Combinations of avibactam and carbapenems exhibit enhanced potencies against drug-resistant Mycobacterium abscessus

In vitro and intracellular activity of vaborbactam combined with β-lactams against Mycobacterium abscessus

OBJECTIVES

Mycobacterium abscessus has emerged as an opportunistic pathogen responsible for lung infections, especially in cystic fibrosis patients. In spite of the production of the broad-spectrum β-lactamase BlaMab, the carbapenem imipenem is recommended in the initial phase of the treatment of pulmonary infections. Here, we determine whether the addition of vaborbactam, a second-generation β-lactamase inhibitor belonging to the boronate family, improves the activity of β-lactams against M. abscessus.

METHODS

The activity of β-lactams, alone or in combination with vaborbactam, was evaluated against M. abscessus CIP104536 by determining MICs, time-killing and intramacrophage activity. Kinetic parameters for the inhibition of BlaMab by vaborbactam were determined by spectrophotometry.

RESULTS

The combination of vaborbactam (8 mg/L) with β-lactams decreased more than 8 times the MIC of amoxicillin (from >1024 to 128 mg/L) and 2 times the MICs of meropenem (from 16 to 8 mg/L) and imipenem (from 4 to 2 mg/L). The reduction of the MICs was less than that obtained with avibactam at 4 mg/L for amoxicillin (from >1024 to 16 mg/L, more than 64 times less) and for meropenem (from 16 to 4 mg/L, 4 times less). In vitro and intracellularly, M. abscessus was not killed by the meropenem/vaborbactam combination, in spite of significant in vitro inhibition of BlaMab by vaborbactam.

CONCLUSIONS

Inhibition of BlaMab by vaborbactam decreases the MIC of β-lactams, including that of meropenem. As meropenem/vaborbactam is clinically available, this combination offers an alternative therapeutic option that should be evaluated for the treatment of pulmonary infections due to M. abscessus.

In vitro effects of the new oral β-lactamase inhibitor xeruborbactam in combination with oral β-lactams against clinical Mycobacterium abscessus isolates

Non-tuberculosis mycobacteria (NTM), particularly Mycobacterium abscessus subsp. abscessus (M. abscessus), are increasingly being recognized as etiological agents of NTM pulmonary disease. However, treatment options for M. abscessus are limited owing to their natural resistance to most antibiotics, including β-lactams. M. abscessus produces a class A β-lactamase, whose activity is inhibited by cyclic boronic acid β-lactamase inhibitors. We aimed to evaluate the in vitro effects of xeruborbactam, a cyclic boronic acid β-lactamase inhibitor, against M. abscessus when combined with five β-lactams (amoxicillin, tebipenem, cefdinir, cefuroxime, and cefoxitin). The drug susceptibilities of 43 M. abscessus clinical isolates obtained from 43 patients between August 2005 and May 2014 were tested. The MIC results for each β-lactam with or without 4 µg/mL xeruborbactam were examined. Xeruborbactam lowered the MIC90 values of tebipenem, amoxicillin, cefuroxime, and cefdinir by 5, ≥4, 3, and 3 dilutions, respectively. The MIC90 values of cefoxitin without xeruborbactam were 32 µg/mL and did not change upon the addition of xeruborbactam. The lowest MIC90 value was obtained for tebipenem with xeruborbactam. Almost all isolates had an MIC of 4 µg/mL; one isolate had an MIC of 2 µg/mL. With respect to the susceptibility to the same family drug, the number of susceptible isolates increased from 1/43 (2%) to 43/43 (100%) for tebipenem with xeruborbactam. Combining tebipenem and xeruborbactam could be considered an effective all-oral regimen that benefits outpatient treatment of M. abscessus pulmonary disease.

