Hydrolysis of digoxin by acid

Strategies To Stabilize Dalbavancin in Aqueous Solutions; Section 3: The Effects of 2 Hydroxypropyl-β-Cyclodextrin and Phosphate Buffer with and without Divalent Metal Ions

PURPOSE

New formulations of the glycopeptide drug dalbavancin containing 2-hydroxpropyl-β-cyclodextrin (2HPβCD) with or without divalent metal ions in phosphate buffer (pH 7.0) were tested to evaluate whether these excipients influence the aqueous solution stability of dalbavancin.

METHOD

Recovery of dalbavancin from phosphate buffered solutions at pH 7.0 with different concentrations of 2HPβCD and a divalent metal ion (Ca2+, Mg2+, or Zn2+) was evaluated by RP-HPLC and HP-SEC after four weeks of storage at 5°C and 55°C. A long-term study of formulations with 2HPβCD and Mg2+ was carried out over six months at 5°C, 25°C, and 40°C using RP-HPLC.

RESULTS

Dalbavancin solutions with either 5.5 mM or 55 mM 2HPβCD were significantly more stable with Mg2+ than with the other divalent metal ions, both at 55°C for four weeks and at 40°C for six months. Dalbavancin was found to be more stable in aqueous solutions at a concentration of 1 mg/mL than at 20 mg/mL with 2HPβCD and Mg2+ at 40°C for six months.

CONCLUSION

The results suggest that 2HPβCD forms an inclusion complex with dalbavancin that slows the formation of the major degradant, mannosyl aglycone (MAG). The effect of 2HPβCD is increased in the presence of Mg2+ and phosphate at pH 7.0, and the complex is more stable at a dalbavancin concentration of 1 mg/mL than at 20 mg/mL. These observations point towards the possibility of formulating a dalbavancin injection solution with a long shelf life at room temperature and physiological pH.

A multiple linear regression approach to the estimation of carboxylic acid ester and lactone alkaline hydrolysis rate constants

Pesticides, pharmaceuticals, and other organic contaminants often undergo hydrolysis when released into the environment; therefore, measured or estimated hydrolysis rates are needed to assess their environmental persistence. An intuitive multiple linear regression (MLR) approach was used to develop robust QSARs for predicting base-catalyzed rate constants of carboxylic acid esters (CAEs) and lactones. We explored various combinations of independent descriptors, resulting in four primary models (two for lactones and two for CAEs), with a total of 15 and 11 parameters included in the CAE and lactone QSAR models, respectively. The most significant descriptors include pKa, electronegativity, charge density, and steric parameters. Model performance is assessed using Drug Theoretics and Cheminformatics Laboratory's DTC-QSAR tool, demonstrating high accuracy for both internal validation (r2 = 0.93 and RMSE = 0.41-0.43 for CAEs; r2 = 0.90-0.93 and RMSE = 0.38-0.46 for lactones) and external validation (r2 = 0.93 and RMSE = 0.43-0.45 for CAEs; r2 = 0.94-0.98 and RMSE = 0.33-0.41 for lactones). The developed models require only low-cost computational resources and have substantially improved performance compared to existing hydrolysis rate prediction models (HYDROWIN and SPARC).

Development of radioimmunoassay for measurement of serum digoxin in digitalized patients using novel anti-digoxin antiserum

There is an antiserum elicited by digoxin 3'-hemisuccinate-bovine serum albumin (BSA) conjugate possessing high specificity for digoxin. Our study focused on development of RIA using this novel antiserum for measurement of digoxin in serum from digitalized patients. The property of the new antiserum was investigated by RIA with digoxin 3'-hemisuccinyl-[3H]leucine. The separation of bound and free fractions was performed using a dextran-coated charcoal suspension. The new antiserum bound approximately 50% of digoxin 3'-hemisuccinyl-[3H]leucine with a final dilution of 1:30000. The intra- and inter-assay coefficients of variation were <9% in the range of 0.52-4.17 ng/ml. The mean digoxin concentration in serum samples (n=35) from digitalized patients was estimated to be 0.68 ng/ml, which was lower than its measurement of digoxin with the commercial antidigoxin BSA serum and monoclonal anti-digoxin. It is apparent that the RIA described here has sufficient precision. The RIA system was available for the measurement of digoxin in serum from digitalized patients.

