Cytotoxic effects of cardiac glycosides in colon cancer cells, alone and in combination with standard chemotherapeutic drugs

Cardiac glycosides have been reported to exhibit cytotoxic activity against several different cancer types, but studies against colorectal cancer are lacking. In a screening procedure aimed at identifying natural products with activity against colon cancer, several cardiac glycosides were shown to be of interest, and five of these were further evaluated in different colorectal cancer cell lines and primary cells from patients. Convallatoxin (1), oleandrin (4), and proscillaridin A (5) were identified as the most potent compounds (submicromolar IC50 values), and digitoxin (2) and digoxin (3), which are used in cardiac disease, exhibited somewhat lower activity (IC50 values 0.27-4.1 microM). Selected cardiac glycosides were tested in combination with four clinically relevant cytotoxic drugs (5-fluorouracil, oxaliplatin, cisplatin, irinotecan). The combination of 2 and oxaliplatin exhibited synergism including the otherwise highly drug-resistant HT29 cell line. A ChemGPS-NP application comparing modes of action of anticancer drugs identified cardiac glycosides as a separate cluster. These findings demonstrate that such substances may exhibit significant activity against colorectal cancer cell lines, by mechanisms disparate from currently used anticancer drugs, but at concentrations generally considered not achievable in patient plasma.

LC-MS assay for quantitative determination of cardio glycoside in human blood samples

A method is described for liquid chromatography-mass spectrometry analysis of the cardio glycosides digoxin and digitoxin in biological samples. The method was optimized for use in the forensic field and, therefore, comprises the determination from whole blood and tissue samples. Sample cleanup by solid phase extraction (SPE) on a functionalized polymeric phase was sufficient to limit matrix suppression to <10% for all analytes. Chromatographic separation was achieved using an RP-8 column. Detection of the cardio glycosides was performed with electrospray ionization in the positive mode. The system was run in single ion monitoring mode, measuring the sodium adducts (M + Na)+ of the analyte and of the internal standard, respectively. The method was fully validated for the analysis of blood samples and was also successfully applied in forensic cases. The method was accurate and precise over a linear concentration range up to 50 ng/g blood. Lower limit of quantitation was 0.2 ng/g for digoxin and 2 ng/g for digitoxin, respectively. As deuterated analyte was used as internal standard, we also present a new microwave-enhanced method for the fast preparation of the labelled analyte within 20 min.

Interference of Uzara glycosides in assays of digitalis glycosides

OBJECTIVE

Presentation of a case report and pharmacokinetic investigation in healthy volunteers on the potential interference between cardiac glycosides and glycosides of Uzara, a herbal antidiarrheal preparation.

METHODS

Pharmacokinetic pilot investigation of apparent digitoxin and digoxin serum concentrations in 4 healthy volunteers after single-dose administration of 30 drops Uzara (approximately 1.5 ml approximately = 22 mg glycosides).

RESULTS

Maximal apparent serum concentrations of digitoxin between 198.0 microg/l and 919.8 microg/l (therapeutic range: 10-25 microg/l) occurred at 4-8 hours after administration. The terminal half-life of the glycosides was 8.87 +/- 2.20 hours. For digoxin, maximal apparent serum concentrations ranged between 1.4 microg/l and 6.34 microg/l (therapeutic range: 0.9-2.0 microg/l) at 6 hours post dosing.

CONCLUSIONS

Administration of a single dose of an Uzara preparation, an over-the-counter product, results in false high serum concentrations of digitoxin and digoxin. As described in the manufacturers Summary of Product Characteristics, this preparation should not be given to patients with cardiac failure or arrhythmia who require treatment with cardiac glycosides because of the demonstrated pharmacological actions of uzara glycosides.

A NOVEL INEXPENSIVE MURINE MODEL OF ORAL CHRONIC DIGITALIZATION

A novel inexpensive murine model of oral administration of digitoxin (100 micro g/kg per day) added to routine chow is described. Serum digitoxin levels achieved after oral (n = 5; 116 +/- 14 ng/mL) and subcutaneous (n = 5; 124 +/- 11 ng/mL) administration were similar. A significant increase in the maximal left ventricular pressure rise of treated (n = 9) compared with control (n = 6) rats (dP/dt: 8956 +/- 233 vs 7980 +/- 234 mmHg/s, respectively; P = 0.01) characterized the positive inotropic action of digitoxin. In addition, no differences were observed in treated compared with control rats with regard to the electrocardiogram and systolic and diastolic left ventricular pressures.

Determination of the cardiac glycosides digoxin and digitoxin by liquid chromatography combined with isotope-dilution mass spectrometry (LC-IDMS)--a candidate reference measurement procedure

This article describes a method of high analytical sensitivity, reproducibility and trueness for the determination of digoxin and digitoxin in serum or plasma at therapeutic levels using a combination of high-pressure liquid chromatography (HPLC), isotope-dilution mass spectrometry (IDMS) and caesium-adduct formation. A method for threefold deuterium substitution in the glycosides was developed, which could be performed within 24 hours without distillation giving yields > 98% of the theoretical value. Extraction from a serum or plasma matrix was performed using a liquid-phase extraction with ammonium acetate buffer/tertiary butylmethyl ether/ethyl acetate at pH 9.5. The HPLC-separation used a 10 x 2 mm LiChrospher RP-18 5 microm guard column in combination with a 125 x 2 mm main column of the same material and a gradient containing methanol, caesium ions and formic acid. Quantification of digoxin and digitoxin was made with IDMS using deuterated internal standards and the system run in single ion monitoring (SIM) mode. The methods had a lower limit of determination of 0.25 microg/l for digoxin and digitoxin, a trueness between 97.5 and 104% for digoxin and between 98 and 101% for digitoxin, respectively and had a coefficient of variation of less than 3% in the therapeutic range for both glycosides. Maximally 1 ml serum or plasma was needed for the procedure. The method is used to set target values for materials used in external quality assessment surveys (EQAS) run by INSTAND as part of a national EQAS-programme.)

