Pharmacokinetic interaction between theophylline and erythromycin

Reduced theophylline clearance due to hepatic congestion secondary to right heart failure - A population pharmacokinetic study

Theophylline, a beneficial drug with bronchodilatory and anti-inflammatory effects, is used for the treatment of respiratory diseases. Pulmonary (PC) and hepatic congestion (HC) are secondary to the development of left- and right-sided heart failure (HF), respectively. This study aimed to evaluate the effects of PC and HC on theophylline clearance (CL) by population pharmacokinetic (PPK) analysis with consideration of the severity of HF assessed by the New York Heart Association (NYHA) functional classification. We obtained 710 minimum steady-state concentrations from 201 Japanese bronchial asthma patients with and without HF. PPK analysis was performed by NONMEM. In the analysis, the left ventricular ejection fraction, smoking (SMK), clarithromycin (CAM), sex, and age were also considered as covariates. The final model of apparent theophylline clearance (CL/F) was as follows: CL/F (L/hr/kg) = 0.0465 × 1.40^SMK × 0.870^CAM × 0.863^{HC(+)NYHA II} × 0.634^{HC(+)NYHA III} × 0.586^{HC(-)NYHA IV} × 0.467^{HC(+)NYHA IV}. SMK is a well-known factor that markedly enhances theophylline clearance through the induction of CYP1A enzymes, while CAM has been reported to inhibit CYP3A4. The final model indicates that HF patients with HC show reduced clearance of theophylline depending on the severity of HF. In this study, no effects of PC were observed.

Differences in Theophylline Clearance Between Patients With Chronic Hepatitis and Those With Liver Cirrhosis

BACKGROUND

Theophylline, a xanthine derivative drug, is used for the treatment of respiratory diseases, such as asthma, and is primarily eliminated by hepatic metabolism. There is marked interindividual variability in theophylline clearance. Therefore, the aim of this study was to evaluate the influence of chronic hepatitis (CH), liver cirrhosis (LC), and other covariates on theophylline clearance by population pharmacokinetic (PPK) analysis.

METHODS

The authors retrospectively obtained 496 trough concentrations of theophylline at steady state from 226 adult patients with bronchial asthma. The liver functions of the patients were classified into 3 categories: normal hepatic function, CH, and LC. The PPK analysis was performed using the NONMEM program. CH, LC, age, smoking status, coadministration of clarithromycin (CAM), and sex were considered as covariates that affected theophylline clearance.

RESULTS

Theophylline clearance (CL/F per kg) was significantly influenced by CH, LC, smoking, and CAM. The final model of theophylline clearance was as follows: CL/F (L/h·kg) = 0.0484 × 1.40 × 0.861 × 0.889 × 0.557. Smoking is a well-known factor that markedly enhances CL/F through the induction of CYP1A enzymes, whereas CAM has been reported to inhibit CYP3A4. The final model for hepatic function showed that CL/F in CH and LC patients was 0.043 and 0.027 L/h/kg, respectively, and it was lower than that in patients with normal hepatic function. As theophylline clearance depends on intrinsic hepatic clearance, lower CL/F in patients with LC than in those with CH may be due to a decrease in the metabolic enzymatic capability of LC patients.

CONCLUSIONS

Differences exist in theophylline clearance between CH and LC patients as per the PPK analysis.

SIME: synthetic insight-based macrolide enumerator to generate the V1B library of 1 billion macrolides

We report on a new cheminformatics enumeration technology—SIME, synthetic insight-based macrolide enumerator—a new and improved software technology. SIME can enumerate fully assembled macrolides with synthetic feasibility by utilizing the constitutional and structural knowledge extracted from biosynthetic aspects of macrolides. Taken into account by the software are key information such as positions in macrolide structures at which chemical components can be inserted, and the types of structural motifs and sugars of interest that can be synthesized and incorporated at those positions. Additionally, we report on the chemical distribution analysis of the newly SIME-generated V1B (virtual 1 billion) library of macrolides. Those compounds were built based on the core of the Erythromycin structure, 13 structural motifs and a library of sugars derived from eighteen bioactive macrolides. This new enumeration technology can be coupled with cheminformatics approaches such as QSAR modeling and molecular docking to aid in drug discovery for rational designing of next generation macrolide therapeutics with desirable pharmacokinetic properties.

