Pharmacophore hybridization approach to discover novel pyrazoline-based hydantoin analogs with anti-tumor efficacy.

In search for new and safer anti-cancer agents, a structurally guided pharmacophore hybridization strategy of two privileged scaffolds, namely diaryl pyrazolines and imidazolidine-2,4-dione (hydantoin), was adopted resulting in a newfangled series of compounds (H1-H22). Herein, a bio-isosteric replacement of "pyrrolidine-2,5-dione" moiety of our recently reported antitumor hybrid incorporating diaryl pyrazoline and pyrrolidine-2,5-dione scaffolds with "imidazoline-2,4-dione" moiety has been incorporated. Complete biological studies revealed the most potent analog among all i.e. compound H13, which was at-least 10-fold more potent compared to the corresponding pyrrolidine-2,5-dione, in colon and breast cancer cells. In-vitro studies showed activation of caspases, arrest of G0/G1 phase of cell cycle, decrease in the expression of anti-apoptotic protein (Bcl-2) and increased DNA damage. In-vivo assay on HT-29 (human colorectal adenocarcinoma) animal xenograft model unveiled the significant anti-tumor efficacy along with oral bioavailability with maximum TGI 36% (i.p.) and 44% (per os) at 50 mg/kg dose. These findings confirm the suitability of hybridized pyrazoline and imidazolidine-2,4-dione analog H13 for its anti-cancer potential and starting-point for the development of more efficacious analogs.

The relationship between stereochemical and both, pharmacological and ADME-Tox, properties of the potent hydantoin 5-HT7R antagonist MF-8.

This study concerns synthesis and evaluation of pharmacodynamic and pharmacokinetic profile for all four stereoisomers of MF-8 (5-(4-fluorophenyl)-3-(2-hydroxy-3-(4-(2-methoxyphenyl)piperazin-1-yl)propyl)-5-methylimidazolidine-2,4-dione), the previously described, highly potent 5-HT7R ligand with antidepressant activity on mice. The combination of DFT calculations of 1H NMR chemical shifts with docking and dynamic simulations, in comparison to experimental screening results, provided prediction of the configuration for one of two present stereogenic centers. The experimental data for stereoisomers (MF-8A-MF-8D) confirmed the significant impact of stereochemistry on both, 5-HT7R affinity and antagonistic action, with Ki and Kb values in the range of 3-366 nM and 0.024-99 µM, respectively. We also indicated the stereochemistry-dependent influence of the tested compounds on P-glycoprotein efflux, absorption in Caco-2 model, metabolic pathway as well as CYP3A4 and CYP2C9 activities.

QSAR characterization of new synthesized hydantoins with antiproliferative activity.

Hydantois have been identified as constituents of a number of pharmacologically active molecules. In the present study, we have examined in vitro antiproliferative activity against human colon cancer cell lines HCT-116 of three series of 3-(4-substituted benzyl)-hydantoins with various substituent attached in position 5 of the hydantoin ring. Since the investigated compounds have recently been synthesized and show antiproliferative activity, a good understanding of the properties of the potential drug responsible for their pharmacokinetics is an important goal for their further development. One of the important properties is lipophilicity. Lipophilicity has been assessed by reversed-phase liquid chromatography (high-performance thin-layer chromatography and high-pressure liquid chromatography) by means of direct and indirect (using calibration curve) methods. Chromatographic lipophilicity indices in addition to calculated logP values were compared by hierarchical cluster analysis. The linear solvation energy relationship approach was used to understand and compare the types and relative strength of the molecular interactions that occur in the chromatographic as well as in the n-octanol-water partitioning systems. Finally, correlation between in silico pharmacokinetic predictors and antiproliferative activity was examined. Preliminary quantitative structure-activity relationship modeling indicates that pharmacokinetic predictors capture only one-quarter of all chemical features that are important for antiproliferative activity itself. Among selected descriptors are chromatographic lipophilicity indices.

Development of a prodrug of hydantoin based TACE inhibitor.

Our research on hydantoin based TNF-α converting enzyme (TACE) inhibitors led to fused bi-heteroaryl hydantoin series that demonstrate sub-nanomolar potency (Ki) as well as excellent activity in human whole blood (hWBA). However, lead compound 2 posed some formulation challenges which prevented it for further development. A prodrug approach was investigated to address this issue. The pivalate prodrug 3 can be formulated as stable neutral form and demonstrated improved DMPK properties when compared with parent compound.

