The ketolides: a critical review

Molecular architecture of DesI: a key enzyme in the biosynthesis of desosamine

Desosamine is a 3-(dimethylamino)-3,4,6-trideoxyhexose found, for example, in such macrolide antibiotics as erthyromycin, azithromycin, and clarithromycin. The efficacies of these macrolide antibiotics are markedly reduced in the absence of desosamine. In the bacterium Streptomyces venezuelae, six enzymes are required for the production of dTDP-desosamine. The focus of this X-ray crystallographic analysis is the third enzyme in the pathway, a PLP-dependent aminotransferase referred to as DesI. The structure of DesI was solved in complex with its product, dTDP-4-amino-4,6-dideoxyglucose, to a nominal resolution of 2.1 A. Each subunit of the dimeric enzyme contains 12 alpha-helices and 14 beta-strands. Three cis-peptides are observed in each subunit, Phe 330, Pro 332, and Pro 339. The two active sites of the enzyme are located in clefts at the subunit/subunit interface. Electron density corresponding to the bound product clearly demonstrates a covalent bond between the amino group of the product and C-4' of the PLP cofactor. Interestingly, there are no hydrogen-bonding interactions between the protein and the dideoxyglucosyl group of the product (within 3.2 A). The only other sugar-modifying aminotransferase whose structure is known in the presence of product is PseC from Helicobacter pylori. This enzyme, as opposed to DesI, catalyzes amino transfer to the axial position of the sugar. A superposition of the two active sites for these proteins reveals that the major differences in ligand binding occur in the orientations of the deoxyglucosyl and phosphoryl groups. Indeed, the nearly 180 degrees difference in hexose orientation explains the equatorial versus axial amino transfer exhibited by DesI and PseC, respectively.

Antibacterial natural products in medicinal chemistry--exodus or revival?

To create a drug, nature's blueprints often have to be improved through semisynthesis or total synthesis (chemical postevolution). Selected contributions from industrial and academic groups highlight the arduous but rewarding path from natural products to drugs. Principle modification types for natural products are discussed herein, such as decoration, substitution, and degradation. The biological, chemical, and socioeconomic environments of antibacterial research are dealt with in context. Natural products, many from soil organisms, have provided the majority of lead structures for marketed anti-infectives. Surprisingly, numerous "old" classes of antibacterial natural products have never been intensively explored by medicinal chemists. Nevertheless, research on antibacterial natural products is flagging. Apparently, the "old fashioned" natural products no longer fit into modern drug discovery. The handling of natural products is cumbersome, requiring nonstandardized workflows and extended timelines. Revisiting natural products with modern chemistry and target-finding tools from biology (reversed genomics) is one option for their revival.

Synthesis and antibacterial activity of 3-keto-6-O-carbamoyl-11,12-cyclic thiocarbamate erythromycin A derivatives

A series of 3-keto-6-O-carbamoyl-11,12-cyclic thiocarbamate erythromycin A derivatives has been synthesized. The best compounds in this series possess potent in vitro antibacterial activity against erythromycin-susceptible and erythromycin-resistant bacteria.

Chemical parameters influencing fine-tuning in the binding of macrolide antibiotics to the ribosomal tunnel

In comparison to existing structural, biochemical, and therapeutical data, the crystal structures of large ribosomal subunit from the eubacterial pathogen model Deinococcus radiodurans in complex with the 14-membered macrolides erythromycylamine, RU69874, and the 16-membered macrolide josamycin, highlighted the similarities and differences in macrolides binding to the ribosomal tunnel. The three compounds occupy the macrolide binding pocket with their desosamine or mycaminose aminosugar, the C4-C7 edge of the macrolactone ring and the cladinose sugar sharing similar positions and orientations, although the latter, known to be unnecessary for antibiotic activity, displays fewer contacts. The macrolactone ring displays altogether few contacts with the ribosome and can, therefore, tilt in order to optimize its interaction with the 23S rRNA. In addition to their contacts with nucleotides of domain V of the 23S RNA, erythromycylamine and RU69874 interact with domain II nucleotide U790, and RU69874 also reaches van der Waals distance from A752, in a fashion similar to that observed for the ketolides telithromycin and cethromycin. The variability in the sequences and consequently the diversity of the conformations of macrolide binding pockets in various bacterial species can explain the drug's altered level of effectiveness on different organisms and is thus an important factor in structure-based drug design.

Cytochrom-P450 mediierte Arzneimittelinteraktionen mit Antibiotika

This review focuses on drug interactions with commonly prescribed antibiotics. With each drug coadministered, the likelihood of an adverse interaction increases exponentially. Thus, poly-pharmacotherapy possesses important clinical challenges for clinicians and exposes patients to potentially life-threatening risks. In particular, following co-administration of drugs such as tricyclic antidepressants, anticoagulants and antiarrhythmics, which are characterized by narrow therapeutic windows, even small changes in plasma levels can cause serious adverse reactions and/or therapeutic failure. The hepatic and intestinal cytochrome, or CYP-450 enzyme system is responsible for the biotransformation of a multitude of drugs and is frequently involved in drug interactions. The present review therefore presents a comprehensive overview on potential drug interactions with antibiotics, which are mediated by the cytochrome-P450-enzymes.

