Contribution of quantitative assay technics to the understanding of the clinical pharmacology of digitalis

[Cancer cell-specific functional relation between Na+,K+-ATPase and volume-regulated anion channel]

Digitoxin and digoxin are plant-derived cardiac glycosides. They are Na⁺,K⁺-ATPase (sodium pump) inhibitors, and have been used clinically for treatment and prevention of heart failure and various tachycardia. On the other hand, some epidemiological studies showed that digoxin users have a lower cancer risk compared to the non-users, and that cancer patients who had been treated with digoxin face on improvement of their survival. In various in vitro studies, cardiac glycosides at sub-μM concentrations, which have no significant effect on enzymatic and ion-transporting activities of Na⁺,K⁺-ATPase, show anti-cancer effects. Na⁺,K⁺-ATPase is ubiquitously expressed, so it remains unclear why low concentrations of cardiac glycosides have cancer-specific effects. Recently, we found that the receptor-type Na⁺,K⁺-ATPase, which has no pumping activity, is associated with leucine-rich repeat-containing 8 family, member A(LRRC8A), one of the components of volume-regulated anion channel (VRAC), in the membrane microdomains of plasma membrane of cancer cells, and that this crosstalk contributes to the inhibition of the cancer cell growth by sub-μM cardiac glycosides. In this mechanism, cardiac glycosides bind to the receptor-type Na⁺,K⁺-ATPase, and then stimulate the production of reactive oxygen species (ROS) via NADPH oxidase. The ROS activate VRAC within the membrane microdomains, thus eliciting anti-proliferative effects. VRAC is ubiquitously expressed, and it is normally activated by cell swelling. However, VRAC is activated by cardiac glycoside without cell swelling. On the other hand, the cardiac glycosides-induced effects were not observed in non-cancer cells. Our findings can partly explain why cardiac glycosides elicit selective effects in cancer cells.

Crosstalk between Na+,K+-ATPase and a volume-regulated anion channel in membrane microdomains of human cancer cells

Low concentrations of cardiac glycosides including ouabain, digoxin, and digitoxin block cancer cell growth without affecting Na⁺,K⁺-ATPase activity, but the mechanism underlying this anti-cancer effect is not fully understood. Volume-regulated anion channel (VRAC) plays an important role in cell death signaling pathway in addition to its fundamental role in the cell volume maintenance. Here, we report cardiac glycosides-induced signaling pathway mediated by the crosstalk between Na⁺,K⁺-ATPase and VRAC in human cancer cells. Submicromolar concentrations of ouabain enhanced VRAC currents concomitantly with a deceleration of cancer cell proliferation. The effects of ouabain were abrogated by a specific inhibitor of VRAC (DCPIB) and knockdown of an essential component of VRAC (LRRC8A), and they were also attenuated by the disruption of membrane microdomains or the inhibition of NADPH oxidase. Digoxin and digitoxin also showed anti-proliferative effects in cancer cells at their therapeutic concentration ranges, and these effects were blocked by DCPIB. In membrane microdomains of cancer cells, LRRC8A was found to be co-immunoprecipitated with Na⁺,K⁺-ATPase α1-isoform. These ouabain-induced effects were not observed in non-cancer cells. Therefore, cardiac glycosides were considered to interact with Na⁺,K⁺-ATPase to stimulate the production of reactive oxygen species, and they also apparently activated VRAC within membrane microdomains, thus producing anti-proliferative effects.

Synergistic effects of ion transporter and MAP kinase pathway inhibitors in melanoma

New therapies are required for melanoma. Here, we report that multiple cardiac glycosides, including digitoxin and digoxin, are significantly more toxic to human melanoma cells than normal human cells. This reflects on-target inhibition of the ATP1A1 Na(+)/K(+) pump, which is highly expressed by melanoma. MEK inhibitor and/or BRAF inhibitor additively or synergistically combined with digitoxin to induce cell death, inhibiting growth of patient-derived melanomas in NSG mice and synergistically extending survival. MEK inhibitor and digitoxin do not induce cell death in human melanocytes or haematopoietic cells in NSG mice. In melanoma, MEK inhibitor reduces ERK phosphorylation, while digitoxin disrupts ion gradients, altering plasma membrane and mitochondrial membrane potentials. MEK inhibitor and digitoxin together cause intracellular acidification, mitochondrial calcium dysregulation and ATP depletion in melanoma cells but not in normal cells. The disruption of ion homoeostasis in cancer cells can thus synergize with targeted agents to promote tumour regression in vivo.

