Antipneumococcal activity of ABT-773 compared to those of 10 other agents

Cethromycin Pharmacokinetics and Pharmacodynamics for Single Dose Cure of Plasmodium berghei Liver Stages

Cethromycin combines a quinoline nucleus and a macrolide for broad spectrum antibacterial and antiprotozoan activity. Here we characterized the murine pharmacokinetics and Plasmodium berghei lifecycle stage pharmacodynamics for the cethromycin base. Liver pharmacokinetic studies in mice show peak mM drug levels in the liver with 20 hour sustained levels above 10 μM. Peak concentrations in the liver were double the lung and about 440 times that of plasma. Immunofluorescence imaging of in vitro cethromycin-treated infected hepatocytes shows complete ablation of the apicoplast. We observed complete cure of P. berghei liver stage infection by single oral dose of 60 mg/kg in mice which is equivalent to the 5 mg/kg human dose of 300 mg a day used in bacterial pneumonia studies. Cethromycin at 60 mg/kg daily for 7 days was curative in the high parasitemic P. berghei mouse model. Both mosquito membrane feeding of P. falciparum gametocytes incubated with 20 μM cethromycin and oral dosing in mice demonstrated no decrease in oocyst numbers. Cethromycin has been evaluated for efficacy against bacterial pneumonia in more than 5,000 patients with good safety profiles. Cethromycin has potential for rapid clinical development for casual malaria prophylaxis and possibly radical cure of dormant liver P. vivax.

In vitro activity of the ketolide cethromycin in multidrug-resistant clinical Neisseria gonorrhoeae isolates and international reference strains

Antimicrobial resistance in Neisseria gonorrhoeae is a major public health problem, which compromises the treatment of gonorrhoea globally. We evaluated the in vitro activity of the ketolide cethromycin against a large panel of clinical gonococcal isolates and international reference strains (n = 254), including numerous multidrug-resistant and extensively drug-resistant isolates. Cethromycin showed potent in vitro activity against most of the gonococcal isolates with the following modal MIC, MIC₅₀ and MIC₉₀: 0.064 mg/L, 0.125 mg/L and 0.5 mg/L, respectively. However, cross-resistance between azithromycin and cethromycin was identified (Spearman's rank correlation coefficient 0.917) and isolates displaying high-level resistance to azithromycin (MIC >256 mg/L; n = 9) also showed high MICs of cethromycin (32-256 mg/L). In conclusion, the cross-resistance with azithromycin indicates that cethromycin may not be considered for empirical first-line monotherapy of gonorrhoea. However, cethromycin might be valuable in combination antimicrobial therapy and for second-line therapy e.g. for cases with ceftriaxone resistance or allergy.

Cethromycin: A New Ketolide Antibiotic

OBJECTIVE

To review the pharmacology, chemistry, microbiology, in vitro susceptibility, mechanism of resistance, pharmacokinetics, pharmacodynamics, clinical efficacy, safety, drug interactions, dosage, and administration of cethromycin, a new ketolide antibiotic.

DATA SOURCES

Literature was obtained through searching PubMed (1950-October 2012), International Pharmaceutical Abstracts (1970-October 2012), and a bibliographic review of published articles. Search terms included cethromycin, ABT-773, ketolide antibiotic, and community-acquired pneumonia.

STUDY SELECTION AND DATA EXTRACTION

All available in vitro and preclinical studies, as well as Phase 1, 2, and 3 clinical studies published in English were evaluated to summarize the pharmacology, chemistry, microbiology, efficacy, and safety of cethromycin in the treatment of respiratory tract infections.

DATA SYNTHESIS

Cethromycin, a new ketolide, has a similar mechanism of action to telithromycin with an apparently better safety profile. Cethromycin displays in vitro activity against selected gram-positive, gram-negative, and atypical bacteria. The proposed indication of cethromycin is treatment of mild to moderate community-acquired bacterial pneumonia in patients aged 18 years or older. Based on clinical studies, the recommended dose is 300 mg orally once a day without regard to meals. Cethromycin has an orphan drug designation for tularemia, plague, and anthrax prophylaxis. The Food and Drug Administration denied approval for the treatment of community-acquired pneumonia in 2009; a recent noninferiority trial showed comparable efficacy between cethromycin and clarithromycin. Preliminary data on adverse effects suggest that cethromycin is safe and gastrointestinal adverse effects appear to be dose-related.

CONCLUSIONS

Cethromycin appears to be a promising ketolide for the treatment of mild to moderate community-acquired pneumonia. It was denied approval by the FDA in 2009 pending more evidence to show its efficacy, with more recent studies showing its noninferiority to antibiotics for the same indication.

Newer antibacterial drugs for a new century

IMPORTANCE OF THE FIELD

Antibacterial drug discovery and development has slowed considerably in recent years, with novel classes discovered decades ago and regulatory approvals tougher to get. Traditional approaches and the newer genomic mining approaches have not yielded novel classes of antibacterial compounds. Instead, improved analogues of existing classes of antibacterial drugs have been developed by improving potency, minimizing resistance and alleviating toxicity.

