ASSOSIATION OF ENDOGENOUS CARDIOTONIC STEROIDS WITH SALT-SENSITIVITY OF BLOOD PRESSURE IN GEORGIAN POPULATION

This investigation differentiates types of essential hypertension in a Georgian population as well as describes endogenous cardiotonic steroids in salt-sensitive and salt-resistant subjects. This case control study included 185 subjects: 94 cases with stage 1 essential hypertension (JNC7) naïve to antihypertensive treatment, and 91 controls. A salt-sensitivity test was used to dichotomize case and control groups into salt-sensitive and salt-resistant subgroups. Blood and urine samples were obtained to categorize participants as consuming high and low salt diets. Endogenous cardiotonic steroids, sodium and plasma-renin activity (PRA) were measured in both samples at the different sodium conditions. Determinants of circulating levels of endogenous sodium pump inhibitors were carried out using the ELISA and RIA methods; PRA was assessed by radioimmunoassay. Descriptive statistics were used to analyze the data. Differences in variables between sodium conditions were assessed using paired t-tests. Salt-sensitivity was found in 60.5% of the total population investigated, with a higher proportion in females. A statistically significant positive correlation was found between salt-sensitivity and age in females (r=0.262, p<0.01), and with 24-hour urine sodium concentration changes (r=0.334, p<0.01). A significant negative correlation was found between salt-sensitivity and PRA. At the high sodium condition, endogenous MBG and OU were high in salt-sensitive subjects compared to those who were salt-resistant. These compounds decreased with a low-salt diet in both salt-sensitive cases and controls but remained the same in salt-resistant individuals. The MBG and OU levels positively correlated with systolic blood pressure in salt-sensitive individuals but no variability was evident among salt-resistant subjects. Our results show that MBG and OU levels start to increase at the normotensive stage and sustained high concentrations can lead to elevated systolic blood pressure, a risk factor for arterial hypertension in salt-sensitive subjects.

The iDigoxin Assay is More Sensitive than LOCI Digoxin Assay for Rapid Detection of Convallatoxin, the Active Cardiac Glycoside of Lily of The Valley

OBJECTIVE

Lily of the valley is a poisonous plant due to the presence of the cardiac glycoside convallatoxin. We compared two immunoassays (LOCI digoxin assay and iDigoxin assay) for rapid detection of convallatoxin if present in human serum.

MATERIALS AND METHODS

Aliquots of a drug free serum pool and a digoxin serum pool were supplemented with microliter amounts of lily of the valley extract or nanogram to microgram quantities of convallatoxin, followed by measurement of apparent digoxin concentrations using the LOCI and iDigxoin assays.

RESULTS

Apparent digoxin concentrations were observed when aliquots of a drug free serum pool were supplemented with convallatoxin or lily of the valley extract using both assays but apparent digoxin concentrations were significantly higher using the iDigoxin assay. In addition, the interference of convallatoxin in serum digoxin measurement was also significantly higher using iDigxoin assay compared to the LOCI digoxin assay.

CONCLUSIONS

The iDigxoin assay is more sensitive in detecting convallatoxin in human serum.

Molecular insights into uremic cardiomyopathy: cardiotonic steroids and Na/K ATPase signaling

Patients with chronic renal failure develop a "uremic" cardiomyopathy characterized by diastolic dysfunction, left ventricular hypertrophy, fibrosis, and systemic oxidant stress. Patients with chronic renal failure also are known to have increases in the circulating concentrations of endogenous cardiotonic steroids (also referred to as endogenous digitalis-like substances.) Endogenous cardiotonic steroids produce reactive oxygen species as part of the signal cascade induced by binding to the plasmalemmal Na/K-ATPase in patients, and this signal cascade appears capable of inducing several key pathophysiologic features of uremic cardiomyopathy. In addition, these patients develop both fibrosis and oxidant stress without a known mechanism. In this review we highlight data supporting the hypothesis that endogenous cardiotonic steroids are a key molecular component involved in the diastolic dysfunction, left ventricular hypertrophy, fibrosis, and systemic oxidant stress associated with chronic kidney disease.

