Determination of didanosine in human serum by on-line solid-phase extraction coupled to high-performance liquid chromatography with electrospray ionization tandem mass spectrometric detection: application to a bioequivalence study

A method based on solid-phase extraction coupled to liquid chromatography with positive ion electrospray ionization and tandem mass spectrometric detection was developed for the determination of didanosine in human serum, using lamivudine as internal standard. The acquisition was performed in the multiple reaction monitoring mode, monitoring the transitions m/z 237 --> 136.7 for didanosine and m/z 230 --> 111.7 for lamivudine. The method was linear over the range studied (10-1500 ng ml(-1)), with r(2) > 0.98, and the run time was 5 min. The intra- and inter-assay precisions were < or =10% and the intra- and inter-assay accuracies were >95%. The absolute recoveries were 99.8% (10 ng ml(-1)), 98.4% (30 ng ml(-1)), 91.5% (700 ng ml(-1)) and 94.7% (1200 ng ml(-1)). The limits of detection and quantitation were 5 and 10 ng ml(-1), respectively. The method was applied to a bioequivalence study, in which 24 healthy adult volunteers (12 men) received single oral doses (200 mg) of reference and test didanosine formulations (buffered powder for oral solutions), in an open, two-way, randomized, crossover protocol. The 90% confidence interval of the individual ratios (test formulation/reference formulation) for C(max) (peak serum concentration) and AUC(0-inf) (area under the serum concentration versus time curve from time zero to infinity) were within the range 80-125%, which supports the conclusion that the two formulations are bioequivalent regarding the rate and extent of didanosine absorption.