IMPORTANCE

Mycobacterium abscessus subsp. abscessus (M. abscessus) disease is treated in two phases; injectable drugs for initial followed by others for continuation. There is a need to develop all-oral treatment methods for M. abscessus infection, especially in the continuation phase. However, treatment options for M. abscessus are limited owing to their natural resistance to most antibiotics. This is the first report to evaluate the in vitro effects of xeruborbactam, a cyclic boronic acid β-lactamase inhibitor capable of inhibiting the class A β-lactamase produced by M. abscessus, against 43 M. abscessus clinical isolates when combined with five β-lactam antibiotics. Xeruborbactam lowered the MIC90 values of tebipenem by five dilutions, and the number of susceptible isolates increased from 1/43 (2%) to 43/43 (100%). We showed that the tebipenem-xeruborbactam combination might be of interest to explore further as a potentially effective oral regimen for outpatient treatment of M. abscessus pulmonary disease.

The in vitro effect of new combinations of carbapenem-β-lactamase inhibitors for Mycobacterium abscessus

As new treatment alternatives for Mycobacterium abscessus complex (MABC) are urgently needed, we determined the minimum inhibitory concentrations (MICs) for novel carbapenem combinations, including imipenem-relebactam and tebipenem-avibactam against 98 MABC isolates by broth microdilution. The MIC50 was reduced from 16 to 8 mg/L by adding relebactam to imipenem, while the addition of avibactam to tebipenem showed a more pronounced reduction from 256 to 16 mg/L, representing a promising non-toxic, oral treatment option for further exploration.

Efficacy of Omadacycline-Containing Regimen in a Mouse Model of Pulmonary Mycobacteroides abscessus Disease

Mycobacteroides abscessus is an opportunistic pathogen in people with structural lung conditions such as bronchiectasis, chronic obstructive pulmonary disease, and cystic fibrosis. Pulmonary M. abscessus infection causes progressive symptomatic and functional decline as well as diminished lung function and is often incurable with existing antibiotics. We investigated the efficacy of a new tetracycline, omadacycline, in combination with existing antibiotics recommended to treat this indication, in a mouse model of M. abscessus lung disease. Amikacin, azithromycin, bedaquiline, biapenem, cefoxitin, clofazimine, imipenem, linezolid, and rifabutin were selected as companions to omadacycline. M. abscessus burden in the lungs of mice over a 4-week treatment duration was considered the endpoint. Omadacycline in combination with linezolid, imipenem, cefoxitin, biapenem, or rifabutin exhibited early bactericidal activity compared to any single drug. Using three M. abscessus isolates, we also determined the in vitro frequency of spontaneous resistance against omadacycline to be between 1.9 × 10-10 and 6.2 × 10-10 and the frequency of persistence against omadacycline to be between 5.3 × 10-6 and 1.3 × 10-5. Based on these findings, the combination of omadacycline and select drugs that are included in the recent treatment guidelines may exhibit improved potency to treat M. abscessus lung disease. IMPORTANCE M. abscessus disease incidence is increasing in the United States. This disease is difficult to cure with existing antibiotics. In this study, we describe the efficacy of a new tetracycline antibiotic, omadacycline, in combination with an existing antibiotic to treat this disease. A mouse model of M. abscessus lung disease was used to assess the efficacies of these experimental treatment regimens. Omadacycline in combination with select existing antibiotics exhibited bactericidal activity during the early phase of treatment.

Activity of Oral Tebipenem-Avibactam in a Mouse Model of Mycobacterium abscessus Lung Infection

The combination of the β-lactam tebipenem and the β-lactamase inhibitor avibactam shows potent bactericidal activity against Mycobacterium abscessus in vitro. Here, we report that the combination of the respective oral prodrugs tebipenem-pivoxil and avibactam ARX-1796 showed efficacy in a mouse model of M. abscessus lung infection. The results suggest that tebipenem-avibactam presents an attractive oral drug candidate pair for the treatment of M. abscessus pulmonary disease and could inform the design of clinical trials.

