In vitro activities of the ketolides ABT-773 and telithromycin and of three macrolides against genetically characterized isolates of Streptococcus pneumoniae, Streptococcus pyogenes, Haemophilus influenzae and Moraxella catarrhalis

F.-J. Schmitz1,2,*, S. Schwarz3, D. Milatovic2, J. Verhoef2 and A. C. Fluit2

1 Institute for Medical Microbiology and Virology, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, Geb. 22.21, D-40225 Düsseldorf; 3 Institut für Tierzucht und Tierverhalten der Bundesforschungsanstalt für Landwirtschaft (FAL), Celle, Germany; 2 Eijkman-Winkler-Institute for Medical Microbiology, Utrecht, The Netherlands

Sir,

Macrolide antibiotics are mainly used for treating community-acquired respiratory tract infections. Ketolides, such as telithromycin and ABT-773, demonstrate in vitro activity and in vivo efficacy in animal models of infection. In addition to the ketone group at position 3, the novel ketolide ABT-773 is modified by an O-allyl-3-quinoline at the 6 position and a cyclic carbamate group between the 11 and 12 positions.1

The present study aimed to compare the in vitro activities of the ketolides ABT-773 and telithromycin with those of azithromycin, clarithromycin and erythromycin. Isolates of Streptococcus pneumoniae (n = 650), Streptococcus pyogenes (n = 100), Moraxella catarrhalis (n = 500) and Haemophilus influenzae (n = 250) derived from community-acquired respiratory tract infections were studied. The isolates were collected at the University hospital of Düsseldorf or during international surveillance studies,2 and were identified and characterized using conventional phenotypic and genotypic methods. Only one isolate per patient was included in this study.

The 550 pneumococcal isolates comprised five groups: (i) those fully susceptible to penicillin, erythromycin, clindamycin, tetracycline, trimethoprim/sulfamethoxazole and levofloxacin (n = 100); (ii) isolates resistant to erythromycin and clindamycin [erm(B) genotype, n = 150]; (iii) those resistant to erythromycin, but susceptible to clindamycin [mef(E) genotype, n = 50]; (iv) isolates resistant to tetracycline [tet(M) genotype, n = 200]; (v) those resistant to trimethoprim and sulfamethoxazole [alterations in the dihydrofolate reductase (Ile-100->Leu) as well as repetitions of one or two amino acids in the region from Arg-58 to Tyr-63 of the dihydropteroate synthase (n = 50)].3

The 80 S. pyogenes clinical isolates comprised three groups of characterized strains: (i) those fully susceptible to penicillin, erythromycin, clindamycin and levofloxacin (n = 60); (ii) isolates resistant to erythromycin, but susceptible to clindamycin, exhibiting a mef(A) gene (n = 10); and (iii) those resistant to erythromycin, carrying an inducible erm(A) gene (n = 10).

In addition, 100 S. pneumoniae and 20 S. pyogenes isolates with a reduced susceptibility to ciprofloxacin (MIC > 4 mg/L) were tested. These 120 isolates were derived from in vitro quinolone resistance selection experiments. All selected isolates demonstrated the well-known key alterations within the quinolone resistance determining regions of ParC and GyrA.4

Of the 500 M. catarrhalis isolates collected, 450 (90%) were ß-lactamase positive. BRO-2 ß-lactamase (detected by isoelectric focusing) and the gene bro-2 (detected by PCR) were found in 22 (5%) of the 450 isolates. The remaining 428 isolates possessed a BRO-1 ß-lactamase encoded by the gene bro-1. Of the 250 H. influenzae isolates collected, 37 (15%) were ß-lactamase positive.5

Antibiotic susceptibilities were determined using a broth microdilution method as defined by the NCCLS.6 Cation-adjusted Mueller–Hinton broth was used for M. catarrhalis, whereas H. influenzae was tested with Haemophilus Test Medium. The microdilution plates were incubated in an atmosphere containing 5% CO2. Quality control results met NCCLS standards.6 The results of susceptibility testing are presented as range, MIC50 and MIC90 values (Table 1).


View this table:
[in this window]
[in a new window]
 
Table 1.  MICs (mg/L) and range of MICs of ABT-773, telithromycin, erythromycin, clarithromycin and azithromycin
 
In general, ABT-773 was at least two- to four-fold more potent than any of the three comparator macrolides, and one dilution step more potent than telithromycin against macrolide-susceptible strains of S. pneumoniae and S. pyogenes. ABT-773 was very active against all macrolide-susceptible streptococcal isolates tested, with MIC50 and MIC90 values of <=0.004–0.03 and 0.004–0.06 mg/L, respectively. With regard to MIC50 and MIC90 values, no significant difference was observed between the different groups of isolates, including those with reduced ciprofloxacin susceptibility derived by in vitro selection experiments.