Pharmacokinetics of telithromycin using bronchoscopic microsampling after single and multiple oral doses

OBJECTIVES

Bronchoscopic microsampling (BMS) is a new technique for repeated sampling of bronchial epithelial lining fluid (ELF) to obtain the pharmacokinetic profile of drugs. We analyzed the time versus concentration profiles of telithromycin in bronchial ELF obtained by BMS and compared these finding to those in plasma and alveolar ELF obtained by bronchoalveolar lavage (BAL).

METHODS

Bronchial ELF samples were obtained from five healthy subjects using BMS probe at 0, 2, 3, 4, 6, 10 and 24h after single or multiple oral doses of 600 mg of telithromycin. Alveolar ELF was also obtained by BAL 3h after single or multiple oral doses of 600 mg of telithromycin.

RESULTS

The areas under the concentration-time curve from 0 to 24h (AUC0-24) of telithromycin in plasma and bronchial ELF were 2.86+/-0.60 and 19.5+/-10.4 mg h/l after single treatment and 3.60+/-0.49 and 42.2+/-22.7 mg h/l after multiple treatments, respectively. Single and multiple oral doses of telithromycin produced significantly (p<0.05) higher AUC0-24 in bronchial ELF compared to those in plasma. While concentrations in bronchial ELF obtained by BMS were significantly lower than those in alveolar ELF obtained by BAL, they tended to be higher than those in plasma after multiple administration. The telithromycin concentrations obtained by BMS method were very consistent in bronchial ELF at different bronchi at one time point and at the same bronchus at different time points.

CONCLUSIONS

Using the BMS technique, we could describe the pharmacokinetics of telithromycin in bronchial ELF. Furthermore, BMS was reasonably validated and reconfirmed to be a feasible and reliable method for measuring antimicrobial concentrations in bronchial ELF.

Drug discovery from natural sources

Organic compounds from terrestrial and marine organisms have extensive past and present use in the treatment of many diseases and serve as compounds of interest both in their natural form and as templates for synthetic modification. Over 20 new drugs launched on the market between 2000 and 2005, originating from terrestrial plants, terrestrial microorganisms, marine organisms, and terrestrial vertebrates and invertebrates, are described. These approved substances, representative of very wide chemical diversity, together with several other natural products or their analogs undergoing clinical trials, continue to demonstrate the importance of compounds from natural sources in modern drug discovery efforts.

Antibiotics for treatment of resistant gram-positive coccal infections

Vancomycin is considered the workhorse for the treatment of most drug-resistant gram-positive bacterial infections. However, concerns have been raised regarding the increasing rates of vancomycin-resistant enterococci and the clinical shortcomings of vancomycin in the treatment of invasive Staphylococcus aureus infections. Resources have been committed to the development of antimicrobial agents with activity against these organisms. This review will focus on the newer antibacterial agents that have been developed for the treatment of resistant gram-positive pathogens. Included in this review are the agents: quinupristin-dalfopristin, linezolid, daptomycin, telithromycin, and tigecycline.

Involvement of the drug transporters p glycoprotein and multidrug resistance-associated protein Mrp2 in telithromycin transport

The present study aims to investigate the role of P glycoprotein and multidrug resistance-associated protein (Mrp2) in the transport of telithromycin, a newly developed ketolide antibiotic, in vitro and in vivo. The in vitro experiments revealed that the intracellular accumulation of telithromycin in adriamycin-resistant human chronic myelogenous leukemia cells (K562/ADR) overexpressing P glycoprotein was significantly lower than that in human chronic myelogenous leukemia cells (K562/S) not expressing P glycoprotein. Cyclosporine significantly increased the intracellular accumulation of telithromycin in K562/ADR cells. When telithromycin was coadministered intravenously with cyclosporine in Sprague-Dawley (SD) rats, cyclosporine significantly delayed the disappearance of telithromycin from plasma and decreased its systemic clearance to 60% of the corresponding control values. Hepatobiliary excretion experiments revealed that cyclosporine almost completely inhibited the biliary clearance of telithromycin, suggesting that telithromycin is a substrate of P glycoprotein and a potential substrate of Mrp2. Moreover, the biliary clearance of telithromycin was significantly decreased by 80% in Eisai hyperbilirubinemic mutant rats with a hereditary deficiency in Mrp2, indicating that Mrp2, as well as P glycoprotein, plays an important role in the biliary excretion of telithromycin. When the effect of telithromycin on the biliary excretion of doxorubicin, a substrate of P glycoprotein and Mrp2, was examined in SD rats, telithromycin significantly decreased the biliary clearance of doxorubicin by 80%. Results obtained from this study indicate that telithromycin is a substrate of both P glycoprotein and Mrp2, and these transporters are involved in the hepatobiliary transport of telithromycin.