Inhibitors of protein arginine deiminases and their efficacy in animal models of multiple sclerosis.

Protein arginine deiminases (PAD) are implicated in a variety of inflammatory and neurodegenerative diseases including multiple sclerosis (MS). Following the discovery of an in silico hit containing hydantoin and a piperidine moiety, we hypothesized that a 2-carbon linker on the hydantoin would be necessary for a 5-membered heterocycle for optimal PAD inhibitory activity. We designed thirteen compounds as potential inhibitors of PAD2 and PAD4 enzymes-two important PAD enzymes implicated in MS. Two compounds, one with an imidazole moiety (22) and the other with a tetrazole moiety (24) showed good inhibition of PAD isozymes in vitro and in the EAE mouse model of MS in vivo. Further experiments suggested that compound 22, a non-covalent inhibitor of PAD2 and PAD4, exhibits dose-dependent efficacy in the EAE mouse model and in the cuprizone-mediated demyelination model.

Bioaugmentation and rhizosphere-assisted biodegradation as strategies for optimization of the dissipation capacity of biobeds.

Biobeds are on-farm biodepuration systems whose efficiency rely on their high pesticide biodegradation capacity. We evaluated two optimization strategies, bioaugmentation and/or rhizosphere-assisted biodegradation, to maximize the dissipation capacity of biobeds. Iprodione was used as a model pesticide. Its dissipation and metabolism was determined in a biobed packing material inoculated with an iprodione-degrading Arthrobacter strain C1 (bioaugmentation, treatments B+C1) and/or seeded with ryegrass (rhizosphere-assisted biodegradation, treatments B+P). The impact of those strategies on the activity and composition of the microbial community was determined. Bioaugmentation accelerated the dissipation of iprodione which was further enhanced in the bioaugmented, rhizosphere-assisted treatment (treatment B+P+C1, Half-life (DT50) = 3.4 d), compared to the non-bioaugmented, non rhizosphere-assisted control (DT50 = 9.5 d, treatment B). Bioaugmentation resulted in the earlier formation of intermediate formation of metabolites I (3,5-dichlorophenyl-carboxamide), II (3,5-dichlorophenylurea acetate) and 3,5-dichloroaniline (3,5-DCA). The latter was further dissipated by the indigenous microbial community. Acid phosphatase (AP) and ß-glucosidase (GLU) were temporarily stimulated in rhizosphere-assisted treatments, whereas a stimulation of the fluorescein diacetate (FDA) hydrolytic activity in the bioaugmented treatments coincided with the hydrolysis of iprodione. q-PCR showed that changes in the abundance of alpha-proteobacteria and firmicutes was driven by the presence of rhizosphere while bioaugmentation had no significant effect.

Evaluation of spiropiperidine hydantoins as a novel class of antimalarial agents.

Given the rise of parasite resistance to all currently used antimalarial drugs, the identification of novel chemotypes with unique mechanisms of action is of paramount importance. Since Plasmodium expresses a number of aspartic proteases necessary for its survival, we have mined antimalarial datasets for drug-like aspartic protease inhibitors. This effort led to the identification of spiropiperidine hydantoins, bearing similarity to known inhibitors of the human aspartic protease ß-secretase (BACE), as new leads for antimalarial drug discovery. Spiropiperidine hydantoins have a dynamic structure-activity relationship profile with positions identified as being tolerant of a variety of substitution patterns as well as a key piperidine N-benzyl phenol pharmacophore. Lead compounds 4e (CWHM-123) and 12k (CWHM-505) are potent antimalarials with IC50 values against Plasmodium falciparum 3D7 of 0.310 µM and 0.099 µM, respectively, and the former features equivalent potency on the chloroquine-resistant Dd2 strain. Remarkably, these compounds do not inhibit human aspartic proteases BACE, cathepsins D and E, or Plasmodium plasmepsins II and IV despite their similarity to known BACE inhibitors. Although the current leads suffer from poor metabolic stability, they do fit into a drug-like chemical property space and provide a new class of potent antimalarial agents for further study.

Synthesis of [(3) H], [(13) C3 , (15) N], and [(14) C]SCH 900567: an inhibitor of TNF-α (tumor necrosis factor alpha) converting enzyme (TACE).

