21-Benzylidene digoxin decreases proliferation by inhibiting the EGFR/ERK signaling pathway and induces apoptosis in HeLa cells

Cardiotonic steroids (CTS) are agents traditionally known for their capacity to bind to the Na,K-ATPase (NKA), affecting the ion transport and the contraction of the heart. Natural CTS have been shown to also have effects on cell signaling pathways. With the goal of developing a new CTS derivative, we synthesized a new digoxin derivative, 21-benzylidene digoxin (21-BD). Previously, we have shown that this compound binds to NKA and has cytotoxic actions on cancer, but not on normal cells. Here, we further studied the mechanisms of actions of 21-BD. Working with HeLa cells, we found that 21-BD decreases the basal, as well as the insulin stimulated proliferation. 21-BD reduces phosphorylation of the epidermal growth factor receptor (EGFR) and extracellular-regulated kinase (ERK), which are involved in pathways that stimulate cell proliferation. In addition, 21-BD promotes apoptosis, which is mediated by the translocation of Bax from the cytosol to mitochondria and the release of mitochondrial cytochrome c to the cytosol. 21-BD also activated caspases-8, -9 and -3, and induced the cleavage of poly (ADP-ribose) polymerase-1 (PARP-1). Altogether, these results show that the new compound that we have synthesized exerts cytotoxic actions on HeLa cells by inhibition of cell proliferation and the activation of both the extrinsic and intrinsic apoptotic pathways. These results support the relevance of the cardiotonic steroid scaffold as modulators of cell signaling pathways and potential agents for their use in cancer.

Evaluation of Doxorubicin Cardiotoxicity in Patients Treated Intermittently with Beta-Methyldigoxin

Twenty-one patients with various advanced neoplasms were treated with 60 to 75 mg/m2 of doxorubicin every 3 to 4 weeks and monitored by ECG and systolic time intervals (PEP/LVET) with the aim to establish whether a pretreatment with beta-methyldigoxin, administered intermittently, could prevent doxorubicin-induced cardiotoxicity. It was found that until patients received digitalis pretreatment the PEP/LVET ratio did not change significantly from mean basal values even after the highest cumulative dosages of doxorubicin. However, after interruption of the therapy with both drugs, PEP/LVET increased reaching a value not significantly different from that observed in a comparable group of patients treated only with doxorubicin. Moreover, of 9 patients who reached the cumulative limiting dose, 2 developed congestive heart failure. These results question the possibility that digitalis administered according to an intermittent treatment scheme may prevent doxorubicin cardiomyopathy.

21-Benzylidene digoxin: a proapoptotic cardenolide of cancer cells that up-regulates Na,K-ATPase and epithelial tight junctions

Cardiotonic steroids are used to treat heart failure and arrhythmia and have promising anticancer effects. The prototypic cardiotonic steroid ouabain may also be a hormone that modulates epithelial cell adhesion. Cardiotonic steroids consist of a steroid nucleus and a lactone ring, and their biological effects depend on the binding to their receptor, Na,K-ATPase, through which, they inhibit Na+ and K+ ion transport and activate of several intracellular signaling pathways. In this study, we added a styrene group to the lactone ring of the cardiotonic steroid digoxin, to obtain 21-benzylidene digoxin (21-BD), and investigated the effects of this synthetic cardiotonic steroid in different cell models. Molecular modeling indicates that 21-BD binds to its target Na,K-ATPase with low affinity, adopting a different pharmacophoric conformation when bound to its receptor than digoxin. Accordingly, 21-DB, at relatively high µM amounts inhibits the activity of Na,K-ATPase α1, but not α2 and α3 isoforms. In addition, 21-BD targets other proteins outside the Na,K-ATPase, inhibiting the multidrug exporter Pdr5p. When used on whole cells at low µM concentrations, 21-BD produces several effects, including: 1) up-regulation of Na,K-ATPase expression and activity in HeLa and RKO cancer cells, which is not found for digoxin, 2) cell specific changes in cell viability, reducing it in HeLa and RKO cancer cells, but increasing it in normal epithelial MDCK cells, which is different from the response to digoxin, and 3) changes in cell-cell interaction, altering the molecular composition of tight junctions and elevating transepithelial electrical resistance of MDCK monolayers, an effect previously found for ouabain. These results indicate that modification of the lactone ring of digoxin provides new properties to the compound, and shows that the structural change introduced could be used for the design of cardiotonic steroid with novel functions.

[Investigation of selective inhibition of digoxin derivative on retinoic acid-related orphan nuclear receptor γt transcription activity using molecular docking]

OBJECTIVE

Psoriasis is an autoimmune-related chronic inflammatory skin disease strongly associated with the dysfunction of Th17 cells. Retinoic acid-related orphan nuclear receptor γt (RORγt) plays a critical role in the differentiation and maturation of Th17 cells and in cell-derived immunologic derangement. We conducted this study to investigate potential mechanism by which the derivative of digoxin selectively antagonizes RORγt transcriptional activity.

METHOD

Using molecular docking in combination with molecular electrostatic potential (MEP), we detected the interaction between the derivative of digoxin (Dhd) and ROR transcription factor (RORα,RORβ and RORγt), and the results were further confirmed by bioluminescent assay.

RESULT

Molecular docking demonstrated that Dhd could exclusively inhibit the conformation of RORγt; bioluminescent assay further indicated that RORγt was selectively antagonized by Dhd in a dose- and time-dependent manner.

