In vitro testing of clarithromycin in combination with ethambutol and rifampicin against Mycobacterium avium complex

Antimycobacterial agents differ with respect to their bacteriostatic versus bactericidal activities in relation to time of exposure, mycobacterial growth phase, and their use in combination

A number of antimycobacterial agents were evaluated with respect to their bacteriostatic activity (growth inhibition) versus the bactericidal activity against a clinical isolate of Mycobacterium avium (Mycobacterium avium complex [MAC] strain 101) in relation to the time of exposure and the growth phase of the mycobacteria. In terms of growth inhibition the MAC in the active phase of growth was susceptible to clarithromycin, ethambutol, rifampin, amikacin, and the quinolones moxifloxacin, ciprofloxacin, and sparfloxacin. In terms of bactericidal activity in relation to the time of exposure these agents differed substantially with respect to the killing rate. An initial high killing capacity at low concentration was observed for amikacin, which in this respect was superior to the other agents. The bactericidal activity of clarithromycin and ethambutol was only seen at relatively high concentrations and increased with time. Killing by rifampin was concentration dependent as well as time dependent. The bactericidal activity of moxifloxacin was marginally dependent on the concentration or the time of exposure. The activity of clarithromycin in combination with ethambutol was not significantly enhanced compared to single-agent exposure. Only an additive effect was observed. The addition of rifampin or moxifloxacin as a third agent only marginally effected increased killing of MAC. However, by addition of amikacin the activity of the clarithromycin-ethambutol combination was significantly improved. The combination of amikacin and amoxicillin-clavulanic acid exhibited synergistic antimycobacterial activity. Towards MAC at low growth rates, only the quinolones exhibited a bactericidal effect.

Commonly used antibacterial and antifungal agents for hospitalised paediatric patients: implications for therapy with an emphasis on clinical pharmacokinetics

Due to normal growth and development, hospitalised paediatric patients with infection require unique consideration of immune function and drug disposition. Specifically, antibacterial and antifungal pharmacokinetics are influenced by volume of distribution, drug binding and elimination, which are a reflection of changing extracellular fluid volume, quantity and quality of plasma proteins, and renal and hepatic function. However, there is a paucity of data in paediatric patients addressing these issues and many empiric treatment practices are based on adult data. The penicillins and cephalosporins continue to be a mainstay of therapy because of their broad spectrum of activity, clinical efficacy and favourable tolerability profile. These antibacterials rapidly reach peak serum concentrations and readily diffuse into body tissues. Good penetration into the cerebrospinal fluid (CSF) has made the third-generation cephalosporins the agents of choice for the treatment of bacterial meningitis. These drugs are excreted primarily by the kidney. The carbapenems are broad-spectrum beta-lactam antibacterials which can potentially replace combination regimens. Vancomycin is a glycopeptide antibacterial with gram-positive activity useful for the treatment of resistant infections, or for those patients allergic to penicillins and cephalosporins. Volume of distribution is affected by age, gender, and bodyweight. It diffuses well across serous membranes and inflamed meninges. Vancomycin is excreted by the kidneys and is not removed by dialysis. The aminoglycosides continue to serve a useful role in the treatment of gram-negative, enterococcal and mycobacterial infections. Their volume of distribution approximates extracellular space. These drugs are also excreted renally and are removed by haemodialysis. Passage across the blood-brain barrier is poor, even in the face of meningeal inflammation. Low pH found in abscess conditions impairs function. Toxicity needs to be considered. Macrolide antibacterials are frequently used in the treatment of respiratory infections. Parenteral erythromycin can cause phlebitis, which limits its use. Parenteral azithromycin is better tolerated but paediatric pharmacokinetic data are lacking. Clindamycin is frequently used when anaerobic infections are suspected. Good oral absorption makes it a good choice for step-down therapy in intra-abdominal and skeletal infections. The use of quinolones in paediatrics has been restricted and most information available is in cystic fibrosis patients. High oral bioavailability is also important for step-down therapy. Amphotericin B has been the cornerstone of antifungal treatment in hospitalised patients. Its metabolism is poorly understood. The half-life increases with time and can be as long as 15 days after prolonged therapy. Oral absorption is poor. The azole antifungals are being used increasingly. Fluconazole is well tolerated, with high bioavailability and good penetration into the CSF. Itraconazole has greater activity against aspergillus, blastomycosis, histoplasmosis and sporotrichosis, although it's pharmacological and toxicity profiles are not as favourable.

Efficacy of macrolides used in combination with ethambutol, with or without other drugs, against Mycobacterium avium within human macrophages

The activities of clarithromycin or roxithromicin used in combination with other antimicrobial drugs were tested in human macrophages experimentally infected with 23 strains of Mycobacterium avium. Overall, clarithromycin-ethambutol-rifampicin was the most active combination tested. The reduction in intracellular viable bacilli was found to be more than 1 log(10) for 95% and more than 2 logs(10) for 65% of the strains. The second most active combination was roxithromycin-ethambutol-rifampicin, which was found to be bactericidal for about 80% and highly bactericidal for 20% of the strains. Others combinations were only bacteriostatic or weakly bactericidal for many of the strains. The addition of a third drug did not necessarily promote enhanced bacterial killing inside the macrophage.

