Stereoselective interaction of phenylbutazone with [12C/13C]warfarin pseudoracemates in man

Pharmacokinetic and pharmacodynamic interactions of aspirin with warfarin in beagle dogs

1. Warfarin and aspirin are widely used in a wide spectrum of thromboembolic and atherothrombotic diseases. Despite the potential efficacy of warfarin-aspirin therapy, the safety and side effect of combined therapy remains unclear. 2. The aim of this study was to investigate the pharmacokinetic and pharmacodynamic interactions between warfarin and aspirin in beagles after single and multiple doses. 3. Coadministration of aspirin had no significant effects on the area under the plasma concentration time curve (AUC(0-t)) and maximum plasma concentration (Cmax) of R- and S-warfarin after a single dose of warfarin, but significantly increase the AUC(0-t) and Cmax and dramatically decrease the clearance (CL) of R- and S-warfarin after multiple dose of warfarin. Accordingly, there was a slight increase in the AUEC(0-t) and Emax of activated partial thromboplastin time (aPTT), prothrombin time (PT) and international normalized ratio (INR) after multiple dose of warfarin. 4. Coadministration of warfarin had no markedly effects on the AUC(0-t) and Cmax of aspirin and its metabolite salicylic acid after single or multiple dose of aspirin. Meanwhile, the AUEC(0-t) and Emax of inhibition of platelet aggregation (IPA) were not significantly affected by warfarin. 5. Our animal study indicated that coadministration of aspirin with warfarin can cause significant pharmacokinetic and pharmacodynamic drug-drug interactions in beagles. However, more studies are urgently needed to assess related information of warfarin-aspirin drug interactions in healthy volunteers or patients.

Oral anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

BACKGROUND

The objective of this article is to summarize the published literature concerning the pharmacokinetics and pharmacodynamics of oral anticoagulant drugs that are currently available for clinical use and other aspects related to their management.

METHODS

We carried out a standard review of published articles focusing on the laboratory and clinical characteristics of the vitamin K antagonists; the direct thrombin inhibitor, dabigatran etexilate; and the direct factor Xa inhibitor, rivaroxaban

RESULTS

The antithrombotic effect of each oral anticoagulant drug, the interactions, and the monitoring of anticoagulation intensity are described in detail and discussed without providing specific recommendations. Moreover, we describe and discuss the clinical applications and optimal dosages of oral anticoagulant therapies, practical issues related to their initiation and monitoring, adverse events such as bleeding and other potential side effects, and available strategies for reversal.

CONCLUSIONS

There is a large amount of evidence on laboratory and clinical characteristics of vitamin K antagonists. A growing body of evidence is becoming available on the first new oral anticoagulant drugs available for clinical use, dabigatran and rivaroxaban.

Human serum albumin: from bench to bedside

Human serum albumin (HSA), the most abundant protein in plasma, is a monomeric multi-domain macromolecule, representing the main determinant of plasma oncotic pressure and the main modulator of fluid distribution between body compartments. HSA displays an extraordinary ligand binding capacity, providing a depot and carrier for many endogenous and exogenous compounds. Indeed, HSA represents the main carrier for fatty acids, affects pharmacokinetics of many drugs, provides the metabolic modification of some ligands, renders potential toxins harmless, accounts for most of the anti-oxidant capacity of human plasma, and displays (pseudo-)enzymatic properties. HSA is a valuable biomarker of many diseases, including cancer, rheumatoid arthritis, ischemia, post-menopausal obesity, severe acute graft-versus-host disease, and diseases that need monitoring of the glycemic control. Moreover, HSA is widely used clinically to treat several diseases, including hypovolemia, shock, burns, surgical blood loss, trauma, hemorrhage, cardiopulmonary bypass, acute respiratory distress syndrome, hemodialysis, acute liver failure, chronic liver disease, nutrition support, resuscitation, and hypoalbuminemia. Recently, biotechnological applications of HSA, including implantable biomaterials, surgical adhesives and sealants, biochromatography, ligand trapping, and fusion proteins, have been reported. Here, genetic, biochemical, biomedical, and biotechnological aspects of HSA are reviewed.

