Digoxin-specific antibodies

Digoxin-specific antibodies: a novel dosing strategy

Digoxin-specific antibodies (digoxin-Fabs) are of value in the treatment of a strongly suspected or a known, potentially life-threatening digoxin toxicity. These antibodies are not registered for use in Europe; therefore Dutch hospital pharmacies are not allowed to keep them in stock. In the Netherlands, digoxin-Fabs are stored in a national calamity stock of emergency medicines at the National Institute for Public Health and the Environment. In the case of a medical emergency, digoxin-Fabs are available after contact with the Dutch Poisons Information Centre. Recent studies have shown that the dose of digoxin-Fabs required to effectively treat digoxin toxicity is lower than previously thought. In this article, we present the adjusted digoxin-Fab dosing strategy currently recommended by the Dutch Poisons Information Centre ( www.vergiftigingen.info ). This new dose titration strategy is safe and effective and has a cost-saving side-effect.

Antibody-powered DNA switches to initiate the hybridization chain reaction for the amplified fluorescence immunoassay

Designing antibody-powered DNA nanodevice switches is crucial and fascinating to perform a variety of functions in response to specific antibodies as regulatory inputs, achieving highly sensitive detection by integration with simple amplified methods. In this work, we report a unique DNA-based conformational switch, powered by a targeted anti-digoxin mouse monoclonal antibody (anti-Dig) as a model, to rationally initiate the hybridization chain reaction (HCR) for enzyme-free signal amplification. As a proof-of-concept, both a fluorophore Cy3-labeled reporter hairpin (RH) in the 3' terminus and a single-stranded helper DNA (HS) were individually hybridized with a recognition single-stranded DNA (RS) modified with Dig hapten, while the unpaired loop of RH was hybridized with the exposed 3'-toehold of HS, isothermally self-assembling an intermediate metastable DNA structure. The introduction of target anti-Dig drove the concurrent conjugation with two tethered Dig haptens, powering the directional switch of this DNA structure into a stable conformation. In this case, the unlocked 3'-stem of RH was implemented to unfold the 5'-stem of the BHQ-2-labeled quench hairpin (QH), rationally initiating the HCR between them by the overlapping complementary hybridization. As a result, numerous pairs of Cy3 and BHQ-2 in the formed long double helix were located in spatial proximity. In response to this, the significant quenching of the fluorescence intensity of Cy3 by BHQ-2 was dependent on the variable concentration of anti-Dig, achieving a highly sensitive quantification down to the picomolar level based on a simplified protocol integrated with enzyme-free amplification.

Adverse Events Associated with Use of Digoxin Immune Fab Reported to the US Food and Drug Administration Adverse Event Reporting System, 1986-2019

Background

Digoxin immune fab products, DigiBind and DigiFab, are antidotes for the treatment of patients with life-threatening or potentially life-threatening digoxin toxicity or overdose. Although approved by the US Food and Drug Administration (FDA) in 1986 (DigiBind) and 2001 (DigiFab), there remains a paucity of literature describing the safety of these products in the postmarketing setting.

Objective

We sought to assess US adverse event (AE) reports submitted to the FDA Adverse Event Reporting System (FAERS) for DigiBind and DigiFab in the postmarketing period.

Patients and Methods

We searched reports for DigiBind and DigiFab submitted from the time of each product approval through December 31, 2019. Descriptive statistics were used to assess AE reports for DigiBind and DigiFab. Empirical Bayes geometric means (EBGMs) and their 90% confidence intervals were computed to identify disproportionate (i.e., at least twice the expected) reporting of DigiBind and DigiFab. Reports describing selected AEs and death outcomes were individually reviewed.

Results

A total of 78 DigiBind and 43 DigiFab reports were identified, of which 68 DigiBind (87.2%) and 27 DigiFab (62.8%) reports were serious. Among the most frequently reported AEs for both products [DigiBind, DigiFab, respectively] were cardiac (bradycardia [3.8%, 3.9%], cardiac arrest [3.3%, 3.9%], and hypotension [2.4%, 2.6%]) and non-cardiac (nausea [1.9%, 2.6%] and hyperkalemia [1.4%, 1.9%]) events. These AEs were labeled events or confounded by indication for use (digoxin toxicity). Nineteen (24.4%) DigiBind and 13 (30.2%) DigiFab reports described an outcome of death, of which seven (53.8%) DigiFab reports were attributed to poisoning with non-digoxin cardiac glycosides. No deaths could be attributed to DigiBind or DigiFab administration.

Conclusions

Our analysis did not identify new safety concerns for DigiBind or DigiFab. Most AEs reported were labeled events or confounded by indication for use.

