Comparison of the effects of the new azalide antibiotic, azithromycin, and erythromycin estolate on rat liver cytochrome P-450

Evaluation of azithromycin in systemic and toothpaste forms for the treatment of ciclosporin-associated gingival overgrowth in dogs

BACKGROUND

Gingival overgrowth is an adverse effect of ciclosporin therapy. Azithromycin (AZI) in capsule and toothpaste form is an effective treatment for ciclosporin-associated gingival overgrowth (CsAGO) in humans.

HYPOTHESIS/OBJECTIVES

To evaluate AZI in a systemic and a toothpaste form for the treatment of CsAGO in dogs. The secondary objective was to determine which treatment is more effective.

ANIMALS

Thirty-six client-owned dogs with CsAGO.

METHODS

Dogs were randomly assigned to the following four groups: AZI capsule; AZI toothpaste; placebo capsule; and placebo toothpaste. Treatments were for 4 weeks, and measurements of gingival sulcus depth, tooth length and subjective global scores were taken at weeks 0, 2, 4 and 8. The AZI dose was 10 mg/kg daily, and brushing (8.5% AZI) was once daily.

RESULTS

There was a significant decrease in gingival sulcus depth for the AZI capsule group at week 8 and for the AZI toothpaste group at weeks 2, 4 and 8. The mean decrease in gingival sulcus depth was significantly greater in active versus placebo groups (P = 0.0356). The tooth length and subjective global scores were not significantly different for any groups. Gastrointestinal adverse events occurred in all groups, but more frequently in the AZI capsule group.

CONCLUSIONS AND CLINICAL IMPORTANCE

Azithromycin improved CsAGO in only one measured parameter, gingival sulcus depth. Only one dog in the AZI capsule group had complete resolution of CsAGO. Further studies are warranted. Azithromycin capsules were associated with the most gastrointestinal adverse effects.

Rhabdomyolysis caused by an unusual interaction between azithromycin and simvastatin

Rhabdomyolysis is an uncommon but life-threatening adverse effect of simvastatin therapy. A 73-year-old male on chronic simvastatin therapy received azithromycin for acute bronchitis. He presented with weakness of all extremities with a significant increase in creatinine phosphokinase levels and acute kidney injury. Simvastatin was stopped and supportive therapy with intravenous saline and bicarbonate was initiated. The serum creatinine and creatine phosphokinase returned to baseline in the next 7 days. Two months later, simvastatin was resumed without any recurrence of symptoms. Our case report highlights the rare description of rhabdomyolysis caused by a drug interaction between simvastatin with azithromycin.

A clinically significant interaction between tacrolimus and multiple proton pump inhibitors in a kidney transplant recipient

The shared metabolism of PPIs and tacrolimus through the CYP enzyme system has been associated with clinically significant drug interactions, especially in patients who are classified as CYP 2C19 PMs. However, existing data are conflicting, indicating that a single mechanism does not account for all interactions. A drug interaction between tacrolimus and omeprazole, esomeprazole, but not lansoprazole, occurred in an 18-yr-old female kidney transplant recipient classified as a CYP 2C19 extensive (normal) metabolizer. This case suggests that further research is needed to establish the definitive mechanism of this potentially serious drug-drug interaction. Physicians prescribing PPIs in organ transplant recipients with tacrolimus immunosuppression should consider close pharmacokinetic monitoring of tacrolimus when starting or switching a PPI.

Cryptosporidium enteritis in solid organ transplant recipients: multicenter retrospective evaluation of 10 cases reveals an association with elevated tacrolimus concentrations

BACKGROUND

Cryptosporidial enteritis, a diarrheal infection of the small intestine caused by the apicomplexan protozoa Cryptosporidium, is infrequently recognized in transplant recipients from developed countries.

METHODS

A retrospective review of all cases of cryptosporidiosis in solid organ transplant (SOT) recipients at 2 centers from January 2001 to October 2010 was performed and compared with transplant recipients with community-onset Clostridium difficile infection (CDI). A literature search was performed with regard to reported cases of cryptosporidiosis in SOT recipients.

RESULTS

Eight renal, 1 liver, and 1 lung transplant recipient were diagnosed with cryptosporidiosis at median 46.0 months (interquartile range [IQR] 25.2-62.8) following SOT. Symptoms existed for a median 14 days (IQR 10.5-14.8) before diagnosis. For the 9 patients receiving tacrolimus (TAC), mean TAC levels increased from 6.3 ± 1.1 to 21.3 ± 9.2 ng/mL (P = 0.0007) and median serum creatinine increased temporarily from 1.3 (IQR 1.1-1.7) to 2.4 (IQR 2.0-4.6) mg/dL (P = 0.008). By comparison, 8 SOT recipients (6 kidney, 2 liver) hospitalized with community-onset CDI had a mean TAC level of 10.8 ± 2.8 ng/dL during disease compared with 9.2 ± 2.3 ng/mL at baseline (P = 0.07) and had no change in median creatinine. All patients recovered from Cryptosporidium enteritis after receiving various chemotherapeutic regimens.