Cheminformatics-based enumeration and analysis of large libraries of macrolide scaffolds

We report on the development of a cheminformatics enumeration technology and the analysis of a resulting large dataset of virtual macrolide scaffolds. Although macrolides have been shown to have valuable biological properties, there is no ready-to-screen virtual library of diverse macrolides in the public domain. Conducting molecular modeling (especially virtual screening) of these complex molecules is highly relevant as the organic synthesis of these compounds, when feasible, typically requires many synthetic steps, and thus dramatically slows the discovery of new bioactive macrolides. Herein, we introduce a cheminformatics approach and associated software that allows for designing and generating libraries of virtual macrocycle/macrolide scaffolds with user-defined constitutional and structural constraints (e.g., types and numbers of structural motifs to be included in the macrocycle, ring size, maximum number of compounds generated). To study the chemical diversity of such generated molecules, we enumerated V1M (Virtual 1 million Macrolide scaffolds) library, each containing twelve common structural motifs. For each macrolide scaffold, we calculated several key properties, such as molecular weight, hydrogen bond donors/acceptors, topological polar surface area. In this study, we discuss (1) the initial concept and current features of our PKS (polyketides) Enumerator software, (2) the chemical diversity and distribution of structural motifs in V1M library, and (3) the unique opportunities for future virtual screening of such enumerated ensembles of macrolides. Importantly, V1M is provided in the Supplementary Material of this paper allowing other researchers to conduct any type of molecular modeling and virtual screening studies. Therefore, this technology for enumerating extremely large libraries of macrolide scaffolds could hold a unique potential in the field of computational chemistry and drug discovery for rational designing of new antibiotics and anti-cancer agents.

Drug interactions in dentistry

BACKGROUND

The hepatic and intestinal cytochrome, or CY, P450 enzyme system is responsible for the biotransformation of a multitude of drugs. Certain medications used in dentistry can act as substrates, inducers or inhibitors of this system.

METHODS

The authors conducted a MEDLINE search of articles appearing between 1976 and the present using the keywords "drug interactions" and "cytochrome P450," and reviewed reports involving dental therapeutic agents using PubMed links from an Indiana University CYP450 drug interaction table on the World Wide Web.

RESULTS

The antibiotics erythromycin and clarithromycin are potent inhibitors of CYP3A4 and can increase blood levels and toxicity of CYP3A4 substrates. Likewise, quinolone antibiotics such as ciprofloxacin inhibit the metabolism of CYP1A2 substrates. Other dental therapeutic agents are substrates for CYP2C9 (celecoxib, ibuprofen and naproxen), CYP2D6 (codeine and tramadol), CYP3A4 (methylprednisolone) and CYP2E1 (acetaminophen). Because codeine and tramadol are prodrugs, inhibition of their metabolism can lead to a diminution of their analgesic effects. While inducers of acetaminophen metabolism, including alcohol, theoretically can increase the proportion of it that is biotransformed into a potentially hepatotoxic metabolite, recent research suggests that concomitant alcohol intake does not increase the hepatotoxic potential of therapeutic doses of acetaminophen.

CONCLUSIONS

A number of clinically significant drug interactions can arise with dental therapeutic agents that act as substrates or inhibitors of the CYP450 system. Clinical Implications. As polypharmacy continues to increase, the likelihood of adverse drug interactions in dentistry will increase as well. Ensuring that patients' medical histories are up to date and acquiring knowledge of the various substrates, inducers and inhibitors of the CYP450 system will help practitioners avoid potentially serious adverse drug interactions.