Digoxin and its related endogenous factors

The digitalis drugs are plant-derived cardenolide compounds used medicinally for several hundred years. These drugs elicit inotropic and chronotropic effects on the heart, but they also affect many other tissues. The mechanism of action involves inhibition of the ion-transport activity of a membrane-associated protein called Na, K-ATPase (sodium pump). Present theory holds that the sodium pump is the principal molecular receptor for the digitalis drugs. Recent evidence indicates the presence of naturally occurring digitalis-like compounds in mammals. It is believed these compounds, collectively known as either digitalis-like (DLF) or ouabain-like (OLF) factors, may be endogenous hormones regulating the biological activity of the sodium pump and its isoforms. The presence of deglycosylated and other congeners of one specific DLF, the digoxin-like immunoreactive factor (DLIF), has very recently been described in humans. Digoxin as a drug is the most widely prescribed digitalis in the U.S., and its measurement in serum has established a model for present-day therapeutic drug monitoring (TDM). Historically, the accurate measurement of digoxin in blood has been difficult. This article focuses on the present understanding of the clinical use of digoxin, factors that affect the accuracy of measuring digoxin, the principle of measuring metabolically active species of digoxin, and the effects of DLIF and other interfering substances in digoxin immunoassay.

Photoaffinity glycoprobes-a new tool for the identification of lectins

One of the proposed functions for the carbohydrate structures on glycoconjugates is the transfer of information through interaction with specific lectin receptors. However, the number of elucidated functional lectin-carbohydrate interactions is still relatively small, largely due to the lack of adequate methods to identify lectin activity in complex biological samples. Aiming to solve this problem, we have developed a method based on the novel group of compounds we named glycoprobes. The glycoprobe consists of three vital parts: (1) glycan, (2) digoxin tag, and (3) photoreactive crosslinker. When incubated in dark, oligosaccharide part of the glycoprobe forms a complex with lectin. After illumination, covalent link between the probe and the lectin is formed resulting in a digoxin-tagged lectin. Using antibodies against digoxin, this complex can easily be identified immuno/cytochemically, or by Western blots. To demonstrate the applicability of glycoprobes we have used Man(9)-glycoprobe (containing Man(9)oligosaccharide) and YEE(ahGalNAc)(3)-glycoprobe (containing a synthetic neoglycopeptide with three terminal N-acetyl-galactosamine residues; Lee and Lee, Glycoconjugate J., 1987,4, 317) to identify lectins in bovine serum and rat liver membranes. The simplicity of the method enables its application in routine monitoring of changes in lectin activity during various developmental or pathological processes. An example of GalNAc-binding analysis in human serum is shown.

Deglycosylated products of endogenous digoxin-like immunoreactive factor in mammalian tissue

Digoxin-like immunoreactive factor (DLIF) from adrenal cortex is an endogenous molecule with structural features remarkably similar to those of digoxin, a plant-derived cardiac glycoside (Shaikh, I. M., Lau, B. W. C., Siegfried, B. A., and Valdes, R., Jr. (1991) J. Biol. Chem. 266, 13672-13678). Two characteristic structural and functional features of digoxin are a lactone ring and three digitoxose sugars attached to a steroid nucleus. Digoxin is known to undergo deglycosylation during metabolism in humans. We now demonstrate the existence of several naturally occurring deglycosylated components of DLIF in human serum. The components are identified as DLIF-genin, DLIF-mono, and DLIF-bis, corresponding to the aglycone, and the aglycone with one and two sugars, respectively. Similar components are produced by acid-induced deglycosylation of DLIF isolated from bovine adrenal cortex. The elution pattern and sequence of DLIF-deglycosylation was identical to that of digoxin suggesting identical sugar stoichiometry. However, analysis of these newly discovered congeners by reverse-phase chromatography, spectrophotometry, antibody reactivity, and kinetics of deglycosylation, demonstrates that subtle structural and physical differences do exist when compared to digoxin. DLIF was chromatographically distinct from digoxin, and interestingly, the mobility of the DLIF-genin was shifted toward increased polarity relative to digoxigenin. DLIF and DLIF-bis, -mono, and -genin congeners have absorbance maxima at 216 nm, whereas digoxin and its congeners absorb at 220 nm. Reaction with specific antibodies directed at the lactone portion of these molecules shows DLIF and its deglycosylated congeners to be 10(3)-fold less reactive than digoxin. Kinetics of sugar removal suggests that DLIF is 8-fold more susceptible to deglycosylation than is digoxin. Two less polar DLIF components produced from the DLIF-genin have lambdamax at 196 nm and are 4-fold less immunoreactive than DLIF. Our data suggest that subtle structural differences exist between DLIF and digoxin at or near the lactone ring as well as in the nature of the sugars. The presence of deglycosylated congeners of DLIF in human serum, including the less polar components, suggests in vivo deglycosylation of these factors. This is the first demonstration of the existence of naturally occurring deglycosylated derivatives of DLIF and establishes the likelihood of active metabolism of DLIF in mammals.

