Lanoxin

[Digoxin]

Table 1: Comparisons of the Systemic Availability and Equivalent Doses for Oral Preparations of LANOXIN Product Absolute Bioavailability Equivalent Doses (mcg)* Among Dosage Forms LANOXIN Tablets 60 -80% 62.5 125 250 500
LANOXIN Elixir Pediatric 70 -85% 62.5 125 250 500 LANOXICAPS 90 -100% 50 100 200 400 LANOXIN Injection/IV 100% 50 100 200 400

* For example, 125-mcg LANOXIN Tablets equivalent to 125-mcg LANOXIN Elixir Pediatric equivalent to 100-mcg LANOXICAPS equivalent to 100-mcg LANOXIN Injection/IV.

In some patients, orally administered digoxin is converted to inactive reduction products (e.g., dihydrodigoxin) by colonic bacteria in the gut. Data suggest that one in ten patients treated with digoxin tablets will degrade 40% or more of the ingested dose. As a result, certain antibiotics may increase the absorption of digoxin in such patients. Although inactivation of these bacteria by antibiotics is rapid, the serum digoxin concentration will rise at a rate consistent with the elimination half-life of digoxin. The magnitude of rise in serum digoxin concentration relates to the extent of bacterial inactivation, and may be as much as two-fold in some cases.

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Distribution:

Following drug administration, a 6-to 8-hour tissue distribution phase is observed. This is followed by a much more gradual decline in the serum concentration of the drug, which is dependent on the elimination of digoxin from the body. The peak height and slope of the early portion (absorption/distribution phases) of the serum concentration-time curve are dependent upon the route of administration and the absorption characteristics of the formulation.

Clinical evidence indicates that the early high serum concentrations do not reflect the concentration of digoxin at its site of action, but that with chronic use, the steady-state post-distribution serum concentrations are in equilibrium with tissue concentrations and correlate with pharmacologic effects. In individual patients, these post-distribution serum concentrations may be useful in evaluating therapeutic and toxic effects (see DOSAGE AND ADMINISTRATION: Serum Digoxin Concentrations).

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