CONCLUSION

Dhd can selectively suppress RORγt transcriptional activity.

Digoxin derivatives substituted by alkylidene at the butenolide part

A series of digoxin derivatives containing the gamma-alkylidene butenolide moiety were synthesised by way of stereoselective vinylogous aldol reaction of the unactivated butenolide in simple conditions. The structures of compounds synthesised were characterised by infrared (IR), nuclear magnetic resonance (NMR) and HR-MS. Preliminary bioassay shows that some of them have cardiac functions, especially compound 2g that induced a marked increase in myocardial contractility at 10ngml(-1) and 20ngml(-1) concentrations without digitalis toxicity.

Effects of Digoxin and Gitoxin on the Enzymatic Activity and Kinetic Parameters of Na+/K+-ATPase

Inhibition of Na+/K+-ATPase activity from human erythrocyte membranes and commercial porcine cerebral cortex by in vitro single and simultaneous exposure to digoxin and gitoxin was investigated to elucidate the difference in the mechanism of the enzyme inhibition by structurally different cardiac glycosides. The drugs exerted a biphasic dose-dependent inhibitory effect on the enzyme activity in both tissues, supporting the existence of two sensitive Na+/K+-ATPase isoforms. The IC50 values for the low and high affinity isoforms were calculated from the inhibition curves using mathematical analysis. The Hill coefficient (n) fulfilled the relationship 1 < n < 3, suggesting cooperative binding of inhibitors to the enzyme. Kinetic analysis showed that digoxin and gitoxin inhibited Na+/K+-ATPase by reducing the maximum enzymatic velocity (Vmax) and Km, implying an uncompetitive mode of interaction. Both the isoforms were always more sensitive to gitoxin. The erythrocyte enzyme was more sensitive to the inhibitors in the range of low concentrations but the commercial cerebral cortex enzyme exerted a higher sensitivity in high inhibitors affinity concentration range. By simultaneous exposure of the enzyme to digoxin and gitoxin in combinations a synergistic effect was achieved by low inhibitor concentrations. An antagonistic effect was obtained with erythrocyte membrane enzyme at high inhibitors concentration.

Detection and quantification of affinity ligand leaching and specific antibody fragment concentration within chromatographic fractions using surface plasmon resonance

Rapid analyses of chromatographic steps within a biopharmaceutical manufacturing process are often desirable to evaluate column performance, provide mass balance data and to permit accurate calculations of yields and recoveries. Using SPR (surface plasmon resonance) biosensor (Biacore) technology, we have developed a sandwich immunoassay to quantify polyclonal anti-digoxin Fab fragments used for the production of the FDA (Food and Drug Administration)-approved biotherapeutic DigiFab. The results show that specific Fab may be quantified in all affinity process streams and accurate yield and mass balance data calculated. Control experiments using sheep Fab and Fc indicate that the assay is specific to DigiFab. The quantification of potential leached ligand within chromatographic fractions may also be technically challenging, particularly when low-molecular-mass ligands are covalently coupled with an affinity absorbent. Typical methods to assess ligand leakage such as DDMA (digoxin-dicarboxymethoxylamine; digoxin analogue) often involve the use of labelled ligands and relatively complex and labour-intensive analytical techniques. Using the same analytical methodologies, an assay to detect leached or eluted ligand off the column was developed. The results indicate minimal levels of leached ligand in all chromatographic fractions, with total levels of leached DDMA calculated to be 1.52 microg. This is less than 0.01% of the total amount of DDMA coupled with the laboratory-scale affinity column. The SPR methods described in the present study may be applicable for the rapid in-process analysis of specific polyclonal Fab fragments (within a polyclonal mixture) and to rapidly assess leakage of small molecule ligands covalently attached to chromatographic supports.

Simple coacervates of zein to encapsulate Gitoxin

This work reports the use of simple coacervates of the hydrophobic protein zein to encapsulate Gitoxin, a cardiotonic glycoside. The microspheres obtained using ethanol, methanol, iso-propyl alcohol were characterized using viscosity index, scanning electron microscopy (SEM) and laser light scattering particle analyzer. Scanning electron micrographs indicated that the zein film was made of microspheres with diameter in the 1-1.5 microm range, which could be controlled. Sizes of Gitoxin-loaded zein microspheres changed little before and after release of the drug because of conglutination among zein microspheres. Release of Gitoxin from zein microspheres, were performed in vitro to investigate the mechanism of model drug release. The results show that the zein microspheres obtained using ethanol are best suited for use as a sustained-release form of Gitoxin. The microspheres may also be useful in drug targeting system since the diameter of the microspheres is appropriate for phagocytosis by macrophages. Both zein film and Gitoxin-loaded zein microsphere film were effective in suppressing platelet adhesion.

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.

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.

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.

Anti-tumour activity of Digitalis purpurea L. subsp. heywoodii

Recent research has shown the anticancer effects of digitalis compounds suggesting their possible use in medical oncology. Four extracts obtained from the leaves of Digitalis purpurea subsp. heywoodii have been assessed for cytotoxic activity against three human cancer cell lines, using the SRB assay. All of them showed high cytotoxicity, producing IC50 values in the 0.78 - 15 microg/mL range with the methanolic extract being the most active, in non toxic concentrations. Steroid glycosides (gitoxigenin derivatives) were detected in this methanolic extract. Gitoxigenin and gitoxin were evaluated in the SRB assay using the three human cancer cell lines, showing IC50 values in the 0.13 - 2.8 microM range, with the renal adenocarcinoma cancer cell line (TK-10) being the most sensitive one. Morphological apoptosis evaluation of the methanolic extract and both compounds on the TK-10 cell line showed that their cytotoxicity was mediated by an apoptotic effect. Finally, possible mechanisms involved in apoptosis induction by digitalis compounds are discussed.