Kinetic bactericidal activity of telithromycin, azithromycin and clarithromycin against respiratory pathogens

The present study assessed the comparative in vitro killing kinetics of telithromycin, azithromycin and clarithromycin. Minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) were determined against Streptococcus pneumoniae, beta-haemolytic streptococci, Haemophilus influenzae and Moraxella catarrhalis strains characterized by different susceptibilities to beta-lactams and macrolides. For each bacterial species, representative strains were chosen for time-kill studies. Telithromycin showed high activity against all the tested strains with MIC ranging from < or = 0.004 to 0.5 mg/L for streptococci, from 0.008 to 8 mg/L for H. influenzae, and from 0.008 to 0.5 mg/L for M. catarrhalis. In time-kill studies, telithromycin showed an overall superior bactericidal activity in respect to macrolides, particularly against resistant strains. In conclusion, telithromycin proved to possess bactericidal activity against a wide range of respiratory pathogens, including strains resistant to common macrolides.

Synthesis and antibacterial activity of 3-O-acyl-6-O-carbamoyl erythromycin A derivatives

A series of 3-O-acyl-6-O-carbamoyl erythromycin A derivatives has been synthesized. Several functional groups were identified as the optimal C3-substituents, and the best compounds in this series possess potent in vitro antibacterial activity against erythromycin-susceptible and erythromycin-resistant bacteria.

Susceptibility of Streptococcus pneumoniae to penicillin, azithromycin and telithromycin (PROTEKT 1999–2003)

Isolates of Streptococcus pneumoniae collected over the first 4 years of the PROTEKT study were tested for susceptibility to penicillin, azithromycin and telithromycin. A total of 20,750 isolates were collected from 39 countries. Penicillin non-susceptibility rates were stable over the study period; overall, 21.8% of isolates were resistant. Azithromycin resistance increased from 31.0% in Year 1 to 36.3% in Year 4. Resistance rates for penicillin and azithromycin varied between countries and were highest in France, Spain, South Africa, USA and the Far East. Multidrug resistance in S. pneumoniae did not change significantly over the 4 years, with an overall rate of 38.6%. Telithromycin retained good activity against S. pneumoniae (0.1% of isolates resistant), including multidrug-resistant isolates.

Telithromycin: a novel agent for the treatment of community-acquired upper respiratory infections

The ketolides are a new subclass of macrolides, and telithromycin is the first of these agents to be approved. Modifications to the basic macrolide structure result in enhanced activity against penicillin- and erythromycin resistant respiratory pathogens. It is therefore an option in the treatment of mild to moderate community-acquired respiratory infections, such as pneumonia, acute exacerbations of chronic bronchitis, pharyngitis/tonsillitis, and sinusitis. Telithromycin also offers the advantages of once-daily dosing and a shorter course of therapy in certain infections. Comparative clinical trials, although limited and involving only a small number of resistant organisms, showed the equivalence of telithromycin to existing therapies, although telithromycin generally had a higher frequency of mild to moderate gastrointestinal adverse effects. Further clinical and safety data, especially in patients with resistant organisms, are needed.

Synthesis and biological activity of new 5-O-sugar modified ketolide and 2-fluoro-ketolide antibiotics

A series of new triazole-containing ketolides and 2-fluoro-ketolides in which the 5-O-desosamine was replaced by unnatural sugars were synthesized and evaluated against relevant macrolide-sensitive and macrolide-resistant respiratory pathogens. Excellent in vitro antibacterial activities were demonstrated for ketolide analogues having the 6'-OBz-3'-dimethylamino-glucose and 6'-OBz-4'-deoxy-3'-dimethylamino-glucose substituents.

[Antibiotherapy in children with atypical bacterial infections]

Atypical bacteria responsible for infections in children are mainly Mycoplasma pneumoniae, Chlamydia pneumoniae and Legionella pneumophila. Atypical pneumonia is a frequent disease in children. Until recently, the outcome was thought to be rather benign and antibiotherapy to have only a minor impact on the prognosis. Recent studies have demonstrated that M. pneumoniae and C. pneumoniae were involved in a variety of infections, including acute upper airway disease, otitis and pharyngitis under five. Antibiotherapy was proven able to decrease the rate of complications and recurrence, notably episodes of wheezing and exacerbations of asthma. Atypical bacteria infections may be severe in immunocompromised children and children with underlying disease such as sickle cell anaemia. Whenever bacteriological documentation is lacking, one of the critical issues in choosing an antibiotic is to consider its activity against Streptococcus pneumoniae, especially in lower respiratory tract infections. The main available molecules are reviewed and discussed, with a special emphasis on ketolides, a newer family of molecules active on both atypical bacteria and S. pneumoniae.