Raising antibodies against epoxyscillirosidine, the toxic principle contained in Moraea pallida Bak. (Iridaceae), in rabbits

Moraea pallida Bak. (yellow tulp) poisoning is the most important plant cardiac glycoside toxicosis in South Africa. The toxic principle, a bufadienolide, is 1α, 2α-epoxyscillirosidine. The aim was to investigate the potential to develop a vaccine against epoxyscillirosidine. Epoxyscillirosidine, proscillaridin and bufalin, were successfully conjugated to hen ovalbumin (OVA), bovine serum albumin (BSA) and keyhole limpet haemocyanin (KLH). There was a low immune response following vaccination of adult male New Zealand White rabbits with epoxyscillirosidine-OVA (n = 3) and OVA (n = 3) using Freund's adjuvant in Trial (T) 1. The immune response improved significantly in T2 following doubling of the dose to 0.8 mg/rabbit and changing the adjuvant to Montanide. In T3, the rabbits (n = 15), allocated into 5 equal groups, vaccinated with proscillaridin-BSA, bufalin-BSA, epoxyscillirosidine-KLH, epoxyscillirosidine-BSA and BSA respectively, using Montanide adjuvant, developed antibodies against the administered immunogens, with epoxyscillirosidine-KLH inducing the highest immune response. Proscillaridin and bufalin antibodies cross-reacted with epoxyscillirosidine in an enzyme linked immunosorbent assay. The conjugation methodology will be adjusted in the future to target optimal conjugation efficiency. Additional vaccination will be conducted in search of neutralizing antibodies against the yellow tulp toxin. The cross-reactivity of proscillaridin and bufalin antibodies with epoxyscillirosidine could be studied in future to explore the potential to prevent yellow tulp poisoning.

Synthesis and Characterization of Hapten-Protein Conjugates for Antibody Production against Cyanogenic Glycosides

Consumption of cyanogenic plants can cause serious health problems for humans. The ability to detect and quantify cyanogenic glycosides, capable of generating cyanide, could contribute to prevention of cyanide poisoning from the consumption of improperly processed cyanogenic plants. Hapten-protein conjugates were synthesized with amygdalin and linamarin by using a novel approach. Polyclonal antibodies were generated by immunizing four New Zealand White rabbits with synthesized amygdalin-bovine serum albumin and linamarin-bovine serum albumin immunogen. This is the first time an antibody was produced against linamarin. Antibody titer curves were obtained from all the four rabbits by using a noncompetitive enzyme-linked immunosorbent assay. High antibody titer was obtained at dilutions greater than 1:50,000 from both immunogens. This new method is an important step forward in preventing ingestion of toxic cyanogenic glycosides.

In vitro selection and characterization of deoxyribonucleic acid aptamers for digoxin

The low therapeutic index of digoxin necessitates careful monitoring of its serum levels. Most of digoxin immunoassays suffer from interferences with digoxin-like immunoreactive substances. Since aptamers have been shown to be highly specific for their targets, the aim of this study was to develop DNA aptamers for this widely used cardiac glycoside. Digoxin was coated onto the surface of streptavidin magnetic beads. DNA aptamers against digoxin were designed using Systematic Evolution of Ligands by Exponential enrichment method (SELEX) by 11 iterative rounds of incubation of digoxin-coated streptavidin magnetic beads with synthetic DNA library, DNA elution, electrophoresis and PCR amplification. The PCR product was cloned and sequenced. Binding affinity was determined using digoxin-BSA conjugate, coated onto ELISA plate. Inhibitory effect of anti-digoxin aptamer was conducted using isolated guinea-pig atrium. Three aptamers (D1, D2 and D3) were identified. Binding studies of fluorescein-labeled truncated (without primer binding region) D1 and D2 and full length D1 anti-digoxin aptamers were performed and their corresponding dissociation constants values were 8.2×10(-9), 44.0×10(-9) and 17.8×10(-9) M, respectively. This is comparable to what other workers have obtained for interaction of monoclonal antibodies raised against digoxin. There was little difference in binding affinity between full length and truncated anti-digoxin D1 aptamer. D1 anti-digoxin aptamer also inhibited the effects of digoxin on the isolated guinea-pig atrium. D1 anti-digoxin aptamer distinguished between digoxin and ouabain in both tissue study and binding experiments. Our finding indicated that D1 anti-digoxin aptamer can selectively bind to digoxin. Further studies might show its suitability for use in digoxin assays and as a therapeutic agent in life-threatening digoxin toxicity.

Production of native bispecific antibodies in rabbits

Background

A natural bispecific antibody, which can be produced by exchanging Fab arms of two IgG4 molecules, was first described in allergic patients receiving therapeutic injections with two distinct allergens. However, no information has been published on the production of natural bispecific antibody in animals. Even more important, establishment of an animal model is a useful approach to investigate and characterize the naturally occurring antibody.

Methodology/Principal Findings

We demonstrated that a natural bispecific antibody can also be generated in New Zealand white rabbits by immunization with synthesized conjugates. These antibodies showed bispecificity to the components that were simultaneously used to immunize the animals. We observed a trend in our test animals that female rabbits exhibited stronger bispecific antibody responses than males. The bispecific antibody was monomeric and primarily belonged to immunoglobulin (Ig) G. Moreover, bispecific antibodies were demonstrated by mixing 2 purified monospecific antibodies in vivo and in vitro.

Conclusions/Significance

Our results extend the context of natural bispecific antibodies on the basis of bispecific IgG4, and may provide insights into the exploration of native bispecific antibodies in immunological diseases.