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.

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.

A single H:CDR3 residue in the anti-digoxin antibody 26-10 modulates specificity for C16-substituted digoxin analogs

We constructed Fab libraries of bacteriophage-displayed H:CDR3 mutants in the high-affinity anti-digoxin antibody 26-10 to determine structural constraints on affinity and specificity for digoxin. Libraries of mutant Fabs randomized at five or 10 contiguous positions were panned against digoxin and three C16-substituted analogs, gitoxin (16-OH), 16-formylgitoxin and 16-acetylgitoxin. The sequence data from 83 different mutant Fabs showed highly restricted consensus patterns at positions H:100, 100a and 100b for binding to digoxin; these residues contact digoxin in the 26-10:digoxin co-crystal structure. Several mutant Fabs obtained following panning on digoxin-BSA showed increased affinity for digoxin compared with 26-10 and retained the wild-type (wt) Trp at position 100. Those Fabs selected following panning on C16-substituted analogs showed enhanced binding to the analogs. Replacement of H:Trp100 by Arg resulted in mutants that bound better to the analogs than to digoxin. This specificity change was unexpected, as C16 lies on the opposite side of digoxin from H:CDR3. Substitution of wt Trp by Arg appears to alter specificity by allowing the hapten to shift toward H:CDR3, thereby providing room for C16 substituents in the region of H:CDR1.

Phage display-selected sequences of the heavy-chain CDR3 loop of the anti-digoxin antibody 26-10 define a high affinity binding site for position 16-substituted analogs of digoxin

The heavy-chain CDR3 region of the high affinity (K(a) = 1.3 x 10(10) M(-)1) anti-digoxin monoclonal antibody 26-10 was modified previously to shift its specificity, by substitution of tryptophan 100 by arginine, toward binding analogs of digoxin containing substitutions at position 16. To further change specificity, two 5-mer libraries of the randomly mutagenized phage-displayed 26-10 HCDR3 region (positions 94-98) were panned against digoxin-bovine serum albumin (BSA) as well as against 16-acetylgitoxin-BSA. When a mutant Fab that binds 16-substituted analogs preferentially was used as a parent sequence, clones were obtained with affinities for digoxin increased 2-4-fold, by panning on digoxin-BSA yet retaining the specificity shift. Selection on 16-acetylgitoxin-BSA, however, resulted in nine clones that bound gitoxin (16-OH) up to 150-fold higher than the wild-type 26-10, due to a consensus mutation of Ser(H95) to Gly(H95). The residues at both position H95 (serine) and position H100 (tryptophan) contact hapten in the crystal structure of the Fab 26-10-digoxin complex. Thus, by mutating hapten contact residues, it is possible to reorder the combining site of a high affinity antibody, resulting in altered specificity, yet retain or substantially increase the relative affinity for the cross-reactive ligand.

Detection of endogenous digitalis-like immunoreactive factors in human blood

Digitalis-like immunoreactive factors (DLIF) are special types of steroids with lactone rings in their structures. Clinically, this type of compound can be used as medicine for heart failure; thus, the elevated endogenous DLIF found under certain pathological conditions are interferent substances in digoxin immunoassay. Endogenous DLIF with biological and immunological properties similar to cardiotonic drugs, such as digoxin, have been found in several tissues and body fluids of animals and humans. Since these endogenous Na+, K(+)-ATPase inhibitors can be considered hormones in nature, immunoassays must be selected detection of them to achieve the required sensitivity and specificity. In this study, we used three sets of in-house formulated immunoassays for DLIF and ouabain-like factors (OLF) detection. Using a polyclonal antibody-based ouabain enzyme immunoassay, the mean +/- S.E.M. of OLF in the sera of 10 healthy individuals were determined to be (9.1 +/- 0.9) x 10(-11) M. Using a monoclonal antibody-based ouabain enzyme immunoassay, the mean +/- S.E.M of OLF in the sera of 10 healthy individuals was (8.2 +/- 1.2) x 10(-11) M while using a antibody fragment Fab-based enzyme immunoassay for digoxin, the mean +/- S.E.M of DLIF in 11 healthy individuals was (4.0 +/- 1.2) x 10(-10) M. In conclusion, our immunological data indicate that DLIFs are normal constituents of human blood. Although DLIF is the major component, coexistence of OLF with DLIF in healthy individuals can not be excluded.

Studies on cardiac ingredients of plants. XIII: Chemical modification of gitoxin to cardiotonic compounds without vascular effect

Nitrated gitoxins (4) and bufotoxin homologues with various lengths of alkyl chain at C-3 of the steroid nucleus (10) were prepared from gitoxin (1). The pharmacological activities of the resulting compounds (4 and 10) were evaluated by measurement of inhibitory effect on NA+, K(+)-adenosine triphosphatase (ATPase) prepared from dog kidney, positive inotropic effect (PIE) on isolated guinea-pig papillary muscle preparations, and the effect on smooth muscle using the mesenteric artery from spontaneously hypertensive rats. Most of the compounds showed a smaller contractile effect on the arterial muscle. Among these compounds, gitoxin 3"-nitrate (4g) exhibited the most desirable biological activities, such as PIE comparable to that of 1, 1.25 times wider concentration-dependent range than 1, and lack of contractile activity on vascular muscle.