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.

Uremic sera contain inhibitors that block digitoxin-valproic acid interaction

BACKGROUND

Digitoxin and valproic acid show strong binding to serum albumin. Thus, when present simultaneously in serum, digitoxin and valproic acid compete for binding sites. We studied digitoxin-valproic acid interaction in normal and uremic sera.

METHODS

Fluorescence polarization immunoassays were used for measuring total digitoxin and total valproic acid concentrations. We used a modified protocol of improved sensitivity to measure free digitoxin concentrations. We supplemented 2 normal and 2 uremic pools with digitoxin and then aliquots of these pools were further supplemented with various concentrations of valproic acid. After incubation at 37 degrees C for 2 hours in a water bath, specimens were allowed to re-equilibrate at room temperature for 20 minutes. Free digitoxin concentrations were measured. We also investigated digoxin-valproic acid interaction using 1 normal and 1 uremic serum pool.

RESULTS

We observed significant increases in free digitoxin concentrations in normal sera in the presence of valproic acid. In contrast, we observed a slight decline in free digitoxin concentration in the presence of valproic acid in uremic sera. We speculated that uremic sera contained inhibitors that block digitoxin-valproic acid interaction and identified indoxyl sulfate as an inhibitor. However, another uremic compound, hippuric acid showed no inhibitory effect. Interestingly, we observed no significant interaction between digoxin and valproic acid in either normal or uremic serum pool. This is probably because of poor protein binding of digoxin.

CONCLUSION

We conclude that valproic acid significantly displaces digitoxin from protein binding sites in normal serum. However, uremic sera contain inhibitors that block digitoxin-valproic acid interaction.

Digitoxin medication and cancer; case control and internal dose-response studies

BACKGROUND

Digitoxin induces apoptosis in different human malignant cell lines in vitro. In this paper we investigated if patients taking digitoxin for cardiac disease have a different cancer incidence compared to the general population.

METHODS

Computer stored data on digitoxin concentrations in plasma from 9271 patients with cardiac disease were used to define a user population. Age and sex matched controls from the Norwegian Cancer Registry were used to calculate the number of expected cancer cases.

RESULTS

The population on digitoxin showed a higher incidence of cancer compared to the control population. However, an additional analysis showed that the population on digitoxin had a general increased risk of cancer already, before the start on digitoxin. Leukemia/lymphoma were the cancer types which stood out with the highest risk in the digitoxin population before starting on digitoxin. This indicates that yet unknown risk factors exist for cardiovascular disease and lymphoproliferative cancer. An internal dose-response analysis revealed a relationship between high plasma concentration of digitoxin and a lower risk for leukemia/lymphoma and for cancer of the kidney/urinary tract.

CONCLUSION

Morbidity and mortality are high in the population on digitoxin, due to high age and cardiac disease. These factors disturb efforts to isolate an eventual anticancer effect of digitoxin in this setting. Still, the results may indicate an anticancer effect of digitoxin for leukemia/lymphoma and kidney/urinary tract cancers. Prospective clinical cancer trials have to be done to find out if digitoxin and other cardiac glycosides are useful as anticancer agents.

Digoxin-like and digitoxin-like immunoreactive substances in elderly people. Impact on therapeutic drug monitoring of digoxin and digitoxin concentrations

We compared digoxin-like (DLIS) and digitoxin-like (DTLIS) immunoreactive substance concentrations for 30 people older than 65 years with those for 25 people younger than 50. None received digoxin or had liver disease, uremia, or volume expansion. We found no DTLIS in any specimen, and only 1 specimen from an elderly person demonstrated a low DLIS concentration. In addition, for 22 non-volume expanded patients (8 younger than 50 years and 14 older than 65) receiving digoxin, the fluorescence polarization (FPIA) and the microparticle enzyme (MEIA) immunoassays revealed comparable serum digoxin concentrations, indicating an insufficient DLIS concentration to interfere with digoxin immunoassay results. Therefore, elderly people who are not volume expanded do not have elevated DLIS or DTLIS concentrations. Furthermore, for patients with liver disease or uremia (18 older than 65 years and 20 younger than 50), the DLIS and DTLIS concentrations were elevated. Finally, for 5 patients with liver disease who received digoxin, serum digoxin concentrations were lower by MEIA and higher by FPIA, indicating the patients had elevated DLIS levels that interfered with the assays. Elevated DLIS and DTLIS concentrations are associated with volume expansion and not age.

Digitoxin-like immunoreactivity in sera of mice after feeding with chinese medicine Chan Su: study of protein binding of Chan Su in normal sera, uremic sera and sera from patients with liver disease

Traditional Chinese medicines are readily available without prescription from herbal drug stores. One such Chinese medicine, Chan Su, which is prepared from the skin gland of toads, has cardiotonic effect due to bufadienolides. Here we report rapid detection of the presence of Chan Su in blood using the fluorescence polarization immunoassay for digitoxin. In our study mice were fed with a dose of 75 mg/kg of Chan Su and blood was drawn before, and 1 and 2 h after feeding. We observed significant digitoxin-like immunoreactivity in the sera. For example in one mouse the digitoxin-like immunoreactivity was undetectable before feeding with Chan Su, but was 19.7 ng/ml 1 h and 8.8 ng/ml 2 h afterwards. The apparent half-life of Chan Su is approximately 1 h in mice. In another experiment, we studied protein binding of Chan Su by measuring total and free Chan Su concentrations (ultrafiltrate prepared by using Centrifree Micropartition Filter, molecular weight cutoff: 30000 Da). Chan Su was strongly bound to serum proteins. We observed higher free fraction in uremic sera and sera from patients with liver disease. We identified albumin as one of the proteins that bind Chan Su in serum.

