Reversal of ouabain and acetyl strophanthidin effects in normal and failing cardiac muscle by specific antibody

Monoclonal antibodies to T-2 toxin. In vitro neutralization of protein synthesis inhibition and protection of rats against lethal toxemia

A murine monoclonal antibody (15H6) against the trichothecene mycotoxin T-2 was capable of neutralizing the in vitro protein synthesis inhibitory effect of T-2 toxin in human B lymphoblastoid cultures. It was further shown that 15H6 given to rats (250 mg/kg) 30 min before or 15 min after a lethal dose (1 mg/kg) of T-2 toxin conferred 100% survival. A lower dose of 15H6 (125 mg/kg), given 15 min after the lethal dose of T-2 toxin, protected 25% of the rats. An increased time to death and 45% survival was seen in rats given the full dose of 15H6 antibody 60 min after lethal toxin. These data are the first demonstration of effective prophylaxis and therapy for T-2 toxemia.

New advances in the assessment and treatment of digitalis toxicity

The narrow margin between the therapeutic and toxic doses and serum levels of cardiac glycosides results in a high incidence of digitalis toxicity. This common problem has led to the development of methods for determining serum glycosides concentrations. It is clear that overlap of serum digoxin levels occurs between groups of patients with and without evidence of toxicity. In spite of these difficulties, use of serum digoxin measurement has been reported to be associated with a lower incidence of digitalis intoxication in clinical practice. When digitalis toxicity does develop, it is generally of two types: disturbances of impulse formation and disturbances of conduction. Therapeutic interventions may include antiarrhythmic drugs, pacemaker placement, and, in the most severe cases, administration of cardiac glycosides-specific antibodies. Recent studies have shown that monoclonal digoxin-specific antibodies and Fab fragments obtained by somatic cell fusion are effective in reversing advanced and otherwise lethal digoxin intoxication. The homogeneity of this antibody offers attractive possibilities for improving our ability to treat advanced digitalis intoxication safely and effectively.

Digoxin specific antibody (Fab) fragments in 34 cases of severe digitalis intoxication

34 patients aged between 2 and 80 years were treated with digoxin specific antibody (Fab) fragments for severe digitalis poisoning. In 27 cases, the glycoside was taken with suicidal intent; in 3 cases accidentally and 4 were iatrogenic. The following criteria were considered to be indications for use of Fab fragments: the appearance of life-threatening arrhythmias such as high-grade atrioventricular conduction disorders (grade 2 and 3 A-V block), multifocal ectopic beats, ventricular tachycardia, and relapsing ventricular fibrillation. Serum digoxin concentrations were between 3.4 and 29ng/ml before the start of treatment. Between 240 and 800mg of Fab were administered; the majority of patients received 480mg. Regression of arrhythmias was seen between 0.5 and 8 hours after Fab infusion. There was a rapid fall in the free digoxin in the serum to concentrations that were no longer measurable and a marked rise in bound digoxin with a simultaneous increase of excretion of bound digoxin in the urine. Fab therapy is considered to be a major advance in the treatment of severe, previously fatal, glycoside poisoning. No notable side effects or allergic reactions were observed.

Digoxin-specific antibody fragments and a calcium antagonist for reversal of digoxin-induced mesenteric vasoconstriction

The effect of digoxin-specific antibody fragments on glycoside-induced mesenteric vasoconstriction were investigated. Digoxin caused a sustained contraction of strips of isolated feline mesenteric artery lasting for several hours, while in anaesthetized cats it produced a significant decrease in blood flow and increase in resistance in the mesenteric artery. In-vitro, digoxin's contractile effect was inhibited by 'prophylactic' addition of antibody to the organ bath, but the clinical use for prophylaxis is not a practical proposition. When the antibodies were added with the contraction of the arterial strip in response to digoxin already established, the tone of the preparation decreased significantly over 3 h, but the effect of the glycoside was not fully reversible. In-vivo, control animals not treated with antibodies developed arrhythmias, mesenteric blood flow fell by more than 50% and resistance increased by more than 80% relative to the initial values. These animals died of ventricular fibrillation before the end of the experiment. Animals treated with digoxin-specific antibody fragments after receiving digoxin injections showed no further decrease in mesenteric blood flow and 90 min after the last dose of digoxin, the flow was recovering and mesenteric resistance decreasing. Furthermore, all the animals that had received antibodies remained in sinus rhythm to the end of the experiment. In view of the latent period to onset of action of the antibodies, valuable time may be lost in impaired mesenteric blood flow. To bridge the gap or, indeed, as primary treatment, calcium antagonists merit consideration; in our experiments mesenteric vasoconstriction was abolished within a few minutes by application of the dihydropyridine calcium antagonist 4-(2,1,3-benzo-oxadiazol-4-yl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylic aid, diethyl ester (PY 108-068).

