Telithromycin: a new ketolide antimicrobial for treatment of respiratory tract infections

The In Vitro Antimicrobial Susceptibility of Borrelia burgdorferi sensu lato: Shedding Light on the Known Unknowns

Human Lyme borreliosis (LB) represents a multisystem disorder that can progress in stages. The causative agents are transmitted by hard ticks of the Ixodes ricinus complex that have been infected with the spirochete Borrelia burgdorferi sensu lato. Today, LB is considered the most important human tick-borne illness in the Northern Hemisphere. The causative agent was identified and successfully isolated in 1982 and, shortly thereafter, antibiotic treatment was found to be safe and efficacious. Since then, various in vitro studies have been conducted in order to improve our knowledge of the activity of antimicrobial agents against B. burgdorferi s. l. The full spectrum of in vitro antibiotic susceptibility has still not been defined for some of the more recently developed compounds. Moreover, our current understanding of the in vitro interactions between B. burgdorferi s. l. and antimicrobial agents, and their possible mechanisms of resistance remains very limited and is largely based on in vitro susceptibility experiments on only a few isolates of Borrelia. Even less is known about the possible mechanisms of the in vitro persistence of spirochetes exposed to antimicrobial agents in the presence of human and animal cell lines. Only a relatively small number of laboratory studies and cell culture experiments have been conducted. This review summarizes what is and what is not known about the in vitro susceptibility of B. burgdorferi s. l. It aims to shed light on the known unknowns that continue to fuel current debates on possible treatment resistance and mechanisms of persistence of Lyme disease spirochetes in the presence of antimicrobial agents.

Antibiotics: natural products essential to human health

For more than 50 years, natural products have served us well in combating infectious bacteria and fungi. Microbial and plant secondary metabolites helped to double our life span during the 20th century, reduced pain and suffering, and revolutionized medicine. Most antibiotics are either (i) natural products of microorganisms, (ii) semi-synthetically produced from natural products, or (iii) chemically synthesized based on the structure of the natural products. Production of antibiotics began with penicillin in the late 1940s and proceeded with great success until the 1970-1980s when it became harder and harder to discover new and useful products. Furthermore, resistance development in pathogens became a major problem, which is still with us today. In addition, new pathogens are continually emerging and there are still bacteria that are not eliminated by any antibiotic, e.g., Pseudomonas aeruginosa. In addition to these problems, many of the major pharmaceutical companies have abandoned the antibiotic field, leaving much of the discovery efforts to small companies, new companies, and the biotechnology industries. Despite these problems, development of new antibiotics has continued, albeit at a much lower pace than in the last century. We have seen the (i) appearance of newly discovered antibiotics (e.g., candins), (ii) development of old but unutilized antibiotics (e.g., daptomycin), (iii) production of new semi-synthetic versions of old antibiotics (e.g., glycylcyclines, streptogrammins), as well as the (iv) very useful application of old but underutilized antibiotics (e.g., teicoplanin).

Antimicrobial susceptibility of Borrelia burgdorferi sensu lato: what we know, what we don't know, and what we need to know

Human Lyme borreliosis is a multisystem disorder that can progress in stages and is transmitted by ticks of the Ixodes ricinus complex infected with the spirochete Borrelia burgdorferi sensu lato. Today, Lyme borreliosis is regarded as the most important human tickborne illness in the northern hemisphere. Soon after the causative agent was correctly identified and successfully isolated in 1982, antibiotic treatment was shown to be effective and since then a variety of in vitro and in vivo studies have been performed to further characterize the activity of antimicrobial agents against B. burgdorferi s.l. Although many antimicrobial agents have been tested for their in vitro activity against borreliae, the full spectrum of antibiotic susceptibility in B. burgdorferi s.l. has not been defined for many compounds. Moreover, our current understanding of possible antimicrobial resistance mechanisms in B. burgdorferi s.l. is limited and is largely founded on in vitro experiments on relatively few borrelial isolates. This review will summarize what is and what is not known about antimicrobial resistance in B. burgdorferi s.l. and will discuss open questions that continue to fuel the current debate on treatment-resistant Lyme borreliosis.

