Pharmacokinetic drug interactions of macrolides

Quantitative prediction of in vivo drug clearance and drug interactions from in vitro data on metabolism, together with binding and transport

It is of great importance to predict in vivo pharmacokinetics in humans based on in vitro data. We summarize recent findings of the quantitative prediction of the hepatic metabolic clearance from in vitro studies using human liver microsomes, hepatocytes, or P450 isozyme recombinant systems. Furthermore, we propose a method to predict pharmacokinetic alterations caused by drug-drug interactions that is based on in vitro metabolic inhibition studies using human liver microsomes or human enzyme expression systems. Although we attempt to avoid the false negative prediction, the inhibitory effect was underestimated in some cases, indicating the possible contribution of the active transport into hepatocytes and/or interactions at the processes other than the hepatic metabolism, such as the metabolism and transport processes during gastrointestinal absorption.

In vitro and in vivo drug interactions involving human CYP3A

Cytochrome P4503A (CYP3A) is importantly involved in the metabolism of many chemically diverse drugs administered to humans. Moreover, its localization in high amounts both in the small intestinal epithelium and liver makes it a major contributor to presystemic elimination following oral drug administration. Drug interactions involving enzyme inhibition or induction are common following the coadministration of two or more CYP3A substrates. Studies using in vitro preparations are useful in identifying such potential interactions and possibly permitting extrapolation of in vitro findings to the likely in vivo situation. Even if accurate quantitative predictions cannot be made, several classes of drugs can be expected to result in a drug interaction based on clinical experience. In many instances, the extent of such drug interactions is sufficiently pronounced to contraindicate the therapeutic use of the involved drugs.

Metabolism of roxithromycin in the isolated perfused rat liver

Roxithromycin is a macrolide antibiotic with high clinical potency. N-Demethylation is considered to be one of the main pathways of roxithromycin metabolism in rats. We have studied the hepatic metabolism of roxithromycin in the isolated perfused rat liver. After addition of roxithromycin (30 microM) to the perfusion medium the parent compound and one major metabolite were detected in bile by high-performance liquid chromatography. The metabolite was identified as monodesmethylated roxithromycin by mass spectrometric analysis. Onset of biliary excretion of native roxithromycin was fast, reaching a maximum (130.52 +/- 43.88 pmol g(-1) min(-1)) after only 10 min, whereas excretion of the metabolite was delayed (maximum 75.83 +/- 11.92 pmol g(-1) min(-1) at 30 min). The cumulative excretion of roxithromycin and its metabolite into bile during the 60 min of application amounted to only 1.09 +/- 0.30 and 0.64 +/- 0.22% of the roxithromycin cleared from the perfusate during the same time. The liver content was 0.48 micromol (g liver)(-1), indicating high retention within the organ. No release of the metabolite into the perfusate was detected. In conclusion, this study has demonstrated the importance of phase-I metabolism for the biliary excretion of roxithromycin in rat liver. These findings might be predictive of roxithromycin biotransformation and biliary excretion in man.

Steady-state pharmacokinetics and electrocardiographic pharmacodynamics of clarithromycin and loratadine after individual or concomitant administration

To evaluate the potential for an interaction between clarithromycin and loratadine, healthy male volunteers (n = 24) received each of the following regimens according to a randomized crossover design: 500 mg of clarithromycin orally every 12 h (q12h) for 10 days, 10 mg of loratadine orally q24h for 10 days, and the combination of clarithromycin and loratadine. A washout interval of 14 days separated regimens. The addition of loratadine did not statistically significantly affect the steady-state pharmacokinetics of clarithromycin or its active metabolite, 14(R)-hydroxy-clarithromycin. However, the addition of clarithromycin statistically significantly altered the steady-state maximum observed plasma concentration and the area under the plasma concentration-time curve over a dosing interval for loratadine (+36 and +76%, respectively) and for descarboethoxyloratadine (DCL), the active metabolite of loratadine (+69 and +49%, respectively). Clarithromycin probably inhibits the oxidative metabolism of loratadine and DCL by the cytochrome P-450 3A subfamily. Electrocardiograms (n = 12) were obtained over 24-h periods at baseline and steady state (day 10). The mean maximum QTc interval and area under the QTc interval-time curve on day 10 were modestly increased (<3%) from baseline for all three regimens, but no QTc interval exceeded 439 ms for any subject. Elevated steady-state concentrations of loratadine and DCL do not appear to be associated with adverse cardiovascular effects related to prolongation of the QTc interval. Loratadine and clarithromycin were well tolerated, alone and in combination.

[New macrolides]

INTRODUCTION

Following their use for a few years, the place of new macrolides can be assessed.

CURRENT KNOWLEDGE AND KEY POINTS

"New macrolides", ie, roxithromycin, clarithromycin, azithromycin and dirithromycin, are derivated from erythromycin and defined by different pharmacokinetic parameters: longer half time with better oral administration and shorter duration of treatment, better tissue concentrations with reduction of dosages and better tolerance, high cellular concentration with good activity. However, new macrolides and erythromycin do not show major differences in their antibacterial spectrum against usual pyogenic strains. Clinical use of new macrolides extends to opportunistic infections, such as atypical mycobacterial infections or toxoplasmosis, occurring in the course of HIV infection. New macrolides are also recommended for the treatment of Helicobacter pylori-related gastroduodenal ulcer.

FUTURE PROSPECTS AND PROJECTS

New macrolides have opened new avenues in the development of anti-infectious strategies. Due to their good efficacy against Chlamydia pneumoniae which is suspected of inducing vascular diseases, the upcoming use of ketolids and the development of their non-antibiotic effects, the future of macrolides is favorable.

Pharmacokinetics of drugs used in critically ill adults

Critically ill patients exhibit a range of organ dysfunctions and often require treatment with a variety of drugs including sedatives, analgesics, neuromuscular blockers, antimicrobials, inotropes and gastric acid suppressants. Understanding how organ dysfunction can alter the pharmacokinetics of drugs is a vital aspect of therapy in this patient group. Many drugs will need to be given intravenously because of gastrointestinal failure. For those occasions on which the oral route is possible, bioavailability may be altered by hypomotility, changes in gastrointestinal pH and enteral feeding. Hepatic and renal dysfunction are the primary determinants of drug clearance, and hence of steady-state drug concentrations, and of efficacy and toxicity in the individual patient. Oxidative metabolism is the main clearance mechanism for many drugs and there is increasing recognition of the importance of decreased activity of the hepatic cytochrome P450 system in critically ill patients. Renal failure is equally important with both filtration and secretion clearance mechanisms being required for the removal of parent drugs and their active metabolites. Changes in the steady-state volume of distribution are often secondary to renal failure and may lower the effective drug concentrations in the body. Failure of the central nervous system, muscle, the endothelial system and endocrine system may also affect the pharmacokinetics of specific drugs. Time-dependency of alterations in pharmacokinetic parameters is well documented for some drugs. Understanding the underlying pathophysiology in the critically ill and applying pharmacokinetic principles in selection of drug and dose regimen is, therefore, crucial to optimising the pharmacodynamic response and outcome.