Hypertension, Na+/Ca2+ exchanger, and Na+, K+-ATPase

Hypertension is the most prevalent risk factor for stroke, myocardial infarction, or end-stage renal failure. The critical importance of excess salt intake in the pathogenesis of hypertension is widely recognized, but the mechanisms whereby salt intake elevates blood pressure have puzzled researchers. Recent studies using Na+/Ca2+ exchange inhibitors and genetically engineered mice provide evidence that vascular Na+/Ca2+ exchanger type 1 (NCX1) is involved in the development of salt-dependent hypertension. Endogenous cardiac glycosides, which may contribute to salt-dependent hypertension, seem to be necessary for NCX1-mediated hypertension. Intriguingly, studies using knock-in mice with modified cardiac glycoside binding affinity of Na+,K+-ATPases provide a clear demonstration that this cardiac glycoside-binding site plays an important role in blood pressure regulation. Taken all together: (1) endogenous cardiac glycosides are secreted after high salt intake; (2) these cardiac glycosides inhibit Na+,K+-ATPase in vascular smooth muscle cells; (3) this inhibition results in the elevation of local Na+ on the submembrane area; and (4) this elevation of local Na+ facilitates Ca2+ entry through NCX1, resulting in vasoconstriction. This proposed pathway may have enabled us to explain how to link dietary salt to hypertension.

Diverse effects of stress and additional adrenocorticotropic hormone on digitalis-like compounds in normal and nude mice

Digitalis-like compounds (DLC) are steroidal hormones that are synthesized in, and released from, the adrenal gland, whose regulation may be directed by the hypothalamic-pituitary-adrenal (HPA) axis. Increasing evidence points to antitumour properties of these compounds and we hypothesized that the establishment of tumours in athymic nude mice may be facilitated by an abnormal synthesis or secretion of DLC. To explore this hypothesis, DLC concentrations were determined in the plasma, and in adrenal and hypothalamic tissues of nude compared to normal mice under basal conditions, and 30 min after a stress stimulus (i.p. injection of 100 micro l saline) with or without additional adrenocorticotropic hormone (ACTH) 1 micro g/per animal. Simultaneously, plasma corticosterone and serum adrenocorticotropic hormone (ACTH) concentrations were analysed. The basal DLC concentrations were similar in the plasma and the hypothalamus of both strains, whereas the basal adrenal DLC concentration was significantly lower in the nude mice compared to normal mice. The stress stimulus induced in normal mice a significant increase in DLC concentrations in the adrenal gland, the plasma and the hypothalamus. However, in nude mice, it caused an increase only in the adrenal gland and the hypothalamus, whereas the plasma DLC concentration was not affected. In both strains, the administration of ACTH in addition to injection stress did not provoke a further increase in DLC concentrations while inducing a significant increase in plasma corticosterone concentration. Regardless of the applied stimulus, the nude mice expressed significant lower DLC concentrations in the adrenal gland and the plasma compared to normal mice. The low basal adrenal DLC concentration in nude mice and their impaired DLC response towards stress- and ACTH stimulation both support an involvement of DLC in tumorigenesis.

Toxicokinetics of cotyledoside following intravenous administration to sheep

Cotyledoside, a bufadienolide cardiac glycoside, was administered intravenously to sheep in 2 studies. In experiment 1, sheep (n = 4) received 0.0135 mg/kg daily on 5 consecutive days and in the 2nd experiment, sheep (n = 4) received 0.027 mg/kg as a single dose. Jugular blood was collected at different time intervals and kinetic parameters were determined. The data fitted a 1-compartmental model. In both experiments a short half-life (t1/2) and mean residence time (MRT), a relative small volume of distribution (Vd(ss)) and rapid clearance were calculated. In the 1st experiment, t1/2 and MRT increased significantly (P < 0.007) from Day (D) 0 to D4. It is suggested that the rapid decline in plasma cotyledoside concentrations in sheep denotes rapid distribution of cotyledoside to the tissues or extracellular spaces and possible accumulation at the biophase.

Acute yellow oleander (Thevetia peruviana) poisoning: cardiac arrhythmias, electrolyte disturbances, and serum cardiac glycoside concentrations on presentation to hospital

OBJECTIVE

To describe the cardiac arrhythmias, electrolyte disturbances, and serum cardiac glycoside levels seen in patients presenting to hospital with acute yellow oleander (Thevetia peruviana) poisoning and to compare these with published reports of digitalis poisoning.

DESIGN

Case series.

SETTING

Medical wards of Anuradhapura District General Hospital, Sri Lanka, and coronary care unit of the Institute of Cardiology, National Hospital of Sri Lanka, Colombo, the national tertiary referral centre for cardiology.

PATIENTS

351 patients with a history of oleander ingestion.

MEASUREMENTS

ECG and blood sample analysis on admission.

RESULTS

Most symptomatic patients had conduction defects affecting the sinus node, the atrioventricular (AV) node, or both. Patients showing cardiac arrhythmias that required transfer for specialised management had significantly higher mean serum cardiac glycoside and potassium but not magnesium concentrations. Although there was considerable overlap between groups, those with conduction defects affecting both sinus and AV nodes had significantly higher mean serum cardiac glycoside levels.

