[Nerium oleander self poisoning treated with digoxin-specific antibodies]

A chronically depressed 44-year-old man was rescued by the French medicalised ambulance service four hours after the ingestion of Nerium oleander leaves in a suicide attempt. Cardiotoxicity was evidenced by the presence of bradycardia with mental confusion and vomiting. The patient was empirically treated in the prehospital phase with a single dose of digoxin-specific Fab antibody fragments (Digidot). In spite of this treatment, the patient presented a new episode of important bradycardia (25 b/minute). Thereafter, the patient's rhythm stabilized and neurological signs and vomiting resolved. The patient recovered uneventfully and was discharged from the intensive care unit two days later.

Pharmacokinetic interaction with digoxin and glucocorticoids in rats detected by radio-immunoassay using a novel specific antiserum

We previously prepared a more specific antiserum (Antiserum-I) to digoxin (Dx) compared with commercially available anti-Dx antiserum (Antiserum-II), clinically used in the therapeutic drug monitoring of Dx. The aims of this study are to compare Dx disposition kinetics by radio-immunoassay (RIA) using Antiserum-I and Antiserum-II, and evaluate the drug-drug interaction with Dx and glucocorticoids in rats. When Dx metabolites were added to rat serum containing Dx, the recovery ratios using Antiserum-I showed 100 to 110% and were remarkably lower than those using Antiserum-II. In rats, serum concentration-time courses of Dx after a single i.v. or p.o. administration of Dx (0.017 mg/kg) by RIA using Antiserum-I were much lower than those using Antiserum-II. The area under the concentration-time course of Dx was significantly lower than that using Antiserum-II and the total body clearance values were significantly higher, while an obvious change of bioavailability was not observed. When using Antiserum-I, rats twice and six times pretreated with dexamethasone (75 mg/kg/day, i.p.) and prednisolone (69 mg/kg/day, i.p.), respectively, showed significant change of the pharmacokinetic parameters of Dx compared with the control rats. In contrast, using Antiserum-II, it took three and nine times of pretreatment with dexamethasone and prednisolone, respectively, to significantly change the parameters of Dx. In conclusion, these results demonstrate that Antiserum-I is very useful not only to more precisely monitor serum Dx levels, but also to determine earlier the drug-drug interaction with glucocorticoids than Antiserum-II.

An enzyme-amplified diffraction-based immunoassay

We have previously shown that a gold-conjugated secondary label can be used to reduce the limit of detection in a diffraction-based assay by more than 40-fold. We now show that by using a combination of a peroxidase-conjugated secondary label and a precipitating substrate the limit of detection in a diffraction-based assay can be reduced by more than 1000-fold. The response to secondary enhancement was linear for concentrations from 50 to 2000 pg/mL of antidigoxin.

Immunologically demonstrable hormones and hormone-like molecules in rat white blood cells and mast cells

The presence or absence of four biologically active hormone or hormone-like molecules was studied in rat immune cells using specific antibodies with flow cytometry and confocal microscopy. Epidermal growth factor (EGF) was not demonstrable at all, digoxin was present only in blood lymphocytes, and insulin was found in the monocyte-macrophage-granulocyte group in peritoneal fluid and thymic lymphocytes. Immunologically demonstrable triiodothyronine (T3) was present in all cells studied (lymphocytes, mast cells and monocyte-macrophage-granulocytes in peritoneal fluid and blood, and thymic lymphocytes). While there is no explanation of the presence of digoxin and insulin, it is assumed that T3 is an extrathyroidal source of the hormone that is needed for maintaining cell proliferation and normal status in the immune system, particularly as it is absent in the case of transitional or durable thyroid deficiency.

