Uremic sera contain inhibitors that block digitoxin-valproic acid interaction.
BACKGROUND
Digitoxin and valproic acid show strong binding to serum albumin. Thus, when present simultaneously in serum, digitoxin and valproic acid compete for binding sites. We studied digitoxin-valproic acid interaction in normal and uremic sera.


METHODS
Fluorescence polarization immunoassays were used for measuring total digitoxin and total valproic acid concentrations. We used a modified protocol of improved sensitivity to measure free digitoxin concentrations. We supplemented 2 normal and 2 uremic pools with digitoxin and then aliquots of these pools were further supplemented with various concentrations of valproic acid. After incubation at 37 degrees C for 2 hours in a water bath, specimens were allowed to re-equilibrate at room temperature for 20 minutes. Free digitoxin concentrations were measured. We also investigated digoxin-valproic acid interaction using 1 normal and 1 uremic serum pool.


RESULTS
We observed significant increases in free digitoxin concentrations in normal sera in the presence of valproic acid. In contrast, we observed a slight decline in free digitoxin concentration in the presence of valproic acid in uremic sera. We speculated that uremic sera contained inhibitors that block digitoxin-valproic acid interaction and identified indoxyl sulfate as an inhibitor. However, another uremic compound, hippuric acid showed no inhibitory effect. Interestingly, we observed no significant interaction between digoxin and valproic acid in either normal or uremic serum pool. This is probably because of poor protein binding of digoxin.


CONCLUSIONS
We conclude that valproic acid significantly displaces digitoxin from protein binding sites in normal serum. However, uremic sera contain inhibitors that block digitoxin-valproic acid interaction.
Dasgupta
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Department of Pathology and Laboratory Medicine, University of Texas Medical School at Houston, 77030, USA. amitava.dasgupta@uth.tmc.edu
Paul
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Wells
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D000927:Anticonvulsants / Q000037:antagonists & inhibitors / Q000097:blood; D001665:Binding Sites; D001667:Binding, Competitive; D002316:Cardiotonic Agents / Q000037:antagonists & inhibitors / Q000097:blood; D004074:Digitoxin / Q000037:antagonists & inhibitors* / Q000097:blood*; D004347:Drug Interactions; D006626:Hippurates / Q000097:blood / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007200:Indican / Q000097:blood / Q000494:pharmacology; D011485:Protein Binding; D012709:Serum Albumin / Q000378:metabolism; D014511:Uremia / Q000097:blood* / Q000188:drug therapy; D014635:Valproic Acid / Q000037:antagonists & inhibitors* / Q000097:blood*
D000927:Anticonvulsants; D002316:Cardiotonic Agents; D006626:Hippurates; D012709:Serum Albumin; D014635:Valproic Acid; D004074:Digitoxin; D007200:Indican; C030514:hippuric acid