Affinity and dose-dependent digoxin Na+K+ATPase dissociation by monoclonal digoxin-specific antibodies

The effect of three monoclonal digoxin-specific antibodies and of polyclonal Digidot as reference on digoxin dissociation from rat brain Na+K+ATPase microsomes was studied to determine the role of the affinity constant (Ka) and dose of the antibody on the rate of digoxin dissociation from Na+K+ATPase. Stoichiometrical doses of 1C10, 6C9, 9F5 IgG, and Digidot (Ka = 6 10(9), 3.1 10(8), 2.5 10(7), and 8.5 10(9) M-1, respectively) resulted in digoxin dissociation related to Ka. When the IgG:digoxin molar ratio increased from 0.25 to 10, digoxin dissociation from Na+K+ATPase sites also increased according to the Hill equation, allowing comparative parameters among the three antibodies to be determined. 1C10 IgG was 2- and 10-fold more efficacious than 6C9 and 9F5, respectively. This in vitro model appears to be a useful predictive screening assay before in vivo experimentation.