Effect of magnesium--aluminum hydroxide and kaolin--pectin on absorption of digoxin from tablets and capsules

Clinically significant drug interactions with antacids: an update

One may consider that drug-drug interactions (DDIs) associated with antacids is an obsolete topic because they are prescribed less frequently by medical professionals due to the advent of drugs that more effectively suppress gastric acidity (i.e. histamine H(2)-receptor antagonists [H2RAs] and proton pump inhibitors [PPIs]). Nevertheless, the use of antacids by ambulant patients may be ever increasing, because they are freely available as over-the-counter (OTC) drugs. Antacids consisting of weak basic substances coupled with polyvalent cations may alter the rate and/or the extent of absorption of concomitantly administered drugs via different mechanisms. Polyvalent cations in antacid formulations may form insoluble chelate complexes with drugs and substantially reduce their bioavailability. Clinical studies demonstrated that two classes of antibacterials (tetracyclines and fluoroquinolones) are susceptible to clinically relevant DDIs with antacids through this mechanism. Countermeasures against this type of DDI include spacing out the dosing interval - taking antacid either 4 hours before or 2 hours after administration of these antibacterials. Bisphosphonates may be susceptible to DDIs with antacids by the same mechanism, as described in the prescription information of most bisphosphonates, but no quantitative data about the DDIs are available. For drugs with solubility critically dependent on pH, neutralization of gastric fluid by antacids may alter the dissolution of these drugs and the rate and/or extent of their absorption. However, the magnitude of DDIs elicited by antacids through this mechanism is less than that produced by H2RAs or PPIs; therefore, the clinical relevance of such DDIs is often obscure. Magnesium ions contained in some antacid formulas may increase gastric emptying, thereby accelerating the rate of absorption of some drugs. However, the clinical relevance of this is unclear in most cases because the difference in plasma drug concentration observed after dosing shortly disappears. Recent reports have indicated that some of the molecular-targeting agents such as the tyrosine kinase inhibitors dasatinib and imatinib, and the thrombopoietin receptor agonist eltrombopag may be susceptible to DDIs with antacids. Finally, the recent trend of developing OTC drugs as combination formulations of an antacid and an H2RA is a concern because these drugs will increase the risk of DDIs by dual mechanisms, i.e. a gastric pH-dependent mechanism by H2RAs and a cation-mediated chelation mechanism by antacids.

Drug, meal and formulation interactions influencing drug absorption after oral administration. Clinical implications

Drug-drug, drug-formulation and drug-meal interactions are of clinical concern for orally administered drugs that possess a narrow therapeutic index. This review presents the current status of information regarding interactions which may influence the gastrointestinal (GI) absorption of orally administered drugs. Absorption interactions have been classified on the basis of rate-limiting processes. These processes are put in the context of drug and formulation physicochemical properties and oral input influences on variable GI physiology. Interaction categorisation makes use of a biopharmaceutical classification system based on drug aqueous solubility and membrane permeability and their contributions towards absorption variability. Overlaying this classification it is important to be aware of the effect that the magnitudes of drug dosage and volume of fluid administration can have on interactions involving a solubility rate limits. GI regional differences in membrane permeability are fundamental to the rational development of extended release dosage forms as well as to predicting interaction effects on absorption from immediate release dosage forms. The effect of meals on the regional-dependent intestinal elimination of drugs and their involvement in drug absorption interactions is also discussed. Although the clinical significance of such interactions is certainly dependent on the narrowness of the drug therapeutic index, clinical aspects of absorption delays and therapeutic failures resulting from various interactions are also important.