Clofazimine as a comparator for preclinical efficacy evaluations of experimental therapeutics against pulmonary M. abscessus infection in mice

Mycobacteroides abscessus (Mab, also known as Mycobacterium abscessus) can cause chronic pulmonary disease in the setting of structural lung conditions. Current treatment recommendations require at least one year of daily therapy with repurposed antibiotics. Yet these therapies are often ineffective and associated with significant adverse events. To address this challenge, research efforts are underway to develop new antibiotics and regimens. During the preclinical phase of treatment development, experimental agents require testing and comparison alongside positive controls that are known agents with clinical history. As there are no FDA approved treatments for this indication, here, we have considered repurposed antibiotics currently included in the recommendation for treating Mab disease as candidates for selection of an ideal standard comparator that can serve as a positive control in preclinical studies. Clofazimine meets the criteria for an ideal positive control as it can be administered via the least invasive route, requires only once-daily dosing, is well tolerated, and is widely available in high purity from independent sources. Using a mouse model of pulmonary Mab disease, we assessed for ideal dosages of clofazimine in C3HeB/FeJ and BALB/c mice in a six-week treatment window. Clofazimine, 25 mg/kg, once daily, produced desired reduction in Mab burden in the lungs of C3HeB/FeJ and BALB/c mice. Based on these findings, we conclude that clofazimine meets the criteria for a positive control comparator in mice for use in preclinical efficacy assessments of agents for treatment of Mab pulmonary disease. Although not included in the current standard-of-care for treating Mab disease, rifabutin, 20 mg/kg, also produced desired reduction in Mab lung burden in C3HeB/FeJ mice but not in BALB/c mice. IMPORTANCE: Mycobacteroides abscessus can cause life-threatening infections in patients with chronic lung conditions. New treatments are needed as cure rate using existing drugs is low. During pre-clinical phase of treatment development, it is important to compare the efficacy of the experimental drug against existing ones with known history. Here, we demonstrate that clofazimine, one of the antibiotics repurposed for treating Mab disease, can serve as a positive control comparator for efficacy assessments of experimental drugs and regimens to treat M. abscessus disease in mice.

In vitro effects of diazabicyclooctane β-lactamase inhibitors relebactam and nacubactam against three subspecies of Mycobacterium abscessus complex

BACKGROUND

Mycobacterium abscessus complex (MABC) pulmonary disease is notoriously difficult to treat due to intrinsic resistance to many common antibiotics. MABC is β-lactam-resistant as it produces class A β-lactamases, such as bla_Mab, which are inhibited by diazabicyclooctane (DBO) β-lactamase inhibitors.

OBJECTIVES

To investigate the microbiological effects of the combination of β-lactam and DBO β-lactamase inhibitors (relebactam and nacubactam) against MABC and determine if the effects are associated with the MABC subspecies and colony morphotype.

METHODS

The antimicrobial susceptibility of three type strains and 20 clinical isolates of MABC to the combination of seven β-lactams with relebactam or nacubactam was evaluated using broth microdilution checkerboard assays. For these strains, expression levels of bla_Mab were assessed using quantitative real-time polymerase chain reaction and genotypic diversity was evaluated using 18-locus variable number tandem repeat assay.

RESULTS

Relebactam and nacubactam lowered the minimum inhibitory concentrations of β-lactams, particularly imipenem, meropenem, and tebipenem, against MABC. There was no difference in efficacy of combination treatment between three subspecies, but rough morphotypes tended to be less susceptible than smooth morphotypes. There were no differences in bla_Mab expression levels and genotypic diversity between the morphotypes.

CONCLUSIONS

The combination of β-lactam with relebactam or nacubactam improved the efficacy of β-lactams against all MABC subspecies, but higher concentrations of β-lactams were needed for rough morphotypes.