Comparative Study of the Effects of Erythromycin and Roxithromycin on Action Potential Duration and Potassium Currents in Canine Purkinje Fibers and Rabbit Myocardium

BACKGROUND: Erythromycin and roxithromycin are macrolide antibiotics in common clinical use. Erythromycin occasionally produces life-threatening arrhythmias (torsades de pointes) by blocking the outward potassium current responsible for repolarization of the cardiac action potential. METHODS AND RESULTS: We used standard cellular electrophysiological and whole-cell patch-clamping techniques to compare the relative efficacy of erythromycin and roxithromycin in prolonging cardiac action potential in canine Purkinje fibers and in blocking individual outward potassium currents in isolated rabbit ventricular myocytes. We demonstrated significant prolongation of action potential duration in canine Purkinje fibers by erythromycin but not roxithromycin at a concentration of 100 µM. The delayed rectifier, the outward potassium current thought to be most sensitive to modulation by drugs, was significantly depressed by both agents at concentrations of >/=30 µM in isolated rabbit ventricular myocytes. Both drugs had similar potencies (26% and 21% reduction by 30 µM erythromycin and roxithromycin, respectively, and 50% and 36% reduction by 100 µM erythromycin and roxithromycin). Neither agent significantly blocked other potassium currents (including the transient outward current). CONCLUSIONS: Taking into account normally observed peak blood concentrations of these agents in clinical use and the fact that roxithromycin is not normally administered intravenously, we conclude that the risk of proarrhythmia during normal clinical use of oral roxithromycin is extremely remote.

Inhibition of nifedipine metabolism in dogs by erythromycin: difference between the gut wall and the liver

The purpose of this study was to evaluate possible interaction of nifedipine with erythromycin or rokitamycin in the intestinal mucosa. Male beagle dogs were orally administered nifedipine (10 mg), with or without oral pre-medication with erythromycin (300 mg), and 300 mg erythromycin or rokitamycin twice a day for 3 days. The experiments were of randomized cross-over design with a two-week wash-out period between dosing regimens. Erythromycin pre-medication for 3 days resulted in a significant increase in the area under the serum nifedipine concentration-time curve (AUC), whereas the curve for one nifedipine metabolite (M-2) decreased significantly. When the effects of erythromycin on the metabolism of nifedipine were studied using dog liver microsomes it was found that erythromycin significantly inhibited formation of M-2 but not of the metabolite M-1. These results indicate that formation of M-2 from M-1 in the liver might be reduced by erythromycin pre-medication. To avoid possible metabolism in the gut, the dogs were then administered 8 mg nifedipine into the peritoneal cavity, with or without multiple dose pre-treatment with erythromycin for 3 days. After intraperitoneal administration of nifedipine, the maximum concentration (Cmax) of nifedipine increased significantly. After pre-administration of erythromycin the relative bioavailability of nifedipine after oral administration was increased compared with injection into the peritoneal cavity. In-vitro study using rat intestinal microsomes and the in-vivo rat intestinal loop technique also showed that pre-administration of erythromycin inhibits nifedipine metabolism in the small intestine.

The effect of erythromycin on the CYP3A component of sertindole clearance in healthy volunteers

The effect of erythromycin on the pharmacokinetic disposition of oral sertindole, a new antipsychotic compound, was investigated. Ten subjects who completed the study received a single 4-mg dose of sertindole without or with concomitant erythromycin 250 mg taken orally 4 times daily. Coadministration of sertindole and erythromycin led to a 33% decrease (P < 0.05) in mean (+/- SD) time to reach maximum plasma concentration (tmax) value and a 15% elevation (P < 0.05) in the mean maximum plasma concentration (Cmax) value of sertindole. The mean area under the concentration-time curve (AUC) value of sertindole did not change significantly in the presence of erythromycin (alone: 159 +/- 111 ng.hr/mL, in combination: 179 +/- 144 ng.hr/mL, P > 0.05). The presence of erythromycin also significantly increased the dehydrosertindole Cmax and AUC means by 16% and 21%, respectively, possibly due to inhibition of the CYP3A metabolic isozyme responsible for the elimination of this metabolite. The rate of absorption of sertindole and the rate of appearance of dehydrosertindole in the systemic circulation after a 4-mg sertindole single dose were slightly enhanced by concomitant dosing of erythromycin. In conclusion, there is a small but noticeable effect of erythromycin on the pharmacokinetic disposition of sertindole. The effects are believed to have little clinical significance.

Potential interaction between clarithromycin and warfarin

OBJECTIVE

To report a possible drug interaction between clarithromycin and warfarin in a patient with chronic atrial fibrillation.

CASE SUMMARY

A patient with chronic atrial fibrillation was placed on warfarin therapy. International normalized ratios (INRs) ranged from 1.61 to 3.99 while the dosage was being adjusted during the first 5 months of warfarin therapy. The dosage was titrated to 20 mg/wk; laboratory tests obtained 2 weeks after this dosage was started indicated an INR of 2.1. The same dosage was continued. Clarithromycin 500 mg bid was started for an acute exacerbation of bronchitis 10 days after the last INR was obtained and was continued for 14 days of therapy. An INR obtained 3 days after completion of the clarithromycin therapy was 16.8. The warfarin was withheld and vitamin K 20 mg im was administered. The INR obtained the next day was 1.52. The warfarin was restarted and the dosage was titrated to between 22.5 and 25 mg/wk, with INRs ranging from 0.85 to 3.14.

DISCUSSION

Many factors influence the metabolism of warfarin, including disease states, medications, age, and diet. Data collected in this case suggested clarithromycin may have contributed to the increase in the effect of warfarin. Inhibition of the cytochrome P450 oxidizing system appears to be the reason for the increase. Numerous drugs and disease states affect the rate at which this system metabolizes drugs.

CONCLUSIONS

The potential interaction between clarithromycin and warfarin warrants prudent monitoring of the INR during concurrent administration of these drugs. Warfarin dosages may need to be reduced during concurrent clarithromycin therapy to prevent bleeding complications. Further controlled clinical trials are needed to substantiate the interaction between clarithromycin and warfarin.

Management of drug interactions in patients with HIV

OBJECTIVE

To provide a complication of relevant information on drug interactions to assist healthcare practitioners in managing complex HIV-related pharmacotherapy.

DATA SOURCES

Information was retrieved via a MEDLINE search (January 1966-December 1996) using MeSH headings "human immunodeficiency virus," "drug interactions," and names of medications commonly prescribed for the management of HIV infection and related opportunistic infections. Abstracts of international and national conferences, review articles, textbooks, and references of all articles were also searched.

STUDY SELECTION AND DATA EXTRACTION

All literature on pharmacokinetic or pharmacodynamic interactions was considered for inclusion. Pertinent information, as assessed by the authors, was selected and summarized for discussion.

