Strategy to Assess the Role of (Inter)active Metabolites in Pharmacodynamic Studies In-vivo: a Model Study with Heptabarbital

Abstract

The purpose of this investigation was to develop a universal experimental strategy by which the role of (inter)active metabolites in in-vivo pharmacodynamic studies can be examined. Heptabarbital was chosen as a model drug and several pharmacokinetic variables which may affect in-vivo concentration-pharmacological response relationships were examined. Adult female rats received an i.v. infusion of the drug at one of three different rates (0.225–1.50 mg min−1) until the animals lost their righting reflex (after 11 ± 1 to 88 ± 8 min of infusion). The serum concentration of the drug at onset of loss of righting reflex (LRR) increased slightly with increasing infusion rate. The drug concentrations in brain tissue and cerebrospinal fluid (CSF), (mean ± s.d.: 67 ± 5 mg kg−1 and 24 ± 4 mg L−1, respectively, for the lowest infusion rate) were not affected by the infusion rate. The possible contribution of (inter)active metabolites to the pharmacological response of heptabarbital was determined by administration of different i.v. bolus doses (14.1–22.5 mg) resulting in widely differing sleeping-times (7 ± 3 to 119 ± 20 min). The concentrations of heptabarbital in serum, brain tissue and CSF at offset of LRR (mean ± s.d.: 77 ± 8 mg L−1, 76 ± 7 mg kg−1 and 29 ± 5 mg L−1, respectively, for the highest dose) were not affected by the administered dose. Kinetic analysis of the relationship between dose and the duration of the pharmacological response revealed an elimination half-life of heptabarbital of 2.8 ± 0.2 h, which is in close agreement with the value determined on the basis of the plasma concentration vs time profile following administration of 22.5 mg i.v. (2.8 ± 0.4 h). In a separate investigation no statistically significant differences were observed in heptabarbital concentrations at onset of LRR during an i.v. infusion (0.563 mg min−1) and at offset of LRR following an i.v. bolus dose (22.5 mg; sleeping time: 100 ± 20 min). These results show that (a) there is a rapid equilibration between the concentrations of heptabarbital (heptabarbitone) in CSF and those at the site of action (i.e. the CSF compartment is pharmacokinetically indistinguishable from the site of action), (b) metabolites do not interfere with the pharmacological effect of heptabarbital and (c) within the time-frame of the experiments there is no development of ‘acute’ tolerance to the anaesthetic effect of heptabarbital. It is concluded that a combination of determination of the concentrations at offset of a certain pharmacological effect following administration of different drug doses, and evaluation of the dose vs duration of pharmacological response relationship, can be a powerful tool in examining the role of unknown (inter)active metabolites.