Interactions between digitoxin and other drugs in man

Digitalis therapy in a 70-year-old population

In the population study "70-year-old people in Gothenburg" 14% of the probands were found to be undergoing treatment with digitalis, 6% with digoxin, 6% with digitoxin and 2% with other glycosides. A comparison between results of the interview method and those of S-digoxin analyses indicates that the interview method was acceptable. As far as can be judged from S-digoxin analyses, only about 60% of the treated patients were on a dosage considered to be effective and free from obvious risks of side-effects. Out of the 130 70-year-olds who were on digitalis treatment, 37% had obvious symptoms of heart disease requiring such treatment, 34% lacked symptoms of arrhythmia and/or congestive failure but had heart volumes larger than those used as reference values in younger age groups, and 29% had no symptoms indicating digitalis treatment. At least 13% of the population had indications for digitalis therapy and about 75% of those apparently needing digitalis were on such treatment. Thus both over- and underdiagnosis of heart disease requiring digitalis therapy were common in this age group.

Antiepileptic drugs. A review of clinically significant drug interactions

Approximately 20 to 30% of patients with active intractable epilepsy are commonly treated with polytherapy antiepileptic drug regimens, and these patients may experience complicated drug interactions. Furthermore, because of the long term nature of treatment, the possibility of drug interactions with drugs used for the treatment of concomitant disease is high. Classically, clinically significant drug interactions, both pharmacokinetic and pharmacodynamic, have been considered to be detrimental to the patient, necessitating dosage adjustment. However, this need not always be the case. With the introduction of new drugs (e.g. vigabatrin and lamotrigine) with known mechanisms of action, the possibility exists that these can be used synergistically. The most commonly observed clinically significant pharmacokinetic interactions can be attributed to interactions at the metabolic and serum protein binding levels. The best known examples relate to induction (e.g. phenobarbital, phenytoin, carbamazepine and primidone) or inhibition [e.g. valproic acid (sodium valproate)] of hepatic monoxygenase enzymes. The extent and direction of interactions between the different antiepileptic drugs are varied and unpredictable. Interactions in which the metabolism of phenobarbital, phenytoin or carbamazepine is inhibited are particularly important since these are commonly associated with toxicity. Some inhibitory drugs include macrolide antibiotics, chloramphenicol, cimetidine, isoniazid and numerous sulphonamides. A reduction in efficacy of antibiotic, cardiovascular, corticosteroid, oral anticoagulant and oral contraceptive drugs occurs during combination therapy with enzyme-inducing antiepileptic drugs. Discontinuation of the enzyme inducer or inhibitor will influence the concentrations of the remaining drug(s) and may necessitate dosage readjustment.

Enzyme induction and inhibition

The rate and extent of drug metabolism significantly influences drug effect. Enzyme induction by increasing the metabolism of drugs may result in important drug interactions. Other implications of enzyme induction include alterations in the metabolism of endogenous substrates, vitamins and activity of extrahepatic enzyme systems. Similarly a wide range of drugs may produce clinically significant drug interactions following enzyme inhibition. Assessment of enzyme induction and inhibition in man involves diverse methods including the use of model drugs.

