintegration - this latter factor being maximal in alkaline pH. The rate of aspirin absorption is dependent not only on the formulation but also on the rate of gastric emptying. Aspirin absorption follows first-order kinetics with an absorption half-life ranging from 5 to 16 minutes. Hydrolysis of aspirin to salicylic acid by nonspecific esterases occurs in the liver and, to a lesser extent, the stomach so that only 68% of the dose reaches the systemic circulation as aspirin. Both aspirin and salicylic acid are bound to serum albumin (aspirin being capable of irreversibly acetylating many proteins), and both are distributed in the synovial cavity, central nervous system, and saliva. The serum half-life of aspirin is approximately 20 minutes. The fall in aspirin concentration is associated with a rapid rise in salicylic acid concentration. Salicylic acid is renally excreted in part unchanged and the rate of elimination is influenced by urinary pH, the presence of organic acids, and the urinary flow rate. Metabolism of salicylic acid occurs through glucuronide formation (to produce salicyluric acid), and salicyl phenolic glucoronide), conjugation with glycine (to produce salicyluric acid), and oxidation to gentisic acid. The rate of formation of salicyl phenolic glucuronide and salicyluric acid are easily saturated at low salicylic acid concentrations and their formation is described by Michaelis-Menten kinetics. The other metabolic products follow first-order kinetics. The serum half-life of salicylic acid is dose-dependent; thus, the larger the dose employed, the longer it will take to reach steady-state. There is also evidence that enzyme induction of salicyluric acid formation occurs. No significant differences exist between the pharmacokinetics of the salicylates in the elderly or in children when compared with young adults. Apart from differences in free versus albumin-bound salicylate in various disease states and physiological conditions associated with low serum albumin, pharmacokinetic parameters in patients with rheumatoid arthritis, osteoarthritis, chronic renal failure or liver disease are essentially the same.(ABSTRACT TRUNCATED AT 400 WORDS)

Preparation and analysis of deuterium-labeled aspirin: application to pharmacokinetic studies

Inhibition of endogenous prostacyclin and thromboxane biosynthesis by aspirin is critically dose-dependent in humans. Gastrointestinal and hepatic hydrolysis may limit systemic availability of aspirin, especially in low doses, perhaps contributing to the biochemical selectivity of aspirin. Existing analytical methods do not permit determination of systemic bioavailability when low (less than 100 mg) doses of aspirin are administered. Deuterium-labeled aspirin (2-acetoxy[3,4,5,6-2H4]benzoic acid) was synthesized from salicylic acid by catalytic exchange and subsequent acetylation. Analysis of the compounds as benzyl esters by GC-MS followed extractive alkylation from plasma. Heptadeuterated compounds were used as internal standards. Simultaneous administration of tetradeuterated aspirin intravenously with native aspirin orally to anesthetized dogs permitted kinetic studies of both aspirin and salicylic acid. The sensitivity of the method is superior to published methods using HPLC and, thus, more applicable to studies of low dose aspirin. Pulse administration of stable isotope-labeled aspirin permits detailed and repeated studies of dose-related aspirin pharmacokinetics in humans.

Community Surveillance Study--perceived response of rheumatoid arthritis patients to NSAIDs

The perceived effectiveness of nonsteroidal anti-inflammatory drugs was studied in a community population (n=380). The population characteristics of a sample were similar to previous studies. Most patients were on a single NSAID. Rheumatoid arthritis (RA) was found to have a significant impact in terms of disability and lifestyle. Active inflammation was the main incentive to comply with therapy. However, the patients perceptions about the efficacy of the medication, their attitude towards the medication and their motivations for taking it, also affected compliance. A decreased effectiveness of therapy may also be due in part to tolerance to some medications and some evidence of the induction of salicylate metabolism has been reported. Considerable variation in the patients' perception of other forms of therapy for RA were also observed.

Dose-related kinetics of aspirin. Presystemic acetylation of platelet cyclooxygenase

When aspirin is administered by mouth in low doses, poor systemic bioavailability may contribute to its apparent dose-related "selective inhibition" of thromboxane A2 formation. Systemic bioavailability of orally administered aspirin is necessary to inhibit prostacyclin synthesis by systemic vascular endothelium, whereas cumulative inhibition of thromboxane A2 formation by platelets may occur in the presystemic (portal) circulation. We simultaneously administered unlabeled aspirin orally and deuterium-labeled aspirin intravenously in five healthy volunteers. This permitted an estimation of the bioavailability of an oral dose from the ratio of plasma drug concentration-time curves for the labeled and the unlabeled species. Systemic bioavailability ranged from 46 to 51 per cent of single oral doses of 20, 40, 325, and 1300 mg of aspirin. Bioavailability was similar after single-dose and long-term oral administration of 325 mg. Thromboxane B2 formation in serum ex vivo after oral administration of 20 mg of unlabeled aspirin was reduced 39 per cent before aspirin was detected in the systemic circulation. Furthermore, incubation of simulated peak plasma aspirin concentrations in whole blood in vitro underestimated the inhibition of thromboxane B2 ex vivo after oral administration of 20 or 40 mg of unlabeled aspirin. These data are consistent with presystemic inhibition of platelets by aspirin and suggest that biochemical "selectivity" might be enhanced by slow administration of very low doses of aspirin, thereby optimizing conditions for cumulative, presystemic acetylation of platelet cyclooxygenase and inhibition of thromboxane formation.

