In vitro plasma protein binding of zileuton and its N-dehydroxylated metabolite.

An ultrafiltration technique or equilibrium dialysis has been used to study the in vitro human plasma protein binding of racemic zileuton, its individual enantiomers, and its pharmacologically inactive metabolite N-dehydroxyzileuton. The plasma protein binding of zileuton and N-dehydroxyzileuton over the concentration range of 0.1 to 100 mg/L averaged 93.1 +/- 0.22 and 92.0 +/- 0.12%, respectively. However, there appeared to be a stereoselective effect, with the R(+) enantiomer of zileuton demonstrating greater binding to plasma proteins than the S(-) enantiomer (96 vs 88%, respectively). Zileuton was bound to both human serum albumin (40 g/L) and alpha 1-acid glycoprotein (1 g/L), although binding affinity to albumin was approximately 3-fold greater. Displacement interactions of zileuton with warfarin, salicylate, theophylline, naproxen, ibuprofen, prednisone, and terfenadine were minimal. The blood to plasma concentration ratio for zileuton and N-dehydroxyzileuton ranged from 0.65 to 0.68, indicating that these compounds were mainly distributed in the plasma. Thus, zileuton is approximately 93% bound to plasma proteins at expected therapeutic concentrations in vitro, and this figure is largely unaffected by several commonly prescribed agents with which the drug may be coadministered.

Stereoselective pharmacokinetics of pazinaclone, a new non-benzodiazepine anxiolytic, and its active metabolite in healthy subjects.

1. Serum and urine concentrations of enantiomers of pazinaclone (DN-2327) and an active metabolite MII, were measured after single and twice daily oral doses of 4 and 8 mg racemic drug to healthy subjects. 2. The kinetics of rac-pazinaclone and rac-MII were dose-independent and no unchanged drug was recovered in urine. 3. The terminal elimination half-lives of the drug isomers were similar (about 10.5 h), but mean steady-state values of AUC were twofold higher for the S-isomer than those of the antipode (e.g., 8 mg dose: 127 vs 69 ng ml(-1) h). However, the corresponding AUC values based upon unbound drug were similar (5.71 vs 5.73 ng ml(-1) h) indicating no stereoselectivity in intrinsic metabolic clearance. 4. The terminal elimination half-lives of S- and R-MII were similar to those of parent compound indicating that the elimination of these metabolites is formation rate-limited. 5. The R:S-ratio for the AUCs of MII was 4:1. Both enantiomers were excreted in the urine mainly as glucuronide conjugates, with stereoselectivity toward S-MII. 6. Since only the S-enantiomers of DN-2327 and MII bind to the benzodiazepine receptor, further measurements of drug effect in patients should be related to combine serum concentrations of the S-enantiomers of both parent drug and MII.

Differences in the stereoselective pharmacokinetics of pazinaclone (DN-2327), a new anxiolytic, and its active metabolite after intravenous and oral single doses to dogs.

The stereoselective pharmacokinetics of the new nonbenzodiazepine anxiolytic compound pazinaclone (DN-2327) were studied in four beagle dogs after oral (2.5 mg/kg) and intravenous (0.5 mg/kg) administration of racemate in a two-way, crossover study design. Racemic pazinaclone was highly cleared after intravenous administration at 2.09 +/- 0.78 l hr-1 kg-1. The total clearance and volumes of distribution (Vc, V beta, and Vss) of (S)-pazinaclone were significantly lower than those of the antipode. The differences in disposition were consistent with stereoselectivity in protein binding, where the unbound fraction of (R)-pazinaclone was almost 5-fold greater than that of the (S)-enantiomer. Lower clearance and distribution for (S)-pazinaclone resulted in comparable elimination half-lives for the two enantiomers. As projected from the intravenous results, the firstpass metabolism of (S)- and (R)-pazinaclone on oral administration of racemic pazinaclone was very extensive and stereoselective, with mean bioavailabilities of 6.0 and 1.2%, respectively, but the rates of absorption of the enantiomers were similar. Simultaneous model-dependent analysis of the intravenous plasma profiles for parent drug and metabolite suggested stereoselectivity of the active metabolite MII with shorter formation half-life for (S)-MII. However, in vitro metabolism by liver slices and our in vivo data indicated exclusive elimination of (S)- and (R)-pazinaclone through complete conversion to the MII metabolite (fm = 1). Thus, the clearances of (S)- and (R)-MII were calculated to be 0.89 and 7.89 l hr-1 kg-1, respectively, indicating pronounced stereoselectivity in the metabolite clearance.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism and disposition of clarithromycin in man.