Effect of 5-fluorouracil on the anticoagulant activity and the pharmacokinetics of warfarin enantiomers in rats

The interaction between the antineoplastic agent 5-fluorouracil (5-FU) and the oral anticoagulant warfarin enantiomers was investigated in rats. An increase in hypoprothrombinaemic response, assessed by means of percent changes of prothrombin complex activity and clotting factor VII activity, to warfarin, was observed following oral administration of 1.5 mg/kg racemic warfarin to rats during a 8-day intraperitoneal dose regimen of 5-FU (13.3 mg/kg daily). 5-FU had no apparent effect on the baseline blood coagulation, the in vitro rat serum protein binding as well as the absorption and distribution of the S- and R-enantiomers of warfarin in rats. Yet treatment with 5-FU produced a significant decrease in the total serum clearance value of S-warfarin in rats. The decreased total clearance was attributed mainly to a significant decrease in the formation rate of the overall oxidative metabolites of the more potent S-enantiomer of warfarin.

Effect of ubidecarenone on warfarin anticoagulation and pharmacokinetics of warfarin enantiomers in rats

Interaction between the antioxidant ubidecarenone and the oral anticoagulant warfarin enantiomers was investigated in rats. The decreased hypoprothrombinemic response, assessed by means of percent changes of prothrombin complex activity and clotting factor VII activity, to warfarin, was observed following oral administration of 1.5 mg/kg racemic warfarin to rats during an 8-day oral regimen (10 mg/kg daily) of ubidecarenone. The antioxidant had no apparent effect on the in vitro rat serum protein binding of warfarin enantiomers. Treatment with ubidecarenone did not affect the absorption and distribution of the S- and R-enantiomers of warfarin, but produced a significant increase in the total serum clearance values of both R- and S-warfarin in rats. This effect was more pronounced with R-warfarin than with S-warfarin. The increased clearance values are attributable to acceleration of certain metabolic pathways and renal excretion of the warfarin enantiomers.

The stereoselective effects of bucolome on the pharmacokinetics and pharmacodynamics of racemic warfarin

The objective of this study was to investigate the stereoselective influence of bucolome on the pharmacokinetics and pharmacodynamics of warfarin in Japanese inpatients with heart disease. Thirty patients were administered a fixed-maintenance dose of warfarin alone once a day for at least 7 days. The other 25 patients were concomitantly administered warfarin and a 300 mg dose of bucolome once a day, and blood samples were collected on days 1, 4, 7, 14, or 21 after administration of bucolome. Serum concentration of warfarin enantiomers was measured by a chiral reversed-phase HPLC-ultraviolet detection method. The PT-INR was used as a measure of the pharmacodynamic effect of warfarin. Coadministration of bucolome and warfarin had no effect on serum (R)-warfarin concentration and significantly increased serum (S)-warfarin concentration compared with warfarin alone. The PT-INR of warfarin alone was significantly lower with bucolome cotreatment. These results indicate that the augmented anticoagulant effect of warfarin by bucolome is due to inhibition of (S)-warfarin metabolism in vivo. When bucolome is added to a stabilized regimen of warfarin therapy, the dose of warfarin should be reduced by about 30% to 60%, and caution should be exercised during the first 7 days after coadministration of bucolome.

The effect of multiple doses of donepezil HCl on the pharmacokinetic and pharmacodynamic profile of warfarin

AIM

The aim of the study was to examine the pharmacokinetic and pharmacodynamic profiles of single doses of warfarin (25 mg) following administration alone, and in combination with multiple doses of donepezil HCl (10 mg day(-1)) in healthy volunteers.

METHODS

This was an open-label, two-period crossover study in 12 healthy male volunteers, aged 18-55 years, who were randomized to one of the following treatment groups: (A) donepezil administered for 19 consecutive days with a single dose of warfarin administered on day 14. On day 20, there was a 21-day washout period after which a single dose of warfarin was again administered, and (B) an initial 13-day period with no medication, then a single dose of warfarin administered alone on day 14, followed by a 14-day washout period. Donepezil was then administered for 19 days (to day 47), with an additional single dose of warfarin administered on day 41. Serial blood samples were collected over 144 h following both warfarin administrations in each treatment group. Pharmacokinetic parameters were assessed for both (R)- and (S)-warfarin concentrations in plasma, and pharmacodynamic analyses utilizing prothrombin time were undertaken. Warfarin concentrations in plasma were determined by HPLC with fluorescence detection. The pharmacokinetic parameters Cmax, AUC(0-infinity), CL(T)/F, Vlambdaz/F and t1/2 of both (R)- and (S)-warfarin, maximum prothrombin time (Rmax) and the area under the prothrombin-time curve (AUC(PT)), were compared in the presence and absence of donepezil by analysis of variance (ANOVA).

