Potentiation of anticoagulant effect of warfarin by phenylbutazone

Androgen Misuse and Abuse

Androgens are potent drugs requiring prescription for valid medical indications but are misused for invalid, unproven, or off-label reasons as well as being abused without prescription for illicit nonmedical application for performance or image enhancement. Following discovery and first clinical application of testosterone in the 1930s, commercialization of testosterone and synthetic androgens proliferated in the decades after World War II. It remains among the oldest marketed drugs in therapeutic use, yet after 8 decades of clinical use, the sole unequivocal indication for testosterone remains in replacement therapy for pathological hypogonadism, organic disorders of the male reproductive system. Nevertheless, wider claims assert unproven, unsafe, or implausible benefits for testosterone, mostly representing wishful thinking about rejuvenation. Over recent decades, this created an epidemic of testosterone misuse involving prescription as a revitalizing tonic for anti-aging, sexual dysfunction and/or obesity, where efficacy and safety remains unproven and doubtful. Androgen abuse originated during the Cold War as an epidemic of androgen doping among elite athletes for performance enhancement before the 1980s when it crossed over into the general community to become an endemic variant of drug abuse in sufficiently affluent communities that support an illicit drug industry geared to bodybuilding and aiming to create a hypermasculine body physique and image. This review focuses on the misuse of testosterone, defined as prescribing without valid clinical indications, and abuse of testosterone or synthetic androgens (androgen abuse), defined as the illicit use of androgens without prescription or valid indications, typically by athletes, bodybuilders and others for image-oriented, cosmetic, or occupational reasons.

Therapeutic Use of Albumin

The clinical use of albumin solutions is a controversial issue, that involves albumin as a volume plasma expander, a supplement of total parenteral nutrition and a substance with pharmacological properties. The aim of this review is to show the present role of albumin in the clinical setting. We reviewed experimental and clinical data collected by the Medline System and the bibliographies of relevant articles. Experimental studies were selected on the basis of their historical value and applicability (hypothetical use, correct use) to the clinical setting. At present, it is possible to assert that: a) exogenous albumin is not an ideal colloid; b) the effects on plasma volume expansion are not entirely predictable, especially in pathologic states accompanied by leaky capillary membranes; c) albumin supplementation shows no benefit on many kinds of tissue edema; d) the supplementation of albumin has no influence on outcome. It is possible to demonstrate the role of albumin as a substance with unique properties that make it useful, but further experimental and clinical data will be necessary and represent new areas for future exploration. Given the protein's cost, the routine use of albumin does not appear to be justified, until new data indicate otherwise.

Influence of Ibuprofen on Oral Anti-Coagulation with Phenprocoumon

Twenty-four patients on long-term anti-coagulant treatment with phenprocoumon (Marcoumar®) took part in a study designed to investigate the possible interaction between phenprocoumon and ibuprofen (Brufen®). Prior to entry into the study, the anti-coagulant state of each patient was adequately controlled on a dose of phenprocoumon which remained unchanged throughout the study.

Under double-blind conditions, half of the total number of patients received ibuprofen 600 mg a day in three divided doses for two weeks followed by two weeks without therapy; then two weeks of placebo, again followed by two weeks without therapy. The other patients received the drugs in the reverse order. The anti-coagulant state was measured using the Thrombotest, the results being expressed as a percentage of the normal.

The results indicated that the effects of a fixed dose of phenprocoumon, as measured by the Thrombotest, did not alter significantly throughout the study, and no serious side-effects were noted. These findings suggest that in general, ibuprofen can safely be given to patients receiving oral anticoagulants without the risk of interfering with their anti-coagulant state.

Albumin-drug interaction and its clinical implication

BACKGROUND

Human serum albumin acts as a reservoir and transport protein for endogenous (e.g. fatty acids or bilirubin) and exogenous compounds (e.g. drugs or nutrients) in the blood. The binding of a drug to albumin is a major determinant of its pharmacokinetic and pharmacodynamic profile.

