Reversal of Advanced Digitoxin Toxicity and Modification of Pharmacokinetics by Specific Antibodies and Fab Fragments

Abstract

A B S T R A C T The effects of Fab fragments of highaffinity specific antibodies have been studied in a canine experimental model of lethal digitoxin toxicity. Selected antiserum from sheep immunized and boosted with a digoxin-serum albumin conjugate contained antibodies that cross-reacted with digitoxin with an average intrinsic association constant of 1.4 x 10"0 M-1 as determined by equilibrium dialysis.Rapid second-order association kinetics (kf = 3.7x 106 M-1 per s) and slow dissociation kinetics (kr = 1.9 X 10-4 per s) were documented for the antibody-digitoxin complex. Eight dogs given 0.5 mg/ kg digitoxin intravenously developed ventricular tachycardia after 23±4 (SEM) min. Control nonspecific Fab fragments were then given. All animals died an average of 101+36 min after digitoxin administration. Another eight dogs given the same digitoxin dose similarly developed ventricular tachycardia after 28+3 min. This group then received a molar equivalent dose of specific Fab fragments intravenously over 3 min, followed by a 30-min infusion of one-third of the initial dose. All dogs survived. Conducted sinus beats reappeared 18+4 min after initial Fab infusion, and stable normal sinus rhythm was present at 54 ± 16 min. Plasma total digitoxin concentrations increased threefold during the hour after initial Fab infusion, while plasma free digitoxin concentration decreased to less than 0.1 ng/ml. Effects on digitoxin pharmacokinetics of these Fab fragments and the antibody population from which they were derived were further investigated in a primate species. Unlike common laboratory animals previously studied, the rhesus monkey was found to have a prolonged elimination half-life, estimated at 135Dr. Ochs was a Fellow of Deutsche Forschungsgemeinschaft.Received for publication 20 January 1977 and in revised form 5 July 1977. and 118 h by radioimmunoassay and [3H]digitoxin measurements, respectively, similar to man and thus providing a clinically relevant experimental model. Intravenous administration of 2 mol of specific Fab fragments per mole of digitoxin 6 h after 0.2 mg of digitoxin produced a rapid 4.3-fold increase in plasma total digitoxin concentration followed by a rapid fall (t1 4 h) accompanied by a 14-fold enhancement of urinary digitoxin excretion over control values during the 6-h period after Fab was given. Analytical studies were consistent with increased excretion of native digitoxin rather than metabolites, and the glycoside was found in equilibrium dialysis studies to be excreted in the urine in Fab-bound form. Administration of 2 mol of specific antibody binding sites per mole of digitoxin as intact IgG caused a greater and more prolonged increase in plasma total digitoxin concentration, peaking 13-fold above control levels. In contrast to the effects of Fab, however, specific IgG reduced the rate of urinary digitoxin excretion substantially below control values. We conclude that Fab fragments of antibodies with high affinity for digitoxin are capable of rapid reversal of advanced, otherwise let...

Immunopharmacotherapeutic manifolds and modulation of cocaine overdose

“…With respect to the former, a few studies have explored the therapeutic benefit of short-acting monovalent antibody fragments over the whole IgG for rapid small-molecule detoxification (e.g. exogenous toxins, pharmaceutical and abused drugs) (Covell et al, 1986; Eddleston and Persson, 2003; Holliger and Hudson, 2005; Ochs and Smith, 1977). Of particular relevance, a comparison between digoxin-specific IgG and Fab fragments in the treatment of acute digoxin toxicity depicted the more rapid and less variable reversal of digoxin-induced cardiac arrhythmias and lethality by Fab infusion than by the equivalent dose of IgG (Lloyd and Smith, 1978).…”

Reversal of Digitalis Effects by Specific Antibodies

“…In experiments with guinea pigs, toxic ventricular fibrillation in duced by /3-methyldigoxin was successfully abolished 12 min after injection of Fab frag ments of a digoxin-specific antibody (26). In a similar fashion toxic ventricular tachycardia could rapidly be terminated in digitoxinintoxicated dogs following intravenous infusion of digoxin-specific Fab fragments cross-reacting with digitoxin (32).…”

“…The influence of Fab fragments of digoxinspecific antibodies on pharmacokinetics and renal excretion o f digitoxin was investigated in detail in rhesus monkeys having an elimination half-life comparable to man (32). Intravenous administration of Fab fragments in those ani mals pretreated with digitoxin resulted in a 4.3-fold increase of total digitoxin plasma con centration, a 14-fold enhancement of urinary digitoxin excretion, and a decrease of serum elimination half-life to 4 h. Thus, in contrast to intact digoxin-specific antibodies Fab fragments arc not only capable of rapidly reversing digoxin-and digitoxininduced toxicity, but are also able to eliminate the glycoside promptly from the organism via tire renal route, much of it in the protein-bound form.…”

Monoclonal antibodies in drug targeting

“…Essentially, all of the digoxin was bound to antibody fragments and therefore rendered inactive. The rise in digoxin concentration suggests that some digoxin is extracted from tissues (102,103). Treatment and management of life threatening digoxin overdose is now by administration of digoxin-specific Fab fragments (104).…”