The reactivity of derivatives of digoxin and digitoxin as measured by the Na-K-atpase displacement assay and by radioimmunoassay.

Derivatives of digoxin and digitoxin were measured by the Na-K-ATPase displacement assay and by radioimmunoassay and the data compared with biological potency of these compounds. There was a slightly higher affinity of digoxigenin-bis-digitoxoside as compared with digoxin using digoxin-specific antiserum and considerably less affinity of digoxigenin and dihydrodigoxin than the parent compound in the same system. A similar trend was observed for the derivatives of digitoxin using digoxin-specific antiserum. The recovery on extraction of some of the derivatives of digoxin and digitoxin differed from that of the parent compounds in the ATPase assay. The potency of the derivatives of these drugs in displacing 3H ouabain also differed from the parent compoundmboth the recovery on extraction and the potency for displacing ouabain must be considered in the estimation of the contribution of the derivatives of digoxin or digitoxin to the result of Na-K-ATPase assay. Quantitative information of the metabolites of digoxin and digitoxin in normal and pathological conditions is needed to properly interpret assay data obtained either by radioimmunoassay or by ATPase assay.