Stereoselective metabolism and pharmacokinetics of disopyramide enantiomers in humans

Metabolism, pharmacokinetics, and influence of alpha 1-acid glycoprotein (alpha 1-AGP) plasma levels on protein binding of (R)-(-) and (S)-(+)-disopyramide (DP) were compared, in six healthy subjects, at the steady state, after oral administration of 100 mg twice daily. The mean unbound clearance of (R)-(-)-DP and (S)-(+)-DP were 8.59 and 14.9 ml/min/kg, respectively (p = 0.003). The mean unbound renal clearance of (R)-(-)-DP and (S)-(+)-DP were 6.26 and 8.75 ml/min/kg, respectively (p = 0.025). The nonrenal clearance, i.e. hepatic metabolic clearance, of (R)-(-)-DP and (S)-(+)-DP averaged 2.32 and 6.19 ml/min/kg, respectively (p = 0.002). The mean unbound volume of distribution of (R)-(-)- and (S)-(+)-DP were 225 and 381 liters, respectively (p = 0.023). The half-life of (R)-(-)-DP and (S)-(+)-DP averaged 4.17 and 3.91 hr, respectively (p = 0.21). The mean unbound renal clearance of (R)-(-)- and (S)-(+)-mono-N-dealkylated disopyramide (MND) were 3.21 and 7.02 ml/min/kg, respectively (p less than 0.001). The unbound fraction at steady state of (R)-(-)-DP and (S)-(+)-DP averaged 12.5 and 7.5%, respectively (p = 0.002). The unbound fraction at steady state of (R)-(-)-DP and (S)-(+)-MND averaged 62.6 and 60.5%, respectively (p = 0.36). The highest alpha 1-AGP plasma concentration resulted in lower unbound fraction for both DP and MND enantiomers, whereas the lowest alpha 1-AGP plasma concentration resulted in higher unbound fraction for (S)-(+)-DP only.     

Stereoselective pharmacokinetics and inversion of suprofen enantiomers in humans.

The stereoselective pharmacokinetics of suprofen enantiomers has been studied in humans by means of stable isotope-labeled pseudoracemate-diastereomer methodology. After a single oral dose of a near equimolar mixture of unlabeled-(R)-(-)- and [2H3]-(S)-(+)-suprofen [or unlabeled-(S)- and [2H3]-(R)-suprofen] to three healthy male subjects, the plasma concentrations of drug were determined by a stereospecific gas chromatography-mass spectrometry method. Racemic [2H7]suprofen was used as an internal standard. The method involved chiral derivatization with (S)-(-)-1-(naphthyl)ethylamine to form the diastereomeric amide. The plasma concentrations were consistently higher for the (R)-isomer than the (S)-isomer. No significant difference in the elimination half-life of the enantiomers was observed. An average of 6.8% of an administered dose of the (R)-isomer was stereospecifically inverted to the (S)-isomer. There was no measurable inversion of the (S)- to (R)-isomer. The present stable isotope-labeled pseudoracemate-diastereomer methodology has made it possible to evaluate the pharmacokinetics of each enantiomer, including the estimation of chiral inversion after administration of the racemic mixture.     

Tissue distribution of mexiletine enantiomers in rats

1. The kinetics of distribution of the enantiomers of mexiletine were studied in various tissues (heart, brain, lungs, liver, kidneys and fat) in male Sprague-Dawley rats after administration of a single i.v. dose (10mg/kg) of racemic mexiletine.

2. The pharmacokinetic parameters calculated from the serum data showed a 32% greater systemic clearance (162ml/min per kg vs 123 ml/rain per kg) and a 22% greater steady-state volume of distribution (9.01/kg vs 7.4 1/kg) for R(-)-mexiletine relative to the S(+)-enantiomer. However, the terminal elimination half-lives of the enantiomers (1.4 and 1.3h for R(-)- and S(+)-mexiletine, respectively) did not exhibit stereoselectivity.

3. Maximum tissue concentrations of the enantiomers were observed at 5?min after dosage in all tissues studied. Stereoselective uptake was evident only in the liver tissue and was 2.4-fold greater for S(+)-mexiletine. High tissue/serum ratios (>20 for both enantiomers) were observed in lungs, brain and kidneys. The cardiac concentrations of R(-)- and S(+)-mexiletine were 8- and 7-fold those of serum, respectively.

4. The results demonstrate that the uptake of mexiletine enantiomers into the target tissue (heart) is not stereoselective. However, the relatively high brain accumulation of the enantiomers may be related to the CNS side-effects commonly associated with mexiletine therapy.     

