Iodinated digoxin derivatives with improved reactivity and stability, for use in radioimmunoassay

We report the synthesis and properties of a new digoxin derivative, prepared by reacting cyanogen bromide-activated digoxin with tyramine. The derivative is extremely stable, and its monoiodinated form can be isolated in 85--90% purity by a single thin-layer chromatographic step. [125I]Tyramine-digoxin is immunologically stable for at least four months (immunoreactivities, 90% or greater throughout), with a shelf life of about six months. Nonspecific binding of less than 10% was observed for several batches during this time.