Abstract
The pharmacodynamic behavior of orally administered chlorzoxazone (CZX) was studied in rats. From the time course of CZX plasma concentration data under alpha-chloralose (80 mg/kg, i. p.) anesthesia, it was found that CZX obeyed a one-compartment model with first-order absorption. The pharmacological response intensity of CZX on the crossed extensor reflex was closely related to the plasma concentration data via Hill's equation under alpha-chloralose (80 mg/kg) anesthesia, but not at a 150 mg/kg dose. The influence of alpha-chloralose at the latter dose on CZX pharmacokinetics and pharmacodynamics appeared to be due to the pharmacodynamic interaction of alpha-chloralose and CZX, thus suggesting that the pharmacokinetic and pharmacodynamic concept proposed by Smolen was not applicable to CZX's behavior at such a dose in rats. Under alphachloralose (80 mg/kg) anesthesia, the biophase compartment was determined to be identical to the central compartment using our proposed model. On the basis of the effect of anesthetics on drug behavior, one may select an appropriate anesthetic dose to evaluate the relationship between the plasma levels and the onset and duration of the drug action. At the higher dose of alpha-chloralose (150 mg/kg), the free fraction of CZX was increased and a possible enhancement in CZX action was suggested.