A novel ketolide class: Synthesis and antibacterial activity of a lead compound

Synthesis and antibacterial activity of a new class of ketolide antibiotics, exemplified by the prototype GW680788X (1), are described. The structure of (1) has been elucidated by spectroscopic analysis. The good antibacterial activity shown by (1) in comparison with clarithromycin prompted us to consider this compound as a lead molecule for further exploration.

Effects of acute renal failure on the pharmacokinetics of telithromycin in rats: negligible effects of increase in CYP3A1 on the metabolism of telithromycin

It was reported that the expression of CYP3A1 increased in rats with acute renal failure induced by uranyl nitrate (rat model of U-ARF) compared with controls. It was shown that telithromycin was mainly metabolized via CYP3A1/2 in rats in this study. Hence, the pharmacokinetic parameters of telithromycin were compared after both intravenous and oral administration at a dose of 50 mg/kg to control rats and a rat model of U-ARF. After intravenous administration of telithromycin to rats with U-ARF, the AUC and renal clearance (Cl(r)) were significantly greater (35.0% increase) and slower (99.1% decrease), respectively, than the controls. Unexpectedly, the nonrenal clearance (Cl(nr)) of telithromycin was comparable between the two groups of rats, suggesting that CYP3A isozyme responsible for the metabolism of telithromycin seemed not to be expressed considerably in the rat model of U-ARF. After oral administration of telithromycin to rats with U-ARF, the AUC was also significantly greater (127% increase) than the controls and the value, 127%, was considerably greater than 35.0% after intravenous administration of telithromycin. This may be due mainly to the decrease in the intestinal first-pass effect of telithromycin compared with controls in addition to significantly slower Cl(r) than controls.

The atypical pneumonias: clinical diagnosis and importance

The most common atypical pneumonias are caused by three zoonotic pathogens, Chlamydia psittaci (psittacosis), Francisella tularensis (tularemia), and Coxiella burnetii (Q fever), and three nonzoonotic pathogens, Chlamydia pneumoniae, Mycoplasma pneumoniae, and Legionella. These atypical agents, unlike the typical pathogens, often cause extrapulmonary manifestations. Atypical CAPs are systemic infectious diseases with a pulmonary component and may be differentiated clinically from typical CAPs by the pattern of extrapulmonary organ involvement which is characteristic for each atypical CAP. Zoonotic pneumonias may be eliminated from diagnostic consideration with a negative contact history. The commonest clinical problem is to differentiate legionnaire's disease from typical CAP as well as from C. pneumoniae or M. pneumonia infection. Legionella is the most important atypical pathogen in terms of severity. It may be clinically differentiated from typical CAP and other atypical pathogens by the use of a weighted point system of syndromic diagnosis based on the characteristic pattern of extrapulmonary features. Because legionnaire's disease often presents as severe CAP, a presumptive diagnosis of Legionella should prompt specific testing and empirical anti-Legionella therapy such as the Winthrop-University Hospital Infectious Disease Division's weighted point score system. Most atypical pathogens are difficult or dangerous to isolate and a definitive laboratory diagnosis is usually based on indirect, i.e., direct flourescent antibody (DFA), indirect flourescent antibody (IFA). Atypical CAP is virtually always monomicrobial; increased IFA IgG tests indicate past exposure and not concurrent infection. Anti-Legionella antibiotics include macrolides, doxycycline, rifampin, quinolones, and telithromycin. The drugs with the highest level of anti-Legionella activity are quinolones and telithromycin. Therapy is usually continued for 2 weeks if potent anti-Legionella drugs are used. In adults, M. pneumoniae and C. pneumoniae may exacerbate or cause asthma. The importance of the atypical pneumonias is not related to their frequency (approximately 15% of CAPs), but to difficulties in their diagnosis, and their nonresponsiveness to beta-lactam therapy. Because of the potential role of C. pneumoniae in coronary artery disease and multiple sclerosis (MS), and the role of M. pneumoniae and C. pneumoniae in causing or exacerbating asthma, atypical CAPs also have public health importance.