SCOPE OF REVIEW

The present review discusses recent findings regarding the nature of drug binding sites, drug-albumin binding in certain diseased states or in the presence of coadministered drugs, and the potential of utilizing albumin-drug interactions in clinical applications.

MAJOR CONCLUSIONS

Drug-albumin interactions appear to predominantly occur at one or two specific binding sites. The nature of these drug binding sites has been fundamentally investigated as to location, size, charge, hydrophobicity or changes that can occur under conditions such as the content of the endogenous substances in question. Such findings can be useful tools for the analysis of drug-drug interactions or protein binding in diseased states. A change in protein binding is not always a problem in terms of drug therapy, but it can be used to enhance the efficacy of therapeutic agents or to enhance the accumulation of radiopharmaceuticals to targets for diagnostic purposes. Furthermore, several extracorporeal dialysis procedures using albumin-containing dialysates have proven to be an effective tool for removing endogenous toxins or overdosed drugs from patients.

GENERAL SIGNIFICANCE

Recent findings related to albumin-drug interactions as described in this review are useful for providing safer and efficient therapies and diagnoses in clinical settings. This article is part of a Special Issue entitled Serum Albumin.

Ultrasound-promoted greener synthesis of novel trifurcate 3-substituted-chroman-2,4-dione derivatives and their drug-likeness evaluation

An efficient and convenient approach for one-pot synthesis of 3-substituted chroman-2,4-diones via a three-component reaction of aromatic aldehydes, 4-hydroxy- coumarins and diverse pyrazolone derivatives was described. The combinatorial synthesis for this methodology was achieved by applying ultrasound irradiation in the absence of activator while making use of water as green solvent. Additionally, novel chroman-2,4-dione derivatives attached to an edaravone moiety represent an exploitable source of brand new anticancer agents. In comparison with conventional methods, experimental simplicity, good functional group tolerance, excellent yields, short routine, and atom efficiency are prominent features of this sonocatalyzed procedure.

Some clinical implications of the protein binding of digoxin

1 The binding of [(3)H]-digoxin to human plasma proteins was studied using both ultrafiltration and frontal analysis, to establish a suitable method for the measurement of digoxin-protein interaction, and to assess the effect of variations in digoxin protein binding on its distribution. 2 Using ultrafiltration, [(3)H]-digoxin was found to bind to plasma proteins to the extent of 30-40% over a concentration range of 0.001-1 ng/ml. Frontal analysis was unsuccessful in the measurement of digoxin protein binding. 3 Dilution experiments using plasma and human serum albumin suggested that variations in digoxin protein binding would be unlikely to cause significant alterations in the levels of free drug in the body. 4 The possible clinical implications of these results are discussed.

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.

Pain control: non-steroidal anti-inflammatory agents

The non-steroidal anti-inflammatory drugs (NSAIDs) and acetaminophen (paracetamol) are the most common analgesic drugs used in neonates and infants despite limited pharmacodynamic data. Both drugs act through inhibition of cyclooxygenase enzymes. Neonatal acetaminophen clearance is reduced in premature neonates (0.7 L h(-1) x 70 kg(-1)) and increases to 5 L h(-1) x 70 kg(-1) at term (40% adult rates); adult rates are reached within the first year of life; NSAID clearance follows similar trends. Volume of distribution is increased in the neonatal period. Dosing of both drug groups is tempered by concerns about toxicity. Acetaminophen hepatotoxicity is less common in neonates than in older children and adults, possibly due to reduced oxidative enzyme activity (e.g. CYP 2E1). Data concerning NSAID adverse effects in the neonatal period are few. Renal function is reduced 20% after NSAID use for patent ductus arteriosus closure in premature neonates and there is no increased frequency of intraventricular haemorrhage. No significant difference in the change in cerebral blood volume, change in cerebral blood flow, or tissue oxygenation index was found between administration of ibuprofen or placebo in neonates. Future studies should define concentration-response relationships for these drugs that are age and pathology specific.