AREAS COVERED IN THIS REVIEW

This article is a comprehensive review of newer classes of antibacterial drugs introduced or approved after year 2000.

WHAT THE READER WILL GAIN

It describes their mechanisms of action/resistance, improved analogues, spectrum of activity and clinical trials. It also discusses new compounds in development with novel mechanisms of action, as well as novel unexploited bacterial targets and strategies that may pave the way for combating drug resistance and emerging pathogens in the twenty-first century.

TAKE HOME MESSAGE

The outlook of antibacterial drug discovery, though challenging, may not be insurmountable in the years ahead, with legislation on incentives and funding introduced for developing an antimicrobial discovery program and efforts to conserve antibacterial drug use.

Antistreptococcal activity of AR-709 compared to that of other agents

Against 300 strains of pneumococci and 100 group A streptococci of differing beta-lactam, macrolide, and quinolone resistance phenotypes, AR-709 was very active, with all MICs being < or =2 microg/ml. Furthermore, AR-709 was active against strains that were both susceptible and resistant to trimethoprim-sulfamethoxazole.

Azithromycin extended release: a review of its use in the treatment of acute bacterial sinusitis and community-acquired pneumonia in the US

Azithromycin is a macrolide antibacterial agent. The novel microspheres oral extended-release formulation (Zmax) is the first antibacterial drug approved in the US for administration as a single dose in adult patients with mild to moderate acute bacterial sinusitis (ABS) or community-acquired pneumonia (CAP). It has a broad spectrum of in vitro antibacterial activity against Gram-positive, Gram-negative and atypical pathogens that cause ABS and CAP infections (including Streptococcus pneumoniae), and achieves good tissue penetration. Azithromycin extended release is an effective and generally well tolerated treatment in patients with ABS or CAP. The clinical cure rates of a single 2.0 g dose of azithromycin extended release were noninferior to those obtained with a 10-day regimen of levofloxacin in patients with ABS, and with 7-day regimens of clarithromycin extended release or levofloxacin in patients with CAP. With a pharmacodynamic and pharmacokinetic profile well suited to administration as a single-dose regimen that may offer the advantage of improved compliance and convenience compared with once-daily longer-course regimens, azithromycin extended release is a new option in the empirical treatment of adult patients with mild or moderate ABS or CAP in the US.

Efficacy of cethromycin, a new ketolide, against Streptococcus pneumoniae susceptible or resistant to erythromycin in a murine pneumonia model

Cethromycin is a ketolide with in vitro activity against macrolide-sensitive and -resistant strains of Streptococcus pneumoniae. We compared its in vivo efficacy to erythromycin in a mouse model of acute pneumonia induced by two virulent clinical strains: a serotype 3 susceptible strain (P-4241) (MICs: erythromycin, 0.03 microg/ml; cethromycin, 0.015 microg/ml) and a serotype 1 strain resistant to erythromycin (P-6254; phenotypically MLSB constitutive) (MICs: erythromycin, 1,024 microg/ml; cethromycin, 0.03 microg/ml). Immunocompetent mice were infected with 10(5) CFU of each strain. Six treatments given either subcutaneously (s.c.) or per os (p.o.) at 12-h intervals were initiated at 6 or 12 h after infection. Against P-4241, cethromycin given s.c. at 25 or 12.5 mg/kg protected 100% of the animals, with lungs and blood completely cleared of bacteria. Given p.o., cethromycin maintained its efficacy with 100 and 86% survival at 25 and 12.5 mg/kg, respectively. Erythromycin, given s.c. at 50 or 37.5 mg/kg, provided 50 and 38% survival rates, respectively. Against P-6254, cethromycin was effective at 25 mg/kg (100% survival) regardless of the administration route, whereas only 25 and 8% of animals survived after a 75-mg/kg erythromycin treatment given s.c. and p.o., respectively. The serum protein binding levels of cethromycin were 94.8 and 88.5% after doses of 12.5 and 25 mg/kg, respectively. The higher in vivo activity of cethromycin compared to erythromycin could be explained by favorable pharmacokinetic/pharmacodynamic indexes against P-6254 but not against P-4241.