Synthesis and in vitro antibiotic activity of 16-membered 9-O-arylalkyloxime macrolides

A series of novel 9-O-arylalkyloxime analogs based on three different 16-membered macrolide scaffolds-5-O-mycaminosyltylonolide (OMT), tilmicosin, and 20-deoxy-20-(3,5-dimethyl-1-piperidin-1-yl)-OMT-was synthesized. In vitro antibiotic activities were assayed against Gram-positive Streptococcus pneumoniae and Staphylococcus aureus and Gram-negative Haemophilus influenzae bacterial strains. Analogs derived from OMT (3-15) showed similar or better antibacterial activities against macrolide-susceptible strains and enhanced activities against macrolide-resistant strains compared with erythromycin A, tylosin, or OMT. Similar results were observed for tilmicosin 9-O-arylalkyloxime analogs (18-24). In contrast, most of the 20-deoxy-20-(3,5-dimethyl-1-piperidin-1-yl)-OMT analogs (25-33) showed reduced antibacterial activities compared with OMT. Ribosome-binding studies were performed on compounds 12 (OMT derivative), 20 (tilmicosin derivative), and 29 [20-deoxy-20-(3,5-dimethyl-1-piperidin-1-yl)-OMT derivative]. It was found that these compounds interacted with both domain V and domain II of the Escherichia coli 23S rRNA.

Pharmacokinetics of telithromycin in plasma and soft tissues after single-dose administration to healthy volunteers

By use of microdialysis we assessed the concentrations of telithromycin in muscle and adipose tissue to test its ability to penetrate soft tissues. The ratios of the area under the concentration-versus-time curve from 0 to 24 h to the MIC indicated that free concentrations of telithromycin in tissue and plasma might be effective against Streptococcus pyogenes but not against staphylococci and human and animal bite pathogens.

Optimizing antibacterial therapy for community-acquired respiratory tract infections in children in an era of bacterial resistance

The spread of antibacterial resistance in bacteria that commonly cause childhood community-acquired respiratory tract infections (RTIs), such as acute otitis media, community-acquired pneumonia, and acute pharyngitis, is a major healthcare problem. One of the foremost concerns is the rapid increase in penicillin, macrolide, and multidrug resistance in Streptococcus pneumoniae. There is also a rising prevalence of macrolide resistance in Streptococcus pyogenes in pockets of the United States, and beta-lactamase production in Haemophilus influenzae is widespread. Although data are limited, some evidence suggests that resistance to antibacterials can impair bacteriologic and clinical outcomes in childhood RTIs. Optimizing antibacterial use is important both in the care of individual patients and within strategies to address the wider problem of antibacterial resistance. This involves encouraging judicious antibacterial use (i.e., reducing overuse for viral infection and prophylaxis), and preventing misuse through the wrong choice, dosage, and duration of therapy. Given that initial therapy is usually empiric, antibacterials used to treat community-acquired RTIs in children should ideally have the following properties: an optimal targeted spectrum of activity; high clinical and bacteriologic efficacy against respiratory pathogens, including resistant strains; simple, short-course therapy; and good tolerability and palatability. New antibacterials will continue to have a role in the treatment of RTIs in children, especially where resistance compromises existing therapies.

Comparison of in vitro activities of ketolides, macrolides, and an azalide against the spirochete Borrelia burgdorferi

Two ketolides, three macrolides, and one azalide were tested in vitro against 17 isolates of the B. burgdorferi s.l. complex. As measured in micrograms per milliliter, activity was highest for cethromycin (MIC at which 90% of the tested isolates were inhibited [MIC(90)], 0.0019 micro g/ml) and telithromycin (MIC(90), 0.0078 micro g/ml). Electron-microscope analysis and time-kill studies also supported enhanced effectiveness of both ketolides.