Falsely elevated serum digitoxin concentrations due to cross-reactivity of water-extractable digitoxin-like immunoreactivity of Chinese medicine Chan SU: elimination of interference by use of a chemiluminescent assay

Chinese medicines are available without prescription in health food stores. One such Chinese preparation, Chan SU, is used as a cardiotonic agent. Digoxin-like immunoreactivity of Chan SU has been reported in the past. In this report we demonstrated significant digitoxin-like immunoreactivity of Chan SU. For example, when a 20-microl aliquot of an aqueous extract of Chan SU (2 mg/ml) was added to drug-free serum, the observed digitoxin-like immunoreactivity was 51.40 ng/ml by the fluorescence polarization assay. In contrast, a new chemiluminescent assay for digitoxin did not show any immunoreactivity. When very small amount of aqueous extract of Chan SU was added into serum containing digitoxin, the observed digitoxin concentrations were falsely elevated when measured by the fluorescence polarization immunoassay (FPIA), but did not change significantly when measured by the chemiluminescent immunoassay (CLIA). Significant digitoxin-like immunoreactivity was also observed (FPIA) in mice after feeding with Chan SU. Because bufalin, cinobufotalin and cinobufagin are major components of Chan SU, digitoxin-like immunoreactivity of these purified compounds was also studied. Bufalin was identified as the major digitoxin-like immunoreactive compound responsible for most of the interference in serum digitoxin measurement using the FPIA.

Sensitive methods for determination of free digitoxin concentration using digitoxin immunoassays: demonstration of elevated free digitoxin concentration caused by digitoxin-phenytoin interaction by applying these new techniques

Digitoxin is very strongly bound to serum albumin. Although free digitoxin is pharmacologically active, it is not monitored because of the lack of a sufficiently sensitive technique. The concentration of free digitoxin in the protein-free ultrafiltrate is usually below the detection limit of digitoxin immunoassays. A modified technique is described by which free digitoxin can be routinely monitored using commercially available immunoassays. The fluorescence polarization immunoassay for determining total digitoxin concentration requires that 100 microL of serum be treated with 300 microL of methanol to precipitate proteins. It is demonstrated that free digitoxin can easily be measured by adding 100 microL of methanol to 300 microL of ultrafiltrate, thus improving the sensitivity of the assay three-fold. The free digitoxin concentration can easily be calculated by dividing the observed value by 3. An attempt to use only ultrafiltrate (no methanol added) caused significant bias in the result, probably as a result of a matrix problem. The chemiluminescent assay for digitoxin does not require any specimen pretreatment and requires only 10 microL of serum. The program was modified and used 50 microL of ultrafiltrate to improve the sensitivity of the free digitoxin assay. If the chemiluminescent assay is used to measure free digitoxin, the true free digitoxin concentration can be calculated by dividing the observed value by 4.3. The free digitoxin concentrations were comparable in eight patients receiving digitoxin as measured by both methods. To show an application of this technique, two serum pools were prepared from patients receiving digitoxin and supplemented with various concentrations of phenytoin. A significant increase in free digitoxin concentration was observed because of the displacement of digitoxin from protein binding sites by phenytoin.

Effect of digoxin fab antibody on the measurement of total and free digitoxin by fluorescence polarization and a new chemiluminescent immunoassay

Digoxin fab antibody (Digibind; Burroughs Wellcome, Research Triangle Park, NC, USA) is used in the treatment of digoxin overdose. The effect of digibind on the measurement of total and free digoxin has been extensively studied. However, the effect of digibind on digitoxin measurements has not been studied thoroughly. The authors studied the effect of digibind on the measurement of total and free digitoxin in vitro using the fluorescence polarization immunoassay and a new chemiluminescent immunoassay. We also studied the capability of digibind to bind digitoxigenin, the major aglycon metabolite of digitoxin. Digibind neutralized both digitoxin and digitoxigenin in vitro, as evidenced by significant reductions in free digitoxin and digitoxigenin (measured as digitoxin equivalent) concentrations. Digibind caused negative interference in the measurement of total digitoxin concentrations by both fluorescence polarization and chemiluminescent assays. However, the magnitude of negative interference was significantly higher with the chemiluminescent assay. For example, in a serum pool supplemented with 80 ng/mL of digitoxin, the concentrations of total and free digitoxin measured by the fluorescence polarization immunoassay were 82.1 ng/mL and 3.3 ng/mL respectively. In the presence of 5 microg/mL of Digibind, the corresponding total and free digitoxin concentrations were 73.9 ng/mL and none detected, respectively. In another serum pool supplemented with 70 ng/mL of digitoxin, the concentrations of total and free digitoxin as measured by the chemiluminescent assay were 69.1 ng/mL and 3.8 ng/mL, respectively. In the presence of 5 microg/mL of Digibind, the corresponding total and free digitoxin concentrations were 29.0 ng/mL and none detected, respectively. Because this effect may also occur in vivo, the progress of Digibind therapy in treating a patient with digitoxin overdose may be monitored by measuring the free digitoxin concentrations.