SCH 900567 is a specific inhibitor of tumor necrosis factor-alpha converting enzyme and is a potential candidate for the treatment of rheumatoid arthritis. [(3) H]SCH 900567 was synthesized to support the initial drug metabolism and pharmacokinetics studies. Stable isotope-labeled [(13) C3 , (15) N]SCH 900567 was requested by the bioanalytical group as an internal standard for Liquid chromatography-tandem mass spectrometry (LC-MS/MS) method development as well as by the drug metabolism and pharmacokinetics group for a potential microdose study. [(13) C3 , (15) N]SCH 900567 is synthesized via a linear sequence of seven steps from commercially available materials in 2.6% overall yield. [(14) C]SCH 900567 was needed for a quantitative whole body autoradiography studies and was prepared from unlabeled Active Pharmaceutical Ingredient (API) via hydrolysis of the hydantoin moiety followed by rebuilding the hydantoin ring using potassium [(14) C]cyanate to give the desired product in 42.8% overall yield. Activation of the hydantoin moiety of SCH 900567 to achieve hydrolysis followed by derivatization of the resulting amino acid to avoid decarboxylation during cyclization is also discussed.

ADME characterization in rats revealed immediate secretion of AZD7903 into the stomach after IV dosing.

The distribution of AZD7903 and/or its metabolites was studied in rats following a single p.o. or i.v. dose using quantitative whole-body autoradiography (QWBA). At 5 min after i.v. administration of the ¹4C compound, high levels of radioactivity were observed in the fundus of the stomach compared to blood and plasma and the rest of the stomach, indicating an active secretion of ¹4C material into the stomach. Also, excretion and pharmacokinetics were studied following p.o. and i.v. dosage in rats. The radioactivity was mainly excreted via feces, and even in i.v. administered bile-duct cannulated (BDC) animals a significant part of radioactivity (26% in males and 57% in females) was recovered in the feces in the form of parent compound and two minor metabolic products. The compound was well absorbed (F% 98 in males and 76 in females), and showed low clearance in plasma (0.14 L/h/kg in males and 0.03 L/h/kg in females) and low-intermediate Vd(ss) (0.7 L/kg). Clear differences in metabolic pathways (qualitative) and rates (quantitative) and consequently in PK parameters between sexes were observed. In summary, the results indicate AZD7903 being substrate for a transporter protein and support the hypothesis that the differences in disposition between sexes are due to differences in metabolic pathways and rates.

Construction of EVA/chitosan based PEG-PCL micelles nanocomposite films with controlled release of iprodione and its application in pre-harvest treatment of grapes.

A novel nanocomposite poly(ethylene-co-vinyl acetate) (EVA) film with controlled in vitro release of iprodione (ID) was prepared. Chitosan (CS) was used as the reinforcement which enhances the water and oxygen permeability of films. ID loaded poly(ethylene glycol)-poly(ε-caprolactone) (PEG-PCL) (IPP) micelles were used as the drug carrier which endows the films with antifungal and controlled release ability. IPP micelles with spherical shape and uniform size were obtained, and the maximum encapsulation efficacy (EE) was 91.17 ± 5.03% by well controlling the feeding amount of ID. Incorporation CS could improve the oxygen and moisture permeability of films, and the maximum oxygen permeability (OP) and water vapor transmission rate (WVTR) were 477.84 ± 13.03 cc/(m2·d·0.1 MPa) and 8.60 ± 0.25 g m-2 d-1, respectively. After loading IPP micelles, the films showed an improved antifungal ability and temperature-sensitive drug release behavior, and were found to enhance the quality of grapes by pre-harvest spraying.

Solid dispersions to enhance the delivery of a potential drug candidate LPSF/FZ4 for the treatment of schistosomiasis.