Digoxin disposition in elderly humans with hypochlorhydria

Digoxin (D3) metabolism is partially mediated by the gastrointestinal tract via acid hydrolysis of digitoxose sugar moieties and bacterial reduction of the lactone. The hypothesis that hypochlorhydria influences digoxin disposition was tested in six normochlorhydric (NC) and four hypochlorhydric (HC) subjects. D3 tablets were administered daily for 19 to 28 days, and quantitative urine and fecal samples were collected over the last 3 days (steady state). Samples were analyzed for D3 and its extractable metabolites by fluorescence-derivatization HPLC. Excretion of D3 in urine increased from 37% of the dose in NC to 46% in HC, whereas excretion of D3 in feces decreased from 29 to 14%. These changes were statistically significant (P < .05) and consistent with decreased hydrolysis of D3 by stomach acid and increased intestinal metabolism in HC. In each subject, D3 was added to anaerobic cultures of both feces and jejunal fluid. Digoxin was reduced in all but two of the fecal incubates, and was not reduced in any jejunal fluid incubates. Because dihydrodigoxin (DHD3) was found in only two hypochlorhydric subjects, in vitro measures of bacterial reduction of D3 were not predictive of in vivo excretion of reduced metabolites. Sugar-hydrolyzed, reduced metabolites were not found in any subjects. It is concluded that D3 disposition is altered by hypochlorhydria, and that an understanding of the metabolic mechanisms requires further study.

Hydroxysteroid sulfotransferase and a specific UDP-glucuronosyltransferase are involved in the metabolism of digitoxin in man

In vitro experiments were performed with cytosolic and microsomal fractions of human liver specimens in order to investigate which enzyme forms of sulfotransferase (ST) and UDP-glucurosyltransferase (GT) are involved in the metabolism of digitoxin (dt-3) and/or its cleavage products. It was found that the cytosolic STs preferentially react with digitoxigenin (dt-0) whereas microsomal GTs conjugate digitoxigenin-monodigitoxoside (dt-1) and in traces the bisdigitoxoside (dt-2). Dt-3 and dt-0 cannot be glucuronidated. By separation of different sulfotransferases it was found that the hydroxysteroid-ST is responsible for dt-0 and 3-epidigitoxigenin (epi-dt-0) sulfation. The hydroxysteroid-ST could be purified and characterized (apparent Km and Vmax for dt-0 sulfation: approx. 17 mumol/l and 2.7 nmol/min mg protein, respectively). Of various model substrates and endogenous compounds (steroids, bilirubin) none caused a competitive inhibition of the microsomal dt-1 glucuronidation except dt-2 and dt-3. Therefore it can be supposed that a new GT form catalyses this reaction. It is characterized by an extraordinarily high affinity towards dt-1 with Km values ranging between 0.7 and 27 mumol/l.