RESULTS

No statistically significant differences in (R)- or (S)-warfarin pharmacokinetics were observed when warfarin administered alone was compared to warfarin administered concurrently with donepezil. Warfarin pharmacodynamic parameters, Rmax and AUC(PT), were also unchanged by concomitant administration ofdonepezil. No clinically significant changes in vital signs, ECG parameters or clinical laboratory tests were observed.

CONCLUSIONS

Concurrent administration of donepezil HCl does not alter the pharmacokinetic or pharmacodynamic profile of single doses of warfarin in healthy volunteers. These findings suggest that donepezil may be safely co-administered with warfarin without the need for dose modification.

Lack of interaction between meloxicam and warfarin in healthy volunteers

OBJECTIVE

The effect of multiple oral doses of meloxicam 15 mg on the pharmacodynamics and pharmacokinetics of warfarin was investigated in healthy male volunteers. Warfarin was administered in an individualized dose to achieve a stable reduction in prothrombin times calculated as International Normalized Ratio (INR) values. Then INR- and a drug concentration-time profile was determined. For the interaction phase, meloxicam was added for 7 days and then INR measurements and the warfarin drug profiles were repeated for comparison. Overall, warfarin treatment lasted for 30 days.

RESULTS

Warfarin and meloxicam were well tolerated by healthy volunteers in this study. Thirteen healthy volunteers with stable INR values entered the interaction phase. Prothrombin times, expressed as mean INR values, were not significantly altered by concomitant meloxicam treatment, being 1.20 for warfarin alone and 1.27 for warfarin with meloxicam cotreatment. R- and S-warfarin pharmacokinetics were similar for both treatments. Geometric mean (% gCV) AUCss values for the more potent S-enantiomer were 5.07 mg.h.l-1 (27.5%) for warfarin alone and 5.64 mg.h.l-1 (28.1%) during the interaction phase. Respective AUCss values for R-warfarin were 7.31 mg.h.l-1 (43.8%) and 7.58 mg.h.l-1 (39.1%).

CONCLUSION

The concomitant administration of the new non-steroidal anti-inflammatory drug (NSAID) meloxicam affected neither the pharmacodynamics nor the pharmacokinetics of a titrated warfarin dose. A combination of both drugs should nevertheless be avoided and, if necessary, INR monitoring is considered mandatory.

Highly sensitive and specific high-performance liquid chromatographic analysis of 7-hydroxywarfarin, a marker for human cytochrome P-4502C9 activity

The formation of 7-hydroxywarfarin in incubations of (S)-warfarin with human liver microsomes reflects their cytochrome P-4502C9 activity. This paper describes a rapid high-performance liquid chromatographic method for the determination of 7-hydroxywarfarin with high sensitivity, selectivity, and a simple sample clean-up procedure. Separation was achieved with a C18 reversed-phase column and quantification by fluorometric detection. The method employs an internal standard resulting in good accuracy and precision. The limit of detection is 150 fmol for 7-hydroxywarfarin.

Nonsteroidal antiinflammatory drug use in patients receiving warfarin: emphasis on nabumetone

Phenylbutazone has been clearly demonstrated to interact pharmacokinetically and clinically with warfarin, although several other nonsteroidal antiinflammatory drugs (NSAIDs) also have the potential to interact with warfarin to cause alterations in prothrombin time. Aspirin is known to inhibit platelet aggregation irreversibly, whereas nonaspirin NSAIDs are thought to inhibit platelet aggregation reversibly. In contrast, nabumetone was not shown to cause significant inhibition of platelet aggregation, which may be related to the fact that nabumetone preferentially inhibits the prostaglandin synthase-2 isozyme instead of the prostaglandin synthase-1 isozyme. Furthermore, in studies in patients and normal volunteers stabilized on warfarin, nabumetone did not cause alterations in the prothrombin time or international normalized ratio. Based on data evaluating the concomitant use of nabumetone and warfarin, the relative lack of platelet inhibition, and the relatively lower risk of nabumetone-induced gastrointestinal mucosal damage as assessed by radiolabeled chromium-51 fecal blood loss studies and endoscopic evaluations, nabumetone may be preferred if concomitant therapy with warfarin is indicated.

