Evaluation of Analytical, Pharmacokinetic and Pharmacodynamic Methods for the Study of Digoxin

The primary objective of the research was to investigate the pharmacodynamics of digoxin in dogs. Initially an assay specific for digoxin in the presence of its major metabolites, viz., digoxigenin, digoxigenin mono-digitoxoside, digoxigenin bis-digitoxoside and dihydrodigoxin was developed using HPLC-RIA. Methodology for non-invasive measurement of left ventricular ejection time (LVET) and other systolic time intervals (STI) in beagle dogs were developed. This involved surgery for exteriorization of the carotid artery in the dogs and subsequent measurements of LVET and STI after recovery. STI, heart rate (HR) and digoxin levels were monitored in normal beagle dogs administered 0.05 mg/kg or 0.025 mg/kg i.v., infused uniformly over a 5 min. period. The STI did not lend itself to pharmacodynamic modelling. The LVET, QS2 and P-R interval were found to be inversely, but linearly, related to the heart rate. Therefore, the bradycardic response to digoxin was extensively investigated in beagle dogs. Pharmacodynamic models evaluated for modelling the bradycardic response to digoxin were: the pharmacokinetic model with a direct linear link, the linear model, the physiologic-pharmacokinetic model with direct linear link and the effect compartment model. The physiologic pharmacokinetic model was simulated using SPICE2 which uses network thermodynamics to simulate biological systems. Criteria for the selection of appropriate models were established. Using the established criteria, the effect compartment model was demonstrated to be the best model. The implications and applications of pharmacodynamic models in general and specifically of the pharmacodynamic model for the bradycardic response to the digoxin are discussed

Phenotypic Differences in Dextromethorphan Metabolism

Polymorphic differences in dextromethorphan metabolism were observed in three studies conducted in a total of 44 subjects (of Dutch origin) administered 60 mg dextromethorphan hydrobromide as an OROS tablet. Mean plasma dextromethorphan (DM) concentrations after a single dose and at steady state were 4–75 times higher in the poor metabolizers (PM) relative to the extensive metabolizers (EM). Following a single dose, the mean areas under the plasma concentration–time curve (AUC, 0–24 hr) of DM, total dextrorphan (DR), and total 3-hydroxymorphinan (HM) were 6.9-fold higher, 17.4-fold lower, and 11-fold lower, respectively, for the PM than for the EM. Correspondingly, steady-state AUC values were 52.8 times higher, 6.7 times lower, and 3.3 times lower for DM, total DR, and total HM, respectively, for the PM relative to the EM. Drug/metabolite ratios (DMR) for amounts excreted in the urine of DR and HM indicated polymorphism in O-demethylation of DM since DMR for PM was 352 and 338 times higher than that for EM for DR and HM, respectively. However, polymorphism in N-demethylation was not observed. Ratios of conjugated/free dextrorphan and 3-hydroxymorphinan excreted in the urine suggest also a lack of conjugative capacity in the PM, relative to the EM. The overall incidence of PM was 9.1% in this populatio