Specific determination of unbound oxacillin in protein solution with cefoperazone by high-performance frontal analysis with chemiluminescence detection

Unbound oxacillin concentrations in human serum albumin (HSA) solutions in the presence or absence of cefoperazone were determined using high-performance frontal analysis coupled with chemiluminescence detection (HPFA-CL). The HPFA was performed on an ISRP column with 67 mM potassium phosphate buffer of pH 7.4 and ionic strength of 0.17 as the mobile phase. The luminol-H(2)O(2)-Co(2+) system was employed in the chemiluminescence detection. The detection was highly specific for oxacillin in the presence of cefoperazone. Although both drugs in HSA solutions co-eluted in the same region in HPFA, cefoperazone did not interfere with the determination of unbound concentration of oxacillin. In the solution of 100 microM HSA and 11.33 micro M oxacillin the bound percentage of oxacillin to HSA was estimated as 80.5%. Addition of 30.98 micro M cefoperazone into the HSA-equilibrated solution produced little effect on the protein binding of oxacillin. In the presence of 154.9 micro M cefoperazone, however, the bound percentage of oxacillin was significantly reduced. This specific method could be applied to the investigation of drug-drug interaction in protein binding.

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 effect of corticosteroids and tamoxifen on suramin protein binding and in vitro activity

Stout and colleagues [Proc Am Assoc Cancer Res 1993, 34, p. 298] previously reported that both hydrocortisone and tamoxifen increased the free fraction of suramin in human plasma. We examined several corticosteroids as well as tamoxifen for their effects on suramin protein binding and also evaluated hydrocortisone for its ability to modulate suramin activity in PC-3 and MCF-7 cells. Greater than 99% of the suramin was protein bound in undiluted human plasma. However, the free fraction of suramin was increased with the reduced plasma protein levels and increased suramin concentrations. At concentrations ranging from 1 to 30 microM, neither tamoxifen, hydrocortisone, prednisone nor dexamethasone had any effect on the binding of suramin to human plasma, regardless of protein concentrations. Similar results were observed with fetal calf serum. Hydrocortisone also had no effect on suramin activity against PC-3 and MCF-7 cell in vitro. We conclude from these studies that neither corticosteroids nor tamoxifen affect suramin protein binding or its cytotoxic activity.

Adverse interactions between warfarin and nonsteroidal antiinflammatory drugs: mechanisms, clinical significance, and avoidance

OBJECTIVE

To review the mechanisms and clinical significance of adverse interactions between warfarin and nonsteroidal anti-inflammatory drugs (NSAIDs) and discuss how these interactions can be avoided.

DATA SOURCES

Previous studies of interactions between warfarin and NSAIDs or reports of adverse interactions were identified from a MEDLINE search (1976 to present) and from the reference lists of pertinent articles.

STUDY SELECTION AND DATA EXTRACTION

All articles were considered for inclusion in the review. Pertinent information was selected for discussion.

DATA SYNTHESIS

All NSAIDs can prolong bleeding time by inhibiting platelet function. High-dose aspirin has a direct hypoprothrombinemic effect. Phenylbutazone and its analogs enhance the hypoprothrombinemic effect of warfarin through a pharmacokinetic interaction by inhibiting the hepatic metabolism of warfarin. Mefenamic acid also enhances the anticoagulant effect of warfarin, but the mechanism is not known. The clinical relevance of protein binding displacement in the interaction between warfarin and NSAIDs has been overstated, although a significant one may be more likely in the presence of high concentrations of NSAIDs in patients with slow elimination of warfarin (e.g., those with severe heart failure or impaired liver function). NSAIDs can induce gastrointestinal bleeding, which is likely to be more severe if warfarin is also given.

CONCLUSIONS

The combined use of warfarin and NSAIDs is generally discouraged because of the increased risk of bleeding in these patients. In patients receiving warfarin who also require NSAIDs, phenylbutazone and its analogs, high-dose aspirin, mefenamic acid, excessive use of topical methyl salicylate, and NSAIDs that are associated with a higher risk of bleeding peptic ulcers should be avoided. Patients should be closely monitored for anticoagulant control and bleeding complications during the combined use of warfarin and NSAIDs.