Clinical toxicology of cardiovascular drugs

Drugs used to treat cardiovascular diseases have low therapeutic indices, may produce the very pathophysiologic effects that it is hoped they will reverse, and are used commonly in aged animals with multisystemic diseases for which they are receiving other compounds. These factors predispose to undesirable drug reactions and interactions. It is difficult to determine, a priori, which animals will respond with profound toxic manifestations; therefore, resuscitative measures, including drugs and devices, must be available at all times, and the clinician must be schooled in their use. In particular, class IA antiarrhythmics, digitalis glycosides, and antineoplastic compounds, all used relatively frequently, have great potential for producing toxicosis. An important role of the clinician with regard to cardiovascular toxicology lies in providing consultation to both the pharmaceutical industry and governmental regulatory agencies. Because the worst aspects of cardiovascular toxicosis lie in electrical disturbances of the heart, and because electrocardiography is the best method for studying these electrical properties, the clinician and adviser to the pharmaceutical industry and the FDA must be well schooled in electrocardiography.

Facilitation of lethal ventricular arrhythmias by therapeutic digoxin in conscious post infarction dogs

The proarrhythmic potential of digoxin, administered in a therapeutic dosage regimen, was evaluated in conscious dogs in the subacute phase of myocardial infarction. In this evaluation, digoxin (0.0125 mg/kg/day intravenously) or vehicle were administered to conscious dogs for periods of 5 to 7 days, commencing 4 to 5 days after anterior myocardial infarction. Before treatment, programmed ventricular stimulation failed to initiate ventricular tachycardia in 26 post infarction dogs. After treatment, programmed stimulation initiated ventricular tachyarrhythmias in only 1 of 13 digoxin-treated dogs (1.36 +/- 0.17 ng/ml serum digoxin) and in 0 of 13 vehicle-treated dogs. However, the incidences of early ventricular fibrilation (4 of 10 digoxin vs 0 of 12 vehicle; p less than 0.05) and of 24-hour mortality (6 of 10 digoxin vs 2 of 12 vehicle; p less than 0.05) occurring in response to the development of posterolateral ischemia in the presence of previous anterior myocardial infarction was significantly greater in digoxin-treated (1.47 +/- 0.19 ng/ml serum digoxin) than in vehicle-treated animals. These findings suggest an enhanced susceptibility toward the development of ischemia-related lethal arrhythmias in the presence of therapeutic digoxin serum concentrations early after myocardial infarction, which is not predicted by programmed ventricular stimulation testing.

A standard approach to compiling clinical pharmacokinetic data

A standard format for a Clinical Pharmacokinetic Summary is proposed. It consists of a heading, tables, notes, and references for each drug reviewed. The table presents a unified and logical set of clinically useful population pharmacokinetic parameters. They concern four major areas: absorption, distribution, elimination, and the relationship of concentration to effect. Within each major group, parameters dealing with extents and rates of processes are given. Each such parameter is really two: a population mea value (for example, average volume of distribution) and the standard deviation of individual values about this mean. The first value allows individual predictions of dosage or drug level to be made; the second allows computation of the likely proximity of subsequently observed quantities to those predictions. The table presents single consensus values for each population parameter, rather than a list of values. A procedure for computing these consensus values, and for revising them in the light of new data, or reinterpreted old data, is given. Examples of Summaries are given. The method appears applicable to a variety of drugs. We suggest our approach as a standard one for preparing Clinical Pharmacokinetic Summaries, and urge our colleagues to consider it for that purpose.

