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Antimicrobial Agents and Chemotherapy, June 1998, p. 1515-1516, Vol. 42, No. 6
Division of Infectious Diseases, Department
of Pediatrics, SUNY Health Science Center at Brooklyn, Brooklyn,
New York 11203
Received 13 January 1998/Returned for modification 4 March
1998/Accepted 26 March 1998
The in vitro activities of HMR 3647, roxithromycin, erythromycin,
and azithromycin against 19 strains of Chlamydia pneumoniae were tested. The MIC at which 90% of the isolates are inhibited and
the minimum bactericidal concentration at which 90% of the isolates
are killed of HMR 3647 were 0.25 µg/ml (range, 0.015 to 2 µg/ml).
Nine recently obtained clinical isolates from children with pneumonia
were more susceptible (MICs, 0.015 to 0.0625 µg/ml) than older
strains that had been passaged more extensively.
The ketolides are a new class of
macrolide antibiotics with a 3-keto function instead of the cladinose
sugar. The ketolides are acid stable and have activity against a broad
range of respiratory pathogens, including multiresistant pneumococci,
Haemophilus influenzae, Legionella species, and
Mycoplasma pneumoniae (1, 4, 5). Chlamydia
pneumoniae is emerging as an important cause of community-acquired respiratory infection in adults and children worldwide. Clinically these infections cannot be readily differentiated from those caused by
"atypical" pathogens, such as M. pneumoniae. Data on the
activity of HMR 647 or other ketolides against C. pneumoniae
are very limited; only two strains, TW 183 and IOL 207, have been
tested so far (2). Therefore, we compared the in vitro
activities of HMR 3647, roxithromycin, erythromycin, and azithromycin
against 19 isolates of C. pneumoniae, including recently
obtained clinical isolates.
Isolates of C. pneumoniae tested included a reference strain
(TW 183), 15 isolates from children and adults with pneumonia from the
United States (AR39, T2023, T2043, W6805, CDC8, T2337, 279, 327, 040, 015, 380, 117, 206, 032, and 428), an isolate from a child with
pneumonia from Japan (J-21), and 2 strains from bronchoalveolar lavage
specimens from patients with human immunodeficiency virus infection and
pneumonia from the United States (BAL15 and BAL16). HMR 3647, roxithromycin, erythromycin, and azithromycin were provided as powders
and solubilized according to the instructions of the manufacturers.
Susceptibility testing of C. pneumoniae was performed with
cycloheximide-treated HEp-2 cells grown in 96-well microtiter plates (6). Each well was inoculated with 0.1 ml of the test strain diluted to yield 103 to 104 inclusion-forming
units per ml; the plates were then centrifuged at 1,700 × g for 1 h and incubated at 35°C for 1 h. Wells
were then aspirated and overlaid with 0.2 ml of medium containing 1 µg of cycloheximide per ml and serial twofold dilutions of the test
drug. After incubation at 35°C for 72 h, cultures were fixed and
stained for inclusions with fluorescein-conjugated antibody to the
chlamydial lipopolysaccharide genus-specific antigen (Pathfinder; Kallestad Diagnostics, Chaska, Minn.). The MIC was the lowest antibiotic concentration at which no inclusions were seen. The minimal
bactericidal concentration (MBC) was determined by aspirating the
antibiotic-containing medium, washing wells twice with
phosphate-buffered saline, and adding antibiotic-free medium. The
infected cells were frozen at The results are shown in Table 1. The MIC
at which 90% of the isolates are inhibited (MIC90) of HMR
3647 was 0.25 µg/ml (range, 0.015 to 2 µg/ml); the
MBC90 was also 0.25 µg/ml (range, 0.015 to 2 µg/ml). The MIC90s for roxithromycin, erythromycin,
and azithromycin were 0.5, 0.25, and 0.25 µg/ml, respectively.
Nine recent clinical isolates of C. pneumoniae were
more susceptible to HMR 3647 than older isolates; the MICs ranged from
0.031 to 0.0625 µg/ml (data not shown). The MIC for the least
susceptible isolate, T2023, was 2 µg/ml. T2023 was first isolated
from a child with pneumonia in 1987 and has been extensively passaged
in our laboratory. This variability of susceptibility between recent
clinical isolates and older isolates was not observed with the other
antibiotics tested.