CONCLUSIONS

Most of these young previously healthy patients had conduction defects affecting the sinus or AV nodes. Relatively few had the atrial or ventricular tachyarrhythmias or ventricular ectopic beats that are typical of digoxin poisoning. Serious yellow oleander induced arrhythmias were associated with higher serum cardiac glycoside concentrations and hyperkalaemia but not with disturbances of magnesium.

Identification and quantification of cardiac glycosides in blood and urine samples by HPLC/MS/MS

Cardiac glycosides (CG) are of forensic importance because of their toxicity and the fact that very limited methods are available for identification of CG in biological samples. In this study, we have developed an identification and quantification method for digoxin, digitoxin, deslanoside, digoxigenin, and digitoxigenin by high-performance liquid chromatography tandem mass spectrometry (HPLC/MS/MS). CG formed abundant [M + NH4]+ ions and much less abundant [M + H]+ ions as observed with electrospray ionization (ESI) source and ammonium formate buffer. Under mild conditions for collision-induced dissociation (CID), each [M + NH4]+ ion fragmented to produce a dominant daughter ion, which was essential to the sensitive method of selected reaction monitoring (SRM) quantification of CG achieved in this study. SRM was compared with selected ion monitoring (SIM) regarding the effects of sample matrixes on the methodology. SRM produced lower detection limits with biological samples than SIM, while both methods produced equal detection limits with CG standards. On the basis of the HPLC/MS/MS results for CG, we have proposed some generalized points for conducting sensitive SRM measurements, in view of the property of analytes as well as instrumental conditions such as the type of HPLC/MS interface and CID parameters. Analytes of which the molecular ion can produce one abundant daughter ion with high yield under CID conditions may be sensitively measured by SRM. ESI is the most soft ionization source developed so far and can afford formation of the fragile molecular ions that are necessary for sensitive SRM detection. Mild CID conditions such as low collision energy and low pressure of collision gas favor production of an abundant daughter ion that is essential to sensitive SRM detection. This knowledge may provide some guidelines for conducting sensitive SRM measurements of very low concentrations of drugs or toxicants in biological samples.

Circulating bufodienolide and cardenolide sodium pump inhibitors in preeclampsia

OBJECTIVE

To determine plasma levels of the endogenous bufodienolide Na+/K+ ATPase inhibitor, marinobufagenin-like factor (MBG), in normotensive pregnancy and in preeclampsia, to compare changes of MBG with that of ouabain-like compound (OLC), and to characterize the purified MBG immunoreactive factor from preeclamptic plasma.

DESIGN AND METHODS

Consecutive sample study. The levels of MBG and OLC compounds were measured in extracted plasma by solid phase fluoroimmunoassays. MBG and ouabain immunoreactive materials were partially purified from preeclamptic plasma via reverse-phase high-performance liquid chromatography (HPLC) and studied for their ability to cross react with MBG and ouabain antibodies, and to inhibit the Na+/K+ ATPase from human mesenteric arteries. Vasoconstrictor effect of authentic MBG was studied in isolated rings of human umbilical arteries.

RESULTS

In 11 nonpregnant control individuals, plasma concentrations of MBG and OLC were 0.190+/-0.04 nmol/l and 0.297+/-0.037 nmol/l, respectively. In the third trimester of noncomplicated pregnancy (n = 6), plasma MBG increased (0.625+/-0.067 nmol/l, P<0.05), and OLC did not (0.32+/-0.07 nmol/l). In 15 patients with preeclampsia, plasma levels of both MBG and OLC increased dramatically (2.63+/-0.10 nmol/l and 0.697+/-0.16 nmol/l, respectively, P<0.01 versus both control groups). When fractionated by reverse phase HPLC, OLC was eluted by 18% acetonitrile, and MBG by 48% acetonitrile. Serially diluted samples of MBG and OLC immunoreactive materials from HPLC fractions reacted with MBG and ouabain antibody in solid phase immunoassay in a concentration dependent fashion. Authentic MBG caused contractile responses of isolated rings of human mesenteric arteries in a concentration-dependent manner. Similarly to the authentic MBG, HPLC purified MBG immunoreactive material from preeclamptic plasma inhibited Na+/K+ ATPase purified from human mesenteric artery.

