ABSTRACT
We evaluated the in vitro activities of the antimicrobial drugs bedaquiline and delamanid against the major pathogenic nontuberculous mycobacteria (NTM). Delamanid showed high MIC values for all NTM except Mycobacterium kansasii. However, bedaquiline showed low MIC values for the major pathogenic NTM, including Mycobacterium avium complex, Mycobacterium abscessus subsp. abscessus, M. abscessus subsp. massiliense, and M. kansasii. Bedaquiline also had low MIC values with macrolide-resistant NTM strains and warrants further investigation as a potential antibiotic for NTM treatment.
INTRODUCTION
The incidence and prevalence of pulmonary disease (PD) associated with nontuberculous mycobacteria (NTM) are increasing worldwide (1, 2). Mycobacterium avium complex (MAC), Mycobacterium abscessus, and Mycobacterium kansasii are the most common pathogens for NTM PD worldwide (3–5). Macrolide antibiotics, such as clarithromycin and azithromycin, are key drugs for treating NTM PD, especially MAC PD (1, 2). Treatment outcomes are still not satisfactory (6–9), however, and the development of acquired resistance to macrolides can further worsen treatment outcomes (10, 11). Moreover, M. abscessus isolates can have intrinsic inducible macrolide resistance or acquired macrolide resistance, and M. abscessus PD is the most difficult-to-treat type of NTM PD (12–14). Therefore, discovery of new and repurposed drugs is urgently needed (15).
Bedaquiline and delamanid are new drugs for the treatment of multidrug-resistant tuberculosis (16–20). Bedaquiline is a diarylquinoline that inhibits the proton pump of mycobacterial ATP synthase, and delamanid is a compound derived from nitrodihydroimidazooxazole that inhibits mycolic acid synthesis (21–23). Previous studies reported that the MICs of bedaquiline and delamanid for Mycobacterium tuberculosis, including multidrug-resistant isolates, were very low (24, 25).
Recently, the MICs of bedaquiline against MAC, including M. avium and Mycobacterium intracellulare, have been reported (26–28). In those studies, most macrolide-sensitive MAC isolates showed low MICs for bedaquiline (26–28). In addition, M. abscessus subsp. abscessus and M. abscessus subsp. massiliense have low MIC values for bedaquiline (28–30).
In contrast to those bedaquiline studies, there has been only one study on the MICs of delamanid for MAC (31), and delamanid MICs for other NTM species have not been reported. In addition, there has been no comparative analysis of MIC values for bedaquiline and delamanid with various NTM, including macrolide-resistant NTM. The purpose of the present study was to evaluate the MICs of bedaquiline and delamanid against major pathogenic NTM clinical isolates, including macrolide-resistant NTM.
For this study, which was initially approved by the institutional review board (IRB) of Samsung Medical Center in 2008 and has received IRB approval once a year (IRB approval no. 2008-09-016; last updated 2 February 2019), we included 251 clinical isolates of five major pathogenic NTM (M. avium, M. intracellulare, M. abscessus subsp. abscessus, M. abscessus subsp. massiliense, and M. kansasii) isolated from patients newly diagnosed with NTM PD. We also included 56 clinical isolates of acquired-macrolide-resistant NTM (M. avium, M. intracellulare, M. abscessus subsp. abscessus, and M. abscessus subsp. massiliense), which were confirmed to have a 23S rRNA gene mutation associated with the acquisition of macrolide resistance (32–34). In vitro susceptibility testing with bedaquiline and delamanid was performed by measuring the MIC using the broth microdilution method, according to Clinical and Laboratory Standards Institute guidelines (35). Mycobacterium peregrinum ATCC 700686, M. abscessus ATCC 19977, M. avium ATCC 700898, and M. kansasii ATCC 12478 were used as controls.
Table 1 shows the MIC, MIC50, and MIC90 values of bedaquiline and delamanid for 251 isolates from newly diagnosed NTM PD patients. All MAC, M. abscessus subsp. massiliense, and M. kansasii isolates were susceptible to macrolides; most M. abscessus subsp. abscessus isolates, except for 11 macrolide-susceptible isolates, had inducible resistance to macrolides, which was confirmed using sequence analysis of the erm(41) gene. MAC and M. kansasii isolates had very low bedaquiline MIC50 (≤0.016 μg/ml) and MIC90 (≤0.016 μg/ml) values. Although the M. abscessus subsp. abscessus and M. abscessus subsp. massiliense isolates also had very low bedaquiline MIC50 (0.062 μg/ml) and MIC90 (0.125 μg/ml) values, the MICs were higher than those for MAC and M. kansasii isolates.