Neutralization of cardiac toxins oleandrin, oleandrigenin, bufalin, and cinobufotalin by digibind: monitoring the effect by measuring free digitoxin concentrations

Oleandrin plant poisoning is common in children and the plant extract is used in Chinese medicines. The toxicity is due to oleandrin and the deglycosylated metabolite oleandrigenin. Bufalin and cinobufotalin (toad cardiac toxins) are also widely used in Chinese medicines like Chan SU, and Lu-Shen -WU. Severe toxicity from bufalin after consumption of toad soup has been reported. Taking advantage of structural similarities of these toxins with digitoxin, we demonstrated that these compounds can be rapidly detected in blood by the fluorescence polarization immunoassay for digitoxin. The cross reactivities of these compounds with digoxin assay were much lower. For example, when a drug free serum was supplemented with 10 microg/ml of oleandrin, we observed 127.7 ng/ml of digitoxin equivalent but only 2.4 ng/ml of digoxin equivalent concentration. Digibind neutralized all cardiac toxins studied as evidenced by significant fall of free concentrations. When aliquots of serum pool containing 50.0 microg/ml of oleandrin were supplemented with 0, 10.0, 25.0, 50.0, 100, and 200 microg/ml of digibind, the mean free concentrations were 30.6, 23.3, 16.0, 10.7, 7.8 and 5.5 microg/ml respectively. Similarly, with 50.0 microg/ml of oleandrigenin (total concentration: 36.2 ng/ml), the free concentration was 14.5 ng/ml digitoxin equivalent in the absence of digibind and 5.4 ng/ml in the presence of 200 microg/ml of digibind. In another specimen containing 500 ng/ml bufalin (total concentration: 156.9 ng/ml), the free concentration was 8.6 ng/ml in the absence of digibind and none detected in the presence of 100.0 microg/ml digibind. Because such neutralization may also occur in vivo, digibind may be useful in treating patients exposed to these toxins.

Effects of digoxin and digitoxin on circadian blood pressure profile in healthy volunteers

BACKGROUND

The aim of the study was to investigate the potential effects of chronic digoxin or digitoxin treatment or circadian blood pressure profile in normotensive subjects.

METHODS

In two randomized double-blind, placebo-controlled cross-over protocols, 22 healthy normotensive subjects were enrolled, 12 subjects in either study. After adequate loading doses, digoxin 0.25 mg twice daily or digitoxin 0.1 mg daily was given for a total of 10 days. Automatic 24-h ambulatory blood pressure measurements were carried out at days 4 and 10 of either glycoside or placebo.

RESULTS

Digoxin treatment significantly decreased heart rate (HR) and diastolic blood pressure (DBP) during the overnight sleeping phase of day 10 compared with placebo (HR, 4 beats min-1; DBP, 8 mmHg; P < 0.05). Digitoxin treatment significantly decreased heart rate and diastolic blood pressure during the overnight sleeping phase of day 4 (HR, 8 beats min-1; DBP, 7 mmHg) and day 10 (HR, 7 beats min-1; DBP, 5 mmHg) compared with placebo (P < 0.05). Neither digoxin nor digitoxin significantly affected systolic blood pressure.

CONCLUSIONS

Both digoxin and digitoxin, within therapeutic steady-state plasma concentrations, reduced diastolic blood pressure and heart rate during overnight sleep, presumably because of increased parasympathetic activity or decreased sympathetic activity.

Unexpected suppression of total digoxin concentrations by cross-reactants in the microparticle enzyme immunoassay: elimination of interference by monitoring free digoxin concentration

The microparticle enzyme immunoassay (MEIA for digoxin (Abbott Laboratories, Abbott Park, Ill) requires no sample pretreatment and is widely used in clinical toxicology laboratories for monitoring serum digoxin concentrations. One advantage of the new MEIA is the lower cross-reactivities with such cross-reactants as digitoxin, oleandrin, and bufalin compared with the fluorescence polarization immunoassay (FPIA)for digoxin. Digitoxin, oleandrin, and bufalin showed positive cross-reactivity with MEIA and FPIAs for digoxin in the absence of the primary analyte, digoxin. A surprising finding was that digoxin concentrations were falsely decreased by these cross-reactants when serum pools containing digoxin were supplemented with various concentrations of these cross-reactants and when digoxin concentrations were measured by the MEIA. In contrast, digoxin concentrations were falsely elevated when measured by the FPIA. For example, when a serum pool containing 2.15 nmol/L of digoxin was supplemented with 129.5 nmol/L of bufalin, the apparent digoxin concentrations were 1.45 nmol/L with the MEIA and 3.00 nmol/L with the FPIA. Taking the advantage of only 25% protein binding of digoxin and more than 95% protein binding of digitoxin and bufalin, we demonstrated that monitoring free digoxin instead of total digoxin eliminated negative interference of digoxin by these cross-reactants in the MEIA and positive interference in the FPIA. Although oleandrin is also strongly bound to serum protein, high concentrations of oleandrin still modestly affect the free digoxin assay for both MEIA and FPIAs.

Bidirectional (positive/negative) interference in a digoxin immunoassay: importance of antibody specificity

The importance of high specificity in immunoassays used in therapeutic monitoring is highlighted by a case study in which therapeutic-to-toxic borderline digoxin levels were measured by a digoxin immunoassay in the serum sample from a patient administered digitoxin rather than digoxin. The sample, mistakenly sent to the laboratory for digoxin analysis, gave discordant results in three digoxin immunoassays: 1.99 and 0.79 ng/ml in assays using polyclonal antibodies (fluorescence-polarization immunoassay and microparticle enzyme immunoassay, respectively), and <0.1 ng/ml in a chemiluminescent immunoassay using more specific monoclonal antibody. The presence of digitoxin (approximately 40 ng/ml) in the sample was confirmed by three different digitoxin immunoassays. Based on these results, the interference of different levels of digitoxin was studied in the presence of 0, 0.85, 1.9, and 4.7 ng/ml digoxin in all three digoxin assays. The chemiluminescent assay showed no significant interference. The fluorescence-polarization immunoassay showed positive interference in all cases; however, the microparticle enzyme immunoassay showed a bidirectional interference: a positive interference observed at digoxin level <1.8 ng/ml, changing to a negative interference at higher digoxin concentrations. The authors conclude that in countries such as Germany, where both digoxin and digitoxin may be prescribed, caution should be used to interpret digoxin immunoassay results. Digoxin assays, with cross-reactivity to digitoxin <0.1% should be used.