Gitaloxin poisoning in a child

Gitaloxin is a digitalis glycoside used for the same indications as digoxin and digitoxin. The successful outcome for a 2 1/2-year-old boy who accidentally ingested 3 mg of gitaloxin (100 times the normal therapeutic dose) is reported. At admission the child presented with irregular heart rhythm. He subsequently started vomiting, even after continuous gastric feeding. Only 48 h after ingestion of gitaloxin he became somnolent and developed bradyarrhythmia. The symptoms disappeared 96 h later; the bradyarrhythmia, however, (second-degree atrioventricular block) decreased progressively only after 120 h. The initial clinical presentation of gitaloxin poisoning may be misleading and careful observation in a pediatric intensive care unit is mandatory. A cross-reaction between the fluorescence polarization immunoassay for digitoxin and the radioimmunoassay for gitaloxin was found and was used as a helpful, but rough, estimate of the severity of gitaloxin poisoning, in the absence of a specific measurement of gitaloxin.

Ultra-specific immunoassays for small molecules: roles of wash steps and multiple binding formats

New immunometric forms of immunoassay are much more flexible to use than competitive-format immunoassays for small molecular analytes. An example of the utility of this flexibility is the ability to wash the capture antibody after it has been exposed to analyte but before addition of the labeled reagent. This simple maneuver has a large impact on the specificity obtained from already highly specific assays. We also show that specificity can be further increased by means of our multiple binding assay approach, in which the final reading reflects analyte binding to two different primary capture monoclonal antibodies.

Evidence for presence of a reduced form of digoxin-like immunoreactive factor (dihydro-DLIF) in mammalian tissues

Digoxin-like immunoreactive factor (DLIF) from adrenal glands is an endogenous ligand structurally related to the plant-derived cardiac glycoside digoxin. Cardiac glycosides regulate the activity of the sodium pump and thus play key roles in disease processes involving regulation of ion transport. We now report the discovery of an endogenous dihydro-DLIF analogous to dihydrodigoxin. We used HPLC, ultraviolet spectrophotometry, and cross-reactivity with two antibodies, one specific for digoxin and one for dihydrodigoxin, to support the hypothesis that dihydro-DLIF contains a chemically reduced lactone ring. The spectral absorbance maximum for dihydro-DLIF is at 196 nm, identical to dihydrodigoxin. DLIF and dihydro-DLIF are 975- and 2588-fold less immunoreactive than digoxin and dihydrodigoxin for their respective antibodies. The molar ratio of dihydro-DLIF to DLIF is approximately 5.3 in bovine adrenocortical tissue and approximately 0.38 in human serum. Dihydrodigoxin (reduced lactone ring) added to microsomes isolated from bovine adrenal cortex produced a 4.5-fold increase in digoxin-like immunoreactivity (oxidized lactone ring) after 3 h of incubation. The biotransformation is likely mediated by a cytochrome P-450 NADPH-dependent process. Our findings demonstrate the presence of a dihydro-DLIF in mammals and suggest a metabolic route for synthesis of endogenous DLIF in mammalian tissue.

Digoxin dialdehyde reductive aminations. Structure proof of the perhydro-1,4-oxazepine product

Digoxin dialdehyde reportedly undergoes reductive amination with primary amines to form a perhydro-1,4-oxazepine; however, no structural proof has been published to substantiate this belief. A digoxin perhydro-1,4-oxazepine derivative has been isolated from the reductive amination of digoxin dialdehyde and its structure determined by mass spectroscopy and NMR measurements. Comparison of the NMR, mass spectroscopy, and HPLC of two compounds obtained from the degradation of the digoxin reductive amination product with synthesized perhydro-1,4-oxazepine diastereomers showed them to be identical. We conclude that, under appropriate conditions, reductive amination products can be obtained, but caution that other products may be produced as well, especially under the conditions of bioconjugation to proteins.

Structure and specificity of the anti-digoxin antibody 40-50

We determined the sequence, specificity for structurally related cardenolides, and three-dimensional structure of the anti-digoxin antibody 40-50 Fab in complex with ouabain. The 40-50 antibody does not share close sequence homology with other high-affinity anti-digoxin antibodies. Measurement of the binding constants of structurally distinct digoxin analogs indicated a well-defined specificity pattern also distinct from other anti-digoxin antibodies. The 40-50-ouabain Fab complex crystallizes in space group C2 with cell dimensions of a = 93.7 A, b = 84.8 A, c = 70.1 A, beta = 128.0 degrees. The structure of the complex was determined by X-ray crystallography and refined at a resolution of 2.7 A. The hapten is bound in a pocket extending as a groove from the center of the combining site across the light chain variable domain, with five of the six complementarity-determining regions involved in interactions with the hapten. Approximately three-quarters of the hapten surface area is buried in the complex; two hydrogen bonds are formed between the antibody and hapten. The surface area of the antibody combining site buried by ouabain is contributed equally by the light and heavy chain variable domains. Over half of the surface area buried on the Fab consists of the aromatic side-chains. The surface complementarity between hapten and antibody is sufficient to make the complex specific for only one lactone ring conformation in the hapten. The crystal structure of the 40-50-ouabain complex allows qualitative explanation of the observed fine specificities of 40-50, including that for the binding of haptens substituted at the 16 and 12 positions. Comparison of the crystal structures of 40-50 complexed with ouabain and the previously determined 26-10 anti-digoxin Fab complexed with digoxin, demonstrates that the antibodies bind these structurally related haptens in different orientations, consistent with their different fine specificities. These results demonstrate that the immune system can generate antibodies that provide diverse structural solutions to the binding of even small molecules.