DATA SYNTHESIS

Drug disposition and/or pharmacologic effect may be affected either by HIV-related physiologic changes or by the presence of concomitant drug therapy. Modifications in drug selection, dosage, dosing regimen, or route of administration may be needed to avoid or manage drug-disease, drug-drug, or drug-food interactions. Management options may depend on the mechanism and the clinical significance of the interaction, the availability of therapeutic alternatives, patient convenience, and cost restrictions. In the absence of specific data, consideration of pharmacokinetic and pharmacodynamic characteristics to assist practitioners in predicting the likelihood of possible interactions was included.

RESULTS

A comprehensive table of clinically significant drug interactions is provided. Drug interaction principles and practical management strategies are also discussed.

CONCLUSIONS

The potential for drug interactions is extremely common, given the increasing complexity of managing patients infected with HIV. To avoid compromising therapeutic efficacy or increasing drug toxicity, practitioners need to be aware of potential interactions and are encouraged to use a systematic approach when managing patient drug therapy.

Clarithromycin-induced digoxin intoxication

OBJECTIVE

To report a case of clarithromycin-induced digoxin intoxication.

CASE SUMMARY

A 78-year-old white man with ischemic cardiomyopathy and chronic renal insufficiency was admitted 4 days after being prescribed clarithromycin for a suspected episode of bronchitis. He reported weakness, asthenia, and gastrointestinal symptoms; the digoxin serum concentration was measured at 3.89 ng/mL. The patient recovered uneventfully after digoxin and clarithromycin were discontinued.

DISCUSSION

Erythromycin frequently interacts with other drugs that are also metabolized by the CYP3A4 isoenzyme. However, erythromycin is hypothesized to interact with digoxin by inhibiting Eubacterium lentum, which is a normal inhabitant of the human gut and is responsible for intestinal metabolism of digoxin in 10% of patients. Since clarithromycin shares a comparable antibacterial spectrum with erythromycin, the possibility of a drug interaction with digoxin remains. Only four cases of clarithromycin interacting with digoxin have been reported to date. Clinically, this interaction may have been more obvious because of our patient's moderate renal dysfunction and serum digoxin concentrations in the upper therapeutic range prior to clarithromycin initiation. Other causes for digoxin intoxication could not be identified.

CONCLUSIONS

Clarithromycin may inhibit the growth of E. lentum, which can lead to an increase in digoxin bioavailability and blood concentrations in patients in whom this intestinal metabolic pathway is present. Patients at risk include those with renal dysfunction, with serum concentrations in the upper therapeutic range, or with measured digoxin concentrations that are much lower than predicted by pharmacokinetic calculations. For these patients, appropriate therapy includes the selection of an alternative, noninteracting antibiotic or, if this is not possible, a temporary reduction of digoxin dosage.

Drug-mediated inactivation of cytochrome P450

1. Multiple forms of cytochrome P450 (CYP) catalyse the oxidation of chemicals of endogenous and exogenous origin, including drugs, carcinogens, steroids and eicosanoids. However, this unusual low substrate specificity also makes CYP susceptible to inhibition by a wide range of drugs, leading to pharmacokinetic interactions of potential clinical significance. 2. Some drugs are converted by CYP to reactive metabolites that bind covalently to sites within the active centre of the same CYP. Such mechanism-based inhibition leads to CYP inactivation or complexation. These processes give rise to long-term effects on drug pharmacokinetics, as the inactivated or complexed CYP must be replaced by newly synthesized CYP protein. 3. Drugs that inactivate CYP generally possess recognizable functional groups that are oxidized to reactive products. Thus, drugs with side chains containing unsaturated carbon-carbon bonds and furan ring systems are associated with CYP inactivation. Nitrogen-containing systems may also inactivate CYP. 4. Metabolites formed from drugs containing alkylamino and methylenedioxy functionalities can trap CYP as inert complexes without eliciting inactivation. However, the functional effects of inactivation and complexation on drug pharmacokinetics are indistinguishable. Drugs that elicit CYP complexation include the first generation macrolide antibiotics, but newer analogues appear much safer. Some antidepressants, antiepileptics and tuberculostatic agents have been associated with CYP complexation.

The treatment of staphylococcal infections with special reference to pharmacokinetic, pharmacodynamic and pharmacoeconomic considerations

The choice of antibiotics for the treatment of staphylococcal infections depends to a high degree on the susceptibility patterns in the hospital in question. These may be highly variable and considerable differences between countries and hospitals exist. The insight into the pharmacodynamic aspects of antimicrobial agents has increased considerably in the last 5 years, resulting in new treatments, such as once daily administration of aminoglycosides and continuous infusion of betalactam antibiotics. The antibiotic policy in Dutch hospitals for the treatment of staphylococcal infections is discussed. In most Western countries with a relatively low incidence of MRSA, penicillin-derivatives, such as flucloxacillin (or cloxacillin, methicillin and nafcillin) will be the drug of choice, because of their good in-vitro activity, low toxicity, good clinical efficacy and relatively low cost. If the incidence of MRSA increases, drugs such as the glycopeptides will be of more importance. This will of course have a clear economic impact, as both vancomycin and teicoplanin are considerably more expensive than agents such as flucloxacillin and oral treatment is not possible. Pharmacoeconomic aspects also play a role. As a rule, intravenous antimicrobial agents are considerably more expensive than the oral formulations. Before oral administration can be recommended, a reliable oral absorption, also in seriously ill patients, must have been demonstrated. Other aspects that influence the cost of therapy are hospital stay and the possibility of outpatient treatment.

Determination of erythromycin concentrations in rat plasma and liver by high-performance liquid chromatography with amperometric detection

A simple method for the quantitative determination of erythromycin (EM) concentrations in rat plasma and liver by high-performance liquid chromatography with amperometric detection was developed. EM was extracted from 200 microl of plasma or liver homogenate sample under sodium hydroxide alkaline conditions with tert.-butyl methyl ether. Oleandomycin was used as an internal standard. The recovery rate of EM was up to 100%. The detector cell potential for the oxidation of EM was +1100 mV. The calibration curves were linear over the concentration ranges 0.1-20.0 microg/ml for plasma and 0.5-100.0 microg/g for liver. The method was applied to the determination of the plasma and liver concentrations of EM in rats after intravenous administration (50 mg/kg dose). The method presented here has proved to be of great use for the investigation of the pharmacokinetic characteristics of EM in small animals such as rats.