ANTIBIOTICS TARGETING RIBOSOMES: Resistance, Selectivity, Synergism, and Cellular Regulation

Antibiotics target ribosomes at distinct locations within functionally relevant sites. They exert their inhibitory action by diverse modes, including competing with substrate binding, interfering with ribosomal dynamics, minimizing ribosomal mobility, facilitating miscoding, hampering the progression of the mRNA chain, and blocking the nascent protein exit tunnel. Although the ribosomes are highly conserved organelles, they possess subtle sequence and/or conformational variations. These enable drug selectivity, thus facilitating clinical usage. The structural implications of these differences were deciphered by comparisons of high-resolution structures of complexes of antibiotics with ribosomal particles from eubacteria resembling pathogens and from an archaeon that shares properties with eukaryotes. The various antibiotic-binding modes detected in these structures demonstrate that members of antibiotic families possessing common chemical elements with minute differences might bind to ribosomal pockets in significantly different modes, governed by their chemical properties. Similarly, the nature of seemingly identical mechanisms of drug resistance is dominated, directly or via cellular effects, by the antibiotics' chemical properties. The observed variability in antibiotic binding and inhibitory modes justifies expectations for structurally based improved properties of existing compounds as well as for the discovery of novel drug classes.

Pharmacodynamics of an 800-mg dose of telithromycin in patients with community-acquired pneumonia caused by extracellular pathogens

The pharmacodynamics of telithromycin, a new ketolide antibacterial, was examined in 115 patients with community-acquired pneumonia (CAP). Patients received telithromycin 800 mg qd for 7-10 days. Pharmacokinetic parameters were determined, and exposure was linked to microbiological outcome using logistic regression analysis. A breakpoint for increased probability of microbiological eradication was developed and was found to be the ratio of area under the concentration-time curve (AUC) to minimum inhibitory concentration (MIC) of 3.375. The final logistic regression model of microbiological outcome included body weight and AUC/MIC ratio breakpoint. This model was found in analyses of the entire population and when Streptococcus pneumoniae and Haemophilus influenzae were examined separately. The AUC/MIC ratio target attainment rate is expected to be >99.9% for S. pneumoniae and Moraxella catarrhalis and 93.1% for H. influenzae. This study demonstrated a relationship between telithromycin drug exposure and microbiological outcome. Telithromycin is expected to achieve the drug exposure breakpoint for the majority of isolates causing CAP.

Ketolides in the treatment of community-acquired respiratory tract infections: A review

BACKGROUND

The increasing prevalence of resistance to established antibiotics among key respiratory bacterial pathogens highlights a need for new antibacterial agents for the treatment of community-acquired respiratory tract infections (RTIs). Ketolides are a new class of antibiotics specifically developed for the treatment of RTIs.

OBJECTIVE

The aim of this review was to present the current status of treatment of RTIs with ketolides, focusing on telithromycin-the first ketolide to be approved by the US Food and Drug Administration for clinical use.

METHODS

TO GATHER EVIDENCE ON THE CURRENT STATUS OF KETOLIDES, A LITERATURE SEARCH WAS CONDUCTED USING MEDLINE (YEARS: 1990-2005; key terms: ketolides, telithromycin, and HMR3647).

RESULTS

Telithromycin shows strong in vitro activity against the major respiratorypathogens, including strains resistant to other antibiotics, as well as the atypical respiratory pathogens. The pharmacokinetic properties of telithromycin are compatible with once-daily dosing. Clinical trials have demonstrated that telithromycin 800 mg QD for 5 to 10 days is effective in the treatment of acute bacterial sinusitis, acute bacterial exacerbations of chronic bronchitis, and mild to moderate community-acquired pneumonia. Overall, telithromycin is well tolerated by patients. Drug-drug interactions are similar to those reported for macrolides.

CONCLUSION

Evidence to date indicates that telithromycin is an effective andwell-tolerated empiric treatment for community-acquired RTIs.

Penetration of telithromycin into the nasal mucosa and ethmoid bone of patients undergoing rhinosurgery for chronic sinusitis

OBJECTIVES

Telithromycin has a broad spectrum of activity against respiratory tract pathogens including penicillin- and macrolide-resistant streptococci. The aim of the study was to investigate the penetration of telithromycin into nasal tissue following a single oral dose of 800 mg.

PATIENTS AND METHODS

A total of 29 patients undergoing rhinosurgery for chronic sinusitis were evaluated. Samples of blood, nasal mucus, nasal mucosa and ethmoid bone were collected during surgery in groups of 5-6 patients after 3, 6, 9, 15 and 24 h following a single oral dose of 800 mg telithromycin. Drug concentrations were determined by HPLC with fluorimetric detection.

