Pharmacokinetics of metildigoxin and digoxin in geriatric patients with normal and elevated serum creatinine levels

The pharmacokinetics of digoxin and metildigoxin were investigated in geriatric patients on maintenance treatment. Minimum serum glycoside concentrations were determined on 3 consecutive days, and the elimination rate over a withdrawal period of 4 to 6 days was studied. In patients with serum creatinine levels of less than or equal to 1.3 mg/dl, the oral standard dose D1) of digoxin necessary for a minimum serum concentration of 1.0ng/ml was 1.4 times higher than that of metildigoxin. There was no significant difference in the elimination rate of both glycosides. The pharmacokinetics of metildigoxin were further investigated in patients with elevated serum creatinine levels. The standard dose was best correlated with the creatinine clearance, calculated from the serum creatinine, age, weight and sex of the patients. The apparent volume of distribution of metildigoxin decreased with the drug's total body clearance.

[Pharmacokinetics of beta-methyldigoxin and beta-acetyldigoxin in patients with cirrhosis of the liver (author's transl)]

In this prospective randomised study 12 patients suffering from cirrhosis of the liver (stable phase) and 12 healthy male volunteers were treated with either 0.3 mg beta-methyldigoxin (Lanitop) or 0.4 mg beta-acetyldigoxin (Novodigal) daily, 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 subjects receiving beta-methyldigoxin therapy the ratio of beta-methyldigoxin to digoxin in the serum was determined by liquid chromatography. The digoxin levels in patients with cirrhosis treated with beta-methyldigoxin 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. Alterations in pharmacokinetics may cause the higher total serum digoxin concentrations in cirrhotic patients. The following factors seem to be important: longer elimination half life, changes in distribution volume and reduced renal clearance. There is greater danger of digitalis toxicity in patients with cirrhosis of the liver on standard dosage of beta-methyldigoxin than on standard dosage of beta-acetyldigoxin.

Effects of cytostatic drugs on plasma level and renal excretion of beta-acetyldigoxin

Mucosal defects decrease digoxin absorption in patients with malabsorption syndromes. Since the intestinal mucosa can be damaged by cytostatic drugs, we investigated their effects on digoxin plasma levels and urinary digoxin excretion. In six patients with malignant lymphoma who received 0.8 mg beta-acetyldigoxin before and 24 hr after treatment with a combination of cyclophosphamide, oncovin, procarbazine, and prednisone (COPP) or cyclophosphamide, oncovin, and prednisone (COP), plasma digoxin concentrations were measured 0 to 8 hr after the dose and areas under the plasma concentration-time curves were calculated. In 15 patients on 0.3 mg of beta-acetyldigoxin daily, plasma glycoside concentrations and renal excretion were measured daily before and after COPP, COP, cyclophosphamide, oncovin, cytosine-arabinosine, and prednisone (COAP), or adriamycin, bleomycin, and prednisone (ABP) treatment schemes. The diminished steady-state glycoside plasma concentrations and daily renal glycoside excretion during the 24 to 168 hr after the cytostatic drug established reversible impairment of digoxin absorption. The delayed time to peak after a single dose of digoxin during cytostatic drug therapy shows that extent and rate of digoxin absorption are reduced. To maintain adequate control of digoxin therapy in patients treated with cytostatic drugs, plasma levels should be monitored.

[Biological availability of digoxin and beta-methyl-digoxin administered in the fasting state or after meals]

Ten healthy volunteers were given 0.75 mg digoxin and 0.5 mg beta-methyl-digoxin (BMD) in tablet form in the fasting state or after breakfast. Serum concentrations and 24-hour urine excretion of glycoside were measured by radioimmunoassay. Neither the mean area under the serum concentration curve nor the mean cumulative urinary excretion was significantly changed by postprandial administration. Peak serum concentrations were higher when the subjects took the tablets while fasting than when they took them postprandially, but the difference was significant only for BMD. After BMD in the fed state the peak serum concentration was reached earlier and with less variation than after digoxin, but -- as after administration in the fasting state -- the differences were not significant. The peak serum concentration and the time when it is achieved are, as parameters for the rate of absorption, only of secondary importance for treatment with cardiac glycosides in medical practice. They suggest faster absorption of BMD compared with digoxin. Both the glycosides can be given equally well before, during or after food, a fact which facilitates prescription.

