Interactions between digitoxin and other drugs in vitro and in vivo

A concentration addition model for the activation of the constitutive androstane receptor by xenobiotic mixtures

The effects of contaminants are typically studied in individual exposures; however, environmental exposures are rarely from a single contaminant. Therefore, the study of chemical mixtures is important in determining the effects of xenobiotics. The constitutive androstane receptor (CAR) responds to endobiotics and xenobiotics, and in turn induces detoxification enzymes involved in their elimination. First, we compared several androgens as inverse agonists, including androgens allegedly used by Bay Area Laboratory Co-operative to enhance athletic performance. CAR inverse agonists ranked in order of potency were dihydroandrosterone (DHA) > tetrahydrogestrinone (THG) > androstanol > norbolethone. Therefore, we used DHA as an inverse agonist during transactivation assays. Next, we examined the effects of several pesticides, plasticizers, steroids, and bile acids on CAR activation. Our data demonstrates that several pesticides and plasticizers, including diethylhexylphthalate, nonylphenol, cypermethrin, and chlorpyrifos activate CAR. Both full and partial CAR activators were discovered, and EC(50) values and Hillslopes were determined for use in the concentration addition models. Concentration addition models with and without restraint values to account for partial activators were developed. Measured results from transactivation assays with a mixture of two to five chemicals indicate that the concentration addition model without restraints correctly predicts activity unless all of the chemicals in the mixture are partial activators, and then restraint values be considered. Overall, our data indicates that it is important to consider that we are exposed to a milieu of chemicals, and the efficacy of each individual chemical is not the sole factor in determining CAR's activity in mixture modeling.

Lack of effect of piroxicam on theophylline clearance in healthy volunteers

This study was conducted to determine if piroxicam alters the clearance or metabolism of theophylline in healthy adults. Six male volunteers were given aminophylline 6 mg/kg iv before and after piroxicam treatment. Piroxicam 20 mg/d po was administered for seven days. Serum samples were collected before and for 24 hours after each aminophylline dose. Theophylline serum concentrations were determined using the Abbott TDx automated immunoassay system. Theophylline and its 1,3-dimethyluric acid metabolite were measured in 24-hour urine collections. Serum theophylline concentration versus time curves were plotted and pharmacokinetic parameters determined for each subject. None of the derived parameters differed as a result of piroxicam exposure. Theophylline metabolites, clearance, half-life, and volume of distribution were compared during the baseline and piroxicam phases and were found to be similar. It was concluded that piroxicam does not alter the clearance, distribution, volume, or metabolism of theophylline in healthy adult volunteers.

Feprazone: an inducer of the P450 II B family of proteins in the rat

The ability of feprazone to induce the hepatic microsomal mixed-function oxidases was investigated in the rat, with emphasis being placed on the nature of the cytochrome P-450 family induced. Treatment with feprazone enhanced the p-hydroxylation of aniline and the dealkylations of benzphetamine and pentoxyresorufin but had no effect on the O-deethylation of ethoxyresorufin. The same treatment had no major effect on total cytochrome P-450 levels but increased the spectral interaction of metyrapone with reduced cytochrome P-450. Immunoblots employing monospecific polyclonal antibodies revealed that feprazone induces the apoprotein levels of the P450 II B, but not of the P450 I, family. It is concluded that feprazone is an inducer of the rat hepatic mixed-function oxidase system showing selectivity toward the P450 II B family.

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.

Inducing effect of feprazone on hepatic drug-metabolizing enzymes in man

The inducing effect of feprazone, a pyrazolone anti-inflammatory agent, on hepatic drug-metabolizing enzymes has been studied in healthy volunteers. The ratio of 6 beta-hydroxycortisol (6 beta-OHF) to 17-hydroxycorticosteroids (17-OHCS) in urine, used as an indicator of oxidative drug-metabolizing enzyme activity, was increased up to 1.6-times the original level after 5 days of oral treatment with feprazone 300 mg/day. This indicates that feprazone induces hepatic drug-metabolising enzymes in man as does phenylbutazone.

