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A method is developed for the specific and sensitive determination of cethromycin concentrations in plasma, bronchoalveolar lavage (BAL), and alveolar cells (AC), using a high-performance liquid chromatographic-tandem mass spectrometry (MS) method. The mobile phase consists of 50% acetonitrile-0.05% acetic acid-5mM ammonium acetate; the column used is a C(8) reversed-phase stationary phase. The preparation of samples requires a solvent extraction step. The retention times for cethromycin and the internal standard are approximately 2.0 and 2.7 min, respectively, with a total run time of 3.5 min. Detection is carried out using electrospray MS in a multiple reaction monitor mode. The detection limits for cethromycin are 1 ng/mL for plasma and 0.2 ng/mL for BAL supernatants and AC suspensions. The assay has excellent performance characteristics and has been used to support a study of the intrapulmonary pharmacokinetics of cethromycin in human subjects.

The in vitro activities of telithromycin and cethromycin (ABT-773) against 412 Streptococcus pyogenes isolates, consecutively collected in 17 Spanish hospitals from different geographical areas, were evaluated and compared with those of erythromycin A, penicillin G, clindamycin and quinupristin-dalfopristin. With a susceptibility testing breakpoint ofor = 1 mg/L for both compounds, 96.1% of isolates were susceptible to telithromycin and 99.8% to cethromycin. Erythromycin non-susceptible isolates (intermediate plus resistant, MICor = 0.5 mg/L) comprised 23% of those tested, and were analysed for the genetic basis of their resistance by PCR. Among these isolates (n = 95), 72.6% harboured mef(A), 8.4%, erm(B), and 3.2%, erm(A), as sole macrolide resistance gene, whereas the presence of mef(A) plus erm(A) (11.6%) or mef(A) plus erm(B) (4.2%) was also observed. Both ketolides displayed a significant in vitro activity against S. pyogenes regardless of the macrolide resistance mechanisms. Nevertheless, in the case of telithromycin, 11 out of 19 of the erm(B)-positive isolates (2.7% of total population) exhibited an MIC range of 4-32 mg/L. According to the present results, telithromycin and cethromycin offer a wide coverage against S. pyogenes isolates in a geographic area with a high incidence of resistance to currently used macrolides.

Objectives: (i) To determine the inhibitory and bactericidal activities of ABT-773, a novel ketolide, against sensitive and erythromycin-resistant pneumococci; (ii) to subdivide erythromycin-resistant pneumococci into resistance phenotypes, more extensive than the conventional M and MLS B groups, by assessing susceptibilities to, and interactions between, erythromycin (14-membered macrolide), clindamycin (lincosamide), rokitamycin (16-membered macrolide), ABT-773 (ketolide), quinupristin (streptogramin B) and dalfopristin (streptogramin A). Methods: MICs and MBCs of ABT-773 were determined for 165 strains of pneumococci (113 resistant to erythromycin). Extended phenotypes for the erythromycin-resistant strains were described in terms of intrinsic susceptibility to, and induction of resistance by, the antibiotics listed above. Results: Erythromycin-resistant strains could be divided into 10 extended phenotypes (designated II-XI), two of which (II and IX) predominated. ABT-773 at 0.12 mg/L inhibited 109 strains (median 0.03 mg/L). MICs for the other four strains (of phenotypes X and XI) were 0.25-1 mg/L. MICs were only slighter higher when measured on agar in CO 2 than by the NCCLS method (in broth in air). MBCs were usually ≤2 x MIC, but for 10 strains (eight of phenotype X, one each of types IX and XI) MBCs were >1 mg/L, and three of the latter (all type X) were tolerant. Clones of reduced susceptibility (MICs 1-8 mg/L, increased by up to 32-fold) could be isolated from some strains of phenotypes VII, IX and X, but not from those of type II (efflux mechanism) or from erythromycin-sensitive strains. Conclusions: ABT-773 was active against all 113 erythromycin-resistant pneumococci tested, which belonged to 10 phenotypes. Extended phenotyping of pneumococci revealed interesting and potentially useful subdivisions of the classical phenotypes.

cin, as the MIC90 of cethromycin was 0.12 mg/L (4-fold lower) compared with an MIC90 of telithromycin of 0.5 mg/L (Table 1). Among penicillin-non-susceptible (MIC 4 mg/L) isolates, the MIC90 of ceftaroline was 0.25 mg/L and the MIC90 of ceftobiprole was 1.0 mg/L, whereas that of ceftriaxone was 2 mg/L (Table 1). Similarly, the MIC90 of cethromycin was 0.12 mg/L, which was 4-fold lower than that of telithromycin. Among erythromycin-non-susceptible (MIC 0.5 mg/L) isolates, the MIC90 of cethromycin was 0.12 mg/L compared with an MIC90 of telithromycin of 0.5 mg/L (Table 1). Similarly, the results showed that ceftaroline, ceftobiprole and cethromycin were more active against emerging MDR serotype 19A than currently available drugs. In summary, among b-lactams tested, ceftaroline was the most potent against MDR S. pneumoniae isolates. Similarly, the ketolide cethromycin was more active than telithromycin.

Objectives To investigate the in vitro activity of cethromycin, a new ketolide, against Chlamydia pneumoniae. Methods The in vitro activity of cethromycin against 20 isolates of C. pneumoniae was compared with the activities of telithromycin, erythromycin A, azithromycin and clarithromycin against those isolates. Results The MIC at which 90% of the isolates were inhibited and the minimal chlamydiacidal concentration at which 90% of the isolates were killed by cethromycin were both 0.016 mg/L (range 0.016-0.031 mg/L). Cethromycin was the most active antibiotic tested in this study. Conclusions Our results appear to indicate that cethromycin is an effective antibiotic that should play some role in the treatment of respiratory tract infections caused by C. pneumoniae.