CONCLUSIONS

Cryptosporidiosis should be recognized as an important cause of diarrhea after SOT and is associated with elevated TAC levels and acute kidney injury. Increased TAC levels may reflect altered drug metabolism in the small intestine.

Influence of ABCB1 gene polymorphisms on the pharmacokinetics of azithromycin among healthy Chinese Han ethnic subjects

The aim of this study was to evaluate the effects of ABCB1 gene polymorphisms on azithromycin pharmacokinetics in Chinese Han ethnic subjects. In total, 20 healthy volunteers with various ABCB1 genotypes (6 with 2677GG/3435CC, 8 with 2677GT/3435CT, 6 with 2677TT/3435TT) were enrolled. Each was given a single oral dose of 500 mg azithromycin. Plasma concentration was measured for up to 96 h by LC/MS/MS. As shown, C(max) was significantly lower among individuals with 2677TT/3435TT genotype (468.0 +/- 173.4 ng x h/ml) than those with 2677GG/3435CC (911.2 +/- 396.4 ng x h/ml, p = 0.013). However, the t(max) value was higher among subjects with 2677TT/3435TT (2.0 +/- 0.5 h) than those with 2677GT/3435CT (1.6 +/- 0.3 h) or 2677GG/3435CC (1.4 +/- 0.4 h) genotypes (p = 0.068 and p = 0.026, respectively). Furthermore, the AUC(last) tended to be higher among subjects with 2677GG/3435CC than those with 2677GT/3435CT or 2677TT/3435TT genotypes (5000.2 +/- 1610.0 vs. 4558.0 +/- 805.0 vs. 4131.0 +/- 995.1 ng/ml). Our results showed for the first time that azithromycin pharmacokinetics may be influenced by particular polymorphisms of the ABCB1 gene. Individualized dosage regimen design incorporating such information may improve the efficacy of the drug while reducing adverse reactions.

Clinically important drug interactions potentially involving mechanism-based inhibition of cytochrome P450 3A4 and the role of therapeutic drug monitoring

Cytochrome P450 (CYP) 3A4 is the most abundant enzyme of CYPs in the liver and gut that metabolizes approximately 50% currently available drugs. A number of important drugs have been identified as substrates, inducers, and/or inhibitors of CYP3A4. The substrates of CYP3A4 considerably overlap with those of P-glycoprotein. Both CYP3A4 and P-glycoprotein are subject to inhibition and induction by a number of factors. Mechanism-based inhibition of CYP3A4 is characterized by NADPH-, time-, and concentration-dependent enzyme inactivation occurring when some xenobiotics or drugs are converted by CYPs to reactive metabolites. Such an inhibition of CYP3A4 is caused by chemical modification of the heme, the protein, or both as a result of covalent binding of modified heme to the protein. To date, the identified clinically important mechanism-based CYP3A4 inhibitors mainly include macrolide antibiotics (eg, clarithromycin and erythromycin), anti-HIV agents (eg, ritonavir and delavirdine), antidepressants (eg, fluoxetine and fluvoxamine), calcium channel blockers (eg, verapamil and diltiazem), steroids and their modulators (eg, gestodene and mifepristone), and several herbal and dietary components. The inactivation of CYP3A4 by drugs often causes unfavorable and long-lasting drug-drug interactions and probably fatal toxicity, depending on many factors associated with the enzyme, drugs, and the patients. Clinicians are encouraged to have a sound knowledge of drug-induced, mechanism-based CYP3A4 inhibition; take proper cautions, and perform close monitoring for possible drug interactions when using drugs that are mechanism-based CYP3A4 inhibitors. To minimize drug-drug interactions involving mechanism-based CYP3A4 inhibition, it is necessary to choose safe drug combination regimens, adjust drug dosages appropriately, and conduct therapeutic drug monitoring for drugs with narrow therapeutic indices.

Identification of severe potential drug-drug interactions using an Italian general-practitioner database

OBJECTIVE

To analyze prescriptions in a general-practitioner database over 1 year to determine the frequency, the characteristics, and the monitoring of the severe potential drug-drug interactions (DDIs).

METHODS

We retrospectively analyzed the clinical records from 16 general practitioners in the Veneto region, an area in northern Italy. The study covered the period from January 1 to December 31, 2004. We selected all severe and well-documented interactions according to the book Drug Interaction Facts by David S. Tatro (Facts and Comparisons, St. Louis, MO, 2006). We grouped severe potential DDIs according to their specific potential risk, and for the most frequently interacting drug pairs, we investigated whether some specific tests had been prescribed by physicians for safety monitoring.