Digoxin immunoassay with cross-reactivity of digoxin metabolites proportional to their biological activity

Our objective was to identify commercially available digoxin immunoassays whose cross-reactivity with digoxin metabolites paralleled the pharmacological activity of the metabolites. We measured the immunoreactivity of digoxigenin bis- and monodigitoxosides, digoxigenin, and dihydrodigoxin in four immunoassays and compared the immunoactivities with pharmacological activities from studies involving whole-animal and receptor (Na,K-ATPase)-based assays. Correlation coefficients for comparisons of immunoassay reactivity and human heart receptor reactivities were: ACS, 0.96; TDx, 0.60; Stratus, 0.57; and Magic, 0.42. Comparison with other biological assays showed a similar trend. The major difference in metabolite cross-reactivities among the immunoassays was that of digoxigenin (ACS, 0.7%; TDx, 103%; Stratus, 108%; Magic, 153%), which has approximately 10% bioactivity relative to digoxin. Measured recovery of mixtures of digoxin and metabolites confirmed these findings. We conclude that the monoclonal antibody in the ACS digoxin assay closely mimics Na,K-ATPase in detecting digoxin and its metabolites. This finding provides a basis for developing therapeutic drug monitoring immunoassays capable of approximating the true pharmacological activity of a mixture of drug metabolites.

Syntheses of europium-labeled digoxin derivatives and their use in time-resolved fluoroimmunoassay

A simple and efficient method of derivatizing and labeling the terminal sugar ring of digoxin (and other steroid glycosides) is described. The terminal sugar ring was oxidized by periodate to dialdehyde, followed by a reaction with the aminooxy group of heterobifunctional spacer reagent. Usually, reductive amination is needed to stabilize the Schiff base-type compounds produced in reactions between amino and aldehyde groups. Here the oxime compounds produced are stable wherefore the reductive amination step can be avoided and the reaction gives high yields without significant side reactions. After characterization the digoxin dioxime derivatives formed were labeled with europium chelates. These labeled digoxin derivatives bearing one or two europium-containing chelates, coupled via different spacers, were purified and tested in a competitive time-resolved fluoroimmunoassay. Several digoxin-specific monoclonal antibodies were also tested to determine the most suitable antibody-tracer combination, which was then employed to develop a simple competitive time-resolved fluoroimmunoassay for digoxin.

Extraordinary efficacy of 16 alpha-gitoxin, an ultra-short acting semi-synthetic digitalis glycoside

The positive inotropic effect of 16 alpha-gitoxin was studied in 52 female and male healthy volunteers between the ages of 21 and 27 years and a body weight between 55 and 76 kg after the single i.v. injection or oral administration in doses between 2.5 and 15 mg. The inotropic effect was evaluated by the use of the systolic time intervals (left ventricular ejection time, LVET, and electromechanic systole, QS2) and by the QT-Interval of the ECG. The glycoside plasma level was measured simultaneously to the pharmacodynamic parameters by a radioimmunoassay, 16 alpha-gitoxin abbreviated dose-dependently the STI (LVET and QS2) for 30 (i.v.) or 15 ms (per os) and the QT interval for 35 ms. A mean plasma concentration of 401 ng/ml was measured 15 minutes after the i.v. injection of 5 mg and a mean plasma level of 402 ng/ml was measured 1 hour after the oral intake of 10 mg 16 alpha-gitoxin as an aqueous-ethanolic solution. The shortening of the STI-values and of the QT-interval correlated with the administered glycoside dose and with the AUC0-6-value. The AUC-values increased linearly with the administered dose. The ratio of the AUC0-6-values after i.v. or oral administration of 7.5 mg 16 alpha-gitoxin amounted to 0.673. Doses of 16 alpha-gitoxin which induce a maximum abbreviation of STI or QT did neither provoke a distinct decrease in heart rate nor cardiac disturbances. Sixteen-alpha-gitoxin develops a positive inotropic effect within a few minutes after i.v. injection and also after oral intake.(ABSTRACT TRUNCATED AT 250 WORDS)

High-performance thin-layer chromatographic determination of digoxin and related compounds, digoxigenin bisdigitoxoside and gitoxin, in digoxin drug substance and tablets

A high-performance thin-layer chromatographic (HPTLC) method for the determination of digoxin and its related compounds digoxigenin bisdigitoxoside (DBD) and gitoxin in digoxin drug substance and tablets was developed. Separation of the three compounds was accomplished on a C18 wettable reversed-phase plate using water-methanol-ethyl acetate (50:48:2, v/v/v) as the mobile phase. The analytes were determined by densitometry using absorbance for digoxin and fluorescence for the two related compounds. All peaks were quantified by peak-height analysis. Linear regression analysis of the data was performed for all three compounds. The calibration range for digoxin was set at 320-480 ng per 5-mm band, equivalent to 80-120% (w/w) of a 400-ng band load, that for DBD was set at 4-12 ng per 5-mm band, equivalent to 1-3% (w/w) of the digoxin load, and that for gitoxin was set at 0.4-1.6 ng per 5-mm band, equivalent to 0.1-0.4% (w/w) of the digoxin load. The limit of quantification (LOQ) for digoxin was 64 ng per 5-mm band with a limit of detection (LOD) of 8 ng per 5-mm band. The LOQs for both DBD and gitoxin were 0.12 ng per 5-mm band with LODs of 0.4 ng per 5-mm band. The linearity range for the digoxin peak height in the absorbance mode was 0-5000 ng per 5-mm band. The linearity range for DBD and gitoxin peak heights in the fluorescence mode was 0-2000 ng per 5-mm band.