MIC ranges and MIC50 and MIC90 values of bedaquiline and delamanid for 251 clinical NTM isolates
In contrast, MAC, M. abscessus subsp. abscessus, and M. abscessus subsp. massiliense isolates had very high delamanid MIC50 (8 to >16 μg/ml) and MIC90 (>16 μg/ml) values. Compared to those NTM, M. kansasii had relatively low delamanid MIC50 (0.25 μg/ml) and MIC90 (1 μg/ml) values.
The MIC, MIC50, and MIC90 values for bedaquiline and delamanid with 56 isolates of macrolide-resistant NTM are shown in Table 2. All macrolide-resistant NTM isolates showed very low bedaquiline MIC50 (≤0.016 to 0.062 μg/ml) and MIC90 (≤0.016 to 0.25 μg/ml) values. For all macrolide-resistant NTM isolates, however, the delamanid MIC50 (4 to >16 μg/ml) and MIC90 (>16 μg/ml) values were very high (Table 2).
MIC ranges and MIC50 and MIC90 values of bedaquiline and delamanid for 56 macrolide-resistant clinical NTM isolates
In this study, we evaluated the bedaquiline and delamanid MICs for major pathogenic NTM clinical isolates, including acquired-macrolide-resistant NTM isolates. Consistent with previous studies, our results showed that MAC, M. abscessus subsp. abscessus, and M. abscessus subsp. massiliense isolates, as well as M. kansasii isolates, had low bedaquiline MIC50 and MIC90 values.
In particular, the low bedaquiline MICs for macrolide-resistant NTM isolates, including MAC, M. abscessus subsp. abscessus, and M. abscessus subsp. massiliense isolates, are notable in this study. Although the clarithromycin MIC50 and MIC90 values for all macrolide-resistant NTM isolates were >64 μg/ml, the macrolide-resistant NTM isolates showed significantly lower bedaquiline MIC50 (≤0.016 to 0.062 μg/ml) and MIC90 (≤0.016 to 0.25 μg/ml) values. These results suggest that bedaquiline may be an effective antimicrobial for treatment of macrolide-resistant NTM strains.
In contrast, in this study, the delamanid MICs were high for most major pathogenic NTM isolates. The exception was M. kansasii, for which the delamanid MIC50 and MIC90 values were relatively low, compared to the values for other NTM isolates. These results, especially the delamanid MICs for MAC isolates, differed from those of one previous study (31). Although studies on delamanid resistance have reported that five genes (ddn, fgd1, fbiA, fbiB, and fbiC) are associated with delamanid resistance in M. tuberculosis (36), a similar association has not yet been reported for NTM. Given the high delamanid MICs that we observed with MAC and M. abscessus clinical isolates, additional studies to identify genes that contribute to delamanid resistance in NTM are needed.
Previous studies reported that mutations within the atpE, Rv0678, and pepQ genes are involved in bedaquiline resistance in M. tuberculosis (37). In addition, recent studies on bedaquiline-resistance-related genes in NTM have been reported. Alexander and colleagues found that mutations in the mmpT5 and atpE genes were associated with bedaquiline resistance in MAC strains (38). In addition, in M. abscessus subsp. abscessus, mutations in the atpE and MAB_2299c genes have been reported to be associated with bedaquiline resistance (39, 40). Therefore, if bedaquiline is used for NTM treatment, then the possibility of bedaquiline resistance due to mutation in a bedaquiline-resistance-related gene should be considered, although most NTM isolates had very low bedaquiline MIC values in this study.
In summary, we evaluated the in vitro activities of bedaquiline and delamanid against major pathogenic NTM clinical isolates. Our results showed that bedaquiline had good in vitro activity against major pathogenic NTM but delamanid did not. Bedaquiline has the potential to be a potent agent for the treatment of NTM PD, including macrolide-resistant NTM PD.
ACKNOWLEDGMENTS
This research was supported by the National Research Foundation of Korea (NRF), funded by the South Korean Ministry of Science, Information, and Communications Technologies (grant NRF-2018R1A2A1A05018309 to W.-J.K.), and by the Basic Science Research Program through the NRF, funded by the Ministry of Education (grant NRF-2016R1A6A3A11930738 to D.H.K.).
C.L.D. has received grants from Insmed, Inc., and served on advisory boards for Otsuka, Insmed, Johnson and Johnson, Spero, and Horizon, not associated with the submitted work. W.-J.K. has received a consultation fee from Insmed, Inc., for the Insmed advisory board meeting, not associated with the submitted work. Otherwise, we have no conflicts of interest to declare.
FOOTNOTES
- Received 28 March 2019.
- Returned for modification 23 April 2019.
- Accepted 4 June 2019.
- Accepted manuscript posted online 10 June 2019.
- Copyright © 2019 American Society for Microbiology.