Steady-state pharmacokinetics of theophylline in COPD patients treated with dirithromycin

Steady-state theophylline pharmacokinetic parameters were studied in a panel of 14 patients with chronic obstructive pulmonary disease (COPD). Pharmacokinetic parameters were evaluated before, during, and after a 10-day regimen of the macrolide antibiotic, dirithromycin. The addition of dirithromycin (500 mg orally once daily at 7:00 AM) to a sustained-release theophylline dosing regimen (every 12 hours) elicited small changes in the steady-state pharmacokinetics of theophylline, which were not statistically significant. Mean steady-state plasma theophylline trough concentrations (Css,min) were invariant before, during, and after dirithromycin treatment. Mean average steady-state plasma theophylline concentrations (Cav) declined by 7% during dirithromycin treatment (NS), and mean peak plasma concentrations (Css,max) declined by 12% (NS). Theophylline clearance (CL/F) also remained relatively unchanged during dirithromycin treatment exhibiting an increase of only 11% (NS). Dirithromycin treatment does not significantly affect the steady-state pharmacokinetics of theophylline, and its use in COPD patients is not likely to modify treatment outcomes with theophylline.

Incidence and cost of hospital admissions secondary to drug interactions involving theophylline

OBJECTIVE

To determine the incidence and cost of hospital admissions for theophylline toxicity, which occurred as a result of the concurrent use of one of the following medications: cimetidine, erythromycin, or ciprofloxacin.

DESIGN

Retrospective chart review (18 months, between June 1989 and November 1990).

SETTING

A Department of Veterans Affairs Medical Center.

PARTICIPANTS

All patients who were receiving theophylline chronically (913 patients) and also had a prescription for cimetidine (124 patients with 140 treatment courses), erythromycin (66 patients with 93 treatment courses), or ciprofloxacin (39 patients with 59 treatment courses) dispensed.

INTERVENTIONS

Each patient's medical record was reviewed to identify hospital admissions within 30 days following the dispensing of the interacting drug.

MAIN OUTCOME MEASURES

Admissions were considered to be related to theophylline toxicity if appropriate signs and symptoms were present and the theophylline concentration was above 20 micrograms/mL or had increased significantly from the concentration obtained prior to introduction of the interacting drug.

RESULTS

One patient who received cimetidine and one who received ciprofloxacin were admitted for theophylline toxicity (2 of 292 potential interactions, 0.81 percent). Admissions were for 16 and 13 days, respectively, and total costs for the two admissions were $12,864.22 or $44.00, respectively, per potential interaction. The entire admission was not for theophylline toxicity; it appeared that iatrogenic factors contributed to the duration.

CONCLUSIONS

The incidence of hospital admissions secondary to theophylline drug interactions with cimetidine, ciprofloxacin, or erythromycin is low, but the admissions represent considerable expense, even when distributed among all patients at risk for the interactions.

