ABSTRACT
The digoxin receptor assays of the late 1960s/early 1970s required large volumes of blood and were very time-consuming. The specificity of the receptor assay using human heart ATPase was significantly better than immunoassays, with minimal interference from digoxin-like immunoreactive factors and cross-reactivity of metabolites being proportional to their pharmacological activity relative to digoxin. The development of immunoassays in the 1950s has greatly improved and enhanced diagnostic ability throughout the entire field of endocrinology. Nevertheless, the receptor assay is tedious, requires human heart ATPase, and is unsuitable for use in the routine clinical laboratory. The metabolite immunosuppressive activity when combined could account for about 25 to 30% of the immunosuppressive activity of the parent cyclosporine present. Many of the drug receptors are membrane-bound, which significantly limits their practical use to develop assays that provide information allowing the therapy of patients receiving these drugs to be optimized.