RESULTS

During the study period, 16,037 patients (55% female) with at least one drug prescription were recorded, and a total of 185,704 prescriptions relating to 1,020 different drugs were analyzed. Ramipril was the most frequently prescribed drug followed by acetylsalicylic acid and atorvastatin. The final number of different types of severe potential DDIs was 119, which occurred 1,037 times in 758 patients (4.7% of the total number of patients). More than 80% of drugs involved in severe potential DDIs were cardiovascular drugs. Digoxin was the most frequently involved drug. Electrolyte disturbances, increase in serum digoxin levels, risk of hemorrhage, severe myopathy or rhabdomyolysis, and cardiac arrhythmias were the most commonly implicated potential risks. When considering patients using digoxin with loop or thiazide diuretics for more than 5 months, 72% had at least one test to monitor potential digoxin toxicity, whereas 28% had no tests. Sixty-four percent of patients using digoxin with amiodarone, verapamil, or propafenone had an ECG and/or digoxin monitoring, and 36% of them did not have any tests.

CONCLUSIONS

The present study revealed that, in a group of Italian general practitioners, the risks of severe potential drug interactions are relatively low and the drugs concerned are few. Analyses of specific tests showed that physicians are generally aware of the potential risks caused by digoxin drug associations. However not all patients were closely monitored and this should be improved.

Severe cryptosporidiosis in a seven-year-old renal transplant recipient: case report and review of the literature

Cryptosporidium is an intracellular protozoa that can cause gastroenteritis in humans. In immunocompromised hosts, infection can be severe, leading to life-threatening persistent diarrhea. There is limited experience in treating this infection in solid organ transplants. Although newer drugs active against Cryptosporidium exist, they are only licensed in the USA for treatment of immunocompetent hosts. Here we describe a seven-year-old renal transplant recipient with severe cryptosporidiosis. He had a protracted course of diarrhea of up to 2 L/day. He was successfully managed with combination antimicrobial therapy including nitazoxanide, paromomycin, and azithromycin. In conjunction with this regimen, he had a reduction in immunosuppression and complete bowel rest. His stool pattern normalized in four weeks and he has had no recurrence after six months of follow up.

Update on clinically significant drug interactions in dermatology

Although there are thousands of drug interactions (DIs) listed in computers and manuals, only about 10% are clinically significant. Equally disturbing is these systems fail to detect up to one third of all dangerous DIs. This update on clinically significant DIs is current through September 2005 and discusses systemic drugs important to dermatologists. In addition, 4 aspects of DIs are discussed that are helpful in replacing rote memory with a step-by-step, logical approach based on scientific evidence.

Current use of anti-infectives in dermatology

Dermatologic diseases encompass a broad category of pathologic situations. Infection remains a significant aspect of the pathology faced in patient encounters, and it is natural to expect that anti-infectives play a major element in the armamentarium utilized by dermatologists. Aside from the treatment of the classic bacterial and fungal infections, there are now new uses for antiviral agents to help suppress recurrent disease, such as herpes simplex. There is also the novel approach of using anti-infectives, or agents that have been thought to have antimicrobial activity, to treat inflammatory diseases. This review describes anti-infectives, beginning with common antibiotics used to treat bacterial infections. The discussion will then cover the current use of antivirals. Finally, the description of antifungals will be separated, starting with the oral agents and ending with the topical antimycotics. The use of anti-infectives in tropical dermatology has been purposefully left out, and perhaps should be the subject of a separate review. Cutaneous bacterial infections consist chiefly of those microorganisms that colonize the skin, such as species of staphylococcus and streptococcus. Propionibacterium acnes and certain other anaerobes can be involved in folliculitis, pyodermas and in chronic conditions such as hidradenitis suppurativa.

Statin-macrolide interaction risk: a population-based study throughout a general practice database

INTRODUCTION

The myopathy risk associated with statin use increases in case of concomitant prescription of certain drugs, such as cytochrome P(450) (CYP) system inhibitors (i.e., macrolides). The aim of this study was to assess whether concurrent statin/macrolide prescriptions at high interaction risk are commonly written in a general practice setting.

METHODS

For this study, 156 general practitioners (GPs) with a patient population of almost 200,000 individuals, and participating in the Arianna database set up by "Caserta-1" Local health-Service Agency (ASL), were recruited. Within such a study sample, subjects receiving at least one statin/macrolide concomitant prescription during the year 2003 were identified. Sensitivity analysis was performed to assess the time distribution of high-risk macrolide prescriptions written within +/-10 days from the statin prescription date.

RESULTS

Among 190,124 patients included in the study, 7,176 (3.8%) received at least one statin prescription during the observation period. Of these, 228 (3.2%) were occasionally co-prescribed with any macrolide on the same date, in 153 cases (2.1% of statin users) the macrolide being of high interaction risk. In particular, 2.1% (55) of simvastatin users and 2.2% (64) of patients on atorvastatin were prescribed with high-risk macrolide on the same date versus 0.6% and 1.8% of patients prescribed with fluvastatin and pravastatin, respectively. Concerning GPs, 99 (63.5%) co-administered statin and macrolide at high interaction risk at least once.

CONCLUSIONS

Most GPs occasionally prescribed statin/macrolide at high interaction risk on the same date, despite the availability of therapeutic alternatives. Prevention strategies targeted to increase awareness of health professionals about the interaction risks of widely prescribed drugs are needed.