Fab antibody fragments: some applications in clinical toxicology

This review provides current information on the use of antigen-binding fragments (Fab) from cleaved antibodies to treat poisoning with digoxin and other potent, low formula mass poisons, such as colchicine and tricyclic antidepressants. Anti-digoxin Fab fragments have been used successfully for many years in the management of severe poisoning with digoxin, digitoxin, and a range of other structurally related compounds, including cardiotoxins from Nerium and Thevetia sp. (oleander) and Bufo sp. (toads). However, their main use remains treating digoxin poisoning. Equimolar doses of anti-digoxin Fab fragments completely bind digoxin in vivo. The approximate dose of Fab fragments (mg) is 80 times the digoxin body burden (mg). If neither the dose ingested nor the plasma digoxin/digitoxin concentration is known, in an adult 380 mg of anti-digoxin Fab fragments should be given. The dose for elderly patients or those with renal impairment should be similar to that for those with normal renal function. Fab fragments have a plasma half-life of 12-20 hours, but this can be prolonged in patients with renal impairment. Analysis of serum ultrafiltrate using an immunoassay shown not to have matrix bias remains the most accurate approach to measuring free digoxin in the presence of anti-digoxin Fab fragments. The antibody fragments are given intravenously over 15-30 minutes after dilution to at least 250 mL with plasma protein solution, 0.9% (w/v) sodium chloride, or deionised water, except in infants where the volume infused can be reduced. Factors limiting the efficacy of Fab fragments are the dose, the duration of the infusion and any delay in administration. Guidelines for Fab fragment administration in children include (i) dilution to a final Fab concentration of 10 g/L in either 5% (w/v) dextrose or 0.9% (w/v) sodium chloride; (ii) infusion through a 0.22 microm filter; (iii) administration of the total dose over a minimum of 30 minutes; and (iv) avoiding coadministration of other drugs and/or electrolyte solutions. Fab fragments are generally well tolerated. Adverse effects attributable to Fab treatment include hypokalaemia and exacerbation of congestive cardiac failure; renal function could be impaired in some patients. Fab fragment preparations for treating acute colchicine and tricyclic antidepressant poisoning have been developed, but are not available commercially. Colchicine poisoning is rare in Western countries, and pharmacological management together with supportive care is usually effective even in severe tricyclic antidepressant overdosage. Attempts have been made to produce anti-paraquat antibodies capable of enhancing paraquat elimination from the lung, but thus far all such attempts have proved unsuccessful.

Application of fluorescence correlation spectroscopy to hapten-antibody binding

Two-photon fluorescence correlation spectroscopy 2P-FCS has received a large amount of attention over the past ten years as a technique that can monitor the concentration, the dynamics, and the interactions of molecules with single molecule sensitivity. In this chapter, we explain how 2P-FCS is carried out for a specific ligand-binding problem. We briefly outline considerations for proper instrument design and instrument calibration. General theory of autocorrelation analysis is explained and straightforward equations are given to analyze simple binding data. Specific concerns in the analytical methods related to IgG, such as the presence of two equivalent sites and fractional quenching of the bound hapten-fluorophore conjugate, are explored and equations are described to account for these issues. We apply these equations to data on two antibody-hapten pairs: antidigoxin IgG with fluorescein-digoxin and antidigitoxin IgG with Alexa488-digitoxin. Digoxin and digitoxin are important cardio glycoside drugs, toxic at higher levels, and their blood concentrations must be monitored carefully. Clearly, concentration assays based on IgG rely on accurate knowledge of the hapten-IgG binding strengths. The protocols for measuring and determining the dissociation constants for both IgG-hapten pairs are outlined and discussed.

Effects of antidigoxin antiserum on endoxin levels, apoptosis and the expression of Bax and Bcl-2 protein in ischaemia-reperfusion myocardium

The aim of the present study was to investigate the effects of antidigoxin antiserum (ADA), an endoxin special antagonist, on endoxin levels, apoptosis and the expression of the apoptosis-related protein bcl-2 and bax in myocardial ischaemia-reperfusion (MIR). The left anterior descending coronary artery was subjected to 30 min ischaemia followed by 45 min reperfusion in open-chest anaesthetized rats. The rats were divided randomly into seven groups: a sham-operated group, an MIR group, a vehicle control (normal saline) group, and groups receiving verapamil (5 mg/kg) or ADA (9, 18 and 36 mg/kg). The drugs were injected into rats via the femoral vein before reperfusion was commenced. Myocardial endoxin levels were measured by radioimmunoassay. Apoptotic cells was detected using the terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick end-labeling method. The expression of the apoptosis-related proteins bcl-2 and bax was detected by immunohistochemistry and their semiquantification scores were recorded by a computer image analysis system. Myocardial endoxin levels, the number of apoptotic cells and bax protein expression were increased in the MIR group compared with the sham group. Although bcl-2 protein expression was elevated in the MIR group, there was no significant difference between the MIR and sham groups. However, the ratio of bcl-2/bax was significantly decreased in the MIR group. In the group receiving 36 mg/kg ADA, myocardial endoxin levels, the number of apoptotic cells and bax protein expression were significantly decreased; bcl-2 protein expression was enhanced. The bcl-2/bax ratio was increased. The results suggest that ADA inhibited myocardial apoptosis induced by MIR in rats. The mechanisms involved require further investigation, but the present study may suggest that ADA prevents bax upregulation and enhances bcl-2 upregulation by antagonizing the effects of endoxin.