Development of glycoside-bound radiopharmaceuticals: novel radioiodination method for digoxin

We combined 2-hydroxy-3-methylbenzoylhydrazide (HMBH) with glycosides as a novel method for the radioiodination of physiologically active glycosides. This method was tested using digoxin, which is one of the cardiac glycosides. A digoxin-HMBH conjugate was synthesized by periodate cleavage of the third sugar ring, and was readily radiolabeled with Na[125I] by the chloramine-T method. 125I labelled digoxin-HMBH conjugate retained Na+, K(+)-ATPase binding in vivo and in vitro, and also retained immunoreactivity to an anti-digoxin antibody. Thus, this 125I labelled digoxin-HMBH conjugate represents a potential radiopharmaceutical for Na+, K(+)-ATPase imaging, as well as for the radioimmunoassay of digoxin.

An anti-digoxin monoclonal antibody seems to express more than one functional paratope

An anti-digoxin monoclonal antibody (mAb 4G3) has been produced and characterized with respect to its fine specificity and affinity. In an independent series of experiments anti-idiotypic monoclonal antibody (mAb 7G9) was selected which reacted with the antigen-binding center of an anti-human chorionic gonadotropin monoclonal antibody (anti-hCG mAb 1B10). In detailed studies on its binding characteristics it has been shown that mAb 4G3 binds to an anti-idiotypic monoclonal antibody mAb 7G9 in solution. Western blotting experiments showed that mAb 4G3 reacted against antiidiotypic antibody under non-reducing conditions, only. Moreover, mAb 4G3 has been shown to express self-binding properties. Absorption with saturating amounts of its specific hapten, i.e. digoxin, did not change the binding of mAb 4G3 to anti-idiotypic antibody and its self-binding ability. It is speculated on the basis of these data that mAb 4G3 possesses more than one functional paratope.

Identification and preliminary characterization of two human digitalis-like substances that are structurally related to digoxin and ouabain

In order to characterize the structure of endogenous digitalis-like immunoreactive factor (DLIF), we utilized peritoneal dialysis fluid from patients with chronic renal failure as a source of endogenous digitalis-like immunoreactive factor (DLIF), and subjected it to one-step ion exchange chromatography, followed by one step reverse HPLC. Crude dialysis fluid contained 0.09 ng/ml of DLIF, and using Amberlite XAD-2 chromatography we extracted 110 ng of DLIF from 800 ml of dialysis fluid. By applying this partially purified DLIF to our HPLC system, we discerned three peaks of DLIF activity, with retention times of 34, 58 and 63 minutes. The first peak overlapped the elution profile of ouabain, and the third peak co-eluted precisely with digoxin. The second DLIF peak was not in proximity to any of the digitalis-like markers employed. Thus, our results indicate that DLIF isolated from peritoneal dialysis fluid exists in three distinct forms, one of which resembles ouabain, and one which is identical to digoxin.

High myocardial accumulation of radioiodinated digoxin derivative: a possible Na,K-ATPase imaging agent

In view of the high binding ability of cardiac glycosides to the myocardial Na,K-ATPase, radioiodinated digoxin derivatives were surveyed as candidates for myocardial imaging, with particular emphasis on the noninvasive monitoring of cardiac glycoside therapy. Among the radioiodinated digoxin derivatives surveyed, 125I-digoxin-iodohistamine(bis(O-carboxymethyloxime)) showed the highest accumulation in the myocardium and similar binding ability to Na,K-ATPase as digoxin itself against ouabain displacement, as indicated by in vivo and in vitro studies. Based on these results, 123I labeling of digoxin-histamine(bis(O-carboxymethyloxime)) and imaging in a dog demonstrated uptake in the myocardium.

Synthesis and structural characterization of a cardioactive biotinylated digoxin analogue

We have biotinylated the terminal glycose of digoxin by reaction of the periodate-oxidized steroid with biotin hydrazide. A biotinylated product (BD-1) was formed which retained significant digoxin receptor (Na+/K+ ATPase) binding activity. Sustained reaction resulted in a second biotinylated product (BD-2) which showed reduced receptor binding activity. The products were characterized by FAB mass spectroscopy and shown to be the mono- and di-biotinylated digoxin conjugates of the oxidized glycose moiety. These analogues may prove useful in determining the subcellular site of digoxin binding.

Human monoclonal antibodies with different fine specificity for digoxin derivatives: cloning of heavy and light chain variable region sequences

Human-mouse hybridoma cell lines producing human monoclonal antibodies against the cardiac glycoside digoxin were established after in vitro immunization or direct immortalization of human peripheral blood lymphocytes with digoxin. Three antibodies, designated MO6, LH92 and LH1114, displayed different patterns of fine specificity against digoxin and several digoxin analogues, as elucidated by inhibition ELISA. All three monoclonal antibodies had mu heavy chains, two of them (MO6 and LH114) had kappa light chains and one (LH92) lambda light chains. DNA encoding the variable regions of both heavy and light chains of the three antibodies were amplified from cDNA using the polymerase chain reaction (PCR). The nucleotide sequences of the amplified DNA were determined after subcloning of PCR fragments in M13 vectors. The deduced amino acid sequences revealed considerable sequence differences in the complementarity determining regions between the three antibodies.