Mechanism-based inhibition of cytochrome P450 3A4 by therapeutic drugs

Consistent with its highest abundance in humans, cytochrome P450 (CYP) 3A is responsible for the metabolism of about 60% of currently known drugs. However, this unusual low substrate specificity also makes CYP3A4 susceptible to reversible or irreversible inhibition by a variety of drugs. Mechanism-based inhibition of CYP3A4 is characterised by nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-, time- and concentration-dependent enzyme inactivation, occurring when some drugs are converted by CYP isoenzymes to reactive metabolites capable of irreversibly binding covalently to CYP3A4. Approaches using in vitro, in silico and in vivo models can be used to study CYP3A4 inactivation by drugs. Human liver microsomes are always used to estimate inactivation kinetic parameters including the concentration required for half-maximal inactivation (K(I)) and the maximal rate of inactivation at saturation (k(inact)). Clinically important mechanism-based CYP3A4 inhibitors include antibacterials (e.g. clarithromycin, erythromycin and isoniazid), anticancer agents (e.g. tamoxifen and irinotecan), anti-HIV agents (e.g. ritonavir and delavirdine), antihypertensives (e.g. dihydralazine, verapamil and diltiazem), sex steroids and their receptor modulators (e.g. gestodene and raloxifene), and several herbal constituents (e.g. bergamottin and glabridin). Drugs inactivating CYP3A4 often possess several common moieties such as a tertiary amine function, furan ring, and acetylene function. It appears that the chemical properties of a drug critical to CYP3A4 inactivation include formation of reactive metabolites by CYP isoenzymes, preponderance of CYP inducers and P-glycoprotein (P-gp) substrate, and occurrence of clinically significant pharmacokinetic interactions with coadministered drugs. Compared with reversible inhibition of CYP3A4, mechanism-based inhibition of CYP3A4 more frequently cause pharmacokinetic-pharmacodynamic drug-drug interactions, as the inactivated CYP3A4 has to be replaced by newly synthesised CYP3A4 protein. The resultant drug interactions may lead to adverse drug effects, including some fatal events. For example, when aforementioned CYP3A4 inhibitors are coadministered with terfenadine, cisapride or astemizole (all CYP3A4 substrates), torsades de pointes (a life-threatening ventricular arrhythmia associated with QT prolongation) may occur.However, predicting drug-drug interactions involving CYP3A4 inactivation is difficult, since the clinical outcomes depend on a number of factors that are associated with drugs and patients. The apparent pharmacokinetic effect of a mechanism-based inhibitor of CYP3A4 would be a function of its K(I), k(inact) and partition ratio and the zero-order synthesis rate of new or replacement enzyme. The inactivators for CYP3A4 can be inducers and P-gp substrates/inhibitors, confounding in vitro-in vivo extrapolation. The clinical significance of CYP3A inhibition for drug safety and efficacy warrants closer understanding of the mechanisms for each inhibitor. Furthermore, such inactivation may be exploited for therapeutic gain in certain circumstances.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Determination of the lack of a drug interaction between azithromycin and warfarin

STUDY OBJECTIVE

To determine the effect on the international normalized ratio (INR) of adding azithromycin to patients receiving stable dosages of warfarin.

DESIGN

Retrospective chart review.

SETTING

Outpatient clinic.

PATIENTS

Ambulatory patients receiving warfarin and azithromycin concurrently who had a documented therapeutic INR value before the start of azithromycin therapy (pre-INR) and a documented INR value within 30 days after the start of azithromycin therapy (post-INR).

MEASUREMENTS AND MAIN RESULTS

Patients given felodipine during long-term warfarin therapy formed a comparative control group. Patient demographics were similar in both treatment groups. Mean age of the azithromycin group (17 patients) was 59 +/- 13 years and of the control group (20 patients) 65 +/- 12 years. All 17 patients in the azithromycin group and 16 of the controls were women. Mean change from pre-INR to post-INR in the azithromycin and control groups, respectively, was 0.14 +/- 0.64 (pre-INR 2.46, post-INR 2.61) and 0.19 +/- 0.54 (pre-INR 2.46, post-INR 2.66) (p = 0.74). A post hoc power analysis based on a pooled standard deviation of 0.60 revealed that the study had 68% power to detect a 0.5 change in the INR value.

CONCLUSION

No interaction between azithromycin and warfarin was observed in ambulatory patients with therapeutic baseline INR values.

Enhanced hypoprothrombinemia with warfarin due to azithromycin

OBJECTIVE

To report a case of probable azithromycin-warfarin drug interaction with enhanced hypoprothrombinemic effect of warfarin.

CASE SUMMARY

An 83-year-old African American man stabilized on warfarin therapy (10 mg on Wednesdays, 7.5 mg on other days) developed a prolonged prothrombin time one day after starting azithromycin 500 mg. The elevated prothrombin time normalized 3 days after azithromycin was discontinued. After the initial increase in the international normalized ratio, the absence of any significant confounding factors affecting the anticoagulant effect of warfarin in our patient and the numerous reports of such interactions indicate that an interaction between azithromycin and warfarin may have been responsible for the elevated prothrombin time seen in this patient. An objective causality assessment revealed that the adverse event was probably related to the combination of these drugs.

