P-glycoprotein-mediated transport of digitoxin, alpha-methyldigoxin and beta-acetyldigoxin.

Digoxin is a drug with a narrow therapeutic index, which is substrate of the ATP-dependent efflux pump P-glycoprotein. Increased or decreased digoxin plasma concentrations occur in humans due to inhibition or induction of this drug transporter in organs with excretory function such as small intestine, liver and kidneys. Whereas particle size, dissolution rate and lipophilic properties have been identified as determinants for absorption of digitalis glycosides, little is known about P-glycoprotein transport characteristics of digitalis glycosides such as digitoxin, alpha-methyldigoxin, beta-acetyldigoxin and ouabain. Using polarized P-glycoprotein-expressing cell lines we therefore studied whether these compounds are substrates of P-glycoprotein. Polarized transport of digitalis glycosides was assessed in P-glycoprotein-expressing Caco-2 and L-MDR1 cells (LLC-PK1 cells stably transfected with the human MDR1 P-glycoprotein). Inhibition of P-glycoprotein-mediated transport of these compounds in Caco-2 cells was determined using the cyclosporine analogue PSC-833 (valspodar) as inhibitor of P-glycoprotein. No polarized transport was observed for ouabain. However, basal-to-apical transport of digitoxin, alpha-methyldigoxin and beta-acetyldigoxin was greater than apical-to-basal transport in Caco-2 and L-MDR1 cells. In Caco-2 cells net transport rates of these compounds were similar to those of digoxin (digoxin: 16.0+/-4.4%, digitoxin: 15.0+/-3.3%, beta-acetyldigoxin: 16.2+/-1.6%, alpha-methyldigoxin: 13.5+/-4.8%). Furthermore, polarized transport of these compounds could be completely inhibited by 1 microM PSC-833. In summary, these data provide evidence that not only digoxin, but also digitoxin, alpha-methyldigoxin and beta-acetyldigoxin are substrates of P-glycoprotein.

The effect of meloxicam on the pharmacokinetics of beta-acetyl-digoxin.

The influence of multiple-dose administration of meloxicam on the pharmacokinetics of oral beta-acetyl-digoxin was studied in 12 healthy male volunteers in a randomized double-blind two-way crossover study. The primary endpoint, Cminss, was within the accepted range for bioequivalence, as were Cmaxss and AUCss. The 90% confidence interval and the point estimator of 98.7 for Cminss were within the equivalence range of 0.8-1.25. MRT and tmax were also unchanged, while the elimination rate constant was decreased slightly by 12%, which is of no therapeutic relevance. It is concluded that co-treatment with meloxicam has no effect on the pharmacokinetics of oral digoxin.

[Absolute bioavailability of beta-acetyldigoxin from tablets and drops in healthy subjects]. / Absolute Bioverfügbarkeit von beta-Acetyldigoxin aus Tabletten und Tropfen bei gesunden Probanden.

In a crossover design in random order 12 healthy male volunteers were given either beta-acetyldigoxin (Novodigal, CAS 5511-98-8) tablets, oral solution or i.v. application at a digoxin equivalent dose of 0.284 mg. To reach steady state each preparation was given for 10 days on a once-daily schedule. On days 8, 9 and 10 of each observation period blood was sampled to determine trough concentrations of digoxin in steady state. In addition, on day 10 blood was collected repeatedly at appropriate time intervals and urine was sampled concomitantly for 24 h. Trough values during steady state and 24 h AUC were used to calculate digoxin bioavailability for tablets and oral solution. From trough values, the mean bioavailability for beta-acetyldigoxin tablets was 91.2% (range 73.1-118.1) and for solution 93.8% (range 65.7-114.8). Using the AUCs 0-24 h at steady state bioavailability was calculated 77.7% for the tablets and 84.5% for the solution. Since trough values in steady state represent the body burden of digoxin which is supposed responsible for the therapeutic effect, trough values should be given priority for the determination of digoxin bioavailability from beta-acetyldigoxin tablets and solution. All formulations were well tolerated. No clinically relevant side effects were observed.

Effect of rioprostil, a methyl-prostaglandin E1, on plasma level and renal excretion of beta-acetyldigoxin.

A single oral dose of rioprostil, a synthetic methyl-prostaglandin E1, together with or 2 h before ingestion of 0.8 mg beta-acetyldigoxin significantly delays the rate but does not change the extent of beta-acetyldigoxin absorption. However, repeated evening doses of rioprostil do not alter either the glycoside steady-state plasma levels or renal excretion.

Pharmacokinetics of pengitoxin and its therapeutic efficacy in congestive heart failure.

In a therapeutic study, 120 inpatients suffering from congestive heart failure were treated with a daily maintenance dose of 0.3 mg pengitoxin (penta-acetyl-gitoxin) over several weeks or months. The plasma level and the glycoside concentration in urine were measured by radioimmunoassay. The therapeutic effects were evaluated considering clinical signs and criteria following the functional capacity according to the New York Heart Association (NYHA). In 27 patients both plasma and urine concentration were measured during 2 weeks after the beginning of the pengitoxin therapy. On the 3rd day of the pengitoxin dosage schedule, a mean plasma level of 18.1 ng.ml-1 (SD 5.1 ng.ml-1) was measured. During this day 26.6% of the daily administered glycoside dose was excreted in urine. In 26 of the 120 patients the mean steady state plasma level was between 7.6 and 22.5 ng.ml-1. A maximum of frequency was found in the 17.6 to 22.5 ng-subclass. In 118 patients the mean urinary excretion of 16-acetyl-gitoxin reached 24.7% (SD 11.8%) of the administered dose. The creatinine clearance and the 16-acetyl-gitoxin plasma level did not correlate, while between the renal clearance values of creatinine and the glycoside a correlation was found, however, it was of no significance for dosage schedules in patients with impaired renal function. After treatment, the NYHA classes III and II were reached in 57 patients; in 3 patients suffering from renal diseases, the NYHA class I remained unchanged. In 90 patients the clinical signs improved and in 27 patients the clinical situation remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)