The relationship between the pharmacokinetics of ibuprofen enantiomers and the dose of racemic ibuprofen in humans

Ibuprofen is a chiral drug which is used clinically as a racemate. The pharmacological properties of ibuprofen reside almost exclusively with the S(+)-enantiomer. However, a portion of R(-)-ibuprofen is metabolically inverted to its pharmacologically active, mirror-image form. To investigate the influence of increasing dose of racemic ibuprofen on the pharmacokinetics of its individual enantiomers, four healthy male volunteers were given racemic ibuprofen (200, 400, 800, and 1200 mg), orally, on four occasions. The study was conducted using a balanced cross-over design. The extent of absorption of ibuprofen, as assessed by the total urinary recovery of ibuprofen and its metabolites, was extensive and independent of the administered dose. At all four doses, the area under the total and unbound plasma concentration-time curves (AUC and AUCu, respectively), and the unbound fraction in plasma, were significantly greater for the S(+)-enantiomer. With increasing ibuprofen dose, there was a less than proportional increase in the AUC of each enantiomer, while the AUCu for both enantiomers increased in direct proportion to the administered dose. The time-averaged unbound fraction of each enantiomer increased significantly with increasing dose, which caused the non-linearity between AUC and dose. It was predicted that the metabolic intrinsic clearance of each enantiomer, and the fraction of R(-)-ibuprofen which was metabolically inverted to S(+)-ibuprofen, was independent of the administered dose.     

Clinical pharmacokinetics of mexiletine

Mexiletine, a class Ib antiarrhythmic agent, is rapidly and completely absorbed following oral administration with a bioavailability of about 90%. Peak plasma concentrations following oral administration occur within 1 to 4 hours and a linear relationship between dose and plasma concentration is observed in the dose range of 100 to 600 mg. Mexiletine is weakly bound to plasma proteins (70%). Its volume of distribution is large and varies from 5 to 9 L/kg in healthy individuals. Mexiletine is eliminated slowly in humans (with an elimination half-life of 10 hours). It undergoes stereoselective disposition caused by extensive metabolism. Eleven metabolites of mexiletine are presently known, but none of these metabolites possesses any pharmacological activity. The major metabolites are hydroxymethyl-mexiletine, p-hydroxy-mexiletine, m-hydroxy-mexiletine and N-hydroxy-mexiletine. Formation of hydroxymethyl-mexiletine, p-hydroxy-mexiletine and m-hydroxy-mexiletine is genetically determined and cosegregates with polymorphic debrisoquine 4-hydroxylase [cytochrome P450 (CYP) 2D6] activity. On the other hand, CYP1A2 seems to be implicated in the N-oxidation of mexiletine. Various physiological, pathological, pharmacological and environmental factors influence the disposition of mexiletine. Myocardial infarction, opioid analgesics, atropine and antacids slow the rate of absorption, whereas metoclopramide enhances it. Rifampicin (rifampin), phenytoin and cigarette smoking significantly enhance the rate of elimination of mexiletine, whereas ciprofloxacin, propafenone and liver cirrhosis decrease it. Cimetidine, ranitidine, fluconazole and omeprazole do not modify the disposition of mexiletine. Conversely, mexiletine is known to alter the disposition of other drugs, such as caffeine and theophylline. Factors affecting the elimination of mexiletine may be clinically important and dosage adjustments are often necessary.     

Tissue distribution of mexiletine enantiomers in rats.

1. The kinetics of distribution of the enantiomers of mexiletine were studied in various tissue (heart, brain, lungs, liver, kidneys and fat) in male Sprague-Dawley rats after administration of a single i.v. dose (10 mg/kg) of racemic mexiletine. 2. The pharmacokinetic parameters calculated from the serum data showed a 32% greater systemic clearance (162 ml/min per kg vs 123 ml/min per kg) and a 22% greater steady-state volume of distribution (9.0 l/kg vs 7.4 l/kg) for R(-)-mexiletine relative to the S(+)-enantiomer. However, the terminal elimination half-lives of the enantiomers (1.4 and 1.3 h for R(-)- and S(+)-mexiletine, respectively) did not exhibit stereoselectivity. 3. Maximum tissue concentrations of the enantiomers were observed at 5 min after dosage in all tissues studied. Stereoselective uptake was evident only in the liver tissue and was 2.4-fold greater for S(+)-mexiletine. High tissue/serum ratios (greater than 20 for both enantiomers) were observed in lungs, brain and kidneys. The cardiac concentrations of R(-)- and S(+)-mexiletine were 8- and 7-fold those of serum, respectively. 4. The results demonstrate that the uptake of mexiletine enantiomers into the target tissue (heart) is not stereoselective. However, the relatively high brain accumulation of the enantiomers may be related to the CNS side-effects commonly associated with mexiletine therapy.     

