Stereoselective pharmacokinetics of oxprenolol and its glucuronides in humans.

OBJECTIVE: To study the pharmacokinetics of R(+)- and S(-)-oxprenolol and their corresponding glucuronide conjugates in healthy subjects. METHODS: An oral dose of 80 mg racemic oxprenolol was given to eight male volunteers. Venous blood samples and urine were collected as a function of time. Oxprenolol enantiomers in plasma and urine were determined by an enantiospecific HPLC method. Oxyprenolol glucuronides in plasma and urine were measured as oxprenolol equivalents after enzymatic hydrolysis. RESULTS: For R-oxprenolol the area under the plasma concentration-time curve was slightly higher (R/S ratio, 1.19) and the oral clearance slightly lower (R/S ratio, 0.84) than those parameters for S-oxprenolol. The free fraction of R-oxprenolol in plasma was 4% higher than that of S-oxprenolol. The intrinsic clearance of S-oxprenolol was 1.5 times larger than that of R-oxprenolol, and a maximum of 3% of the dose was excreted as unchanged enantiomers in the urine. The plasma concentrations of S-oxprenolol glucuronide were more than three times higher than those of R-oxprenolol glucuronide. Twenty-five percent of the dose of the R-enantiomer was excreted in the urine as R-oxprenolol glucuronide; 29% of the S-enantiomer dose was excreted as S-oxprenolol glucuronide. The renal clearance of R-oxprenolol glucuronide was, on average, 172 ml/min, suggesting active tubular secretion. In contrast, the renal clearance of S-oxprenolol glucuronide was only 49 ml/min, which can be explained by the plasma binding of the compound. CONCLUSIONS: Our results show small differences in disposition between R- and S-oxprenolol but a marked difference in disposition between the glucuronides. The difference in plasma concentrations between the oxprenolol glucuronides is mainly attributable to the stereoselectivity of the renal excretion.

Direct high-performance liquid chromatography determination of diastereomeric oxprenolol glucuronides.

The beta-blocking agent oxprenolol is used therapeutically as the racemate. In humans and animals it is metabolized i.a. to ether glucuronide diastereomers. A stereoselective HPLC assay was developed to determine directly, without hydrolysis to their parent enantiomers, the oxprenolol glucuronides in biological samples. The glucuronide standards for this direct assay are prepared by incubation of rabbit liver microsomes with RS-oxprenolol. The glucuronides obtained are purified and concentrated with solid-phase extraction, and their concentration is measured by an indirect method, i.e. HPLC assay of the oxprenolol enantiomers after enzymatic hydrolysis with beta-glucuronidase. The direct assay involves separation by HPLC using a C18-reversed-phase column, with UV detection at 274 nm; nalorphine is used as internal standard. On injection onto the column, without previous hydrolysis, the limit of detection is 20 ng for both glucuronides. The assay is sensitive, accurate and reproducible. The method is suitable for the assay of glucuronides in liver microsomal incubates and plasma.

Influence of endotoxin on the stereoselective pharmacokinetics of oxprenolol, propranolol, and verapamil in the rat.

The influence of endotoxin-induced inflammation on the enantioselective pharmacokinetics of propranolol, oxprenolol, and verapamil, which bind to alpha 1-acid glycoprotein, was studied in the rat. The racemic mixtures were given orally. In the control animals, for propranolol and oxprenolol, the plasma concentrations of the (R)-enantiomer were higher than those of the (S)-enantiomer, while for verapamil the reverse was true. Protein binding and intrinsic clearance are the main factors responsible for this enantioselectivity. After endotoxin treatment, for the three drugs tested the plasma concentrations and the plasma binding of both enantiomers were significantly increased. This effect was more pronounced for (R)-propranolol, (R)-oxprenolol, and (S)-verapamil than for their respective antipodes. The enantioselective effect of endotoxin on the plasma concentrations of the drugs studied seems mainly due to the enantioselective increase in binding to alpha 1-acid glycoprotein.

Chiral high-performance liquid chromatographic determination of oxprenolol in plasma.

A sensitive, stereospecific high-performance liquid chromatographic assay for oxprenolol enantiomers in rat plasma was developed, using a chiral derivatization agent. Racemic oxprenolol and the internal standard (racemic propranolol) are extracted with dichloromethane after alkalinization of the plasma. Quantitation of R(+)- and S(-)-oxprenolol is based on derivatization with the chiral agent S(-)-1-(1-naphthyl)-ethyl isocyanate, followed by chromatographic separation on a C18 reversed-phase column, with fluorometric detection (excitation at 226 nm, emission at 333 nm). The assay is reproducible as judged by a coefficient of variation of less than 17.5% for both enantiomers at all concentrations used. Preliminary experiments in the rat demonstrate that the method is sufficiently sensitive for pharmacokinetic studies in that species.

Stereoselective pharmacokinetics of oxprenolol, propranolol, and verapamil: species differences and influence of endotoxin.

The influence of endotoxin-induced inflammation was studied on the pharmacokinetics of the enantiomers of the racemic drugs oxprenolol, propranolol, and verapamil in rabbits and dogs. Enantioselective pharmacokinetics were seen for oxprenolol and propranolol in the rabbit and for propranolol and verapamil in the dog. In the dog, the enantioselective differences in plasma concentrations are due to differences in both protein binding and metabolism, whereas in the rabbit the differences are due solely to differences in metabolism. In both species endotoxin treatment increases the plasma concentrations of the enantiomers of the three drugs; both protein binding and metabolism are influenced. In rabbits and in dogs, the influence of endotoxin on the disposition of the three drugs is less enantioselective than was previously observed in the rat.

Effect of probenecid on the enantioselective pharmacokinetics of oxprenolol and its glucuronides in the rabbit.

PURPOSE: To study the effect of probenecid on the stereoselective pharmacokinetics of oxprenolol and its glucuronides in the rabbit. METHODS: An oral dose of 50 mg/kg racemic oxprenolol was given to nine rabbits twice, in random sequence with and without the concurrent administration of probenecid. Oxprenolol enantiomers were determined in plasma and urine by an enantioselective HPLC method. Oxprenolol glucuronides were measured in plasma and urine after enzymatic hydrolysis. RESULTS: The disposition of the oxprenolol enantiomers in rabbits is stereoselective, mainly due to a difference in metabolism. Renal excretion is only a minor elimination route for unchanged oxprenolol, and the renal clearances of the enantiomers are similar. Pretreatment with probenecid did not affect the plasma concentrations of the oxprenolol enantiomers, but there was a slight decrease in their urinary excretion. The plasma concentrations of the oxprenolol glucuronides are much higher than those of the parent enantiomers, and those of (S)-glucuronide are about twice those of its antipode. About 10% of the oxprenolol dose is excreted in the urine as glucuronides. The renal clearances of both glucuronides are similar, and markedly higher than the creatinine clearance. After probenecid, the mean glucuronide plasma levels were markedly higher, with for both glucuronides a more than twofold increase in mean AUC. Probenecid decreased the renal clearance of both glucuronides to about 30%. Moreover, it decreased slightly the formation clearance of (S)-glucuronide, while the formation clearance of (R)-glucuronide was not significantly influenced. CONCLUSIONS: Our results show that in the rabbit, both oxprenolol glucuronide diastereomers are actively secreted by the kidney, and that this process is inhibited by probenecid.