History of macrolide use in pediatrics

Erythromycin, the prototypical macrolide, has been widely used since the 1950s in the management of pediatric infections. Erythromycin is the drug of choice for infants and children with Legionnaire's disease, pertussis, diphtheria, lower respiratory tract infections caused by Mycoplasma pneumoniae, Chlamydia pneumoniae and Chlamydia trachomatis and enteritis caused by Campylobacter jejuni. It is also indicated for treatment of syphilis; for streptococcal, staphylococcal and pneumococcal infections; genital infections caused by Ureaplasma urealyticum; and for the prevention of rheumatic fever and endocarditis in patients who are allergic to beta-lactam antibiotics. The new macrolides azithromycin and clarithromycin are also active against Borrelia burgdorferi, Helicobacter pylori, Mycobacterium avium-intracellulare complex, Cryptosporidium spp. and Toxoplasma gondii. Erythromycin is associated with a low risk of serious side effects, although gastric distress occurs in a significant proportion of patients. Drug interactions with theophylline, carbamazepine, warfarin, cyclosporine, terfenadine and digoxin limit erythromycin use. The newer macrolides azithromycin and clarithromycin are more stable, better absorbed and better tolerated than erythromycin. Azithromycin is more active than erythromycin against Haemophilus influenzae. Excellent tissue and intracellular penetration may contribute to their clinical efficacy. In children both azithromycin and clarithromycin are indicated for acute otitis media caused by Streptococcus pneumoniae, H. influenzae and Moraxella catarrhalis and for pharyngitis/tonsillitis caused by Streptococcus pyogenes. (As of December, 1996, azithromycin for oral suspension was approved for community-acquired pneumonia in children caused by C. pneumoniae, H. influenzae, M. pneumoniae and S. pneumoniae.) Claritromycin is also indicated for acute maxillary sinusitis, uncomplicated skin and skin structure infections, pneumonia and disseminated mycobacterial infections. Azithromycin and clarithromycin are associated with a lower incidence of gastrointestinal side effects, a low rate of drug discontinuation caused by side effects and a low potential for interaction with other drugs.

Pediatric dermatology: advances in therapy

Several recent therapeutic advances in pediatric dermatology have been made. Of particular importance are new developments in the use of antimicrobials, antivirals, antifungals, retinoids, calcipotriene, and intravenous gamma globulin. We review safety and efficacy data of these drugs in their use in children with cutaneous disease.

Pharmacokinetics and pharmacodynamics of newer macrolides

Requirements for the antimicrobial activity of an antibiotic are: (1) binding of the drug to a specific site in the bacteria; (2) occupation of a critical number of binding sites; and (3) persistence at these binding sites for a sufficient time. With concentration-dependent antibiotics the ratio of the peak serum drug concentration to the MIC of a pathogen is the primary determinant of bacterial killing; with concentration-independent antibiotics it is the length of time serum concentration remains above the MIC, rather than the peak level. The pharmacokinetics of the new macrolides azithromycin and clarithromycin differ notably from those of conventional antibiotics in a more rapid and extensive distribution to body tissues. Because of these unique tissue pharmacokinetics, the pharmacodynamic models that apply to other classes of antibiotics may not explain the antimicrobial activity and clinical efficacy of azithromycin and clarithromycin.

Drug interactions of macrolides: emphasis on dirithromycin

OBJECTIVE

To describe the drug interactions of dirithromycin, a new macrolide, and to compare them with those of other macrolides.

DATA SOURCES

A literature search was performed using MEDLINE to identify articles published between January 1980 and July 1995 concerning the drug interactions of macrolides. Published abstracts were also examined. All studies using dirithromycin were performed under the sponsorship of Eli Lilly and Company.

DATA SYNTHESIS

Erythromycin, the first macrolide discovered, is metabolized by the cytochrome P450 enzyme system. By decreasing their metabolism, erythromycin can interact with other drugs metabolized by the cytochrome P450 enzymes. The lack of such interactions would be a desirable feature in a newer macrolide. We describe studies performed to detect any interactions of dirithromycin with cyclosporine, theophylline, terfenadine, warfarin, and ethinyl estradiol. The studies showed that dirithromycin, like azithromycin, is much less likely to cause the interactions detected with clarithromycin and erythromycin. A review of the literature showed differences among macrolides in their abilities to inhibit cytochrome P450 enzymes and, thus, to cause drug-drug interactions. Erythromycin and clarithromycin inhibit cytochrome P450 enzymes, and have been implicated in clinically significant interactions. Azithromycin and dirithromycin neither inhibit cytochrome P450 enzymes nor are implicated in clinically significant drug-drug interactions.

CONCLUSIONS

Dirithromycin, a new macrolide, does not inhibit the cytochrome P450 enzyme system. The concomitant use of dirithromycin with cyclosporine, theophylline, terfenadine, warfarin, or ethinyl estradiol was studied in pharmacokinetic and pharmacodynamic studies. In vitro, dirithromycin did not bind cytochrome P450. In healthy subjects, erythromycin increases the clearance of cyclosporine by 51%, whereas dirithromycin causes no significant changes in the pharmacokinetics of cyclosporine. In kidney transplant recipients, administration of dirithromycin was associated with a significant (p < 0.003) decrease of 17.4% in the clearance of cyclosporine. In patients taking low-dose estradiol, the administration of dirithromycin caused a significant (p < 0.03) increase of 9.9% in the clearance of ethinyl estradiol; escape ovulation did not occur. Unlike erythromycin and clarithromycin, dirithromycin had no significant effects on the pharmacokinetics of theophylline, terfenadine, or warfarin. The alterations typical of drug interactions that are based on inhibition of the cytochrome P450 system occurring with erythromycin and clarithromycin were not observed with dirithromycin.

Nonlinear mixed effects modeling of single dose and multiple dose data for an immediate release (IR) and a controlled release (CR) dosage form of alprazolam

PURPOSE

NONMEM was applied to single dose and multiple dose bioavailability data for an immediate release (IR) and a controlled release (CR) dosage form of alprazolam to acquire additional information from the data which are not easily obtainable by traditional means.

METHODS

The objective function value (OBJ) and diagnostic plots were used as measures of goodness of fit of the model to the data. A change in the OBJ value of 7.9 was necessary to show statistical significance (p < 0.005) between two models when the two models differed by 1 parameter.

RESULTS

A two-compartment linear model with first-order absorption and elimination best describes the data. Including a lag time, two different rates of absorption (KAIR and KACR), and bioavailability for the CR relative to the IR dosage form significantly improved the fit of the model to the data. Cigarette smoking was associated with a 100% increase in clearance of alprazolam as compared to non-smokers. The higher residual variability observed in this study, where interoccasion variability (IOV) was not initially modeled, could be explained to a large extent by the presence of significant interoccasion variability (IOV).

CONCLUSIONS

Since alprazolam has been suggested to be mainly metabolized by the CYP3A4 isozyme in humans, it appears that tobacco could be an inducer of CYP3A4 and/or alprazolam may be metabolized by other isozyme(s) (specifically, CYP1A1/1A2) that are induced by cigarette smoke. The population pharmacokinetic model approach combined with exploratory graphical data analysis is capable of identifying important covariates from well-controlled "data rich" Phase I studies early in drug development.