Antipneumococcal activity of DW-224a, a new quinolone, compared to those of eight other agents

DW-224a is a new broad-spectrum quinolone with excellent antipneumococcal activity. Agar dilution MIC was used to test the activity of DW-224a compared to those of penicillin, ciprofloxacin, levofloxacin, gatifloxacin, moxifloxacin, gemifloxacin, amoxicillin-clavulanate, cefuroxime, and azithromycin against 353 quinolone-susceptible pneumococci. The MICs of 29 quinolone-resistant pneumococci with defined quinolone resistance mechanisms against seven quinolones and an efflux mechanism were also tested. DW-224a was the most potent quinolone against quinolone-susceptible pneumococci (MIC(50), 0.016 microg/ml; MIC(90), 0.03 microg/ml), followed by gemifloxacin, moxifloxacin, gatifloxacin, levofloxacin, and ciprofloxacin. beta-Lactam MICs rose with those of penicillin G, and azithromycin resistance was seen mainly in strains with raised penicillin G MICs. Against the 29 quinolone-resistant strains, DW-224a had the lowest MICs (0.06 to 1 microg/ml) compared to those of gemifloxacin, clinafloxacin, moxifloxacin, gatifloxacin, levofloxacin, and ciprofloxacin. DW-224a at 2x MIC was bactericidal after 24 h against eight of nine strains tested. Other quinolones gave similar kill kinetics relative to higher MICs. Serial passages of nine strains in the presence of sub-MIC concentrations of DW-224a, moxifloxacin, levofloxacin, ciprofloxacin, gatifloxacin, gemifloxacin, amoxicillin-clavulanate, cefuroxime, and azithromycin were performed. DW-224a yielded resistant clones similar to moxifloxacin and gemifloxacin but also yielded lower MICs. Azithromycin selected resistant clones in three of the five parents tested. Amoxicillin-clavulanate and cefuroxime did not yield resistant clones after 50 days.

Telithromycin: The first ketolide for the treatment of respiratory infections

PURPOSE

The pharmacology, mechanisms of resistance, in vitro activity, clinical efficacy, pharmacokinetics, indications, adverse effects, dosage and administration, and place in therapy of telithromycin in the treatment of respiratory infections are reviewed.

SUMMARY

Telithromycin is the first ketolide to be approved in the United States for use against common respiratory pathogens. The unique structure of telithromycin allows for enhanced binding to bacterial ribosomal RNA, thereby blocking protein synthesis. Its spectrum of activity includes pathogens implicated in common respiratory infections (Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Mycoplasma pneumonia, and Chlamydia pneumoniae) and multidrug-resistant isolates of pneumococcus. Clinical efficacy has been documented in several multicenter, comparative trials for the treatment of community-acquired pneumonia, acute exacerbation of chronic bronchitis, acute maxillary sinusitis, and pharyngitis tonsillitis. Although studies have demonstrated that the clinical efficacy of telithromycin is comparable to macrolides, telithromycin is unique in that it provides activity against penicillin- and macrolide-resistant respiratory pathogens. The recommended dosage of telithromycin is 800 mg p.o. once daily. The most common adverse events resulting from telithromycin use include diarrhea, nausea, headache, dizziness, vomiting, loose stools, dysgeusia, and dyspepsia. The drug's adverse-event profile is comparable to that of similar agents. Telithromycin is a strong inhibitor of cytochrome P-450 isoenzyme 3A4; therefore, it can affect the efficacy and toxicity profile of medications that are metabolized by this isoenzyme.

CONCLUSION

Telithromycin is a reasonable addition to the current treatment options for upper-respiratory-tract infections. Its use should be restricted to infections caused by penicillin- and macrolide-resistant pathogens.

Antipneumococcal activities of two novel macrolides, GW 773546 and GW 708408, compared with those of erythromycin, azithromycin, clarithromycin, clindamycin, and telithromycin

The MICs of GW 773546, GW 708408, and telithromycin for 164 macrolide-susceptible and 161 macrolide-resistant pneumococci were low. The MICs of GW 773546, GW 708408, and telithromycin for macrolide-resistant strains were similar, irrespective of the resistance genotypes of the strains. Clindamycin was active against all macrolide-resistant strains except those with erm(B) and one strain with a 23S rRNA mutation. GW 773546, GW 708408, and telithromycin at two times their MICs were bactericidal after 24 h for 7 to 8 of 12 strains. Serial passages of 12 strains in the presence of sub-MICs yielded 54 mutants, 29 of which had changes in the L4 or L22 protein or the 23S rRNA sequence. Among the macrolide-susceptible strains, resistant mutants developed most rapidly after passage in the presence of clindamycin, GW 773546, erythromycin, azithromycin, and clarithromycin and slowest after passage in the presence of GW 708408 and telithromycin. Selection of strains for which MICs were >/=0.5 microg/ml from susceptible parents occurred only with erythromycin, azithromycin, clarithromycin, and clindamycin; 36 resistant clones from susceptible parent strains had changes in the sequences of the L4 or L22 protein or 23S rRNA. No mef(E) strains yielded resistant clones after passage in the presence of erythromycin and azithromycin. Selection with GW 773546, GW 708408, telithromycin, and clindamycin in two mef(E) strains did not raise the erythromycin, azithromycin, and clarithromycin MICs more than twofold. There were no change in the ribosomal protein (L4 or L22) or 23S rRNA sequences for 15 of 18 mutants selected for macrolide resistance; 3 mutants had changes in the L22-protein sequence. GW 773546, GW 708408, and telithromycin selected clones for which MICs were 0.03 to >2.0 microg/ml. Single-step studies showed mutation frequencies <5.0 x 10(-10) to 3.5 x 10(-7) for GW 773546, GW 708408, and telithromycin for macrolide-susceptible strains and 1.1 x 10(-7) to >4.3 x 10(-3) for resistant strains. The postantibiotic effects of GW 773546, GW 708408, and telithromycin were 2.4 to 9.8 h.