Application of an ELISA-elution assay to dissociate digoxin-antibody complexes in immunoaffinity chromatography

In this study, we used a modified enzyme-linked immunosorbent assay (ELISA)-elution technique as a screening tool to select specific elution conditions. We examined 12 different elution conditions for the removal of antibodies from a complex on an ELISA plate; 0.2 mol/l glycine-HCl (pH 2.5), 1.0 mol/l acetic acid (pH 2.5), 25% methanol (pH 2.5) and 3 mol/l NaSCN showed a higher elution efficiency. We conducted affinity chromatography with these four conditions for the purification of anti-digoxin antibodies from hyperimmune sera with a digoxin-specific column using omega-aminoalkyl derivatives of Sepharose 4B, whose elution efficiency was similar to that of ELISA. We also monitored the relative specific activities during elution from the digoxin-specific column. The optimum, general-purpose dissociation reagent for this immunoaffinity system was identified as 25% methanol (pH 2.5) with an elution efficiency and relative specific activity of 88.40% and 62.25%, respectively. The high purity of the purified antibodies was demonstrated with sodium dodecyl sulphate-polyacrylamide gel electrophoresis.

Positive inotropic drugs--digitalis

Digitalis glycosides inhibit Na+K+-ATPase in the cells and have been used for scientific studies of cation transport over cell membranes. Furthermore, digitalis has a positive inotropic and antiarrhythmogenic effect. Specific binding sites for digitalis glycosides have been observed in erythrocytes, the myocardium and the central nervous system. Transcellular transport of digoxin has been found in the kidney, since digoxin is excreted by tubular secretion. Recent studies have discovered an endogenous digitalis-like substance both inhibiting Na-K-ATPase and displacing digoxin from specific binding sites at the erythrocytes. The concentration of this component in plasma seems to be higher in hypertension than in normotension. Future studies will have to disclose other effects of this substance in order to evaluate whether it can be used as a drug in heart failure.

Structural and functional study of rabbit polyclonal antibody for immunoassay purposes

In this study, we investigated the effects of some denaturants, such as urea and heat, on structure and function of rabbit polyclonal antibody and its Fab fragments. Thermal unfolding studies by circular dichroism of antibody and Fab fragments showed that in acidic pH, antibody has multi-transitions whereas Fab fragments have one transition curve; however in neutral pH, thermal unfolding of both had one transition. Effects of urea on the structure of antibody and Fab were studied through fluorescence spectroscopy. Despite exposure of protein to high concentration of denaturant, partial unfolding occurred in both antibody and Fab, but the denaturation of Fab was more considerable than that of antibody. Functional studies indicated that urea and heat causes a decrease in affinity in both antibody and Fab, but deactivation of Fab is more considerable in comparison with the antibody molecule. Turbidity study of antibody and Fab showed that aggregation of Fab occurred at lower temperatures than that of antibody. Our results indicate that Fab has higher sensitivity in comparison with antibody in the unfolding, deactivation, and aggregation processes. Therefore, our data proposes a stabilizing role for Fc.

Properties of Novel Anti-digoxin Antisera in Radioimmunoassay Using Homologous and Site Heterologous Tritium-Labeled Antigens Involving a [3H]-Leucine Moiety

The specificities of antisera against digoxin C-3' or C-3'' hemisuccinate-bovine serum albumin (BSA) conjugate were assessed by cross-reactivity studies with digoxin metabolites by radioimmunoassay (RIA) using the homologous and the site heterologous tritium-labeled antigens. One of the tracers used was digoxin 3'-hemisuccinyl-[3H]-leucine; the other was digoxin 3''-hemisuccinyl-[3H]-leucine, which had been prepared from digoxin 3''-hemisuccinate. When the tracer with [3H]-leucine at the C-3' position was used, antisera (I-1, I-3) elicited by digoxin 3'-hemisuccinate-BSA conjugate showed the following cross-reactivity: digoxigenin bisdigitoxoside (0.34%, 76%), digoxigenin monodigitoxoside (0.11%, 65%), digoxigenin (0.02%, 26%) and dihydrodigoxin (9.4%, 1.2%). However, when using the homologous antigen, antiserum (I-1) was highly specific against the digitoxose chain. When the site heterologous antigen, digoxin 3''-hemisuccinyl-[3H]-leucine was combined, this antiserum showed high cross-reactivity to digoxin degradation products. This digoxin RIA using antiserum (I-1) with the homologous antigen measures unmetabolized digoxin. On the other hand, the RIA system using antiserum (I-3) with the homologous antigen had cross-reactivity with the metabolites in accordance with their relative cardio-activities, so this system would be useful in therapeutic drug monitoring of digoxin.