Drug candidate LPSF/FZ4 with promising schistosomicidal properties in vitro was previously synthesized. However, LPSF/FZ4 has limited aqueous solubility (<1 µg/mL), leading to ineffective dissolution and, therefore, no meaningful in vivo comparative studies could be pursued. This study was aimed to develop a proper amorphous solid dispersion (SD) to enhance the solubility and dissolution rate of LPSF/FZ4 such that its biological activity could be investigated. To better understand its physiological behavior, the pKa of LPSF/FZ4, a monoprotic weak acid with NH group at the imidazolidine ring, was first determined to be 8.13 using an automated SiriusT3. The development of SD systems for LPSF/FZ4 involved the evaluation of various water-soluble polymer carriers such as PVP K-29/32, PVP K-90, HPMC K4M, PVPVA 64 and SOLUPLUS®. The most promising SD systems were selected through in vitro dissolution studies under nonsink conditions, together with physicochemical characterization as well as accelerated stability study. It was shown that SD of 10% LPSF/FZ4 in SOLUPLUS® and PVP K-90 could significantly increase the area-under-the-curve value of the nonsink dissolution profile (AUC values of the SD in SOLUPLUS® and PVP K-90 were 1381.03 and 1342.34 µL/mL·min, respectively, and that of the pure crystalline drug was 0.02 µL/mL·min), a useful surrogate for the in vivo bioavailability. Cmax values for the SD in SOLUPLUS® (12.50 µL/mL) and PVP K-90 (25.86 µL/mL) were also higher than the one of the crystalline drug (0.02 µL/mL). The SD system of LPSF/FZ4 in SOLUPLUS® showed a significant increase in schistosomicidal activity in an animal model as compared with the conventional treatment using crystalline drug, consistent with the AUC trend from the nonsink dissolution. Thus this SD system of LPSF/FZ4 could be useful as a potential formulation for treating schistosomiasis.

A pediatric phase I and pharmacokinetic study of spirohydantoin mustard.

A pediatric Phase I and pharmacokinetic study of the lipophilic alkylating agent spirohydantoin mustard (SHM) was conducted in 23 patients. The dose-limiting toxicity of SHM was neurological with disorientation, delirium, or hallucinations occurring in 9 of 23 patients. These symptoms were partially reversible and preventable with physostigmine. In 17 patients who were evaluable for response to treatment (14 of whom had central nervous system malignancies), no objective tumor responses were observed. Pharmacokinetic evaluation of SHM revealed a t1/2 alpha of 1.7 +/- 0.7 min, t1/2 beta of 16 +/- 8.3 min, and total body clearance of 2134 +/- 735 ml/min/m2. Measureable peak plasma levels were less than 40% of that which produces cytotoxicity in vitro against monolayer cultures of rat 9L brain tumor. Over 90% of SHM was protein bound, greatly limiting the free drug available for central nervous system penetration. SHM cerebrospinal fluid to plasma ratios were less than 0.047. The above suggests that in spite of its lipophilicity, SHM may not reach clinically significant levels in the central nervous system at clinically tolerable doses.

In Silico Study of Chromatographic Lipophilicity Parameters of 3-(4-Substituted Benzyl)-5-Phenylhydantoins.

New synthesized compounds, particularly those with biological activity, are potential drug candidates. This article describes experimental studies performed to estimate lipophilicity parameters of new 3-(4-substituted benzyl)-5-phenylhydantoins. Lipophilicity, as one of the most important molecular characteristics for the activity, was determined using the reversed-phase liquid chromatography (RP-18 stationary phase and methanol-water mobile phase). Molecular structures were used to generate in silico data which were used to estimate pharmacokinetic properties of the investigated compounds. The results show that generally, the investigated compounds attain good bioavailability properties. A more detailed analysis shows that the presence of a nitro, methoxy and tert-butyl group in the molecule is indicated as unfavorable for the oral bioavailability of hydantoins. Multivariate exploratory analysis was used in order to visualize grouping patterns among molecular descriptors as well as among the investigated compounds. Molecular docking study performed for two hydantoins with the highest bioavailability scores shows high binding affinity to tyrosine kinase receptor IGF-1R. The results achieved can be useful as a template for future development and further derivation or modification to obtain more potent and selective antitumor agents.

Use of Osmotic Pumps to Establish the Pharmacokinetic-Pharmacodynamic Relationship and Define Desirable Human Performance Characteristics for Aggrecanase Inhibitors.

The development of reliable relationships between in vivo target engagement, pharmacodynamic activity, and efficacy in chronic disease models is beneficial for enabling hypothesis-driven drug discovery and facilitating the development of patient-focused candidate selection criteria. Toward those ends, osmotic infusion pumps can be useful for overcoming limitations in the PK properties of proof-of-concept (POC) compounds to accelerate the development of such relationships. In this report, we describe the application of this strategy to the development of hydantoin-derived aggrecanase inhibitors (eg, 3) for the treatment of osteoarthiritis (OA). Potent, selective inhibitors were efficacious in both chemical and surgical models of OA when exposures were sustained in excess of 10 times the plasma IC50. The use of these data for establishing patient-focused candidate selection criteria is exemplified with the characterization of compound 8, which is projected to sustain the desired level of target engagement at a dose of 45 mg qd.