RESULTS

The highest telithromycin concentrations were observed after 3 h in plasma as well as in all tissues sampled. The mean plasma concentrations were 0.73 mg/L in the 3 h group and 0.02 mg/L in the 24 h group. The concomitant tissue concentrations were higher. The tissue penetration, expressed by the ratio of the area under the concentration-time curve in tissue versus plasma, was 1.0 for nasal mucus, 5.9 for nasal mucosa and 1.6 for ethmoid bone.

CONCLUSIONS

Telithromycin achieved tissue concentrations that were generally above the MIC(90) for common pathogens in upper respiratory tract infections. These results indicate that telithromycin diffuses rapidly into the nasal tissues and achieves high and prolonged concentrations in nasal mucosa and ethmoid bone.

Clinical efficacy of newer agents in short-duration therapy for community-acquired pneumonia

Streptococcus pneumoniae, the most important respiratory tract pathogen implicated in community-acquired pneumonia (CAP), is becoming increasingly resistant in vitro to the beta -lactams and macrolides, and fluoroquinolone resistance has been detected. A growing body of evidence suggests that prolonged antimicrobial use may contribute directly and indirectly to increased antimicrobial resistance among common respiratory pathogens. Long-term exposure to antimicrobial agents, especially less-potent agents, directly increases selection pressure for resistance. Indirectly, poor patient compliance, multiple daily dosing, and the increased risk of adverse events further complicate the resistance issue and diminish the efficacy of long-term antimicrobial use. Controlled clinical trials addressing the appropriate duration of therapy for CAP are lacking. However, available data suggest that with appropriate antibiotic selection, based on appropriate spectrum, potency, and pharmacokinetic/pharmacodynamic profile, lower respiratory tract infections in outpatients can be successfully treated in <7 days rather than the 7-14 days currently recommended.

Natural products to drugs: natural product derived compounds in clinical trials

Natural product and natural product-derived compounds that are being evaluated in clinical trials or in registration (current 31 December 2004) have been reviewed. Natural product derived drugs launched in the United States of America, Europe and Japan since 1998 and new natural product templates discovered since 1990 are discussed.

Production of hybrid 16-membered macrolides by expressing combinations of polyketide synthase genes in engineered Streptomyces fradiae hosts

Combinations of the five polyketide synthase (PKS) genes for biosynthesis of tylosin in Streptomyces fradiae (tylG), spiramycin in Streptomyces ambofaciens (srmG), or chalcomycin in Streptomyces bikiniensis (chmG) were expressed in engineered hosts derived from a tylosin-producing strain of S. fradiae. Surprisingly efficient synthesis of compounds predicted from the expressed hybrid PKS was obtained. The post-PKS tailoring enzymes of tylosin biosynthesis acted efficiently on the hybrid intermediates with the exception of TylH-catalyzed hydroxylation of the methyl group at C14, which was efficient if C4 bore a methyl group, but inefficient if a methoxyl was present. Moreover, for some compounds, oxidation of the C6 ethyl side chain to an unprecedented carboxylic acid was observed. By also expressing chmH, a homolog of tylH from the chalcomycin gene cluster, efficient hydroxylation of the 14-methyl group was restored.

Antibiotic activity of telithromycin and comparators against bacterial pathogens isolated from 3,043 patients with acute exacerbation of chronic bronchitis

Background

Antimicrobial therapy is considered an important component in the medical management of most patients with acute exacerbation of chronic bronchitis (AECB). The three predominant bacterial species isolated are nontypeable Haemophilus influenzae, Moraxella catarrhalis, and Streptococcus pneumoniae. Staphylococcus aureus is also frequently isolated while atypical bacteria are thought to cause up to 10% of exacerbations. Antibacterial resistance is increasing worldwide and little surveillance data exist concerning pathogens isolated from patients with AECB.

Methods

This study examines the prevalence of antibacterial resistance in isolates obtained from patients with clinically diagnosed AECB. A total of 3043 isolates were obtained from 85 centres in 29 countries, between 1999–2003, and were tested against the new ketolide telithromycin and a panel of commonly used antibiotics.

Results and Discussion

Of the S. pneumoniae isolates, 99.9% were susceptible to telithromycin, but only 71% were susceptible to erythromycin and 75.3% to penicillin. Of the H. influenzae isolates, 99.6% were susceptible to telithromycin. 11.7% of these isolates produced β-lactamase. Almost 10% of S. pneumoniae were multidrug-resistant; 99.0% of these isolates were susceptible to telithromycin. Telithromycin also demonstrated good in vitro activity against M. catarrhalis (MIC₉₀ = 0.12 mg/L) and was the most active compound against methicillin-susceptible S. aureus (98.9% susceptible).