Effects of antacids on the clinical pharmacokinetics of drugs. An update

Since a previous review by Hurwitz was published in 1977 a large number of reports on drug interactions with antacids have appeared, few of which are of clinical relevance. Tetracyclines form insoluble complex molecules by metal ion chelation with various antacids; tetracycline absorption may be decreased by more than 90% by this interaction. Of the new class of quinolone antibiotics, the absorption of ciprofloxacin and ofloxacin is reduced by 50 to 90% in the presence of aluminium- and magnesium hydroxide-containing antacids. In contrast to early work showing inhibition of the absorption of beta-adrenergic blocking drugs by antacids, subsequent studies did not confirm a reduction in the bioavailability of either atenolol or propranolol during antacid treatment; indeed, they showed an increase in the plasma concentrations of metoprolol when the drug was coadministered with an antacid. The bioavailability of captopril was significantly reduced in the presence of an antacid, and lower plasma concentrations of this angiotensin-converting enzyme inhibitor were accompanied by a reduction of its effect on the systolic blood pressure of the patients. The absorption of the cardiac glycosides digoxin and digitoxin is not inhibited by antacids to a significant degree, although earlier studies had shown a positive effect when the dissolution of the glycoside preparations was relatively poor. Antacids reduce the bioavailability of the H2-receptor antagonists cimetidine and ranitidine only when high antacid doses are used and when the drugs are administered simultaneously. The bioavailability of famotidine was not significantly altered by a potent antacid preparation, although a trend towards reduced absorption was observed. Iron absorption is significantly decreased in the presence of sodium bicarbonate and calcium carbonate, but is nearly complete when coadministered with aluminium-magnesium hydroxide. Nonsteroidal anti-inflammatory drugs such as naproxen, tenoxicam, ketoprofen, ibuprofen and piroxicam are not affected in their absorption by antacid treatment. Theophylline bioavailability is unchanged when the drug is given together with antacids, although its rate of absorption may be altered, leading to a reduction or an increase in the time of the occurrence of peak plasma drug concentrations.

Pharmacokinetic interactions with digoxin

Numerous pharmacological agents have been shown to produce clinically significant pharmacokinetic interactions with digoxin. Drugs which reduce digoxin absorption include the antacids aluminium hydroxide, magnesium hydroxide and magnesium trisilicate, the antidiarrhoeals kaolin and pectin, the hypocholesterolaemic agent cholestyramine and the chemotoxins cyclophosphamide, vincristine and bleomycin. Certain antibiotics including sulphasalazine, neomycin and aminosalicylic acid reduce digoxin absorption while others, including erythromycin and tetracycline, increase the bioavailability of digoxin in some patients. Capsule preparations of digoxin in solution are less subject to several of the interactions which affect the absorption and bioavailability of digoxin tablets. Various drugs induce alterations in the volume of distribution and clearance of digoxin. Cardiac patients receiving digoxin therapy are particularly prone to interactions with commonly co-administered medications such as the antiarrhythmics quinidine and amiodarone, the calcium channel blockers verapamil and nifedipine, and possibly some vasodilating agents. Studies of digoxin interactions have yielded discrepant results, indicating the need for careful analysis of investigational design before arriving at clinical conclusions.

The effect of dietary fiber on the bioavailability of digoxin in capsules

Sixteen healthy volunteers were regularly given 0.4 mg of digoxin daily as two capsules with breakfast. After ten days during which breakfast was supplemented with 11 g of bran fiber, steady-state predose mean serum digoxin was lower (0.89 +/- 0.19 versus 0.84 +/- 0.18 ng/mL, P less than .05) and mean 24-hour area under curve determination was lower (30.5 +/- 6.1 versus 28.4 +/- 6.0 ng X hr/mL, P less than .05) than during the control period without bran. Height and time of peak serum digoxin, and 24-hour urinary digoxin were not significantly different. The 6 to 7% reduction in digoxin absorption from capsules is less than that reported from tablets and is probably clinically unimportant.

Drug-antacid interactions: assessment of clinical importance

Antacids and adsorbents are commonly used preparations that are generally considered to be pharmacologically inert and free from adverse effects. They may, however, interact with a diverse range of primary drugs and the sequelae can be disadvantageous to the efficacy of the primary medication. Many such reports in the literature are based on animal experiments, or on single-dose studies in healthy subjects. Some reports are anecdotal and are unconfirmed; others are based solely on in vitro evidence. Potentially important interactions have been suggested for a relatively small group of drugs: tetracyclines, phenytoin, digoxin, chloroquine, cimetidine, quinidine, nonsteroidal antiinflammatory drugs, and beta-blocking agents. The evidence for these has been critically evaluated, as well as for antacid-anticoagulant and antacid-nitrofurantoin interactions that have been wrongly emphasized in the literature. The majority of literature reports on interactions with antacids have been overemphasized; only ferrous sulfate-, isoniazid-, and tetracycline-antacid interactions fall into a category I importance (scale I-III of descending importance). This category is for those interactions with good evidence of actual or potential importance in patients or in relevant studies on normal subjects.

