Digoxin and metabolites in urine: a derivatization--high-performance liquid chromatographic method capable of quantitating individual epimers of dihydrodigoxin

A high-performance liquid chromatographic method is described for the determination of digoxigenin, digoxigenin monodigitoxoside, digoxigenin bis-digitoxoside, digoxin, and dihydrodigoxin as the 3,5-dinitrobenzoyl esters. The method is applied to a 10 ml urine sample by adding digitoxigenin as internal standard, extracting with methylene chloride, derivatizing with 3,5-dinitrobenzoyl chloride in pyridine, chromatographing with a normal-phase system and detecting at 254 nm. Derivatized digoxigenin, digoxigenin mono- and bis-digitoxoside, and digoxin each yielded one symmetrical peak with the limit of sensitivity of the method being approximately 100 ng/ml. Analysis of a commercially obtained sample of dihydrodigoxin resulted in two well-separated, symmetrical peaks that represent the two epimers of derivatized dihydrodigoxin. Data indicate rapid and complete esterification of all primary and secondary alcohol moieties in the various molecules and the derivatives are shown to be stable in chloroform for at least four days. The procedure appears to be suitable for metabolic investigations and as a prototype for future analytical developments.

The inotropic activity of digitalis genins is dependent upon C(17) side-group carbonyl oxygen position

The inotropic potencies of four digitalis genins were studied utilizing cat left atrial strips. The genin concentration required to induce a 50% increase of isometric tension (T50) was found to closely correlate with the degree of displacement (D) of the C(17) side-group carbonyl oxygen from the position of that atom in digitoxigenin. The line of regression was: log T50 = 0.54D - 6.85, r2 = 0.98, P less than 0.008. These observations were related to recently reported cat ventricular Na+, K+ -ATPase inhibitory potencies of the same genins [expressed as 50% inhibitory (I50) concentrations]. I50 correlated strongly with T50: log I50 = 0.78 log T50 - 1.68, r2 = 0.99, P less than 0.003. Thus, the activity of digitalis genins towards their receptor in intact cardiac tissue is closely related to genin carbonyl oxygen position as well as to Na+, K+ -ATPase inhibitory activity. These results further support our earlier conclusions, based upon isolated Na+, K+ -ATPase studies, that the digitalis genin C(17) side-group carbonyl oxygen position versus activity relationship is biologically relevant and may prove to be a useful unifying structural model in the further elucidation of the mechanism of digitalis-receptor interactions.

Kinetics of partitioning and binding of digoxin and its analogues in the subcompartments of blood

The kinetics of digoxin and analogues (metabolites) in the subcompartments of blood, erythrocytes and plasma, were first order and concentration independent. The rate and extent of red blood cell partitioning of the tested compounds were, according to first-order kinetics, identical in the presence and absence of structurally related compounds and dependent on lipophilicity and temperature; red blood cell penetration was a reversible process. Erythrocyte partitioning of digoxin and its analogues was postulated to be by passive diffusion. There was significant intracellular binding of the compounds, and hemoglobin was the major ligand. The kinetics of red blood cell partitioning of the individual compounds were fitted according to a closed two- or three-compartment modeL. The latter indicated the existence of two kinetically separable compartments within the red blood cells. Model-independent mean transit times of red blood cell-plasma water partitioning of the compounds depended largely on the lipophilicity and were a thousand times greater for the least lipophilic analogues than for the most lipophilic derivatives. Red blood cell partitioning of digoxin and its metabolites under in vivo and in vitro conditions, were equivalent. Plasma protein binding of the tested compounds was concentration independent, unaltered in the presence of the analogues, and temperature independent between 24 degrees C and 37 degrees C. Binding to albumin in buffer solution was significantly larger than to albumin in plasma for all the compounds. Binding of digoxin and its analogues to albumin in buffer solution increased with increasing lipophilicity of the compounds.

Variable cross-reactivity of digoxin metabolites in digoxin immunoassays

The authors investigated the cross-reactivity of the major known digoxin metabolites--digoxigenin, digoxigenin monodigitoxoside, digoxigenin bisdigitoxoside, and dihydrodigoxin--and of digitoxin in three 125I-radioimmunoassays and one enzyme immunoassay for digoxin. Digitoxin and dihydrodigoxin exhibit low cross-reactivity and nonparallel dilution responses for these assays. The cross-reactivities of the other three substances are significant for all assays studied with digoxigenin and monodigitoxoside having nonparallel and enhanced tracer displacement compared with digoxin itself. The authors demonstrate that because of nonparallel tracer displacement estimates of cross-reactivity calculated by the 50% displacement method fail to adequately predict the error induced in digoxin assays by digitoxin. They conclude that digoxin metabolites in serum are measured to various extents as the parent digoxin compound by all of the immunoassays they studied. In view of the varying biologic activity of digoxin metabolites and the large patient to patient variations in digoxin metabolism, the cross-reactivities the authors observe may help to explain the discrepancies in correlation of clinical response to measured serum digoxin values reported in other studies.