Comparison of dirithromycin and penicillin for treatment of streptococcal pharyngitis

In the treatment of group A beta-hemolytic streptococcal pharyngitis, penicillin is the drug of choice and erythromycin is the alternative. In a double-blind, randomized study, dirithromycin, a new macrolide, was compared with penicillin for the treatment of streptococcal pharyngitis. Of 121 patients who were treated with dirithromycin, 96.7% manifested a favorable clinical response, and of 136 patients treated with penicillin, 94.2% manifested a favorable clinical response. Streptococci were eradicated from the pharynges of 85.3% of 116 dirithromycin-treated patients and 82.5% of 126 penicillin-treated patients who returned for follow-up. There were no statistically significant differences in efficacy between the two groups. The incidence of abdominal symptoms was higher in dirithromycin-treated patients. Being as efficacious as penicillin and having the advantages over erythromycin of once-daily dosing and the lack of drug interactions, dirithromycin is an alternative to penicillin in the treatment of streptococcal pharyngitis for patients 12 years of age and older.

Medical issues and emergencies in the dermatology office

We review the medical issues and emergencies potentially encountered in the practice of general or surgical dermatology. Traditional guidelines have largely consisted of dated extrapolations from the nondermatologic literature concerning procedures that are primarily irrelevant to dermatology. This article outlines a rational approach to organizing an office emergency plan for anaphylaxis, stroke, status epilepticus, myocardial infarction, and hypertensive crisis. We discuss the literature that has influenced current office behavior regarding endocarditis prophylaxis, the use of electrosurgery with pacemakers, arrhythmogenic drug interactions, vasovagal syncope, lidocaine "allergy," and bleeding complications from oral anticoagulants. Recommendations for managing these issues in a dermatologic context are provided.

Clinical efficacy of dirithromycin in patients with bacteremic pneumonia

Dirithromycin is a new macrolide antimicrobial drug with a long half-life (44 hours) that reaches high tissue concentrations, thus permitting once-daily oral dosing and shorter courses of therapy. Soon after absorption, dirithromycin enters the tissue so rapidly that serum concentrations are comparatively low. It could be hypothesized that these low serum levels could endanger the outcome in patients with bacteremic pneumonia. We reviewed the database on dirithromycin pneumonia (consisting of 1108 patients randomized to receive dirithromycin or erythromycin in two double-masked trials) to ascertain its efficacy in patients with community-acquired pneumonia and concomitant bacteremia. Fourteen (2.5%) of 555 dirithromycin-treated patients and 10 (1.8%) of 553 erythromycin-treated patients had bacteremia. A favorable clinical response posttherapy was observed in 92.3% and 88.9% of these patients with a response assigned, respectively. Overall, favorable response rates were comparable between the two groups in the bacteremic subsets: patients with pneumococcal bacteremia, patients with nonbacteremic pneumococcal pneumonia, and all patients enrolled with acute pneumonia who had a posttherapy clinical response. In the treatment of patients with mild or moderate community-acquired pneumonia, including those with unsuspected and incidental bacteremia, dirithromycin is an effective macrolide antimicrobial drug.

Effects of roxithromycin, erythromycin and troleandomycin on their N-demethylation by rat and human cytochrome P450 enzymes

1. The effects of treatment of rat with roxithromycin, erythromycin and troleandomycin as well as other chemicals including typical cytochrome P450 inducers were examined in rat and human liver microsomes. 2. Erythromycin and troleandomycin but not roxithromycin caused slight increases in CYP3A1 levels and the N-demethylation of roxithromycin, erythromycin and troleandomycin and oxidation of nifedipine in rat, but none of these chemicals induced significantly CYP2B1 levels or benzphetamine N-demethylation activities. 3. Dexamethasone and pregnenolone 16 alpha-carbonitrile induced CYP3A1 levels and N-demethylation of roxithromycin, erythromycin and troleandomycin but not of benzphetamine, in rat liver microsomes. Treatment of rat with phenobarbital caused increases in both CYP2B1 and 3A1 levels and all of the N-demethylation activities examined. Phenytoin and metyrapone produced increases in contents of 2B1 and activities of benzphetamine N-demethylation as well as of roxithromycin, erythromycin and troleandomycin, although these two inducers did not induce 3A1 protein significantly. 4. In man, a liver sample that was high in CYP3A4 and nifedipine oxidation activity was found to be the most active in N-demethylation activities towards these substrates examined. In addition, recombinant 3A4 catalysed very efficiently the N-demethylation of roxithromycin, erythromycin and troleandomycin in reconstituted monooxygenase systems. 5. These data suggest that erythromycin and troleandomycin, but not roxithromycin, were able to induce CYP3A1 in rat liver microsomes, and that N-demethylation of roxithromycin, erythromycin and troleandomycin were catalysed mainly by 3A1 (and partly by 2B1) in rat and by 3A4 in man.

Roxithromycin-induced digoxin toxicity

A 76-year-old women presented with digoxin toxicity four days after starting a course of roxithromycin. In 10%-15% of the population, digoxin is degraded extensively by bacteria in the gastrointestinal tract, and antibiotic-induced changes in the bacterial flora may lead to digoxin toxicity.

Properties of azithromycin that enhance the potential for compliance in children with upper respiratory tract infections

BACKGROUND

Azithromycin, the prototypical azalide antibiotic, has a wide spectrum of activity that is characterized by resistance to beta-lactamase-producing microbes and efficacy against Gram-positive and Gram-negative pathogens, including Haemophilus influenzae. Tissue-directed pharmacokinetics include tissue concentrations up to 100-fold higher than those in plasma and a tissue half-life of up to 4 days. Pharmacokinetics of azithromycin permits a reduction in dosage frequency and duration while maintaining efficacy comparable to that of conventional 7- to 10-day three or four times daily regimens. Dosage interval, duration of treatment, side effects and palatability can affect compliance and thus clinical outcome. Compliance among children is important in light of the high incidence of community-acquired infections such as otitis media and streptococcal pharyngitis.

OBJECTIVE

To compare the flavor, taste acceptability and color preference of oral antibiotic suspensions given to children.

METHODS

The taste and acceptability of the oral suspension form of azithromycin vs. cefixime, cefpodoxime proxetil, cefprozil, clarithromycin or loracarbef were rated by children during blinded taste tests and with acceptability/ preference questionnaires.

RESULTS

Analysis of the mean acceptability/ preference rating from 769 children demonstrated that the flavor of azithromycin was rated significantly higher than that of cefpodoxime (4.3 vs. 2.8), cefprozil (4.0 vs. 3.4) and clarithromycin (4.3 vs. 2.7) and was comparable to that of cefixime (4.0 vs. 4.2) and loracarbef (4.4 vs. 4.5). A greater percentage of children preferred the taste of azithromycin to that of cefpodoxime (90.0% vs. 5.2%), cefprozil (63.0% vs. 33.1%) and clarithromycin (89.0% vs. 11.0%). The taste of azithromycin was not preferred to that of cefixime (39.0% vs. 53.9%) or loracarbef (36% vs. 58.5%).