Endoxin Antagonist Lessens Myocardial Ischemia Reperfusion Injury

OBJECTIVE

To elucidate whether endoxin is one of important factors involved in myocardial ischemia reperfusion (MIR) injury, the change of myocardial endoxin level was determined in rats with MIR injury model and the effects of anti-digoxin antiserum (ADA), an endoxin specific antagonist, on MIR injury were studied.

METHODS

MIR injury model was obtained by ligating left anterior descending coronary artery 30 min followed by 45 min reperfusion. Sprague-Dawley rats were randomly divided into six groups of 10 rats, each. Sham group, MIR group, normal saline group, ADA 9, 18 and 36 mg.kg(-1). ECG was continuously recorded. After reperfusion left ventricular myocardium samples of ischemic area were processed immediately. Myocardial endoxin level, Na(+)-K(+)-ATPase, Ca(2+)-ATPase, Mg(2+)-ATPase activities, and intramitochondrial Ca(2+) content were measured.

RESULTS

Myocardial endoxin level was significantly increased; Na(+)-K(+)-ATPase, Ca(2+)-ATPase, and Mg(2+)-ATPase activities were remarkably decreased; intramitochondrial Ca(2+) content was remarkably raised; ST segments of ECG were significantly elevated and occurrence and scores of ventricular arrhythmias were significantly increased in early stage of reperfusion in rats with MIR. In all groups with ADA, myocardial endoxin level was remarkably decreased; Na(+)-K(+)-ATPase, Ca(2+)-ATPase and Mg(2+)-ATPase activities were drastically increased; intramitochondrial Ca(2+) content was declined; ST segments and ventricular arrhythmias were improved.

CONCLUSION

Myocardial endoxin level was increased in MIR, which implies that the elevated endoxin may be one of major factors inducing MIR injury. This postulate is supported by the observation that ADA has protective and therapeutic effects against MIR injury probably by antagonizing the action of endoxin. The underlying mechanism may be ascribed to restoration of energy metabolism, and attenuation of intracellular Ca(2+) overload.

Neutralization of free digoxin-like immunoreactive components of oriental medicines Dan Shen and Lu-Shen-Wan by the Fab fragment of antidigoxin antibody (Digibind)

Dan Shen and Lu-Shen-Wan, traditional Chinese medicines used as remedies for heart diseases, demonstrate digoxin-like immunoreactivity. The digoxin-like immunoreactive components of Lu-Shen-Wan show approximately 55% protein binding, while Dan Shen demonstrates concentration-dependent protein binding (68% bound at lower concentrations but only 25% bound at higher concentrations). Because Dan Shen and Lu-Shen-Wan can cause substantial toxic effects in patients, we studied the potential use of Digibind (Fab fragment of polyclonal antidigoxin antibody; Burroughs Wellcome, Research Triangle Park, NC) for neutralizing the pharmacologically active free fractions of Dan Shen and Lu-Shen-Wan. Drug-free serum pools were supplemented with Dan Shen or Lu-Shen-Wan to achieve apparent digoxin concentrations expected in severe overdoses. Aliquots of supplemented serum pools were supplemented further with aqueous Digibind solution to achieve final Digibind concentrations between 5 and 20 microg/mL (expected in vivo range in patients overdosed with digoxin and being treated with Digibind). We observed complete removal of the free apparent digoxin in the presence of Digibind for Dan Shen and Lu-Shen-Wan. Digibind binds free digoxin-like immunoreactive components of Dan Shen and Lu-Shen-Wan in vitro.

