These findings qualitatively confirm the known potency difference between warfarin enantiomers. Furthermore, although phenylbutazone and secobarbital altered the pharmacokinetics of warfarin, these compounds do not appear to influence its pharmacodynamics. Simulation studies indicate that, after racemate administration, the continual presence of the more potent (S)-enantiomer precludes accurate assessment of Cu50,R. Analysis indicates that use of racemic (rather than enantiomer) warfarin concentration data in drug interaction studies may lead to misinterpretation of pharmacodynamic data.

The pharmacology of ketoconazole was studied in patients with fungal infections. After administration of 50-, 100-, and 200-mg doses of ketoconazole, there was a linear increase in the area under the curve of serum concentrations; this was not apparent when higher doses of ketoconazole were given. An increase in the area under the curve occurred in patients receiving 200 mg daily who were restudied after 1 to 12 months of therapy. However, normalized area under the curve appeared to decrease after higher doses were administered chronically. The half life ranged from 2.0 to 3.3 h. Peak serum concentrations up to 50 ,ug/ml were detected in this study, and potentially therapeutic concentrations were detectable up to 26 h after high doses. Ketoconazole penetrated the saliva and inflamed joint fluid and meninges, although variably, and could be demonstrated in some other tissue compartments. In the presence of renal failure, ketoconazole disposition was not altered, whereas in the presence of hepatic insufficiency, an alteration in disposition was suggested. The interactions of ketoconazole and other drugs were studied. Of note, antacids did not significantly affect ketoconazole pharmacokinetics (nor did meals), and ketoconazole and warfarin did not appear to affect the pharmacokinetics of the other.Ketoconazole is a new, broad-spectrum, orally administered, antifungal agent. The therapeutic efficacy of ketoconazole has been previously reported (7), but scant data on the pharmacokinetics of ketoconazole in patients have been published. We present here the results of our studies of the pharmacokinetics of ketoconazole. MATERIALS AND METHODSSingle-dose studies. Pharmacokinetic studies were performed in patients upon entry into the ketoconazole therapeutic trial. The first six adult patients received 50, 100, or 200 mg after overnight fasting. The order of these doses was determined by a balanced crossover design. Each dose was administered 48 h after the previous dose. To give 50 mg of ketoconazole, one-half of a scored, 200-mg tablet was dissolved in 60 ml of orange juice; 30 ml of this solution was administered to the patients. Before dosage, a catheter was placed in an antecubital vein and flushed with 3 ml of sterile, normal saline containing 1 U of heparin USP per ml. After the first 3 ml of blood was discarded, 10 ml of blood was collected, and the serum was separated after clotting occurred. Samples were taken before administration of the drug and at 0

The kinetics of many pharmacologic effects of drugs in man can be described by mathematical expressions that are based on apparent first-order elimination of the drug and on an essentially linear relationship (over the clinically significant range) between the intensity of the elicited effect and the logarithm of the dose or concentration of the drug in the blood. The maximum prothrombinopenic response to the coumarin anticoagulant drug, warfarin, appears 2 to 4 days after the administration of a single dose of the drug and the occurrence of the peak plasma levels. However, it is shown in the present study that the pharmacologic effect of warfarin, when expressed in terms of the degree of inhibition of "prothrombin complex activity synthesis rate," follows the classical relationship previously described. Thus, there is a linear relationship between the logarithm of drug concentration in plasma at a given time and the pharmacologic effect at that time, and the pharmacologic effect declines at a constant rate following cessation of therapy. The prothrombinopenic effect of warfarin as a function of time after drug administration can now be predicted effectively by use of a mathematical relationship based on the dose or the initial concentration of the drug, the rate constants for warfarin elimination and for prothrombin complex activity decline, and the slope of the log-plasma concentration-response plot for the drug. The pharmacokinetic approach used in this study may also be applicable in principle to the kinetic analysis of other types of apparently delayed pharmacologic effects.

Link (1) suggested in 1948 that the coumarin compound warfarin be used as a rodenticide. Its first human use was in a suicide attempt in 1952 (2). Although warfarin has been employed increasingly in the treatment of thromboembolic disorders (3)(4)(5), no published reports of its physiologic disposition in man have appeared, probably owing largely to the lack of a suitable method for its measurement in biologic fluids (6).Weiner, Brodie, and Burns (7,8) stated that the half-life of warfarin in man is 90 hours, but did not include the assay method, or give supporting data. In 1953, Yuyama, Goto, and Umezu (9) described a colorimetric method for the estimation of warfarin in rat plasma, but apparently this method was never applied to man. Since 1954, several investigations of warfarin in the rat have appeared, including the studies of Garner (10), Eble (11), and Lin (12). Their methods, however, were unpublished except as doctoral theses, or were not readily adaptable for studies in man.Recently, we described a spectrophotometric method for the estimation of warfarin in biologic fluids (13), and have used this method in the present study to investigate the pharmiacodynamics of warfarin in man. We determined the concentrations of warfarin in the plasma and of a warfarin metabolite in the urine of normal subjects after both oral and intravenous administration of the drug. Analysis of the data provided information on its absorption, elimination, apparent volume of distribution, and excretion. Differences in its biologic effect were evaluated by simultaneous measurements of its plasma concentration and of prothrombin complex activity. * Work supported by U. S. Public Health Service grant H-2754. METHODSThe subjects, all volunteers, were 14 normal men and women, ages 27 to 63 years, and one patient, age 72 years, with coronary artery disease who was included because he was unusually resistant to the prothrombinopenic effects of the drug.Warfarin sodium 1 was administered either orally in the form of tablets (starch base) or by iv injection. The tablets were swallowed whole in the morning by subjects in the postabsorptive state, and no food was taken for at least 2 hours thereafter. For iv administration, lyophilized warfarin sodium was reconstituted in distilled water. The total dose was injected into the antecubital vein in less than 1 minute. The dose of warfarin given by either route was based on body weight in an attempt to equalize the effect of the drug on subjects with different plasma volumes, assumed to be a function of body weight. A standard dose of 1.5 mg per kg of body weight was selected so that drug levels and prothrombin complex responses would be in a clearly measurable range for several days after administration of warfarin.Test specimens were prepared as follows. Blood obtaimed by clean venipuncture was mixed in glass tubes in a proportion of 9: 1 with 3.2% sodium citrate in 0.7%c saline and centrifuged at 2,000 rpm for 20 minutes at 4°C.