Stereoisomers and drug toxicity. The value of single stereoisomer therapy

All drugs produce adverse effects, though the risk varies widely between different compounds. Many toxic reactions are an extension of the mechanism responsible for the therapeutic effect and can be avoided by careful dose adjustment. Other adverse events are not related to the beneficial action of the drug. Recent interest has focused on the role of the different properties of individual drug enantiomers in causing drug toxicity. For drugs with a single chiral centre, both enantiomers may be therapeutically active. However, if the main therapeutic benefit is in only 1 enantiomer, several possibilities exist for the other enantiomer--inactive, a qualitatively different effect, an antagonistic effect or greater toxicity.

Adverse drug interactions with nonsteroidal anti-inflammatory drugs (NSAIDs). Recognition, management and avoidance

The prevalence and incidence of adverse drug interactions involving nonsteroidal anti-inflammatory drugs (NSAIDs) remains unknown. To identify those proposed drug interactions of greatest clinical significance, it is appropriate to focus on interactions between commonly used and/or commonly coprescribed drugs, interactions for which there are numerous well documented case reports in reputable journals, interactions validated by well designed in vivo human studies and those affecting high-risk drugs and/or high-risk patients. While most interactions between NSAIDs and other drugs are pharmacokinetic, NSAID-related pharmacodynamic interactions may be considerably more important in the clinical context, and prescriber ignorance is likely to be a major determinant of many adverse drug interactions. Prescribing NSAIDs is relatively contraindicated for patients on oral anticoagulants due to the risk of haemorrhage, and for patients taking high-dose methotrexate due to the dangers of bone marrow toxicity, renal failure and hepatic dysfunction. Combination NSAID therapy cannot be justified as toxicity may be increased without any improvement in efficacy. Where lithium or anti-hypertensives are coprescribed with NSAIDs, close monitoring is mandatory for lithium toxicity and hypertension, respectively, and aspirin (acetylsalicylic acid) or sulindac are preferred. Phenytoin or oral hypoglycaemic agents may be administered with NSAIDs other than pyrazoles and salicylates provided that patients are monitored carefully at the initiation and cessation of NSAID treatment. Digoxin, aminoglycosides and probenecid may be coprescribed with NSAIDs, but close monitoring is required, particularly for high-risk patients such as the elderly. Indomethacin and triamterene should be avoided due to the risk of renal failure. High dose aspirin should be replaced by naproxen in patients on valproic acid (sodium valproate) and care is required when corticosteroids are administered to patients taking salicylates long term in high dosage. Interactions between NSAIDs and antacids or cholestyramine are generally avoidable. Adverse drug interactions involving NSAIDs may be limited by rational prescribing and by careful monitoring, particularly for high-risk patients, drugs and therapy periods.

Warfarin metabolites: stereochemical aspects of protein binding and displacement by phenylbutazone

The in vitro human serum albumin binding characteristics of the enantiomers of the major metabolites of warfarin [6-hydroxywarfarin (6-HW), 7-hydroxywarfarin (7-HW), (S)-warfarin alcohols [(S,S)- and (S,R)-WA], and (R,S)-warfarin alcohol [(R,S)-WA]] have been studied, using a stereospecific HPLC assay. Warfarin metabolites are less bound both within plasma and a 40 g/liter solution of human serum albumin than the enantiomers of warfarin. The reduced warfarin metabolites have a lower fraction unbound [1.33% for (S,R)-WA, 2.09% for (S,S)-WA, and 1.04% for (R,S)-WA] than hydroxylated metabolites [3.24% for (R)-6-HW, 4.26% (S)-6-HW, 4.49% for (R)-7-HW and 4.27% for (S)-7-HW] to HSA. Phenylbutazone produced a concentration-dependent increase in the unbound fraction of all metabolites. It was possible to predict the unbound fraction of warfarin metabolites based on the unbound fraction of warfarin enantiomers.

Lack of interaction of ketoprofen with warfarin

We gave ketoprofen (100 mg bid) for 7 days, on a placebo-controlled, double-blind basis, to 15 healthy male volunteers already stabilized on warfarin in dosages which lowered the prothrombin time by about 60%. Ketoprofen did not affect the prothrombin time, there was no change in coagulation cascade parameters, and there was no clinical evidence of bleeding. We conclude that ketoprofen in this dosage has no significance effect on the anticoagulant effect of warfarin.

Chirality in antirheumatic drugs

Use of chiral molecules in clinical practice may cause problems because different chiral forms of a drug (enantiomers) may have different biological activities--yet clinicians have little awareness of these risks. After discussion of the chemical conventions used to describe chirality, examples of the influence of chirality on the efficacy and toxicity of antirheumatic drugs are given. It is recommended that single enantiomers should be used in biological experiments and clinical trials.