The improved potency of ABT-773 is due to its enhanced affinity for ribosomes.7 In addition, ABT-773 retained potent activity against macrolide-resistant S. pneumoniae and S. pyogenes, irrespective of whether resistance was due to a mef efflux gene [mef(E) in S. pneumoniae or mef(A) in S. pyogenes, respectively] or an erm gene [erm(B) or inducible erm(A), respectively]. This activity derives from an affinity for methylated streptococcal ribosomes, and the inability of the Mef efflux pump to clear ABT-773.7

ABT-773 was approximately two-fold less potent than azithromycin against M. catarrhalis and H. influenzae, irrespective of their ß-lactamase production. The corresponding MIC50 and MIC90 values for ABT-773 were 0.06 and 0.12 mg/L, respectively, for M. catarrhalis, and 2 and 4 mg/L, respectively, for H. influenzae. The interpretation of these MICs for H. influenzae must await clinical response data.

Overall, our data, based on analyses of genetically or phenotypically characterized isolates, confirm and extend previous investigations of the in vitro activity of this new ketolide.8 The rank order of activity demonstrates that the novel ketolide ABT-773 was the most active agent against all streptococcal isolates tested, followed by telithromycin and the three macrolides. In addition, the in vitro activities of the two ketolides against M. catarrhalis and H. influenzae were comparable to those of the three macrolides tested. Thus, the novel ketolide ABT-773 appears to be a promising new antimicrobial agent for respiratory tract infections.

Footnotes

* Corresponding author. Tel/Fax: +49-2132-72040; E-mail: schmitfj{at}uni-duesseldorf.de Back

References

1 . Or, Y. S., Clark, R. F., Wang, S., Chu, D. T. W., Nilius, A. M., Flamm, R. K. et al. (2000). Design, synthesis, and antimicrobial activity of the 6-O-substituted ketolides active against respiratory tract pathogens. Journal of Medical Chemistry 43, 1045–9.

2 . Milatovic, D., Schmitz, F. J., Brisse, S., Verhoef, J. & Fluit, A. C. (2000). In vitro activities of sitafloxacin (DU-6859a) and six other fluoroquinolones against 8,796 clinical bacterial isolates. Antimicrobial Agents and Chemotherapy 44, 1102–7.[Abstract/Free Full Text]

3 . Schmitz, F.-J., Perdikouli, M., Beeck, A., Verhoef, J. & Fluit, A. C. (2001). Molecular surveillance of macrolide, tetracycline, and quinolone resistance mechanisms in 1191 clinical European Streptococcus pneumoniae isolates. International Journal of Antimicrobial Agents 18, 433–6.[ISI][Medline]

4 . Boos, M., Mayer, S., Fischer, A., Köhrer, K., Scheuring, S., Heisig, P. et al. (2001). In-vitro development of resistance to six quinolones in Streptococcus pneumoniae, Streptococcus pyogenes, and Staphylococcus aureus. Antimicrobial Agents and Chemotherapy 45, 938–42.[Abstract/Free Full Text]

5 . Schmitz, F.-J., Boos, M., Mayer, S., Verhoef, J., Milatovic, D. & Fluit, A. C. (2002). In-vitro activity of faropenem and 20 other compounds against ß-lactamase-positive and -negative Moraxella catarrhalis and Haemophilus influenzae isolates and the effect of serum on faropenem MICs. Journal of Antimicrobial Chemotherapy 49, 220–3.[Free Full Text]

6 . National Committee for Clinical Laboratory Standards. (2000). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically—Fifth Edition: Approved Document M7-A5. NCCLS, Villanova, PA.

7 . Capobianco, J. O., Cao, Z. S., Shortridge, V. D., Ma, Z., Flamm, R. K. & Zhong, P. (2000). Studies on the novel ketolide ABT-773: transport, binding to ribosomes, and inhibition of protein synthesis in Streptococcus pneumoniae. Antimicrobial Agents and Chemotherapy 44, 1562–7.[Abstract/Free Full Text]

8 . Barry, A. L., Fuchs, P. C. & Brown, S. D. (2001). In vitro activity of the ketolide ABT-773. Antimicrobial Agents and Chemotherapy 45, 2922–4.[Abstract/Free Full Text]