Purification of a rat liver cytosolic sulfotransferase responsible for the conjugation of digitoxigenin

Previous investigations on the digitoxin metabolism hardly considered the role of the sulfate ester conjugation. Therefore, this study examined whether digitoxin (dt-3) or one of its cleavage products might be sulfated in vitro. It was proven that digitoxigenin (dt-0) is by far the best substrate for the cytosolic sulfotransferases (ST). Digitoxigenin-monodigitoxoside (dt-1) and digitoxigenin-bisdigitoxoside (dt-2) are sulfated in trace amounts whereas dt-3 is not sulfated at all. The purification of the responsible enzyme was performed by liquid chromatography on Q-Sepharose and hydroxyapatite. During the purification procedure this enzymatic activity corresponded exactly to that towards dehydroepiandrosterone (DHEA). The 134-fold purified and gel electrophoretically homogeneous enzyme protein (Mr 33,000) showed a Vmax of 12.5 nmoles dt-0 sulfate/min mg protein and a KM of 37 mumol/L. The purified enzyme conjugated dt-1 and dt-2 in trace amounts only and was inhibited competitively by DHEA. It can be concluded that in the rat a 3 beta-hydroxy-steroid sulfotransferase is responsible for the sulfation of dt-0. The purified enzyme reacts with dt-1, dt-2 and digoxigenin (dg-0) in traces only, a sulfation of dt-3 is not detectable.

Reduction of digoxin to 20R-dihydrodigoxin by cultures of Eubacterium lentum

The anaerobic bacterium Eubacterium lentum, a common constituent of the intestinal microflora, inactivates digoxin by reducing the unsaturated lactone ring. Reduction of the cardiac glycoside by growing cultures of E. lentum ATCC 25559 proceeded in a stereospecific manner, with the 20R-dihydrodigoxin constituting more than 99% of the product formed. This is in contrast to the 3:1 ratio of 20R and 20S epimers formed in the chemical catalytic hydrogenation. Formation of the reduced glycosides proceeded quantitatively when an overall concentration of 10 micrograms/ml was added to the cultures. E. lentum did not hydrolyze the digitoxose sugars from C-3 of the parent glycoside. However, the synthetically prepared sugar-hydrolyzed metabolites (digoxigenin, digoxigenin monodigitoxoside, and digoxigenin bisdigitoxoside) were reduced to the corresponding dihydro metabolites. Repetition of the experiments with a feces sample from a volunteer who was known to be a converter of digoxin to dihydrodigoxin gave results identical to those obtained with pure E. lentum cultures.

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)

Fasting and postprandial absorption of digoxin from a microencapsulated formulation

The absorption of digoxin from a capsule preparation containing a large number of small, enteric-coated granules of the glycoside (Preparation CR) was compared in 10 volunteers with that from a rapidly dissolving tablet (Preparation L). Plasma and urine digoxin concentrations were measured by radioimmunoassay. In the fasting state, after a loading dose of digoxin (0.76 mg), peak plasma concentrations were significantly (p less than 0.001) lower after CR (2.0 +/- 0.5 nmol/l, mean +/- SD) than L (4.7 +/- 1.1 nmol/l). Peak concentrations after CR were significantly (p less than 0.001) delayed compared to L (3.3 +/- 0.6 h vs 1.1 +/- 0.4 h). Also, postprandial peak plasma concentrations at steady state, were significantly (p less than 0.01) lower after CR (1.0 +/- 0.3 nmol/l) than L (2.7 +/- 0.5 nmol/l), and the peak concentrations occurred later (3.9 +/- 1.7 h vs 1.4 +/- 0.9 h). The area under the plasma concentration-time curves was smaller (p less than 0.01) for CR (17.7 +/- 5.9 nmol X 1(-1) X h) than for L (22.4 +/- 4.1 nmol X 1(-1) X h), and so was the amount of drug excreted in urine (174 +/- 25 micrograms vs 190 +/- 31 micrograms; p less than 0.005). Thus, the absorption rate of digoxin from the enteric-coated formulation was markedly reduced but at the cost of a variable reduction in the amount absorbed.