The metabolic fate and pharmacokinetics of clarithromycin following a single 250- or 1200-mg oral dose of 14C-clarithromycin were studied in six healthy adult males. Peak plasma levels of clarithromycin averaged 0.6 microgram/ml after the low dose and 2.7 micrograms/ml after the high dose. The AUC of clarithromycin increased 13-fold, with the 4.8-fold increase in dose, while the plasma half-life increased from 4.4 hr to 11.3 hr. The major metabolite in plasma and urine was the microbiologically active 14-hydroxylated-R epimer of clarithromycin. After 5 days, a mean of 38% of the low dose (18% as clarithromycin) and 46% of the high dose (29% as clarithromycin) was recovered in the urine, with approximately one-third eliminated during the first 24 hr. The nature of the urinary and fecal metabolites revealed the involvement of three metabolic pathways, viz. 1) hydroxylation at the 14-position to form the R and S epimers, 2) N-demethylation, and 3) hydrolysis of the cladinose sugar. Secondary metabolism via these pathways was also evident. The overall recovery of metabolites, but not total radioactivity, decreased 42% after the high dose. The nonlinear pharmacokinetic behavior of clarithromycin and the decrease in metabolite production suggest that clarithromycin metabolism can be saturated at high doses.

Renin inhibitors. Dipeptide analogues of angiotensinogen utilizing a structurally modified phenylalanine residue to impart proteolytic stability.

A series of renin inhibitors have been prepared and evaluated for their susceptibility to cleavage by the serine protease chymotrypsin. The compounds were designed by consideration of the structural requirements in the active-site region of renin and chymotrypsin. By systematic alteration of the P3 phenylalanine residue, compounds with varying degrees of renin inhibitory potency and chymotrypsin susceptibility were obtained. Selected analogues from this group were examined in vivo for both their hypotensive effects and metabolic patterns.

Optimization and in vivo evaluations of a series of small, potent, and specific renin inhibitors containing a novel Leu-Val replacement.

Further structure-activity relationships (SAR) for a novel dipeptide series inhibitors are reported. These inhibitors retain the Phe8-His9 portion of angiotensinogen and employ a unique Leu10-Val11 replacement [(LVR), ref 2]. SAR at the Leu10 side chain revealed that the LVR derived from cyclohexylalanine provided a nearly 10-fold boost in potency for the final inhibitor. In addition SAR work was carried out to delineate the relationships between binding potency and (1) the size, shape, and charge of the side chain at the His9 position; (2) the size and topology of the side chain at the Phe8 site; and (3) the size of the Phe8 N-protecting group. One of the more potent inhibitors, 12, was shown to provide a substantial antihypertensive effect in a sodium depleted monkey model when administered intravenously. Metabolism work, in Sprague-Dawley rats, provided insights into the susceptibility of 12 to significant hepatic clearance and provided encouraging evidence for intestinal absorption.

Metabolism of 14C-estazolam in dogs and humans.

14C-Estazolam (2 mg) administered orally to dogs and human subjects was rapidly and completely absorbed with peak plasma levels occurring within one hour. In humans, plasma levels peaked at 103 +/- 18 ng/ml and declined monoexponentially with a half-life of 14 h. The mean concn. of estazolam in dog plasma at 0.5 h was 186 ng/ml. Six metabolites were found in dog plasma at 0.5 and 8 h, whereas only two metabolites were detected in human plasma up to 18 h. Metabolites common to both species were 1-oxo-estazolam (I) and 4-hydroxy-estazolam (IV). Major metabolites in dog and human plasma were free and conjugated 4-hydroxy-estazolam; the concn. were higher in dogs. After five days, 79% and 87% of the administered radioactivity was excreted in dog and human urine, respectively. Faecal excretion accounted for 19% of the dose in dog and 4% in man. Eleven metabolites were found in the 0-72 h urine of dogs and humans; less than 4% dose was excreted unchanged. Four metabolites were identified as: 1-oxo-estazolam (I), 4'-hydroxy-estazolam (II), 4-hydroxy-estazolam (IV) and the benzophenone (VII), as free metabolites and glucuronides. The major metabolite in dog urine was 4-hydroxy-estazolam (20% of the dose), while the predominant metabolite in human urine (17%) has not been identified, but is likely to be a metabolite of 4-hydroxy-estazolam. The metabolism of estazolam is similar in dog and man.

Toxicological aspects of cyclamate and cyclohexylamine.

In the late 1960s the artificial sweetener cyclamate was implicated as a bladder carcinogen in rats. This finding and other concerns about its safety ultimately led to a ban on cyclamate in the U.S. and restrictions on its use in many other countries. Since that time, the carcinogenic potential of cyclamate and cyclohexylamine, its principal metabolite, has been reevaluated in a group of well-controlled, well-designed bioassays that have failed to substantiate the earlier findings. This review of the published and unpublished literature on cyclamate attempts to evaluate the carcinogenicity question and other important aspects of the toxicity of cyclamate and cyclohexylamine, including their effects on various organ systems, their genotoxic potential, and their effects on reproduction. In addition, the physiological disposition of cyclamate is reviewed, with particular attention directed toward the site and extent of its conversion to cyclohexylamine.

