Influence of kaolin--pectin suspension on digoxin bioavailability

Effects of medicines used to treat gastrointestinal diseases on the pharmacokinetics of coadministered drugs: a PEARRL Review

Objectives

Drugs used to treat gastrointestinal diseases (GI drugs) are widely used either as prescription or over-the-counter (OTC) medications and belong to both the 10 most prescribed and 10 most sold OTC medications worldwide. The objective of this review article is to discuss the most frequent interactions between GI and other drugs, including identification of the mechanisms behind these interactions, where possible.

Key findings

Current clinical practice shows that in many cases, these drugs are administered concomitantly with other drug products. Due to their metabolic properties and mechanisms of action, the drugs used to treat gastrointestinal diseases can change the pharmacokinetics of some coadministered drugs. In certain cases, these interactions can lead to failure of treatment or to the occurrence of serious adverse events. The mechanism of interaction depends highly on drug properties and differs among therapeutic categories. Understanding these interactions is essential to providing recommendations for optimal drug therapy.

Summary

Interactions with GI drugs are numerous and can be highly significant clinically in some cases. While alterations in bioavailability due to changes in solubility, dissolution rate, GI transit and metabolic interactions can be (for the most part) easily identified, interactions that are mediated through other mechanisms, such as permeability or microbiota, are less well-understood. Future work should focus on characterising these aspects.

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.

Pharmacokinetic and pharmacodynamic studies of drug interaction following oral administration of imipramine and sodium alginate in rats

Recently, the use of health foods has increased due to growing interest in health maintenance. Previous in vitro studies have shown some drugs to be adsorbed by sodium alginate, a dietary fiber, and that such adsorption was marked with tricyclic antidepressants, such as imipramine. This study investigated the pharmacokinetic and pharmacological interactions between imipramine and sodium alginate in rats. The simultaneous administration of imipramine (30 mg/kg, oral (p.o.)) and sodium alginate (3.0%, p.o.) decreased the antidepressant-like activity of imipramine in a forced swimming test. In the rats administrated imipramine and 0.3%, 1.0%, or 3.0% sodium alginate, the geometric mean ratio of the Cmax values of imipramine was 72% [90% confidence intervals (CI) = 53-91%], 64% (90% CI = 47-80%), and 58% (90% CI = 50-67%), respectively. The geometric mean ratio of the AUC(0-6) values of imipramine were 68% (90% CI = 56-80%), 74% (90% CI = 60-89%), and 87% (90% CI = 73-102%), respectively. The decrease in Cmax and AUC(0-6) was judged to be significant with a 90% CI outside the 80-125% boundaries. In addition, the Tmax value of imipramine significantly increased (P < 0.05) by coadministration with 3.0% sodium alginate. These results suggested that simultaneous administration of sodium alginate decreased the serum concentration and pharmacological action of imipramine, through a delay in its absorption. Although the clinical relevance of these findings is unclear, it is important to pay considerable attention to the interactions between imipramine and sodium alginate.

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.

Effect of troglitazone on steady-state pharmacokinetics of digoxin

Twelve healthy subjects participated in a study to determine the effect of multiple doses of troglitazone on the steady-state pharmacokinetics of digoxin. Subjects received digoxin 0.25 mg orally once daily on days 1 through 20 and 400 mg of troglitazone orally once daily on days 11 through 20. Serial plasma samples and 24-hour urine samples collected before and after the doses on days 10 and 20 were analyzed for digoxin using a radioimmunoassay method. Eleven subjects completed the study. Administration of multiple oral doses of digoxin and troglitazone was well tolerated. Mean values for maximum concentration (Cmax), time to Cmax (tmax), and area under the concentration-time curve from 0 to 24 hours (AUC0-24) of digoxin on day 10 were similar to those on day 20. Mean day 10 digoxin values for minimum concentration (Cmin), apparent oral clearance (Cl/F), total urinary excretion from 0 to 24 hours (Ae0-24), and renal clearance (Clr) were also similar to corresponding values on day 20. Thus, concomitant administration of multiple-dose troglitazone does not alter the steady-state pharmacokinetics of digoxin.

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.

Inter- and intrasubject variation of digoxin renal clearance in normal adult males

Three methods of estimating inter- and intrasubject variances and coefficients of variation of digoxin renal clearance are discussed and illustrated. All three methods gave essentially the same values for intrasubject variation; two of the methods (II and III), based on two-factor and one-factor analyses of variance, respectively, gave the same estimates of intersubject variation. The preferred method I involves directly calculating variances from the rows and columns of the tabular data, where the rows are repetitive treatments in the same subject and the mean variance represents intrasubject variance; a column represents different subjects' results on a given treatment and the mean of the column variances represents intersubject variance. Problems in attempting to use components of variance are discussed. Intrasubject coefficients of variation of digoxin renal clearance averaged 23.8 percent by method I and ranged from 15.2 to 28.6 percent. Intersubject coefficients of variation averaged 30.5 percent with a range of 17.5-41.8 percent by method I and averaged 42.1 percent with a range of 19.2-49.7 percent by methods II and III.

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.

