title: Pharmacokinetics of thiopental enantiomers during and following prolonged high-dose therapy.
abstract: BACKGROUND
  Thiopental is used as a racemate; however, this is not generally recognized. During conditions of prolonged high-dose therapy, the pharmacokinetics of thiopental may become nonlinear, but whether this derives from one or both enantiomers has not been evaluated. The authors determined the pharmacokinetics of R- and S-thiopental and serum concentrations of R- and S-pentobarbital from prolonged high-dose infusion of thiopental for neuroprotection.
  
  
  METHODS
  Twenty patients received a mean thiopental dose of 41.2 g over a mean duration of 95 h. R- and S-thiopental enantiomer serum concentration-time data from 18 patients were fitted with two models: a linear one-compartment model with first-order output, and a nonlinear one-compartment model with Michaelis-Menten output.
  
  
  RESULTS
  Nonlinear models were preferred in 16 of 18 patients. Paired analysis indicated that steady state clearance (Clss) and volume of distribution (Vd) were higher for R-thiopental (0.108 vs. 0.096 l/min, P < 0.0001; and 313 vs. 273 l, P < 0.0005, respectively); maximal rate of metabolism (Vm) was higher for S- than for R-thiopental (1.01 vs. 0.86 mg x l(-1) x h(-1), P = 0.02); elimination half-lives did not differ (14.6 vs. 14.7 h, P = 0.8); unbound fractions (f(u)) of R- and S-thiopental were 0.20 and 0.18, respectively, P < 0.0001). The differences in mean Clss, Vd and Vm were not significant when adjusted by f(u). Plasma concentrations of R- and S-pentobarbital were relatively small and unlikely to be of clinical significance.
  
  
  CONCLUSIONS
  The pharmacokinetics of R- and S-thiopental became nonlinear at these doses. The pharmacokinetic differences between R- and S-thiopental, although small, were statistically significant and were influenced by the higher f(u) of R-thiopental.
authors.affiliation: Department of Neurology, University of Sydney at Royal North Shore Hospital, St. Leonards, Australia.
authors.forename: D J
authors.initials: DJ
authors.lastname: Cordato
authors.forename: L E
authors.initials: LE
authors.lastname: Mather
authors.forename: A S
authors.initials: AS
authors.lastname: Gross
authors.forename: G K
authors.initials: GK
authors.lastname: Herkes
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000465:Algorithms; D018686:Anesthetics, Intravenous / Q000008:administration & dosage / Q000737:chemistry / Q000493:pharmacokinetics*; D005260:Female; D006207:Half-Life; D006801:Humans; D007262:Infusions, Intravenous; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D018696:Neuroprotective Agents / Q000008:administration & dosage / Q000737:chemistry / Q000493:pharmacokinetics*; D017711:Nonlinear Dynamics; D013237:Stereoisomerism; D013874:Thiopental / Q000008:administration & dosage / Q000737:chemistry / Q000493:pharmacokinetics*
chemical_list: D018686:Anesthetics, Intravenous; D018696:Neuroprotective Agents; D013874:Thiopental