Comparison of digitoxin and its major glucuronidated metabolite: inhibition of Na-K-ATPase and reactivity with the radioimmunoassay.

The metabolism of digitoxin is known to occur through several major pathways including oxidation and glucuronidation. Several studies have compared the behavior of the various unconjugated metabolites with respect to reactivity toward Na-K-ATPase and toward the radioimmunoassay (RIA). Other studies with conjugated products synthesized chemically have also been performed. However, the activity of the conjugated products produced in vivo has not been reported and these metabolites are generally assumed to be inactive. In the present studies we have prepared and purified the glucuronide conjugate of digitoxigenin monodigitoxoside formed by rat liver microsomes and determined the activity of this metabolite as measured by inhibition of Na-K-ATPase and by the RIA. The concentration of the conjugated product needed to cause a fifty per cent inhibition of Na-K-ATPase was 5.40 microM compared to 0.68 for digitoxin and 0.08 for digitoxigenin monodigitoxoside. However, the conjugated product had a two-fold greater affinity for the antibody in the RIA procedure than did either digitoxin or digitoxigenin monodigitoxoside. Although these data cannot be extrapolated to the effects of these drugs on cardiac contractility, the results suggest that the contribution of the glucuronide conjugate to the therapeutic or toxic effect of digitoxin may be minimal. This metabolite may, however, lead to inaccurate estimation of blood levels by the RIA.