Interactions between anticonvulsants and other commonly prescribed drugs

Many drug interactions can be demonstrated, but only a few are so clinically significant that they necessitate adjusting drug dosages. The same drug combination may produce changes of variable extent or direction in different individuals. The reasons for this variability include genetic control of the rate and inducibility of drug metabolism, and environmental factors such as contact with chemicals. Among antimicrobial agents, chloramphenicol may cause accumulation of phenytoin (PHT) and phenobarbital (PB), and isoniazid may cause PHT, carbamazepine (CBZ), and primidone (PRM) to accumulate. Erythromycin may cause accumulation of CBZ. Among anti-ulcer agents, antacids may reduce PHT concentration while cimetidine may cause accumulation of PHT, CBZ, and diazepam (DZP). Salicylates displace strongly binding drugs such as PHT, DZP, or valproate (VPA) from the binding sites in plasma proteins, which may lead to some decline of the total plasma level with an increase in the unbound drug percentage. Conversely, anticonvulsants may influence the dosage requirements of oral anticoagulants by inducing their metabolism. Failures of oral contraceptives have been attributed to anticonvulsants in some patients. Probably the most predictable interaction that necessitates dosage adjustment is accumulation of PB caused by VPA. Intentional inhibition of PRM metabolism by nicotinamide serves as an example of attempts to utilize an interaction for improved therapeutic effect.

Metabolic interactions of phenytoin in the rat: effect of coadministration with the anticonvulsant drugs sodium valproate, sulthiame, ethosuximide or phenobarbital

Rats were administered with 50 mg/kg phenytoin (PHT), twice a day, for five consecutive days and with a second anticonvulsant drug in addition for a further five days. Analysis of 24 hr urine samples for content of unmetabolized PHT and its major metabolite 5-(p-hydroxyphenyl)-5-phenylhydantoin (pHPPH) indicates that PHT hydroxylation was significantly inhibited by sulthiame coadministration since during the test period (days 6-10) the concentrations of PHT and pHPPH in urine were significantly increased and decreased respectively. In contrast, sodium valproate, ethosuximide and phenobarbital had no significant effect on the urinary excretion pattern of PHT. These data correlate with changes in plasma and brain PHT concentrations.

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.

Drug interactions with phenytoin

Drug interactions with phenytoin are a frequent occurrence, although their clinical relevance has often been overemphasised. Probably the most important of such interactions are those resulting in inhibition of phenytoin metabolism: due to the saturable nature of phenytoin biotransformation even minor degrees of inhibition can produce disproportionate changes in both steady-state serum concentration and the magnitude of pharmacological effect. Phenytoin has marked enzyme-inducing properties and can stimulate the metabolism of many concurrently administered drugs, thereby reducing their therapeutic efficacy. Clinically important examples of such interactions include a reduction of the anticoagulant effect of dicoumarol, a decrease in the prophylactic efficacy of the contraceptive pill and failure of response to various corticosteroid agents when administered therapeutically or diagnostically. Unless complicated by additional mechanisms, plasma protein binding interactions with phenytoin are seldom of clinical significance. However, they may alter considerably the relationship between serum drug concentration and clinical response, a possibility which needs to be taken into account when interpreting serum phenytoin levels in clinical practice.

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.

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.

Interindividual differences in the pharmacokinetics of digitoxin and digoxin during long-term treatment

Patients suffering from congestive heart failure received maintenance doses of digitoxin (N = 10) or digoxin (N = 8). The plasma glycoside concentration was determined, and after a single dose of 3H-digitoxin or 3H-digoxin, the decline and excretion of radioactivity were measured over a period of 7 (digitoxin) and 3 days (digoxin). Plasma radioactivity declined with a T1/2 beta between 77 and 234 h (mean 138 h) in the case of digitoxin and with a T1/2 beta between 9.2 and 38.6 h (mean 23.5 h) for digoxin. A close correlation between T1/2 beta and excreted radioactivity and T1/2 beta and total plasma level was found for digitoxin. In 4 patients TLC of urine showed that interindividual variations in digitoxin elimination could possibly be attributed to variation in metabolism, resulting in the production of different metabolites. Predicted digitoxin plasma levels agreed well with measured values. The maintenance dose could be calculated from the total body clearance (VCl) and a presumed plasma glycoside level. The recommended technique facilitates dosage calculations in patients treated with digitoxin.