Conclusion

Telithromycin demonstrated similar or better activity against the bacterial species investigated than the other agents, with the most complete coverage overall. These species are the predominant causative bacterial pathogens in AECB and thus the spectrum of activity of telithromycin makes it a potential alternative for the empirical treatment of AECB.

In vitro activities of novel 2-fluoro-naphthyridine-containing ketolides

In vitro activities of erythromycin A, telithromycin, and two investigational ketolides, JNJ-17155437 and JNJ-17155528, were evaluated against clinical bacterial strains, including selected common respiratory tract pathogens. Against 46 macrolide-susceptible and -resistant Streptococcus pneumoniae strains, the MIC(90) (MIC at which 90% of the isolates tested were inhibited) of the investigational ketolides was 0.25 microg/ml, twofold lower than that of telithromycin and at least 64-fold lower than that of erythromycin A. Against erm(B)-containing pneumococci, the MIC(90) of all the ketolides was 0.06 microg/ml. The MIC(90) of the investigational ketolides against mef(A)-containing pneumococci or pneumococci with both mef(A) and erm(B) was 0.25 microg/ml, two-and fourfold lower, respectively, than that of telithromycin. In contrast, the MICs of the investigational ketolides against macrolide-resistant S. pneumoniae strains with ribosomal mutations were similar to or, in some cases, as much as eightfold higher than those of telithromycin. Against Haemophilus influenzae, MICs of all the ketolides were < or =2 microg/ml. Against three Moraxella catarrhalis isolates, the MIC of the ketolides was 0.25 microg/ml. The ketolides inhibited in vitro protein synthesis, with 50% inhibitory concentrations ranging from 0.23 to 0.27 microM. In time-kill studies against macrolide-susceptible and erm- or mef-containing pneumococci, the ketolides were bacteriostatic to slowly bactericidal, with 24-h log(10) decreases ranging from 2.0 to 4.1 CFU. Intervals of postantibiotic effects for the ketolides against macrolide-susceptible and -resistant S. pneumoniae were 3.0 to 8.1 h.

Update on treatment guidelines for acute bacterial sinusitis

Acute bacterial sinusitis (ABS) is a common complication of viral upper respiratory tract infections and represents a considerable social burden both in terms of diminished quality of life for the patient and the economic implications of decreased productivity and treatment costs. Several national health authorities have developed guidelines for the management of ABS, which aim to promote rational selection of anti-bacterial therapy to optimise clinical outcomes while minimising the potential for selection of anti-bacterial resistance as a result of inappropriate anti-bacterial usage. This article provides an overview of current guidelines, with particular focus on the clinical significance of variations in treatment recommendations and new treatment options, such as the ketolide telithromycin, which was recently added to a number of national treatment guidelines.

Synthesis and antibacterial activity of C2-fluoro, C6-carbamate ketolides, and their C9-oximes

Novel C6-carbamate ketolides with C2-fluorination and C9-oximation have been synthesized. The best compounds in this series displayed MIC values of 0.03-0.12 microg/mL against streptococci containing erm and mef resistance determinants and 2-4 microg/mL against Haemophilus influenzae. Several compounds also showed measurable activity against erm(B)-containing enterococci with MIC values of 2-8 microg/mL. In vivo activity was adversely affected by fluorination, possibly as a result of increased serum protein binding.

Telithromycin new product overview

Community-acquired respiratory tract infections (CARTIs), including community-acquired pneumonia, acute exacerbations of chronic bronchitis, and acute bacterial sinusitis, contribute substantially to health care costs in the United States. Although many prescriptions for antibiotics are written each year for the treatment of CARTIs, most are prescribed on an empiric basis. Concerns about the increasing prevalence of antimicrobial resistance and the changing pattern of pathogens isolated from subjects with CARTIs have raised questions about the empiric treatment paradigm. When choosing appropriate antimicrobial therapy for CARTIs, physicians must consider not only the spectrum of activity of antibiotics but also the potential risk of resistance. Telithromycin is the first member of the ketolide class, a new family of antimicrobials structurally related to the macrolides, to be approved by the US Food and Drug Administration for the treatment of CARTIs. The spectrum of activity of telithromycin includes common typical and atypical causative pathogens associated with community-acquired respiratory tract infections, including antibiotic-resistant strains of Streptococcus pneumoniae. Clinical trials have shown that telithromycin is as effective as traditionally used antimicrobial agents in the treatment of mild-to-moderate community-acquired pneumonia, acute exacerbations of chronic bronchitis, and acute bacterial sinusitis.