Metabolism of [14C]Cefmenoxime in normal subjects after intramuscular administration.

The metabolism of cefmenoxime (SCE-1365) was studied in four healthy male volunteers after intramuscular administration of a single 500-mg dose of the 14C-labeled drug. Plasma levels of total radioactivity and cefmenoxime peaked at 0.5 and 1.0 h, corresponding to 16.5 micrograms eq/ml and 15.8 micrograms/ml, respectively. Thereafter, parent drug levels declined rapidly, with a terminal elimination half-life of ca. 1.5 h. No significant differences were noted between total radioactivity and parent drug levels up to 2 h after drug administration. After 3 h, low but persistent levels of radioactivity were significantly greater than parent drug levels, indicating metabolism or degradation of cefmenoxime. The terminal elimination half-life of total radioactivity was estimated to be ca. 40 h. The radioactive plasma metabolite(s) remaining at the end of the 5-day study represented only 1% of the administered dose. Urinary excretion was the major route of elimination of cefmenoxime, accounting for ca. 86% of the dose in 12 h. Analysis of cefmenoxime in urine by total radioactivity, high-pressure liquid chromatography, and a microbiological assay showed that 80 to 92% of the excreted dose was parent drug. Radioactivity was also excreted into the feces via the bile and represented ca. 11% of the dose after 5 days. Although extensive degradation of cefmenoxime was found in fecal samples, it was proposed that this may be due to the metabolic activity of the intestinal flora rather than in vivo biotransformation in the liver. This study supports the concept that cefmenoxime undergoes minimal metabolism in humans and is excreted largely as unchanged drug.

Pharmacokinetics of fostedil, a new calcium antagonist, in beagle dogs following oral and intravenous administration.

Twelve adult beagle dogs received both an oral and intravenous dose (12 mg/kg) of fostedil in a cross-over design. The plasma levels and urinary excretion of intact fostedil were measured, and the pharmacokinetic parameters of the drug were defined. The results indicate that fostedil was at least 68% bioavailable after an oral dose given as a suspension or solution. The terminal half-life was about 7-8 h. The in vitro protein binding, at concentrations of 0.1-100 micrograms/mL, ranged from 95 to 97%. The binding was not concentration dependent, and saturation of the binding sites was not apparent at concentrations up to 100 micrograms/mL. Excretion of unchanged drug from the kidneys accounted for only a small percentage of drug clearance.

Intraocular penetration of topically applied [14C]fosfonet sodium in rabbits.

The intraocular penetration of [14C]fosfonet was studied following topical application of 25 mg of an ointment containing 5% [14C]fosfonet sodium onto the intact and abraded eyes of New Zealand white rabbits. Radioactivity penetrated rapidly through the abraded cornea and entered the anterior chamber. The concentration of fosfonet in the aqueous humor peaked at 7.2 microgram/ml by 90 min. Assuming an aqueous humor volume of 300 microliter, this level would correspond to approximately 0.2% of the applied dose. The highest concentration of fosfonet found in the abraded cornea was 0.3 microgram/mg, or 0.4% of the applied dose. The half-lives for the elimination of fosfonet from the aqueous humor and cornea were about 2.5 and 2.7 hr, respectively. The fosfonet levels in the iris were extremely low throughout the 6-hr period. The penetration of [14C]fosfonet through the intact cornea was considerably less than that found in the abraded eye. The peak concentrations of fosfonet in the aqueous humor and cornea of the intact eye were 0.26 microgram/ml and 0.02 microgram/mg, respectively, and occurred within 10 min of application of ointment.

Local anesthetic activity and acute toxicity of N-substituted 1,2,3,4-tetrahydro-1- and 2-naphthylamines.

Seven N-substituted 1,2,3,4-tetrahydro-1- and three 2-naphthylamines were prepared and tested for local anesthetic activity in the rabbit corneal reflex test and the mouse sciatic nerve block test. At 0.1 and 1%, three 1-alkylamino compounds had durations of action comparable to that of tetracaine in the rabbit corneal reflex test and were considerably more potent than lidocaine. The other four 1-alkylamino derivatives were inactive or at best minimally active. The durations of action of 1% concentrations of the three 2-alkylamino compounds were equivalent to that of 1% lidocaine in the corneal reflex test. In the mouse sciatic nerve block test, the three active 1-alkylamino compounds were considerably longer acting than either tetracaine or lidocaine. Three 1-alkylamino and the three 2-alkylamino compounds showed toxicity equal to or greater than lidocaine, while two 1-alkylamino and two 2-alkylamino compounds showed toxicity equal to or greater than tetracaine by the intraperitoneal route in mice. N-Heptyl-1,2,3,4-tetrahydro-6-methoxy-1-naphthylamine methanesulfonate was the most promising local anesthetic in these series.