Measurement of beta-methyldigoxin level in serum from patients by enzyme immunoassay using novel specific antiserum with a phenyl boric acid column

The authors compared serum beta-methyldigoxin (MDx) levels in digitalized patients by enzyme immunoassay (EIA) using anti-MDx 3'-hemisuccinate BSA antiserum (antiserum-I) with commercial antidigoxin antiserum (antiserum-II). The usefulness of a phenyl boric acid (PBA) column for pretreatment of the serum samples was also investigated. The assay using antiserum-I demonstrated good accuracy and precision in the concentration range of 0.5 to 5 ng/mL. When the specificities of antiserum-I and antiserum-II were assessed by cross-reactivity studies with various related compounds, antiserum-I was much more specific for MDx antiserum-II. Using a phenyl boric acid (PBA) column, MDx, and digoxigenin, which exhibits a negligible cross-reactivity, were separated from serum, including MDx and its metabolites. The recovery tests of MDx using antiserum-I with a PBA column in human serum were satisfactory and no interference of metabolites of MDx was observed. Mean MDx concentrations in serum samples (n = 30) from digitalized patients by EIA using antiserum-I with PBA column, antiserum-I, and antiserum-II were 1.06, 1.30, and 1.74 ng/mL, respectively. These results indicate that our EIA system using antiserum-I with a PBA column for pretreatment of serum samples is useful to more precisely measure the unchanged type of MDx in patients.

Three-dimensional structure-activity relationship modeling of cocaine binding to two monoclonal antibodies by comparative molecular field analysis

Successful immunotherapy of cocaine addiction and overdoses requires cocaine-binding antibodies with specific properties, such as high affinity and selectivity for cocaine. We have determined the affinities of two cocaine-binding murine monoclonal antibodies (mAb: clones 3P1A6 and MM0240PA) for cocaine and its metabolites by [3H]-radioligand binding assays. mAb 3P1A6 (K(d) = 0.22 nM) displayed a 50-fold higher affinity for cocaine than mAb MM0240PA (K(d) = 11 nM) and also had a greater specificity for cocaine. For the systematic exploration of both antibodies' binding specificities, we used a set of approximately 35 cocaine analogues as structural probes by determining their relative binding affinities (RBAs) using an enzyme-linked immunosorbent competition assay. Three-dimensional quantitative structure-activity relationship (3D-QSAR) models on the basis of comparative molecular field analysis (CoMFA) techniques correlated the binding data with structural features of the ligands. The analysis indicated that despite the mAbs' differing specificities for cocaine, the relative contributions of the steric (approximately 80%) and electrostatic (approximately 20%) field interactions to ligand-binding were similar. Generated three-dimensional CoMFA contour plots then located the specific regions about cocaine where the ligand/receptor interactions occurred. While the overall binding patterns of the two mAbs had many features in common, distinct differences were observed about the phenyl ring and the methylester group of cocaine. Furthermore, using previously published data, a 3D-QSAR model was developed for cocaine binding to the dopamine reuptake transporter (DAT) that was compared to the mAb models. Although the relative steric and electrostatic field contributions were similar to those of the mAbs, the DAT cocaine-binding site showed a preference for negatively charged ligands. Besides establishing molecular level insight into the interactions that govern cocaine binding specificity by biopolymers, the three-dimensional images obtained reflect the properties of the mAbs binding pockets and provide the initial information needed for the possible design of novel antibodies with properties optimized for immunotherapy.

Preparation and characterization of monoclonal antibody against digoxin

Mouse-mouse hybridoma cell lines producing stable, highly specific and with good affinity monoclonal antibody (MAb) against the cardiac glycoside digoxin were established. Balb/c mice were immunized via injection of digoxin-3'-bovine serum albumin (BSA). The spleens of which were fused with myeloma cells of SP2/0 origin. Three clones designated as BBA, MBE, and BMG producing good antibodies displayed different patterns of fine specificity for digoxin and low cross-reaction with several digoxin analogues as elucidated by inhibition enzyme-linked immunosorbant assay (ELISA). All three MAbs were of the same class and subclass (IgG(1)). Affinity purification was performed for the selected clone BBA displaying the highest affinity and nearly no cross-reactivity with any of the structurally related molecules. Ultrafiltered concentrated hybrid cell supernatant was also purified by polyethylene glycol (PEG) 6000 precipitation for large-scale preparation and coated onto the wells of microtiter plates. The standard curve was constructed with a sensitivity of 10 pg/well covering up to 10 ng/well.

Three-dimensional quantitative structure-activity relationship analysis of ligand binding to human sequence antidigoxin monoclonal antibodies using comparative molecular field analysis

The present study indicates that the newly generated human sequence antidigoxin monoclonal antibody (mAb), 1B3, binds digoxin with a different fine specificity binding than our previously obtained human sequence monoclonal antibodies (mAbs) (Ball, W. J.; et al. J. Immunol. 1999, 163, 2291-2298). Uniquely, 1B3 has a higher affinity for digitoxin than digoxin, the immunizing hapten, and a strong requirement for at least one sugar residue linked to the aglycone (-genin). By means of comparative molecular field analysis (CoMFA), the results of competition binding studies for 56 cardiotonic and hormonal steroids were employed to develop three-dimensional quantitative structure-activity relationship (3D-QSAR) models for ligand binding to 1B3 and to three additional human sequence mAbs, as well as the murine antidigoxin mAb 40-50 (Mudgett-Hunter, M.; et al. Mol. Immunol. 1985, 22, 447-488). All five 3D-QSAR models yielded cross-validated q(2) values greater than 0.5, which indicates that they have significant predictive ability. The CoMFA StDevCoeff contour plots, as well as the competition results, indicate that 1B3 binds ligands in a manner distinct from the other four mAbs. The CoMFA contour plots for 40-50 were also compared with the known X-ray crystallographic structure of the 40-50-ouabain complex (Jeffrey, P. D.; et al. J. Mol. Biol. 1995, 248, 344-360) in order to identify correlations between residues in the mAb binding site and specific contour plot regions. These 3D-QSAR models and their respective contour plots should be useful tools to further understand the molecular nature of antibody-antigen interactions and to aid in the redesign or enhancement of therapeutic antibodies.