CONCLUSIONS

Our observations demonstrate the coexistence of two endogenous cardiotonic steroids in preeclamptic plasma, a more polar OLC and a less polar MBG-like compound. Substantial increases in plasma OLC and MBG immunoreactivity in preeclampsia, along with the vasoconstrictor properties of authentic MBG and Na+,K+ ATPase inhibitory activity of human MBG immunoreactive factor, suggest, that in preeclampsia, plasma concentrations of MBG are enough to substantially inhibit the sodium pump in cardiovascular tissues, and are in accordance with the views attributing endogenous digitalis-like factors a pathogenic role in the preeclamptic hypertension.

A specific binding protein for cardiac glycosides exists in bovine serum

Searching for a binding protein in blood, which may be involved in the specific transport of cardiac glycosides to their receptor sites on the sodium pump, we isolated a cardiac glycoside-binding protein (CGBG) of 26 kDa from the globulin fraction of bovine serum by affinity chromatography and on a ouabain-Sepharose 4B column by a purification factor of 5000. The cardiac glycoside-binding globulin was labeled specifically and covalently by the protein-reactive digoxigenin derivative HDMA (N-hydroxysuccimidyldigoxigenin-3-O-methylcarbonyl-epsilon-+ ++aminocapro ate). Even very high concentrations of other steroids, such as estrogen, testosterone, progesterone, and cortisone, did not prevent HDMA-labeling (at 5 and 100 nM) of CGBG, but the cardenolides ouabain and digoxin or the bufadienolide proscillaridin A did so. CGBG is a homodimer of two 26-kDa subunits forming disulfide bonds, since HDMA labeling of a protein of 53 kDa was observed in SDS-polyacrylamide gel electrophoresis when beta-mercaptoethanol was absent during SDS denaturation. The N-terminal amino acid sequence K-D-V-Y-R-A-P-D-G-T-Q-S-A showed no sequence similarity with proteins recorded in gene and protein sequence data banks. A 90-kDa cytosolic CGBG exists in bovine kidneys and reacts with antibodies against CGBG. Binding of ouabain to the cardiac glycoside-binding globulin was monitored by quenching of intrinsic tryptophan fluorescence. Such studies reveal two negatively cooperative ouabain binding sites with Kd' of 1.52 nM and Kd' = 75 nM and with an interaction factor of 50 using a Koshland-Némethy-Filmer model. The demonstration of a cardiac glycoside-binding globulin in plasma is consistent with the recent finding of endogenous cardiac glycosides in mammals.

Standards of laboratory practice: cardiac drug monitoring. National Academy of Clinical Biochemistry

In this Standard of Laboratory Practice we recommend guidelines for therapeutic monitoring of cardiac drugs. Cardiac drugs are primarily used for treatment of angina, arrhythmias, and congestive heart failure. Digoxin, used in congestive heart failure, is widely prescribed and therapeutically monitored. Monitoring and use of antiarrhythmics such as disopyramide and lidocaine have been steadily declining. Immunoassay techniques are currently the most popular methods for measuring cardiac drugs. Several reasons make measurement of cardiac drugs in serum important: their narrow therapeutic index, similarity in clinical complications and presentation of under- and overmedicated patients, need for dosage adjustments, and confirmation of patient compliance. Monitoring may also be necessary in other circumstances, such as assessment of acetylator phenotypes. We present recommendations for measuring digoxin, quinidine, procainamide (and N-acetylprocainamide), lidocaine, and flecainide. We discuss guidelines for measuring unbound digoxin in the presence of an antidote (Fab fragments), for characterizing the impact of digoxin-like immunoreactive factor (DLIF) and other cross-reactants on immunoassays, and for moni-toring the unbound (free fraction) of drugs that bind to alpha1-acid glycoprotein. We also discuss logistic, clinical, hospital, and laboratory practice guidelines needed for implementation of a successful therapeutic drug monitoring service for cardiac drugs.

Confirmation of oleander poisoning by HPLC/MS

A non-fatal case of Nerium oleander (common oleander) self-poisoning in a 45-year-old female is presented. Initial symptoms were nausea and vomiting, abdominal pain, phosphenes, cardiovascular shock and sinus brady-cardia. Blood and urine were assayed for oleandrin, the major cardiac glycoside of N. oleander, using a highly specific HPLC/MS procedure. The blood concentration of oleandrin at admission was 1.1 ng/ml. This is the first report of an oleander intoxication ascertained by the mass spectrometric identification of oleandrin in blood. HPLC/MS appears to be the method of choice for the forensic-toxicological investigation of poisonings by cardiac glycosides.

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.

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.