Strongly Bactericidal All-Oral β-Lactam Combinations for the Treatment of Mycobacterium abscessus Lung Disease

Bioactive forms of oral β-lactams were screened in vitro against Mycobacterium abscessus with and without the bioactive form of the oral β-lactamase inhibitor avibactam ARX1796. Sulopenem was equally active without avibactam, while tebipenem, cefuroxime, and amoxicillin required avibactam for optimal activity. Systematic pairwise combination of the four β-lactams revealed strong bactericidal synergy for each of sulopenem, tebipenem, and cefuroxime combined with amoxicillin in the presence of avibactam. These all-oral β-lactam combinations warrant clinical evaluation.

Arabinosyltransferase C Mediates Multiple Drugs Intrinsic Resistance by Altering Cell Envelope Permeability in Mycobacterium abscessus

Mycobacterium abscessus is an emerging human pathogen leading to significant morbidity and even mortality, intrinsically resistant to almost all the antibiotics available and so can be a nightmare. Mechanisms of its intrinsic resistance remain not fully understood. Here, we selected and confirmed an M. abscessus transposon mutant that is hypersensitive to multiple drugs including rifampin, rifabutin, vancomycin, clofazimine, linezolid, imipenem, levofloxacin, cefoxitin, and clarithromycin. The gene MAB_0189c encoding a putative arabinosyltransferase C was found to be disrupted, using a newly developed highly-efficient strategy combining next-generation sequencing and multiple PCR. Furthermore, selectable marker-free deletion of MAB_0189c recapitulated the hypersensitive phenotype. Disruption of MAB_0189c resulted in an inability to synthesize lipoarabinomannan and markedly enhanced its cell envelope permeability. Complementing MAB_0189c or M. tuberculosisembC restored the resistance phenotype. Importantly, treatment of M. abscessus with ethambutol, a first-line antituberculosis drug targeting arabinosyltransferases of M. tuberculosis, largely sensitized M. abscessus to multiple antibiotics in vitro. We finally tested activities of six selected drugs using a murine model of sustained M. abscessus infection and found that linezolid, rifabutin, and imipenem were active against the MAB_0189c deletion strain. These results identified MAB_0189 as a crucial determinant of intrinsic resistance of M. abscessus, and optimizing inhibitors targeting MAB_0189 might be a strategy to disarm the intrinsic multiple antibiotic resistance of M. abscessus.

IMPORTANCEMycobacterium abscessus is intrinsically resistant to most antibiotics, and treatment of its infections is highly challenging. The mechanisms of its intrinsic resistance remain not fully understood. Here we found a transposon mutant hypersensitive to a variety of drugs and identified the transposon inserted into the MAB_0189c (orthologous embC coding arabinosyltransferase, EmbC) gene by using a newly developed rapid and efficient approach. We further verified that the MAB_0189c gene played a significant role in its intrinsic resistance by decreasing the cell envelope permeability through affecting the production of lipoarabinomannan in its cell envelope. Lastly, we found the arabinosyltransferases inhibitor, ethambutol, increased activities of nine selected drugs in vitro. Knockout of MAB_0189c made M. abscessus become susceptible to 3 drugs in mice. These findings indicated that potential powerful M. abscessus EmbC inhibitor might be used to reverse the intrinsic resistance of M. abscessus to multiple drugs.

T405, a New Penem, Exhibits In Vivo Efficacy against M. abscessus and Synergy with β-Lactams Imipenem and Cefditoren

Mycobacteroides abscessus (Mab) is an emerging environmental microbe that causes chronic lung disease in patients with compromised lung function such as cystic fibrosis and bronchiectasis. It is intrinsically resistant to most antibiotics, therefore there are only few antibiotics that can be repurposed to treat Mab disease. Although current recommendations require daily intake of multiple antibiotics for more than a year, cure rate is low and often associated with significant adverse events. Here, we describe in vivo efficacy of T405, a recently discovered β-lactam antibiotic of the penem subclass, in a mouse model of pulmonary Mab infection. Imipenem, one of the standard-of-care drugs to treat Mab disease, and also a β-lactam antibiotic from a chemical class similar to T405, was included as a comparator. Probenecid was included with both T405 and imipenem to reduce the rate of their renal clearance. T405 exhibited bactericidal activity against Mab from the onset of treatment and reduced Mab lung burden at a rate similar to that exhibited by imipenem. The MIC of T405 against Mab was unaltered after 4 weeks of exposure to T405 in the lungs of mice. Using an in vitro assay, we also demonstrate that T405 in combination with imipenem, cefditoren or avibactam exhibits synergism against Mab. Additionally, we describe a scheme for synthesis and purification of T405 on an industrial scale. These attributes make T405 a promising candidate for further preclinical assessment to treat Mab disease.