Severe digoxin intoxication in a child treated by infusion of digoxin-specific Fab-antibody-fragments

Accidental digitalis ingestion in children is a rare, but potentially life-threatening emergency. We report the case of a 2 10/12-year-old boy with accidental ingestion of 6 mg beta-Acetyl-digoxin. Soon after admission, the boy developed sinus bradycardia, SA and AV-block, of increasing severity without circulatory impairment. As the serum digoxin level reached 21.7 ng/ml digoxin-specific Fab-antibody-fragments were used to bind free serum digoxin. Immediately after infusion, serum free digoxin was below the detection limit, whereas total digoxin peaked at 219 ng/ml 5 h thereafter. The arrhythmias did not subside totally, so that in addition, a transvenous pacemaker was placed, but never used. The antibody-infusion was well tolerated and the boy was discharged in good health.

Effect of digoxin on serum and urinary cation changes on acute induction to high altitude

Forty subjects each were rapidly inducted by road to altitudes of 3200 and 3771 meters (m). Half of subjects at each altitude were maintained on digoxin and the other half received placebos. The digoxin group showed higher serum potassium at both altitudes. Serum sodium remained unaltered. When compared to control groups, urinary volume changes were less marked in the digoxin group. Similarly, the alterations in calcium and magnesium in serum and urine at 3771 m were less pronounced. However, none of these changes in the digoxin groups were statistically significant when compared with control groups.

Importance of the urinary excretion in the total elimination of SC4453 in the guinea-pig

Administration of SC4453 (30 micrograms/kg) in the guinea-pig increases the rate of urinary excretion from 35 ml/kg/24 h to 55 ml/kg/24 h. Ligature of ureters modifies parameters of SC4453 ; t 1/2 beta increases from 4.2 h to 5.6 h and the body clearance diminishes from 622 ml/h/kg (normal guinea-pig) to 530 ml/h/kg in the ligatured ureters guinea-pig. Analysis of the radioactive material excreted in the urine after administration of tritiated SC4453 shows that 58.4% is dichloromethane (DCM) soluble and 41.6% is water soluble. Thin layer chromatography (TLC) of the DCM soluble fraction shows that 74.0% is unchanged SC4453 . After enzymatic hydrolysis of the water soluble fraction, it is shown that 28% is again extractable by DCM of which 57% consists of unchanged SC4453 which is therefore the main partner of conjugated products. Following an i.v. injection of tritiated SC4453 , only 17% of the dose was recovered in urine, against 14% after an intragastric administration. Gastro-intestinal absorption of SC4453 was estimated to be 82.2%.

Changes in metildigoxin pharmacokinetics in cirrhosis of the liver: a comparison with beta-acetyldigoxin

In a prospective randomized study 12 patients suffering from cirrhosis of the liver (stable phase) and 12 healthy volunteers were treated daily with either 0.3 mg metildigoxin (Lanitop) or 0.4 mg beta-acetyldigoxin (Novodigal) orally. Every day the total serum digoxin concentrations of the patients and volunteers were measured by radioimmunoassay. Both digoxin and beta-methyldigoxin are measured by this method. In patients receiving metildigoxin therapy the ratio of beta-methyldigoxin/digoxin in the serum was determined by HPLC. The digoxin levels in patients with cirrhosis treated with metildigoxin were statistically significantly higher than in healthy volunteers. In patients with cirrhosis the proportion of serum beta-methyldigoxin averaged 77.7% of the total digoxin concentration, whereas the proportion was only 37.5% in healthy volunteers. With beta-acetyldigoxin there was no statistically significant difference between patients with cirrhosis and healthy volunteers. The higher total serum-digoxin levels in patients with cirrhosis of the liver after moderate saturation with metildigoxin are caused by reduced demethylation of beta-methyldigoxin to digoxin due to impaired liver function. A comparison with healthy volunteers showed that the reduced hepatic metabolism in the cirrhotic patients caused changes in the pharmacokinetics: a reduced metildigoxin clearance and a smaller distribution volume were found. According to our findings there is more danger of digitalis toxicity in patients with cirrhosis of the liver on a standard dosage of metildigoxin than on a standard dosage of beta-acetyldigoxin.