Immediate and delayed allergic reactions to Crotalidae polyvalent immune Fab (ovine) antivenom

Allergic reactions are the most commonly reported adverse events after administration of antivenoms. Conventional horse serum-based crotalid antivenom used in the United States (Antivenin [Crotalidae] polyvalent) can lead to both immediate and delayed hypersensitivity reactions. Crotalidae polyvalent immune Fab (ovine) (CroFab; FabAV) has recently been approved for use in the United States. Experience from premarketing trials of this product and in the administration of other types of Fab, such as in digoxin poisoning, has demonstrated these fragments to be safe and effective, with a low incidence of sequella; however, allergic reactions can occur when any animal-protein derivatives are administered to human subjects. We report in detail the nature and course of allergic reactions that occurred in 4 patients treated with FabAV. Cases of anaphylaxis, acute urticaria, angioedema, and delayed serum sickness are described. All reactions were easily treated with some combination of antihistamines, epinephrine, and steroids, with prompt resolution of signs and symptoms enabling further dosing of antivenom as required. Several of these cases may have resulted from batches of antivenom contaminated with Fc fragments. The overall incidence of immediate and delayed allergic reactions to this product appears so far to be lower than that reported with conventional whole-immunoglobulin G (IgG) antivenom, but postmarketing surveillance is warranted.

Development of radioimmunoassay for measurement of serum digoxin in digitalized patients using novel anti-digoxin antiserum

There is an antiserum elicited by digoxin 3'-hemisuccinate-bovine serum albumin (BSA) conjugate possessing high specificity for digoxin. Our study focused on development of RIA using this novel antiserum for measurement of digoxin in serum from digitalized patients. The property of the new antiserum was investigated by RIA with digoxin 3'-hemisuccinyl-[3H]leucine. The separation of bound and free fractions was performed using a dextran-coated charcoal suspension. The new antiserum bound approximately 50% of digoxin 3'-hemisuccinyl-[3H]leucine with a final dilution of 1:30000. The intra- and inter-assay coefficients of variation were <9% in the range of 0.52-4.17 ng/ml. The mean digoxin concentration in serum samples (n=35) from digitalized patients was estimated to be 0.68 ng/ml, which was lower than its measurement of digoxin with the commercial antidigoxin BSA serum and monoclonal anti-digoxin. It is apparent that the RIA described here has sufficient precision. The RIA system was available for the measurement of digoxin in serum from digitalized patients.

Three-dimensional quantitative structure-activity relationship study of the inhibition of Na(+),K(+)-ATPase by cardiotonic steroids using comparative molecular field analysis

Na(+),K(+)-ATPase is a transmembrane protein that transports sodium and potassium ions across cell membranes during an activity cycle that uses the energy released by ATP hydrolysis. Cardiotonic steroids (digitalis) inhibit this activity and consequently produce a positive inotropic response in the heart. To identify the structural features of the steroids that are important for this inhibition, we have tested the inhibitory properties of 47 cardiotonic and hormonal steroids and developed a three-dimensional quantitative structure-activity relationship (3D-QSAR) model for the inhibition of Na(+),K(+)-ATPase using comparative molecular field analysis (CoMFA). We also developed a 3D-QSAR model for the binding of digoxin to the murine anti-digoxin monoclonal antibody (mAb) 26-10 because we have previously shown that the environment of the binding sites of 26-10 and the enzyme are similar (Kasturi et al. (1998) Biochemistry 37, 6658-6666). These statistically predictive 3D-QSAR models indicate that both binding sites are about 20 A long and have a close fit or complementarity about the beta side of the lactone ring of digitalis. Furthermore, steric bulk about the lactone ring and the alpha sugar may be critical for drug binding. However, the binding site of Na(+),K(+)-ATPase differs from that of mAb in that it has a greater number of electrostatic interactions along the alpha-sugar, steroid, and lactone moieties. In addition, the availability of the structure of the 26-10 Fab-digoxin complex (Jeffrey et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 10310-10314) enabled us to compare the CoMFA-derived contour maps with the known locations for amino acid residues comprising the mAb ligand binding site.

Measurement of serum digitoxin in patients by radioimmunoassay using specific antiserum

INTRODUCTION

Digitoxin is used to treat patients with heart failure.

METHODS

A radioimmunoassay procedure for the specific determination for digitoxin in serum was developed using the antiserum (antiserum (A)) raised against digitoxin 3'-hemisuccinate-BSA conjugate.