Effect of quinidine on digoxin bioavailability

To evaluate the possible effect of quinidine on digoxin bioavailability, the steady state digoxin kinetics was examined with and without concomitant quinidine therapy, in 7 cardiac patients after simultaneous administration of oral digoxin and intravenous [3H]-digoxin. In the presence of quinidine, the absorption rate constant of digoxin (ka) increased from 2.72 +/- 1.04 to 3.53 +/- 1.34 h-1 (p less than 0.05), whereas lag time and peak time decreased from 0.16 +/- 0.10 to 0.05 +/- 0.04 h (p less than 0.05) and from 0.92 +/- 0.27 to 0.69 +/- 0.19 h (p less than 0.02), respectively. Predose plasma digoxin increased from 0.41 +/- 0.25 to 0.70 +/- 0.31 ng/ml (p less than 0.02), while peak plasma digoxin increased from 0.93 +/- 0.34 to 1.63 +/- 0.46 ng/ml (p less than 0.02). The systemic availability of digoxin increased from 68.48 +/- 13.35 to 79.09 +/- 14.89% (p less than 0.05) in the presence of quinidine. Quinidine had no effect on the biotransformation pattern of digoxin, as assessed by thin layer chromatography. Quinidine increases the rate and extent of digoxin absorption, and this interaction contributes significantly to the elevation in plasma digoxin during both its distribution and elimination phases.

Effect of cyclodextrins on the acid hydrolysis of digoxin

The effects of three cyclodextrins (alpha-, beta-, gamma-CyD) on the acid hydrolysis of digoxin were examined. From the high performance liquid chromatographic tracing of each of the four components (digoxin, bisdigitoxoside, monodigitoxoside, digoxigenin) in reaction mixtures, the individual rate constants (K1-K6) were determined by analogue computer simulation. The hydrolysis was suppressed by CyDs in the order of beta-great than gamma-greater than alpha-greater than-CyD, where beta-CyD inhibited the appearance rates of digoxigenin (k3, K5, and K6) significantly. In the dissolution study of digoxin tablets, the increase in dissolution rate and decrease in acid hydrolysis were attained by inclusion complexation. The data are presented suggesting that CyDs are useful for improving the oral bioavailability of digoxin.

Excretion of digoxin and its metabolites in urine after a single oral dose in healthy subjects

The 3-day urinary excretion of digoxin, its conjugated and unconjugated hydrolytic metabolites and dihydrodigoxin, was studied in 8 healthy men after oral administration of tritiated digoxin. Analysis was performed by high pressure liquid chromatography (HPLC). The total radioactivity corresponded to 45.4 +/- 2.0 per cent (mean +/- S.E.M.) of the dose. By HPLC 42.4 +/- 2.7 per cent was recovered before and 44.0 +/- 2.7 per cent after deconjugation of the samples. Digoxin and dihydrodigoxin constituted 40.3 +/- 2.9 per cent; of this 0.7 +/- 0.4 per cent was dihydrodigoxin. The sum of the hydrolytic metabolites was 2.1 +/- 0.3 per cent before and 3.4 +/- 0.5 per cent after deconjugation. No correlation was found between gastric pH and the production of hydrolytic metabolites. The relative amount of these metabolites was maximal (mean 13.4 per cent of the excretion) in the 4-8 h sampling period. During the first 8 h an average of 8.6 per cent of the radioactivity was not recovered by HPLC. The metabolism of digoxin as judged by urinary excretion was limited and showed great variation during the early hours after treatment. The excretion of unchanged digoxin in some individuals constituted as little as 60 per cent over the first 12 h after dosing.

Separation of digoxin, digitoxin and their potential metabolites, impurities or degradation products by high-performance liquid chromatography

A rapid and versatile series of high-performance liquid chromatographic systems are described for the resolution of digoxin, digitoxin and their potential metabolites or degradation products and impurities. These systems consist of isocratic, single-step gradient and linear gradient modes that provide resolution of the glycosides in 25, 17 and 14 min respectively. Digoxin, its mono- and bisdigitoxosides, digoxigenin and gitoxin, a potential impurity, may be isocratically separated in 11 min. The two semi-synthetic glycosides alpha- and beta-acetyldigoxin are resolved and separated from digoxin and its metabolites in a chromatographic time of 23 min. Digitoxin and its metabolites or degradation products may be separated in as little as 9 min using an isocratic system. The solvent systems employ varying proportions of methanol, water, isopropanol and dichloromethane and a conventional 5 micrometers bonded, octadecyl phase. Detection was accompanied using a variable wavelength detector set at 220 nm.

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.

