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ER-2 is a new investigational quinoline derivative with the formula 7a,13b-8,8-dimethylpyrrolo[2,3-d]pyrimidine-[2,1-a]pyrrolo[4,3:2,3]-7a,8,13,13b-tetrahydroquinoline 7 (compound 7), synthesized by Department of Organic Chemistry, University of Chennai, Chennai, India (Fig. (Fig.1).1). This drug has shown significant activity against gram-positive and gram-negative organisms (7).
Mycoplasma pneumoniae is a significant cause of upper and lower respiratory tract infections in persons of all age groups (6, 8-10). In this study, we compared the in vitro antibacterial activity of ER-2 against a collection of mycoplasma species known to cause disease in humans to those of antimicrobial agents known to have in vitro activity against these organisms.
Sixty-one M. pneumoniae isolates tested were collected from clinical specimens of persons with pneumonia. Forty M. hominis isolates were derived from clinical specimens from the urogenital tract. Ten of the isolates were resistant to tetracycline (MIC90, ≥8 μg/ml).
Moxifloxacin, ciprofloxacin, levofloxacin, gatifloxacin, gemifloxacin, azithromycin, doxycycline, tetracycline, and clindamycin were purchased from Sigma Aldrich (St. Louis, MO). Azithromycin was tested only against M. pneumoniae since Mycoplasma hominis is mostly resistant to macrolides (9), except for the 16-member ring of this class, and clindamycin was tested only against M. hominis since it is not recommended for the treatment of M. pneumoniae (2).
Antimicrobial powders were used according to the manufacturer's protocol. Working dilutions of the drugs were prepared fresh on the day of the assay.
Mycoplasmas were tested by the agar dilution method using a Steers replicator as described previously (4, 5). M. pneumoniae ATCC 29342 and M. hominis ATCC 43521 were used as reference strains. Five isolates of M. pneumoniae were randomly chosen for further testing to determine minimum bactericidal concentrations (MBCs) according to the protocol previously described (9).
MICs of ER-2 and other antimicrobial agents are shown in Table Table1.1. ER-2 exhibited excellent in vitro activity against 61 isolates of M. pneumoniae (MIC50, 0.001 μg/ml; MIC90, 0.016 μg/ml), which was the same as that of azithromycin (MIC50, 0.001 μg/ml; MIC90, 0.016 μg/ml) and which had the lowest MIC of any drug tested. Moxifloxacin (MIC50, 0.001 μg/ml; MIC90, 0.12 μg/ml) was more active than levofloxacin (MIC50, 1 μg/ml; MIC90, 1 μg/ml), ciprofloxacin (MIC50, 4 μg/ml; MIC90, 4 μg/ml), gatifloxacin (MIC50, 0.125 μg/ml; MIC90, 0.125 μg/ml), gemifloxacin (MIC50, 0.032 μg/ml; MIC90, 0.125 μg/ml), and doxycycline (MIC50, 0.25 μg/ml; MIC90, 0.25 μg/ml). MBCs of ER-2 (MBC50, 0.001 μg/ml; MBC90, 0.016 μg/ml) and moxifloxacin (MBC50, 0.06 μg/ml; MBC90, 0.12 μg/ml) showed that they were bactericidal against all of the M. pneumoniae isolates tested.
M. hominis was highly susceptible to ER-2 (MIC90, 0.06 μg/ml) and had the same susceptibility to moxifloxacin (Table (Table1).1). ER-2 was more highly active than gatifloxacin (MIC90, 0.125 μg/ml), clindamycin (MIC90, 0.032 μg/ml), levofloxacin (MIC90, 0.5 μg/ml), and ciprofloxacin (MIC90, 1 μg/ml). In vitro data indicate that ER-2 may have considerable promise for both genital and respiratory infections with mycoplasmas, but its clinical utility will depend upon its toxicity and pharmacokinetics (3). Our study indicates that ER-2 is active in vitro against mycoplasmal species that are clinically important in humans, with activity that was comparable to that of azithromycin and superior to those of the fluoroquinolones tested. Earlier work demonstrated that ER-2 inhibited gyrase supercoiling with potency similar to ciprofloxacin (7). Pyrimidine as a motif is found to be present in a variety of therapeutic molecules exhibiting antibacterial, antiviral, anticancer and anti-inflammatory properties (1). ER-2 consists of a uracil moiety that might be responsible for the potent activity of the compound (1). Overall, the excellent in vitro and in vivo activities of ER-2 suggest its usefulness in the treatment of mycoplasma infections. Further study for the potential use of this drug is needed.
Published ahead of print on 8 September 2009.