Chiral stereoisomeric molecules in the treatment of arthritis

Pharmacokinetic and pharmacodynamic properties of drugs and their ultimate therapeutic effects are often significantly influenced by interactions between the geometry of host receptors, host enzymes, and the three-dimensional structure of drugs. Drug molecules that are mirror images of each other are chiral stereoisomers, and such chiral isomer compounds are commonly used as therapeutic agents by rheumatologists either as racemates (mixtures of chiral isomers) or as pure stereoisomers. Understanding and using such stereoisomeric drugs may lead to lower risks of drug toxicity, better therapeutic indices, and newer approaches for the treatment of articular disorders. A review of the properties of these special isomers is presented, and their therapeutic advantages are discussed.

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.

Matching the Drug to the Patient: The rational use of antiarthritic drugs

Antirheumatic drugs now available are often effective in helping control the more serious rheumatic disorders, but are not curative. All have potentially serious side effects and need to be used with caution in the presence of recognized risk factors. Treatment should be individualized for the diagnosis and severity of the underlying disease, which must be of sufficient magnitude to justify the risk of potential adverse reactions from the prescribed medication.

Pharmacokinetic drug interactions with nonsteroidal anti-inflammatory drugs

Nonsteroidal anti-inflammatory drugs (NSAIDs) are among the most widely used drugs. Drug interactions with this class of compounds are frequently reported and can be pharmacokinetic and/or pharmacodynamic in nature. The pharmacokinetic interactions can be divided into 3 classes: (1) drugs affecting the pharmacokinetics of an NSAID. (2) an NSAID interfering with the pharmacokinetics of another NSAID and (3) NSAIDs altering the pharmacokinetics of another drug. Although the pharmacokinetics of some NSAIDs may be significantly affected by the concurrent administration of certain other drugs (including other NSAIDs), this type of interaction only occasionally leads to serious complications. Concurrent administration of antacids or sucralfate may delay the rate of oral absorption of NSAIDs but generally has little effect on the extent. Use of antacids increases urinary pH, leading to increased renal excretion of unchanged salicylic acid and decreased plasma concentrations of this antirheumatic agent. The H2-receptor blocking agent cimetidine inhibits the oxidative metabolism of many concurrently administered drugs, including certain NSAIDs. Probenecid inhibits the renal secretion of drug glucuronides and this will lead to accumulation in plasma of those NSAIDs eliminated primarily by the formation of labile acyl glucuronides such as naproxen, ketoprofen, indomethacin, carprofen. Cholestyramine decreases the oral absorption of many concurrently administered drugs, including NSAIDs. It may also decrease plasma concentrations of those NSAIDs undergoing enterohepatic circulation (e.g. piroxicam, tenoxicam) by interrupting the enterohepatic cycle. Corticosteroids stimulate the clearance of salicylic acid, leading to low plasma salicylate concentrations. Plasma concentrations of many NSAIDs are significantly reduced when the NSAID is coadministered with aspirin. The clinical relevance of most of these interactions is not well established. However, in those cases where the interaction results in elevated plasma concentrations of the NSAID, special caution should be exercised to avoid excessive accumulation of the NSAID especially in elderly and/or very sick patients who may be more sensitive to the more serious gastroduodenal and renal side-effects of these agents. By virtue of their pharmacokinetic and pharmacodynamic properties, NSAIDs may significantly affect the disposition kinetics of a number of other drugs. They can displace other drugs from their plasma protein binding sites, inhibit their metabolism or interfere with their renal excretion. If the affected drug has a narrow therapeutic index, the interaction may be clinically significant. The pyrazole NSAIDs (phenylbutazone, oxyphenbutazone, azapropazone) inhibit the metabolism of many drugs such as the coumarin anticoagulants, oral antidiabetics and anticonvulsants such as phenytoin. Salicylates displace oral anticoagulants from their plasma protein binding sites.(ABSTRACT TRUNCATED AT 400 WORDS)