Characterization of the neutralizing activity of digoxin-specific Fab toward ouabain-like steroids

Digoxin-specific Fab (Digibind) is a mixture of antidigoxin Fab fragments prepared from sheep sera and is used as a treatment for digoxin poisoning. Digoxin-specific Fab has been shown to neutralize an endogenous Na+/K+ ATPase inhibitor (endogenous digoxin-like Na+/K+ ATPase regulatory factor; EDLF) in rats and humans and to lower blood pressure. Although the exact structure of EDLF is unknown, compounds identical to or structurally related to ouabain, bufalin, and marinobufagenin have been detected in mammalian plasma. In this study, some structural characteristics of EDLF were inferred from the ability of digoxin-specific Fab to neutralize the Na+/K+ ATPase inhibitory activity of several known cardenolides and bufodienolides. Additional structural information was obtained from [3H]ouabain binding and enzyme-linked immunosorbent assay experiments. Digoxin-specific Fab had the ability to interact to some extent with all of the cardenolides and bufodienolides tested. However, digoxin-specific Fab was more than 20-fold more potent in neutralizing ouabain and bufalin than marinobufagenin. The antihypertensive effect of digoxin-specific Fab seen in preeclampsia and animal models of hypertension may therefore be due to a molecule identical to or structurally similar to ouabain or bufalin.

Digoxin-Specific Antibody Fragments

Digitalis glycoside poisoning is an important clinical problem and the development of digoxin-specific antibody fragments (Fab) 30 years ago has changed clinical practice. Nevertheless, doubts still exist as to the appropriate dose indications for therapy. This paper reviews relevant literature, describes the difficulties associated with current treatment protocols and proposes an approach to therapy, which is based on theoretical principles and evidence gleaned from currently available clinical data sets. In patients with 'acute' poisoning, serum digoxin concentrations do not equate to the total body burden, as tissue distribution will not have occurred, and the calculations for present protocols, which use serum concentrations, are therefore likely to result in too much antibody being administered. Since a therapeutic quantity of digoxin will have little effect in a normal individual, complete neutralisation of all digoxin is also unnecessary. The pharmacokinetic and dynamic logic of using a smaller initial loading dose than predicted from total body calculations is rational. It is recommended that half the calculated loading dose, either based on serum concentration or history, should be administered and the impact on clinical features observed. If a clinical response is not seen within 1-2 hours, a further similar dose should be given. In the event of a full response, patients should be monitored for 6-12 hours; a second dose should only be given in the event of recurrence of toxicity. In patients with 'chronic' digoxin poisoning, the serum digoxin concentration will reflect the total body load. However, since such patients are invariably receiving digoxin for therapeutic purposes, full neutralisation is again not indicated. In addition, tissue redistribution of digoxin from deeper stores will occur following the binding of biologically active digoxin in the circulation. This process will occur over a number of hours and if the total calculated dose of antibody is administered in a single bolus, significant quantities will be excreted prior to redistribution of digoxin. Pharmacokinetic logic, therefore, suggests that half the calculated loading dose, based on serum concentration, should be administered and the impact on clinical features observed; a second dose should be given in the event of recurrence of toxicity.

Digoxin antibody prevents cerebral hemorrhage-induced hypertension

BACKGROUND

Brain injury may induce hypertension. Because serum ouabain-like compound (OLC) has vasoconstrictor activity, digoxin antibody antihypertensive effects were evaluated using an intracerebroventricular (ICV) hemorrhage rat model.

METHODS

Four ICV infused Wistar rat groups were studied: control; blood; blood plus digoxin antibody, and cerebrospinal fluid-like solution. Tail-cuff blood pressure, cumulative sodium balance, and serum OLC were measured.

RESULTS

The ICV blood infusion increased blood pressure (BP) and OLC without sodium balance change. Digoxin antibody prevented BP and OLC rise. Blood pressure was positively correlated with OLC in blood and blood plus digoxin antibody rats (R = 0.63; P <.05).

CONCLUSIONS

Cerebral hemorrhage increased OLC and BP, which were reversed by digoxin antibody administration.