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.

Mechanisms of resistance among respiratory tract pathogens

Antimicrobial resistance among respiratory tract pathogens represents a significant health care threat. Identifying the antimicrobial agents that remain effective in the presence of resistance, and knowing why, requires a thorough understanding of the mechanisms of action of the various agents as well as the mechanisms of resistance demonstrated among respiratory tract pathogens. The primary goal of antimicrobial therapy is to eradicate the pathogen, via killing or inhibiting bacteria, from the site of infection; the defenses of the body are required for killing any remaining bacteria. Targeting a cellular process or function specific to bacteria and not to the host limits the toxicity to patients. Currently, there are four general cellular targets to which antimicrobials are targeted: cell wall formation and maintenance, protein synthesis, DNA replication, and folic acid metabolism. Resistance mechanisms among respiratory tract pathogens have been demonstrated for all four targets. In general, the mechanisms of resistance used by these pathogens fall into one of three categories: enzymatic inactivation of the antimicrobial, prevention of intracellular accumulation, and modification of the target site to which agents bind to exert an antimicrobial effect. Resistance to some agents can be overcome by modifying the dosage regimens (e.g., using high-dose therapy) or inhibiting the resistance mechanism (e.g., b-lactamase inhibitors), whereas other mechanisms of resistance can only be overcome by using an agent from a different class. Understanding the mechanisms of action of the various agents and the mechanisms of resistance used by respiratory tract pathogens can help clinicians identify the agents that will increase the likelihood of achieving optimal outcomes.

Telithromycin susceptibility and genomic diversity of macrolide-resistant serotype III group B streptococci isolated in perinatal infections

We studied the telithromycin, erythromycin, azithromycin, and clindamycin susceptibilities of serotype III macrolide-resistant group B streptococci, together with genetic mechanisms of resistance and genomic diversity. ermB, ermA, and mefA were found in, respectively, 57, 32, and 9% of isolates. The telithromycin MIC at which 90% of isolates were inhibited was 0.5 micro g/ml. Macrolide resistance was associated with dissemination of resistance determinants among isolates of different genetic backgrounds.

Activity of telithromycin against penicillin-resistant Streptococcus pneumoniae isolates recovered from French children with invasive and noninvasive infections

We compared the activities of telithromycin, erythromycin, azithromycin, josamycin, penicillin G, amoxicillin, cefpodoxime, and ceftriaxone against invasive and noninvasive non-penicillin-susceptible Streptococcus pneumoniae isolates recovered from children. Of the 186 isolates tested, 89% were positive for erm(B) by PCR. Telithromycin had the lowest MICs, with MICs at which 90% of the isolates tested are inhibited of 0.032 and 0.25 micro g/ml for erythromycin-sensitive and -resistant isolates, respectively.

Structural basis for the antibiotic activity of ketolides and azalides

The azalide azithromycin and the ketolide ABT-773, which were derived by chemical modifications of erythromycin, exhibit elevated activity against a number of penicillin- and macrolide-resistant pathogenic bacteria. Analysis of the crystal structures of the large ribosomal subunit from Deinococcus radiodurans complexed with azithromycin or ABT-773 indicates that, despite differences in the number and nature of their contacts with the ribosome, both compounds exert their antimicrobial activity by blocking the protein exit tunnel. In contrast to all macrolides studied so far, two molecules of azithromycin bind simultaneously to the tunnel. The additional molecule also interacts with two proteins, L4 and L22, implicated in macrolide resistance. These studies illuminated and rationalized the enhanced activity of the drugs against specific macrolide-resistant bacteria.

In vitro bactericidal activities of ABT-773 against ermB strains of Streptococcus pneumoniae

The bactericidal activities of ABT-773, a new ketolide, were compared to those of cefuroxime and amoxicillin-clavulanate against 10 strains of Streptococcus pneumoniae containing the ermB gene. MICs and time-kill curves were determined in duplicate per NCCLS guidelines with cation-adjusted Mueller-Hinton broth with 3% lysed horse blood. Viable counts were done at 0, 2, 6, and 24 h. Antibiotic concentrations tested were two and eight times the MIC. ABT-773 MICs ranged from 0.008 to 1.0 micro g/ml. Bactericidal activity was observed with ABT-773 at eight times the MIC against 4 of 10 strains at 24 h compared to 10 of 10 strains with the beta-lactam antibiotics.