CONCLUSIONS

The efficacy and safety of azithromycin in otitis media and streptococcal pharyngitis, the simple dosing regimen and a highly palatable oral suspension formulation should increase compliance among pediatric patients and thereby improve clinical outcomes.

Effect of combination therapy with ciprofloxacin and clarithromycin on theophylline pharmacokinetics in healthy volunteers

Five adults completed this four-way randomized crossover study to compare the effects of oral treatment with ciprofloxacin, clarithromycin, and a combination of the two drugs on theophylline pharmacokinetics. The area under the concentration-time curve for theophylline during combination therapy was not different from that for ciprofloxacin alone. Beta error may explain this finding, but any real effect from combination treatment appears to be clinically unimportant.

Antibiotic use in the emergency department. II The aminoglycosides, macrolides, tetracyclines, sulfa drugs, and urinary antiseptics

The aminoglycoside, macrolide, tetracycline, and sulfa classes of antibiotics provide antimicrobial coverage pertinent to many infectious diseases diagnosed in the emergency department (ED). The aminoglycosides are parenteral agents that are useful in Gram-negative infections and as synergistic drugs in the management of some Gram-positive infections. The macrolides, of which erythromycin is the prototype, are used for a number of Gram-positive and atypical bacterial infections, while the tetracyclines are appropriate for ED treatment of a diverse group of infections such as chlamydiae, spirochetes, and rickettsiae. The sulfa agents are appropriate for many urinary and respiratory tract infections, and also have particular utility in some infections encountered primarily in patients with AIDS. The urinary antiseptics are a group of antimicrobials that may be effective for cystitis but have no systemic efficacy. This article, which is the second in a four-part series on antibiotic use in the ED, reviews the pharmacology and clinical utility of these diverse agents for the emergency physician.

A kinetic and dynamic study of oral alprazolam with and without erythromycin in humans: in vivo evidence for the involvement of CYP3A4 in alprazolam metabolism

OBJECTIVE

To assess the possible involvement of CYP3A4 in the metabolism of alprazolam in vivo.

METHOD

Twelve healthy male volunteers were randomly allocated to one of the two different treatment sequences, placebo-erythromycin or erythromycin-placebo, with an at least 6-week washout period between the two trial phases. Each volunteer received 400 mg erythromycin or matched placebo given orally three times a day for 10 days and an oral dose (0.8 mg) of alprazolam on the posttreatment day 8. Plasma concentration of alprazolam was measured up to 48 hours after the administration, and psychomotor function was assessed at each time of blood samplings with use of the Digit Symbol Substitution Test, visual analog scale, and Udvalg for kliniske undersøgelser side effect rating scale.

RESULTS

Erythromycin significantly (p < 0.001) increased the area under the plasma concentration-time curves (200 +/- 43 versus 322 +/- 49 ng . hr/ml from 0 to 48 hours and 229 +/- 52 versus 566 +/- 161 ng . hr/ml from 0 hour to infinity), decreased the apparent oral clearance (1.02 +/- 0.31 versus 0.41 +/- 0.12 ml/min/kg), and prolonged the elimination half-life (16.0 +/- 4.5 versus 40.3 +/- 14.4 hours) of alprazolam. However, any psychomotor function variables did not differ significantly between the erythromycin and placebo trial phases.

CONCLUSION

This study suggests that erythromycin, an inhibitor of CYP3A4, inhibits the metabolism of alprazolam, providing an in vivo evidence for the involvement of CYP3A4 in its metabolism. However, the kinetic change of alprazolam by erythromycin does not result in the pharmacodynamic change of this triazolobenzodiazepine, at least after single dosing.

Macrolide antibiotics in paediatric infectious diseases

Erythromycin and other macrolides have enjoyed a renaissance in the 1970s, 1980s and 1990s secondary to the discovery of "new' pathogens such as Chlamydia, Legionella, Campylobacter and Mycoplasma spp. Erythromycin is an important therapeutic agent in the paediatric age group for several reasons: (a) it exhibits proven efficacy for a wide range of infections (upper and lower respiratory tract infections, skin/skin structure infections, prophylaxis of endocarditis/acute rheumatic fever/ophthalmia neonatorum and pre-colonic surgery, campylobacteriosis, chlamydial and ureaplasmal infections, diphtheria, whooping cough, streptococcal pharyngitis) and gastrointestinal (GI) dysmotility states; (b) intravenous formulations are widely available; and (c) it is available in a number of formulations as a generic product, which is likely to result in significant cost savings. Nevertheless, erythromycin and similar earlier macrolides are characterised by a number of drawbacks including a narrow spectrum of antimicrobial activity, unfavourable pharmacokinetic properties and poor GI tolerability. Newer macrolides such as clarithromycin and azithromycin are useful in serving the needs of paediatric patients who are erythromycin-intolerant or who have infections caused by organisms that are intrinsically erythromycin-resistant, or for which a high percentage of strains are resistant (e.g. Haemophilus influenzae, Helicobacter pylori, Mycobacterium avium complex). In addition, these newer macrolides may be considered as alternatives to oral amoxicillin-clavulanic acid, second or third generation cephalosporins, or erythromycin plus sulphonamide in this patient population. Selection between specific macrolides and between macrolides and other antibiotics in the paediatric population is likely to depend, at least for the immediate future, on separate comparisons of product availability, cost, effectiveness and tolerability profiles.

Dextromethorphan as an in vivo probe for the simultaneous determination of CYP2D6 and CYP3A activity

Dextromethorphan (DM) is O-demethylated into dextrorphan (DEX) in humans by the cytochrome P450 designated as CYP2D6 and N-demethylated into 3-methoxymorphinan (3MM) via CYP3As. Clinically, DM has been successfully used as an index of CYP2D6 and this paper describes analytical and clinical data that will help evaluate the use of DM hydrobromide as a probe of CYP3A activity. DM and its three demethylated metabolites were measured in a 4-h spot urine sample using a HPLC method employing solid-phase extraction (C(18)), analysis on a phenyl column [mobile phase, methanol-acetonitrile-phosphate buffer (10 mM, pH 3.5, 20:25:55, v/v)] and fluorescence detection (excitation at lambda=228 nm, no emission cut-off filter). The urinary molar ratio DM-DEX was used to assess CYP2D6 activity while DM-3MM was used for CYP3As. The DM-3MM ratios were sensitive to the co-administration of selective CYP3A inhibitors grapefruit juice and erythromycin. In addition, in healthy volunteers and cancer patients, the N-demethylation of DM correlated with the CYP3A-mediated metabolism of verapamil and tamoxifen. DM appears to be a promising way to simultaneously phenotype patients for CYP2D6 and CYP3As.