Adverse effects of hypolipidaemic drugs

Cholestyramine, colestipol, clofibrate, gemfibrozil, nicotinic acid (niacin), probucol, neomycin, and dextrothyroxine are the most commonly used drugs in the treatment of hyperlipoproteinaemic disorders. While adverse reaction data are available for all of them, definitive data regarding the frequency and severity of potential adverse effects from well-controlled trials using large numbers of patients (greater than 1000) are available only for cholestyramine, clofibrate, nicotinic acid and dextrothyroxine. In adult patients treated with cholestyramine, gastrointestinal complaints, especially constipation, abdominal pain and unpalatability are most frequently observed. Continued administration along with dietary manipulation (e.g. addition of dietary fibre) and/or stool softeners results in diminished complaints during long term therapy. Large doses of cholestyramine (greater than 32 g/day) may be associated with malabsorption of fat-soluble vitamins. Most significantly, osteomalacia and, on rare occasions, haemorrhagic diathesis are reported with cholestyramine impairment of vitamin D and vitamin K absorption, respectively. Paediatric patients have been reported to experience hyperchloraemic metabolic acidosis or gastrointestinal obstruction. Concurrent administration of acidic drugs may result in their reduced bioavailability. Serious adverse reactions to clofibrate will probably limit its role in the future. Of particular concern are ventricular arrhythmias, induction of cholelithiasis and cholecystitis, and the potential for promoting gastrointestinal malignancy which far outweigh the reported benefits in preventing new or recurrent myocardial infarction, cardiovascular death and overall death. Patients with renal disease are particularly prone to myositis, secondary to alterations in protein binding and impaired renal excretion of clofibrate. Drug interactions with coumarin anticoagulants and sulphonylurea compounds may produce bleeding episodes and hypoglycaemia, respectively. Nicotinic acid produces frequent adverse effects, but they are usually not serious, tend to decrease with time, and can be managed easily. Dermal and gastrointestinal reactions are most common. Truncal and facial flushing are reported in 90 to 100% of treated patients in large clinical trials. Significant elevations of liver enzymes, serum glucose, and serum uric acid are occasionally seen with nicotinic acid therapy. Liver enzyme elevations are more common in patients given large dosage increases over short periods of time, and in patients treated with sustained release formulations.(ABSTRACT TRUNCATED AT 400 WORDS)

Drug-drug and drug-disease interactions with nonsteroidal anti-inflammatory drugs

Nonsteroidal anti-inflammatory drugs may cause a number of drug interactions. They can displace other drugs from serum proteins, and some can affect the metabolism or decrease the renal elimination of other drugs. In addition, they can attenuate the pharmacologic effect of other drugs, such as diuretic and antihypertensive agents, without affecting their disposition. Lastly, many disease states and aging can affect the handling of nonsteroidal anti-inflammatory drugs, mandating dose adjustment of some of these agents in certain clinical conditions. Some drugs may require more of these adjustments than others.

Pharmacokinetics, bioavailability and serum levels of cardiac glycosides

Digoxin, the cardiac glycoside most frequently used in clinical practice in the United States, can be given orally or intravenously and has an excretory half-life of 36 to 48 hours in patients with serum creatinine and blood urea nitrogen values in the normal range. Since the drug is excreted predominantly by the kidney, the half-life is prolonged progressively with diminishing renal function, reaching about 5 days on average in patients who are essentially anephric. Serum protein binding of digoxin is only about 20%, and differs markedly in this regard from that of digitoxin, which is 97% bound by serum albumin at usual therapeutic levels. Digitoxin is nearly completely absorbed from the normal gastrointestinal tract and has a half-life averaging 5 to 6 days in patients receiving usual doses irrespective of renal function. The bioavailability of digoxin is appreciably less than that of digitoxin, averaging about two-thirds to three-fourths of the equivalent dose given intravenously in the case of currently available tablet formulations. Recent studies have shown that gut flora of about 10% of patients reduce digoxin to a less bioactive dihydro derivative. This process is sensitive to antibiotic administration, creating the potential for important interactions among drugs. Serum or plasma concentrations of digitalis glycosides can be measured by radioimmunoassay methods that are now widely available, but knowledge of serum levels does not substitute for a sound working knowledge of the clinical pharmacology of the preparation used and careful patient follow-up.

Acute digitalis toxicity during TURP

Water entrainment into opened prostate venous sinuses during transurethral resection of the prostate (TURP) may lead to dilution of serum electrolytes. Dilutional hypokalemia may precipitate digitalis toxicity in the digitalized patient. Successful resuscitation of such a patient is reported.

Cholestyramine resin in the treatment of digitoxin toxicity

Cholestyramine, an ion-exchange resin, has been used in the treatment of digitoxin toxicity. A case report describing the use of a therapeutic regimen of this resin in a toxic patient is presented. Although cholestyramine aided in binding digitoxin in the intestine, its effect was not so pronounced as in past studies. Further investigation of this agent as adjunctive therapy in digitoxin toxicity is warranted.