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.

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.

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.

Stereoselectivity: an issue of significant importance in clinical pharmacology

Many drugs have one or more asymmetric centers and are administered as a racemate containing an equal mixture of enantiomers with different pharmacologic properties, routes, and rates of disposition in humans. For example, S-warfarin is more potent than R-warfarin, and is metabolized by different pathways. The S-enantiomer is primarily oxidized, and the R-enantiomer is metabolized by both oxidation and reduction. Nevertheless, because of the difficulty in separating and analyzing individual enantiomers, most pharmacokinetic and pharmacodynamic studies on drugs have been performed without considering the stereochemical factors. This is unfortunate, because a nonstereoselective approach to the study of chiral drugs precludes insight into potential valuable information that may be relevant to drug development and evaluation. On the other hand, when the pharmacologic properties (including activity, disposition, and interaction with the other enantiomer) of enantiomers have been defined, manipulation of the enantiomeric ratio or use of the pure enantiomer can be pursued to optimize therapeutic efficacy.

Warfarin: metabolism and mode of action

The various stages involved in the transport, pharmacological action and elimination of warfarin involve the specific binding of warfarin to a chiral macromolecular complex. However, it seems that the degree of stereoselectivity is variable, which presumably reflects the importance of the side-chain in binding to each type of macromolecule. It would appear that there is greater stereoselective control in the interaction of warfarin with cytochrome P-450 enzymes than that observed for interaction with the receptor, vitamin K1 epoxide reductase. Indeed, warfarin has been developed as a powerful stereochemical probe for in vitro studies of the terminal enzyme in the mixed-function oxidase system, cytochrome P-450. Warfarin undergoes hydroxylation in the 6, 7 and 8-positions of the aromatic ring which must interact with the active (haemoprotein) portion of the molecule, leaving the side-chain, which contains the chiral centre, free for recognition by the substrate binding site. In vitro studies indicate that the interaction of warfarin at its receptor, vitamin K1 epoxide reductase, is completely non-stereoselective. This suggests that only the 4-hydroxycoumarin ring portion of the drug binds to the enzyme. Consistent with this hypothesis, salicylate, which can mimic part of the 4-hydroxycoumarin ring system, produces hypothrombinaemia by inhibition of vitamin K1 epoxide reductase. These findings suggest that the coumarin ring system is largely responsible for the pharmacodynamic properties of warfarin, whereas the side-chain dictates the disposition and metabolism of the drug.

Pharmacokinetics and toxicity of high-dose human alpha 1-acid glycoprotein infusion in the rat

The pharmacokinetics of high-dose human alpha 1-acid glycoprotein (AAG) was studied in rats to determine the feasibility of using AAG to alter the tissue distribution of basic drugs. alpha 1-Acid glycoprotein (2.2 g/kg) was administered iv to six male Holtzman rats over a period of 30 min, and serum AAG concentrations were measured by a specific radial immunodiffusion assay. The AAG concentrations were computer fit to a biexponential equation to generate pharmacokinetic constants for an open two-compartment model. The peak serum AAG concentration was 1830 +/- 180 mg/dL at the end of infusion; greater than 20 times the normal value for rats. The central volume of distribution and steady state volume of distribution were 0.09 +/- .02 and 0.15 +/- 0.02 L/kg, respectively. Total body clearance of AAG was 0.065 +/- 0.005 L/kg/h, and the terminal elimination half-life was 19.3 +/- 1.5 h. The AAG administration was tolerated without adverse effect and did not alter systolic blood pressure, the electrocardiogram, creatinine clearance, weight gain, or survival. The results of the histologic examination of various tissues by light microscopy at 30 d post AAG treatment were normal. These data demonstrate that high doses of human AAG can be safely administered to rats and that they produce supraphysiologic serum AAG concentrations.