Azithromycin. A review of its pharmacological properties and use as 3-day therapy in respiratory tract infections

The azalide antibacterial agent azithromycin is a semisynthetic acid-stable erythromycin derivative with an expanded spectrum of activity and improved tissue pharmacokinetic characteristics relative to erythromycin. The drug is noted for its activity against some Gram-negative organisms associated with respiratory tract infections, particularly Haemophilus influenzae. Azithromycin has similar activity to other macrolides against Streptococcus pneumoniae and Moraxella catarrhalis, and is active against atypical pathogens such as Legionella pneumophila, Chlamydia pneumoniae and Mycoplasma pneumoniae. Once-daily administration of azithromycin is made possible by the long elimination half-life of the drug from tissue. Azithromycin is rapidly and highly concentrated in a number of cell types after absorption, including leucocytes, monocytes and macrophages. It undergoes extensive distribution into tissue, from where it is subsequently eliminated slowly. A 3-day oral regimen of once-daily azithromycin has been shown to be as effective as 5- to 10-day courses of other more frequently administered antibacterial agents [such as erythromycin, amoxicillin-clavulanic acid and phenoxymethylpenicillin (penicillin V)] in patients with acute exacerbations of chronic bronchitis, pneumonia, sinusitis, pharyngitis, tonsillitis and otitis media. Adverse effects of azithromycin are mainly gastrointestinal in nature and occur less frequently than with erythromycin. Azithromycin is likely to prove most useful as a 3-day regimen in the empirical management of respiratory tract infections in the community. Its ease of administration and 3-day duration of therapy, together with its good gastrointestinal tolerability, should optimise patient compliance (the highest level of which is achieved with once-daily regimens). Azithromycin is also likely to be useful in the hospital setting, particularly for outpatients and for those unable to tolerate erythromycin.

Interaction between erythromycin and the benzodiazepines diazepam and flunitrazepam

The effect of erythromycin on the pharmacokinetics and pharmacodynamics of diazepam and flunitrazepam was investigated in two randomized, double-blind, cross-over studies. Healthy volunteers ingested erythromycin for one week 500 mg t.i.d. On the 4th day they ingested a single 5 mg dose of diazepam (6 subject, Study 1) or 1 mg dose of flunitrazepam (5 subjects, Study 2), respectively. Plasma drug concentrations and psychomotor effects were measured during 42 hr after the ingestion of diazepam or flunitrazepam. In Study 1 erythromycin increased the area under the diazepam plasma concentration-time curve [AUC (0-42 hr)] by 15% (P < 0.05) and the concentration of diazepam in plasma at 42 hr by 63% (P < 0.05). The median peak concentration (Cmax) and the half-life (t1/2) of diazepam were increased but they did not change significantly (P = 0.17 and 0.12, respectively). Plasma N-desmethyldiazepam concentrations were slightly reduced during erythromycin treatment up to 8 hr (P < 0.05). In Study 2 the AUC (0-42 hr) of flunitrazepam was increased by 25% (P < 0.05) during the erythromycin treatment. The t1/2 of flunitrazepam increased significantly (P < 0.05), but the Cmax remained unchanged. The psychomotor effects of diazepam or flunitrazepam were not changed significantly by erythromycin. These pharmacokinetic interactions can be explained by the reduced metabolic elimination of diazepam and flunitrazepam. The interactions of erythromycin with diazepam and flunitrazepam seem to be slight and of limited clinical significance only.

Treatment and prophylaxis of Mycobacterium avium complex

The most common pathogens involved in disseminated bacterial infection in people with acquired immunodeficiency syndrome (AIDS) are organisms of the Mycobacterium avium-intracellulare complex (MAC). Azithromycin and clarithromycin, a new azalide and macrolide, respectively, are among the most potent monotherapies for MAC bacteraemia, Although many bloodstream isolates demonstrate increased minimum inhibitory concentrations after 4 months of treatment. Current recommended prophylaxis, based on the results of two randomized, double-blind, prospective studies, is rifabutin 300 mg daily for people with AIDS with < 100 CD4 lymphocytes/mm3. In the beige mouse model, we have shown that both azithromycin and clarithromycin prevent MAC bacteraemia following repetitive oral challenge. Clinical trails are now underway to confirm these effects in man; comparative treatments include placebo, rifabutin and an azalide/macrolide plus rifabutin. While combinations might be more effective and reduce the emergence of resistance, the spectre of cytochrome P-450 drug interactions necessitates careful study before combination prophylactic approaches are accepted. Such interactions are associated with rifabutin and some macrolides, although azithromycin may be less problematic in this respect as it appears to have little potential to interact with other antimicrobial agents.

Macrolides versus azalides: a drug interaction update

OBJECTIVE

To describe the current drug interaction profiles for all approved and investigational macrolide and azalide antimicrobials, and to comment on the clinical impact of these interactions when appropriate.

DATA SOURCES

MEDLINE was searched to identify all pertinent studies, review articles, and case reports from 1975 to 1995. When appropriate information was not available in the literature, data were obtained from the product manufacturers.

STUDY SELECTION

All available data were reviewed to give an unbiased account of possible drug interactions.

DATA EXTRACTION

Data for some of the interactions were not available from the literature, but were available from abstracts or from company-supplied materials. Although the data were not always entirely explicative, the best attempt was made to deliver the pertinent information that clinical practitioners would need to formulate practice opinions. When more in-depth information was supplied in the form of a review or study report, a thorough explanation of pertinent methodology was supplied.

DATA SYNTHESIS

Since the introduction of erythromycin into clinical practice, there have been several clinically significant drug interactions identified throughout the literature associated with this drug. These interactions have been caused mostly by inhibition of the CYP3A subclass of hepatic enzymes, thereby decreasing the metabolism of any other agent given concurrently that is also cleared through this mechanism. With the development and marketing of several new macrolides, it was hoped that the drug interaction profile associated with this class would improve. This has been met with variable success. Although some of the extensions of the 14-membered ring macrolides have shown an incidence of interactions equal to that of erythromycin, others have shown improved profiles. In contrast, the 16-membered ring macrolides have demonstrated a much improved, though not absent, interaction profile. The most success in avoiding drug interactions through structure modification has been accomplished with the development of the azalide class, of which azithromycin is the first to be approved for marketing. This agent has to date produced none of the classic drug interactions that most macrolides have demonstrated in patient care.