Synthesis and characterization of digoxin-phospholipid conjugates

The preparation of immunoreactive derivatives of digoxin for analytical applications is most often carried out by periodate cleavage of the terminal sugar ring (digitoxose) followed by reaction with an enzyme, protein, carrier, or related biological molecules. Here we report an improved and more efficient synthesis which was developed to provide digoxin-phospholipid conjugates useful for liposome immunoassay. The approach used involved the linking of the cleaved digitoxose through a carboxymethyl oxime functionality, which provides much improved yields of readily purified products. The synthetic modification should be applicable to the preparation of analogous phospholipid conjugates involving linkage through a sugar ring (digitoxin, ouabain, and related cardiac glycosides) or to those involving steroids (i.e., 3-digoxigenone) which can be modified to form oxime derivatives remote from key functionalities important for immunorecognition by specific antibody. The characterization of the digoxin-phospholipid conjugates with high-resolution NMR and fast atom bombardment mass spectrophotometry will also be discussed.

Identification of enterotoxigenic Escherichia coli by colony hybridization with nonradioactive digoxigenin-labeled DNA probes

Enterotoxigenic Escherichia coli (ETEC) strains were readily identified in pure and mixed cultures with nonradioactive, digoxigenin-labeled DNA probes coding for heat-labile (LTI) and heat-stable (STaI, STaII, and STb) enterotoxins. Digoxigenin-labeled ETEC fragments were more sensitive than and exhibited less nonspecific background contamination than biotinylated ETEC probes.

Lectin--digoxigenin conjugates: a new hapten system for glycoconjugate cytochemistry

We report the use of a novel hapten system for lectin cytochemistry. Various lectins conjugated to the steroid hapten digoxigenin (DIG) and monospecific anti-digoxigenin antibodies were applied for the light and electron microscopic detection of glycoconjugates in tissue sections. Both IgG and Fab' anti-DIG antibodies were complexed to particles of colloidal gold and compared to commercially available alkaline phosphatase and horseradish peroxidase conjugated Fab' as general second step reagents. The three different markers performed equally well on paraffin sections whereas the gold-labeled antibodies were superior reagents for semithin and ultrathin sections of Lowicryl K4M embedded tissues. In conjunction with the latter marker, no pretreatment to abolish endogenous enzyme activity was necessary. At the light microscope level, gold signal amplification by the photochemical silver reaction was required. DIG, in contrast to biotin, does not occur in animal tissues thus eliminating the need for blocking reactions prior to lectin incubation. Compared to affinity techniques using glycoprotein-gold complexes as second step reagent the DIG hapten system required smaller amounts of lectins. The staining patterns were indistinguishable from those obtained in other lectin-gold techniques and the specificity of the labeling could be demonstrated in sugar inhibition tests.

Cross-resistance and biochemical characteristics of second-step mutants of HeLa cells resistant to cardiac glycosides

From ouabain-resistant (OuaR) mutants of HeLa cells which do not show any cross resistance to the digoxin analog SC4453, stable second-step mutants resistant to either SC4453 or those exhibiting increased resistance to digoxin have been isolated. The mutants obtained exhibited highly specific cross resistance towards different cardiac glycosides (CGs) and, based on their cross-resistance patterns, contained more than one type of genetic lesion. Biochemical studies with these mutants showed that cellular uptake of 86Rb was inhibited by specific CGs to which they showed increased resistance. The mutants showed reduced binding of [3H]ouabain and [3H]digoxin in comparison with the parental OuaR cells and about 50-60% of the Na+, K(+)-ATPase activity in the mutant cell extract was highly resistant to inhibition by ouabain and digoxin. In contrast to the above changes, these mutants showed no evidence of amplification, enhanced transcription, or gross alterations in the genes for the alpha or beta subunits of Na+, K(+)-ATPase. These observations indicated that these mutants involved a second-specific alteration in Na+, K(+)-ATPase. In contrast to these mutants, Chinese hamster ovary cells, which naturally exhibit comparable levels of resistance to CGs, showed no significant binding of either [3H]ouabain or [3H]digoxin and all of their Na+, K(+)-ATPase activity was resistant to inhibition by CGs.

Detection of messenger RNA using a digoxigenin end labelled oligodeoxynucleotide probe

A synthetic oligodeoxynucleotide sequence complementary to the mRNA for the adrenocorticotrophin (ACTH) precursor pro-opiomelanocortin (POMC) was end labelled using digoxigenin. The probe was used to detect POMC mRNA both on nitrocellulose filters and by non-isotopic in situ hybridisation (NISH) in tissue sections. Digoxigenin was identified using anti-digoxigenin alkaline phosphatase. The model system examined was the rat pituitary gland. Removal of both adrenal glands and dexamethasone administration were used to change the concentrations of POMC mRNA in the rat anterior lobe. The labelled probe reacted with a single band of appropriate molecular weight in Northern blot analysis. The distribution of signal in tissue sections and the changes induced by experimental manipulation were as predicted. The results indicate that this method of NISH will prove useful in the detection of specific messenger RNAs in tissue sections of buffered, formalin fixed, paraffin wax embedded material.