DISCUSSION

Azithromycin, unlike erythromycin and clarithromycin, is not known to inhibit the cytochrome P450 enzyme system and is presumed to be the macrolide of choice in patients already on warfarin. However, previously reported cases of azithromycin-warfarin interactions support the possibility that azithromycin does interact with warfarin, although the exact mechanism is not understood.

CONCLUSIONS

Azithromycin may interact with warfarin and enhance its hypoprothrombinemic effects. This effect may be delayed for 4-8 days after a course of azithromycin has been completed. Periodic monitoring of the prothrombin time is recommended when using azithromycin in patients taking warfarin.

Protective Effect of Sesbania grandiflora Against Erythromycin Estolate-Induced Hepatotoxicity

Sesbania grandiflora, commonly known as 'sesbania', is widely used in Indian folk medicine for the treatment of liver disorders. Oral administration of an ethanolic extract of S. grandiflora leaves (200 mg/kg/day) for 15 days produced significant hepatoprotection against erythromycin estolate (800 mg/kg/day)-induced hepatotoxicity in rats. The increased level of serum enzymes (aspartate transaminase, alanine transaminase, alkaline phosphatase), bilirubin, cholesterol, triglycerides, phospholipids, free fatty acids, plasma thiobarbituric acid reactive substances and hydroperoxides observed in rats treated with erythromycin estolate were significantly decreased in rats treated concomitantly with sesbania extract and erythromycin estolate. The sesbania extract also restored the depressed levels of antioxidants to near normal. The results of the study reveal that sesbania could afford a significant protective effect against erythromycin estolate-induced hepatotoxicity. The effect of sesbania was compared with that of silymarin, a reference hepatoprotective drug.

Antibiotic treatment of incipient drug-induced gingival overgrowth in adult renal transplant patients

BACKGROUND

Drug-induced gingival overgrowth (GO) remains a challenge in periodontics. Partial and total regressions of this GO have been reported after a short course of antibiotics.

METHODS

We conducted a double-blinded controlled randomised study to determine the effect of metronidazole (MNZ) or azithromycin (AZM) on the regression of incipient cyclosporin A-induced GO in 40 adult renal transplanted patients. The quantitation of the GO was performed with Image Digital Analysis.

RESULTS

None of the patients with GO showed complete remission after 30 days. The pretreatment GO index was 0.895 +/- 0.16 in the metronidazole treatment group (MNZ group, n = 13), 0.932 +/- 0.11 in the azithromycin treatment group (AZM group, n = 14), and 1.073 +/- 0.32 in the controls (placebo group, n = 13). At the end of the study (30 days), the GO index score was lower in 54.4% and 62.3% of the MNZ and AZM groups, respectively, and the mean score differences were statistically significant between the groups (0.897 +/- 0.28, MNZ group vs. 0.909 +/- 0.15, AZM group vs. 1.130 +/- 0.3, placebo group, P < 0.05 ANOVA).

CONCLUSIONS

A 7-day course of MNZ or AZM does not induce remission of CsA-induced GO, although it acts on concomitant bacterial over-infection and gingival inflammation.

Azithromycin metabolite identification in plasma, bile, and tissues of the ball python (Python regius)

Azithromycin is the first of a class of antibiotics classified as azalides. Six ball pythons (Python regius) were given a single dose of azithromycin at 10 mg/kg p.o. and i.v. in a crossover design. Serial blood samples were collected for unchanged azithromycin and to determine, if possible, the structure and number of circulating azithromycin metabolites. After a 4-month wash-out period, the snakes were given azithromycin p.o. as a single dose of 10 mg/kg for the study of azithromycin metabolism and metabolite tissue distribution. Bile, liver, lung, kidney, and skin samples were analyzed for the metabolites identified from the first experiment. Unchanged azithromycin accounted for 80, 68, and 60% of the total material at 12, 24, and 48 h postadministration in plasma, independent of route of administration. At both 24 and 72 h postadministration, azithromycin accounted for 70% of total azithromycin- associated material in bile. In liver and kidney, unchanged azithromycin accounted for 40% of the total azithromycin-associated material; this doubled in lung and skin. Fifteen metabolites were positively or tentatively identified in plasma, bile, or tissues of all snakes. Four of these possible metabolites: 3'-desamine-3-ene-azithromycin, descladinose dehydroxy-2-ene-azithromycin, 3'-desamine-3-ene descladinose-azithromycin, and 3'-N-nitroso,9a-N-desmethyl-azithromycin are unique to this species. Descladinose-azithromycin, 3'-N-desmethyl,9a-N-desmethyl-azithromycin, and 3'-N-desmethyl, 3'-O-desmethyl-azithromycin were the only metabolites identified in skin. Kidney tissue contained a greater number of metabolites than liver tissue, with 3'-N-didesmethyl-azithromycin being identified only in the kidney. Compared with the dog and cat, a greater number of metabolites were identified in ball python plasma. The percentage of unchanged azithromycin in bile is not different between the three species.