Skeletal muscle digoxin concentration during digitalization and during withdrawal of digoxin treatment

Blood samples and skeletal muscle biopsies (m. quadriceps femoris, vastus lateralis) were taken from 15 patients during digitalization or during withdrawal of digoxin treatment for analysis of serum and skeletal muscle digoxin concentrations. A percutaneous needle biopsy technique was used for muscle sampling and digoxin was analysed by radioimmunoassay. During "slow" digitalization with 0.25 mg digoxin daily the skeletal muscle digoxin concentrations after 2 and 4 days were 45% (range 19%--62%; n = 3) and 78% (range 56%--92%; n= 3) respectively, of the steady state concentration (defined as the digoxin concentration after 25--40 days of treatment). After 9 and 11 days of treatment the skeletal muscle digoxin concentrations were 106% (range 84%--133%; n = 5) and 116% (range 72%--164%; n = 3) respectively, of the steady state concentration. A doubling of the digoxin dose gave a proportional increase in skeletal muscle digoxin concentration (three patients). The magnitude of the estimated half-life of skeletal muscle digoxin was the same as previously reportedly in healthy subjects. No significant correlations were found between changes in systolic time intervals and steady state serum or skeletal muscle digoxin concentrations.

Dosage of digoxin in premature infants

The pharmacokinetics and pharmacodynamics of digoxin in premature infants was studied. During maintenance therapy, after a total digitalizing dose of 30 microgram/kg, the measured digoxin level was related inversely to body weight at birth and to estimated gestational age. The serum digoxin levels found in the immature and smaller infants were two to three times the values usually reported to be toxic in older children. Based on these findings, a second group of premature infants was digitalized with 20 microgram/kg; in this group, the serum digoxin levels were below the toxic range, irrespective of gestational age or birth weight. The cardiac effects of digoxin, i.e., shortened left ventricular pre-ejection period and ejection time as determined by echocardiography, were similar in the two groups. For both groups, the half-life of digoxin in the serum was twice that reported for term infants and children. Since digitalis effect is obtained with lower dose and serum concentration, we recommend that this dose be used in premature infants.

[A simplified radioimmunoassay for digoxin determination using a 125-j-labelled, solid-phase kit (author's transl)]

Our experience with a commercially available kit (Radioimmunoassay DIGOXIN, Boehringer, Mannheim) using (125J)-labelled digoxin and antibody-coated tubes is reported. This simplified method requires only two pepetting steps per sample and results can be obtained in 70 min. The intra- and interassay coefficient of variation ranged between 7% and 8%. The specific digoxin antibody gave no clinical relevant cross-reactions with spironolactone or prednisone (less than 0.0007%). Of the digoxin metabolites the aglucone digoxigenin showed 31% cross-reaction while the more important cardioactive metabolites digoxigenin-bis- and mono-digitoxide had the same binding affinity to the antibody as digoxin, beta-methyldigoxin and beta-acetyldigoxin. Cross-reaction with digitoxin was 6.8%. More than double-fold dilution of serum protein concentration showed little influence on the digoxin values measured. The results obtained by this new kit compare closely with those obtained by our tritium-labelled kit (r = 0.94, p less than 0.001). Therefore, the upper therapeutic limit of 1.9 ng/ml can be adopted for this method.

The effect of intravenous digoxin on the occurrence of ventricular tachyarrhythmias in acute myocardial infarction in man

