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

Bioavailability of digoxin from rapidly dissolving preparations

1 Intestinal absorption of digoxin was assessed by determination of peak plasma concentrations, areas under plasma concentration curves over 80 h, and 10 day urinary excretion. Absorption was equal after ingestion of single doses of standard Lanoxin (Wellcome) tablets, tablets and capsules of ultra-rapid dissolution rate material, or an oral solution of digoxin in water. 2 Mean plasma concentrations and dosage-interval urinary excretion were highly similar during 14 day courses of either Lanoxin or ultra-rapid dissolution tablets. Increased bioavailability does not result from encapsulation of solid dosage presentations, nor from increasing tablet dissolution rate beyond 75% in 15 minutes. 3 Fourteen day courses of tablets of slow dissolution rate produced lower and less consistent mean plasma concentrations and urinary excretion. Slow dissolution rates are associated with greater individual variability in absorption.

Evaluation of hydrodynamics in the basket dissolution apparatus using computational fluid dynamics—Dissolution rate implications

The aim of this work was to simulate the fluid flow in the basket dissolution apparatus using computational fluid dynamics (CFD) and to use the resulting velocity data (in combination with velocity data from simulated flow fields of the paddle dissolution apparatus) to relate velocities in the vicinity of a dissolving surface to dissolution rate. A further objective of the work was to compare fluid velocities between the basket and paddle dissolution apparatuses. CFD simulations of the basket apparatus were carried out using Fluent software. Flow field solutions were compared with results from flow visualisation techniques and with published ultrasound-pulse-echo velocity data. Velocity data from the flow field solution revealed velocities within the basket to be of the same order as those at the base of the paddle apparatus at the same rotation speed, supporting equivalent dissolution rate data from these locations. Dissolution rates were obtained for compacts of benzoic acid dissolved in 0.1 M HCl at 37 degrees C in the basket apparatus at 50 rpm. The relationship between maximum velocity in the vicinity of a dissolving surface and dissolution rate data from both the paddle and basket apparatuses was in good agreement with theoretical predictions. Analysis of the dissolution rates suggests a significant contribution from free convection in regions of low velocity at the base of the vessel of the basket apparatus.

Absorption of digoxin from tablets and capsules in subjects with malabsorption syndromes

The relative steady-state bioavailability of two oral digoxin dosage forms was studied in 17 subjects with malabsorption syndromes. Male subjects received the following treatments in randomized crossover fashion for 14 days: three 0.125-mg digoxin tablets or three 0.1-mg digoxin capsules once daily. Female subjects received digoxin on the same schedule but at two-thirds the dose. Serum and urine samples were collected and analyzed for digoxin by radioimmunoassay, and treatments were compared by evaluating pharmacokinetic parameters. The mean area under the serum concentration versus time curve for tablets (28.1 h.nmol/L [21.9 h.ng/mL]) was smaller (p less than 0.03) than that for capsules (31.1 h.nmol/L [24.3 h.ng/mL]), and the mean maximum serum digoxin concentration for tablets (2.9 nmol/L [2.3 ng/mL]) was lower (p less than 0.02) than that for capsules (4.0 nmol/L [3.1 ng/mL]). There was no difference in cumulative urinary excretion of digoxin between the two treatments. In contrast to previous reports, we observed that digoxin from Lanoxin Tablets appears to be well absorbed in subjects with malabsorption. Nevertheless, these subjects absorbed digoxin from capsules better than from tablets, with the greatest differences occurring in subjects without a colon and in those subjects with the lowest serum carotene concentrations.

Solid dispersions of nitrofurantoin, ethotoin, and coumarin with polyethylene glycol 6000 and their coprecipitates with providone 25,000

The influence of two linear polymers, polyethylene glycol 6000 and povidone 25,000, on the dissolution rate of three poorly soluble drugs, nitrofurantoin, ethotoin, and coumarin, was studied. Povidone 25,000 produced better drug solubilization than polyethylene glycol 6000 with the drug-polymer ratios studied, and an increase in the weight fraction of either polymer gave more rapid dissolution. TLC and IR studies ruled out any interaction between the drugs and polyethylene glycol 6000. IR spectrophotometry provided evidence that there was complex formation between nitrofurantoin and povidone 25,000, probably via hydrogen bonding.

Bioavailability of drugs: the digoxin dilemma

The absorption of oral digoxin preparations has been a topic of much concern during the last 5 years. The completeness of digoxin absorption is proportional to the area under the serum concentration time curve and to the urinary excretion of digoxin after single doses. During chronic therapy the completeness of absorption is proportional to these values and also to the steady state serum concentration. Determination of absolute bioavailability of a given digoxin preparation requires a comparative study using intravenous digoxin as a standard. Oral digoxin solutions are incompletely absorbed, but have biological availability greater than or equal to that of tablets. The absorption of digoxin tablets depends upon their dissolution rate which in turn is related to drug particle size. Digoxin tablets with small drug particles have rapid rates of dissolution and can be absorbed as completely as oral solutions. The bioavailability of digoxin from tablets can be influenced by changes in gastro-intestinal motility, malabsorption syndromes, and by co-administration of food or other drugs. New regulations now insure that all marketed digoxin tablet preparations have satisfactory bioavailability. Problems with biological availability at present are unlikely to account for unexpected clinical results during digoxin therapy.

Effect of particle size on the bioavailability of digoxin

The bioavailability of digoxin in three tablets prepared from materials with different particle sizes was measured in healthy volunteers in a cross-over study using an alcoholic solution of digoxin as a reference standard. Its bioavailability in tablets with particle sizes of 7 mu or 13 mu was 78-97% of that of digoxin in solution. The tablet with largest particle size (102 mu) showed markedly lower bioavailability than the reference solution, namely 39%. It is obvious that particle size is an important determinant of the dissolution rate and bioavailability of digoxin.

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.

The uptake of digoxin and digitoxin by some antacids

The adsorption of digoxin and digitoxin by some antacids has been investigated. Magnesium trisilicate showed the highest adsorptive effect, the extent of adsorption being up to 99 % for the two glycosides. The calculated values of monolayer adsorption were 0·93 and 1·36 mg g-1 for digoxin and digitoxin respectively. At 37° only partial elution of adsorbed glycosides occurred in 0·2N HCl due to re-adsorption on the hydrated silica gel formed. Antacid preparations containing magnesium trisilicate adsorbed digoxin from a paediatric elixir to the extent of about 95%. Dissolution of digoxin tablets was similarly affected since almost complete suppression of dissolution occurred in the presence of 0·5 g magnesium trisilicate per 0·25 mg digoxin tablet. The effect of antacids, when concurrently administered with oral digoxin, on the bioavailability of the drug, is discussed.

Biological availability and in vitro dissolution of oxytetracycline dihydrate tablets

1 The concentration of oxytetracycline in plasma was studied by microbiological assay after oral administration of four different preparations of oxytetracycline dihydrate tablets. 2 There were statistically significant differences in biological availability between the four preparations, as assessed by the peak plasma level, the area under the plasma concentration-time curve, or the cumulative fraction of the dose excreted in urine at 405 minutes. In contrast, differences between the subjects were not statistically significant. 3 The differences in biological availability were not predictably related to the in vitro dissolution of the tablets.