The pharmacokinetics of ethosuximide enantiomers in the rat.

A chiral gas chromatographic assay previously developed for quantitative analysis of ethosuximide and its major metabolites in rat urine has been adapted for the analysis of the drug in plasma. Ethosuximide, both as a racemic mixture and as the individual enantiomers, was administered to conscious rats by the intravenous (i.v.) and intraperitoneal (i.p.) routes. Pharmacokinetic parameters were estimated using standard non-compartmental methods. Comparison of the pharmacokinetic parameters of (S)-ethosuximide and (R)-ethosuximide showed that total body clearance of (R)-ethosuximide was significantly larger than that of (S)-ethosuximide and that elimination half-life was significantly shorter following administration of both 40 mg i.v. and i.p. doses, indicating that there is stereoselective elimination of ethosuximide. However, no significant differences were found between apparent volumes of distribution. In addition, no significant differences were found for either enantiomer between the estimates of the pharmacokinetic parameters obtained following administration as the individual enantiomer and as a constituent of the racemic mixture. This indicates that, at the doses studied, the preferential faster elimination of (R)-ethosuximide is not dependent upon the presence of the (S)-enantiomer. Also, for each enantiomer, the lack of any significant difference between estimates of clearance when administered as part of a racemic mixture and when administered separately indicates that neither enantiomer affects the clearance of the other.     

Resolution and electrophysiological effects of mexiletine enantiomers.

Resolution of mexiletine enantiomers from the racemic mixture has been achieved by fractional crystallization through the formation of diastereoisomeric p-toluoyl tartrate salts. Following three crystallization steps in methanol, R-(-)- and S-(+)-mexiletine were resolved with an optical purity greater than 98% (yield approximately 30%) and their hydrochloride salts formed. Incremental doses of mexiletine enantiomers were administered to dogs with experimentally-induced arrhythmias to investigate the stereoselective antiarrhythmic and electrophysiological effects of these compounds. Using up to three extrastimuli, programmed electrical stimulation was performed in conscious animals 7-30 days after coronary ligation. R-(-)-Mexiletine prevented ventricular tachycardia in 3/6 dogs (2 after 0.5 mg kg-1, 1 after 8 mg kg-1); two animals died after 1 and 8 mg kg-1, respectively; one remained unchanged even at the highest dosage (16 mg kg-1). S-(+)-Mexiletine prevented ventricular tachycardia in only one dog (after 1 mg kg-1); two died after 4 and 8 mg kg-1, respectively; 2/5 remained unchanged even after the administration of 16 mg kg-1. No significant changes in any electrocardiographic intervals (PR, QRS, QTc) or refractory periods were induced by mexiletine enantiomers at any doses used (0.5-16.0 mg kg-1). These results suggest that R-(-)-mexiletine possesses greater antiarrhythmic properties than the opposite enantiomer.     

Effects of age on the pharmacokinetics of mexiletine

Mexiletine (1-methyl-2-(2, 6-xylyloxy)ethylamine hydrochloride) is an antiarrhythmic drug eliminated primarily by hepatic metabolism. The influence of age on the plasma pharmacokinetics of mexiletine was assessed in seven young and ten elderly healthy subjects. Mexiletine (50 mg) was administered orally three times daily for 10 days. The use of low doses of mexiletine was possible owing to the development of a new fluorescence high performance liquid chromatography method with a limit of sensitivity lower than 20 ng/ml. No differences in the pharmacokinetic parameters of mexiletine related to age were observed. At steady-state plasma concentrations were 0.121 +/- 0.033 microgram-ml in the young volunteers, and 0.135 +/- 0.150 microgram/ml in the older subjects. The elimination t 1/2 was 11.4 +/- 1.78 h in the young subjects and 10.48 +/- 3.06 h in the elderly. The data provide no justification for lowering the recommended dose of mexiletine for older patients.     