[Report of experiences in the treatment of 16 cases of severe glycoside poisoning with digitalis antibody fragments (Fab)]

To date, 16 patients with severe glycoside poisoning have been treated with Fab as part of the clinical trial for efficacy and tolerance. The ages of the patients ranged from 4 to 77 years. In 13 cases, the substance was taken with suicidal intent. The following were considered to be indications for the use of Fab: the appearance of life-threatening arrhythmias as high-grade atrioventricular conduction disorders, multifocal ectopic beats, ventricular tachycardia, and relapsing ventricular fibrillation in 5 cases. Serum digoxin concentrations were between 3.8 and 78 ng/ml before the start of treatment. The amount of Fab administered was 240-800 mg, in the majority of cases 480 mg. Regression of the arrhythmias was seen in all patients during or shortly after the Fab infusion. There was a rapid fall in the free digoxin in the serum to levels that were no longer measurable and a marked rise in bound digoxin with a simultaneous intensive excretion of bound digoxin in the urine. Fab therapy is considered to be a major advance in the treatment of severe, previously fatal, glycoside poisoning. No notable side effects were observed, nor were there any allergic reactions to foreign protein.

[Gitoformate and digitoxin as alternatives to kidney-dependent glycosides in the therapy of cardiac insufficiency]

Kidney independent glycosides offer a high measure of therapeutic safety in comparison with kidney dependent glycosides. The intoxication rate lies between 4 and 6%. The pharmacokinetic properties of pentaformylgitoxin (INN: gitoformate) are comparable with those for digitoxin. The active glycoside 16-formylgitoxin (INN: gitaloxin) is formed by rapid deformylation of the formyl residue on the sugar chain. The maintenance dose of 0.06 mg daily, based on the half-life, produces therapeutic concentrations in the range 6-30 ng/ml. The required loading dose, as for digitoxin, amounts to 10 times the maintenance dose.

Metabolism and excretion in bile of SC4453, a new semi-synthetic derivative of digoxin following an i.v. bolus injection in the guinea-pig

After a single bolus injection of tritiated SC4453 in the guinea-pig, 36% of the dose was excreted in the bile during the first 6 hours after administration. Solvent partition has shown that 88% of the total radioactivity excreted was soluble in dichloromethane (DCM). Thin layer chromatography analysis of the DCM soluble fraction has shown that SC4453 was mainly excreted unchanged in the bile. Enzymatic hydrolysis of the water soluble fraction has shown that the main partner of conjugation was SC4453 itself. This study has demonstrated a significant enterohepatic circulation for SC4453, suppression of which modified the pharmacokinetic parameters of SC4453 in the guinea-pig. The half-life of elimination decreases from 4.2 hours to 3.22 hours and the volume of distribution (Vdarea) was equal to 2680 ml/kg versus 3692 ml/kg for normal guinea-pig.

Positive inotropic effects of digitoxin- and digoxin-glucuronide in human isolated ventricular heart muscle preparations

The effect on force of contraction of human isolated ventricular heart muscle preparations was studied with the 16'-glucuronides of digitoxin (dt-gluc) and digoxin (dg-gluc). The parent compounds digitoxin (dt) and digoxin (dg) were studied for comparison and all compounds were also investigated in guinea-pig isolated papillary muscles. All compounds increased myocardial force of contraction in human preparations in a concentration-dependent manner. The positive inotropic effect of dt-gluc and dg-gluc began at 0.1 mumol 1(-1) and was maximal at 1 mumol 1(-1). The metabolites were slightly less potent but, on the other hand, slightly more effective than the parent compounds dt and dg. The positive inotropic effects of all compounds peaked after about 60 minutes. In contrast, the decline of the positive inotropic effect upon washing was about twice as fast in the case of the hydrophilic metabolites. Similar results were obtained in guinea-pig papillary muscles. It is concluded that the glucuronides of digitoxin and digoxin are biologically active in the human heart. This may be of clinical importance with respect to the positive inotropic action and the plasma level determination of the cardiac glycosides.