Inhibition of Na,K-ATPase by oleandrin and oleandrigenin, and their detection by digoxin immunoassays

Ingestion of oleander plant, containing the cardiac glycoside oleandrin, has been reported to induce fatal poisonings. Derivatives of oleandrin are structurally similar to digoxin. We investigated the cross-reactivities of oleandrin and its aglycone metabolite, oleandrigenin, in several commercially available digoxin immunoassays; assessed their ability to inhibit Na,K-ATPase catalytic activity; and measured their binding to proteins in serum. As assayed with ACS:180, Stratus, RIA, On-Line, and TDx digoxin assays, oleandrin at 100 micromol/L in digoxin-free serum gave apparent digoxin values of 0, 0.83, 2.24, 2.37, and 5.34 nmol/L, respectively, whereas oleandrigenin at that concentration gave results of 0, 0.52, 0.77, 4.94, and 1.40 nmol/L. Study of Na,K-ATPase inhibition showed IC50 values (micromol/L) of 0.22 for ouabain, 0.62 for oleandrin, 1.23 for oleandrigenin, and 2.69 for digoxin. At 25 degrees C, 96% of oleandrin and 48% of oleandrigenin were bound to serum proteins. Because detection of oleandrin and oleandrigenin by digoxin immunoassays is variable between assays as well as between congeners, assessment of cross-reactivity is warranted for each assay. The inhibition of Na,K-ATPase by oleandrin and oleandrigenin confirms that they likely exert their toxic effects through inhibition of sodium pump activity. In cases of digitalis-like poisoning with suspicion of oleander ingestion, a combination of digoxin immunoassays may be useful to effectively rule out the presence of oleander.

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.

Specific binding of cardiac glycoside drugs and endogenous digitalis-like substances to particulate membrane fractions from human placenta

We studied the characteristics of binding of cardiac glycosides to particulate membrane fractions from human placenta, to demonstrate that placental tissue is a suitable source of receptors for digitalis drugs. Moreover, we performed preliminary experiments with 125I-labeled digoxin and placental particulates to develop a radioreceptor assay for measurement of endogenous substances with activity similar to cardiac glycoside drugs (EDLS). Placental membrane fractions were incubated with [3H]ouabain (10 nmol/L) or 125I-labeled digoxin (50 pmol/L). With both ligands, binding followed a pseudo-first-order reaction kinetics and was saturable. Scatchard analysis revealed a single class of sites [for ouabain, KD = 20.2 +/- 5.8 nmol/L (mean +/- SEM), Bmax = 3.1 +/- 0.9 nmol per gram of protein; for digoxin, KD = 29.7 +/- 1.9 nmol/L, Bmax = 24.3 +/- 1.1 nmol per gram of protein]. As expected, digoxin was less potent than ouabain in displacing both tracers from digitalis drugs receptors; progesterone, cortisone, digitoxose, furosemide, bumetanide, and propranolol had no or little effect. Specific 125I-labeled digoxin binding was competitively inhibited by plasma and (or) urine extracts from newborns, adults, pregnant women, and patients with renal insufficiency. Inhibition of binding and volume of plasma and urine assayed were linearly related. These findings support the hypothesis that cardiac glycosides and EDLS can interact with the human placenta and suggest placental tissue to be a suitable source of receptors for cardiac glycosides.

Detection of poisoning by plant-origin cardiac glycoside with the Abbott TDx analyzer

Cardiac glycoside poisoning caused by ingestion of plant material is common in tropical and sub-tropical areas. In evaluating the use of the Abbott TDx Digoxin II assay to detect such cases of poisoning, we found it a rapid and convenient method for confirming the ingestion of glycosides from the plants Nerium oleander, Thevetia peruviana, and Adonis microcarpa, and from the toad Bufo marinus. Here we report some clinical cases illustrating our experience with the use of this assay, and describe results of cross-reactivity studies with compounds structurally similar to digoxin. Because of the competitive nature of the immunoassay as well as the complexity of the mixture of cross-reacting cardiac glycosides present in the plant material, the measured apparent digoxin concentration is not linearly related to the cardiac glycoside concentration.

Distribution of SC-4453, a new semi-synthetic derivative of digoxin, following an infusion preceded by an i.v. loading dose in the guinea-pig. Binding to plasma proteins

The tissue distribution of 12 alpha-[3H]SC-4453 was studied in the guinea-pig after a 15 hours infusion at the rate of 4 micrograms/h/kg, preceded by an IV loading dose. At the end of the infusion, at the steady state, the skeletal muscle apparently presented the highest concentration of SC-4453 (24 ng/g) and the brain the lowest (1.2 ng/g), which demonstrates the presence of a blood-brain barrier to this compound. Our results also show that this new semi-synthetic cardiotonic presents a weak affinity to plasma proteins (25%).