In vitro activities of cethromycin (ABT-773), a new ketolide, against Streptococcus pneumoniae strains that are not susceptible to penicillin or macrolides

Pneumococcal resistance to antimicrobials presents problems to physicians for empirical treatment of acute otitis media (AOM). Three hundred thirty-three isolates of Streptococcus pneumoniae selected for nonsusceptibility to penicillin (MIC >0.1 microg/ml) from the middle ear (n = 325) or mastoid (n = 8) of children seen between 1994 and 2000 at four children's hospitals in the United States were tested by broth microdilution for susceptibility to nine antibiotics. Using NCCLS 2002 breakpoints, resistance to the following drugs was as indicated: amoxicillin, 1%; azithromycin, 71%; cefprozil, 71%; ceftriaxone, 2%; cefdinir, 98%; erythromycin, 70%; levofloxacin, 0%; and trimethoprim-sulfamethoxazole, 93%. Of the penicillin- and erythromycin-nonsusceptible isolates, 97% were inhibited by cethromycin (ABT-773) and 83% were inhibited by telithromycin at a concentration of <or=0.125 microg/ml. Macrolide resistance among penicillin-nonsusceptible pneumococci increased from 44 to 80% in the 6 years of the study from which the isolates were selected; however, the proportion of isolates with M or MLS(B) phenotypes remained constant over the time period (53 and 18%, respectively). Prior treatment with a macrolide or clindamycin alone or in combination with a beta-lactam resulted in 94 or 85% of isolates causing infections being macrolide and or clindamycin resistant. No prior individual macrolide (azithromycin, erythromycin, or clarithromycin) resulted in more macrolide resistance or in a more prevalent resistance phenotype. The ketolides appear to be active antimicrobials against penicillin- and macrolide-resistant pneumococci.

The ketolides: a critical review

Ketolides are a new class of macrolides designed particularly to combat respiratory tract pathogens that have acquired resistance to macrolides. The ketolides are semi-synthetic derivatives of the 14-membered macrolide erythromycin A, and retain the erythromycin macrolactone ring structure as well as the D-desosamine sugar attached at position 5. The defining characteristic of the ketolides is the removal of the neutral sugar, L-cladinose from the 3 position of the ring and the subsequent oxidation of the 3-hydroxyl to a 3-keto functional group. The ketolides presently under development additionally contain an 11, 12 cyclic carbamate linkage in place of the two hydroxyl groups of erythromycin A and an arylalkyl or an arylallyl chain, imparting in vitro activity equal to or better than the newer macrolides. Telithromycin is the first member of this new class to be approved for clinical use, while ABT-773 is presently in phase III of development. Ketolides have a mechanism of action very similar to erythromycin A from which they have been derived. They potently inhibit protein synthesis by interacting close to the peptidyl transferase site of the bacterial 50S ribosomal subunit. Ketolides bind to ribosomes with higher affinity than macrolides. The ketolides exhibit good activity against Gram-positive aerobes and some Gram-negative aerobes, and have excellent activity against drug-resistant Streptococcus pneumoniae, including macrolide-resistant (mefA and ermB strains of S. pneumoniae). Ketolides such as telithromycin display excellent pharmacokinetics allowing once daily dose administration and extensive tissue distribution relative to serum. Evidence suggests the ketolides are primarily metabolised in the liver and that elimination is by a combination of biliary, hepatic and urinary excretion. Pharmacodynamically, ketolides display an element of concentration dependent killing unlike macrolides which are considered time dependent killers. Clinical trial data are only available for telithromycin and have focused on respiratory infections including community-acquired pneumonia, acute exacerbations of chronic bronchitis, sinusitis and streptococcal pharyngitis. Bacteriological and clinical cure rates have been similar to comparators. Limited data suggest very good eradication of macrolide-resistant and penicillin-resistant S. pneumoniae. As a class, the macrolides are well tolerated and can be used safely. Limited clinical trial data suggest that ketolides have similar safety profiles to the newer macrolides. Telithromycin interacts with the cytochrome P450 enzyme system (specifically CYP 3A4) in a reversible fashion and limited clinically significant drug interactions occur. In summary, clinical trials support the clinical efficacy of the ketolides in upper and lower respiratory tract infections caused by typical and atypical pathogens including strains resistant to penicillins and macrolides. Considerations such as local epidemiology, patterns of resistance and ketolide adverse effects, drug interactions and cost relative to existing agents will define the role of these agents. The addition of the ketolides in the era of antibacterial resistance provides clinicians with more options in the treatment of respiratory infections.