Extracellular versus intracellular digoxin action on bovine myocardium, using a digoxin antibody and intracellular glycoside application

1. The actions of externally and internally applied digoxin in heart ventricular muscle have been compared. 2. External application of digoxin (5 x 10(-8)--10(-7) M) had an inotropic effect, a steady level of twitch tension being reached at the end of about 3 hr. 3. Addition of an anti-digoxin antibody to the bathing solution prevented or reversed the digoxin effect, depending on the time of application. 4. The efflux of the antibody-[3H]digoxin-complex could be fitted by a single exponential with a half-time of 18 min. 5. In the absence of antibody, [3H]digoxin washout was two-compartmental with half-times of 4 and 72 min respectively. It is thought that the fast process signifies efflux from the extracellular space while the slow process reflects the washout of initially membrane bound glycoside. 6. When digoxin was applied by a cut end method, there was no effect on contractile strength. 7. The profile of radioactivity several hours following exposure to [3H]digoxin clearly indicated movement from cell to cell, the concentration of [3H]digoxin being above 10(-7) M in half the preparation at the end of 6 hr. Longitudinal diffusivity averaged 8.6 x 10(-8) cm-2 sec-1. 8. We conclude that digoxin has an inotropic effect when reaching the surface membrane of cardiac cells from the outside but is ineffective when applied from the inside.

The effectiveness of digoxin-specific F(ab')2-antibody fragments in the treatment of digitoxin poisoning: experimental investigations in the cat

In animal experiments heterologous digoxin-specific F(ab')2 antibody fragments have been found to be effective for the treatment of arrhythmias induced by toxic doses of digoxin. So far they have been successfully employed in three patients suffering from digoxin poisoning. The present study was undertaken to test whether these antibodies are also effective in the treatment of digitoxin poisoning. Ventricular tachycardia, induced in digitalized cats by intravenous injections of digitoxin, was fatal in four out of five controls. However, sinus rhythm was reinstated in all the animals treated with digoxin-specific F(ab')2 after onset of the arrhythmia. Cross-reactivity between the antibodies used and digitoxin, while slight in vitro, is, nevertheless, sufficient to justify their clinical use in cases of digitoxin poisoning.

Immunogenicity and kinetics of distribution and elimination of sheep digoxin-specific IgG and Fab fragments in the rabbit and baboon

To evaluate the relative merits of purified IgG and Fab preparations of defined specificity for potential clinical use, immunogenicity studies were carried out in baboon and rabbit experimental models. Distribution and elimination kinetics of purified sheep digoxin-specific IgG and Fab fragments were also studied following intravenous administration to baboons. Serial plasma and urine Fab concentrations were determined from trichloroacetic acid-precipitable 125I counts from pre-labelled preparations and also by measurement of the antibody's functional 3H-digoxin binding capacity. Results were compared with data obtained from IgG by 3H-digoxin binding. Kinetic data analysed by computer-fitted functions demonstrated that plasma Fab disappearance was best described by a tri-exponential function, whereas a bi-exponential function best described the IgG data. Initial distribution half-life (t 1/2) of Fab (0.28-0.32 hr) was considerably shorter than that of IgG (4.0 hr) and contributed a greater proportion of the total fall in plasma level over 24 hr. Fab elimination t 1/2 (9-13 hr) was also shorter than IgG (61 hr), but appreciably longer than earlier estimates in rabbits, guinea-pigs, rats and mice. The total volume of distribution of Fab was 8.7 times greater than that of IgG measured by the same method. Over the first 24 hr after administration 30-45% of administered Fab was recoverable in active form in urine, while 93% of total administered 125I counts from 125I-Fab preparations (bound and free) could be recovered. Less than 1% of administered IgG binding activity was recovered in urine during the initial 24 hr. The relative immunogenicities of sheep digoxin-specific IgG and Fab fragments were studied in six baboons. Both IgG and Fab elicited prompt immune responses when injected intramuscularly with Freund's complete adjuvant. Intravenous injection of soluble sheep IgG resulted in a prompt immune response in one baboon while repeated injections caused only a late, weak response in a second animal. Soluble sheep Fab fragments elicited only delayed and weak responses in the two baboons thus challenged. Further immunogenicity studies in ninteen rabbits showed significantly earlier and greater antibody responses to intravenously administered sheep IgG antigen than to Fab fragments derived from the same IgG population. These studies demonstrate that digoxin-specific Fab fragments undergo more rapid and extensive distribution to the extra vascular compartment and also more rapid renal excretion than IgG. Furthermore, Fab fragments are significantly less immunogenic than the parent IgG population. These data indicate potentially important therapeutic advantages for digoxin-specific Fab compared with IgG when administered for the reversal of life-threatening digitlis toxicity.