Exogenous ouabain is accumulated in the adrenals and mimics the kinetics of endogenous digitalis-like factor in rats

Ouabain has been isolated as an endogenous pathogenetic factor in salt-induced hypertension and has been shown to be rich in the adrenals. In this study, organ accumulation of orally administered [3H]ouabain was examined in rats. Exogenous [3H]ouabain was accumulated in high levels in the adrenals, especially in the zona intermedia, and was not metabolized in the rat. Accumulated [3H]ouabain mimicked the movement of "endogenous" digitalis-like factor, since 1) the plasma [3H]ouabain level decreased in bilaterally adrenalectomized rats, 2) the plasma [3H]ouabain level increased accompanied by a decrease in [3H]ouabain content in the adrenals in reduced renal mass hypertensive rats, and 3) [3H]ouabain levels in plasma and in the adrenals increased in spontaneously hypertensive rats, as compared with those in respective control animals. Moreover, the rat diet contained a relatively high amount of ouabain-like immunoreactivity (OLI), and the ratio of the [3H]ouabain content to OLI in each organ was comparable to that of the daily intake of dietary [3H]ouabain to OLI. Furthermore, high 3H-radioactivities were also observed in the adrenals of rats that ingested [3H]digoxin and [3H]digitoxin. These data suggest that exogenous ouabain, related cardiotonic glycosides of plant origin, or both accumulate in the adrenals and, at least in part, act as "endogenous" digitalis-like factor(s).

[Digitalis poisoning treated with a specific antidote]

Toxicity by digitalis is a common problem in everyday clinical practice. In this paper three cases of severe poisoning with digitoxin with maximal S-digitoxin levels of 115, 150 and 239 nmol/l are described. All patients received specific digoxin Fab-fragment intravenously. Administration of antidotes resulted in a favourable outcome in all three patients. So far, the use of digoxin-specific antibodies has been limited to a few cases of severe intoxication where life-threatening arrhythmias and hyperkalaemia were present. We discuss whether a more liberal indication should be accepted.

Rapid detection of oleander poisoning using fluorescence polarization immunoassay for digitoxin. Effect of treatment with digoxin-specific Fab antibody fragment (ovine)

Poisoning from the oleander plant is common. Taking advantage of the high cross-reactivity of oleandrin, the major cardiac glycoside found in the oleander plant, we demonstrated that the serum digitoxin assay can be successfully used for the rapid diagnosis of oleander poisoning. Digitoxin is rarely used for treatment of cardiac disorders in the United States and has a therapeutic range of 19.7 to 39.3 nmol/L. In a typical oleander poisoning, serum oleandrin concentrations may reach 174 mmol/L or more. A serum specimen supplemented with 174 mmol/L of oleandrin containing no digitoxin showed an apparent digitoxin concentration of 1,272.1 nmol/L, a very high value compared with the range of the serum digitoxin assay, which is 2.6 to 104.8 nmol/L. Moreover, the response of the serum digitoxin assay with serum specimens containing various concentrations of oleandrin (and no digitoxin) is linear. Therefore, the oleandrin concentration in serum can be calculated from the apparent digitoxin concentration to access the severity of poisoning. Recently, the usefulness of the digoxin-specific Fab antibody fragment in the treatment of oleander poisoning has been described; however, no laboratory test was performed to demonstrate the progress of therapy. We demonstrated that the digoxin-specific Fab antibody can bind oleandrin in vitro, thus reducing the pharmacologically active free oleandrin. Because Fab and oleandrin bound to Fab are absent in the protein-free ultrafiltrates, monitoring the activity of free oleandrin in the ultrafiltrates can be used for monitoring the effectiveness of therapy.

Interference of oleandrin and oleandrigenin in digitoxin immunoassays: minimal cross reactivity with a new monoclonal chemiluminescent assay and high cross reactivity with the fluorescence polarization assay

Toxicity from ingestion of the oleander plant is common. Oleandrin, the oleander glycoside, has structural similarity to cardiac glycoside digoxin and is known to cross react with various digoxin immunoassays. The authors studied the cross reactivity of oleandrin and its deglycosylated congener oleandrigenin with a fluorescence polarization immunoassay for digitoxin and compared their results with a new chemiluminescent assay for digitoxin on the Automated Chemiluminescent System (ACS:180 Plus) from Chiron Diagnostics. Even though the chemiluminescent assay has been reported to be comparable with the fluorescence polarization assay among normal patient population, oleandrin and oleandrigenin showed very high cross reactivities with the fluorescence polarization immunoassay and minimal cross reactivity with the new chemiluminescent assay. When the authors supplemented a serum specimen containing no digitoxin with 50 micrograms/ml of oleandrin, the fluorescence polarization assay recorded a value of 535.7 ng/ml of digitoxin equivalent, whereas the new chemiluminescent assay recorded a value of 10.3 ng/ml of digitoxin equivalent. The cross reactivity of oleandrigenin with the fluorescence polarization immunoassay for digitoxin was significantly lower than oleandrin. The presence of oleandrin also falsely elevated total digitoxin level in a specimen supplemented with digitoxin and oleandrin. The authors also measured free digitoxin concentration by the fluorescence polarization immunoassay in the ultrafiltrate of serum supplemented with digitoxin and oleandrin. Because digitoxin and oleandrin are bound strongly to protein, monitoring free digitoxin concentration by the fluorescence polarization immunoassay instead of total digitoxin concentration does not eliminate oleandrin interference. The authors conclude that fluorescence polarization immunoassay for digitoxin has a high cross reactivity with oleandrin and can falsely elevate digitoxin concentration in the presence of oleandrin, whereas the new chemiluminescent assay for digitoxin is almost free from interferences from oleandrin.