Pharmacokinetic drug interactions of macrolides

The macrolide antibiotics include natural members, prodrugs and semisynthetic derivatives. These drugs are indicated in a variety of infections and are often combined with other drug therapies, thus creating the potential for pharmacokinetic interactions. Macrolides can both inhibit drug metabolism in the liver by complex formation and inactivation of microsomal drug oxidising enzymes and also interfere with microorganisms of the enteric flora through their antibiotic effects. Over the past 20 years, a number of reports have incriminated macrolides as a potential source of clinically severe drug interactions. However, differences have been found between the various macrolides in this regard and not all macrolides are responsible for drug interactions. With the recent advent of many semisynthetic macrolide antibiotics it is now evident that they may be classified into 3 different groups in causing drug interactions. The first group (e.g. troleandomycin, erythromycins) are those prone to forming nitrosoalkanes and the consequent formation of inactive cytochrome P450-metabolite complexes. The second group (e.g. josamycin, flurithromycin, roxithromycin, clarithromycin, miocamycin and midecamycin) form complexes to a lesser extent and rarely produce drug interactions. The last group (e.g. spiramycin, rokitamycin, dirithromycin and azithromycin) do not inactivate cytochrome P450 and are unable to modify the pharmacokinetics of other compounds. It appears that 2 structural factors are important for a macrolide antibiotic to lead to the induction of cytochrome P450 and the formation in vivo or in vitro of an inhibitory cytochrome P450-iron-nitrosoalkane metabolite complex: the presence in the macrolide molecules of a non-hindered readily accessible N-dimethylamino group and the hydrophobic character of the drug. Troleandomycin ranks first as a potent inhibitor of microsomal liver enzymes, causing a significant decrease of the metabolism of methylprednisolone, theophylline, carbamazepine, phenazone (antipyrine) and triazolam. Troleandomycin can cause ergotism in patients receiving ergot alkaloids and cholestatic jaundice in those taking oral contraceptives. Erythromycin and its different prodrugs appear to be less potent inhibitors of drug metabolism. Case reports and controlled studies have, however, shown that erythromycins may interact with theophylline, carbamazepine, methylprednisolone, warfarin, cyclosporin, triazolam, midazolam, alfentanil, disopyramide and bromocriptine, decreasing drug clearance. The bioavailability of digoxin appears also to be increased by erythromycin in patients excreting high amounts of reduced digoxin metabolites, probably due to destruction of enteric flora responsible for the formation of these compounds. These incriminated macrolide antibiotics should not be administered concomitantly with other drugs known to be affected metabolically by them, or at the very least, combined administration should be carried out only with careful patient monitoring.(ABSTRACT TRUNCATED AT 400 WORDS)

Double-blind, randomized comparative study of the antihypertensive effect of nicardipine slow-release and nifedipine slow-release in hypertensive patients with coronary heart disease

The main aim of this study was to investigate whether a new slow-release formation of nicardipine can control hypertension and whether its antihypertensive effect is manifest throughout the dose intervals. In a randomized, double-blind placebo-controlled study, the antihypertensive effect of two calcium antagonists (Type II) was investigated in two independent groups of hypertensive patients with coronary artery disease. One group of patients received 40 mg nicardipine slow-release b.i.d. and the other 20 mg nifedipine slow-release b.i.d. The effect of the active drugs on blood pressure (BP), heart rate, and hemodynamics was compared with placebo within each group. In addition, a group comparison was made to establish whether nicardipine had any advantage over nifedipine. Twenty-eight patients [27 female, 1 male; 55 (41-72) years old], 18 with previous myocardial infarction (MI) entered the study (nicardipine, 15 patients; nifedipine, 13 patients). A placebo period of 3 days was followed by a 13-day drug treatment period. From the first to last trial day, BP and heart rate were measured three times daily. At the end of the placebo and the active drug periods, the following measurements were carried out: ambulatory BP monitoring by half-hourly recording for 12.5 hours with the Remler system, ergometric tests with ECG, and right heart catheterization. Both drugs lowered the BP at rest, during exercise, and during usual daily activities. The antihypertensive effect of nicardipine was significant for the daytime mean arterial BP (MAP) and for systolic BP and diastolic BP at various stages of the exercise tests. The difference between the effect of nicardipine and nifedipine was not significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic interactions between theophylline and other medication (Part II)

Part I of this article, which appeared in the previous issue of the Journal, covered the effects or lack of effects on theophylline clearance of sympathomimetics, corticosteroids, antihistamines and other antiallergy drugs, antimicrobial agents, phenytoin, carbamazepine, barbiturates, antacids and activated charcoal. In Part II, this discussion is extended to the effects of other agents. Overall summaries, both textual and tabular, appear in Part I.

Changes in the steady-state pharmacokinetics of theophylline during treatment with dirithromycin

The steady-state plasma concentrations and pharmacokinetic characteristics of theophylline were studied during intermittent treatment with dirithromycin. The addition of dirithromycin (500 mg orally once daily at 7:00 AM) to a sustained-release theophylline dosing regimen (200 mg every 12 hours) elicited small changes in the steady-state pharmacokinetics of theophylline. Mean steady-state plasma theophylline trough concentrations (Cmin) were invariant before, during, and after dirithromycin treatment; however, mean average steady-state plasma theophylline concentrations (Cav) declined by 18% during dirithromycin treatment (P less than .05), and mean peak plasma concentrations (Css,max) declined by 26% (P less than .01). Theophylline clearance (CL/F) exhibited an increase of comparable magnitude during dirithromycin treatment, although the increase in CL/F was not statistically significant (.05 less than P less than .1). Dirithromycin treatment alters the steady-state pharmacokinetics of theophylline; however, the magnitude of the changes is small and is not likely to modify treatment outcomes.