The pharmacokinetics of etodolac enantiomers in the rat. Lack of pharmacokinetic interaction between enantiomers

Pharmacokinetics of the enantiomers of the nonsteroidal anti-inflammatory drug etodolac (ET, (+/- )-1,8-diethyl-1,3,4,9-tetrahydropyrano[3,4-b]indole-1-acetic acid), which is marketed as a racemate, were studied in male Sprague-Dawley rats. Following administration of iv racemate, plasma concentrations of inactive R-ET were much greater than those of active S-ET. After iv doses of individual enantiomers, similar results were found, with significantly greater t1/2, CL, and Vdss, and lower AUC, for S- than for R-ET. No evidence of a pharmacokinetic interaction between the enantiomers was observed. Secondary peaks indicative of extensive enterohepatic recirculation were seen in plasma time courses of S-ET. In bile duct-cannulated rats, the AUC of S- but not R-ET, was significantly reduced, and secondary peaks were absent in plasma profiles. The differences between enantiomers were attributed to a greater extent of plasma protein binding of R-ET, and to preferential conjugation and biliary excretion of S-ET. Complete recovery of S-ET was achieved in bile, whereas only 30% of the R-enantiomer was recovered via this route of elimination. Urine was a minor route of elimination of ET. It was concluded that the rat may be a suitable pharmacokinetic model for the study of stereoselective pharmacokinetics of ET because, in some aspects, the results closely paralleled those of ET in man.     

Stereoselective pharmacokinetics and metabolism of the enantiomers of cyclophosphamide. Preliminary results in humans and rabbits

[R(+),S(-)]-Cyclophosphamide [(R,S)-CP] is an anticancer drug, containing a chiral phosphorous atom, which is prepared and used clinically as the racemic mixture. A new high-performance liquid chromatographic assay suitable for pharmacokinetic studies of CP enantiomers in plasma has been reported recently by this laboratory (Reid et al., Anal Chem 61: 441-446, 1989). Briefly, the assay involves ethyl acetate extraction of CP enantiomers from plasma followed by derivatization to diastereomers in a two-step process utilizing chloral and (+)-naproxen acid chloride. Chromatographic analysis was performed on a reversed phase (ODS) column with detection at 232 nm. In the present study, preliminary results on the applicability of this assay to pharmacokinetic studies are presented. Several rabbits were used to compare the influence of i.p., i.v., and oral routes of administration on the stereoselective disposition of (R,S)-CP. Following i.p. administration, S-CP was cleared faster than R-CP. Following oral administration, only R-CP was detectable in plasma, while i.v. administration resulted in minor or no stereoselective disposition. These results indicated that there was a marked stereoselective metabolism of the S-CP enantiomer, with the i.p. and oral routes producing the greatest differences due to first-pass metabolism. Incubation of rabbit-liver microsomes with (R,S)-CP demonstrated that the monooxygenase system can exhibit marked stereoselectivity in its metabolism of CP. The ratio of R-CP to S-CP in the incubation medium increased during the incubation period from 1:1 initially to 4.5:1 after 60 min. The results from the experiments with rabbits indicate that the first-pass metabolism of this drug is highly stereoselective; in contrast, cancer patients who had received (R,S)-CP as an i.v. infusion showed no stereoselectivity in the elimination of the enantiomers. Pharmacokinetic studies with cancer patients, receiving (R,S)-CP as an oral dose, are in progress in order to determine if stereoselective first-pass metabolism of this drug also occurs in humans.     

The pharmacokinetics of pranoprofen in humans

The pharmacokinetics of pranoprofen, 2-(5H-[1]benzopyrano[2,3-b]pyridin-7-yl) propionic acid (I) in humans were examined. 1-O-Acylglucuronide of I (II) and its isomer (III) were isolated from the human urine after oral administration of I. The stability of II was tested in order to establish the suitable conditions for the storage and handling of biological samples. Maximum stability of II was found at pH 3-4. Unchanged drugs, II and III were detected in the human plasma after oral administration of I. Plasma concentrations of these compounds reached the maximum at 1-2 h after the administration and thereafter decreased biphasically. From the urinary sample, 1-O-acylglucoside of I (IV) was detected in addition to unchanged drugs, II and III. Within 24 h after dosing, 1.3, 84.0, 7.0 and 0.6% of the dose were excreted as I, II, III and IV, respectively and the total urinary excretion amounted to 92.9% in term of unchanged drug.     

Dose independent pharmacokinetics of mexiletine in healthy volunteers.

