Stereoselective distribution and stereoconversion of zopiclone enantiomers in plasma and brain tissues in rats.

Concentrations of (-)-zopiclone and (+)-zopiclone were determined in plasma and brain after oral administration, to investigate the stereoselectivity of distribution in rats. Zopiclone enantiomers were administered separately to rats and concentrations were determined by chiral HPLC in plasma and brain. In initial experiments, rats were treated with urethane before cannulation for blood sampling but as this drug modified zopiclone pharmacokinetics, it was not used in subsequent studies. This study showed that zopiclone pharmacokinetics after oral gavage in rats are stereoselective. After oral administration of (+)-zopiclone, no stereoconversion was observed in plasma. Conversely, after administration of (-)-zopiclone, both enantiomers were found in plasma and brain with (+)-zopiclone/(-)-zopiclone ratios of 1 and 8.4 in plasma and brain, respectively. Our findings suggest that zopiclone undergoes stereoconversion and that it is stereospecifically distributed to the brain.

Pharmacokinetics of teicoplanin during plasma exchange.

OBJECTIVE: To study the elimination of teicoplanin during plasma exchange, a procedure currently used to treat a variety of disorders involving immune complexes. Teicoplanin is a glycopeptide antibiotic that exhibits a long terminal half-life (100-150 h) and is highly bound to plasma proteins (unbound fraction (fu)=0.2). METHODS: Twelve adults with systemic polyarteritis nodosa, cryoglobulinemia-induced vasculitis or dysglobulinemic neuropathy undergoing plasma exchange were studied. Each patient received intravenous teicoplanin, 6 mg/kg body weight, immediately before plasma exchange. Plasma was assayed for teicoplanin by high pressure liquid chromatography. RESULTS: A high level of protein binding of teicoplanin was measured within this patient population (98%). The mean quantity of teicoplanin eliminated (+/-SD) was 74.6+/-34.6 mg. The mean drug fraction eliminated by plasma exchange (+/-SD) was 19.5+/-5.6%. Mean fu value as determined by ultrafiltration (+/-SD) was 2.2+/-1.7%. CONCLUSIONS: These results show that plasma exchange influences teicoplanin pharmacokinetics, with a clinically significant quantity being eliminated. If trough teicoplanin concentrations of around 10 mg/L are desired, it is recommended that teicoplanin dosage be supplemented or given after plasma exchange.

Pharmacokinetics of once-daily amikacin in pediatric patients.

OBJECTIVE: To study the pharmacokinetic parameters of a once-daily regimen of amikacin (15 mg/kg) in association with other antimicrobial agents in 35 children with severe Gram-negative infections. METHODS: A Bayesian approach was developed to optimize the amikacin regimen. The predictive performance was assessed by computing bias and precision. Each patient was evaluated for toxicity after 5 days of treatment. RESULTS: Peak amikacin concentrations on days 2 and 5 of therapy averaged 31.3 plus minus 9.0 mg/L and 32.4 plus minus 7.4 mg/L, respectively. To achieve peak serum concentrations between 30 and 40 mg/L, individualized dosage was necessary in 19 of 35 children. The pharmacokinetic parameters showed large interindividual variations, with a mean half-life of 2 h and a mean volume of distribution of 0.36 L/kg. No nephrotoxicity was observed in any of the children. After individualization of dosage on the basis of one measurement of peak concentration, no significant differences were observed between predicted and subsequently measured amikacin concentrations. CONCLUSIONS: Once-daily dosage of amikacin (15 mg/kg) is well tolerated in pediatric patients; however, a loading dose of 20 mg/kg is recommended to achieve a therapeutic peak value between 30 and 40 mg/L. Initial serum monitoring is essential in a population such as children, with wide interpatient variability. Using the Bayesian approach, the amikacin regimen in children can then be predicted with minimal bias and good precision.