The impact of novel immunosuppressive agents on infections in organ transplant recipients and the interactions of these agents with antimicrobials

Several of the new immunosuppressive agents that are used to treat transplant recipients possess in vitro activity against specific pathogens, enhance the activity of antimicrobial agents, or have unique drug interactions with antimicrobial agents. Mycophenolate mofetil may have a protective effect against Pneumocystis carinii; it also enhances the activity of ganciclovir and has strong antiviral activity against human immunodeficiency virus type 1. High doses of mycophenolate mofetil have been associated with a higher frequency of tissue-invasive cytomegalovirus disease but not with asymptomatic cytomegalovirus infection. Rapamycin exhibits potent in vitro fungicidal activity against Cryptococcus neoformans and several pathogenic fungi in transplant recipients; however, it is not known whether its immunosuppressive effect in organ transplant recipients outweighs its antifungal activity. Recognition of the unique characteristics of these agents and the evolving spectrum of opportunistic infections has implications for the differential diagnosis, management, and prophylaxis of infections in organ transplant recipients in the modern immunosuppressive era.

Possible interaction between intravenous azithromycin and oral cyclosporine

A 42-year-old man who had received a cadaveric kidney transplant 9 years earlier was admitted to the hospital with pneumonia. His oral cyclosporine dosage for the past 2 years was stabilized at 100 mg twice/day; his cyclosporine whole blood trough levels 15 days earlier and on the day he was admitted were both 178 ng/ml. The patient was treated with intravenous ceftriaxone and intravenous azithromycin and continued to receive the same dosage of oral cyclosporine. On hospital day 3, his cyclosporine trough level rose to 400 ng/ml and his dosage was reduced by 50%. Trough levels were 181 ng/ml and 175 ng/ml on hospital days 6 and 9, respectively On hospital day 9, the patient stopped receiving azithromycin. On hospital day 14, his cyclosporine trough level dropped to 76 ng/ml, and his cyclosporine dosage was increased back to 100 mg twice/day. The dosage produced trough levels consistent with those before he had been admitted. The patient was discharged on day 20, and a follow-up cyclosporine trough level determined 3 weeks later was 175 ng/ml. Administration of azithromycin may have caused the increased cyclosporine concentrations in this patient through p-glycoprotein inhibition and/or competition for biliary excretion. Azithromycin's interference may be inferred by the increase in cyclosporine levels after administration of this drug and the decrease in cyclosporine levels after its discontinuation-both consistent with the pharmacokinetic properties of cyclosporine. Ceftriaxone and acute-phase reactant activation during infection, however, also may have interfered with the patient's cyclosporine elimination. Azithromycin generally is considered unlikely to interact with cyclosporine. Nonetheless, practitioners should be aware of this possibility and should monitor cyclosporine levels closely, especially in critically ill patients who have other complications.

Systemic antibiotic agents

Understanding the breadth of systemic antimicrobial agents available for use by the dermatologist and their associated side-effect profiles and drug interactions allows the clinician to offer patients optimal care in the management of cutaneous infectious disease.

Potentially fatal interaction between azithromycin and disopyramide

A patient on disopyramide developed disopyramide toxicity when treated concurrently with azithromycin. Evidence of toxicity included an elevated serum disopyramide level and ventricular tachycardia requiring cardioversion. The azalide antibiotic presumably inhibited dealkylation of disopyramide to its major metabolite, mono-N-dealkyldisopyramide. Physicians should avoid using azithromycin in patients on disopyramide. If this drug combination is unavoidable, disopyramide levels must be closely monitored.

Retrospective evaluation of a potential interaction between azithromycine and warfarin in patients stabilized on warfarin

STUDY OBJECTIVE

To investigate a potential interaction between azithromycin and warfarin.

DESIGN

Retrospective case-control study.

SETTING

Veterans Affairs medical center.

PATIENTS

Fifty-two patients stable on anticoagulation therapy.

INTERVENTION

Patients who received a prescription for azithromycin and warfarin at any time since the hospital was opened, June 1, 1995, to July 22, 1999, were identified through a computerized report generated from the pharmacy prescription package.

MEASUREMENTS AND MAIN RESULTS

Patients having a stable international normalized ratio (INR; defined as a therapeutic INR +/- 0.2) for at least two consecutive visits before receiving an azithromycin prescription were reviewed. Changes in INR from before and after addition of azithromycin were compared with changes in a control group. Controls were identified from a computer-generated report of patients who received a prescription for terazosin and warfarin at any time since the hospital was opened to July 22, 1999 (terazosin was chosen as it has no known interaction with warfarin). These patients also had a stable INR for at least two consecutive visits before receiving the terazosin prescription. In patients with INRs on record within 14 days after starting azithromycin or terazosin (9 patients/group), the average change in INR was 0.18 +/- 0.48 in the azithromycin group and 0.07 +/- 0.49 in the terazosin group (p=0.60). For patients with an INR on record within 30 days after starting azithromycin or terazosin (26 patients/group), the average change in INR was 0.25 +/- 0.67 in the azithromycin group and 0.05 +/- 0.55 in the terazosin group (p=0.18).