S-mephenytoin hydroxylation phenotypes in a Swedish population determined after coadministration with debrisoquin.

Mephenytoin (100 mg) and debrisoquin (10 mg) were administered orally, both separately and together, to 41 healthy subjects. The ratios between the S and R enantiomers of mephenytoin and between debrisoquin and 4-OH-debrisoquin in urine were determined by use of GC. These ratios were used as measures of drug hydroxylation. There was no change in the phenotypic trait values of the two drugs when they were coadministered. Mephenytoin and debrisoquin then were coadministered to 253 healthy Swedish subjects, before bedtime, and urine samples were collected at periods of 0 to 8, 8 to 24, and 24 to 32 hours after drug administration. In the first sample, seven of the 253 subjects (2.8%, 95% confidence interval 0.8% to 4.8%) had an S/R ratio of greater than 0.8; this indicated that they were poor hydroxylators of S-mephenytoin. In the two consecutive samples, the S/R ratios of mephenytoin did not change in these seven persons, whereas it decreased to less than 0.2 in the third sample in the extensive hydroxylators. As was reported before, there was no relationship between the mephenytoin S/R ratio and the debrisoquin metabolic ratio (rs = 0.01). Coadministration of debrisoquin and mephenytoin before bedtime and urine collection during two consecutive nights allow for an accurate determination of both phenotypes in the population.

Discovery of novel 2,8-diazaspiro[4.5]decanes as orally active glycoprotein IIb-IIIa antagonists.

In our efforts to develop orally active GPIIb-IIIa antagonists with improved pharmaceutical properties, we have utilized a novel 2,8-diazaspiro[4.5]decane scaffold as a template. We describe here our investigation of a variety of templates including spiropiperidinyl-gamma-lactams, spiropiperidinylimide, spiropiperidinylureas, and spiropiperidinylhydantoins. With the appropriate acidic and basic pharmacophores in place, each template yielded analogues with potent GPIIb-IIIa inhibitory activity. One of the compounds, 59 (CT50787), was also used to demonstrate for the first time the use of a pharmacological agent which is alphaIIbbeta3 specific to display biological activity in a lower species such as mouse and to extend bleeding times. Evaluation of the pharmacokinetic properties of selected compounds from each series in rat, dog, and cynomolgus monkey has led to the identification of 22 (CT51464), a double prodrug, with excellent pharmacokinetic properties. It exhibited good pharmacokinetic profile across species (F% = 33 (Cyno), 73 (dog), 22 (rat); t(1/2)(beta)() = 14.2 h (Cyno), 8.97 h (dog), 1.81 h (rat)). The biologically active form, 23 (CT50728), displayed inhibition of platelet aggregation in platelet rich plasma (PRP) with an IC(50) value of 53 nM in citrate buffer, 110 nM in PPACK anticoagulated PRP, and 4 nM in solid-phase GPIIb-IIIa competition binding assay (ELISA). Both 23 and 22 were stable in human liver microsomes, did not inhibit the P450 3A4 isozyme, and had low protein binding (18.22% for 23) and a desirable log P (0.45 +/- 0.06 for 22, and -0.91 +/- 0.32 for 23). It is predicted that the high oral bioavailability for these compounds in multiple species should translate into lower intra- and intersubject variability in man. The long plasma half-life of the lead is consistent with once or twice daily administration for chronic therapy. Analogue 22 (CT51464) thus appears to be a promising oral GPIIb-IIIa inhibitor with significantly improved pharmacokinetic properties over the previously described clinical candidates and may be found useful in the treatment of arterial occlusive disorders.

Disposition of the aldose reductase inhibitor AL01576 in rats.

