In Vitro Activity of New Tetracycline Analogs Omadacycline and Eravacycline against Drug-Resistant Clinical Isolates of Mycobacterium abscessus

Tigecycline is used in multidrug regimens for salvage therapy of Mycobacterium abscessus infections but is often poorly tolerated and has no oral formulation. Here, we report similar in vitro activity of two newly approved tetracycline analogs, omadacycline and eravacycline, against 28 drug-resistant clinical isolates of M. abscessus complex.

ABSTRACT

Tigecycline is used in multidrug regimens for salvage therapy of Mycobacterium abscessus infections but is often poorly tolerated and has no oral formulation. Here, we report similar in vitro activity of two newly approved tetracycline analogs, omadacycline and eravacycline, against 28 drug-resistant clinical isolates of M. abscessus complex. Since omadacycline and eravacycline appear to be better tolerated than tigecycline and since omadacycline is also formulated for oral dosing, these tetracycline analogs may represent new treatment options for M. abscessus infections.

INTRODUCTION

Mycobacterium abscessus complex, consisting of the subspecies abscessus, massiliense, and bolletii, is a group of rapidly growing, nontuberculous mycobacteria (NTM) known for its extensive intrinsic and acquired drug resistance (1). It can cause treatment-refractory lung infections (especially among cystic fibrosis patients), as well as other serious infections (2). Increasing prevalence of pulmonary NTM infections over the last several decades has been reported from several parts of the world, including the United States and Europe (3–6). M. abscessus complex is now the most common rapid-growing NTM causing lung infection and the second most common among all NTM after Mycobacterium avium complex. It is also the most difficult-to-treat NTM lung infection (3–6). A typical multidrug treatment regimen for cystic fibrosis patients with M. abscessus infection consists of an oral macrolide, intravenous amikacin, along with one or more additional intravenous antibiotics, such as cefoxitin, imipenem, or tigecycline (7). Tigecycline (a glycylcycline of the tetracycline class) is active in vitro against most clinical isolates of M. abscessus and has been used clinically for M. abscessus lung infections with some success, but nausea and vomiting are frequent, often treatment-limiting, adverse effects (8, 9). In addition, tigecycline’s intravenous mode of administration is undesirable for a disease that is often treated for more than a year (7, 9). Therefore, new antibiotics with similar or better efficacy, fewer adverse effects, preferably with oral bioavailability, are desperately needed to improve the treatment of M. abscessus infections.

Omadacycline (an aminomethylcycline) is a new tetracycline analog, approved for the treatment of acute bacterial skin and skin-structure infections (ABSSSI) and community-acquired bacterial pneumonia (CABP). It is available in both intravenous and oral formulations (10, 11). Eravacycline (a fluorocycline) is a new tetracycline analog approved for the treatment of complicated intraabdominal infections in an intravenous formulation (12). In the present study, we evaluated the activity of omadacycline and eravacycline against a panel of drug-resistant M. abscessus complex organisms.

Omadacycline, eravacycline, and tigecycline were purchased from MedChem Express, Monmouth Junction, NJ (purity, >95%). All antimicrobials were received in powdered form, stored at −20°C, and dissolved in dimethyl sulfoxide or deionized water in accordance with the manufacturer’s recommendations. M. abscessus strain ATCC 19977 was purchased from the American Type Culture Collection (Manassas, VA) and used as a reference strain. Twenty-eight unique clinical isolates of M. abscessus complex were obtained from the Johns Hopkins Hospital Clinical Microbiology Laboratory from 2005 to 2015, as described previously (13, 14). Isolates were identifies to the subspecies level based on the length of erm(41), which is truncated in M. abscessus subsp. massiliense, and the rpoB sequence (15–17). Reference genomes for each subspecies were as follows: abscessus strain ATCC 19977 (NCBI accession NC_010397), massiliense strain GO 06 (NCBI accession NC_018150), and bolletii strain CIP 198541 (NCBI accession NZ_JRMF00000000). These isolates are resistant to nearly all drugs used to treat M. abscessus infection (amikacin, clarithromycin, imipenem, sulfamethoxazole/trimethoprim, linezolid, and moxifloxacin). The MICs were determined using the broth microdilution method in cation-adjusted Mueller-Hinton broth (CAMHB) in accordance with Clinical and Laboratory Standards Institute (CLSI) guidelines (18). In brief, CAMHB (100 μl/well) was added in each well of 96-well, U-bottom, polystyrene plates (Corning, Inc., Corning, NY). Serial 2-fold dilutions of compounds were prepared. M. abscessus strains were grown to the mid-log phase. An inoculum adjusted to 1 × 10⁴ to 5 × 10⁴ CFU in a 0.1-ml volume was added in each well except the medium control. Plates were sealed and incubated at 30°C for 3 days. Plates were incubated up to 5 days if the pellet size in control wells without drug was small on days 3 and 4. MICs were determined on the basis of presence or absence of pellet with unaided eyes (13). Drug susceptibility assays were repeated to confirm the MICs.

Against M. abscessus strain ATCC 19977, the MIC of omadacycline was similar to that of tigecycline (1 μg/ml), whereas the eravacycline MIC was 2-fold lower (Table 1). Likewise, omadacycline and tigecycline had the same MIC₅₀ and MIC₉₀ against 28 drug-resistant clinical isolates (2 μg/ml), while the MIC₅₀ and MIC₉₀ of eravacycline were 2-fold lower. Interestingly, while the present study was under review, a newly published study reported similar MICs for tigecycline and omadacycline against M. abscessus complex clinical isolates (19).

TABLE 1.