CONCLUSION

An interaction between azithromycin and warfarin was not observed in this retrospective review of patients with a stable INR receiving the combination.

Macrolide drug interactions: an update

OBJECTIVE

To describe the current drug interaction profiles for the commonly used macrolides in the US and Europe, and to comment on the clinical impact of these interactions.

DATA SOURCES

A MEDLINE search (1975-1998) was performed to identify all pertinent studies, review articles, and case reports. When appropriate information was not available in the literature, data were obtained from the product manufacturers.

STUDY SELECTION

All available data were reviewed to provide 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 company-supplied materials. Although the data were not always explicit, the best attempt was made to deliver 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

Several clinically significant drug interactions have been identified since the approval of erythromycin. These interactions usually were related to the inhibition of the cytochrome P450 enzyme systems, which are responsible for the metabolism of many drugs. The decreased metabolism by the macrolides has in some instances resulted in potentially severe adverse events. The development and marketing of newer macrolides are hoped to improve the drug interaction profile associated with this class. However, this has produced variable success. Some of the newer macrolides demonstrated an interaction profile similar to that of erythromycin; others have improved profiles. The most success in avoiding drug interactions related to the inhibition of cytochrome P450 has been through the development of the azalide subclass, of which azithromycin is the first and only to be marketed. Azithromycin has not been demonstrated to inhibit the cytochrome P450 system in studies using a human liver microsome model, and to date has produced none of the classic drug interactions characteristic of the macrolides.

CONCLUSIONS

Most of the available data regarding macrolide drug interactions are from studies in healthy volunteers and case reports. These data suggest that clarithromycin appears to have an interaction profile similar to that of erythromycin. Given this similarity, it is important to consider the interaction profile of clarithromycin when using erythromycin. This is especially necessary as funds for further studies of a medication available in generic form (e.g., erythromycin) are limited. Azithromycin has produced few clinically significant interactions with any agent cleared through the cytochrome P450 enzyme system. Although the available data are promising, the final test should come from studies conducted in patients who are taking potentially interacting compounds on a chronic basis.

Potential interaction between azithromycin and warfarin

Azithromycin is considered unlikely to interact with warfarin. Unlike other macrolide antibiotics, it is not hepatically metabolized and did not produce an interaction with warfarin in a single-dose study. A 71-year-old woman with a prosthetic heart valve, stabilized with warfarin, had international normalized ratios (INRs) maintained between 2.5 and 3.5. Six days after she received a prescription for a 5-day course of azithromycin, her INR was 15.16. Phytonadione 10 mg was administered subcutaneously, and warfarin was held for 3 days until her INR fell to 2.10. She then was restabilized with warfarin. Until more information is known about the safety of warfarin and azithromycin, caution is advised when the agents are given together. Close monitoring of INR is recommended, and warfarin dosage adjustment may be necessary.

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.

Antimicrobial agents for the dermatologist. II. Macrolides, fluoroquinolones, rifamycins, tetracyclines, trimethoprim-sulfamethoxazole, and clindamycin

This article is the second of a two-part series reviewing antimicrobial agents that are used by the dermatologist. In part I we reviewed beta-lactam antibiotics and related compounds. In this section we again emphasize some newer agents (macrolides, fluoroquinolones) as well as some of the more commonly employed older agents (rifamycins, tetracyclines, trimethoprim-sulfamethoxazole, and clindamycin.

Protective effect of Livex, a herbal formulation against erythromycin estolate induced hepatotoxicity in rats

Livex, a compound herbal formulation, was investigated for its possible hepatoprotective effect in Wistar rats against erythromycin estolate induced toxicity. Oral administration of Livex significantly prevented the occurrence of erythromycin estolate induced hepatic damage. The increased level of serum enzymes (aspartate transaminase, alanine transaminase, alkaline phosphatase), bilirubin, serum and tissue cholesterol, triglycerides, phospholipids and free fatty acids observed in rats treated with erythromycin estolate were very much reduced in rats treated with Livex and erythromycin estolate. These biochemical observations were supplemented by histopathological examination of liver sections. Results of this study revealed that Livex could afford a significant protection against erythromycin estolate induced hepatocellular damage.

