Specific and sensitive assays for digoxin in plasma, urine and heart tissue.

A combined internal standard and marker was added directly to urine, then after mixing, an aliquot was injected directly onto a C18 mu-Bondapak HPLC column. The 4.5 ml digoxin fraction was collected over 1.5 min starting 1 min after the appearance of the marker peak (UV detection), and the digoxin determined in the fraction by radioimmunoassay using 3H-digoxin. Digoxin was extracted from alkalinized plasma into dichloromethane. After evaporating the extract and addition of internal standard the remainder of the assay was as described above for urine. Multiple samples of urine and plasma collected in a single dose bioavailability trial in normal human volunteers were assayed by one of these specific methods as well as by the usual direct nonspecific RIA method. There were no significant differences in results by the specific and nonspecific methods. Heart tissue was homogenized with internal standard and phosphate buffer than the mixture was centrifuged. The supernatant was extracted with dichloromethane, the solvent evaporated from the extract, the residue redissolved in mobile phase and the remainder of the assay carried out as for urine and plasma.