Clinical pharmacokinetic considerations in the elderly. An update

There are numerous studies of drug handling in the elderly, but it is difficult to assess the significance of changes seen in vitro, or after single-dose administration, because they are often compensated by other mechanisms at steady-state. However, a knowledge of these studies is important as the results alert the investigator to possible treatment problems. The high incidence of adverse drug reaction in the elderly population leaves no doubt that improvements in therapy are needed. Research has been directed at seeking patterns of abnormality in the elderly on which to base recommendations for alterations in dosage regimens. The major shortcoming of this approach has been the failure to distinguish between the effect of chronological age on drug pharmacokinetics, and drug kinetics in elderly people with multiple pathology. The latter concern appreciates the variety of factors involved and the importance of treating each patient as an individual: presentation of mean data is confusing and misleading. The objective of drug treatment in any age group, but particularly in the elderly, is to administer the smallest possible dose which gives adequate therapeutic benefit throughout the entire dosage interval with the minimum of side effects. For most drugs the safe starting dose in the elderly is one-third to half that recommended in the young. Vigilance for potential side effects with plasma concentration monitoring, if available, should help keep toxicity to a minimum. When other medications are added or changed, the possibility of interaction should be anticipated. Methods for individualisation of dosage regimens and the use of sustained-release formulations in the elderly are discussed. Dosage alteration in the elderly in terms of reduced dose frequency, rather than dose size, may help improve compliance. A knowledge of the pharmacokinetics of a drug helps determine which approach will be most beneficial.

Stereoselective disposition and pharmacologic activity of propafenone enantiomers

Propafenone is an antiarrhythmic drug that produces a variable degree of beta-blockade in humans and is administered as a racemate. To examine the relative contribution of the individual enantiomers to pharmacologic effects seen during treatment with propafenone, we assessed the steady-state plasma concentrations of (+)-S-propafenone and (-)-R-propafenone in seven patients who were on long-term oral therapy, and we evaluated the electrophysiologic and beta-blocking properties of both enantiomers in vitro. The metabolism of propafenone is known to be polymorphic and to cosegregate with that of debrisoquine-4-hydroxylation. Among five patients with the extensive metabolizer phenotype (EM), the ratio of the area under the plasma concentration-time curve of (+)-S-propafenone to (-)-R-propafenone was 1.73 +/- 0.15 (mean +/- SD). In the other two patients, who had the poor metabolizer phenotype (PM), the concentrations of both enantiomers were elevated but the S/R ratios were similar to those seen in patients with EM. In canine cardiac Purkinje fibers, both enantiomers produced similar frequency-dependent depression of maximum upstroke of phase 0. In contrast, the affinity of the human lymphocyte beta 2-adrenoceptor was approximately 100-fold greater for (+)-S-propafenone (Ki, 7.2 +/- 2.9 nM) than for the (-)-R-enantiomer (Ki, 571 +/- 141 nM). We conclude that during long-term oral therapy, propafenone undergoes stereoselective disposition in patients with either EM or PM. beta-Blockade during propafenone therapy is likely related to accumulation of (+)-S-propafenone.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of nonsteroidal anti-inflammatory agents and their effect on binding of racemic warfarin and its enantiomers to human serum albumin

The binding of racemic warfarin, its enantiomers, and several nonsteroidal anti-inflammatory agents to human serum albumin was investigated by equilibrium dialysis at 4 degrees C in pH 7.4 phosphate buffer. The primary binding constant for the S(-) enantiomer of warfarin was approximately two times greater than the corresponding binding of the R(+) enantiomer. The effect of azapropazone, phenylbutazone, naproxen, ibuprofen, mefenamic acid, and tolmetin on the binding of racemic warfarin and its enantiomers was studied. Warfarin was displaced by all of the nonsteroidal anti-inflammatory agents except tolmetin. Azapropazone caused the largest displacement of warfarin (39 to 46% free warfarin versus 2.5 to 6% free warfarin without competing drug), followed by phenylbutazone (23 to 43% free warfarin), naproxen (9 to 24% free warfarin), mefenamic acid (5 to 11.5% free warfarin), and ibuprofen (5 to 9% free warfarin). Azapropazone and phenylbutazone competed with warfarin for the same primary binding site on the albumin molecule. Naproxen appeared to affect warfarin binding at both primary and secondary sites. Ibuprofen and mefenamic acid interfered with the binding of warfarin at its secondary sites. In contrast to the other drugs studied, tolmetin caused an increase in the primary binding constant of warfarin. Structural analysis indicated that a common feature of those compounds which primarily bind at the warfarin site is a hydrophobic area bearing a widely delocalized negative charge.

Interactions of non-steroidal anti-inflammatory drugs

As NSAIDs are commonly used in patients receiving concomitant drug therapy, there is a risk of clinically significant drug interactions. Important interactions with NSAIDs involve one or both of two major mechanisms: pharmacokinetic (e.g. lithium, phenytoin and barbiturates) and pharmacodynamic (e.g. antihypertensive agents, diuretics). Prescription of a NSAID should be preceded by a careful evaluation of any coexisting pathology (such as renal dysfunction or hypertension) or concurrent drug therapy (such as anticonvulsant or anticoagulant agents) which may predispose a patient to the development of an interaction with potentially severe effects.