The macrolides: erythromycin, clarithromycin, and azithromycin

In addition to erythromycin, macrolides now available in the United States include azithromycin and clarithromycin. These two new macrolides are more chemically stable and better tolerated than erythromycin, and they have a broader antimicrobial spectrum than erythromycin against Mycobacterium avium complex (MAC), Haemophilus influenzae, nontuberculous mycobacteria, and Chlamydia trachomatis. All three macrolides have excellent activity against the atypical respiratory pathogens (C. pneumoniae and Mycoplasma species) and the Legionella species. Azithromycin and clarithromycin have pharmacokinetics that allow shorter dosing schedules because of prolonged tissue levels. Both azithromycin and clarithromycin are active agents for MAC prophylaxis in patients with late-stage acquired immunodeficiency syndrome (AIDS), although azithromycin may be the preferable agent because of fewer drug-drug interactions. Clarithromycin is the most active MAC antimicrobial agent and should be part of any drug regimen for treating active MAC disease in patients with or without AIDS. Although both azithromycin and clarithromycin are well tolerated by children, azithromycin has the advantage of shorter treatment regimens and improved tolerance, potentially improving compliance in the treatment of respiratory tract and skin or soft tissue infections. Intravenously administered azithromycin has been approved for treatment of adults with mild to moderate community-acquired pneumonia or pelvic inflammatory diseases. An area of concern is the increasing macrolide resistance that is being reported with some of the common pathogens, particularly Streptococcus pneumoniae, group A streptococci, and H. influenzae. The emergence of macrolide resistance with these common pathogens may limit the clinical usefulness of this class of antimicrobial agents in the future.

In vitro susceptibility of Mycobacterium ulcerans to clarithromycin

Buruli ulcer (BU), caused by Mycobacterium ulcerans, was recently recognized by the World Health Organization as an important emerging disease. While antimycobacterial therapy is often effective for the earliest nodular or ulcerative lesions, medical management of BU lesions in patients presenting for treatment is usually disappointing, leaving wide surgical excision the only alternative. Advanced ulcerated lesions of BU rarely respond to antimycobacterial agents; however, perioperative administration of such drugs may prevent relapses or disseminated infections. Clarithromycin possesses strong activity in vitro and in vivo against most nontuberculous mycobacteria. In this study we determined the antimycobacterial activity of this drug in vitro against 46 strains of M. ulcerans isolated from 11 countries. The MIC of clarithromycin was determined at pH 6.6 (on 7H11 agar) and at pH 7.4 (on Mueller-Hinton agar). The MICs ranged from 0.125 to 2 microg/ml at pH 6.6 and from <0.125 to 0.5 microg/ml at pH 7.4. For the majority of the strains, geographic origin did not play a significant role. Thirty-eight strains (83%) were inhibited by 0.5 microg/ml at pH 7.4. These MICs are below peak therapeutic concentrations of clarithromycin obtainable in blood. These results suggest that clarithromycin is a promising drug both for the treatment of early lesions of M. ulcerans and for the prevention of hematogenous dissemination of the etiologic agent during and after surgery. Studies should be initiated to evaluate the effects of clarithromycin in combination with ethambutol and rifampin on M. ulcerans both in vitro and in experimentally infected mice. Multidrug regimens containing clarithromycin may also help control the secondary bacterial infections sometimes seen in BU patients, most importantly osteomyelitis.

French multicenter study involving eight test sites for radiometric determination of activities of 10 antimicrobial agents against Mycobacterium avium complex

The radiometric BACTEC 460-TB methodology has filled an increased need in the screening of a wide range of antimicrobial agents against Mycobacterium avium (MAC) isolates on a patient-to-patient basis. In this context, a multicenter study involving eight test sites across France was performed to determine the MICs of 10 antimicrobial agents for MAC organisms. The aim of the investigation was to compare the in vitro activities of D-cycloserine, ethambutol, ethionamide, rifampin, amikacin, streptomycin, ciprofloxacin, sparfloxacin, clofazimine, and clarithromycin against MAC isolates. All of the test sites received the same clinical isolates of MAC, and the MICs were determined by a common protocol. The overall interlaboratory reproducibility of the MICs within +/- 1 dilution of the modal MICs varied from 79.70 to 100% (mean, 95.2% +/- 2.1%), whereas overall agreement of the MICs among the test sites varied from a mean of 91% +/- 4.1% to a mean of 98 +/- 1.3%. We confirmed that the proposed methodology is easy, accurate, and sufficiently reproducible to be used routinely in a clinical laboratory. Despite variations in the MICs of the same drug among strains, no link between the origin of MAC isolates (from human immunodeficiency virus-positive or -negative patients) and their drug susceptibilities was established. On the basis of the MICs that inhibited 50 and 90% of isolates tested for the drugs used, clarithromycin, clofazimine, ethambutol, and streptomycin were the most uniformly active against MAC; this was followed by amikacin, rifampin, and sparfloxacin. On the other hand, ciprofloxacin, D-cycloserine, and ethionamide showed only marginal in vitro activities.