Increased metabolism to dihydrodigoxin after intake of a microencapsulated formulation of digoxin

A capsule preparation containing small, enteric-coated granules of digoxin was developed to prevent acid hydrolysis of the drug in the stomach and to diminish the variation in plasma glycoside concentration during the intervals between doses. The absorption and metabolism of tritiated digoxin after a single oral loading dose of this formulation (Formulation C) were compared to those after ingestion of a digoxin solution (Formulation S) by 8 healthy men. Drug concentrations were measured by radioimmunoassay (RIA) and liquid chromatography (LC). The percentage of the digoxin dose excreted in the urine during 72 h, as measured by RIA, was significantly lower after the capsule (20.5 +/- 2.0% vs 36.2 +/- 3.0% after S, mean +/- SEM) but total urinary radioactivity after the two treatments was similar (C 35.3 +/- 5.2 and S 41.2 +/- 2.6%; p greater than 0.05). The discrepancy was mainly due to significantly greater excretion of dihydrodigoxin after the capsule (m 12.8%, range 0-28.6% of the dose) than after the digoxin solution (m 5.4%, range 0-14.5%). Dihydrodigoxin was not measured by the RIA. The recovery of hydrolysis metabolites (LC) was greater during the first 24 h after S (2.3 +/- 0.6% vs 0.9 +/- 0.3% after C; p less than 0.05). The peak plasma concentration of digoxin (RIA) was significantly reduced and delayed after intake of C (2.5 +/- 0.4 nmol/l at 3.8 +/- 0.3 h vs. 8.3 +/- 0.8 nmol/l at 0.9 +/- 0.1 h after S), and so was the shortening of electromechanical systole at 1.5 h, 2.5 h, and 3 h.(ABSTRACT TRUNCATED AT 250 WORDS)

[Cardiac effects of a single daily dose of 0.15 mg and 0.20 mg of beta-methyldigoxin]

In this placebo-controlled study the cardiac effects of once daily doses of beta-methyl digoxin 0.15 mg and 0.20 mg were evaluated using noninvasive methods. In double-blind fashion, 3 groups of 15 healthy volunteers each took 0.15 mg, or 0.20 mg beta-methyl digoxin, or placebo for a period of 14 days. On the first 2 days, all volunteers received a double amount of tablets for digoxin loading. Before the start of the study and 12 h after the last dose, the following parameters were recorded: electrocardiogram (QTc, T wave amplitude), systolic time intervals (STI) (QS2c, PEP, PEPc, LVETc), and impedance cardiogram (Heather index). The mean digoxin serum concentrations were 0.648 ng/ml and 0.975 ng/ml respectively for the two doses. The typical glycoside effects were seen and there was shortening of the STI (QS2c: - 12 ms/ - 14 ms resp.), flattening of the T wave, and a rise in the Heather index. The variations between baseline values and the 2 doses were statistically significant (p less than 0.05). In most parameters, the more intense effects corresponded with the higher serum level from the 0.20 mg dose; however, the difference between the 2 doses was not statistically significant. In conclusion, the results of this study show that a positive digitalis effect is demonstrable following the relatively small dose of beta-methyl digoxin 0.15 mg once daily under long-term application in healthy volunteers.

[Renal and extracorporeal elimination of digoxin and its methylated and acetylated derivatives]

This study was performed to get more informations on the renal and extracorporeal elimination of digoxin. The first part of this study demonstrated that a radioimmunoassay for digoxin or a specific tritium label of digoxin is necessary to measure renal digoxin clearances. Randomly labeled 3H-digoxin may loose its label and thus give incoherent results. A comparison of digoxin and inulin clearances in patients demonstrates glomerular filtration as the major renal excretion pathway of digoxin, but a major fraction of the glycoside is excreted by tubular secretion. Opposite to digoxin, beta-methyl-digoxin undergoes less tubular secretion but eventually additional tubular reabsorption. The insertion of digoxin and quinidine may be a further prove for the existence of a tubular secretion of digoxin. While the renal clearance of digoxin is significantly bigger than the renal clearance of creatinine, additional quinidine therapy reduces the renal clearance of digoxin to the one of creatinine. We studied the renal excretion mechanism of digoxin additionally in an animal model of acute prerenal failure. After a 33% reduction of renal arterial pressure glomerular filtrate dropped 68%. Under these circumstances the renal excretion mechanism of digoxin measured as the digoxin to inulin clearance ratio did not change. We evaluated the efficiency of hemodialysis, hemofiltration and hemoperfusion to eliminate digoxin. Although digoxin is eliminated by all three methods, even the most effective, hemoperfusion, can reduce total body content of digoxin less than 3%. Thus we conclude that all these methods have no indication in the treatment of digoxin intoxications.