[Antidigoxin Fab fragments and digoxin monitoring: a challenge for the biologist]

Following administration of anti-digoxin Fab fragments, monitoring unbound digoxin concentrations may help ensure appropriate dosing, and prevent recrudescent toxicity. Ultrafiltration by using Centrifree system and measurement of digoxin in the ultrafiltrate is considered as reference technique. However, ultrafiltration method is cumbersome, costly, and some immunoassays are affected by matrix differences. Another approach is to analyse the serum directly by digoxin immunoassays without ultrafiltering it. The validity of results obtained depends on the architecture of the immunoassay and the amount of Fab in the sample. The old radioimmunoassays and usually the other competitive immunoassays give inaccurate results. The fluorescence polarization immunoassay (FPIA) slightly underestimates the total digoxin concentrations. Total digoxin levels obtained at 24 hours and 48 hours after treatment permit measurement of the half-life of digoxin Fab complexes and can be used to estimate when the patient can be redigitalized, if necessary. The sequential immunoassays usually overestimate the free digoxin concentrations. The differences observed are >25% and cannot be explained solely by albumin binding (normal range, 20% +/- 5%). To date, ultrafiltration remains the best strategy for accurate determination of digoxin concentrations in the presence of antidigoxin Fab fragments.

Immediate control of life-threatening digoxin intoxication in a child by use of digoxin-specific antibody fragments (Fab)

Digoxin-immune antibody fragments (Fab) for treatment of digitalis intoxication was introduced in 1976. Many reports have been published concerning this therapy for children, but few have focused on its immediate reversal of cardiac as well as extracardiac life-threatening manifestations of digoxin toxicity. We present a case of life-threatening digitalis intoxication in a child with postoperative renal insufficiency, after a Sennings procedure for transposition of the great arteries. Digoxin administration according to the nationally recommended dosage and intervals unexpectedly resulted in serum levels in the toxic range. Severe cardiac arrhythmias, haemodynamic instability and a rapid-increasing serum potassium level resulted. This report demonstrates how administration of Fab according to the manufacturer's dosage recommendation reversed the tachyarrhythmia immediately and re-established a normal level of serum potassium within minutes.

Permanently oriented antibody immobilization for digoxin determination with a flow-through fluoroimmunosensor

This paper reports a new flow-through fluoroimmunosensor, the function of which is based on antibodies immobilized on an inmunoreactor of controlled-pore glass (CPG), for determination of digoxin, used in the treatment of congestive heart failure and artery disease. The immunosensor has a detection limit of 1.20 microg L(-1) and provides high reproducibility (RSD=4.5% for a concentration of 0.0025 mg L(-1), and RSD=6.7% for 0.01 mg L(-1)). The optimum working concentration range was found to be 1.2 x 10(-3)-4.0 x 10(-2) mg L(-1). The lifetime of the immunosensor was about 50 immunoassays; if stored unused its lifetime can be extended to three months. A sample speed of about 10-12 samples per hour can be attained. Possible interference from substances with structures similar to digoxin (morphine, heroin, tebaine, codeine, pentazocine and narcotine) was investigated. No cross-reactivity was seen at the highest digoxin: interferent ratio studied (1:100). The proposed fluoroimmunosensor was successfully used to determine digoxin concentrations in human serum samples.

Effect of the traditional Chinese medicines Chan Su, Lu-Shen-Wan, Dan Shen, and Asian ginseng on serum digoxin measurement by Tina-quant (Roche) and Synchron LX system (Beckman) digoxin immunoassays

Chan Su, Lu-Shen-Wan, Dan Shen, and Asian ginseng are traditionally used to treat a number of conditions, including cardiovascular disease. All of these traditional Chinese medicines exhibit cardioactive properties. Digoxin is a cardioactive drug with a narrow therapeutic range (0.8-1.9 ng/mL). A patient taking digoxin may also take these Chinese medicines for their cardiotonic effects. Moreover, the active components of these medicines that are responsible for cardiotonic effects bear structural similarities to digoxin. Therefore, we studied the potential interference of these Chinese medicines with two digoxin immunoassays--the Tina-quant (Roche Diagnostics) and the Beckman (Synchron LX system)--and compared the values with the fluorescence polarization immunoassay (FPIA; Abbott Laboratories). When very small amounts (2-5 microL) of aqueous extract of Chan Su or Lu-Shen-Wan were added to drug-free serum, we observed high digoxin-like immunoreactivity with the FPIA. In contrast, when ethyl acetate extract of Dan Shen or microliter amounts of ginseng extract were added to drug-free serum, we observed modest digoxin-like immunoreactivity with the FPIA, but no apparent digoxin activity with the Roche and Beckman digoxin immunoassays. When aliquots of a digoxin pool prepared from patients receiving digoxin were supplemented with these Chinese medicines, we observed the most significant interference with the FPIA. The presence of endogenous digoxin-like immunoreactive substances can have additive effects with these Chinese medicines and falsely increase apparent digoxin levels by the FPIA. On the other hand, the Roche and Beckman assays were free from interference from DLIS but showed significant interference from Chan Su and Lu-Shen-Wan. We conclude that the FPIA showed the most significant interference from all four of the Chinese medicines we studied. However, the Roche and Beckman assays showed no interference from two (Dan Shen and Asian ginseng) of the four Chinese medicines we studied.