Telithromycin

Telithromycin, the first member of the ketolide antibacterials, has good activity against community-acquired respiratory pathogens, including multiple-drug-resistant strains of Streptococcus pneumoniae. Telithromycin 800 mg once daily has been US FDA approved for the treatment of acute bacterial sinusitis (ABS; treatment duration 5 days), acute bacterial exacerbations of chronic bronchitis (AECB; 5 days) and mild-to-moderate community-acquired pneumonia (CAP; 7-10 days). In patients with CAP, telithromycin was as effective as amoxicillin 1000 mg three times daily for 10 days, clarithromycin 500 mg twice daily for 10 days or trovafloxacin 200 mg once daily for 7-10 days. In patients with AECB, telithromycin was as effective as a 10-day regimen of amoxicillin/clavulanic acid 500/125 mg three times daily, cefuroxime axetil 500 mg twice daily or clarithromycin 500 mg twice daily. In patients with ABS, telithromycin was as effective as a 10-day course of amoxicillin/clavulanic acid 500/125 mg three times daily or cefuroxime axetil 250 mg twice daily. Telithromycin was generally well tolerated and most adverse events were of mild-to-moderate severity and transitory. The most common adverse events with telithromycin were diarrhoea and nausea (10.8% and 7.9% of 2702 patients in clinical trials); these events occurred in 8.6% and 4.6% of 2139 comparator-treated patients.

Fluoroquinolone AUIC Break Points and the Link to Bacterial Killing Rates Part 2: Human Trials

OBJECTIVE

To review clinical trials with fluoroquinolones and the pharmacokinetic and pharmacodynamic parameters predictive of clinical and microbiologic outcomes and resistance. Data on fluoroquinolones are summarized and the premise that a single AUIC target >125 may be used for all fluoroquinolones against all target organisms is examined.

DATA SOURCES

Primary articles were identified by a MEDLINE search (1966-February 2002) and through secondary sources.

STUDY SELECTION AND DATA EXTRACTION

All of the articles identified from the data sources were evaluated and all information deemed relevant was included.

DATA SYNTHESIS

The fluoroquinolones exhibit concentration-dependent killing. This effect clearly depends upon concentrations achieved and outcomes depend upon endpoints established by individual investigators. With AUIC values <60, the actions of fluoroquinolones are essentially bacteriostatic; any observed bacterial killing is the combined effect of low concentrations in relation to minimum inhibitory concentration and the action of host factors such as neutrophils and macrophages. AUIC values >100 but <250 yield bacterial killing at a slow rate, but usually by day 7 of treatment. AUICs >250 produce rapid killing, and bacterial eradication occurs within 24 hours. Disagreements regarding target endpoints are the expected consequences of comparing microbial and clinical outcomes across animal models, in vitro experiments (Part 1), and humans when the endpoints are clearly not equivalent. Careful attention to time-related events such as speed of bacterial killing versus global endpoints such as bacteriologic cure allows optimal break points to be defined.

CONCLUSIONS

Evidence from human trials favors the use of AUIC values >250 for rapid bactericidal action, regardless of whether the organism is gram-negative or gram-positive.

Dead bugs don't mutate: susceptibility issues in the emergence of bacterial resistance

The global emergence of antibacterial resistance among common and atypical respiratory pathogens in the last decade necessitates the strategic application of antibacterial agents. The use of bactericidal rather than bacteriostatic agents as first-line therapy is recommended because the eradication of microorganisms serves to curtail, although not avoid, the development of bacterial resistance. Bactericidal activity is achieved with specific classes of antimicrobial agents as well as by combination therapy. Newer classes of antibacterial agents, such as the fluoroquinolones and certain members of the macrolide/lincosamine/streptogramin class have increased bactericidal activity compared with traditional agents. More recently, the ketolides (novel, semisynthetic, erythromycin-A derivatives) have demonstrated potent bactericidal activity against key respiratory pathogens, including Streptococcus pneumoniae, Haemophilus influenzae, Chlamydia pneumoniae, and Moraxella catarrhalis. Moreover, the ketolides are associated with a low potential for inducing resistance, making them promising first-line agents for respiratory tract infections.