Patients with acute myocardial infarction were allocated to two groups according to a double blind-system of radomization. The patients (n = 18) in one of the groups received digoxin intravenously as an injection of 0.01 mg. per kilogram of body weight during 10 minutes. The patients in the other group (n = 15) received saline and served as controls. A continuous ECG record was obtained from each patient during 1 hour preceding the administration of digoxin or saline and was continued for 3 hours following the injection. No antiarrhythmic treatment was given during the time of the study. Based on the continuous ECG, calculations were made of the relative incidence of patients with different types of ventricular tachyarrhythmias during the period of observation as well as the percentage of arrhythmia-containing 1 minute intervals observed during this period. There was no statistical difference between the incidence of ventricular tachyarrhythmias in the two groups in the 1 hour period preceding drug injection. The administration of digoxin and saline did not change the incidence of ventricular tachyarrhythmias and there was also no statistically significant difference between the two groups as regards the incidence of patients showing different types of ventricular tachyarrhythmias during the 3 hour period following drug administration, Considering the 1-minute intervals, those without any ventricular premature contractions were less in the digoxin group (92 per cent) than in the saline group (88 per cent; p less than 0.001). Serum levels of digoxin at the end of the observation period were well above what is considered the minimum therapeutic level and in three patients the level approached or reached the toxic range. In these three patients there was still no increased incidence of ventricular tachyarrhythmias. It is concluded that patients with acute myocardial infarction complicated by incipient left ventricular failure do not show an increased sensitivity to an ordinary dose of digoxin as measured by the occurrence of ventricular tachyarrhythmia.

Therapeutic and toxic lanatoside C serum concentrations in hospital patients

Serum lanatoside C concentrations were measured by radioimmunoassay in 56 patients on oral maintenance therapy. Seven patients were judged to have digitalis toxicity and the lanatoside C concentration in this group was 6.99+/-2.00 ng/ml (mean +/-1 SD). 49 patients were non-toxic and the mean serum lanatoside C concentration in this group corresponded to 2.32+/-1.08 ng/ml.

Serum digitalis measurements in the assessment of digitalis resistance and sensitivity

Antibodies to digitalis glycosides have been elicited in experimental animals and have been utilized in the development of rapid, sensitive, specific and convenient radioimmunoassay methods for the clinical measurement of digoxin and other cardiac glycosides in man. The use of these assay methods has supplemented earlier studies with radiolabeled digitalis preparations and has made it possible to obtain much new information concerning factors which may contribute to the well known patient to patient variability in digitalis dosage requirements and in sensitivity to the toxic effects of cardiac glycosides. In some patients with a poor clinical response to digitalis, the finding of a serum concentration which is relatively low for the dose prescribed may suggest that true digitalis resistance is not present and may raise questions of poor patient compliance, tablet inadequacies, intestinal malabsorption, increased metabolic degradation or hyperthyroidism; if the cause of the low serum level cannot be identified or corrected, serial serum measurements should enable safe and rational upward adjustment of dosage. In some patients with digitalis toxicity, the finding of a serum level which is relativity high for the dose prescribed may suggest that the patient is not sensitive to digitalis but rather is excreting it slowly; in such instances in elderly patients (with decreased glomerular filtration rates) and in patients with renal disease, serial digitalis measurements are useful adjuncts to clinical observation in determining optimal digitalis dosage schedules. A knowledge of serum digitalis concentrations should enable us to develop sound principles for a more rational approach to the clinical administration of cardiac glycosides, especially in patients with unusually high dosage requirements or with unusual sensitivity to relatively small doses of digitalis.

Pharmacokinetics of digoxin and digitoxin in patients undergoing hemodialysis

The pharmacokinetics of digoxin and digitoxin in patients undergoing long-term hemodialysis were examined to determine which is the preferred cardiac glycoside in this patient population. Absorption curves from 0 to 24 hours after an oral dose of digitoxin were similar in dialyzed patients and in control patients. Serum glycoside concentrations after an oral dose of digoxin were higher in dialyzed patients than in control patients, significantly so from 2 to 24 hours, reflecting the absence of the predominantly renal route of excretion of digoxin. When nine dialyzed patients were placed on a maintenance dose of digoxin, 0.125 mg 5 days a week, serum levels plateaued at 30 days at a mean concentration (plus or minus SE) of 0.84 plus or minus 0.05 ng/ml. Maintenance therapy with 0.1 mg digitoxin 5 days a week resulted in stabilization of serum levels within 30 days at a mean concentration of 19 plus or minus 1 ng/ml. Variability in the serum glycoside concentrations was determined after stabilization of levels during 2 to 19 week follow-up periods with each drug. Variability in serum levels was somewhat increased during maintenance therapy with digitoxin. On the basis of the parmacokinetic data obtained in this study, no clear cut preference for one glycoside over the other could be established.