Theophylline toxicity due to drug interaction

Acute theophylline toxicity is usually due to overdose. However, it may also be brought about by interference of its metabolism secondary to the concurrent administration of other drugs. Erythromycin is important in this regard as illustrated in the following case of a 16-year-old girl who developed theophylline toxicity while on therapy with both of these drugs. As well as the potential for theophylline toxicity, coadministration of these two drugs may result in subtherapeutic serum erythromycin concentrations. Thus, if at all possible, this practice should be avoided. If unavoidable, then serial serum theophylline concentrations should be monitored. The occurrence of this interaction is unpredictable. Thus the previous recommendation of decreasing the theophylline dosage by 25% to prevent toxicity during erythromycin therapy is irrational and should be avoided. Drug interactions should be considered in the differential diagnosis of theophylline toxicity.

[Pharmacokinetic interactions of macrolides and theophylline]

Interaction between macrolides and theophylline has given rise to numerous studies which are reviewed in this paper. The results of these studies are often conflicting. When such interactions occur, the question is whether they have clinical significance.

The theophylline-erythromycin interaction

Since its publication in 1976, the original report of an interaction between erythromycin and theophylline by Cummin, Kozak, and Gillman has generated considerable interest and controversy. Many studies with considerably different designs have been performed to address this question. Those studies that most closely simulate the clinical setting suggest that a 7- to 10-day course of concurrent theophylline and erythromycin therapy will result in variable changes in theophylline clearance. It may be that as many as 25% of patients, especially when maintained with serum theophylline concentrations at the upper portion of the therapeutic range, display elevations in serum theophylline concentrations that might lead to clinical symptoms of theophylline toxicity. There has been a suggestion, based on the mean changes in several studies, that the interaction may lead to a 25% increase in serum theophylline concentrations; however, it is clear that there may be a much larger increase in some patients. This toxicity can be anticipated and avoided if careful attention is paid to monitoring the serum theophylline concentrations of such high-risk patients when erythromycin therapy is contemplated as an addition to theophylline therapy. Other macrolide antibiotics may display interactions with theophylline, which may be due in part to the ability of the various antibiotics to form complexes with isoenzymes of the cytochromes P-450. The growing impression of the importance of mycoplasma in asthmatics and the introduction of new macrolides onto the market make the appreciation of this possible interaction of extreme importance to primary care and chest physicians.

The interaction between chronic oral slow-release theophylline and single-dose intravenous erythromycin

Eight volunteers were each given 300 mg of erythromycin lactobionate by i.v. infusion over 15 min in the presence and absence of chronic dosing with slow-release theophylline. Pharmacokinetic profiles were obtained for theophylline in the presence and absence of erythromycin and for erythromycin in the presence and absence of theophylline. A very small, clinically unimportant, but statistically significant increase occurred in mean (+/- S.E.M.) serum theophylline concentration from 4.9 +/- 0.3 mg/l to 5.2 +/- 0.3 mg/l in the presence of erythromycin (P = less than 0.01). The theophylline pharmacokinetic parameters did not change significantly. The only changes in erythromycin pharmacokinetics were an increase in the renal excretion (0-12 h) from 5.5 +/- 4.0 mg to 11.2 +/- 6.0 mg (P less than 0.03) and an increase in renal clearance (0-2 h) from 9.0 +/- 6.0 ml/min to 21.6 +/- 15 ml/min (P less than 0.05) in the presence of theophylline.