Pharmacokinetic interactions with antiepileptic drugs

A large number of pharmacokinetic interactions with antiepileptic drugs have been reported in recent years. Among the interactions affecting the disposition of anticonvulsants, the most important are probably those resulting in inhibition of the metabolism of phenytoin, phenobarbitone and carbamazepine. Drugs which have been shown to inhibit the metabolism of these anticonvulsants and to precipitate clinical signs of intoxication in epileptic patients include sulthiame, valproic acid, chloramphenicol, certain sulphonamides, phenylbutazone, isoniazid and propoxyphene. Interactions affecting the plasma protein binding of antiepileptic drugs are less likely to cause long-lasting alterations in response, but they are important because they change the relationship between serum drug concentrations and clinical effect. Anticonvulsant agents may induce important alterations in the pharmacokinetics of other drugs. Phenytoin and phenobarbitone may decrease the gastrointestinal absorption of frusemide and griseofulvin, respectively. Many of the drugs used in the treatment of the adult epilepsies, including phenytoin, phenobarbitone, primidone and carbamazepine, are potent inducers of the hepatic microsomal enzymes. This results in an increased rate of metabolism and decreased clinical efficacy of a number of drugs, including dicoumarol, steroid oral contraceptives, metyrapone, glucocorticoid agents, doxycycline, quinidine and vitamin D.

Acute, massive poisoning with digitoxin: report of seven cases and discussion of treatment

Severe digitoxin poisoning in seven patients is reported. Doses taken varied from 2 to 20 mg, and maximal plasma concentrations of digitoxin from 50 to 237 nmol/L. One patient died from ventricular fibrillation, and the course in another was considerably protracted due to severe complications. The course in all patients was more dependent on underlying heart disease than on the plasma digitoxin concentration. Based on our own experiences and survey of the literature the following treatment is proposed: Gastric aspiration and lavage followed by instillation of activated charcoal should even be performed many hours after drug intake. In order to interrupt the enterohepatic circulation of digitoxin, repeated doses of charcoal should be given. Charcoal is preferable to cholestyramine because of its better tolerability. Ventricular arrhythmias should not be treated unless they are serious, because most antiarrhythmic drugs may further impede the AV-conduction. Phenytoin is the drug of choice, because the AV-conduction is less affected or even improved, and because the metabolism of digitoxin is accelerated. Conduction disturbances with bradycardia are frequently seen and may occur suddenly. Prophylactic introduction of a transvenous pacing catheter is therefore recommended as a routine procedure.

Effect of phenobarbital on serum digoxin concentration during administration of liposome-entrapped digoxin: an examination of possible drug interactions

The effect of concomitant administration in normal dogs of phenobarbital with either oral free digoxin, oral liposome-entrapped digoxin were examined. Phenobarbital induced a transient rise in serum digoxin concentrations during concomitant administration of oral free digoxin. Phenobarbital did not affect serum concentrations of either oral or intravenous liposome-entrapped digoxin. The results suggest that liposome-entrapped digoxin may be protected against transient fluctuations in serum concentrations during concomitant administration of phenobarbital. The protective liposome encapsulation of digoxin may prevent digoxin toxicosis in dogs induced by elevations in serum digoxin concentrations during concomitant administration of phenobarbital.

Clinical implications of enzyme induction and enzyme inhibition

The pharmacological effect of a drug is partly dependent upon its concentration at its site of action, which in turn is partly dependent upon its rate of elimination. The rate of elimination of many lipophilic drugs is governed by the activity of the hepatic microsomal mixed-function oxidases. Consequently any alteration in the activity of these enzymes may result in a modification of drug action. A wide range of chemically unrelated substances may stimulate the activity of the mixed-function oxidases by enzyme induction. The drugs most frequently encountered as enzyme-inducing agents in man are barbiturates, rifampicin and phenytoin. Enhancement of drug metabolism by ethanol, tobacco smoking and diet may also involve enzyme induction. Enzyme induction is normally associated with a reduction in the drug efficacy but may also alter the toxicity of certain substances. Enzyme induction has been assessed in man by measuring changes in the pharmacokinetics of a marker drug, or changes in the disposition of endogenous compounds such as gamma-glutamyltranspeptidase, D-glucaric acid and 6beta-hydroxycortisol. The therapeutic problems associated with enzyme inhibition have received much less attention than those associated with enzyme induction. The effect on the rate of elimination of a particular drug will depend upon the fraction of the dose that is normally metabolised by the inhibited enzyme and on the affinity of the enzyme for the drug and the inhibitor. An alteration in the dosage schedule is usually only necessary for drugs with a small therapeutic ratio.

