In Vitro Activities of Omadacycline (PTK 0796) and Other Antimicrobial Agents against Human Mycoplasmas and Ureaplasmas

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Omadacycline (9-neopentylaminomethylminocycline) is a novel, first-in-class aminomethylcycline, with both intravenous (i.v.) and oral formulations, that is in clinical development for use against acute skin and skin structure infections, community-acquired pneumonias, and urinary tract infections. Omadacycline has potent activity against Gram-positive and Gram-negative bacteria, including important skin and respiratory tract pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA), β-hemolytic streptococci, penicillin-resistant Streptococcus pneumoniae, Haemophilus influenzae, and Legionella pneumophila, as well as vancomycin-resistant Enterococcus spp. (VRE) and anaerobes (1, 2). Although the omadacycline binding site is similar to that of tetracycline, a significant advantage of this agent is that it retains activity against microorganisms with the two main tetracycline resistance mechanisms, efflux and ribosomal protection (3).

Mycoplasma pneumoniae is an important cause of tracheobronchitis and community-acquired pneumonia in children and adults (4). Mycoplasma hominis and Ureaplasma spp. cause various urogenital conditions in adults and are systemic pathogens in infants (5). Invasive disease may result from these organisms in children and adults who are immunosuppressed (4, 5). Treatment options for mycoplasmal and ureaplasmal infections have been complicated by the emergence of macrolide resistance in M. pneumoniae in Asia and its spread to Europe and North America (6, 7). Tetracycline resistance sometimes occurs in M. hominis and Ureaplasma spp., and resistance to macrolides and fluoroquinolones has been well documented in these organisms (6). People who are immunosuppressed and those who have received numerous courses of antibiotics over time are at greater risk for systemic infections with drug-resistant organisms (6). For these reasons, new agents that are not affected by cross-resistance to other drug classes are needed for treatment of Mycoplasma and Ureaplasma infections.

We performed an in vitro study testing the activity of omadacycline against a collection of reference strains and clinical isolates of Mycoplasma and Ureaplasma spp. obtained from various U.S. states and from Shanghai, China. Organisms included 20 M. pneumoniae, 20 M. hominis, 10 Ureaplasma parvum, and 10 Ureaplasma urealyticum isolates. Organisms tested included strains known to contain tet(M), the only known mechanism for tetracycline resistance in human mycoplasmas and ureaplasmas; strains with rRNA mutations conferring macrolide resistance; and strains with parC/parE mutations conferring fluoroquinolone resistance. Comparator agents used were tetracycline, doxycycline, azithromycin, and moxifloxacin. Clindamycin was substituted for azithromycin for M. hominis. Antimicrobial agents were obtained in powdered form of known purity and diluted in accordance with their respective manufacturer's instructions. Inoculum was prepared, MICs were determined by broth microdilution, and quality control was performed in accordance with Clinical and Laboratory Standards Institute Guideline M43-A (8). The mycoplasmacidal concentration (MBC) for omadacycline was determined for 10 isolates of M. pneumoniae, including 1 macrolide-resistant strain, 1 M. hominis, and 1 U. parvum, as previously described by subculturing fluid from the growth control well and test wells of the broth microdilution MIC system that did not show color change at the time the growth control first showed color change into a larger volume of broth to dilute the antibiotic beyond the MIC (9). Broths were incubated for at least twice the time necessary to determine the MIC. The MBC was defined as the lowest concentration of antimicrobial at which there was no evidence of broth color change after prolonged incubation. Positive and negative controls for the MBC assay consisted of tetracycline (bacteriostatic) and moxifloxacin (bactericidal). When the MBC was ≥3 dilutions greater than the MIC, the drug was considered bacteriostatic. When the MBC was ≤2 dilutions greater than the MIC, it was considered bactericidal (9). This work was performed with approval from the Institutional Review Board for Human Use. Table 1 summarizes the MIC data.

Omadacycline had the lowest MIC₅₀ and MIC₉₀ values (0.032 and 0.063 μg/ml, respectively) among all 5 drugs tested against M. hominis, one 2-fold dilution lower than clindamycin and moxifloxacin with all omadacycline MICs of ≤0.125 μg/ml. Omadacycline MICs were not affected by the presence of tet(M) in 4 isolates of M. hominis. These organisms had elevated MICs for tetracycline ranging from 8 to 32 μg/ml, while the omadacycline MICs were 0.032 to 0.125 μg/ml.

For M. pneumoniae, the omadacycline MIC₉₀ (0.25 μg/ml) was one 2-fold dilution greater than that of moxifloxacin and one 2-fold dilution lower than the MIC₉₀s for tetracycline and doxycycline. Azithromycin had the lowest MICs of all drugs tested against macrolide-susceptible M. pneumoniae (≤0.005 μg/ml), but omadacycline MICs remained low (0.125 μg/ml) for 10 isolates with high-level macrolide resistance (azithromycin MICs, ≥16 μg/ml).

Omadacycline was somewhat less active against Ureaplasma spp. than against Mycoplasma spp. The presence of macrolide resistance with azithromycin MICs of 16 to 32 μg/ml (2 isolates), high-level tetracycline resistance with MICs of 8 to 16 μg/ml (4 isolates), or fluoroquinolone resistance (2 isolates with moxifloxacin MICs of 4 to 16 μg/ml) had no effect on omadacycline MICs, all of which were 0.25 to 2 μg/ml. However, among the 16 tetracycline-susceptible Ureaplasma isolates, omadacycline activity was comparable to tetracycline, but its MICs were consistently 1 to 3 dilutions higher than those for doxycycline.

Omadacycline MBCs tested against M. pneumoniae, M. hominis, and U. parvum were ≥3 dilutions greater than the corresponding MICs, indicating that this drug is bacteriostatic against all of these organisms, consistent with previously reported results for tetracycline and doxycycline (6).

This comparative in vitro evaluation of omadacycline against a collection of reference strains and clinical isolates of human Mycoplasma and Ureaplasma spp. from several locations within the United States and from China demonstrated that this agent is active against Mycoplasma spp., with all MICs of ≤0.25 μg/liter, and against Ureaplasma spp., with all MICs of ≤2 μg/ml. Its activity in vitro is not affected by any of the known mechanisms of macrolide, tetracycline, or fluoroquinolone resistance. No omadacycline MIC breakpoints have been established thus far. Therefore, it is not possible to provide MIC data categorized as susceptible or resistant. However, pharmacodynamic assessments in animal models of infection have indicated that the omadacycline AUC/MIC (area under the concentration-time curve over 24 h in the steady state divided by the MIC) is the most important index for prediction of the therapeutic efficacy in humans (2). The serum concentrations for omadacycline following a 100-mg i.v. or 300-mg oral dose have resulted in AUC/MIC values that would be expected to provide clinical efficacy against several bacteria that commonly cause pneumonias and skin infections, including various Gram-positive cocci, Chlamydia, and Legionella spp. (2). MIC ranges for Mycoplasma and Ureaplasma spp. obtained in the present study are similar to those reported for these other organisms. The results of this study suggest that omadacycline should be studied further as a treatment for infections of the respiratory tract and/or urogenital tract that may be caused by Mycoplasma or Ureaplasma spp.