Sir, Historically, methicillin-resistant Staphylococcus aureus (MRSA) has been regarded as a nosocomial pathogen responsible for severe toxin-mediated disease or invasive pyogenic infections. In recent years, however, community-associated MRSA (CA-MRSA) has been reported from around the world. Although the first CA-MRSA strains were susceptible to most antimicrobials, antimicrobial resistance has increased. One CA-MRSA clone, identified by the CDC as pulsotype USA300, has been implicated in outbreaks within the USA and is resistant to many currently marketed antimicrobial agents. This clone carries both the SCCmec IV element and the Panton–Valentine leukocidin (PVL) locus. The aim of this study was to compare the activity of a novel ketolide, cethromycin (previously ABT-773), with potentially improved pharmacodynamic and therapeutic benefits, three other macrolides (azithromycin, clarithromycin and erythromycin) and one lincosamide (clindamycin) against the 170 MRSA USA300 isolates that are in the Center for Biological Defense (CBD) collection. Seventeen MRSA isolates were procured from two hospital laboratories in Seattle, WA, USA. These were isolated from cultures of blood, skin lesions and tissue abscesses of patients seen in their clinics or emergency department in 2003 and 2004. Of the remaining 153 MRSA, 121 isolates were obtained from cultures of lesions, abscesses and nasopharyngeal specimens from outpatients seen at west central Florida clinical laboratories and doctor’s offices, whereas 32 MRSA were isolated from lesions, abscesses, nasopharyngeal and blood specimens acquired from patients seen at hospitals in the Tampa Bay area from 2004 to 2006. These isolates were previously characterized as USA300 by PFGE using Network on Antimicrobial Resistance in Staphylococcus aureus (NARSA) standards for comparison. Cethromycin (obtained from Advanced Life Sciences, Woodridge, IL, USA) was prepared and diluted with calciumadjusted Mueller–Hinton broth (Remel, Lenexa, KS, USA) using 96-well microtitre plates following standard protocols. The MIC and MBC were determined using orthodox procedures. Clarithromycin, erythromycin and clindamycin were tested with Sensititre by TREK Diagnostic Systems (Cleveland, OH, USA), whereas azithromycin, erythromycin and clindamycin were tested using Etest (AB Biodisk, North America Inc., Piscataway, NJ, USA), according to manufacturer’s instructions. All MIC interpretations followed CLSI guidelines. Clindamycin resistance was examined further with the D-test. Controls were S. aureus ATCC 29213, S. aureus ATCC BAA977 and S. aureus ATCC BAA976. Cethromycin was effective against all 170 isolates (Table 1). The cethromycin MIC range was 0.002–0.125 mg/L, whereas the MIC50 and MIC90 were both 0.002 mg/L. The cethromycin MBC range was 0.002–1.0 mg/L, with the MBC50 of 0.002 mg/L and the MBC90 of 0.008 mg/L. Of the 170 CA-MRSA, 169 (99.4%) were intermediate or resistant to at least one macrolide. In contrast, 164 (96.5%) isolates were susceptible to clindamycin. Only six (3.5%) isolates demonstrated either constitutive resistance [4 (2.4%)] or inducible resistance [2 (1.2%)] to clindamycin. This is unlike the USA300 isolates in Boston where 57% of the isolates were clindamycin-resistant. Against 10 erythromycin-susceptible isolates, the cethromycin MIC and MBC ranges were 0.002–0.008 mg/L, compared with the MIC range of 0.002–0.25 mg/L and MBC range of 0.002–1.0 mg/L obtained with the erythromycin-resistant isolates. However, MIC50 and MBC50 values, as well as MIC90 and MBC90 values, did not change regardless of erythromycin susceptibility. There were six isolates resistant to both erythromycin and clindamycin, against which cethromycin MIC and MBC ranges were 0.002–0.06 mg/L. This population’s susceptibility to cethromycin is especially important in the light of the fact that most isolates were resistant to one or more macrolides. This collection of MRSA USA300 isolates was overwhelmingly macrolide-resistant and clindamycinsusceptible. The study of Chavez-Bueno et al. demonstrated a similar pattern in CA-MRSA isolates among children in Texas where the inducible resistance to clindamycin was slowly replaced by clindamycin susceptibility (non-inducible resistance). Most of the CA-MRSA isolates in the Texas study carried the erm(C) gene, with others carried erm(A) or msr(A) with or without erm(C). This phenotypic change suggests an alteration in the proportion of the macrolide resistance determinants in the population. Tenover et al. reported the msr(A) gene in isolates comprising a specific clone (USA300-0114). Accordingly, we hypothesized that most of the CBD collection of MRSA USA300 isolates (only one being USA300-0114) also carried a different determinant, putatively the msr(A)/msr(B) gene encoding an efflux protein that normally renders a bacterial cell resistant only to macrolides and streptogramin B compounds (MS phenotype) instead of the MLSB resistance genes [i.e. erm(A) and erm(C)]. Using the oligonucleotides described by Sutcliffe et al. for PCR primers and a well-characterized MRSA strain NARSA 384 (USA300-0114) as positive control, we performed a PCR assay for the msr(A) gene on DNA extracted from all 170 isolates. After the amplicons were electrophoresed on a 1% agarose gel, a positive PCR reaction yielded a DNA band of 399 bp. All negative and questionable results were performed three times. The assay produced a positive reaction in 160 (94.1%) isolates, whereas only 10 (5.9%) of the 170 MRSA yielded negative PCR reactions. These Research letters