Contrasting rates of reversal of digoxin toxicity by digoxin-specific IgG and Fab fragments

Both heterologous IgG and Fab fragments of appropriate affinity and specificity have been shown capable of reversing advanced cardiac glycoside toxicity. Fab fragments are more rapidly excreted and theoretically have a smaller risk of unwanted immunologic effects, but relative rates of toxicity reversal have not been established. Rates of reversal of advanced digoxin toxicity by digoxin-specific IgG and Fab fragments were therefore compared in a dog model of advanced digoxin intoxication. Initial studies confirmed more rapid distribution of sheep Fab fragments (M.W. 50,000) than of the parent IgG molecule (M.W. 150,000) after intravenous injection. Twenty-five pentobarbital-anesthetized dogs were given 0.3 mg/kg digoxin intravenously, resulting in rapid onset of ventricular tachycardia in all animals. Eight dogs subsequently given nonspecific IgG or Fab died in asystole or ventricular fibrillation an average of 55 minutes after digoxin administration. Ten of 11 dogs given 1.33 moles of binding sites per mole of digoxin as intact IgG returned to sinus rhythm at a mean time of 85 minutes after antibody infusion. In contrast, six of six dogs given an equivalent dose of specific Fab fragments returned to sinus rhythm in a significantly shorter mean time of 36 minutes (P less than 0.01). Variability of time to arrhythmia reversion was less in Fab-treated dogs. These data demonstrate a decisive advantage of specific Fab fragments over intact IgG for potential clinical use in advanced, life-threatening digoxin intoxication.

Reversal of advanced digitoxin toxicity and modification of pharmacokinetics by specific antibodies and Fab fragments

The effects of Fab fragments of high-affinity 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(10) M(-1) as determined by equilibrium dialysis. Rapid second-order association kinetics (k(f) = 3.7 x 10(6) M(-1) per s) and slow dissociation kinetics (k(r) = 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 135 and 118 h by radioimmunoassay and [(3)H]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 (t((1/2)) 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 lethal digitoxin toxicity, and are capable of reducing the plasma half-life and accelerating urinary excretion of digitoxin.

Effects of sheep digoxin-specific antibodies and their Fab fragments on digoxin pharmacokinetics in dogs

Intact sheep antidigoxin antibodies and their Fab fragments have both been found to exert profound effects on digoxin pharmacokinetics in [3H] digoxin-treated dogs. Both classes of molecule remove digoxin from the extravascular space and sequester it in the circulation in protein-bound form, a form in which the digoxin is presumably inactive. These two classes of molecule differ, however, in that the intact antibody molecules interfere with digoxin excretion, thereby promoting the retention of the glycoside; this retained digoxin is eventually released in free, active form when the administered antibody is metabolically degraded. In contrast, urinary excretion of digoxin continues in Fab-treated dogs, with significant quantities of digoxin being excreted promptly in the urine in complex with Fab fragments. These differences in urinary excretion, together with the probable decreased immunogenicity of sheep antidigoxin Fab fragments, suggest that such fragments possess potential advantages over intact antibody molecules for use in the therapy of life-threatening digoxin intoxication in man.

Reversal of advanced digoxin intoxication with Fab fragments of digoxin-specific antibodies

Purified Fab fragments of ovine digoxin-specific antibodies reversed severe digoxin intoxication in a patient who had taken 22.5 mg of the drug with suicidal intent. Atrioventricular block with extreme bradycardia was temporarily managed by pacing, but progressive, intractable hyperkalemia (serum potassium of 8.7 meq per liter) with increasing pacing threshold and progressive intraventricular conduction delay was controlled only after infusion of 1100 mg of Fab. Sinus rhythm returned 10 minutes after completion of Fab infusion. Within five hours, the serum potassium concentration fell to 4.0 meq per liter. Free digoxin concentrations in serum fell sharply to undetectable levels, whereas total serum digoxin concentration concomitantly increased 12-fold. Renal excretion of digoxin bound to Fab was documented. Reversal of toxicity was not accompanied by hemodynamic instability, and antibodies to sheep Fab fragments were not detected in the patient's serum after treatment. Thus, purified digoxin-specific Fab fragments are capable of rapid reversal of advanced digoxin toxicity.