The interaction of erythromycin with theophylline

We have studied the interaction of erythromycin with theophylline. We gave ten healthy volunteers theophylline as an intravenous loading dose (5 mg X kg-1) over 1 h, followed by a maintenance infusion (0.5 mg X kg-1 X h-1) for 5 h. A second infusion of theophylline was given after 9 days of treatment with 1 g erythromycin base daily, and the concentrations of theophylline were determined during the infusion periods. The concentrations of erythromycin were measured for 8 h, after one week of treatment, and also after the last erythromycin dose, simultaneously with the second theophylline infusion. Concentrations within the therapeutic range were obtained with both drugs. A significant increase in both AUC and mean plasma concentrations of theophylline was seen during erythromycin treatment. The plasma clearance of theophylline was reduced in 9 of the 10 subjects. Renal clearance increased correspondingly, but the change was not statistically significant. Serum concentrations of erythromycin fell significantly, by more than 30%, with concurrent theophylline medication. We conclude that an interaction between theophylline and erythromycin, affecting both drugs, can be shown with concentrations of the drugs within the therapeutic range.

Theophylline poisoning. Pharmacological considerations and clinical management

The recent marketing of slow release preparations of theophylline and new indications for the use of the drug have resulted in a marked increase in the sale of theophylline products. This phenomenon combined with the drug's highly variable pharmacokinetics has led to an increase in the number of theophylline intoxications. The morbidity and mortality rates associated with theophylline intoxication are significant. Therefore it is essential that clinicians are aware of the pathophysiology, clinical presentation and treatment of this poisoning. Theophylline intoxication mainly affects the gastrointestinal, cardiovascular and central nervous systems. Signs and symptoms range from mild gastrointestinal upset to serious central nervous system manifestations such as seizures, a symptom often associated with a bad prognosis. Theophylline serum concentrations are very useful for making decisions regarding treatment. However, their interpretation should take into account several factors such as the age of the patient and the type of intoxication (acute versus chronic). Prevention of gastrointestinal absorption should be the principal objective of treatment of an oral theophylline poisoning. The repetitive administration of activated charcoal not only prevents theophylline absorption but also increases its rate of Once absorbed, external methods such as haemodialysis and haemoperfusion can significantly accelerate the elimination of the drug from the body. Finally, the rapid suppression of seizures and cardiac arrhythmias are essential to prevent severe neurological sequelae and death. Since theophylline intoxication can be potentially life-threatening, its administration should be monitored with regular measurements of the serum theophylline concentration, especially in the very young and the very old.

The interaction between i.v. theophylline and chronic oral dosing with slow release nifedipine in volunteers

Eight healthy volunteers received a 5 min i.v. infusion of lysine theophylline, equivalent to 197 mg anhydrous theophylline, both before (day 1) and during (day 5) steady state chronic oral dosing with slow release nifedipine 20 mg 12 hourly. A theophylline pharmacokinetic profile was performed on day 1 and day 5 and a nifedipine pharmacokinetic profile was performed on day 4 and day 5. The greatest difference in serum theophylline concentrations was seen at the first sampling time (5 min after completion of the infusion) with a mean concentration of 9.9 mg l-1 during nifedipine administration and 14.6 mg l-1 with theophylline alone. Thereafter, the difference fell to approximately 1 mg l-1 until 6 h when they became almost identical. Repeated measures analysis of variance using the theophylline serum concentrations at each of ten time points over 8 h as the repeated measures showed a small but significant effect of nifedipine (F(1,151) = 7.0, P less than 0.01) on serum theophylline concentrations. Mean volume of distribution (V) rose from 0.33 +/- 0.07 to 0.39 +/- 0.06 1 kg-1 corrected body weight (CBW) in the presence of nifedipine (t = 2.23, P = 0.052). Theophylline clearance, area under the curve to 8 h AUC (0-8), area under the curve to infinity AUC (0-infinity) and elimination half-life (t1/2) did not change appreciably. No statistically significant changes in nifedipine pharmacokinetics occurred in the presence of theophylline.

