Saliva concentrations of disopyramide cannot substitute the drug's plasma concentrations

For many drugs the salivary concentration corresponds to the free plasma drug concentration, which may be more closely related to drug activity or toxicity than the total plasma drug concentration. In this study a preliminary investigation was undertaken to determine the feasibility of monitoring saliva levels of disopyramide, an antiarrhythmic drug, for clinical and toxicological purposes. Single oral doses of this compound were administered to healthy volunteers. Stimulated mixed saliva and plasma levels were measured by the EMIT technique. The concentrations of disopyramide in the stimulated mixed saliva tended to be lower than those found in the corresponding plasma sample (fp 0.3-0.5), and the saliva-to-plasma concentration ratio increased with a decreasing salivary pH (pH 6.89, S/P = 0.25; pH 8.15, S/P = 0.08). The correlation between the saliva and the total plasma concentrations was significant but relatively poor, however. Consequently, mixed salivary disopyramide concentrations are a poor indicator of plasma concentrations, even if correction is made for pH change.