A steady-state evaluation of the effects of propantheline bromide and cholestyramine on the bioavailability of digoxin when administered as tablets or capsules

Drug interactions can profoundly alter the absorption of digoxin in tablet form. This study evaluated whether digoxin solution in capsules, a new dosage form with 90% to 100% bioavailability, would reduce such alterations, specifically those caused by cholestyramine and propantheline bromide. The investigation used a six-treatment, steady-state, balanced, incomplete block design with 18 healthy adults studied for four continuous two-week treatment periods. Treatments were either two 0.25 mg digoxin tablets or two 0.20 mg digoxin capsules administered alone, with propantheline, 15 mg qid, or with cholestyramine, 8 g qd. Bioavailability was determined from steady-state, 24-hour area under the serum concentration-time curve (AUC, ng X h/mL) and from 0- and 24-hour trough serum digoxin concentrations (ng/mL). The AUCs for tablets alone, with cholestyramine, and with propantheline were 32.8 +/- 13.3 (+/- SD), 22.4 +/- 12.1, and 40.6 +/- 13.9, respectively, while corresponding values for capsules were 31.7 +/- 9.3, 24.7 +/- 7.9, and 35.9 +/- 12.8. The trough concentrations for tablets alone, with cholestyramine, and with propantheline were 0.88 +/- 0.47, 0.61 +/- 0.38, and 1.09 +/- 0.35, respectively; trough concentrations for capsules were 0.77 +/- 0.28, 0.74 +/- 0.28, and 0.96 +/- 0.48, respectively. The only significant differences in AUC were seen when comparing tablets alone versus tablets with cholestyramine (P less than .0005) and tablets with propantheline (P less than .01). A significant finding was also observed when comparing trough concentrations for tablets alone versus tablets with cholestyramine (P less than .005).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic interactions between digoxin and other drugs

Drug interactions with digoxin are important because of this agent's narrow therapeutic index. Among the drugs that can decrease digoxin bioavailability are cholestyramine, antacid gels, kaolin-pectate, certain antimicrobial drugs and cancer chemotherapeutic agents. In selected patients, antibiotics may enhance digoxin bioavailability by eliminating intestinal flora that metabolize digoxin. Antiarrhythmic drugs, such as quinidine and amiodarone, can markedly increase steady state serum digoxin levels. Certain calcium channel blocking drugs, particularly verapamil, have a similar effect. Potassium-sparing diuretic drugs, such as spironolactone, can alter digoxin pharmacokinetics. Indomethacin may decrease renal excretion of digoxin in preterm infants. Finally, rifampin, an antibiotic used in the treatment of tuberculosis, may lower steady state serum digoxin levels in patients with severe renal disease. Physicians must maintain constant vigilance whenever medications are added to or withdrawn from a therapeutic regimen that includes digoxin.

Absolute bioavailability and effect of food and antacid on diazepam absorption from a slow-release preparation

A series of healthy volunteers received a single 7.5-mg intravenous dose of diazepam on one occasion and a single 15-mg oral dose of slow-release diazepam (DZ-SR) on another occasion. Diazepam concentrations were measured by gas chromatography in multiple plasma samples drawn during seven days after each dose. Absorption of diazepam from DZ-SR was slow, with mean +/- S.E. peak concentrations attained at 3.8 +/- 0.5 hours after dosage. Absolute bioavailability of DZ-SR averaged 0.98 +/- 0.06. In two other studies, diazepam absorption from DZ-SR was evaluated when coadministered with a standard breakfast or with an antacid preparation (Maalox). Neither food nor antacid altered the rate of diazepam absorption and did not impair the completeness of absorption. Higher peak total plasma diazepam concentrations occurred in the postprandial as opposed to the fasting state, but this was an artifact of reduced protein binding (increased free fraction) due to fasting. Thus, diazepam absorption from DZ-SR is slow and essentially complete.