Monitoring of unbound digoxin in patients treated with anti-digoxin antigen-binding fragments: a model for the future?

Digoxin is one of the few therapeutic drugs for which antidotal therapy is available (1). Administration of Fab fragments has successfully been used to reverse the effects of life-threatening digoxin overdoses for over 20 years. Digoxin-specific Fab fragments are produced by cleaving sheep digoxin-specific IgG with papain to form two Fab fragments (50 000 Da each) and one 50 000-Da Fc fragment (2). As determined by equilibrium dialysis with one such preparation of Fab fragments, the intrinsic affinity constant for binding of digoxin was 10 L/mol (3). Depending on the experimental conditions used (e.g., K concentration and type of membrane isolated), the affinity constant for binding of digoxin to its receptor (the sodium pump) has been reported to be in the range 10-10 L/mol (4, 5). The rationale for using Fab fragments to reduce toxic effects of digoxin is their greater affinity for digoxin when compared with the sodium pump. As a consequence, the extracellular unbound digoxin concentration is lowered, and further equilibration of receptor-bound digoxin with the fluid in the extracellular space leads to the eventual release of digoxin from its receptor sites. The affinity of digitoxin for binding to Fab fragments is 10-fold lower than that of digoxin; however, it is high enough to allow clinical utility of Fab fragments in cases of digitoxin intoxication as well (3). The dose of Fab fragments should be approximately equimolar to the total body digoxin load, which is determined according to the serum digoxin concentration and/or patient’s medical history (6). Digibind (Glaxo Wellcome) is the most common brand of anti-digoxin Fab fragments used in the United States and worldwide. Boehringer-Mannheim also produces anti-digoxin Fab fragments intended for use as an antidote. Anti-digoxin Fab fragments have been suggested and used as antidotes in intoxications caused by other digitalis-like molecules such as oleandrin (7–9), bufadienolide-containing aphrodisiacs (10), digitoxin (11), and foxglove extract (12); they have also been successfully used to reverse hypertension believed linked to increased digoxin-like immunoreactive factors in blood (13). In 90% of digitalis-induced intoxicated patients, a median initial favorable response time of 19 min was achieved after administration of Digibind (14). Hyperkalemia associated with digitalis poisoning is also reversed after administration of Fab fragments (2). Fab fragments have a volume of distribution of 0.4 L/kg, an elimination half-life of 16–20 h, and a systemic clearance of 0.324 mL z min z kg (15). Both renal and nonrenal (i.e., metabolism and/or removal by the reticuloendothelial system) routes are responsible for the elimination of Fab fragments with ;65% of both Fab as well as Fab-digoxin complexes eliminated by the kidneys (15). Such complexes are not removed by hemodialysis or even by continuous arteriovenous hemofiltration (16). In patients with reduced or no renal function, Fab fragments can remain in plasma for 2–3 weeks after administration because of decreased renal elimination. After administration of Fab fragments, there is approximately a 10to 30-fold increase in total digoxin concentration in the serum, whereas the unbound fraction of the drug decreases rapidly (17). Because the unbound fraction of digoxin is the pharmacologically active form, its accurate and reliable measurement in serum collected at various times after administration of Fab fragments may be clinically important (18). The rationale here is that rapid removal of all bioactive digoxin from a patient in need of the drug is not clinically efficacious, and an ability to titer the unbound digoxin would seem optimal. Although no definitive evidence has yet been presented, the concept seems rational. Furthermore, the pharmacokinetics of digoxin after administration of Fab fragments becomes dependent on the disposition of Fab (19). Monitoring of unbound digoxin concentrations after administration of Fab fragments may be appropriate to establish recrudescent toxicity in renal failure patients, to assess the need for more Fab to be administered, and to establish the need for reintroduction of digoxin (15). However, specimens collected from patients treated with digoxin Fab fragments usually give grossly erroneous and misleading values for digoxin concentrations by most immunoassays (20). Assay antibody and Fab fragments have similar affinities for digoxin and the tracer. Therefore, the most likely mechanism of interference of Fab fragments in digoxin assays is by binding of the assay tracer components. In fact, over the last few years, over a dozen publications have addressed the issue of discrepant digoxin values caused by the presence of this antidote in blood. One study in particular noted discrepancies between several immunoassays for digoxin as long as 14 days after a patient had been treated with Fab (21). Generally, immunoassays involving wash steps are less prone to interference by Fab fragments because the unbound Fab fragments are removed before the addition of tracer during the wash step (17). In some digoxin immunoassays that require acid precipitation of proteins, pretreatment of sample with acid releases the Fab-sequestered digoxin; subsequently, total digoxin present in the serum including the bound and the unbound fractions is measured. This usually gives very large measured values. There appears to be no clinical value in measuring total serum digoxin concentrations in patients treated with the antidote. One method for measuring the unbound digoxin is the use of ultrafiltration before the immunoassay. Although this procedure is cumbersome, it has been used successfully (18, 22). In the study reported in this issue, Ocal and Green (23) have used the Stratus and AxSYM digoxin immunoassays to analyze three samples from a patient treated with Digibind. In addition, they also analyzed several samples to which Digibind was added at different Editorial