[Serum digitoxin in concomitant use of antiepileptics in routine therapy]

Concomitant use of digitoxin and enzyme-inducing antiepileptics may lower serum levels, and accordingly the effect of digitoxin, unless the higher metabolic clearance is compensated for by higher dosage. Use of digitoxin is almost always guided by serum concentration measurements. Information on a possible enzyme-inducing effect of phenobarbital, phenytoin and carbamazepine is easily accessible. Compilation of serum level measurements for digitoxin showed that serum levels shifted towards lower values during concomitant use of phenytoin or carbamazepine than when digitoxin was used alone. As a consequence, the fraction of patients with serum levels below the therapeutic range was doubled. Concomitant use of phenobarbital did not cause a shift in the levels of digitoxin. In fact, in this group, a larger fraction of the serum level measurements were within the therapeutic range. Thus, the dosage of digitoxin appears to be fully compensated during concomitant use of phenobarbital, but obviously deserves attention during concomitant use of phenytoin or carbamazepine.

High-performance liquid chromatography-ionspray mass spectrometry for the specific determination of digoxin and some related cardiac glycosides in human plasma

An original method based upon high-performance liquid chromatography coupled to ionspray mass spectrometry (HPLC-ISP-MS) has been developed for the identification and quantification in plasma of several cardiac glycosides, namely digoxin, digitoxin, lanatoside C and acetyldigitoxin. After single-step liquid-liquid extraction by chloroform-2-propanol (95:5, v/v) at pH 9.5 using oleandrin as an internal standard, solutes are separated on a 4 microm NovaPak C18 (Waters) column (150x2.0 mm, I.D.), using a gradient of acetonitrile-2 mM NH4COOH, pH 3 buffer (flow-rate 200 microl/min, post-column split 1:3). Detection is done by a Perkin-Elmer Sciex API-100 mass analyzer equipped with an ISP interface. In most instances the major ion observed is not [M+H]+ as expected, but [M+NH4]+. The mean retention times (min) are: lanatoside C, 5.74; digoxin, 6.00; digitoxin, 8.08, oleandrin, 8.30, acetyldigitoxin, 8.66 and 9.01 (isomers alpha and beta, respectively). The lower limits of detection in single ion monitoring mode range from 0.15 ng/ml (alpha- and beta-acetyldigitoxin) to 0.60 ng/ml (lanatoside C), making the method less sensitive than radioimmunoassay, whereas it is much more specific.

Digitoxinlike and digoxinlike immunoreactivitis in sera of patients with uremia and liver disease as measured by fluorescence polarization immunoassays: poor correlation between digitoxinlike and digoxinlike immunoreactivities

Digoxinlike immunoreactive substances (DLIS) cross-react with antidigoxin antibodies and falsely elevante total digoxin levels. Cross-reactivity of DLIS with various immunoassays for digoxin has been extensively studied in the past. The digitoxin molecule differs from digoxin by having one extra hydroxyl group in the aglycone ring. Therefore, DLIS may also falsely elevate total digitoxin concentrations. However, in the past, limited studies have shown the presence of digitoxinlike immunoreactive substances (DTLIS) in cord blood and sera of neonates. We compared DLIS and DTLIS concentrations in sera of patients with uremia, liver disease, and hypoalbuminemia. We found measurable DLIS concentrations in 11 of 45 patients by the fluorescence polarization immunoassay (FPIA) for digoxin (range, 0.20-0.89 ng/ml digoxin equivalent). These patients did not receive any digoxin or digitoxin. Using the FPIA, we also found elevated DTLIS concentrations in 10 of these 45 patients (range, 2.88-21.24 ng/ml digitoxin equivalent). Given the narrow therapeutic range of digitoxin (15-30 ng/ml), this cross-reactivity is significant. Elevated concentrations of DTLIS in sera falsely elevated the measured concentrations of digitoxin (positive interference) when known amounts of digitoxin were added to such sera. Interestingly, we found a poor correlation between DLIS and DTLIS concentrations in sera of patients with liver disease and uremia (r = 0.58), with some patients having no measurable DLIS activity but measurable DTLIS activity and vice versa. Digoxin showed only 4-8% cross-reactivity against digitoxin antibody with a wide range of concentration. Some proposed DLIS compounds (nonesterified fatty acids, cholic acid, lysophospholipid, and DHEA-sulfate) did not show any cross-reactivity with the digitoxin assay at a concentrations much higher than the physiologic range. We conclude that DTLIS activity is present in patients with liver disease and uremia, and the correlation between DLIS activity and DTLIS activity is poor. Moreover, some proposed DLIS do not explain the DTLIS activity detected in serum.

Binding characteristics of KNI-272 to plasma proteins, a new potent tripeptide HIV protease inhibitor

The binding characteristics of KNI-272, a potent and selective human immunodeficiency virus (HIV) protease inhibitor, were evaluated in rat and human plasma, and in solutions of human alpha 1-acid glycoprotein (AAG) and human serum albumin (HSA). The unbound fractions (Fu) of KNI-272 were 12.13 and 2.24% in rat and human plasma, respectively, at the drug concentration of 1.0 microgram mL-1. Although KNI-272 binds to both AAG and HSA, the Fu of KNI-272 in AAG solution was 1.83%, and only one-quarter of that in HSA solution (Fu = 6.78%). Binding displacing agents, such as disopyramide, warfarin, diazepam, and digitoxin, were used to determine the binding site of KNI-272 on these plasma proteins. The Fu of KNI-272 in AAG solution increased 14-fold when disopyramide was added to the AAG solution. In addition, warfarin, diazepam, and digitoxin were added to HSA solution as representative drugs bound to distinct binding sites on HSA, namely sites I, II, and III, respectively. The Fu values of KNI-272 in HSA solution significantly increased when warfarin and diazepam were added. In particular, with the addition of warfarin to HSA solution, the Fu of KNI-272 increased to 16%. The modified Scatchard plots of KNI-272 binding to AAG and HSA both showed biphasic curves, and the KNI-272 binding sites at low concentration range on AAG and HSA disappeared with the addition of disopyramide and warfarin, respectively. Therefore, it is considered that KNI-272 binds to the identical site as disopyramide on AAG and site I on HSA in the low KNI-272 concentration range. By comparing the KNI-272 binding parameters obtained in human plasma and these protein solutions, we can assume that KNI-272 binding at low concentration in human plasma is mainly concerned with the binding on AAG. As KNI-272 concentration in plasma increases, HSA becomes concerned with KNI-272 binding.