RESULTS

The intra- and interassay variability were <10% in the range of 5-100 ng/ml. The specificities of antiserum (A) and the commercial anti-digitoxin antiserum (antiserum (B)) were assessed by cross-reactivity studies with various related compounds. Antiserum (A) was highly specific for digitoxin. Mean digitoxin concentrations in serum samples (n=34) from digitalized patients by RIA using these antisera were 10.0 and 12.4 ng/ml, respectively.

CONCLUSION

This RIA using antiserum (A) measure unmetabolized digitoxin and may be applicable for pharmacokinetic studies.

Selection of peptidic mimics of digoxin from phage-displayed peptide libraries by anti-digoxin antibodies

Since the initial report of the development of methodology to generate high-affinity digitalis-specific (digoxin) antibodies, these antibodies have proven extremely useful tools to monitor digoxin levels in digitalized patients and, as Fab fragments, to reverse toxic digoxin effects in life-threatening digoxin overdoses. These antibodies (both digoxin-specific and ouabain-specific) have been used extensively by investigators for the identification and characterization of putative endogenous digitalis-like factors. In this study, we used two well-characterized mouse anti-digoxin monoclonal antibodies (mAbs), designated 26-10 and 45-20, as binding templates with which to select short bacteriophage-displayed (pIII protein inserted) peptides that are capable of binding to these mAbs and mimicking the conformational structure of digoxin. Selective enrichment from two phage-displayed random peptide libraries enabled us to isolate and identify distinct 15 and 26 amino acid residue peptide inserts that bind with high avidity and idiotypic specificity to the selecting mAbs. Among these displayed inserts a subset was identified whose mAb binding is inhibited by digoxin and whose corresponding synthetic peptides inhibit phage binding. They, therefore, appear to bind at the mAbs digoxin-binding sites. These data provide the first clear evidence that short polypeptides can serve as surrogates for the low molecular mass hapten digoxin.

Isolation and Characterization of Human Monoclonal Antibodies to Digoxin

Fab preparations of sheep polyclonal anti-digoxin Abs have proven useful for reversal of the toxic effects of digoxin overdoses in patients. Unfortunately, the use of foreign species proteins in humans is limited because of the potential for immunological responses that include hypersensitivity reactions and acute anaphylaxis. Immunization of recently developed transgenic mice, whose endogenous μ heavy and κ light chain Ig genes are inactivated and which carry human Ig gene segments, with a digoxin-protein conjugate has enabled us to generate and isolate eight hybridoma cell lines secreting human sequence anti-digoxin mAbs. Six of the mAbs have been partially characterized and shown to have high specificity and low nanomolar affinities for digoxin. In addition, detailed competition binding studies performed with three of these mAbs have shown them to have distinct differences in their digoxin binding, and that all three structural moieties of the drug, the primary digitoxose sugar, steroid, and five-member unsaturated lactone ring, contribute to Ab recognition.

Functional antagonism by a monoclonal antibody to digoxin in a test system of cultured rat heart myocytes

A monoclonal antibody with a high affinity for digitoxin (KA = 0.50 nM) and digoxin (KA = 0.55 nM) was produced by somatic cell fusion. This antibody, designated 2A3(47), displayed little cross reactivity with other glycosides. In cultured rat heart myocytes, 2A3(47), antagonized the positive chronotropic effect exerted by digitoxin but did not alter that of ouabain. Our results suggest that this monoclonal antibody may prove to be useful in treating digoxin and digitoxin intoxication.

High-performance capillary electrophoresis for characterization of hapten-protein conjugates used for production of antibodies against soyasaponin I

Micellar electrokinetic capillary chromatography using sodium cholate as the micellar phase has been investigated for characterization of hapten-protein conjugates. Special focus has been placed on the hapten soyasaponin I which is a quantitatively dominating glycoside in seeds of several legumes including pea (Pisum sativum L.) and soybean [Glycine max (L.) Merr.]. Soyasaponin I has been isolated from pea and used as hapten for production of anti-saponin specific polyclonal antibodies. Soyasaponin I was coupled to Kunitz soybean trypsin inhibitor (KSTI) and bovine serum albumin. The degree of coupling was determined by high-performance capillary electrophoresis (HPCE). Capillaries dynamically coated with zwitterions were found to be efficient for reduction of interaction between the silica capillary surface and the proteins. The applicability of HPCE for determination of coupling density was confirmed by investigation of a model hapten (p-nitrophenyl-alpha-D-galactoside; PNPG) coupled to KSTI. The PNPG-KSTI conjugates were examined by both HPCE and by spectrophotometric determination of the PNPG density on KSTI. The HPCE method was shown to be efficient in studies of the formation of hapten-protein conjugates and to be more specific than alternative techniques applied for determination of coupling densities.

Enzyme-linked immunosorbent assay for the phytotoxin thevetin

An enzyme-linked immunosorbent assay is reported for monitoring thevetin, an active constituent of the highly poisonous plant Thevetia nerifolia. A thevetin-BSA conjugate was employed as the immunogen and the antibodies raised in rabbits were used for the development of an ELISA. Penicillinase served as the marker enzyme and its conjugation to thevetin by the periodate method is reported for the first time. The present ELISA method could detect 2 ng/ml of thevetin. Cross-reactivity studies with structural analogues and other phytotoxins and drugs of common occurrence in clinical and forensic toxicology established the superiority of the ELISA over the existing analytical methods for determining thevetin in various biospecimens.