Electrochemical detection of the 3,5-dinitrobenzoyl derivative of digoxin by high-performance liquid chromatography

Electrochemical detection of 3,5-dinitrobenzoyl derivatives of digoxin and its metabolites following high-performance liquid chromatography is reported. Partial resolution of derivatized digoxin and dihydrodigoxin was obtained using a Spherisorb ODS II analytical column. Both single- and dual-electrode detection were investigated and a maximum sensitivity equivalent to 0.39 ng of digoxin was found with the dual-electrode method. This system has the necessary sensitivity and selectivity for development into a therapeutic monitoring assay method.

Detection of human papillomavirus DNA in genital condylomata in women and their male partners by using in situ hybridization with digoxygenin labeled probes

Twelve couples (12 women and their male partners) presenting genital warts were investigated in order to evaluate the sexual transmission of human papillomavirus (HPV) in mutual partners and the localization of HPV DNA. Formalin-fixed, paraffin-embedded biopsy samples of 12 vulvar condylomata, and 12 penile condylomata from male partners were analyzed for the presence of HPV DNA-6, -11, and 16/18 by using in situ hybridization with digoxygenin labeled DNA probes. HPV DNA was identified in 9 women (75%) and in 9 men (75%). HPV-6 was frequently identified, being revealed in 42% of the vulvar specimens, in 67% of the cervical specimens and 58% of the penile specimens. Seven of 9 (77%) positive couples shared the same HPV DNA, and 2 couples harbored different HPV DNA types between the partners. The signal intensity of the HPV DNA was generally strong in superficial cell layers, weak in parabasal or basal cell layers. No malignant lesions resulted from the condyloma acuminatum caused by HPV-6 or -11. There were only mild dysplasia in the both sexes.

Detection of provirus in an HTLV-II producer CD8+ T cell line by polymerase chain reaction combined with digoxigenin-ELISA method

A human T-cell leukemia virus type II (HTLV-II) producer cell line, designated HTLV-IIA, was established by cocultivation with leukocytes from an anti-human T-cell leukemia type I (HTLV-I) antibody-positive white male intravenous drug abuser and a healthy Japanese female. The cell line was examined for viral antigens by the indirect immunofluorescence method. The cytoplasm of over 80% of the cells was brilliantly stained. Cytogenetically, the cell line has a normal female karyotype. Electron microscopy of the HTLV-IIA disclosed many C-type retrovirus particles of mature, immature and non-cored types in the extracellular spaces. The surface markers of the transformed cells are CD2+, CD3+, CD4- and CD8+. To distinguish between HTLV-I and HTLV-II infection in the cell line, a method for detection of the HTLV-II provirus was developed by combining the polymerase chain reaction method with digoxigenin-enzyme-linked immunosorbent assay method.

Detection of Epstein-Barr virus DNA in nasopharyngeal carcinoma using a non-radioactive digoxigenin-labelled probe

The presence of Epstein Barr virus (EBV) DNA in biopsies from the post-nasal space (PNS) of patients suspected of nasopharyngeal carcinoma (NPC) was detected by in situ cytohybridization with an EBV DNA probe labelled with the novel labelling compound digoxigenin. The digoxigenin probe was hybridised to cryostat sections of NPC biopsies and subsequently detected by an enzyme immunoassay procedure. It was found that in situ cytohybridization using the digoxigenin probe was much more rapid and sensitive (96 h compared to five weeks) than the current method of using 3H-labelled probe. Using the digoxigenin EBV probe, it was found that in all the eighteen NPC biopsies tested, EBV DNA was detected in malignant epithelial cells and infiltrating lymphocytes. EBV DNA was also detected in some normal epithelial cells in these NPC biopsies. EBV DNA was not detected in epithelial cells of non-malignant biopsies.

[Detection of human papillomavirus genome in nasolaryngeal papillomas using digoxigenin labeled DNA probes]

It is being reported that human papillomavirus (HPV) has been implicated in the pathogenesis of various neoplastic lesions of the genital organs. To investigate the etiological role of HPV and its types in nasolaryngeal papillomas, we retrospectively analyzed HPV genomes by nucleic acid hybridization methods; for detecting DNA and mRNA, we employed the recently developed nonradioactive (digoxigenin labeled) DNA probes and compared the results by radioisotope methods. In total, 43 cases of papillomatous lesions were examined. They were verruca vulgaris of the nasal vestibule (Nr = 2), nasal inverted papilloma (IP, Nr = 26), and laryngeal papilloma (Nr = 15). HPV types examined were type 2, 6, 11, 16 and 18. Two cases of verruca vulgaris were shown to contain HPV-2 DNA and its mRNA by in situ hybridization. HPV-11 DNA was detected in 3 cases (12%) of nasal inverted papilloma whereas HPV-16 was detected in 1 case (4%); the latter case was associated with squamous cell carcinoma. These results suggest that HPV may be implicated in the development of IP, and HPV-16 may play an important role in the malignant transformation of IP. In the cases of multiple laryngeal papilloma (Nr = 8, one juvenile type and 7 adult type), either HPV-6 or HPV-11 was detected at the high rate (6/8, 75%). The presence of the HPV genomes provides strong evidence for the HPV etiology of these laryngeal papillomas. Whereas in the cases of adult single laryngeal papilloma (Nr = 7), HPV was not detected. Technically, the sensitivity of digoxigenin (DIG) labeled DNA probe was almost same as 35S labeled probe by dot blot hybridization, thus we applied DIG labeled probe to Southern blot hybridization with low background. By in situ hybridization using digoxigenin labeled probes, the rates of HPV detection were almost equal to those by 35S labeled probes.(ABSTRACT TRUNCATED AT 250 WORDS)