In vitro activity of telithromycin against mefA and ermB erythromycin-resistant viridans streptococci isolated from bacteremia of oral origin in Spain

Erythromycin-resistant viridans streptococci are often responsible of bacteremias following dental manipulations. The aim of this study was to evaluate the in vitro activity of telithromycin against erythromycin-susceptible and erythromycin-resistant viridans streptococci. Venous blood samples were collected from 84 patients, 30 s after dental extractions. Viridans streptococci were identified by conventional microbiological techniques. A set of 36 viridans streptococci were selected to investigate the in vitro activity of telithromycin by the agar dilution method. Macrolide resistance genes were amplified with specific primers for the mefA and ermB genes and visualized by electrophoresis. For erythromycin-susceptible viridans streptococci, telithromycin minimal inhibitory concentration90 (MIC90) was <0.008 microg/ml. For erythromycin-resistant viridans streptococci, telithromycin MIC90 was 1 microg/ml. The MIC90 to telithromycin of the mefA-positive and ermB-negative viridans streptococci was lower than that of the mefA-negative and ermB-positive viridans streptococci (0.128 microg/ml versus 1 microg/ml). The in vitro activity of telithromycin was high, irrespective of the erythromycin susceptibility and the mechanism of erythromycin resistance identified.

Chalcomycin biosynthesis gene cluster from Streptomyces bikiniensis: novel features of an unusual ketolide produced through expression of the chm polyketide synthase in Streptomyces fradiae

Chalcomycin, a 16-membered macrolide antibiotic made by the bacterium Streptomyces bikiniensis, contains a 2,3-trans double bond and the neutral sugar D-chalcose in place of the amino sugar mycaminose found in most other 16-membered macrolides. Degenerate polyketide synthase (PKS)-specific primers were used to amplify DNA fragments from S. bikiniensis with very high identity to a unique ketosynthase domain of the tylosin PKS. The resulting amplimers were used to identify two overlapping cosmids encompassing the chm PKS. Sequencing revealed a contiguous segment of >60 kb carrying 25 putative genes for biosynthesis of the polyketide backbone, the two deoxysugars, and enzymes involved in modification of precursors of chalcomycin or resistance to it. The chm PKS lacks the ketoreductase and dehydratase domains in the seventh module expected to produce the 2,3-double bond in chalcomycin. Expression of PKS in the heterologous host Streptomyces fradiae, from which the tyl genes encoding the PKS had been removed, resulted in production of at least one novel compound, characterized as a 3-keto 16-membered macrolactone in equilibrium with its 3-trans enol tautomer and containing the sugar mycaminose at the C-5 position, in agreement with the structure predicted on the basis of the domain organization of the chm PKS. The production of a 3-keto macrolide from the chm PKS indicates that a discrete set of enzymes is responsible for the introduction of the 2,3-trans double bond in chalcomycin. From comparisons of the open reading frames to sequences in databases, a pathway for the synthesis of nucleoside diphosphate-D-chalcose was proposed.

Short-Course Antimicrobial Therapy for Community-Acquired Pneumonia

Management of community-acquired pneumonia (CAP) remains surprisingly controversial. Optimal duration of antimicrobial therapy reflects one of the open questions due to the lack of sufficient randomized clinical trial data. Recently, there have been efforts to rationalize antimicrobial therapy of this disease. Trials addressing the issue of short-course antimicrobial therapy for CAP have revealed no adverse outcomes with a treatment duration of 5 days when compared with conventional courses of 7-10 days. There is accumulating evidence that a shorter duration of antimicrobial therapy may have benefits in patients with CAP, as it might enhance compliance, decrease the development of antimicrobial resistance, decrease the incidence and shorten the duration of adverse drug effects, reduce treatment costs and improve patient satisfaction with therapy. Nevertheless, remaining questions regarding the influence of patient selection, disease severity or choice of antimicrobial for short-course therapy indicate the need for further randomized controlled clinical trials in this area of research. This article summarizes current evidence for short-course therapy in patients with CAP and draws conclusions for clinical practice.

Synthesis and antibacterial activity of 6-O-arylpropargyl-9-oxime-11,12-carbamate ketolides

A series of novel 6-O-arylpropargyl-9-oxime-ketolides was synthesized and evaluated against various pathogens. These new compounds show promising in vitro antibacterial potency and in vivo efficacy against macrolide resistant strains.

Ketolides: an emerging treatment for macrolide-resistant respiratory infections, focusing on S. pneumoniae

Resistance to antibiotics in community acquired respiratory infections is increasing worldwide. Resistance to the macrolides can be class-specific, as in efflux or ribosomal mutations, or, in the case of erythromycin ribosomal methylase (erm)-mediated resistance, may generate cross-resistance to other related classes. The ketolides are a new subclass of macrolides specifically designed to combat macrolide-resistant respiratory pathogens. X-ray crystallography indicates that ketolides bind to a secondary region in domain II of the 23S rRNA subunit, resulting in an improved structure-activity relationship. Telithromycin and cethromycin (formerly ABT-773) are the two most clinically advanced ketolides, exhibiting greater activity towards both typical and atypical respiratory pathogens. As a subclass of macrolides, ketolides demonstrate potent activity against most macrolide-resistant streptococci, including ermB- and macrolide efflux (mef)A-positive Streptococcus pneumoniae. Their pharmacokinetics display a long half-life as well as extensive tissue distribution and uptake into respiratory tissues and fluids, allowing for once-daily dosing. Clinical trials focusing on respiratory infections indicate bacteriological and clinical cure rates similar to comparators, even in patients infected with macrolide-resistant strains.