Extrarenal clearance, distribution volume, and elimination rate of digoxin and metildigoxin in anuric patients

The pharmacokinetics of 3H-labeled digoxin and metildigoxin were compared in six anuric patients. The following means +/- s.e.m. were obtained: extrarenal clearance of digoxin, 43.3 +/- 5.4 ml/min, of metildigoxin, 30.3 +/- 2.9 ml/min; distribution volume of digoxin, 315 +/- 29 1, of metildigoxin, 258 +/- 22 1; rate constant for elimination of digoxin, 0.0086 +/- 0.0013 h-1, of metildogixon, 0.0071 +/- 0.0007 h-1. The elimination rates correspond to half-lives of 80 h for digoxin and of 97 h for metildigoxin. From our investigations and published data a weighed mean of 47 ml/min was calculated for the extrarenal clearance of metildigoxin. This is not significantly different from the mean extrarenal clearance of 40 ml/min reported for digoxin. A total body clearance of 40 ml/min and a daily intravenous dose of 0.1 mg correspond to an average steady-state glycoside concentration of 1.74 ng/ml.

[Influence of azosemide on renal metildigoxin excretion (author's transl)]

The influence of the diuretic azosemide on metildigoxin blood levels and on renal excretion was assessed in 20 patients with chronic cardiac failure. Two groups, each of 10 patients, were treated for 14 days with 0.1 and 0.2 mg metildigoxin, respectively. All patients received 2 tablets of azosemide, 80 mg each, from day 12 to 14 of treatment. There was no significant influence of the diuretic either on blood levels or renal excretion of metildigoxin.

7 beta -hydroxycardenolide glycosides

The microbial transformation of digitoxin (I) by Streptomyces sp. yielded digoxin (III) as main product along with the by-products 7beta -hydroxydigitoxin (II) and 7beta -hydroxydigoxin (IV). The present paper is concerned with the structure elucidation of 7beta-hydroxycardenolides as well as with the formation of some of their derivatives.

Urinary excretion of reduced metabolites of digoxin

The urinary excretion of the relatively cardioinactive reduced metabolites of digoxin, dihydrodigoxin and related compounds was measured by radioimmunoassay in 131 normal subjects during studies of the bioavailability of digoxin preparations. Digoxin reduction products (DRP) constitute more than 5 percent of the excretion of digoxin and its metabolites in one-third of the volunteers after the administration of single or multiple doses of digoxin. There was little or no output of DRP during the first 8 hours after a single dose, with maximal excretion usually occurring on the second day. Most subjects who excreted more than 5 percent DRP on one occasion did so with each subsequent exposure to digoxin. Six volunteers, however, in whom substantial amounts of DRP had previously been found, failed to excrete detectable quantities after subsequent doses. In two, this change occurred shortly after they took erythromycin. Urinary DRP were less after the intravenous administration compared to the oral administration of digoxin. After oral doses, DRP excretion tended to vary inversely with the bioavailability of the preparation. The findings are consistent with the hypothesis that DRP are formed as the result of the activity of a variable component of the intestinal flora. Prospective studies will be necessary to prove this hypothesis.

Determination of tritiated digoxin and metabolites in urine by liquid chromatography

A liquid chromatographic method for the determination of digoxin, digoxigenin, its mono- and bisdigitoxoside and dihydrodigoxin in urine is described. Doses of 100 muCi of [12 alpha-3H]digoxin and 0.5 mg (640 nmol) of digoxin were administered orally to eight healthy volunteers. The compounds were extracted from urine with methylene chloride containing 3% of heptafluorobutanol. After separation, fractions corresponding to digoxin and the metabolites were measured by liquid scintillation counting. Conjugates of the glycoside metabolites were determined indirectly after pre-treatment of the samples with beta-glucuronidase-arylsulphatase. The detection limit was 0.1 nmol/l. Metabolites amounting to 0.5% of digoxin were assayed with a relative standard deviation of 5%. The advantages of the method are a high recovery in the extraction step, short separation times and the possibility of separate assay of dihydrodigoxin.

On the plasma protein binding of 16-acetyl-gitoxin

The plasma protein binding of 16-acetyl-gitoxin (16-AG) was estimated in comparison with that of digoxin by the use of equilibrium dialysis, ultracentrifugation, and gel filtration techniques. Depending on temperature, the binding of 16-AG reached values between 81 and 90%. Calculations of the reaction enthalpy and entropy show that hydrophobic interaction results. The corresponding values of digoxin amount to 13 and 15%.

Digoxin concentration in cerebrospinal fluid-a study carried out after 9 days of treatment of beta-methyldigoxin or beta-acetyldigoxin

Two groups of seven healthy volunteers were treated for 9 days with either 0.3 mg beta-methyldigoxin or 0.4 mg beta-acetyldigoxin daily, applied orally. On the 10th day, digoxin concentrations in plasma and cerebrospinal fluid (CSF) were determined by radioimmunoassay. After therapy with beta-methyldigoxin the plasma/CSF digoxin concentration ratio was 3.7:1; after therapy with beta-acetyldigoxin it was 3.2:1. There was no significant difference in the plasma/CSF digoxin concentration ratio after 9 days of treatment with equipotent doses of beta-methyldigoxin and beta-acetyldigoxin.