MICs of tigecycline, omadacycline, and eravacycline against Mycobacterium abscessus ATCC 19977 and 28 drug-resistant M. abscessus complex clinical isolates in CAMHB

Isolate or MIC	M. abscessus subspecies	MIC (μg/ml)
		Tigecycline	Omadacycline	Eravacycline
Isolates
Strain ATCC 19977a	abscessus	1	1	0.5
1N	abscessus	1	1	0.5
2N	massiliense-bolletiib	1	1	0.25
3N	abscessus	2	2	1
4N	massiliense	1	1	0.25
5N	massiliense	1	0.5	0.25
6N	abscessus	2	4	1
11N	abscessus	1	2	2
12N	abscessus	1	0.5	0.25
13N	massiliense-bolletii	1	2	0.5
14N	massiliense-bolletii	2	2	1
19N	abscessus	1	0.5	0.25
201	abscessus	1	0.5	0.25
202	abscessus	1	2	0.5
203	massiliense-bolletii	1	2	0.5
204	massiliense	1	1	0.5
206	massiliense	0.5	0.5	0.125
208	massiliense	2	2	0.5
210	abscessus	2	2	0.5
211	abscessus	2	2	0.5
212	massiliense-bolletii	1	1	0.25
214	massiliense	1	1	0.5
215	abscessus	1	1	0.25
216	massiliense	1	1	0.25
218	abscessus	4	4	2
JHH2	abscessus	1	1	0.25
JHH4	abscessus	1	1	0.25
JHH9	abscessus	2	2	0.5
JHHKB	abscessus	2	2	0.5
MIC data
MIC range		0.5–4	0.5–4	0.125–2
MIC50		1	1	0.5
MIC90		2	2	1

^aM. abscessus strain ATCC 19977 is included as a reference strain, and the MIC values for this strain were not included when determining the MIC range, MIC₅₀, and MIC₉₀.

^bFive isolates had truncated erm(41) genes, indicating subsp. massiliense, but had rpoB sequences matching subsp. bolletii.

While no formal susceptibility breakpoint has been established for tigecycline against M. abscessus, breakpoints ranging from 0.5 to 4 μg/ml have been proposed (8, 20). Clinical isolates of rapidly growing mycobacteria are susceptible to tigecycline concentrations of ≤2 μg/ml (21–24), which is the approved susceptibility breakpoint against Enterobacteriaceae (25). The MIC₅₀ and MIC₉₀ of omadacycline reported here are 4- and 2-fold lower, respectively, than the susceptibility breakpoint for Enterobacteriaceae (26). The MIC₅₀ of eravacycline reported here matches its susceptibility breakpoint for Enterobacteriaceae and anaerobes (27). It is noteworthy that steady-state plasma concentrations equivalent to our observed MIC₉₀ for drug-resistant M. abscessus clinical isolates are achievable with intravenous dosing of omadacycline and eravacycline (28–30). At an intravenous omadacycline dose of 100 mg/day (approved marketed dose for CABP and ABSSSI), the steady-state plasma C_max and AUC_0–24 are 2.12 μg/ml and 12.14 μg ⋅ h/ml, respectively, compared to 0.87 μg/ml and 4.7 μg ⋅ h/ml for tigecycline at 50 mg twice daily (29, 31, 32). Oral omadacycline doses of 300 to 450 mg produced C_max values of 9.52 to 10.8 μg/ml and AUC_0–24 values of 11.2 to 13.4 μg ⋅ h/ml, respectively (29). Eravacycline after intravenous dosing 1.0 mg/kg every 12 h produced a plasma C_max of 1.83 μg/ml and an AUC_0–24 of at least 12.6 μg ⋅ h/ml (25). Although there is no marketed oral formulation of eravacycline, a single oral dose of 100 mg produced a C_max of 0.17 μg/ml and an AUC_0–∞ of 2.25 μg ⋅ h/ml (33). In addition, omadacycline has a low protein binding (21%) compared to eravacycline (79 to 87%) and tigecycline (69 to 87%) (28).

The free drug AUC/MIC ratio was the pharmacokinetic/pharmacodynamic parameter most closely correlated with tigecycline activity in an in vitro hollow fiber model of M. abscessus infection (8). Considering the steady-state AUC and protein binding data described above and the MICs obtained in our study against M. abscessus, the free drug AUC/MIC ratios for omadacycline and eravacycline given intravenously are expected to be approximately 8 to 10 times higher and 2 times higher, respectively, compared to tigecycline. This preliminary comparison suggests that eravacycline and, especially, omadacycline could be more efficacious clinically than tigecycline. These hypotheses should be evaluated further in nonclinical models of M. abscessus infection.

Despite tigecycline’s appreciable activity as a component of multidrug regimen for M. abscessus infections, its clinical utility is limited by significant nausea and vomiting (7, 9), especially at the 200-mg daily dose identified as the optimal dose in the hollow fiber infection model (8). Omadacycline and eravacycline appear better tolerated. Omadacycline was associated with significantly less nausea and fewer treatment-emergent adverse events (TEAEs) compared to tigecycline in one study (31). Omadacycline also demonstrated similar safety and side effect profiles to linezolid (for treatment of ABSSI) and moxifloxacin (for CABP) in pivotal trials (10, 11). In IGNITE1 and IGNITE4 trials, eravacycline-treated patients experienced only slightly more TEAEs compared to ertapenem- and meropenem-treated patients (12, 34).

In conclusion, omadacycline and eravacycline may represent new options for treatment of M. abscessus complex infections. The results presented here support further investigation of their efficacy and exposure-response profiles in animal models and clinical trials to better understand their potential clinical utility.