Direct separation of drug enantiomers by high-performance liquid chromatography with chiral stationary phases

The development of chiral stationary phases for HPLC has resulted in renewed interest in methods for the separation of drug enantiomers. This paper provides a brief overview of some of the more recent approaches to the direct resolution of drug enantiomers by HPLC with particular emphasis on their quantification in biological fluids.

Pharmacokinetic and pharmacodynamic consequences of stereoselective drug metabolism in man

The examples discussed demonstrate the importance of stereoselective drug metabolism and raise the question of whether the therapeutic use of racemic drugs is still justified. There is no straightforward answer to this question. If only quantitative differences in therapeutic activity exist and the less active enantiomer is not predominantly responsible for side effects, the therapeutic benefit gained by using the more active enantiomer is only marginal and does not justify the substantial increase in costs involved in manufacturing such a drug preparation. However, if stereoselectivity in therapeutic activity is pronounced and adverse drug reactions are caused mainly by the less active isomer then an isomeric pure drug preparation should be used.

Substrate and product stereoselectivity in monooxygenase-mediated drug activation and inactivation

In this overview, stereoselective aspects of drug metabolism have been examined in a biochemical and pharmacodynamic perspective. From the facts and concepts presented, the conclusion to emerge is that the pharmacokinetic behaviour of mixtures of stereoisomers (e.g. racemates) is not always the simple addition of the behaviour of individual stereoisomers; as a consequence, stereoisomeric mixtures might display pharmacodynamic effects differing somewhat from those caused by the separate eutomers and distomers. In some circles, the notion of "isomeric ballast" is being mentioned with increasing regularity, leading almost fatally to the conclusion that eutomers should be purified from their distomeric ballast for therapeutic use. A number of examples discussed here show that in vitro and also in vivo, a racemate often displays a pharmacokinetic and pharmacodynamic behaviour which is not the mere addition of the behaviour of its separate enantiomers. This may seem as an additional argument for the therapeutic use of pure eutomers since a number of interactions are thus avoided. But does this imply that distomers must always be considered as detrimental ballast? Enforcing compulsory resolution of stereoisomeric mixtures, in particular racemates, would increase severalfold the cost of many drugs. This is a small price to pay if the benefit is an improved therapeutic index. But, to reword the question, would such a legislation automatically result in therapeutic benefits?

Protein binding as a primary determinant of the clinical pharmacokinetic properties of non-steroidal anti-inflammatory drugs

The ability of a wide variety of anionic, cationic, and neutral drugs to bind in a reversible manner to plasma proteins has long been recognised. Non-steroidal anti-inflammatory drugs (NSAIDs) are distinguished as a class by the high degree to which they bind to plasma protein. Plasma protein binding properties are primary determinants of the pharmacokinetic properties of the NSAIDs. Theoretical relationships are reviewed in order to define quantitatively the impact of plasma protein binding on clearance, half-life, apparent volume of distribution, and the duration and intensity of pharmacological effect. The quantitative relationships governing competitive displacement binding interactions are also presented. Experimental methods for in vitro and in vivo determination of the degree of plasma protein binding are discussed. The more common in vitro methods are equilibrium dialysis and ultrafiltration. Methods for characterising the degree of plasma protein binding in vivo consist of either measuring the concentration of drug at equilibrium in an implanted semipermeable vessel or measuring the relative drug concentrations in two body spaces with different protein content. Emphasis is given to the comparative advantages and disadvantages of experimental application of the various in vitro and in vivo methods. Plasma protein binding is discussed as a determinant of the trans-synovial transport of NSAIDs. Trans-synovial transport of NSAIDs appears to be a diffusional process. Limited data in humans receiving ibuprofen, indomethacin, aspirin, carprofen, alclofenac, or diclofenac suggest that clearance of each of these NSAIDs from the synovium is slower than clearance from plasma. The clinical data relevant to the relationship between plasma NSAID concentration and various measures of anti-inflammatory effect are reviewed. A positive correlation between plasma NSAID concentration and anti-inflammatory effect has been observed in only one study on naproxen and one study on piroxicam. In several other studies, the lack of concentration-response correlations is generally attributed to the relatively subjective, quantitatively inexact methods used to assess anti-inflammatory effect and analgesia in arthritic patients, as well as the substantial interpatient variabilities in the fraction of unbound NSAID and the unbound plasma NSAID concentration. In view of the generally poor correlation between concentration and therapeutic response, routine therapeutic monitoring of total plasma NSAID concentration is not recommended as a means of titrating individual dosages to the desired effect in each patient.(ABSTRACT TRUNCATED AT 400 WORDS)