Amiodarone decreased the total body clearance of both (R)- and (S)-warfarin in normal subjects but did not change volumes of distribution. Warfarin excretion products were quantified and clearance and formation clearance values calculated. Amiodarone and metabolites inhibited the reduction of (R)-warfarin to (R,S)-warfarin alcohol-1 and the oxidation of both (R)- and (S)-warfarin to phenolic metabolites. Inhibition of warfarin hydroxylation by amiodarone in human liver microsomes was compared with the in vivo results. In agreement, the in vitro data indicates that amiodarone is a general inhibitor of the cytochrome P450 catalyzed oxidation of both enantiomers of warfarin, but the metabolism of (S)-warfarin is more strongly inhibited than that of (R)-warfarin. These data suggest that the enhanced anticoagulant effect observed when amiodarone and warfarin are coadministered is attributable to inhibition of P4502C9, the isozyme of P-450 primarily responsible for the conversion of (S)-warfarin to its major metabolite, (S)-7-hydroxywarfarin.

Because of the known interaction of warfarin and disulfiram and the "disulfiram effect" of metronidazole, the interaction with metronidazole of commercial racemic warfarin and its separated enantiomorphs was evaluated in eight normal subjects. Single oral doses of racemate, S(-)-warfarin, and R (+)-warfarin, were administered in the amounts of 1.5, 0.75 and 1.5 mg per kilogram of body weight, respectively, with and without metronidazole, 750 mg by mouth, beginning seven days before the warfarin dose and continuing seven days before the warfarin dose and continuing daily throughout the hypoprothrombinemia. Daily plasma samples were analyzed for warfarin in content and one-stage prothrombin time. A highly significant (P less than 0.01) augmentation of the mean warfarin level and hypoprothrombinemia with metronidazole occurred for racemic and S (-)-warfarin; none occurred with R (+)-warfarin. Thus, the interaction of racemic warfarin and metronidazole is stereoselective and can be lessened or even avoided by use of R (+)-warfarin alone for long-term therapy.

To allow the simultaneous evaluation of the interaction between sulfinpyrazone and each enantiomer of racemic warfarin, pseudoracemic warfarin (1:1 12C-R(+) and 13C-S(-)warfarin) was given to six normal subjects both before and during oral sulfinpyrazone dosing. Serial blood and urine samples were analyzed for unchanged warfarin and its metabolic products by GC/MS. A mass balance of an oral dose of pseudoracemic warfarin, containing a tracer quantity of 14C-warfarin, was carried out in one of the subjects by monitoring 14C levels in urine and feces for 15 days. Concomitant sulfinpyrazone dosing markedly increased hypoprothrombinemia, decreased clearance of (S)-warfarin, and increased clearance of (R)-warfarin. Sulfinpyrazone also decreased the urinary excretion of warfarin-related products but increased their fecal excretion by an equivalent amount. Virtually all of the administered warfarin dose could be accounted for either as unchanged drug or known metabolites. Pharmacokinetic analysis of the data suggests the following: At least four distinct enzymes (two oxidases and two reductases) are involved in the metabolism of warfarin. Sulfinpyrazone increases the hypoprothrombinemia caused by warfarin primarily by inhibition of the cytochrome P-450-mediated oxidation of (S)-warfarin, the biologically more potent enantiomer. The increased clearance of (R)-warfarin results not from induction, but from its selective displacement from plasma protein binding sites.

The mechanism of the drug interaction in humans between warfarin and rifampin was investigated by monitoring the elimination kinetics and metabolic disposition of a single oral dose of pseudoracemic warfarin by GC/MS. The decrease in hypoprothrombinemia observed with concomitant administration of therapeutic doses of rifampin was accompanied by a substantial decrease in the elimination half-lives of both warfarin enantiomers. Rifampin increased the clearance of (R)-warfarin threefold and the clearance of (S)-warfarin twofold. The excretion profiles for warfarin and its metabolites in urine and feces were similar for both control and treated subjects with the exception that 4'-hydroxywarfarin (stereoselective for the (S)-enantiomer) was observed when rifampin was administered. 4'-Hydroxywarfarin is a metabolite of the drug hitherto undetected in vivo in humans. Based on formation clearance values estimated for 6-, 7-, and 8-hydroxywarfarin, rifampin appears to increase the clearance of the parent drug by induction of the cytochrome P-450 isozyme(s) responsible for aromatic hydroxylation.