Monitoring of warfarin therapy by saliva analysis: an open issue on the way to clinical practice

In many clinical conditions, warfarin (WAR) is an essential and not replaceable drug that requires a therapeutic monitoring due to its narrow therapeutic range. The evaluation of the International Normalized Ratio (INR) is the primary assay used for monitoring WAR therapy. New anticoagulants drugs (NOACs) have appeared on the market in the last decade, but they are not expected to replace WAR for a large number of patients due to the following drawbacks: i) cost, ii) collateral effects, iii) absence of specific antidotes and iv) quick loss of the anticoagulant effect if a single dose is missed. The aim of this study was to verify if safety and effectiveness of the WAR therapy could be improved by monitoring the plasma and oral fluid (OF) concentrations of both the WAR and its active metabolites (i.e. warfarin alcohols, WAROHs). For this purpose, nine patients (5 males, 4 females) undergoing WAR therapy were prospectively enrolled for a period of three months. WAR and WAROHs were determined in stimulated OF and plasma samples (both unbound fraction and total content) by high performance liquid chromatography with fluorescence detection after a suitable sample preparation. We found in each patient a different but constant dose-effect ratio of WAR, and that each patient attained the desired anticoagulant effect with a different WAR plasma concentration largely independent from the dose. The INR variations over time mirror the variations of the plasma concentrations of WAR, and in particular of its RS/SR metabolite. Even if statistic is insufficient to draw a firm conclusion, these results suggest that potentially useful clinical information can be obtained from this kind of measurements. For a clinical application, OF analysis would allow minimum invasiveness and easy sampling, which would represent an advantage for the quality of life of patients undergoing a long-term therapy.