CONCLUSIONS

The introduction of new 14- and 16-membered ring macrolides appears to have had a variable effect in modifying the incidence of drug interactions associated with this class. Azithromycin, a member of the new azalide class, has to date produced fewer clinically significant interactions than other azalides with any agent that is cleared through the CYP3A system.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of omeprazole on concentrations of clarithromycin in plasma and gastric tissue at steady state

This study was conducted to determine (i) the effect of omeprazole on steady-state concentrations of clarithromycin and 14-(R)-hydroxyclarithromycin in plasma and gastric mucosa, (ii) the effect of clarithromycin on steady-state concentrations of omeprazole in plasma, and (iii) the effect of clarithromycin on the suppression of gastric acid secretion by omeprazole. Twenty healthy, Helicobacter pylori-negative male subjects completed this three-period, double-blind, randomized crossover study. In period 1, all subjects received 40 mg of omeprazole each morning for 6 days. Twenty-four-hour gastric pH monitoring took place on days -1 and 6. Pharmacokinetic sampling took place on day 6. In periods 2 and 3, subjects were randomly assigned to receive either 40 mg of omeprazole or omeprazole placebo daily for 6 days plus clarithromycin (500 mg) every 8 h for 5 days with a single 500-mg dose on day 6. Gastric tissue and mucus samples were obtained via endoscopy on day 5. Gastric pH monitoring and pharmacokinetic sampling took place on day 6. Two-week washout intervals separated the three study periods. Clarithromycin increased mean omeprazole area under the concentration-time curve from 0 to 24 h from 3.3 +/- 2.0 to 6.3 +/- 4.5 micrograms.h/ml (P < 0.05) and harmonic mean half-life from 1.2 to 1.6 h (P < 0.05) but did not significantly alter the effect of omeprazole on gastric pH. Mean clarithromycin area under the concentration-time curve from 0 to 8 h increased from 22.9 +/- 5.5 (placebo) to 26.4 +/- 5.7 micrograms.h/ml (omeprazole) (P < 0.05) when clarithromycin was administered with omeprazole. Analysis of variance revealed that mean concentrations of clarithromycin in tissue and mucus were statistically significantly higher when clarithromycin was given with omeprazole than when clarithromycin was given with placebo (P <0.001). Mean maximum observed concentrations of clarithromycin in the gastric fundus increased from 20.8 +/- 7.6 (placebo) to 24.3 +/- 6.4 micrograms/g (omeprazole), and those in the gastric mucous from 4.2 +/- 7.7 placebo to 39.3 +/- 32.8 micrograms/g (omeprazole). Similar increases were observed for the 14-(R)-hydroxyclarithromycin. These results show that omeprazole increases concentrations of clarithromycin in gastric tissue and mucus and may provide a mechanism for synergy between clarithromycin ad omeprazole that explains the excellent eradication of H. pylori seen in clinical trials.

Macrolide antibacterials. Drug interactions of clinical significance

Macrolide antibiotics can interact adversely with commonly used drugs, usually by altering metabolism due to complex formation and inhibition of cytochrome P-450 IIIA4 (CYP3A4) in the liver and enterocytes. In addition, pharmacokinetic drug interactions with macrolides can result from their antibiotic effect on microorganisms of the enteric flora, and through enhanced gastric emptying due to a motilin-like effect. Macrolides may be classified into 3 different groups according to their affinity for CYP3A4, and thus their propensity to cause pharmacokinetic drug interactions. Troleandomycin, erythromycin and its prodrugs decrease drug metabolism and may produce drug interactions (group 1). Others, including clarithromycin, flurithromycin, midecamycin, midecamycin acetate (miocamycin; ponsinomycin), josamycin and roxithromycin (group 2) rarely cause interactions. Azithromycin, dirithromycin, rikamycin and spiramycin (group 3) do not inactivate CYP3A4 and do not engender these adverse effects. Drug interactions with carbamazepine, cyclosporin, terfenadine, astemizole and theophylline represent the most frequently encountered interactions with macrolide antibiotics. If the combination of a macrolide and one of these compounds cannot be avoided, serum concentrations of concurrently administered drugs should be monitored and patients observed for signs of toxicity. Rare interactions and those of dubious clinical importance are those with alfentanil and sufentanil, antacids and cimetidine, oral anticoagulants, bromocriptine, clozapine, oral contraceptive steroids, digoxin, disopyramide, ergot alkaloids, felodipine, glibenclamide (glyburide), levodopa/carbidopa, lovastatin, methylprednisolone, phenazone (antipyrine), phenytoin, rifabutin and rifampicin (rifampin), triazolam and midazolam, valproic acid (sodium valproate) and zidovudine.

Delirium probably induced by clarithromycin in a patient receiving fluoxetine

BACKGROUND

Clarithromycin is a macrolide antibiotic very similar to erythromycin in structure and spectrum of activity. It has gained increasing use since its release in Canada in May 1992, partly because it is promoted as having less potential for drug interactions and adverse effects. However, as with all new medications, a high degree of vigilance for unreported adverse effects is advisable.

CASE SUMMARY

A healthy 53-year-old lawyer was receiving long-term fluoxetine 80 mg hs and nitrazepam 10 mg hs for depression and mild sleep apnea. Subsequent to initiation of treatment with clarithromycin for a respiratory infection, he rapidly developed delirium, which cleared quickly after stopping all 3 medications. The delirium and psychosis did not recur when the infection was treated with erythromycin alone or after restarting fluoxetine and nitrazepam therapy at previous dosages in the absence of antibiotics.

DISCUSSION

This man's delirium is consistent with fluoxetine intoxication, which appears to have resulted from inhibition of hepatic cytochrome P450 metabolism by clarithromycin. Undiagnosed, this serious drug reaction could have lead to serious medical and social consequences.

CONCLUSIONS

As the use of clarithromycin increases, the potential for interactions with other drugs metabolized by the P450 enzyme system may be realized. Clinicians should consider which other medications a patient is receiving before prescribing clarithromycin or any macrolide antibiotic with potential to influence the P450 system.

Interaction between erythromycin and nitrazepam in healthy volunteers

Interaction between erythromycin, a strong inhibitor of CYP3A4, and nitrazepam, a long-acting benzodiazepine, was investigated in a double-blind and randomized cross-over study of two phases. Ten healthy volunteers received erythromycin (500 mg x 3) orally or placebo for 6 days. On the fourth day they were given a challenge dose of 5 mg nitrazepam. Plasma samples were collected and psychomotor effects were measured during 42 hr after intake of nitrazepam. There was a statistically significant pharmacokinetic interaction between erythromycin and nitrazepam. Erythromycin increased the area under the nitrazepam concentration-time curve by 25% (P < 0.05) and the peak concentration by 30% (P < 0.05). The concentration peak time of nitrazepam was shortened by over 50% (P < 0.05). The elimination half-lives did not change. Accordingly, as far as the metabolism of nitrazepam is concerned, erythromycin does not cause any major changes in the metabolism of nitrazepam. In psychomotor performance only minor differences were seen. It is concluded that the interaction between erythromycin and nitrazepam is of little clinical significance.

Helicobacter pylori gastritis therapy with omeprazole and clarithromycin increases serum carbamazepine levels

An antibiotic combination that includes a proton pump inhibitor such as omeprazole and an antibiotic such as clarithromycin is likely to become the new standard regimen for treatment of Helicobacter pylori gastritis because this combination is extremely effective and very well tolerated. The current report highlights a potentially significant pharmakokinetic drug interaction between clarithromycin and carbamazepine in two patients with long-standing epilepsy who were given such therapy for Helicobacter pylori gastritis. In both cases, clarithromycin therapy was temporally related to an increase in serum carbamazepine levels, which returned to the therapeutic range following cessation of clarithromycin therapy. The potential implications of this newly recognized drug interaction are discussed.