Digoxin-Specific Antibodies
Pharmacology. Digoxin-specific antibodies are produced in immunized sheep and have a high binding affinity for digoxin and, to a lesser extent, digitoxin and other cardiac glycosides. The Fab fragments used to treat poisoning are derived by cleaving the whole antibodies. Once the digoxin-Fab complex is formed, the digoxin molecule is no longer pharmacologically active. The complex enters the circulation, is renally eliminated and cleared by the reticuloendothelial system, and has a half-life of 14–20 hours (may increase 10-fold with renal impairment). Reversal of signs of digitalis intoxication usually occurs within 30–60 minutes of administration (average initial response, 19 minutes), with complete reversal varying up to 24 hours (average, 88 minutes). There are two commercially available products, Digibind and DigiFab, that should be considered therapeutic equivalents. The decision to use either product may be based on availability and cost.
Indications. Digoxin-specific antibodies are used for life-threatening arrhythmias or hyperkalemia (≥5 mEq/L) caused by acute and chronic cardiac glycoside intoxication (See Digoxin and Other Cardiac Glycosides). Treatment should be based on elevated levels that are at steady state (or are post-distributional), as well as the presence of significant symptoms (eg, hyperkalemia, ventricular arrhythmias, bradyarrhythmias, and hypotension).
Contraindications. No contraindications are known. Caution is warranted in patients with known sensitivity to ovine (sheep) products; a skin test for hypersensitivity may be performed in such patients, with the use of diluted reconstituted drug. There are no reports of hypersensitivity reactions in patients who have received the drug more than once (although this is a theoretical risk). Product may contain traces of papain; therefore, caution is advised in patients with allergies to papain, chymopapain, papaya extracts, and the pineapple enzyme bromelain.
Adverse effects
Monitor the patient for potential hypersensitivity reactions and serum sickness. A dose- and rate-related (anaphylactoid) reaction may occur with rapid IV administration.
In patients with renal insufficiency and impaired clearance of the digitalis-Fab complex, a delayed rebound of free serum digoxin levels may occur for up to 130 hours.
Removal of the inotropic effect of digitalis may exacerbate preexisting heart failure.
With removal of the digitalis effect, patients with preexisting atrial fibrillation may develop an accelerated ventricular response.
Removal of the digitalis effect may reactivate sodium-potassium-ATPase and shift potassium into cells, causing a drop in the serum potassium level.
Use in pregnancy. FDA Category C (indeterminate). This does not preclude its acute, short-term use for a seriously symptomatic patient (See Introduction in Section III).
Drug or laboratory interactions
Digoxin-specific Fab fragments will bind other cardiac glycosides, including digitoxin, ouabain, oleander glycosides, and possibly glycosides in lily-of-the-valley, Strophanthus, squill, and toad venom (Bufo species cardenolides).
The digoxin-Fab complex cross-reacts with the antibody commonly used in quantitative immunoassay techniques. This results in falsely high serum concentrations of digoxin owing to measurement of the inactive Fab complex (total serum digoxin levels may increase 10- to 21-fold). However, some assays and procedures may measure free digoxin levels, which may be useful for patients with renal impairment (to monitor a rebound in free serum digoxin levels after administration of Fab fragments).
Dosage and method of administration. Each vial of either digoxin–immune Fab product binds 0.5 mg of digoxin.
Complete neutralization/equimolar dosing; known level or amount ingested. Estimation of the dose of Fab is based on the body burden of digitalis. This may be calculated if the approximate amount ingested is known (Table III–6) or if the steady-state (post-distributional) serum drug concentration is known (Table III–7
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