Modifying specificity of antidigoxin antibodies using insertional mutagenesis

Certain antibodies (Abs) elicited using the cardiac glycoside digoxin (digoxigenin tridigitoxoside) bind preferentially to analogs that differ from digoxin by substitutions on the cardenolide rings, the lactone, or by the presence or absence of attached sugars. Antibody 26-10 binds equally well to digoxin and digitoxin, which differ only by the presence in the former and the absence in the latter of an hydroxyl group at C12. Other antidigoxin Abs, however, can distinguish between these ligands by three orders of magnitude in binding. Inspection of the structure of Fab 26-10 complexed with digoxin shows a gap in complementarity in the region between the digoxin O12 and LCDR3. We proposed that insertions in LCDR3 might result in Abs that bind digitoxin preferentially. We produced libraries of mutants displayed on bacteriophage which were randomized at LCDR3 and contained LCDR3 insertions. Mutants were selected by panning against digoxin and analogs. The mutants bound digitoxin preferentially up to 47-fold greater than digoxin. The mutants that bound well to digitoxin demonstrated a consensus sequence including the substitution of Trp at position L:94. Using site-directed mutagenesis, the binding to digitoxin was shown to be maximized by the combination of an insertion and L:Trp94 mutation, moving the L 94 side chain closer to digoxin. We also selected mutants that bound preferentially to gitoxin, which, like digitoxin, lacks the 12-hydroxyl, increasing relative binding to gitoxin up to 600-fold compared to the unmutated Ab 26-10.

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.

Measuring antibody affinity and performing immunoassay at the single molecule level

Fluorescence correlation spectroscopy (FCS) enables direct observation of the translational diffusion of single fluorescent molecules in solution. When fluorescent hapten binds to antibody, analysis of FCS data yields the fractional amounts of free and bound hapten, allowing determination of the equilibrium binding constant. Equilibrium dissociation constants of anti-digoxin antibodies and corresponding fluorescein-labeled digoxigenin obtained by FCS and fluorescence polarization measurements are identical. It is also possible to follow a competitive displacement of the tracer from the antibody by unlabeled hapten using FCS in an immunoassay format. The fluorescence polarization immunoassay for vancomycin detection was used to test the FCS approach. Fitting of the FCS data for the molar fractions of free and bound fluorescein-labeled vancomycin yielded a calibration curve which could serve for determination of the vancomycin concentration in biological samples.

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.

Complementary combining site contact residue mutations of the anti-digoxin Fab 26-10 permit high affinity wild-type binding

Antibody 26-10, obtained in a secondary immune response, binds digoxin with high affinity (K(a) = 1.3 x 10(10) M(-1)) because of extensive shape complementarity. We demonstrated previously that mutations of the hapten contact residue HTrp-100 to Arg (where H refers to the heavy chain) resulted in increased specificity for digoxin analogs substituted at the cardenolide 16 position. However, mutagenesis of H:CDR1 did not result in such a specificity change despite the proximity of the H:CDR1 hapten contact residue Asn-35 to the cardenolide 16 position. Here we constructed a bacteriophage-displayed library containing randomized mutations at H chain residues 30-35 in a 26-10 mutant containing Arg-100 (26-10-RRALD). Phage were selected by panning against digoxin, gitoxin (16-OH), and 16-acetylgitoxin coupled to bovine serum albumin. Clones that retained wild-type Asn at position 35 showed preferred binding to gitoxin, like the 26-10-RRALD parent. In contrast, clones containing Val-35 selected mainly on digoxin-bovine serum albumin demonstrated a shift back to wild-type specificity. Several clones containing Val-35 bound digoxin with increased affinity, approaching that of the wild type in a few instances, in contrast to the mutation Val-35 in the wild-type 26-10 background, which reduces affinity for digoxin 90-fold. It has therefore proven possible to reorder the 26-10 binding site by mutations including two major contact residues on opposite sides of the site and yet to retain high affinity for binding for digoxin. Thus, even among antibodies that have undergone affinity maturation in vivo, different structural solutions to high affinity binding may be revealed.