Effects of phenobarbital on digitoxin metabolism in guinea-pig liver slices

1. [3H]Digitoxin was incubated for 2-5 h with liver slices from control and phenobarbital-pretreated guinea-pigs. Metabolites were the same in both groups. 2. After 5 h incubation, a 6-fold increase in unmetabolized digitoxin was found in the phenobarbital-pretreated group compared with controls. 3. The major metabolite extracted by chloroform is formed by hydroxylation at an unknown position of the cardenolide nucleus, which is not the 12 beta-hydroxy position. Only half the amount of this metabolite was formed in livers of pretreated animals, compared to controls, in 2 h, after 5 h incubation similar amounts occurred in both groups. 4. Water-soluble metabolites decreased 2-fold in phenobarbital-pretreated animals at 2 and 5 h of incubation.

Studies on digitalis. XIII. A prospective study of 649 patients on maintenance treatment with digitoxin

In a prospective study of digitalis intoxication in 649 patients on maintenance treatment with digitoxin a low incidence of digitalis toxicity was found, namely, 5.8 per cent. This is mainly due to a more careful use to digitalis glycosides. It is especially important to reduce the dose of digitoxin in the liver and partly excreted metabolized in the liver and partly excreted through the kidneys as metabolities. Serum half-time of digitoxin is shortened in patients with impaired renal function. Patients with reduced renal function may be treated with digitoxin in the same doses as individuals with normal renal function. This is in contrast to patients treated with digoxin. Digitoxin should therefore be the cardiac glycoside of choice in treatment of patients with renal failure. Digitoxin is further rapidly eliminated in patients with reduced liver function in spite of its extensive hepatic metabolism. In this study extracardia symptoms were found equally often as cardiac signs of toxicity. Patients intoxicated usually had several symptoms and signs of toxicity at the same time. The specificity of commonly used symptoms and signs a digitalis intoxication is very low. In this study atrial tachycardia with block, which has been considered to be an important cardiotoxic arrhythmia, very seldom was found in digitalis intoxication. There is an overlap in digitalis serum concentration between toxic and nontoxic patients. The diagnosis of toxicity was made on clinical grounds. Most of the intoxicated patients had high serum concentrations, but some had concentrations in the normal or low range. Apart from being a guide to the diagnosis of digitalis intoxication, serum digitalis levels may further be a guide to underdigitalization of cardiac patients, especially patients in sinus rhythm.

Interactions of CNS drugs--hypnotics and sedatives

The physicochemical properties of the barbiturates and a brief review of microsomal enzyme induction introduces this literature review of the interactions of sedative and hypnotic drugs. Food has been shown to delay absorption of barbiturates; barbiturates, in turn, may interfere with the absorption of griseofulvin, dicumarol, and folic acid. Barbiturate-produced enzyme induction may result in interactions with the oral anticoagulants, the anticonvulsants, vitamin D, bilirubin, digitoxin, doxycycline, and perhaps other drugs. The problem of additive central nervous system depression and the relative lack of documented evidence is considered. Finally, the interactions of nonbarbiturate hypnotics, such as glutethimide, chloral hydrate, and others are reviewed.

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.

Cardiovascular drug interactions

Drug interactions may be responsible for certain changes in therapeutic response and toxicity of cardiac drugs. Interactions occur at the sites of drug absorption and elimination as well as at the receptor sites in the pacemaker cells, specialized conducting tissue, and myocardium. Studies of the kinetics of cardiac drugs are being applied clinically in an effort to reduce the danger of adverse drug interactions in heart patients.