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.

[Serum levels of beta-methyldigoxin and contractile efficiency of the myocardium evaluated with systolic polygraphy and determination of cardiac output]

0.3 mg/day betamethyldigoxin was given per os in three daily administrations to 8 healthy subjects, and 8 compensated and 8 decompensated heart patients. Prior to the treatment, and 6 hr after the last administration, blood digoxin values were determined radio-immunologically, together with cardiac output, systolic stroke volume, cardiac index (dilution of indocyanine green), and systolic time intervals, by simultaneous recording of the ECG, carotid pulse, and the phonocardiogram. No significant change in output, stroke volume and cardiac index was noted in the healthy subjects, whereas these parameters were distinctly improved in the decompensated patients. Changes in the systolic intervals after treatment were significant in all cases though there was no significant correlation with the blood digoxin levels reached. In particular, the healthy and compensated subjects displayed a reduction in the corrected electromechanical systole (delta Q-S2), the corrected pre-ejection period (delta PEP), the corrected left ventricular ejection time (delta LVET), and their ratio (PEP/LVET), whereas in the decompensated patients the picture differed to the extent that the LVET increased owing to an augment-systolic stroke volume, the other parameters being reduced. In the healthy subjects, the polygraphic data were normal prior to the treatment, while in the compensated patients delta PEP and the PEP/LVET ratio were enhanced, and the delta LVET was less than in the normal subjects. It is felt that recording of the systolic intervals may be regarded as a sound method, owing to its simplicity and its ability to demonstrate latent cardiac failure before haemodynamic changes appear. Simultaneous determination of serum digoxin and the polygraphic data, therefore, opens the way to the commencement of appropriate, safe and timely management of as yet non-decompensated heart patients.

Influence of gastric pH on digoxin biotransformation. II. Extractable urinary metabolites

High-performance liquid chromatography (HPLC) analysis was performed on methylene chloride extracts of urine from six subjects after administration of 3H-digoxin-12 alpha and unlabeled digoxin by nasogastric tube under four conditions: pentagastrin and control saline infusions, each in the supine and ambulatory states. There were no differences with change in position. With pentagastrin stimulation of acid secretion, there was extensive intragastric hydrolysis, mainly to digoxigenin; there was further extensive biotransformation leading to an increase in both extractable and unextractable metabolites in urine, particularly the latter. In the first 5 hr mean digoxin was only 17% and unextractable metabolites were 54% of total urine radioactivity. Extractable radioactivity was found under HPLC peaks with retention times of digoxin, digoxigenin, and its mono- and bis-digitoxosides. There were also three other peaks that were not identified; two correlated with gastric H+ activity and with the peak for digoxigenin, which is probably their precursor since similar peaks were found after ingestion of digoxigenin. The third unidentified peak eluted immediately after the digoxin, with which it correlated; it may have a close structural relationship to digoxin. Gastric acid stimulation induced a major increase in the production of urinary metabolites and may prove a useful model for the study of digoxin biotransformation, which is not yet well defined.

Cleavage of digoxigenin digitoxosides by rat liver microsomes

Microsomal monoxygenases can oxidize the axial hydroxyl of the terminal digitoxosyl of digoxin (dg-3), digoxigenin bis-, and digoxigenin mono-digitoxoside (dg-2 and dg-1, respectively) to an oxo-group. The corresponding metabolites (15'-dehydro-dg-3, 9'-dehydro-dg-2, and 3'-dehydro-1, respectively) have been identified by chromatographic and chemical methods. Only after this oxibation the terminal sugar can be split off, presumably by beta-elimination. Therefore, for the degradation of dg-3 three successive cytochrome P450 catalyzed oxidations are necessary before digoxigenin (dg-0) can be obtained. The highest oxibation rate was observed with dg-1 (120-150 pmoles/mg microsomal protein/min) and by far the lowest with dg-2 (6-7 pmoles/min) as the substrate (concentration was 30/microM). The latter may contribute to the effect that dg-2 is the main dg-3 metabolite in vivo. Pretreatment of rats with canrenoate enhanced the microsomal oxidation of dg-3, dg-2, and dg-1 by a factor of 3.2, 2.3 and 1.3, respectively. In contrast, there was no increase after pretreatment with phenobarbital.