Metabolism of digitoxin in man and its modification by spironolactone

The effect of spironolactone on the metabolism of intravenously administered 3H-digitoxin (80 muCi) was investigated in eight patients. In three of them the labelled glycoside was given on a second occasion after spironolactone treatment had been discontinued for at least 65 days. Of total urinary radioactivity 79% was unaltered drug and 12% consisted of water soluble compounds. No digitoxigenin or digoxigenin and only trace amounts (less than 2 %) of digoxin and the bis- and monoglycosides of digoxigenin were found. After spironolactone total urinary radioactivity was unchanged but the fraction eliminated as unchanged digitoxin fell from 79 to 66 % and the water soluble compounds increased from 12 to 26 % (p less than 0.05). In addition spironolactone caused a 20 ( reduction in the half-life of serum radioactivity (p less than 0.01) and a 16 % reduction in the volume of distribution (p less than 0.05). Induction of hepatic enzymes by spironolactone is proposed to explain the alteration in the metabolism of digitoxin in man. Both the altered metabolic pattern and the reduction in the volume of distribution appear to contribute to the reduction in half-life.

Interactions of antiepileptic drugs

Several interactions involving antiepileptic drugs are based on changes in the rate of their metabolism and elimination, with concomitant rise or fall of plasma levels. Thus, phenobarbital generally induces the production of the DPH metabolizing enzyme, but its presence inhibits the action of that enzyme. The net result depends upon the balance between these factors in individual patients. Either a decline, a rise, or no change of the DPH plasma level may occur after the onset of administration of phenobarbital. Drugs that may cause elevation of the DPH plasma level include disulfiram, sulthiame, bishydroxycoumarin, chloramphenicol, phenyramidol, benzodiazepines, sulfamethizole, and isoniazid. Isoniazid has been shown experimentally to be a strong inhibitor of DPH metabolism. The extent of DPH plasma level elevation by INH is related to the genetic make-up of individual patients. The highest and frequently toxic DPH plasma levels were seen in very slow INH inactivators. The incidence of clinically significant interactions is not high with most drug combinations; marked changes of antiepileptic drug levels occur only in apparently susceptible individuals. The effects of interactions are not necessarily detrimental; elevation of a low ineffective level may improve seizure control. A rise to a toxic level range requires reduction of the dose of primary drug or elimination of interfering drugs. Monitoring the blood levels of anti-epileptic drugs provides the best means to anticipate interactions and to regulate the doses when multiple medications have to be used.

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.

Spectroscopic techniques in the study of protein binding. A fluorescence technique for the evaluation of the albumin binding and displacement of warfarin and warfarin-alcohol

1. The binding of racemic mixtures of warfarin and warfarin-alcohol to human serum albumin (HSA) is accompanied by an increase in the fluorescence quantum yield of these compounds. This property has been used to measure the characteristics of the binding of warfarin and warfarin-alcohol to HSA at 22 degrees C and 37 degrees C. Within the limits of the technique, no significant differences between the number of binding sites and strength of binding at the tight site at either temperature were observed. 2. The fluorescence of warfarin and warfarin-alcohol was used to label their binding site on HSA and to study the effects of other drugs on their binding. The results indicate that these two molecules are bound to the same site on HSA. 3. The validity of using changes in the fluorescence of warfarin as a measure of its displacement from HSA was investigated. Good correlations were observed between drug-induced decreases in the fluorescence of bound warfarin and displacement as measured by equilibrium dialysis. The displacement of warfarfin, as detected by fluorescence, correlates well with the increase in free warfarin resulting from addition of therapeutic drug concentrations to undiluted human serum. 4. The most potent displacing agents, by all the methods used, were iophenoxic acid, phenylbutazone and oxyphenylbutazone. The first of these is no longer used clinically, but the latter two are and have been reported to cause hypoprothrominaemia by displacing warfarin from HSA. The present study indicates that changes in the fluorescence of warfarin bound to HSA can be used to measure displacement of bound warfarin and to screen drugs that may cause clinically significant interactions by this mechanism.

Metabolic interactions among environmental chemicals and drugs

It is evident that metabolic interactions can occur among drugs, insecticides, food additives, carcinogenic hydrocarbons, and a variety of environmental chemicals. A common denominator governing these effects is the versatile nature of the liver microsomal enzymes that metabolize chemicals with diverse structures and biological activities, and the fact that these enzymes can be stimulated or inhibited by other chemicals administered simultaneously. The discovery of these particular enzymes in the 1950's laid the groundwork for the current research on metabolic interactions. Such research provides information that is helpful in the evaluation of the safety and efficacy of drugs and environmental chemicals, and suggests new directions for further research. Some examples are as follows.