Antibiotics in the treatment of acute exacerbations of chronic bronchitis

The benefit of antimicrobial therapy for patients with an acute exacerbation of chronic bronchitis (AECB) remains controversial for two main reasons. First, the distal airways of patients with chronic bronchitis are persistently colonised, even during clinically stable periods, with the same bacteria that have been associated with AECB. Second, bacterial infection is only one of several causes of AECB. These factors have led to conflicting analyses on the role of bacterial agents and the response to antimicrobial therapy of patients with AECB. An episode of AECB is said to be present when a patient with chronic obstructive pulmonary disease (COPD) experiences some combination of increased dyspnoea, increased sputum volume, increased sputum purulence and worsening lung function. While the average COPD patient experiences 2 - 4 episodes of AECB per year, some patients, particularly those with more severe airway obstruction, are more susceptible to these attacks than others. Bacterial agents appear to be particularly associated with AECB in patients with low lung function and those with frequent episodes accompanied by purulent sputum. Non-typeable Haemophilus influenzae, Streptococcus pneumoniae and Moraxella catarrhalis account for up to 50% of episodes of AECB. Gram-negative bacilli are more likely to occur in patients with more severe lung disease. Antibiotics have been used to ameliorate AECB, to prevent AECB and to prevent the long-term loss of lung function that characterises COPD. Numerous prevention trials have been conducted with fairly consistent results; antibiotics do not lessen the number of episodes of AECB but do reduce the number of days lost from work. Most antibiotic trials have studied the impact of treatment on episodes of AECB and results have been inconsistent, largely due to patient selection and end point definition. In patients with severe airway obstruction, especially in the presence of purulent sputum, antibiotic therapy significantly shortens the duration of symptoms and can be cost-effective. Over the past 50 years, virtually all classes of antimicrobial agents have been studied in AECB. Important considerations include penetration into respiratory secretions, spectrum of activity and antimicrobial resistance. These factors limit the usefulness of drugs such as amoxicillin, erythromycin and trimethoprim-sulfamethoxazole. Extended-spectrum oral cephalosporins, newer macrolides and doxycycline have demonstrated efficacy in clinical trials. Amoxicillin-clavulanate and flouoroquinolones should generally be reserved for patients with more severe disease. A number of investigational agents, including ketolides and newer quinolones, hold promise for treatment of AECB.

Therapies in development for community-acquired pneumonia

The current use of antimicrobials has become more complex due to the extensive emergence of antibiotic resistance. The single most important approach in resistance control is probably the judicious use of chemotherapeutic agents. New agents that may be of use in the treatment of community-acquired pneumonia are currently in development. Antimicrobials can be grouped according to their mechanism of action. These include protein synthesis inhibitors (ketolides, oxazolidinones, streptogramins and glycylcyclines), nucleic acid synthesis inhibitors (fluoroquinolones), peptidoglycan synthesis inhibitors (beta-lactams and glycopeptides) and agents interfering with membrane function (cationic peptides and lipopeptides). Among those agents under development, only the oxazolidinones, the cationic peptides and the lipopeptide antibiotics can be truly regarded as structurally novel inhibitors as the other agents are analogues of existing compounds which have been in clinical use for many years.

Advances in the research and development of chemotherapeutic agents for respiratory tract bacterial infections

The activity of existing antibiotics is diminishing due to the increasing number of resistant strains and by the increase of infections with naturally resistant microorganisms. New agents are urgently needed to meet this challenge and the molecular strategies adopted for the discovery of these compounds must focus on minimizing the emergence of future resistance to them. Novel compounds can be grouped on the basis of their mechanism of action: inhibitors of nucleic acid synthesis (fluoroquinolones), inhibitors of protein synthesis (ketolides, oxazolidinones, streptogramins, and glycylcyclines), inhibitors of peptidoglycan synthesis (beta-lactams and glycopeptides), and agents interfering with membrane function (cationic peptides, and lipopeptides). Regarding the agents that are already in the research and development pipeline, only the oxazolidinones, the cationic peptides and the lipopeptide antibiotics can be truly considered as structurally novel inhibitors because the other agents are analogues of existing compounds that have been in use for many years.