Uremic sera contain inhibitors that block digitoxin-valproic acid interaction

Background: Digitoxin and valproic acid show strong binding to serum albumi n. Thus, when present simultaneously in serum, digitoxin and valproic acid compete for binding sites. We studied digitoxin-valproic acid interaction i n normal and uremic sera. Methods: Fluorescence polarization immunoassays w ere used for measuring total digitoxin and total valproic acid concentratio ns. We used a modified protocol of improved sensitivity to measure free dig itoxin concentrations. We supplemented 2 normal and 2 uremic pools with dig itoxin and then aliquots of these pools were further supplemented with vari ous concentrations of valproic acid. After incubation at 37 degreesC for 2 hours in a water bath, specimens were allowed to re-equilibrate at room tem perature for 20 minutes. Free digitoxin concentrations were measured. We al so investigated digoxin-valproic acid interaction using 1 normal and 1 urem ic serum pool. Results: We observed significant increases in free digitoxin concentrations in normal sera in the presence of valproic acid. In contras t, we observed a slight decline in free digitoxin concentration in the pres ence of valproic acid in uremic sera. We speculated that uremic sera contai ned inhibitors that block digitoxin-valproic acid interaction and identifie d indoxyl sulfate as an inhibitor. However, another uremic compound, hippur ic acid showed no inhibitory effect. Interestingly, we observed no signific ant interaction between digoxin and valproic acid in either normal or uremi c serum pool. This is probably because of poor protein binding of digoxin. Conclusion: We conclude that valproic acid significantly displaces digitoxi n from protein binding sites in normal serum. However, uremic sera contain inhibitors that block digitoxin-valproic acid interaction.