Digitoxin intoxication with lethal outcome

A 65-year old woman with known history of reactive depression and failed suicide attempts ingested 7 mg digitoxin at 09.00 h. After vomiting 4 hours later, she reported the drug intake to her husband who thereupon summoned a physician. Arriving at 16.00 h, the physician was informed about the suicide attempt, but failed to initiate any specific measures. After a second doctor's visit at 22.00 h, the patient was rushed to hospital in a moribund state. In spite of a gastric lavage, treatment with activated charcoal and insertion of a transvenous pacemaker, the patient died at 23.45 h with signs of total atrioventricular block. Digitalis fab fragments could not be administered in time. A calculation based on the plasma digitoxin concentration of 212 ng.ml-1 measured at 23.00 h indicated that nearly the entire ingested dose had been absorbed. Thus, neither the vomiting nor the gastric lavage eliminated significant amounts of the drug which had left the stomach without delay. Under these circumstances, the failure to initiate timely therapy with specific digitalis fab fragments ultimately contributed to the lethal outcome.

Microspheres based detoxification system: a new method in convective blood purification

A variety of protein-bound or hydrophobic substances, accumulating as a result of pathologic conditions such as exogenous or endogenous intoxications, are removed poorly by conventional detoxification methods because of low accessibility (hemodialysis), insufficient adsorption capabilities (hemosorption), low efficiency (peritoneal dialysis), or economic limitations (high-volume plasmapheresis). Combining advantages of existing methods with microspheric technology, a module-based system was designed. Major operating parameters of the latter can be modified to allow for adjustment to individual clinical situations. An extracorporeal blood circuit including a plasmafilter is combined with a secondary high-velocity plasma circuit driven by a centrifugal pump. Different microspheric adsorbers can be combined in one circuit or applied in sequence. Thus, a prolonged treatment can be tailored using specially designed selective adsorber materials. Comparing this system with existing methods (high-flux hemodialysis, molecular adsorbent recycling system), results from our in vitro studies and animal experiments demonstrate the superior efficiency of substance removal.

Prospective, randomized, placebo-controlled, double-blind testing of colour vision and electroretinogram at therapeutic and subtherapeutic digitoxin serum levels

In a prospective, randomized, double-blind study with 10 healthy probands, changes in colour discrimination and in the pattern electroretinogram (P-ERG) and visually evoked cortical potentials (P-VECP) were monitored at therapeutic and subtherapeutic digitoxin serum levels. There was a slight increase in the total error score in the Farnsworth-Munsell 100-hue test at 0.1 mg digitoxin per day in comparison with the placebo. P-ERG and P-VECP did not show any significant changes.

Protein binding of digitoxin, valproate and phenytoin in sera from diabetics

Chronic hyperglycaemia results in glycation of serum albumin and might affect the binding of drugs. The aim of the present study was to compare, using an equilibrium dialysis method, the protein binding of therapeutic concentrations digitoxin, valproate and phenytoin in sera from 70 insulin-dependent diabetics and 25 controls. Drug concentrations were measured by fluorescence immunopolarisation. Glycated albumin was measured by laser nephelometry after affinity chromatography. In sera from diabetics, protein binding of digitoxin (88.8 versus 89.9%) was unchanged; the protein binding of valproate (75.2 versus 80.7%) and phenytoin (67.9 versus 75.3%) was significantly decreased, but with no correlation with the concentration of glycated albumin. We conclude that the difference in protein binding between diabetic and control sera is due to glucose-independent modification of albumin in diabetics.

Treatment of foxglove extract poisoning with digoxin-specific Fab fragments

A 22-year-old man presented to our emergency department after an intentional overdose of a homemade foxglove extract. Clinical symptoms with symptomatic bradyarrhythmia and ECG changes were consistent with cardiac glycoside poisoning. Treatment with digoxin-specific Fab fragments resulted in transient clinical and ECG improvement. Serum immunoassay demonstrated a digitoxin-like glycoside. The serum levels showed no evidence of altered elimination or distribution with Fab therapy despite temporary improvements in the clinical course. The use of Fab did not result in a shortened clinical course in this episode of foxglove poisoning, as one would expect in the setting of commercial glycoside product poisoning.

Evaluation of a fully mechanised immunoassay--Enzymun-Test System ES 300--and comparison with in-house methods for 8 analytes

The following analytes were determined with the automated enzyme immunoassay unit, Enzymun-Test System ES 300, and by routine in-house tests, and the results were compared: thyrotropin, free thyroxine, cortisol, immunoglobulin E, digoxin, digitoxin, insulin and carcinoembryonic antigen. The methods used for comparison included two radioimmunoassays, one with enzyme labelling, two with fluorescence enhanced enzyme reactions and 3 luminescence immunoassays. In most cases, the precision of the ES 300 lay between 2 and 5% in all assays in the concentration ranges of clinical interest. The inter-assay variation was almost identical to the intra-assay precision, which reflects the constancy of reagent quality and machine performance. The correlation between in-house and Enzymun-Test was excellent, even though the slope of the regression line was sometimes far from unity, due to the calibration materials used. ES 300 can be used as a stand alone unit and connected to a host computer. As a "walk-away" machine, it is suitable for laboratories with medium length series. It is a multi-batch analyser capable of taking 12 analytes and a maximal series length of 136 tubes excluding standards and controls.