Pharmacokinetic interactions of the macrolide antibiotics

The macrolide antibiotics erythromycin and triacetyloleandomycin (troleandomycin) are prescribed for many types of infections. As such they are often added to other preexisting drug therapy. Thus, there are frequent opportunities for the interaction of these antibiotics with other drugs. Both erythromycin and triacetyloleandomycin appear to have the potential to inhibit drug metabolism in the liver and also drug metabolism by micro-organisms in the gut, either through their antibiotic effect or through complex formation and inactivation of microsomal drug oxidising enzymes. Of the two agents, triacetyloleandomycin appears to be the more potent inhibitor of microsomal drug metabolism. Published studies indicate that triacetyloleandomycin can significantly decrease the metabolism of methylprednisolone, theophylline and carbamazepine. Its ability to cause ergotism in patients receiving ergot alkaloids and cholestatic jaundice in patients on oral contraceptives may also be related to its inhibitory effect on drug metabolism. Erythromycin appears to be a much weaker inhibitor of drug metabolism. There are numerous reports describing apparent interactions of erythromycin with theophylline and a lesser number of reports dealing with carbamazepine, warfarin methylprednisolone and digoxin. There are sufficient data to suggest that erythromycin can, in some individuals, inhibit the elimination of methylprednisolone, theophylline, carbamazepine and warfarin. The mean change in drug clearance is about 20 to 25% in most cases, with some patients having a much larger change than others. Like tetracycline, erythromycin also appears to have the potential for increasing the bioavailability of digoxin in patients who excrete high amounts of reduced digoxin metabolites, apparently through destruction of the gut flora that form these compounds. Concurrent administration of triacetyloleandomycin with drugs whose metabolism is known to be affected or that could potentially be affected should be avoided unless appropriate adjustments in dosage are made. Coadministration of erythromycin with drugs believed to interact should be undertaken with caution and with appropriate patient monitoring. Among the other macrolide antibiotics, josamycin has seldom been involved in causing drug interactions, while midecamycin and the older derivative spiramycin have not so far been incriminated.

Pharmacokinetic drug interactions with theophylline

Since up to 90% of a theophylline dose is biotransformed, drugs influencing microsomal enzyme systems in the liver may affect the elimination of theophylline. Other integrated mechanisms (e.g. hepatic uptake) may also be altered by concurrent administration of other drugs. Whatever the mechanism, the interaction may be sufficient to necessitate adjustment of the theophylline dosage, preferably guided by plasma theophylline determinations. Comedication with phenobarbitone may require an increase of the theophylline dose by about 30% due to increased clearance resulting from enzyme induction. Similarly, with phenytoin and carbamazepine a dose increase of about 40 to 50% may be required. In the case of rifampicin, isoniazid or sulphinpyrazone comedication, an increase of the theophylline dose by about 20 to 25% may be needed. On the other hand, other drugs decrease theophylline clearance, making a reduction in the dose of concurrent theophylline advisable: with usual doses of erythromycin, propranolol and isoprenaline (isoproterenol), a reduction of about 25% is needed; with cimetidine and oral contraceptives by about 30% or more; and with triacetyloleandomycin (troleandomycin) by about 50%. In high doses, the xanthine oxidase inhibitor allopurinol can also retard theophylline elimination, and a reduction of the theophylline dose by about 20% may be advisable. Conflicting results have been reported on the influence of frusemide (furosemide) and influenza vaccines, while data regarding the effect of corticosteroids, benzodiazepines and verapamil on theophylline kinetics are not yet conclusive. Many drugs, however, appear not to significantly affect theophylline clearance. Some are from the same therapeutic group as the drugs mentioned above and offer clinical alternatives for coadministration with theophylline. Examples of drugs not found to have a significant effect on theophylline pharmacokinetics are ranitidine, josamycin, midecamycin, amoxycillin, tetracycline, cephalexin, cefaclor, orciprenaline, metoprolol, antacids, medroxyprogesterone acetate, metoclopramide and metronidazole. Most of the drugs discussed in this review appear not to affect the volume of distribution of theophylline significantly.