In Vitro Activities of Omadacycline against Rapidly Growing Mycobacteria

INTRODUCTION

Infections caused by Mycobacterium abscessus, Mycobacterium chelonae, and Mycobacterium fortuitum occur more frequently in immunocompromised individuals than in healthy humans. When they do occur, they are difficult to treat and do not respond well to antimicrobials used for other mycobacterial infections (1).

M. abscessus is the third most common nontuberculous mycobacterial (NTM) respiratory pathogen recovered from lungs in the United States (1). Clinical infections can range from asymptomatic to severe bronchiectasis and/or cavitary lung disease. M. abscessus is inherently multidrug resistant. Drug regimens are selected on the basis of results from in vitro susceptibility studies. The optimal therapeutic regimen and treatment duration are not well established. M. chelonae causes a range of diseases, including cellulitis and eye infections. M. chelonae is often found in water heaters, pedicure beds, and tattoo parlors (2). It is often treated with azithromycin or clarithromycin. M. fortuitum is usually associated with soft tissue infections and hospital-acquired postoperative infections in immunocompromised hosts.

Omadacycline (OMC) is a new intravenous and oral tetracycline derivative with potent in vitro and in vivo activities against many drug-resistant pathogens, including common Gram-positive aerobes, many Gram-negative aerobes, anaerobes, and atypical bacterial pathogens (3, 4). The activity of OMC in the presence of tetracycline-specific efflux pumps and ribosomal protection is conferred by substitutions at C-7 and C-9, respectively (Fig. 1).

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FIG 1

Chemical structures of omadacycline, tigecycline, and doxycycline.

The purpose of the present study was to evaluate the in vitro activities of OMC, doxycycline (DOX), and tigecycline (TGC) against clinical isolates of M. abscessus complex, M. chelonae, and M. fortuitum. Amikacin (AMK) served as a control drug.

OMC, TGC, DOX, and AMK were obtained from Paratek Pharmaceuticals (Boston, MA), Carbosynth (San Diego, CA), Sigma Chemical Co. (St. Louis, MO), and Bristol Meyers Squibb (Princeton, NJ), respectively. OMC, TGC, and DOX were dissolved in double-distilled water at a final concentration of 2 mg/ml and sterile filtered through a 0.22-μm filter (Corning, Inc., Corning, NY). AMK was dissolved in dimethyl sufoxide at a final concentration of 5 mg/ml. The drugs were frozen at −20°C until used.

M. abscessus complex and M. fortuitum isolates were kindly provided by Barbara Body (LabCorp, Burlington, NC) and Barbara Brown-Elliot (University of Texas Health Science Center, Tyler, TX). The isolates were grown in cation-adjusted Mueller-Hinton broth (CAMHB; Becton-Dickinson, Sparks, MD) on a rotary shaker at 37°C for 2 or 5 days, respectively. The cultures were diltuted to 10 Klett units (equivalent to 5 × 10⁷ CFU/ml) (photoelectric colorimeter; Manostat Corp., New York, NY) and frozen at −70°C until used. M. chelonae isolates were provided by Barbara Brown-Elliot. The isolates were grown on cation-adjusted Mueller-Hinton agar at 32°C for 3 to 5 days prior to testing.

Polystyrene 96-well round-bottom microtiter plates (Corning, Inc., Corning, NY) were prepared by adding 50 μl of CAMHB to each well. The antimicrobial compounds were thawed and diluted in CAMHB. Fifty microliters of each solution was added to the first well and serially diluted, leaving the last well with broth only (positive-control well). M. abscessus and M. fortuitum isolates were thawed and diluted in CAMHB to a final concentration of approximately 1 × 10⁵ CFU/ml (inoculum range for M. abscessus, 1.2 × 10⁴ to 2.6 × 10⁵ CFU/ml, and for M. fortuitum, 1.3 × 10⁴ to 2.4 × 10⁶ CFU/ml). M. chelonae was prepared by placing several colonies of each isolate in CAMHB, sonicating for 5 to 10 min, and diluting to approximately 1 × 10⁵ CFU/ml (inoculum range, 2.6 × 10⁴ to 3.2 × 10⁶ CFU/ml). To each well, 50 μl of the appropriate mycobacterial cell suspension was added. Plates were sealed and incubated at 37°C in ambient air for 2 days (M. fortuitum) or 5 days (M. abscessus) or incubated at 32°C for 4 days (M. chelonae) prior to reading. Each isolate was tested in duplicate, with one isolate of each species serving as a control strain. The MIC was defined as the lowest concentration of antimicrobial agent yielding no visible turbidity. The MIC₅₀ and MIC₉₀ values are defined as the concentrations at which 50% and 90% of the isolates were inhibited, respectively.

The MICs of OMC, TGC, DOX, and AMK against 24 isolates of the M. abscessus complex are presented in Table 1. The MIC₅₀ and MIC₉₀ of OMC, DOX, and AMK against M. abscessus were 1 μg/ml and 2 μg/ml, >64 μg/ml and >64 μg/ml, and 2 μg/ml and 8 μg/ml, respectively. TGC was tested against a subset of these organisms (n = 14) and was found to have activity similar to that of OMC.

The MICs of OMC, TGC, DOX, and AMK against 22 isolates of M. chelonae are presented in Table 2. The MIC₅₀ and MIC₉₀ of OMC, TGC, DOX, and AMK against M. chelonae were 0.125 μg/ml and 0.25 μg/ml, 0.06 μg/ml and 0.25 μg/ml, 32 μg/ml and 64 μg/ml, and 4 μg/ml and 8 μg/ml, respectively.

The MICs of OMC, TGC, DOX, and AMK against 20 isolates of M. fortuitum are presented in Table 3. The MIC₅₀ and MIC₉₀ of OMC, TGC, DOX, and AMK against M. fortuitum were 0.125 μg/ml and 0.5 μg/ml, 0.25 μg/ml and 0.5 μg/ml, 8 μg/ml and 64 μg/ml, and 0.5 μg/ml and 4 μg/ml, respectively.

OMC and TGC had similar in vitro activities against M. abscessus, M. chelonae, and M. fortuitum. The MICs of OMC were lower than those of DOX and were less than or equal to those of AMK for all isolates tested. In vitro evaluation of OMC against a larger group of isolates would be useful, as would in vivo studies in a murine test system. The in vitro activity of OMC suggests that further development as a potential therapy for infections caused by rapidly growing mycobacteria is warranted.