Antimicrobial resistance among clinical isolates of Streptococcus pneumoniae in Canada during 2000

A total of 2,245 clinical isolates of Streptococcus pneumoniae were collected from 63 microbiology laboratories from across Canada during 2000 and characterized at a central laboratory. Of these isolates, 12.4% were not susceptible to penicillin (penicillin MIC, >or=0.12 microg/ml) and 5.8% were resistant (MIC, >or=2 microg/ml). Resistance rates among non-beta-lactam agents were the following: macrolides, 11.1%; clindamycin, 5.7%; chloramphenicol, 2.2%; levofloxacin, 0.9%; gatifloxacin, 0.8%; moxifloxacin, 0.4%; and trimethoprim-sulfamethoxazole, 11.3%. The MICs at which 90% of the isolates were inhibited (MIC90s) of the fluoroquinolones were the following: gemifloxacin, 0.03 microg/ml; BMS-284756, 0.06 microg/ml; moxifloxacin, 0.12 microg/ml; gatifloxacin, 0.25 microg/ml; levofloxacin, 1 microg/ml; and ciprofloxacin, 1 microg/ml. Of 578 isolates from the lower respiratory tract, 21 (3.6%) were inhibited at ciprofloxacin MICs of >or=4 microg/ml. None of the 768 isolates from children were inhibited at ciprofloxacin MICs of >or=4 microg/ml, compared to 3 of 731 (0.6%) from those ages 15 to 64 (all of these >60 years old), and 27 of 707 (3.8%) from those over 65. The MIC90s for ABT-773 and telithromycin were 0.015 microg/ml for macrolide-susceptible isolates and 0.12 and 0.5 microg/ml, respectively, for macrolide-resistant isolates. The MIC of linezolid was <or=2 microg/ml for all isolates. Many of the new antimicrobial agents tested in this study appear to have potential for the treatment of multidrug-resistant strains of pneumococci.

Comparison of in vitro activities of ABT-773 and telithromycin against macrolide-susceptible and -resistant streptococci and staphylococci

The activity of a new ketolide, ABT-773, was compared to the activity of the ketolide telithromycin (HMR-3647) against over 600 gram-positive clinical isolates, including 356 Streptococcus pneumoniae, 167 Staphylococcus aureus, and 136 Streptococcus pyogenes isolates. Macrolide-susceptible isolates as well as macrolide-resistant isolates with ribosomal methylase (Erm), macrolide efflux (Mef), and ribosomal mutations were tested using the NCCLS reference broth microdilution method. Both compounds were extremely active against macrolide-susceptible isolates, with the minimum inhibitory concentrations at which 90% of the isolates tested were inhibited (MIC90s) for susceptible streptococci and staphylococci ranging from 0.002 to 0.03 microg/ml for ABT-773 and 0.008 to 0.06 microg/ml for telithromycin. ABT-773 had increased activities against macrolide-resistant S. pneumoniae (Erm MIC90, 0.015 microg/ml; Mef MIC90, 0.12 microg/ml) compared to those of telithromycin (Erm MIC90, 0.12 microg/ml; Mef MIC90, 1 microg/ml). Both compounds were active against strains with rRNA or ribosomal protein mutations (MIC90, 0.12 microg/ml). ABT-773 was also more active against macrolide-resistant S. pyogenes (ABT-773 Erm MIC90, 0.5 microg/ml; ABT-773 Mef MIC90, 0.12 microg/ml; telithromycin Erm MIC90, >8 microg/ml; telithromycin Mef MIC90, 1.0 microg/ml). Both compounds lacked activity against constitutive macrolide-resistant Staphylococcus aureus but had good activities against inducibly resistant Staphylococcus aureus (ABT-773 MIC90, 0.06 microg/ml; telithromycin MIC90, 0.5 microg/ml). ABT-773 has superior activity against macrolide-resistant streptococci compared to that of telithromycin.

Susceptibilities to telithromycin and six other agents and prevalence of macrolide resistance due to L4 ribosomal protein mutation among 992 Pneumococci from 10 central and Eastern European countries

The macrolide and levofloxacin susceptibilities of 992 isolates of Streptococcus pneumoniae from clinical specimens collected in 1999 and 2000 were determined in 10 centers in Central and Eastern European countries. The prevalences of penicillin G-intermediate (MICs, 0.125 to 1 microg/ml) and penicillin-resistant (MICs, < or =2 microg/ml) Streptococcus pneumoniae isolates were 14.3 and 16.6%, respectively. The MICs at which 50% of isolates are inhibited (MIC(50)s) and the MIC(90)s of telithromycin were 0.016 and 0.06 microg/ml, respectively; those of erythromycin were 0.06 and >64 microg/ml, respectively; those of azithromycin were 0.125 and >64 microg/ml, respectively; those of clarithromycin were 0.03 and >64 microg/ml, respectively; and those of clindamycin were 0.06 and >64 microg/ml, respectively. Erythromycin resistance was found in 180 S. pneumoniae isolates (18.1%); the highest prevalence of erythromycin-resistant S. pneumoniae was observed in Hungary (35.5%). Among erythromycin-resistant S. pneumoniae isolates, strains harboring erm(B) genes (125 strains [69.4%]) were found to be predominant over strains with mef(E) genes (25 strains [13.4%]), L4 protein mutations (28 strains [15.6%]), and erm(A) genes (2 strains [1.1%]). Similar pulsed-field gel electrophoresis patterns suggested that some strains containing L4 mutations from the Slovak Republic, Bulgaria, and Latvia were clonally related. Of nine strains highly resistant to levofloxacin (MICs, >8 microg/ml) six were isolated from Zagreb, Croatia. Telithromycin at < or =0.5 microg/ml was active against 99.8% of S. pneumoniae isolates tested and may be useful for the treatment of respiratory tract infections caused by macrolide-resistant S. pneumoniae isolates.