The role of advanced generation macrolides in the prophylaxis and treatment of Mycobacterium avium complex (MAC) infections

Since the start of the acquired immunodeficiency syndrome (AIDS) epidemic, the role of Mycobacterium avium complex (MAC) as an opportunistic pathogen in advanced AIDS patients has become more and more clear. Once identified in an advanced AIDS patient it is possible to find evidence that the MAC organism and infection is not only present in the pulmonary tree, but has also disseminated to a wide variety of body organs. Treatment of MAC or disseminated MAC (DMAC) infections has historically been very difficult due to the inherent resistance of the MAC pathogen to most standard antimycobacterial agents. This has resulted in the development of new agents for the prevention of DMAC infection as well as combinations of both new and standard agents for its treatment. Three drugs are currently approved for single-agent DMAC prophylaxis, including rifabutin, azithromycin and clarithromycin. Combinations of agents for DMAC treatment are highly variable in content but most experts agree that all combinations should contain one of the advanced generation macrolides (azithromycin or clarithromycin). Both of these agents have favourable intracellular pharmacokinetics and pharmacodynamics which maximise their effects against this mostly intracellular pathogen. Due to the paucity of comparative data, no one macrolide can be recommended over the other. However, the expected increase in compliance, lower weekly and annual costs, and lack of any drug interactions may make azithromycin a preferable choice, but this should be decided on a case-by-case basis.

Lovastatin-induced rhabdomyolysis possibly associated with clarithromycin and azithromycin

OBJECTIVE

To describe two cases of rhabdomyolysis in patients taking lovastatin that were precipitated by the use of the newer macrolide antibiotics clarithromycin and azithromycin.

CASE SUMMARIES

In each case, the patients were treated over 5 years with lovastatin and developed rhabdomyolysis that coincided with the completion of a prescribed regimen of a newer macrolide antibiotic. Following intravenous hydration and administration of bicarbonate, the patients' condition resolved without permanent' sequelae.

DISCUSSION

Rhabdomyolysis is a clinical syndrome resulting from the destruction of skeletal muscle that may progress to renal failure Several drugs have been associated with rhabdomyolysis, including lovastatin, a hydroxymethylglutaryl-coenzyme A reductase inhibitor. Erythromycin is a macrolide antibiotic that may increase the risk of lovastatin-induced rhabdomyolysis. To our knowledge, these cases are the first published reports of lovastatin-induced rhabdomyolysis associated with azithromycin and clarithromycin.

CONCLUSIONS

The risk of drug-induced rhabdomyolysis due to the potential interaction between lovastatin and azithromycin or clarithromycin should be considered before the concomitant use of these agents.

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.

Choosing the right macrolide antibiotic. A guide to selection

Macrolide antibiotics have proven to be valuable alternatives to penicillins and cephalosporins for the treatment of a number of infections. Currently, a number of macrolides are available. When choosing a particular macrolide, the types of organisms causing the infection, the tolerability of the drug, convenience of dosing and possible drug interactions all must be taken into account. Erythromycin, azithromycin and clarithromycin are equally effective against most gram-positive organisms. However, clarithromycin and azithromycin have much better activity against Haemophilus influenza and Moraxella catarrhalis. Thus, these 2 drugs are better choices for the treatment of community-acquired pneumonia. However, the low serum concentrations of azithromycin may be a problem in patients with bacteraemia associated with with community-acquired pneumonia. Clarithromycin appears to be effective for the treatment and prophylaxis of Mycobacterium avium complex (MAC) in patients with AIDS, while azithromycin appears to be effective for prophylaxis. Treatment of MAC with azithromycin is currently undergoing study. Although clarithromycin is the macrolide of choice for the treatment of Helicobacter pylori, azithromycin is the preferred macrolide for the treatment of Chlamydia trachomatis infections. The major factor limiting the use of azithromycin and clarithromycin has been their cost. However, these drugs may be cost effective if compliance is improved due to better tolerability and more convenient dosing regimens.

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)

Azithromycin and terfenadine: lack of drug interaction

A double-blind placebo-controlled study was conducted in healthy men to determine the effect of coadministration of azithromycin on the pharmacodynamics and pharmacokinetics of terfenadine. Administration of 500 mg azithromycin for 1 day and 250 mg on 4 subsequent days did not affect the pharmacokinetics of the pharmacologically active terfenadine carboxylate metabolite when 60 mg terfenadine was given twice daily for 12 days, starting 7 days before azithromycin administration. Terfenadine alone resulted in a 0.010 msec increase in the rate-corrected QT interval (QTc), but the incremental effects of azithromycin and placebo on QTc in volunteers receiving terfenadine were not statistically different. It is concluded that the potentially life-threatening disorders that have been attributed to a pharmacokinetic interaction between macrolide antibiotics and terfenadine are unlikely to take place in patients treated simultaneously with azithromycin and terfenadine.

An open-label, noncomparative study to evaluate the efficacy, safety, and tolerability of azithromycin in the treatment of patients with acute sinusitis

The efficacy, safety, and tolerability of a 5-day, once-daily course of azithromycin were assessed in patients with acute sinusitis. Patients received two 250-mg capsules of azithromycin on day 1 and one 250-mg capsule on days 2 through 5. Of 102 clinically assessable patients, 27 (26.5%) were cured and 69 (67.6%) were improved on days 5 to 7. At days 12 to 16, 88 (86.3%) had a favorable clinical response. A total of 64 patients experienced adverse events; in all but two patients, adverse events were of mild or moderate severity. Thus azithromycin given once daily for 5 days was an effective treatment for patients with acute sinusitis.