Basic evaluation of 67Ga labeled digoxin derivative as a metal-labeled bifunctional radiopharmaceutical

To develop metal-labeled digoxin radiopharmaceuticals with affinity with anti-digoxin antibody as well as Na+,K(+)-ATPase, a digoxin derivative conjugated with deferoxamine was synthesized. The derivative had a high binding affinity with 67Ga at deferoxamine introduced to the terminal sugar ring of digoxin. The 67Ga labeled digoxin derivative showed enough in vitro binding affinity and selectivity to anti-digoxin antibody as well as Na+,K(+)-ATPase. The 67Ga labeled digoxin derivative is considered to be a potential metal-labeled bifunctional radiopharmaceutical for digoxin RIA as well as myocardial Na+,K(+)-ATPase imaging.

Digoxin Immune Fab therapy in the management of digitalis intoxication: safety and efficacy results of an observational surveillance study

An observational surveillance study was conducted to monitor the safety and effectiveness of treatment with Digoxin Immune Fab (Ovine) (Digibind) in patients with digitalis intoxication. Before April 1986, a relatively limited number of patients received treatment with digoxin-specific Fab fragments through a multicenter clinical trial. Beginning with commercial availability in July 1986, this study sought additional, voluntarily reported clinical data pertaining to treatment through a 3 week follow-up. The study included 717 adults who received Digoxin Immune Fab (Ovine). Most patients were greater than or equal to 70 years old and developed toxicity during maintenance dosing with digoxin. Fifty percent of patients were reported to have a complete response to treatment, 24% a partial response and 12% no response. The response for 14% of patients was not reported or reported as uncertain. Six patients (0.8%, 95% confidence interval 0.3% to 1.8%) had an allergic reaction to digoxin-specific antibody fragments. Three of the six had a history of allergy to antibiotic drugs. Twenty patients (2.8%, 95% confidence interval 1.7% to 4.3%) developed recrudescent toxicity. Risk of recrudescent toxicity increased sixfold when less than 50% of the estimated dose of antibody was administered. A total of 215 patients experienced posttreatment adverse events. The events for 163 patients (76%) were judged to result from manifestations of underlying disease and thus considered unrelated to Fab treatment. Digoxin-specific antibody fragments were generally well tolerated and clinically effective in patients judged by treating physicians to have potentially life-threatening digitalis intoxication.

Treatment of digitalis intoxication with emphasis on the clinical use of digoxin immune Fab

Many studies and cases of digitalis intoxication have been reported since the time of William Withering's first publication in 1785. Recognition and management of digitalis toxicity is challenging. Before digoxin immune Fab was commercially available, treatment consisted of managing the signs and symptoms of toxicity until the digitalis was eliminated. Digoxin immune Fab offers a safe, effective, and specific method of quickly reversing digitalis toxicity. Factors that must be considered with the clinical use of this agent include the dosage calculation, administration technique, postdose monitoring, pharmacokinetics, mechanism of action, interference with commercially available digoxin assays, partial neutralizing dosing, rebound of free digoxin, and indications for use. For severe, life-threatening toxicity, digoxin immune Fab is the treatment of choice.

Characterization of digoxin-like materials in human cord serum

Human cord serum contains substantial amounts of materials that cross-react in radioimmunoassays for digoxin. The average content of digoxin-like immunoreactivity in mixed cord serum is 0.3 +/- 0.05 ngE/ml compared to less than 0.05 ngE/ml in serum from normal adults. After solvent extraction, the major digoxin-like materials present in cord serum coelute on Sephadex LH-20 column with the steroid sulfate fraction. The steroid sulfates are digoxin-like materials because they cross-react in the digoxin RIA. If the amount of dehydroepiandrosterone sulfate, 16 alpha-hydroxydehydroepiandrosterone sulfate, and 15 beta-hydroxydehydroepiandrosterone sulfate present in cord serum and the amount of digoxin-like material present in the same serum are considered, then up to 90% of the digoxin-like materials present can be accounted for on the basis of the steroid sulfates present. However, although steroid sulfates contribute to the digoxin immunoreactivity in both serum and breast cyst fluid, digoxin-like materials are present in breast cyst fluid that are not present in cord serum.