[Binding of cardiac glycosides by the erythrocytes of patients]

Digoxin and strophanthin K uptake by patients' red cells was investigated by the Rb-86 assay. It was found that digoxin and strophanthin K levels in red cells increased with augmentation of the drugs concentrations in plasma, the ratio of cardiac glycoside levels in red cells to their concentrations in plasma decreased. The degree of binding to red cells is more pronounced for strophanthin K than for digoxin.

Naturally occurring cardiac glycosides

Cardiac glycoside poisoning from the ingestion of plants, particularly of oleanders, occurs with reasonable frequency in tropical and subtropical areas. We have assessed a variety of plant specimens for their cardiac glycoside content by means of radioimmunoassays with antibodies that differ in their specificity for cardiac glycosides. Significant amounts of immunoreactive cardiac glycoside were found to be present in the ornamental shrubs: yellow oleander (Thevetia peruviana); oleander (Nerium oleander); wintersweet (Carissa spectabilis); bushman's poison (Carissa acokanthera); sea-mango (Cerbera manghas); and frangipani (Plumeria rubra); and in the milkweeds: redheaded cotton-bush (Asclepias curassavica); balloon cotton (Asclepias fruiticosa); king's crown (Calotropis procera); and rubber vine (Cryptostegia grandifolia). The venom gland of the cane toad (Bufo marinus) also contained large quantities of cardiac glycosides. The competitive immunoassay method permits the rapid screening of specimens that are suspected to contain cardiac glycosides. Awareness of the existence of these plant and animal toxins and their dangers allows them to be avoided and poisoning prevented. The method is also useful for the confirmation of the presence of cardiac glycosides in serum in cases of poisoning.

Pharmacokinetics, bioavailability and serum levels of cardiac glycosides

Digoxin, the cardiac glycoside most frequently used in clinical practice in the United States, can be given orally or intravenously and has an excretory half-life of 36 to 48 hours in patients with serum creatinine and blood urea nitrogen values in the normal range. Since the drug is excreted predominantly by the kidney, the half-life is prolonged progressively with diminishing renal function, reaching about 5 days on average in patients who are essentially anephric. Serum protein binding of digoxin is only about 20%, and differs markedly in this regard from that of digitoxin, which is 97% bound by serum albumin at usual therapeutic levels. Digitoxin is nearly completely absorbed from the normal gastrointestinal tract and has a half-life averaging 5 to 6 days in patients receiving usual doses irrespective of renal function. The bioavailability of digoxin is appreciably less than that of digitoxin, averaging about two-thirds to three-fourths of the equivalent dose given intravenously in the case of currently available tablet formulations. Recent studies have shown that gut flora of about 10% of patients reduce digoxin to a less bioactive dihydro derivative. This process is sensitive to antibiotic administration, creating the potential for important interactions among drugs. Serum or plasma concentrations of digitalis glycosides can be measured by radioimmunoassay methods that are now widely available, but knowledge of serum levels does not substitute for a sound working knowledge of the clinical pharmacology of the preparation used and careful patient follow-up.

Oleander tea: herbal draught of death

A woman died after drinking herbal tea prepared from oleander (Nerium oleander) leaves. This case demonstrates the cross-reactivity between the cardiac glycosides in oleander and the digoxin radioimmunoassay. Digoxin-specific Fab antibody fragments have not been used in oleander poisoning, but these might prove to be lifesaving. Treatment of oleander toxicity is aimed at controlling arrhythmias and hyperkalemia; inactivation of the Na-K ATPase pump, however, can make treatment difficult.

[Therapy control in cardiology: serum level monitoring]

Treatment of cardiac patients with drugs of low therapeutic ratio (i.e. cardiac glycosides, antiarrhythmics) must be individualized to avoid undertreatment or intoxication with their often deleterious consequences. However, in most cases the dose-response effect cannot be predicted, especially in those instances in which the desired effect is hard to measure (e.g. intermittent arrhythmias). Very often a potentially useful drug is not effective because the applied dose is either too low or administered at incorrect intervals. An effective medication can also be incorrectly assumed to be intolerable when the dose administered is too high relative to the patient's impaired renal or liver function. Fixed application schedules will not be successful in the majority of cases due to the following large interpatient variables: absorption, distribution, elimination, biotransformation, protein and tissue binding, and effect on the target organ. If drug efficacy cannot be proven by clinical observation the determination of blood levels of substances with a narrow therapeutic ratio cen be helpful. However, an interpretation should only be made by considering the clinical condition of the patient and the inherent kinetic variables of the drug.

