Biol. Pharm. Bull. 28(2) 340—343 (2005)

the treatment of congestive heart failure and atrial fibrillation. It is important to measure digoxin in serum from digitalized patients because the therapeutic range of serum digoxin concentration is narrow. Immunoassays are routinely used for therapeutic drug monitoring (TDM) of digoxin. The antibody with cross-reactivity to metabolites of digoxin (digoxigenin bis and monodigitoxosides, digoxigenin, and dihydrodigoxin) matching their relative cardio-activities should be used, and a good correlation must exist between the assay value and the pharmacological effect to avoid toxic dosage in the TDM of digoxin. A receptor radioligand assay, using semi-purified Na /K -ATPase from a human heart, and a commercial immunoassay, the ACS Digoxin assay, using a monoclonal anti-digoxin antibody, satisfied almost all these conditions. We recently reported the preparation of digoxin C-3 and C-3 hemisuccinate–bovine serum albumin (BSA) conjugates, the former of which was then used for generating a new anti-digoxin antiserum. Digoxin concentration in serum from digitalized patients was measured by the radioimmunoassay (RIA) using this antiserum. In the present study, we prepared new anti-digoxin antisera against the BSA conjugates as well as new labeled antigens in which the C-3 or C-3 hemisuccinate derivative of digoxin was coupled with a [H]-leucine. RIA was carried out in homologous and site heterologous combinations between the antiserum and labeled antigen, and one of these systems was found to have a desirable recognition pattern for digoxin metabolites that matches the metabolite pharmacological activity.