Treatment of 150 cases of life-threatening digitalis intoxication with digoxin-specific Fab antibody fragments. Final report of a multicenter study

One hundred fifty patients with potentially life-threatening digitalis toxicity were treated with digoxin-specific antibody fragments (Fab) purified from immunoglobulin G produced in sheep. The dose of Fab fragments was equal to the amount of digoxin or digitoxin in the patient's body as estimated from medical histories or determinations of serum digoxin or digitoxin concentrations. The youngest patient received Fab fragments within several hours of birth, and the oldest patient was 94 years old. Seventy-five patients (50%) were receiving long-term digitalis therapy, 15 (10%) had taken a large overdose of digitalis accidentally, and 59 (39%) had ingested an overdose of digitalis with suicidal intent. The clinical response to Fab was unspecified in two cases, leaving 148 patients who could be evaluated. One hundred nineteen patients (80%) had resolution of all signs and symptoms of digitalis toxicity, 14 (10%) improved, and 15 (10%) showed no response. After termination of the Fab infusion, the median time to initial response was 19 minutes, and 75% of the patients had some evidence of a response by 60 minutes. There were only 14 patients with adverse events considered to possibly or probably have been caused by Fab; the most common events were rapid development of hypokalemia and exacerbation of congestive heart failure. No allergic reactions were identified in response to Fab treatment. Of patients who experienced cardiac arrest as a manifestation of digitalis toxicity, 54% survived hospitalization.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissociation of digoxin-like immunoreactivity and Na+,K+-ATPase inhibitory activity in rat plasma

We measured endogenous digitalis-like factor (EDF) in rat plasma during acute saline infusion by two different procedures. Na+,K+-ATPase inhibitory activity in the rat plasma significantly increased during saline loading (7.8 +/- 2.2 vs 2.5 +/- 0.9%, with and without acute saline loading, respectively, p less than 0.05). On the other hand, the plasma digoxin-like immunoreactivity significantly decreased during acute saline loading (16.9 +/- 1.6 vs 32.0 +/- 2.8 pg digoxin equivalents/ml, with and without acute saline loading, respectively, p less than 0.01). These results indicate that the major substances detected by digoxin-like immunoreactivity and direct Na+,K+-ATPase inhibitory activity are completely different, at least in rat plasma.

Specific binding characteristics of high affinity monoclonal antidigitoxin antibodies

The specificity of various monoclonal antidigitoxin antibodies was characterized using 6 cardiac glycoside analogs. Spleen cells from BALB/c mice, immunized with BSA- or KLH-digitoxin conjugates, were fused with NS1 myeloma cells, and antibody-producing hybrids were identified by radioimmunoassay. Twenty-one monoclonal antidigitoxin-specific antibodies were obtained, 10 of which were cloned and characterized for affinity and specificity. All the antibodies had a high affinity constant, ranging from 8.10(8) to 2.5.10(10) 1/M. On the basis of their binding specificities, the antibodies could be classified into 3 groups: the first contained 7 antibodies exhibiting high cross reactivity (42-100%) with digitoxigenin, whereas the second and third groups did not recognize this analog (cross-reactivity of 1%). In the former group, the absence of the sugar moiety only slightly affected the binding reaction, although for the two other groups, this structure did appear to be involved in antibody recognition. Changes in the functional groups of the hapten molecule led to considerable changes in the antibody-antigen reaction. For all the antibodies except one, saturation of the lactone ring considerably affected binding. These results demonstrated that monoclonal antibodies of different specificities with respect to both the steroid backbone and the sugar moiety of digitoxin can be induced using a digitoxin-protein conjugate.

Human monoclonal antibodies produced by primary in vitro immunization of peripheral blood lymphocytes

A general procedure is described for the production of human monoclonal antibodies from peripheral blood lymphocytes immunized in vitro against T-cell-dependent antigens. These lymphocytes immunized in culture were used to produce human-human or human-mouse hybridomas secreting monoclonal antibodies specific for digoxin, hemocyanin, a recombinant fragment of the gp120 envelope glycoprotein of human immunodeficiency virus (PB1), or a melanoma-associated antigen (p97). Depletion of a lysosome-rich cell population, containing large granular lymphocytes, monocytes, cytotoxic T cells, and a subset of CD8-positive T cells, was shown to be crucial before the cells could be immunized in vitro. This depletion was accomplished by treating the peripheral blood lymphocytes with the lysosomotropic agent L-leucine methyl ester. In addition, the in vitro immunization had to be supported by interleukin 2, gamma-interferon, and B-cell growth and differentiation factors, derived from irradiated, pokeweed-mitogen-stimulated human T cells. The production of human monoclonal antibodies from primary, antigen-specifically activated peripheral lymphocytes might obviate the need to immunize volunteers or patients.

Digitalis toxicity: mechanisms, diagnosis, and management

Digitalis glycosides have a narrow margin between therapeutic and toxic levels. Although the incidence of digitalis toxicity appears to be decreasing, continuation of digoxin therapy in the face of overt toxicity carries a substantial risk of life-threatening cardiac rhythm disturbances. This review will focus primarily on toxicity produced by digoxin and discuss the mechanisms, clinical manifestations, and current management of digitalis toxicity. The appropriate methodology for measurement and interpretation of serum digoxin levels is emphasized. The varied cardiac dysrhythmias induced by digoxin are detailed and their specific management outlined. Immunological (antibody) treatment has now been established and approved for the therapy of advanced, life-threatening digitalis toxicity and the multicenter trial of digitalis antibodies is described. Proper understanding of the pharmacokinetics of digitalis glycosides and careful followup of digitalis treated patients for early evidence of toxicity should help to decrease further the prevalence of toxic reactions.