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
In Vitro Activity of a New Ketolide Antibiotic, HMR
3647, against Chlamydia pneumoniae
ABSTRACT
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70°C, thawed, passed onto new cells,
incubated for 72 h, and then fixed and stained as described above.
The MBC was the lowest antibiotic concentration which resulted in no
inclusions after passage. All tests were run in triplicate.
TABLE 1.
Activities of HMR 3647 and other antibiotics against
19 isolates of C. pneumoniae
HMR 3647 had overall activity in vitro against C. pneumoniae comparable to those of the other macrolides tested. However, there was wide interstrain variation in susceptibility that appeared to be related to the age of the isolates. HMR 3647 was 10-fold more active against recent clinical isolates of C. pneumoniae than against older isolates that had been passaged more intensively in the laboratory. The older isolates, including T2023, had been passaged as many as 100 times, compared to only 5 to 6 times for the recent clinical isolates. HMR 3647 was also 10-fold more active against these isolates than were the other macrolides tested. This degree of variability was not observed with the other macrolides tested in this study or previously with other macrolides (3). Studies comparing the in vitro activities of several quinolones, including ofloxacin, levofoloxacin, and trovafloxacin, against many of the same strains tested here found very little variability, with MICs ranging from only 0.5 to 1 µg/ml (3, 7, 8).
The available data on the activity of ketolides against
Chlamydia species are very limited. Haider et al. evaluated
the activity of another ketolide compound, RU 64004, against one strain
of Chlamydia psittaci and two strains each of
Chlamydia trachomatis and C. pneumoniae
(2). The C. pneumoniae strains tested were two
reference strains, TW 183 and IOL 207. The MICs and MBCs were 
0.01 to
0.05 µg/ml for all three species. These results are dramatically different from those of the present study, where we found the MIC of
HMR 3647 against TW 183 to be 0.125 µg/ml, 10-fold less than that
found by Haider et al. We also tested RU 64004 side by side in vitro
against the same isolates, and it was less active than HMR 3647, with a
MIC90 of 0.5 µg/ml and an MBC90 of 1 µg/ml (data not shown). We also saw the same patterns of susceptibility seen
with HMR 3647; recent clinical isolates were more susceptible, with
MICs ranging from 0.015 to 0.25 µg/ml. One possible explanation for
this discrepancy is the different tissue culture system used by Haider
et al. They used McCoy cells, which we have found to be 3 log units
less susceptible to infection with C. pneumoniae than
HEp-2 cells (6). This might lead to an overall lower rate of
infection in the McCoy cell monolayers, resulting in lower MICs.
HMR 3647 shows promising activity in vitro against C. pneumoniae, especially against recent clinical isolates. The potential role of this antibiotic in the treatment of respiratory infection due to C. pneumoniae will depend on the results of treatment studies utilizing culture for diagnosis and evaluation of microbiologic efficacy.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Pediatrics, Box 49, SUNY Health Science Center at Brooklyn, 450 Clarkson Ave., Brooklyn, NY 11203-2098. Phone: (718) 245-4075. Fax: (718) 245-2118. E-mail: mhammerschlag{at}pol.net.
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REFERENCES |
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| 2. | Haider, F., F. Eb, and J. Orfila. 1995. Ketolides and Chlamydia: in vitro evaluation of RU 004, p. 142. , abstr. F165. In Abstracts of the 35th Interscience Conference on Antimicrobial Agents and Chemotherapy. American Society for Microbiology, Washington, D.C. |
| 3. |
Hammerschlag, M. R.,
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Antimicrob. Agents Chemother.
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| 4. | Jamjian, C., D. J. Biedenbach, and R. N. Jones. 1997. In vitro evaluation of a novel ketolide antimicrobial agent, RU-64004. Antimicrob. Agents Chemother. 41:454-459[Abstract]. |
| 5. | Jones, R. N., and D. J. Biedenbach. 1997. Antimicrobial activity of RU-66647, a new ketolide. Diagn. Microbiol. Infect. Dis. 27:7-12[Medline]. |
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Roblin, P. M.,
W. Dumornay, and M. R. Hammerschlag.
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Roblin, P. M.,
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