Evaluation of medigoxin in outpatients

We compared our ability to predict the dose of medigoxin and of digoxin required to achieve a fixed serum concentration (the dose requirement) in 33 outpatients. Preliminary work supported the assumptions that the steady state glycoside concentration achieved was proportional to the daily dose given to an individual, and that the bioavailability of the different tablet presentations was similar for either glycoside. We were not able to predict the dose requirement from patient characteristics with any more certainty for medigoxin than for digoxin. Not only the between-patient variability in dose requirement, but also the within-patient variability, was similar for the two glycosides. However the digoxin used had a dissolution rate of over 90% in 1 h. When comparing medigoxin with digoxin of lower, or more variable dissolution rate, medigoxin may be preferable.

[Effective concentration and dosage for cardiac glycosides]

The pharmacokinetic fundaments of the glycoside therapy with strophanthin, digoxin and digitoxin are described and compared with the traditional clinically oriented parameters total body stores, decay ratio and so on. Newly introduced is the parameter medium remaining duration in the circulation. A critical analysis of the hitherto cited values of the total body stores and the decay ratio established from the clinical effect shows that on the basis of pharmacokinetic calculations the corresponding values for digoxin and digotoxin need a great correction. The fully active quantity for digoxin lies between 0.5 and 0.8 mg and the subsiding share is 50%. The values for digitoxin lie at 0.9 mg and 9%. Furthermore the causes of the interindividual variability of the maintenance dose are discussed. According to the already existing international experiences an essential improvement of the therapy is possible, when the pharmacokinetically established effective constants are taken into consideration. Examinations of the inter- and intraindividual differences in the kinetics of the glycosides and the development of digoxin preparations with a high (i.e. constant) bioavailability may contribute to a further decrease of the frequency of toxic side-effects in future.

Investigation of cardiac glycoside levels in human post mortem blood and tissues determined by a special radioimmunoassay procedure

Even after the introduction of radioimmunological methods the question of a cardiac glycoside causing or contributing to the death of a patient can not be answered satisfactorily. By means of a special radioimmunoassay procedure for digoxin as well as for the structurally related methyl- and acetylderivatives we measured the concentrations in human blood and post mortem tissues. We investigated the glycoside contents in the blood of intravenously digitalised (Novodigal) al) patients before and after death. At autopsy blood specimens were taken from the heart and the femoral vein. We found an increase of the glycoside level up to a highly toxic range (7--15 ng/ml) especially in the heart blood. Thus post mortem blood levels of digoxin and its derivatives are not suitable for a final decision in alleged cases of fatal poisonings. Measuring various concentrations in tussues and body fluids of the above cardiac glycosides mentioned revealed the kidney concentration to be of high value in confirming a digitalis poisoning. This organ and the heart show the highest tissue concentrations. Interpretations of fatal digitalis poisonings should be based on the additional knowlege of these concentrations. Individual cardiac glycosides may be analyzed by a combination of thin layer chromatography and radioimmunoassay.

[Plasma concentrations of digoxin in patients under intensive care conditions and in patients undergoing anesthesia and operation (author' transl)]

Pharmacokinetic behavior of digoxin or beta-acetyldigoxin was examined in 66 patients (27 patiets under intensive care conditions, partially with controlled breathing, 22 patients undergoing extirpation of the uterus and 17 patients treated with radium or chemotherapeutics; 19 males and 47 females) by determining plasma concentrations of digoxin (PDC). After intravenous and oral application with a maintenance dose of 0.20--0.50 mg/day blood was taken daily during a 2 to 3 week period, resulting in 510 determinations. 24 hours after the first application of 0.50 mg digoxin i.v. the mean values of PDC amounted to 0.62 +/- 0.08 ng/ml. After 0.40 or 0.25 mg digoxin per day i.v. therapeutical concentrations could be observed at the third vs fifth day. An equilibrium of PDC was reached on the 6th day after starting digitalization using maintenance doses. Intravenous application of 0.25, 0.40 or 0.50 mg digoxin per day resulted in a mean steady state of 0.68 +/- 0.37, 0.86 +/- 0.33 or 1.27 +/- 0.49 ng/ml PDC, respectively. The results were significantly different (p less than 0.01--0.001). Serial measurements indicated a great variation of PDC. In patients without renal failure the intraindividual variation of the plasma concentrations was maximal 37.4% referring to the mean steady state, interindividual 37.1% and the evaluation of the inter- and intraindividual differences amounted to 54.1%. After oral administration of digoxin (maintenance dose: 0.50 mg/day) or beta-acetyldigoxin (maintenance doses: 0.20--0.40 mg/day) differences in PDC of 38.3% and 29.7% were obtained. Body weight, age and serum creatinine concentration were partly responsible for the variance of PDC. Multiple linear regression between stead state PDC and dose, age, body weight and serum creatinine concentration revealed 62.1% of the variance of the PDC after intravenous administration of digoxin. After oral administration of beta-acetyldigoxin 39.9% were obtained. Thus, 40% of the variance were caused by differences in distribution and elimination of digoxin after i.v. application. After oral application additional 20% of the variance could be attributed to resorption and possible disturbances.