Antibiotic resistance: where do ketolides fit?

Burgeoning resistance to antibiotics among common respiratory pathogens poses a very real risk to public health. A need therefore exists for new antibiotics that not only target all common respiratory pathogens, including problematic strains such as penicillin- and macrolide-resistant Streptococcus pneumoniae, but also resist resistance. The ketolides are a new class of antibiotics, of which telithromycin is the first to undergo clinical evaluation, designed to address such issues. These agents possess several innovative structural modifications that not only confer activity against common respiratory pathogens, irrespective of their beta-lactam or macrolide susceptibility, but also minimize the risk of emergent resistance. Ketolides such as telithromycin therefore represent important new options for the empiric treatment of community-acquired respiratory tract infections in an era of increasing resistance. They may be especially useful in areas where macrolide resistance among S. pneumoniae is common because current macrolide treatments against such pathogens are far from optimal.

In vitro activity of ABT-773, telithromycin and eight other antimicrobials against erythromycin-resistant Streptococcus pneumoniae respiratory isolates of children

The activity of the ketolide ABT-773 against 180 erythromycin-resistant Streptococcus pneumoniae obtained from children was compared with telithromycin, azithromycin, clarithromyin, roxithromycin, clindamycin, penicillin, levofloxacin and gatifloxacin. Ketolide MICs were all < or =1 mg/l, with ABT-773 being the most potent of all drugs tested. MIC(90)s for macrolides and azithromycin in mefE+ isolates were 16-32 compared with >128 mg/l for ermB+ isolates. ABT-773 and telithromycin MIC(90)s for mefE+ isolates were 0.125 and 0.5, compared with 0.032 and 0.016 mg/l for ermB+ isolates and 0.5 and 1 mg/l, respectively, for isolates containing both genes. Clindamycin was active against mefE+ but not ermB+ isolates. 155 isolates were resistant to penicillin. All fluoroquinolone MICs were < 1 mg/l. Further studies of ketolides for treatment of paediatric S. pneumoniae infections are warranted.

Postantibiotic effects of ABT-773 and amoxicillin-clavulanate against Streptococcus pneumoniae and Haemophilus influenzae

This study determined the postantibiotic effect (PAE) of ABT-773 versus that of amoxicillin-clavulanate against clinical isolates of Streptococcus pneumoniae and Haemophilus influenzae. The PAEs of ABT-773 and amoxicillin-clavulanate ranged from 2.3 to 6.0 h and 0 to 2.2 h against S. pneumoniae and from 2.7 to 9.1 h and 0 to 0.8 h against H. influenzae, respectively.

In vitro activity of ABT-773 against Legionella pneumophila, its pharmacokinetics in guinea pigs, and its use to treat guinea pigs with L. pneumophila pneumonia

The activity of ABT-773 was studied against extracellular and intracellular Legionella pneumophila and for the treatment of guinea pigs with L. pneumophila pneumonia. The ABT-773 MIC at which 50% of isolates are inhibited (MIC(50)) for 20 different Legionella sp. strains was 0.016 microg/ml, whereas the MIC(50)s of clarithromycin and erythromycin were 0.032 and 0.125 microg/ml, respectively. ABT-773 (1 microg/ml) was bactericidal for two L. pneumophila strains grown in guinea pig alveolar macrophages. In contrast, erythromycin and clarithromycin had easily reversible static activity only. Therapy studies of ABT-773 and erythromycin were performed with guinea pigs with L. pneumophila pneumonia. When ABT-773 was given to infected guinea pigs by the intraperitoneal route (10 mg/kg of body weight), mean peak levels in plasma were 0.49 microg/ml at 0.5 h and 0.30 microg/ml at 1 h postinjection. The terminal half-life phase of elimination from plasma was 0.55 h, and the area under the concentration-time curve from 0 to 24 h (AUC(0-24)) was 0.65 microg. h/ml. For the same drug dose, mean levels in the lung were 15.9 and 13.2 microg/g at 0.5 and 1 h, respectively, with a half-life of 0.68 h and an AUC(0-24) of 37.0 microg. h/ml. Ten of 15 L. pneumophila-infected guinea pigs treated with ABT-773 (15 mg/kg/dose given intraperitoneally once daily) for 5 days survived for 9 days post-antimicrobial therapy, as did 14 of 15 guinea pigs treated with erythromycin (30 mg/kg given intraperitoneally twice daily) for 5 days. All of the ABT-773-treated animals that died appeared to do so because of drug-induced peritonitis rather than overwhelming pneumonia. None of 12 animals treated with saline survived. ABT-773 is as effective as erythromycin against L. pneumophila in infected macrophages and in a guinea pig model of Legionnaires' disease. These data support studies of the clinical effectiveness of ABT-773 for the treatment of Legionnaires' disease.