Immunoaffinity screening with capillary electrochromatography

Highly efficient capillary electrochromatographic separations of cardiac glycosides and other steroids are presented. Employing butyl-derivatized silica particles as stationary phase resulted in a nearly three times faster electroosmotic flow (EOF) compared to capillary electrochromatography (CEC) with octadecyl silica particles. On-column focusing with a preconcentration factor of 180 was performed and separation efficiencies of up to 240,000 plates per meter were obtained. Using label-free standard UV absorbance, detection limits of 10-80 nM were reached for all steroids tested. For screening of cardiac glycosides, e.g., digoxin and digitoxin in mixtures of steroids, CEC was combined with immunoaffinity extraction using immobilized polyclonal anti-digoxigenin antibodies and F(ab) fragments. Simply adding small amounts of antibody carrying particles to the samples and comparing chromatograms before and after antibody addition allowed screening for high affinity antigens in mixtures with moderate numbers of compounds. Under conditions of competing antigens, affinity fingerprints of immobilized anti-digoxigenin and anti-digitoxin antibodies were obtained, reflecting the cross-reactivity of eleven steroids. The method provides high selectivity due to the combination of bioaffinity interaction with highly efficient CEC separation and UV detection at several wavelengths in parallel. This selectivity was exploited for the detection of four cardiac glycosides in submicromolar concentrations in an untreated urine sample.

In vivo digoxin-like immunoreactivity in mice and interference of Chinese medicine Danshen in serum digoxin measurement: elimination of interference by using a chemiluminescent assay

BACKGROUND

Danshen, a traditional Chinese medicine used in the management of cardiovascular diseases, is available without prescription in the US. Because Danshen is used to treat cardiovascular diseases, we studied the potential interference of Danshen with serum digoxin measurement using various immunoassays.

METHODS

Blood was collected 1 day before and then 1 and 2 h after feeding mice with Danshen. The apparent digitoxin activities were measured by the fluorescence polarization immunoassay (FPIA). We also added microliter amounts of Danshen extract to digoxin pools prepared from patients receiving digoxin. The digoxin concentrations were measured using the fluorescence polarization immunoassay (FPIA), microparticle enzyme immunoassay (MEIA) and chemiluminescent assay (CLIA). The observed values were compared with original values. We also fed mice with Danshen.

RESULTS

We observed measurable digoxin-like immunoreactivity in sera of mice after feeding with Danshen. We also observed falsely lower digoxin concentrations (negative interference) when MEIA was used for digoxin measurement. However, serum digoxin concentrations were falsely elevated with FPIA. We observed no interference of Danshen in serum digoxin measurement using the CLIA.

CONCLUSION

Danshen appears to contain digoxin-like immunoreactivity but does not interfere with serum digoxin measurement when CLIA was used.

Cysteine-free mutant of aequorin as a photolabel in immunoassay development

The bioluminescent protein aequorin is a sensitive label that has been employed in a number of analytical applications. A mutant of aequorin with enhanced stability produced recombinantly in our laboratory has been employed as a label in the development of an immunoassay for digoxin. Digoxin is a cardiac glycoside used in the treatment of congestive heart failure. This drug has a very narrow therapeutic range of 0.8-2.0 ng/mL (1.0-2.5 nmol/L), thus requiring therapeutic drug monitoring. In this study, a derivative of digoxigenin was chemically conjugated to the mutant aequorin, and the resulting protein-digoxigenin derivative conjugates were characterized in terms of their luminescence properties. A solid-phase immunoassay for digoxin was then developed. The detection limit of the assay for digoxin was 1 x 10(-12) M. To demonstrate the use of this mutant aequorin as a label in biological sample analysis without any need for pretreatment of the samples, the assay was tested in serum spiked with digoxin. Interference from digoxin analogues was also evaluated to determine the specificity of the assay.