Arrhythmias following cardiac surgery: relation to serum digoxin levels

Arrhythmias were analyzed in 50 patients undergoing cardiac surgery: 27 with valve surgery, 15 with coronary artery bypass (CAB), 5 with CAB and valve surgery, and 3 with miscellaneous procedures. The role of electrolyte abnormalities, pericarditis, serum osmolarity, digoxin level, and the type of surgery performed was evaluated. Thirty-seven out of 50 patients (74 per cent) had a postoperative arrhythmia, and a total of 78 different arrhythmias were noted. Twenty-six out of 27 patients with valve surgery had an arrhythmia vs. six out of 15 patients with CAB (p less than 0.001). Atrial fibrillation was the most common arrhythmia in all groups. Although postoperative hypocalcemia, hypomagnesemia, pericarditis, and wide shifts in osmolarity were common, they did not correlate with arrhythmias. Seventeen patients developed postoperative arrhythmias compatible with digitalis toxicity, including junctional rhythm, atrioventricular dissociation, or atrial tachycardia with block. However, the range of serum digoxin levels in these patients was zero to 2.80 ng. per milliliter. This suggests increased sensitivity to digitalis glycosides or the effects of surgical trauma as the etiology of arrhythmia in many patients. The distinction between digitalis-induced arrhythmia and spontaneously occurring arrhythmia cannot be made with certainty in most postoperative patients. Therapy should reflect an awareness of the potential for postoperative digitoxicity.

Removing substances from blood by affinity chromatography. I. Removing bilirubin and other albumin-bound substances from plasma and blood with albumin-conjugated agarose beads

Substances such as bilirubin that bind tightly to plasma proteins cannot readily be removed from blood. We describe here the use of affinity chromatography as a new approach to the removal of proteinbound metabolites and toxins from blood. Agarose beads were coupled via cyanogen bromide to human serum albumin so as to contain 30-50 mg of albumin/g wet wt. Such beads, when exposed to plasma from a patient with congenital nonhemolytic jaundice labeled with [(14)C]-bilirubin, bound more than 150 mug bilirubin/g of beads. The binding was saturable, concentration-dependent, relatively independent of flow rate, and reversible by elution with plasma, albumin, or 50% (vol/vol) ethanol. The beads could be repeatedly reused without loss of efficiency after ethanol elution and long storage in the cold. Salicylate, cortisol, and taurocholate, which bind weakly to albumin, were retarded by the beads but eluted with neutral buffer. Thyroxine, taurolithocholate, chenodeoxycholate, and digitoxin bound tightly but were eluted with 50% ethanol. Digoxin did not bind at all. When whole blood was passed over agarose-albumin beads, bilirubin was removed, calcium and magnesium fell slightly, but red cells, white cells, platelets, clotting factors, and a variety of electrolytes and proteins were substantially unchanged. Agarose-albumin beads may be useful for removing protein-bound substances from the blood of patients with liver failure, intoxication with protein-bound drugs, or specific metabolic deficits. Furthermore, it may be possible to make useful adsorbents by attaching other proteins to agarose or other polymer beads.

Therapeutic non-equivalence of digoxin tablets in the United Kingdom: correlation with tablet dissolution rate

Seven types of digoxin 0.25 mg tablet in common use in the United Kingdom were administered to a total of 38 patients. Significant differences were found in the mean plasma digoxin levels and in the control of atrial fibrillation achieved with these brands. There was a close correlation between the dissolution rate of the tablets and the plasma digoxin levels. Measurement of in-vitro dissolution rate appears to be a valid method of ensuring that different tablets of digoxin are of equal efficacy. However, in some patients absorption of the drug is markedly sensitive to changes in dissolution rate and new pharmacopoeal standards should not be defined until very rapidly-dissolving formulations have been studied.