Bioavailability of dextromethorphan (as dextrorphan) from sustained release formulations in the presence of guaifenesin in human volunteers.

A multiple dose bioavailability study with six healthy male human volunteers was conducted. The bioavailability of an experimental sustained release tablet containing dextromethorphan hydrobromide (DXP-HBr), was compared with a marketed sustained release DXP-HBr suspension in a three-way crossover study. Plasma samples, collected serially after oral drug administration, were analysed for the major metabolite of dextromethorphan (DXP), dextrorphan (DX), using a specific HPLC method with fluorescence detection. The bioavailability parameters; area under the concentration-time curve (AUC), maximum plasma concentration (Cmax), and time to peak (Tmax), were obtained from the plasma concentration-time data. Additionally, pharmacokinetic parameters such as mean residence time (MRT), accumulation factor (R), fluctuation index (Fi), total body clearance (Cl), and the average concentration (C) were estimated by using model independent kinetics approach. Analysis of variance of the data revealed that the presence of guaifenesin in the test formulation does not appear to have a statistically significant (p > 0.05) effect on the bioavailability of dextromethorphan as dextrorphan. The relative bioavailability of the tablet dosage form with respect to the suspension was found to be 113% on Day 1 and 110% on Day 6.