Warfarin binding to plasma albumin, measured in patients and related to fatty acid concentrations

A method for determination of reserve albumin equivalent for binding of warfarin as previously described [1] has been used for assessing the influence of non-esterified fatty acid concentration (NEFA) on binding of warfarin to human serum albumin (HSA). Reserve albumin concentration can be used for calculation of the expected fraction of bound warfarin in serum. It is shown in vitro that binding of warfarin increases with added oleate up to 4 mol of oleate per mol of albumin and then decreases. Twenty-four patients on permanent warfarin treatment showed no correlation of serum albumin and reserve albumin concentrations (r = 0.10, P greater than 0.50) indicating that warfarin binding is governed by other factors. However, in the same patients there was a significant correlation between reserve albumin concentration and NEFA/HSA (r = 0.54, P less than 0.01). In one human volunteer changes of NEFA were provoked by strenuous work and it was found that reserve albumin concentration increased with NEFA concentration as expected from the in vitro findings (r = 0.90, P less than 0.001). Five uraemic patients on permanent warfarin treatment showed increasing reserve albumin concentration with increasing NEFA concentration induced by heparin. These findings indicate that, both in vitro and in vivo, the reserve albumin concentration for binding of warfarin and hence the free warfarin concentration is markedly influenced by NEFA concentration. This may add to the understanding of warfarin dose requirement during anticoagulant therapy.

Drug-drug and drug-disease interactions with nonsteroidal anti-inflammatory drugs

Nonsteroidal anti-inflammatory drugs may cause a number of drug interactions. They can displace other drugs from serum proteins, and some can affect the metabolism or decrease the renal elimination of other drugs. In addition, they can attenuate the pharmacologic effect of other drugs, such as diuretic and antihypertensive agents, without affecting their disposition. Lastly, many disease states and aging can affect the handling of nonsteroidal anti-inflammatory drugs, mandating dose adjustment of some of these agents in certain clinical conditions. Some drugs may require more of these adjustments than others.

Drug-binding properties of human alpha-foetoprotein

The drug-binding properties of human alpha-foetoprotein (alpha FP) were investigated by a fluorescence-spectral method. Human alpha FP was shown to bind to albumin's site I marker (warfarin, phenylbutazone), site II marker (L-tryptophan), but not site III marker (cholic acid, digoxin). The binding of human alpha FP towards lower alcohols was examined, and this binding seems to depend partly on the hydrophobicity of the ligands. The binding of human alpha FP is discussed in comparison with human serum albumin or rat alpha FP.

Dose-dependent pharmacokinetics and hepatobiliary transport of bromophenol blue in the beagle

The pharmacokinetic profile of bromophenol blue (I) in the plasma, urine, and bile of beagle dogs was determined after intravenous administration of 5-, 20-, and 30-mg/kg doses. In addition, two competitors, probenecid and phenylbutazone, were interacted with I in vivo and with I and rat liver cytoplasmic protein fractions Y and Z in vitro as a means of elucidating the mechanism of intrahepatic transport of I. Compound I was determined spectrophotometrically at 587 nm. In plasma, I displayed apparent first-order dose-dependent kinetics. The percentage of I bound to plasma proteins was approximately 92.5% over the dose range studied. Consecutive injections of equal doses of I produced statistically different terminal half-lives (p less than 0.05), suggesting the possibility of a saturable uptake process. In the presence of each competitor, the disposition of I was altered significantly (p less than 0.05): phenylbutazone displaced I from plasma protein, while probenecid decreased the binding of I to liver proteins in the Z-fraction. The Z-fraction bound a larger amount of I than the Y-fraction, suggesting a larger binding capacity. Under no circumstances was the binding of I to the Y-fraction altered. Cumulative biliary excretion data showed that the elimination of I in bile accounted for 92-99% of the dose delivered. The biliary excretion sigma- plots displayed no dose dependency, suggesting that the dose-dependent plasma half-life is due to a dose-dependent liver uptake (as opposed to elimination) process.

Study of binding of warfarin to serum albumins by high-performance liquid chromatography

The binding of warfarin to human serum albumin and bovine serum albumin, respectively, was studied by high-performance liquid chromatography (HPLC). Based upon the Hummel - Dreyer method, two techniques were developed: the internal calibration and the external calibration. The results obtained by the HPLC method and those obtained by the classical method (equilibrium dialysis) were compared. The external calibration method seems to be superior to others for its simplicity, speed and convenience.