Activities of roxithromycin used alone and in combination with ethambutol, rifampin, amikacin, ofloxacin, and clofazimine against Mycobacterium avium complex

Preliminary studies showed that roxithromycin possessed significant in vitro activity against a variety of atypical mycobacteria such as the Mycobacterium avium complex, M. scrofulaceum, M. szulgai, M. malmoense, M. xenopi, M. marinum, and M. kansasii and rare pathogens such as M. chelonae and M. fortuitum. In this investigation, radiometric MICs of roxithromycin, ethambutol, rifampin, amikacin, ofloxacin, and clofazimine for 10 clinical isolates of the M. avium complex (5 each from human immunodeficiency virus [HIV]-positive and HIV-negative patients) were determined. Roxithromycin MICs against all the isolates were below the reported maximum concentration of drug in serum at the routine pH of 6.8, and the MICs were further lowered by 1 to 2 dilutions at a pH of 7.4. In vitro enhancement of roxithromycin activity against all strains was further investigated by the previously established Bactec 460-TB method by combining the drugs at sub-MIC levels. Antibacterial activity of roxithromycin was enhanced in all 10 strains by ethambutol, in 3 strains each by rifampin and clofazimine, in 2 strains by amikacin, and in 1 strain by ofloxacin. In vitro screening of three-drug combinations showed that combinations of roxithromycin, ethambutol, and a third potential anti-M. avium drug (rifampin, amikacin, ofloxacin, or clofazimine) resulted in further enhancement of activity in 13 out of 20 drug combinations screened.

The Mycobacterium avium complex

Mycobacterium avium complex (MAC) disease emerged early in the epidemic of AIDS as one of the common opportunistic infections afflicting human immunodeficiency virus-infected patients. However, only over the past few years has a consensus developed about its significance to the morbidity and mortality of AIDS. M. avium was well known to mycobacteriologists decades before AIDS, and the MAC was known to cause disease, albeit uncommon, in humans and animals. The early interest in the MAC provided a basis for an explosion of studies over the past 10 years largely in response to the role of the MAC in AIDS opportunistic infection. Molecular techniques have been applied to the epidemiology of MAC disease as well as to a better understanding of the genetics of antimicrobial resistance. The interaction of the MAC with the immune system is complex, and putative MAC virulence factors appear to have a direct effect on the components of cellular immunity, including the regulation of cytokine expression and function. There now is compelling evidence that disseminated MAC disease in humans contributes to both a decrease in the quality of life and survival. Disseminated disease most commonly develops late in the course of AIDS as the CD4 cells are depleted below a critical threshold, but new therapies for prophylaxis and treatment offer considerable promise. These new therapeutic modalities are likely to be useful in the treatment of other forms of MAC disease in patients without AIDS. The laboratory diagnosis of MAC disease has focused on the detection of mycobacteria in the blood and tissues, and although the existing methods are largely adequate, there is need for improvement. Indeed, the successful treatment of MAC disease clearly will require an early and rapid detection of the MAC in clinical specimens long before the establishment of the characteristic overwhelming infection of bone marrow, liver, spleen, and other tissue. Also, a standard method of susceptibility testing is of increasing interest and importance as new effective antimicrobial agents are identified and evaluated. Antimicrobial resistance has already emerged as an important problem, and methods for circumventing resistance that use combination therapies are now being studied.

Semi-synthetic derivatives of erythromycin

Semi-synthetic derivatives of erythromycin have played an important role in antimicrobial chemotherapy. First generation derivatives such as 2'-esters and acid-addition salts significantly improved the chemical stability and oral bioavailability of erythromycin. A second generation of erythronolide-modified derivatives: roxithromycin, clarithromycin, azithromycin, dirithromycin and flurithromycin, have been synthesized and have exhibited significant improvements in pharmacokinetic and/or microbiological features. In addition, erythromycin itself has expanded its utility as an effective antibiotic against a variety of newly emerged pathogens. As a result of these developments, macrolide antibiotics have enjoyed a resurgence in clinical interest and use during the past half-dozen years, and semi-synthetic derivatives of erythromycin should continue to be important contributors to this macrolide renaissance. Despite these recent successes, other useful niches for macrolide antibiotics will remain unfilled. Consequently, the search for new semi-synthetic derivatives of erythromycin possessing even better antimicrobial properties should be pursued.