In 12 healthy volunteers who received orally 100, 200, 300, 400 and 600 mg mexiletine at weekly intervals, the maximum plasma concentration of mexiletine and AUC increased linearly with the dose of mexiletine. Between doses there were no significant differences in the values for clearance and volume of distribution of mexiletine but there were for plasma elimination half-life. These results indicate that the kinetics of mexiletine are linear.     

Human pharmacokinetics of betaxolol enantiomers

Betaxolol is a cardioselective beta-adrenergic antagonist effective in the treatment of hypertension. The pharmacokinetic behavior of betaxolol enantiomers in healthy male subjects is reported. Betaxolol enantiomer concentrations were determined in samples collected up to 48 h after iv administration of a 10-mg dose over a 30-min period by constant-rate infusion in 12 subjects and after oral administration of 40-mg capsules to eight of the same subjects. Betaxolol extracted from whole blood was reacted with (+) or (-)-1-naphthylethyl isocyanate. The resulting diastereoisomeric derivatives were analyzed by reversed-phase HPLC with fluorimetric detection. Following the iv dose, there were no differences in clearance or volume of distribution for the two enantiomers (15.6 +/- 4.4 versus 16.4 +/- 4.1 L/h and 342 +/- 62 versus 340 +/- 65 L, respectively). Likewise, after the oral dose, there were no differences in the maximum concentration, time of maximum concentration, bioavailability, or apparent absorption rate constant (41.0 +/- 8.6 versus 42.0 +/- 7.0 ng/mL, 214 +/- 59 versus 215 +/- 56 min, 0.89 +/- 0.26 versus 0.94 +/- 0.23, and 1.0 +/- 0.6 versus 1.2 +/- 0.6 h-1, respectively). Thus, the pharmacokinetic behavior of racemic betaxolol accurately reflects the behavior of betaxolol enantiomers in this subject group.     

Influence of mexiletine on the pharmacokinetics of theophylline in healthy volunteers

Preliminary reports suggest an interaction exists between theophylline and mexiletine. We conducted a two-way crossover study in 15 healthy male subjects to assess the magnitude of the pharmacokinetic interaction between mexiletine and theophylline. Twelve subjects completed 5 days of therapy on sustained-release theophylline 200 mg every 12 hours alone and 5 days of therapy with theophylline and mexiletine 150 mg every 8 hours. The two treatment periods were separated by a minimum of 7 days. On the morning of day 5 of each treatment period, blood samples for theophylline to be assayed by fluorescence immunoassay were collected over 24 hours. Mexiletine significantly increased the mean AUC, Cmax, t1/2 beta, and Cl of theophylline. Mexiletine did not affect tmax or Vd. Side effects occurred in 4 subjects during treatment with theophylline alone all of which were judged to be mild in intensity. During concomitant theophylline-mexiletine therapy, 10 subjects reported side effects of which 4 were judged to be severe, 1 moderate, and 5 mild. The magnitude of the increase in theophylline plasma concentrations induced by mexiletine as measured by AUC (0-24) was 58%, which is similar to other preliminary reports of this interaction. The authors conclude that the magnitude of the pharmacokinetic interaction between theophylline and mexiletine may be clinically significant in patients in light of the increased incidence of side effects in our healthy subjects. Theophylline dosage adjustments will be required in patients who receive concomitant mexiletine therapy.     

Influence of debrisoquine hydroxylation phenotype on the pharmacokinetics of mexiletine

Summary Marked interindividual variation has been observed in the pharmacokinetics of the antiarrhythmic agent mexiletine. The fact that its urinary excretion is dependent on urinary pH may account, in part, for such variation. The influence that genetic differences in hepatic metabolism of the debrisoquine-type may have on mexiletine pharmacokinetics was considered in this study. The pharmacokinetics and urinary excretion of mexiletine (250 mg administered intravenously) were investigated in 5 rapid extensive metabolisers (EM), 5 slow EM and 5 poor metabolisers (PM) of debrisoquine, under conditions of controlled urinary pH.Mexiletine disposition kinetics was found to be altered in PM individuals. These subjects showed higher total area under the curve (AUC), (15.7 versus 8.16 g · h · ml^–1) prolonged elimination half-lives (in serum and urine) (serum: 18.5 versus 11.6 h, urine: 19.2 versus 11.7 h) and lower total clearance values compared with EM (216 versus 450 ml · min^–1). In this respect, slow EM individuals generally presented intermediate values of those pharmacokinetic parameters. A higher incidence of adverse-effects was also observed among slow EM and PM subjects.It is concluded that genetic differences in mexiletine oxidation of the debrisoquine-type have an influence on its observed pharmacokinetic variability. The clinical consequences are discussed.     