In vitro activity of the ketolide ABT-773

The in vitro activities of ABT-773, azithromycin, erythromycin, and clindamycin were compared by testing 1,223 clinical isolates selected to represent different species and phenotypes. ABT-773 was particularly potent against staphylococci (the MIC at which 90% of the strains tested were inhibited [MIC(90)] was < or =0.06 microg/ml), including all strains that were macrolide resistant but clindamycin susceptible. Streptococcus pneumoniae and other streptococci were inhibited by low concentrations of ABT-773, and that included most erythromycin-resistant strains. Against Haemophilus influenzae, ABT-773 and azithromycin were similar in their antibacterial potency (MIC(90), 4.0 and 2.0 microg/ml, respectively).

Macrolide-resistant Streptococcus pneumoniae in Canada during 1998-1999: prevalence of mef(A) and erm(B) and susceptibilities to ketolides

In this study (1998-1999), we collected 215 macrolide-resistant Streptococcus pneumoniae isolates from an ongoing Canadian Respiratory Organism Surveillance Study involving 23 centers representing all regions of Canada. The prevalence of erythromycin-resistant S. pneumoniae was 8% (215 of 2,688). Of the 215 isolates, 48.8% (105 of 215) were PCR positive for mef(A) and 46.5% (100 of 215) were PCR positive for erm(B). The ketolides telithromycin and ABT-773 demonstrated excellent activity against both mef(A) (MIC for 90% of strains [MIC(90)], 0.06 and 0.03 microg/ml, respectively) and erm(B) (MIC(90), 0.06 and 0.03 microg/ml, respectively) strains of S. pneumoniae.

Comparative antimicrobial activity of ABT-773, a novel ketolide, tested against drug-resistant Gram-positive cocci and Haemophilus influenzae

The antimicrobial activity of ABT-773, a novel ketolide, was tested against 618 Gram-positive strains collected from various surveillance programmes between 1997 and 2000. ABT-773 has potent activity against Streptococcus pneumoniae (MIC90, < or = 0.03-0.12 mg/l), beta-haemolytic streptococci (MIC90, < or = 0.03 mg/l) and viridans group streptococci (MIC90, < or = 0.03 mg/l), including erythromycin-resistant strains. In contrast, ABT-773 was less active against erythromycin-resistant Staphylococcus aureus (31% susceptible at < or = 0.25 mg/l), coagulase-negative staphylococci (41% susceptible) and enterococci (30% susceptible). Haemophilus influenzae (MIC90, 4 mg/l) was less inhibited by the two ketolides tested, and ABT-773 was generally two- to fourfold more potent than telithromycin. The ketolides appear to have potential clinical use against some Gram-positive species resistant to macrolides.

Overcoming antimicrobial resistance by targeting resistance mechanisms

Three mechanisms of antimicrobial resistance predominate in bacteria: antibiotic inactivation, target site modification, and altered uptake by way of restricted entry and/or enhanced efflux. Many of these involve enzymes or transport proteins whose activity can be targeted directly in an attemptto compromise resistance and, thus, potentiate antimicrobial activity. Alternatively, novel agents unaffected by these resistance mechanisms can be developed. Given the ongoing challenge posed by antimicrobial resistance in bacteria, targeting resistance in this way may be our best hope at prolonging the antibiotic era.

Mechanisms of macrolide resistance in clinical pneumococcal isolates in France

The genetic basis of macrolide resistance was investigated in a collection of 48 genotypically unrelated clinical isolates of Streptococcus pneumoniae obtained between 1987 and 1997 in France. All strains were resistant to erythromycin, clindamycin, and streptogramin B, exhibiting a macrolide-lincosamide-streptogramin B resistance phenotype, and harbored the erm(B) gene. None of the strains carried the mef(A) or erm(A) subclass erm(TR) gene.

ABT-773: a new ketolide antibiotic

ABT-773 is a new semisynthetic derivative of erythromycin A, the ketolide class of broad spectrum antibacterial agents, in Phase II development by Abbott. With good broad spectrum activity against Gram-positive, some Gram-negative organisms and intracellular bacteria, ABT-773 is being developed as a respiratory agent. Structural changes in the ketolide class agents such as ABT-773 provides expanded activity in vitro against macrolide-resistant strains of Streptococcus pneumoniae and improved activity against MLS(B) (macrolide-lincosamide-streptogramin) constitutive expressing streptococci.