The disposition of the aldose reductase inhibitor AL01576 was studied in rats following intravenous and oral dosing. Single 4-mg/kg intravenous bolus and oral doses of [14C] AL01576 were administered and levels of radioactivity in blood, excreta, and various tissues were determined over a 168-h period. The decline of radioactivity in blood was quite similar for the two routes of administration, with an apparent half-life of approximately 30 h. At 120 to 144 h, a second, slower elimination phase began that was not fully characterized in the 168-h duration of the study. The HPLC analysis of plasma samples revealed intact AL01576 as the only compound in plasma. The mean plasma parent and radioactivity concentrations are in agreement; suggesting the absence of or an insignificant amount of metabolite in the plasma. The urinary and fecal excretion rate data showed a kinetic pattern similar to that of blood radioactivity. Fecal excretion was the primary route of elimination following both intravenous and oral dosing, accounting for 59% of the administered intravenous dose and 61% of the oral dose. Urinary excretion accounted for 32% of the intravenous dose and 29% of the oral dose. Negligible amounts of radioactivity were recovered as expired 14CO2. Experiments with bile-duct cannulated rats confirmed that the major route of elimination of the drug is biliary excretion. The pattern of distribution of [14C] AL01576 in tissues was quite similar following the two routes of administration. Tissue radioactivity concentration peaked at 4 h (the first sampling time) following both routes of administration in all tissues except the GI tract.(ABSTRACT TRUNCATED AT 250 WORDS)

High-performance liquid chromatographic assay of the aldose reductase inhibitor spiro-(2-fluoro-9H-fluorene-9,4'-imidazolidine)-2',5'-dione (AL01567) in plasma and urine and its pharmacokinetics in humans.

Two selective high-performance liquid chromatographic (HPLC) methods have been developed for the quantitative determination of spiro-(2-fluoro-9H-fluorene-9,4'-imidazolidine)-2',5'-dione (AL01567; 1) in plasma and urine, with an assay sensitivity of 0.25 micrograms/mL for plasma and 0.13 micrograms/mL for urine. The plasma assay procedure involved precipitation of proteins with acetonitrile followed by dilution with water. The diluted supernatant was analyzed on an ODS column eluting with acetonitrile:0.5% phosphoric acid (30:70) adjusted to pH 7.2 with concentrated ammonium hydroxide. The urine assay procedure involved extraction of 1 with 10% n-butanol in hexane, followed by back extraction with 0.05 M sodium hydroxide. The basic extract was neutralized and analyzed on a phenyl column eluting with acetonitrile:10 mM potassium phosphate (30:70; monobasic, pH 5.6). The pharmacokinetics of 1 was investigated in humans following single and multiple oral doses. The elimination half-life from 12 normal subjects following single 100-400-mg oral doses was independent of dose, and the overall mean half-life was 66 +/- 9 h. The overall mean oral clearance (assuming a bioavailability of 100%) was 11 +/- 3 mL/min, and the mean apparent volume of distribution was 59 +/- 13 L. The mean urinary recovery of intact drug during the first 24 h after dosing was 1.2 +/- 0.4% of the administered dose. During once daily 100-mg oral dosing of 1 to five subjects for 21 d, plasma concentrations of 1 reached apparent steady-state by 7 d.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic fate of FCE 22716, a new antihypertensive agent, and of its N-oxide (FCE 24220) in the rat.

[3H]-FCE 22716 and [3H]-FCE 24220 were given both orally and intravenously to the rat. Radioactivity was mainly eliminated by the faecal route after oral administration in both cases. After intravenous administration, renal excretion was twice the faecal one in the case of FCE 22716, whereas for FCE 24220 the two routes were equal. In urine FCE 22716 was eliminated almost completely unchanged after both oral and intravenous administration. FCE 24220 was extensively reduced to FCE 22716 after oral administration, whereas after intravenous treatment, this reduction, although important, was not complete.

Mephenytoin stereoselective elimination in the rat: I. Enantiomeric disposition following intravenous administration.

The stereoselective disposition of mephenytoin was characterized after an intravenous bolus dose of racemic mephenytoin to rats being infused with 50% polyethylene glycol 400/50% saline via the jugular and hepatic portal vein. No significant influence on mephenytoin disposition was noted due to the site selected for the administration of the 50% polyethylene glycol 400 solution. The mean (+/- SD) clearance of R- and S-mephenytoin were 171 +/- 58 ml/hr (R) and 110 +/- 37 ml/hr (S), and the mean (+/- SD) volumes of distribution were 325 +/- 75 ml (R) and 359 +/- 72 ml (S). The clearance of R-mephenytoin was significantly larger than the clearance of S-mephenytoin, but this stereoselective difference is of opposite stereochemistry and of much smaller magnitude than the stereoselective difference reported for these enantiomers in man. The difference in the volumes of distribution of R- and S-mephenytoin was not significant.