Pharmacokinetics of clinafloxacin enantiomers in humans

The pharmacokinetics of R-clinafloxacin and S-clinafloxacin enantiomers of the broad-spectrum fluoroquinolone antibiotic, clinafloxacin, were characterized in selected volunteer subjects and patients after the administration of oral and intravenous doses of racemic drug. The absorption of each enantiomer was rapid and nearly complete after a single, oral 400 mg racemic dose. The mean (+/- SD) bioavailability of R-clinafloxacin was 87.5% +/- 4.8% compared to 86.2% +/- 5.8% for S-clinafloxacin. The mean Cmax of each enantiomer was 1.19 micrograms/mL, with plasma concentrations of each enantiomer remaining above 0.1 microgram/mL for at least 12 hours. No notable differences in the disposition of R-clinafloxacin and S-clinafloxacin were observed. After a single 400 mg intravenous dose of racemic drug, mean (+/- SD) t1/2 was 5.6 +/- 0.3 hours and 5.7 +/- 0.4 hours, plasma Cl was 329 +/- 49 mL/min and 314 +/- 45 mL/min, and Vdss was 138 +/- 18 L and 134 +/- 16 L for R- and S-clinafloxacin, respectively. Two healthy volunteers each received a single 400 mg oral dose of racemic clinafloxacin (alone) and with oral administration of 1 gm probenecid separated by a 1-week washout period between treatments. With probenecid coadministration, the increase in AUC0-infinity was 75% and 83% for R-clinafloxacin and was 71% and 75% for S-clinafloxacin in each subject, respectively. Probenecid increased the total exposure (AUC) of both R-clinafloxacin and S-clinafloxacin, although it had no stereo-selective effects on the disposition of either enantiomer. The antimicrobial potency of the isomers was also evaluated. In vitro susceptibility testing showed that the two compounds were comparable in their inhibitory activities, as all MICs were within twofold for each organism tested. These results demonstrate that in addition to their similar antimicrobial potency, R- and S-clinafloxacin have nearly identical disposition characteristics and are eliminated by similar mechanisms that display no apparent enantioselectivity in man.     

Neuroimaging of mirtazapine enantiomers in humans

INTRODUCTION: Mirtazapine is a racemic antidepressant with a multireceptor profile. Previous studies have shown that the enantiomers of mirtazapine have different pharmacologic effects in the brain of laboratory animals. MATERIALS AND METHODS: In the present study, we used positron emission tomography (PET) and autoradiography to study effects of (R)- and (S)-[(11)C]mirtazapine in the human brain. Detailed brain imaging by PET using three methods of kinetic data analysis showed no reliable differences between regional binding potentials of (R)- and (S)-[(11)C]mirtazapine in healthy subjects. RESULTS: Autoradiographic studies carried out in whole hemispheres of human brain tissue showed, however, that (R)- and (S)-mirtazapine differ markedly as inhibitors of [(3)H]clonidine binding at alpha(2)-adrenoceptors. CONCLUSION: The multireceptor binding profiles of mirtazapine enantiomers, along with individual differences between subjects, may preclude PET neuroimaging from demonstrating reliable differences between the regional distribution and binding of (R)- and (S)-[(11)C]mirtazapine in the living human brain.     

The pharmacokinetics of furazlocillin in healthy humans

The pharmacokinetics of the novel acylureidopenicillin furazlocillin, 6-[D-2-(3-furfurylidenamino-2-oxo-imidazolidine-1-carboxamido)-2-(4-hydroxyphenyl)-acetamido]-penicillanic acid and of its penicilloic acid derivative were investigated in five healthy male volunteers after intravenous administration of 2 and 4 g dosages. The volunteers were either in a lying or sitting position throughout the duration of the studies. The concentrations of the drug in plasma and urine were measured by two different methods in parallel: a microbiological assay and a newly developed high pressure liquid chromatography method. The latter method was also applicable for quantitation of the penicilloic acid derivative in these biological fluids. The drug's plasma protein binding (66%) and apparent red cell-plasma partition coefficient (0.055) were concentration independent. The pharmacokinetics of the drug were first order only at the lower dose level The apparent half lives of three distinguishable phases were, respectively, 4 \((t_{1/2_1 } )\) , 18 \((t_{1/2_2 } )\) , and 64 \((t_{1/2_z } )\) .