The importance of prospective planning of pharmacokinetic trials. Considerations of studies on the phenytoin-digoxin-(P-D) and phenytoin-digitoxin-(P-DT) interaction

To study possible pharmacokinetic interactions between two compounds a single-dose or a multiple dose experimental design may be used. Two prospective trials on the possible pharmacokinetic interaction between phenytoin (P) and digoxin (D) or digitoxin (DT) were performed in healthy volunteers. It is demonstrated that an statistically significant pharmacokinetic interaction was found only after multiple dosing under conditions of steady-state, whereas after single dosing no interaction was observed. In case of investigating possible pharmacokinetic interactions a multiple dosing trial design appears to be advantageous.

Treatment of a patient with severe digitoxin intoxication by Fab fragments of anti-digitalis antibodies

A massive digitoxin (DGTX) intoxication in a 36-year-old man (35 mg DGTX) was treated by prolonged and repeated i.v.-infusions of Fab fragments of anti-digitalis antibodies (FAB). Blood and urine samples were collected over a 98 h period for monitoring the efficacy and adequacy of FAB treatment. DGTX concentrations were determined after protein precipitation (release of FAB-bound and protein-bound DGTX) in unprocessed serum and urine samples, and after aliquots of these samples had been dialysed in vitro against DGTX-free buffer (elimination of DGTX not bound to FAB). The difference in DGTX concentration between the unprocessed and dialysed samples was the amount of DGTX bound to plasma proteins and the small fraction of unbound DGTX being relevant for the therapeutic and toxic effects of the drug. Before FAB therapy was started, the total serum DGTX concentration was 535 nmol/l. The first FAB infusion (320 mg) was started 11 h after drug ingestion. Since this amount of FAB was insufficient to bind all DGTX present in the serum, cardiac DGTX toxicity (total AV-block) persisted. During a second FAB infusion (400 mg) the patient reverted to regular AV-conduction. At this time most of the DGTX in serum was FAB-bound. Toxic symptoms (sinus arrest) reappeared twice and were accompanied by increasing amounts of non-antibody-bound DGTX in the serum. Additional application of FAB (2 x 80 mg) resulted in the immediate disappearance of arrhythmia. During FAB-treatment total DGTX serum concentrations and renal DGTX clearance rose, indicating redistribution of drug from tissue to serum and urinary elimination of FAB-bound DGTX, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between digitoxin and some antiarrhythmic drugs

In the present study the pharmacokinetic interactions between digitoxin and the antiarrhythmic drugs amiodarone, mexiletine and propafenone have been examined. Experiments were performed on rabbits in which serum digitoxin concentration was used as indicator to detect drug interactions. The radioimmunoassay "Coat-A-Count" procedure of DPC was used for the quantitative measurement of digitoxin. It was determined that in order to achieve a considerable level of serum digitoxin, it was necessary to administer a multiple dose rather than the one tolerated by humans. It was also observed that serum contained digitalis like immunoreactive factor(s) (DLIF) measured as digitoxin. The mean (+/- SE) digitoxin equivalent value of the DLIF, measured by the "Coat-A-Count" radioimmunoassay in the serum of rabbits (n = 34) was 4.15 +/- 0.059 ng/ml. Each of the three antiarrhythmic drugs increased serum digitoxin levels; its values were almost double in relation to the control group where only digitoxin was administered. This increased digitoxin value was detected one hour after administration of the first dose of the antiarrhythmic drug and remained at a higher level than that of the control group for 6-8 hours. Rabbits given a single high dose of digitoxin and some of the antiarrhythmic drugs and those given a small dose of digitoxin for only four days, presented a retrogressive increase of digitoxin level in serum 5-6 days later. This mechanism needs to be further investigated.

Determination of thevetin B in serum by fluorescence polarization immunoassay

Thevetin B, a cardiac glycoside of Thevetia neriifolia Juss. seeds, was determined in serum by fluorescence polarization immunoassay. Anti-digitoxin antibody was used, thevetin B genin being structurally identical to digitoxigenin. Cross-reactivity of 94% was found by this method, for concentrations from 5 to 80 ng ml-1.

The effect of ageing on plasma albumin and plasma protein binding of diazepam, salicylic acid and digitoxin in healthy subjects and patients with renal impairment

1. Plasma albumin concentration was measured in 118 healthy subjects (aged between 18 and 87 years), in 95 renal patients with creatinine clearances between 15 and 50 ml min-1 (aged between 14 and 79 years) and in 101 uraemic patients maintained on chronic haemodialysis (aged between 27 and 83 years). 2. There was a significant (P less than 0.001) negative correlation between albumin concentration and age in healthy subjects, but no correlation in patients with low creatinine clearance or in uraemic patients. 3. The ex vivo plasma binding of diazepam (1 microM), salicylic acid (2 mM) and digitoxin (37 nM) was studied in groups of age-selected young and aged healthy subjects in patients with low creatinine clearance and in patients with uraemia. The unbound fractions of diazepam and salicylic acid were about double in old compared with young healthy subjects whereas they were similar in young and old patients with lowered creatinine clearance. In uraemic patients, ageing did not affect the binding of salicylic acid whereas the unbound fraction of diazepam was slightly but significantly greater in elderly subjects. The unbound fraction of digitoxin was independent of age in both healthy subjects and in those with renal disease. 4. Decreased plasma binding of diazepam and salicylic acid was partially corrected by extensive dialysis of plasma. The lower plasma binding of diazepam and salicylic acid associated with ageing may be ascribed to the effects of endogenous displacers and to hypoalbuminaemia. The influence of these two factors appears to be drug-dependent.