Absorption of digoxin from a new microencapsulated formulation

The absorption of digoxin from two capsule preparations containing a large number of small, enteric-coated granules of the glycoside (0.38 mg) was compared with that of the same amount from ultrarapidly dissolving commercial tablets. Eight volunteers were studied during steady state conditions. Digoxin concentrations in plasma and urine were measured by radioimmunoassay. Peak plasma concentrations of digoxin were significantly (p < 0.01) delayed after taking the capsules (2.6 +/- 1 h and 2.6 +/- 0.9 h, mean +/- SD) as compared to the tablets (1.3 +/- 0.7 h). The peak concentrations produced by the capsules were 3.1 +/- 1.0 and 2.6 +/- 1.1 nmol/l; only the latter was significantly (p less than 0.05) lower than after the tablets (3.4 +/- 1.0 nmol/l). Areas under the plasma concentration-time curves during a 24 h dosage interval were similar for the three preparations, and so was the 24 h urinary excretion of digoxin, which averaged 60-63% of the daily dose. Thus, this particular enteric coating of digoxin delayed absorption without reducing the amount absorbed.

Digoxin degradation in acidic dissolution medium

The release of digoxin and its simultaneous conversion to digoxigenin bisdigitoxoside, digoxigenin monodigitoxoside, and digoxigenin in a USP dissolution test medium were followed by high-pressure liquid chromatography. Two products, Tablets A and B, were manufactured by solvent deposition and simple blending methods, respectively. Tablet A released digoxin faster than Tablet B in distilled water and in artificial intestinal juice, and no decomposition was observed. In the USP dissolution test medium, the rate of hydrolysis to digoxigenin bisdigitoxoside was almost equal to that of hydrolysis to digoxigenin monodigitoxoside, and a comparatively large formation rate of digoxigenin was observed. Concentrations of digoxin and its decomposition products were described by differential equations that included dissolution rates of digoxin (rapidly dissolving digoxin and digoxin crystals) and an apparent hydrolysis rate. In the earlier stage of dissolution, hydrolysis was rate determining; in the later stage, dissolution became the rate-determining step for overall digoxin degradation. To suppress digoxin hydrolysis in the USP dissolution test medium, a developmental formulation study was performed. The incorporation of magnesium oxide and magnesium hydroxide-aluminum hydroxide in the tablet formulations inhibited digoxin hydrolysis by 15.3 and 14.5%, respectively, after dissolution for 30 min without serious delay of drug release.

Interaction of digoxin and montmorillonite: mechanism of adsorption and degradation

IR, X-ray diffraction, and absorption studies showed that digoxin is adsorbed onto montmorillonite by a reversible adsorption mechanism at pH 2 and 6. Degradation studies indicated abnormally high acid hydrolysis rates for digoxin interacted with montmorillonite. Accelerated digoxin degradation is attributed to the ability of the clay surface to concentrate both digoxin and protons. The effective pH at the clay surface appeared to be 1.5 pH unites lower than the bulk suspension pH. Bisdigoxigenin was the major adsorbed degradation product. A similar catalytic effect also may occur with other neutral drugs that degrade by acid hydrolysis and should be considered in the formulation of clay-containing drug products or their coadministration with other drugs.

Instability of digoxin in acid medium using a nonisotopic method

A selective nonisotopic assay was used to investigate the digoxin hydrolysis rates at 37 +/- 0.1 degrees over the pH 1.1--2.2 range. The colorimetric method adopted is based on the use of a xanthydrol reagent after extraction with chloroform. The spectrofluorometric method specified in the dissolution test for digoxin tablets was nonspecific because of digoxigenin interference. Digoxin hydrolysis followed specific acid hydrolysis, and K values of the apparent first-order reaction varied from 0.0357 to 0.0027 min-1 over the pH range used. The effect of the dissolution medium on digoxin stability during the dissolution tests of the tablets also was studied. Water (the BP medium) and 0.6% HCl (the USP medium) were compared using the fluorometric method and the xanthydrol method. In the USP medium (pH 1.3), no hydrolysis was revealed by the fluorometric estimation whereas the xanthydrol method showed about 74% hydrolysis. In water, the two methods revealed no hydrolysis. The extent of hydrolysis after 1 hr in the USP medium was studied using three brands of digoxin tablets of differing dissolution characteristics. The fast dissolving brand showed relatively more hydrolysis than the slow dissolving tablets.