Phenylbutazone-warfarin interaction in man: further stereochemical and metabolic considerations

The pharmacokinetics and urinary metabolic profile of R and S-warfarin, following administration of a 1.5 mg/kg oral dose of racemic warfarin, alone and 4 days into an oral regimen of 100 mg phenylbutazone three times a day, was investigated in three volunteers using a stereospecific h.p.l.c. fluorescent assay. The mean elimination half-life of S-warfarin was increased from 25 to 46 h during phenylbutazone administration, whilst that of the R-isomer was decreased from 37 to 25 h. The peak unbound concentrations of both warfarin enantiomers were higher during phenylbutazone administration, due to displacement. Displacement was not stereoselective. The unbound clearance of more potent S-warfarin is decreased by four-fold during phenylbutazone administration, due to substantial inhibition of both 6- and 7-hydroxylation, significant pathways of elimination of S-warfarin in the absence of phenylbutazone. The unbound clearance of R-warfarin is almost unchanged during phenylbutazone administration, due to the marginal effect of phenylbutazone on 6- and 7-hydroxylation, themselves minor pathways of elimination of this enantiomer in the absence of phenylbutazone. The stereoselective reduction of S- and R-warfarin, to their respective SS and RS-alcohols, is also substantially inhibited during phenylbutazone administration. Collectively the data point to the complex effect of phenylbutazone administration on warfarin's pharmacokinetics.

Trauma and oral anticoagulants

Patients on oral anticoagulants who present to the emergency department following trauma pose a special problem. Careful attention must be given to exclude any sites of overt or occult bleeding, particularly in the CNS, where there may be no external evidence of injury. A PT or thrombotest should be obtained as minimum laboratory workup in all cases. In specific situations, emergency reversal of anticoagulation must be undertaken.

Mechanism for potentiation of warfarin by phenylbutazone. Inhibition of vitamin K-dependent carboxylation and prothrombin synthesis by phenylbutazone in preparations from rat liver

Phenylbutazone potentiated the anticoagulant effects of racemic warfarin and of the individual enantiomers to similar extents in the rat. This indicates that the phenylbutazone did not act stereospecifically on the enantiomers, as it does in humans. Phenylbutazone doubled the turnover rate of warfarin in plasma, but it did not increase the amount of the anticoagulant in liver or the amount excreted in urine. The drug had no effect on plasma disappearance of [3H] or on hepatic levels of [3H] vitamin K1 or of its chief metabolite, [3H] vitamin K1 epoxide, after injection of [3H] vitamin K1. Phenylbutazone, however, at concentrations of 0.5 to 2.8 mM inhibited vitamin K-dependent carboxylation of a synthetic pentapeptide substrate in liver microsomes by 40-88 per cent. Vitamin K-dependent protein carboxylation was also inhibited by about 40 per cent in microsomes and post-mitochondrial supernatant fluid at drug concentrations of 2.8 to 4.8 mM. Most importantly, prothrombin synthesis was inhibited in post-mitochondrial supernatant fractions by 19 and 39 per cent at drug concentrations of 2.8 and 4.8 mM respectively. The inhibition of both carboxylation and prothrombin synthesis appears to have been of sufficient magnitude to account for the potentiation by phenylbutazone observed in vivo. The calculated hepatic level of phenylbutazone during potentiation was around 3 mM, a concentration that produced inhibition in vitro.

Kinetics of drug displacement interactions

A simple model simulating the kinetics of drugs displacement kinetics is investigated. It is demonstrated that for highly bound, lowly cleared drugs, displacement interactions are transitory. Consequently, the kinetics of the interaction have to be considered as well as the in vitro interaction. It is possible to have a significant in vitro displacement interaction with no in vivo counterpart. Methods of moderating drug displacement by adjusting the rate and the timing of administration of the displacing agent are discussed.