Digoxin-specific antibody fragments in the treatment of digoxin toxicity.

CONTEXT: Digoxin-specific antibody fragments (digoxin-Fab) are widely regarded as a safe and effective treatment for the management of acute and chronic digoxin poisoning. Calculated equimolar doses of digoxin-Fab are high, very expensive, and infrequently used.
OBJECTIVE: To review the pharmacology, efficacy, effectiveness, indications, safety and the dosage of digoxin-specific antibody fragments.
METHODS: Pubmed, Embase, Medline and Cochrane were searched from 1946 to May 2013 using the terms digoxin, digoxin-specific Fab, and digoxin antibody. Pharmacology and kinetics of digoxin and digoxin-Fab. Digoxin acts via inhibition of Na⁺/K⁺ ATPase. It has a narrow therapeutic index. Digoxin has 60-80% bioavailability, a mean plasma half-life of 40 h and a volume of distribution (Vd) of 5-10 L/kg and low protein binding (20%). A 40-mg vial of digoxin-Fab (DigiFab) binds 0.5 mg digoxin. Digoxin-Fab has a mean plasma half-life of 19-30 h and a Vd of 0.4 L/kg. The half-lives of both digoxin and digoxin-Fab are prolonged in renal failure to over 100 h. Efficacy and effectiveness of digoxin-Fab. There were no randomised clinical trials examining the use of digoxin-Fab for acute or chronic digoxin poisonings. Ten case series with a total of 2,080 patients have reported on the use of digoxin-Fab in digoxin poisoning. In three large case series of 430 acute and 1308 chronic poisonings, response rates to digoxin-Fab vary from 80-90% to 50%. The time for reversal of digoxin toxicity is reported to be 30-45 min. Studies with pharmacokinetic data showed that free digoxin concentration fell to almost zero within a few minutes following the administration of digoxin-Fab. Digoxin-Fab was used more frequently in acute than chronic digoxin poisoning with a higher reported success rate when used in acute overdose. It is sometimes recommended to use full neutralisation doses (based on serum concentration × Vd or ingested dose). It has also been proposed to use half this dose. Indications for digoxin-Fab. Patients who have life-threatening tachy-bradyarrhythmias, hyperkalaemia (> 6 mmol/L) or haemodynamic instability with an elevated digoxin concentration (> 2 μg/L or 2.6 nmol/L). The lowest effective digoxin-Fab dosing regimen has not been established. Safety of digoxin-Fab. Adverse events such as exacerbation of heart failure, increased ventricular rate and hypokalaemia are uncommon (< 10%). Recrudescence of digoxin toxicity and allergic reactions are infrequent. Digoxin-Fab dosing in acute poisoning. Digoxin load based on ingested dose will generally overestimate digoxin-Fab doses as bioavailability is 60-80%, and further reduced by vomiting and activated charcoal. Digoxin load based on concentration also will be overestimated when the concentration is taken before distribution is complete (around 6 h). Much smaller doses of digoxin-Fab can eliminate the digoxin in the central compartment (Vd ≈ 55 L). In imminent cardiac arrest, it may be justified to give a full neutralising dose. Otherwise, based on pharmacokinetic modelling, it is recommended to give 80 mg bolus digoxin-Fab, repeated as required according to clinical parameters because the onset of clinical response is usually rapid. Most patients would be expected to require a total of less than half of the calculated neutralising dose using this strategy. Digoxin-Fab dosing in chronic poisoning. Even if digoxin load is estimated following distribution (> 6 h), excessive neutralisation doses may still be calculated because of variation in Vd due to equations failing to account for lean body weight, age and renal failure. In practice, it is suggested to give 40 mg (1 vial) digoxin-Fab at a time and repeat after 60 min if patient is still symptomatic, sooner if patient is clinically unstable. In general, 40-120 mg (1-3 vials) should be sufficient.
CONCLUSIONS: Digoxin-Fab is safe and indicated in all patients with life-threatening arrhythmias and an elevated digoxin concentration. However, calculated full neutralising doses of digoxin-Fab are expensive and may not be required. In acute poisoning, a small bolus of 80 mg, repeat if necessary, titrated against clinical effect, is likely to achieve equivalent benefits with much lower total doses. With chronic poisoning, it may be simplest to give 40 mg (1 vial) digoxin-Fab at a time and repeat after 60 min if there is no response.