Drug interactions in surgical patients

Drug interactions, defined as when the administration of a single substance (drug, nutrient, or tobacco) modifies the response to a drug, occur relatively frequently in surgical patients and may result in increased morbidity and lengthened hospital stay. Drug interactions also account for some instances of drug ineffectiveness or exaggerated pharmacologic response. There are many types of drug interactions. However, most of them are related to altered drug pharmacokinetic properties, where there are alterations in drug absorption, distribution, metabolism, or elimination; or altered drug pharmacodynamic actions, where two agents may have synergistic, additive, or antagonistic pharmacologic effects. The term, drug interaction, usually refers to pairs of drug substances administered concurrently, but more than two agents may be involved. When patients are taking a large number of different medications, there may be multiple drug interactions with additive or antagonistic effects, the overall effects of which are difficult to predict. There are hundreds of reported drug interactions, and some may be of important clinical consequence. In surgical patients, the majority of drug interactions involve histamine-2 blockers (particularly cimetidine), digoxin, warfarin, or a variety of agents that may be administered during anesthesia. Recognition of the potential for adverse drug interactions is of primary importance in minimizing their effects. Usually, potentially interacting drugs may be administered concurrently as long as appropriate patient or laboratory assessments are performed. For some agents, such as digoxin or theophylline, serum drug concentrations may aid in the avoidance of adverse drug interactions.

Clinical pharmacokinetics and pharmacodynamics of warfarin. Understanding the dose-effect relationship

The simplest complete system accounting for the time-course of changes in the prothrombin time induced by warfarin requires the combination of 4 independent models: A pharmacokinetic model for the absorption, distribution, and elimination of warfarin. Warfarin is essentially completely absorbed, reaching a maximum plasma concentration between 2 and 6 hours. It distributes into a small volume of distribution (10 L/70kg) and is eliminated by hepatic metabolism with a very small clearance (0.2 L/h/70kg). The elimination half-life is about 35 hours. A pharmacodynamic model for the effect of warfarin on the synthesis of clotting factors (prothrombin complex). Prothrombin complex synthesis is inhibited 50% at a warfarin concentration of about 1.5 mg/L. Warfarin concentrations associated with therapeutic anticoagulation are of similar magnitude. A physiological model for the synthesis and degradation of the prothrombin complex. The synthesis rate is about 5%/h/70kg and the elimination half-life estimated from changes in prothrombin time is approximately 17 hours. On average it will take 3 days for the anticoagulant effect of warfarin to reach a stable value when warfarin concentrations are constant. A model for the relationship between the activity of prothrombin complex and the prothrombin time. In general there is a hyperbolic relationship between these quantities. Its exact shape depends upon the method used for measuring the prothrombin time. Attempts to integrate these models into a single system have used essentially the same pharmacokinetic, physiological, and prothrombin activity models. Four distinct pharmacodynamic models have been proposed: linear, log-linear, power and Emax. One might be preferred on theoretical grounds (Emax) but its performance is not clearly different from the others. Empirical methods for warfarin dose prediction as well as those based on the combined pharmacokinetic-pharmacodynamic-physiological-prothrombin system have been proposed. Only one (which was also the first) [Sheiner 1969] has been adequately described and compared with the performance of an unaided physician. The programme compared favourably with decisions made by those physicians normally responsible for adjusting warfarin dose, but was not tested prospectively. A sizeable body of theoretical and experimental observations has contributed to our understanding of the warfarin dose-effect relationship. It remains to be demonstrated that any alternative method is superior to the traditional empirical approach to warfarin dose adjustment.

Pharmacokinetic drug interactions

Drugs labeled with stable isotopes have been successfully used in pharmacokinetic drug interaction studies. This technique provides precise information on the mechanisms responsible for drug interactions, e.g., changes in clearance due to induction or inhibition of drug metabolism, bioavailability, and volume of distribution. It offers the advantage that detailed studies on the pharmacokinetics and metabolism of a drug can be conducted in a clinical setting during steady-state conditions, thus avoiding problems in patient management that can arise from studies in which drug is withdrawn.