[Pharmacokinetics of digoxin and methyldigoxin in patients with acute hepatitis (author's transl)]

After i.v. application of 3h-digoxin or 3H-methyldigoxin to 5 healthy volunteers and 5 patients with acute hepatitis, respectively (0.75 mg daily for 3 days and 0.375 for the following 2 days) total radioactivity in urine and plasma were determined. Chloroform-soluble and -insoluble glycosides were separated and the chloroform-soluble fraction was determined by TLC. 3 days after methyldigoxin application plasma levels reached toxic values in the patient group (2.73 +/- 0.48 ng/ml), whereas in patients receiving digoxin a mean plasma level of 0.91 +/- 0.21 ng/ml was obtained. During the first 24 hours following administration of digoxin 44 +/- 12% of the dose were recovered in urine of control subjects and 48 +/- 13% in patients with acute hepatitis, after methyldigoxin 34 +/- 5% and 34 +/- 8%, respectively. Metabolism of digoxin in patients with acute hepatitis was unaltered, whereas a diminished demethylation rate of methyldigoxin could be observed. 16 patients with acute hepatitis and 7 healthy volunteers received unlabelled digoxin p.o. as maintenance dose. Plasma glycoside concentrations were studied by radioimmunoassay. The average glycoside plasma concentrations were 0.59 +/- 0.21 ng/ml and 0.63 +/- 0.24 ng/ml, respectively.

[The clinical significance of plasma glycoside concentrations in patients with cardiac pacemakers (author's transl)]

308 digitalized out-patients with artificial cardiac pacemakers were explored for signs of glycoside toxicity with simultaneous determination of digoxin plasma levels 12 hours after the last dose. The incidence of different side effects commonly attributed to overdigitalization did not allow prediction of toxic plasma levels. 55% of all glycoside levels were within the therapeutic range, 34% were below 0.7 ng/ml and only 11% above 2.0 ng/ml. With the most commonly prescribed maintenance doses of the glycosides used (digoxin 0.5 mg, beta-acetyldigoxin 0.4 mg, beta-methyldigoxin 0.2 mg, lanatosid C 1.0 mg) therapeutic plasma levels were reached regularly in 60-65% of the patients. A significant correlation existed between plasma glycoside concentrations and renal function as well as age, but glycoside concentrations could not be correlated with the age of the patients. There were no indications for interactions of the different glycosides prescribed with diuretics or oral antidiabetics.

Protein binding of cardiac glycosides in disease states

Digitoxin in 97% bound to serum albumin and digoxin only to the extent of 24%. Hypoalbuminaemia significantly changes the protein binding of digoxin in Kwashorkor serum and the binding of digitoxin in patients with chronic active hepatitis and the nephrotic syndrome. Sprue patiens with normal albumin values have normal binding of digitoxin. Preliminary data in patients with thyrotoxicosis and myxoedema show digitoxin binding within the normal range. The effect of uraemia per se on digitoxin binding is controversial as both normal and slightly decreased values have been reported. In uraemic patients on treatment with haemodialysis, heparin administration has been shown to be a powerful serum binding displacing agent for both digitoxin and digoxin, the mechanism probably being a heparin-induced release of free fatty acids. Patients with a significant decrease in serum protein binding of digitoxin or digoxin should be maintained on a total serum concentration lower than usually considered within the therapeutic range.

[Pharmacokinetics of cardiac glycosides and clinical consequences]

The purpose of pharmacokinetics of cardiac glycosides is to study the time courses of glycosides in biological fluids, tissues and excreta. The extent of accumulation of a given dose at uniform time intervals depends only from the overall elimination rate constant. By knowing the elimination rate constant the extent to which a cardiac glycoside would accumulate in the body following a fixed dosing regimen can be calculated. The higher accumulation in the central nervous system requires a much longer time. Therefore it may be assumed that the brain is a deep compartment for cardiac glycosides and this compartment cannot be detected by analysis of plasma glycoside concentrations. Central side effects of cardiac glycosides may occur at therapeutic plasma levels. In renal disease a lower maintenance dose of digoxin and methyldigoxin should be administered or the same dose less frequently. Digitoxin does not accumulate in patients with renal failure or in anuria since the extrarenal elimination of digitoxin is much higher compared to digoxin and methyldigoxin.