[Increased digitalis-like activity in combined administration of calcium antagonists and digitalis preparations]

It is well recognized that the co-administration of some drugs to persons receiving digoxin results in an increase of serum digoxin levels: it occasionally precipitates digoxin toxicity. There are some possible mechanisms by which different drugs may increase serum glycoside levels: 1) by displacing digoxin from tissue binding sites, 2) by decreasing digoxin renal excretion, 3) by enhancing digoxin absorption, 4) by increasing the inhibition of the ATP-dependent pump. Combined administration of calcium antagonists and B-methyldigoxin is common in patients with heart diseases. Other Authors have found that the co-administration of these drugs with digoxin increases glycoside serum levels, without signs of intoxication. We have examined the effects of this combination on digitalis activity, by Rubidium 86 uptake in erythrocytes. Thirty patients aged 52 to 66 with congestive heart failure were studied. All of these had normal serum creatinine, blood urea and serum protein levels. These subjects received 0,20 mg of B-methyldigoxin daily for nine days; afterwards, ten of them were given in addition 320 mg of verapamil for seven days; the other ten received in addition 60 mg of nifedipine, for seven days; the remaining ten patients received in addition 320 mg of verapamil and 60 mg of nifedipine together, for seven days. Digoxin activity was measured on the eighth, ninth and sixteenth day of treatment by Rubidium 86 uptake in erythrocytes. The technique was that of Bertler. Means were compared with the t test, with the assumption of the equal variances in the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Ischemia-induced enhancement of digitalis sensitivity in isolated guinea-pig heart

Digitalis sensitivity of the heart is increased in patients with ischemic heart disease. Whether this elevation of digitalis sensitivity occurs as the result of ischemia-induced changes in the cardiac tissue and whether changes in the sarcolemmal Na,K-adenosine triphosphatase (ATPase) or reserve capacity of the sodium pump are responsible for the increased digitalis sensitivity were examined using isolated heart preparations obtained from guinea pigs. Ligation of the left anterior descending coronary artery (LAD) in Langendorff preparations 40 min before perfusion with a toxic concentration (either 1.8 or 2.5 microM) of digoxin decreased the time to the onset of arrhythmias. LAD-ligation by itself did not cause arrhythmias. The time to the onset of arrhythmias during digoxin perfusion was slightly longer in preparations obtained from reserpine-treated animals; however, the reserpine pretreatment failed to alter the effect of LAD ligation on digitalis sensitivity, indicating that the release of catecholamines is not involved in the sensitization. The effects of ischemia on Na,K-ATPase and sodium pump activities, glycoside binding to the enzyme and reserve capacity of the sodium pump were examined in globally ischemic Langendorff preparations. The preparations were perfused with a Krebs-Henseleit bicarbonate buffer solution (pH 7.4) saturated with a 95% O2-5% CO2 gas mixture at a control flow rate of 2.5 ml/g of tissue per min or at 5 or 0% of the control flow rate. After 6 hr of zero perfusion, Na,K-ATPase activity and the number of specific ouabain binding sites were reduced in ventricular muscle homogenates. However, the remaining Na,K-ATPase was not altered in its sensitivity to dihydrodigoxin-induced inhibition or affinity of binding sites for ouabain, sodium or potassium. Similar results were observed after reperfusion following 2 or 5 hr of zero perfusion. A 5% perfusion for 2 or 6 hr, or zero perfusion for 2 hr failed to affect Na,K-ATPase activity in muscle homogenates. Sodium pump activity in ventricular slices, estimated from the ouabain-sensitive 86Rb+ uptake, was unchanged after 5% perfusion or zero perfusion for 2 hr, but was significantly reduced after a 20-min reperfusion following 2 hr of zero perfusion. Reserve capacity of the sodium pump, as estimated from the differences in 42K+ uptake by right ventricular strips under 1.5 and 7 Hz stimulation, was unaffected by 2 hr of 5% perfusion. These results indicate that coronary artery occlusion enhances the arrhythmogenic action of digitalis in isolated heart preparations. This change in digitalis sensitivity produced by 40 min of occlusion cannot be explained by reductions in Na,K-ATPase activity or sodium pump reserve capacity as 2 hr of zero perfusion does not alter Na,K-ATPase or sodium pump activity in ventricular tissue.