Efficacy and safety of oral telithromycin once daily for 5 days versus moxifloxacin once daily for 10 days in the treatment of acute bacterial rhinosinusitis

OBJECTIVE

To compare the clinical and bacteriologic efficacy and safety of short-duration treatment with telithromycin given for 5 days with moxifloxacin given for 10 days in adults with acute bacterial rhinosinusitis (ABRS).

STUDY DESIGN

In this prospective, double-blind, parallel-group, randomized, multicenter study, adult patients (N = 349) with ABRS were randomized to oral telithromycin (800 mg once daily for 5 days) or to oral moxifloxacin (400 mg once daily for 10 days) and followed for 31 to 36 days. Clinical outcome was determined by the investigator at the posttherapy/test of cure (TOC) visit. Bacteriologic outcome was determined by comparing cultures taken at the pretreatment visit with cultures obtained at the posttherapy/TOC visit. The primary objective was to demonstrate equivalence of clinical cure rates in the per-protocol population between treatment groups at the posttherapy/TOC visit.

RESULTS

Clinical success at TOC (primary endpoint) was achieved in 87.4% of patients in the telithromycin group compared with 86.9% for moxifloxacin (per-protocol patients; 0.5% difference between treatment groups; 95% confidence interval [CI], -8.1, 9.2; P = 0.8930). The bacteriologic success rates were 94.1% and 93.9%, respectively (0.2% difference between treatment groups; 95% CI, -14.2, 14.5; P = 0.9734). Overall treatment-emergent adverse events for both drugs (mostly gastrointestinal) were mild to moderate in intensity.

CONCLUSION AND SIGNIFICANCE

The clinical and bacteriologic efficacy of telithromycin 800 mg once daily for 5 days was equivalent to that of moxifloxacin 400 mg once daily for 10 days, establishing telithromycin as an important treatment option for ABRS.

Impact of cethromycin (ABT-773) therapy on microbiological, histologic, immunologic, and respiratory indices in a murine model of Mycoplasma pneumoniae lower respiratory infection

Mycoplasma pneumoniae is a major etiologic agent of acute lower respiratory infections. We evaluated the antimicrobial and immunologic effects of cethromycin (ABT-773), a ketolide antibiotic, for the treatment of M. pneumoniae pneumonia in a mouse model. Eight-week-old BALB/c mice were inoculated intranasally once with 10(6) CFU of M. pneumoniae on day 0. Treatment was started 24 h after inoculation. Groups of mice were treated subcutaneously with cethromycin at 25 mg/kg of body weight or with placebo daily until sacrifice. Five to ten mice per group were evaluated at days 1, 4, 7, and 10 after inoculation. Outcome variables included bronchoalveolar lavage (BAL) for M. pneumoniae quantitative culture and cytokine and chemokine concentration determinations by enzyme-linked immunosorbent assay (tumor necrosis factor alpha [TNF-alpha], gamma interferon [IFN-gamma], interleukin-1beta [IL-1beta], IL-2, IL-4, IL-12, granulocyte-macrophage colony-stimulating factor, IL-8, monocyte chemoattractant protein 1 [MCP-1], and macrophage inflammatory protein 1alpha [MIP-1alpha]), histopathologic score of the lungs (HPS), and pulmonary function tests (PFT) using whole-body, unrestrained plethysmography at the baseline and post-methacholine exposure as indicators of airway obstruction (AO) and airway hyperresponsiveness (AHR), respectively. The cethromycin-treated mice had a greater reduction in M. pneumoniae culture titers than placebo-treated mice, reaching statistical significance on days 7 and 10 (P < 0.05). HPS was significantly reduced in cethromycin-treated mice compared with placebo-treated mice on days 4, 7, and 10 (P < 0.05). Cytokine concentrations in BAL samples were reduced in mice that received cethromycin, and the differences were statistically significant for 7 of the 10 cytokines measured (TNF-alpha, IFN-gamma, IL-1beta, IL-8, IL-12, MCP-1, and MIP-1alpha) on day 4 (P < 0.05). PFT values were improved in the cethromycin-treated mice, with AO and AHR significantly reduced on day 4 (P < 0.05). In this mouse model, treatment with cethromycin significantly reduced M. pneumoniae culture titers in BAL samples, cytokine and chemokine concentrations in BAL samples, histologic inflammation in the lungs, and disease severity as defined by AO and AHR.