Steady-state bioavailability of a sustained-release quinidine gluconate formulation was compared with that of a sustained-release quinidine sulfate preparation in a crossover study. Sixteen healthy men were given multiple doses (two tablets every 12 hours) of the two drugs in randomized sequence. Blood samples were obtained immediately before administration of the seventh dose (hour 70) and at 1, 2, 3, 4, 5, 6, 8, 10, and 12 hours after administration. Plasma samples were assayed for quinidine content by high-performance liquid chromatography, with the analyst unaware of the identity of the drug in the sample. On a tablet-for-tablet basis, the sustained-release quinidine gluconate tablets achieved significantly higher plasma levels between two and six hours, higher mean area under the curve, and higher mean maximum concentrations than did the sustained-release quinidine sulfate tablets. When the data were adjusted to correct for the 23% higher quinidine content in the quinidine sulfate tablets, plasma levels from the sustained-release quinidine gluconate were significantly higher than those from quinidine sulfate at all sampling points. Moreover, the sustained-release quinidine gluconate provided significantly greater bioavailability as determined by all pharmacokinetic parameters. Steady-state pharmacokinetics of sustained-release quinidine products cannot be predicted from single-dose studies. The present multiple-dose study demonstrated that, under steady-state conditions, sustained-release quinidine gluconate tablets are more available systemically than sustained-release quinidine sulfate tablets.
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