Effect of Indian Ayurvedic medicine Ashwagandha on measurement of serum digoxin and 11 commonly monitored drugs using immunoassays: study of protein binding and interaction with Digibind.

CONTEXT Ashwagandha, a popular Ayurvedic medicine, is now available in the United States. Alkaloids found in this herb have structural similarity with digoxin. OBJECTIVE To study potential interference of Ashwagandha with serum digoxin measurement by immunoassays. Potential interference was also investigated with immunoassays for 11 other commonly monitored drugs. In addition, interaction of components of Ashwagandha with the Fab fragment of antidigoxin antibody (Digibind) was investigated. DESIGN Two different brands of liquid extract and 1 dry powdered form of Ashwagandha were used for this investigation. Aliquots of drug-free serum were supplemented with various concentrations of Ashwagandha and apparent digoxin concentrations were measured by 3 digoxin immunoassays. Mice were fed with Ashwagandha and apparent digoxin concentrations were measured 1 and 3 hours after feeding. Potential interference of Ashwagandha with immunoassays of 11 other drugs was also investigated. Interaction of components of Ashwagandha with Digibind was studied in vitro. RESULTS Significant apparent digoxin concentrations were observed both in vitro and in vivo using the fluorescence polarization immunoassay of digoxin, whereas the Beckman and the microparticle enzyme immunoassay digoxin assay demonstrated minimal interference. Immunoassays of 11 other drugs tested were unaffected. When Ashwagandha extract was added to a serum pool containing digoxin, falsely elevated digoxin value was observed with fluorescence polarization immunoassay, but values were falsely lowered when measured by the microparticle enzyme immunoassay. Digibind neutralized digoxin-like immunoreactive components of Ashwagandha in vitro. CONCLUSIONS Components of Ashwagandha interfered with serum digoxin measurements using immunoassays. Digibind neutralized free digoxin-like immunoreactive components of Ashwagandha.