Induction of salicyluric acid formation in rheumatoid arthritis patients treated with salicylates

Average steady-state serum salicylate concentrations and salicyluric acid (SU) formation rates were measured in 4 subjects with rheumatoid arthritis. After a salicylate washout period (1 month), the mean observed maximum formation rate of SU was determined by collecting frequent urine samples after a single oral dose of salicylate (35 mg/kg). The patients were then commenced on appropriate high dose salicylate therapy. Two and 5 weeks later, the mean observed maximum rates of SU formation were re-determined along with the average steady-state serum salicylate concentrations. Mean observed maximum SU excretion rates increased significantly between the single-dose study (0.96 +/- 0.22 mg/kg/h) and the last dose of the high dose therapy at 2 weeks (1.65 +/- 0.30 mg/kg/h; p less than 0.01); however, there was no further increase at week 5. Similar increases in the theoretical maximum rate of SU formation (Vmax) were observed between the single-dose study and after 2 weeks of high dose salicylate therapy. Average steady-state serum salicylate concentrations showed no decline between weeks 2 and 5. High dose salicylate therapy leads to acceleration of the rate of SU formation in patients with rheumatoid arthritis and this occurs largely during the first 2 weeks of therapy.

Decreasing serum salicylate concentrations during long-term administration of acetylsalicylic acid in healthy volunteers. Discussion of possible clinical implications

The suitability for multiple dosing of two acetylsalicylic acid (ASA) preparations was compared in 8 healthy volunteers. During this study a marked decrease in serum salicylate concentrations with time was observed in 7 of the subjects. These results are presented here. On day 16 of medications, levels were 60-80% of those on day 6. On day 38, when the study was discontinued, the levels had fallen to 50-65% of the values on day 6. The decrease in salicylate concentrations is probably due to induction of the salicyluric acid formation, one of the saturable pathways of salicylate elimination. Because of a narrow therapeutic range and large interindividual differences in salicylate concentrations with the same dose, determination of serum salicylate levels is an important tool in the adjustment of optimum individual ASA therapy. Our findings further emphasize this fact. They indicate that serum salicylate determinations should preferably be repeated a few times after initiation of ASA therapy.

Sodium salicylate teratogenicity in vitro

Rat embryos were exposed to sodium salicylate in vitro at concentrations comparable to those used to produce terata in vivo. Embryos treated with 400, 600, or 800 micrograms Na salicylate/ml were compared with embryos treated with an equimolar concentration of NaCl. When compared to growth of the controls, a significant dose-dependent decrease in growth parameters was observed. The incidence of abnormal embryos characterized by generalized swelling (particularly in the area of the rhombencephalon) was also dose dependent. This study demonstrates the teratogenicity of salicylate in the rat independent of maternal factors.

Monitoring plasma concentrations of salicylate

Measurement of the plasma concentration of salicylate has been made in the majority of clinical settings where salicylates, mostly in the form of acetylsalicylic acid (aspirin), are used extensively to treat chronic rheumatic conditions. At the same time, studies on the kinetics of salicylates have increased in the specialised literature. Analysis of published papers suggests a surprising difference between principles documented in controlled situations and what happens in routine clinical practice. Recommended dosage regimens are based on a rather narrow amount of data, as relevant studies include few patients and have not used readily comparable protocols. The wealth of information available on the kinetics and metabolism of salicylates has not yet produced generally accepted agreement on how to achieve optimum therapeutic results. Plasma concentration monitoring programmes aim mainly at avoiding side effects and checking compliance, but do not deal specifically with overall evaluation of therapeutic outcome. In the longer term, aspirin may possibly become less favoured as the initial drug of choice for the treatment of chronic rheumatic conditions, and the merits of monitoring plasma salicylate levels could be even less favoured. In the meantime, however, as clinical practice still largely involves use of aspirin, prospective collection of more controlled data on larger population groups seems warranted, not only for the benefit of treated patients but also for the purpose of assessing more thoroughly the clinical relevance of therapeutic drug monitoring in this area.