Interactions affecting drug absorption

The influence of drug-drug and drug-food interactions affecting the absorption of orally administered medication is reviewed. Drug-drug interactions can be classified in terms of indirect effects by one drug on gastrointestinal tract physiology influencing the absorption of other drugs, or direct interactions involving altered pH, adsorption, absorption, or chelation. Most, but not all, drug-drug interactions result in reduced or delayed systemic drug availability. Drug-food interactions may result in reduced, delayed, or increased systemic drug availability. The absorption of only a small number of drugs is unaffected by concomitant food intake. The degree of interaction and whether it positively or negatively affects drug absorption depends on a number of factors including the physical and chemical nature of the drug, the formulation, the type of meal, and the time interval between eating and dosing. Mechanisms of drug-food interactions are not well characterised. They clearly involve both direct and indirect factors in a similar fashion to drug-drug interactions, but indirect factors probably predominate. Reduced or delayed drug absorption is generally attributed, at least in part, to delayed stomach-emptying due to food. Increased absorption may also result from delayed stomach-emptying facilitating greater drug dissolution before it passes from the stomach into the small intestine. Increased bioavailability of some drugs, e.g. propranolol, metoprolol and labetalol, may be related to reduced presystemic clearance. The potential clinical implications of drug-drug and drug-food interactions must be taken into account with oral medications in order to minimise variations in systemic drug availability and hence in clinical efficacy.

Influence of kaolin-pectin suspension on steady-state plasma digoxin levels

The effect of a kaolin-pectin antidiarrheal mixture on steady-state plasma levels of orally administered digoxin in subjects receiving chronic digoxin therapy was evaluated when the antidiarrheal and the cardiac glycoside were given concomitantly and when two doses of antidiarrheal were given, one 2 hours before and the other 2 hours after digoxin. Although simultaneous administration of both products decreased peak digoxin levels by 36 per cent, 24-hour areas under the curve were reduced by only 15 per cent, indicative of a slight decrease in digoxin bioavailability. In contrast, when their times of administration were separated by 2 hours, no evidence of a drug interaction was noted. Hence, the effect of one or two doses of kaolin-pectin suspension on steady-state plasma levels of digoxin appears inconsequential in patients on chronic digoxin therapy. Saliva levels were poorly correlated with plasma levels, presumably because of complexation in the oral cavity.

In vitro interaction of quinidine with kaolin and pectin

The adsorption of quinidine onto kaolin was studied as a function of pH in aqueous solutions in which the ionic strength was adjusted to 0.1. The interaction of quinidine with pectin also was investigated in water and in phosphate buffer; the buffer pH and ionic strength were adjusted to 6.5 and 0.1, respectively. The in vitro results indicated that quinidine was adsorbed onto kaolin. At the highest concentration studied, the extent of adsorption increased from 3.64 mg of quinidine adsorbed/g of adsorbent at pH 2.4 to an average of 5.81 mg/g in the pH 5.5-7.5 range. In the presence of electrolytes, the interaction of quinidine with pectin was relatively small (3-13% bound) as compared to studies performed in water (66-90% bound). The data indicate that some quinidine may be adsorbed when this drug is administered concurrently with kaolin-pectin preparations.

A standard approach to compiling clinical pharmacokinetic data

A standard format for a Clinical Pharmacokinetic Summary is proposed. It consists of a heading, tables, notes, and references for each drug reviewed. The table presents a unified and logical set of clinically useful population pharmacokinetic parameters. They concern four major areas: absorption, distribution, elimination, and the relationship of concentration to effect. Within each major group, parameters dealing with extents and rates of processes are given. Each such parameter is really two: a population mea value (for example, average volume of distribution) and the standard deviation of individual values about this mean. The first value allows individual predictions of dosage or drug level to be made; the second allows computation of the likely proximity of subsequently observed quantities to those predictions. The table presents single consensus values for each population parameter, rather than a list of values. A procedure for computing these consensus values, and for revising them in the light of new data, or reinterpreted old data, is given. Examples of Summaries are given. The method appears applicable to a variety of drugs. We suggest our approach as a standard one for preparing Clinical Pharmacokinetic Summaries, and urge our colleagues to consider it for that purpose.

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

Twelve healthy fasting volunteers received two 0.2-mg digoxin capsules or tablets with 60 ml water, 60 ml Maalox, or 60 ml Kaopectate in a randomized, single-dose, six-way crossover study. Concentrations of digoxin in multiple plasma samples and in all urine collected during the 24 hours after each dose were determined by radioimmunoassay. Compared to the water treatment, administration of both tablets and capsules with Maalox or Kaopectate reduced the peak digoxin plasma concentrations but did not significantly influence the time of peak concentration. Neither Maalox nor Kaopectate influenced the area under the 24-hour plasma concentration--time curve for either tablets or capsules. However, 24-hour urinary recovery of digoxin from tablets tended to be reduced by Maalox and Kaopectate; this was not the case with capsules. Digoxin capsules may have an advantage over currently available tablets in clinical situations requiring digoxin coadministration with nonabsorbable gastrointestinal preparations.