In Vitro Activity of Bedaquiline and Imipenem against Actively Growing, Nutrient-Starved, and Intracellular Mycobacterium abscessus

Mycobacterium abscessus lung disease is difficult to treat due to intrinsic drug resistance and the persistence of drug-tolerant bacteria. Currently, the standard of care is a multidrug regimen with at least 3 active drugs, preferably including a β-lactam (imipenem or cefoxitin). These regimens are lengthy and toxic and have limited efficacy. The search for more efficacious regimens led us to evaluate bedaquiline, a diarylquinoline licensed for treatment of multidrug-resistant tuberculosis. We performed in vitro time-kill experiments to evaluate the activity of bedaquiline alone and in combination with the first-line drug imipenem against M. abscessus under various conditions. Against actively growing bacteria, bedaquiline was largely bacteriostatic and antagonized the bactericidal activity of imipenem. Contrarily, against nutrient-starved persisters, bedaquiline was bactericidal, while imipenem was not, and bedaquiline drove the activity of the combination. In an intracellular infection model, bedaquiline and imipenem had additive bactericidal effects. Correlations between ATP levels and the bactericidal activity of imipenem and its antagonism by bedaquiline were observed. Interestingly, the presence of Tween 80 in the media affected the activity of both drugs, enhancing the activity of imipenem and reducing that of bedaquiline. Overall, these results show that bedaquiline and imipenem interact differently depending on culture conditions. Previously reported antagonistic effects of bedaquiline on imipenem were limited to conditions with actively multiplying bacteria and/or the presence of Tween 80, whereas the combination was additive or indifferent against nutrient-starved and intracellular M. abscessus, where promising bactericidal activity of the combination suggests it may have a role in future treatment regimens.

In vitro efficacy of relebactam versus avibactam against Mycobacterium abscessus complex

Infections resulting from Mycobacterium abscessus are increasing in prevalence worldwide, with the greatest risk posed to patients with underlying respiratory conditions. Treatment for infections is difficult due to wide ranging intrinsic antimicrobial resistance, which is compounded by the existence of a range of subspecies within the M. abscessus complex, each with varying additional antimicrobial resistance profiles. Previously, the use of β-lactam/β-lactamase inhibitors within a combination therapy has been proposed as an effective treatment option for pulmonary M. abscessus infections. Here, we assess the in vitro efficacy of two non-β-lactam based inhibitors, relebactam and avibactam, as agents against M. abscessus with their respective partner drugs imipenem and ceftazidime, as well as in triplicate combinations with additional β-lactam antibiotics against the M. abscessus complex. We have shown that the commercially available ratio of imipenem to relebactam is the appropriate ratio for bactericidal activity against M. abscessus, whereas the ratio between ceftazidime and avibactam is redundant, due to inactivity of ceftazidime to inhibit the bacteria. We have identified that the use of imipenem and meropenem alongside either relebactam or avibactam yield low minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) for each M. abscessus subspecies, which are within the therapeutically achievable concentration ranges within the epithelial lining fluid of the lungs. We propose the implementation of imipenem with relebactam in place of stand-alone imipenem into the current treatment regime, alongside meropenem, as a future front-line treatment option for M. abscessus complex infections.

Current Molecular Therapeutic Agents and Drug Candidates for Mycobacterium abscessus

Mycobacterium abscessus has been recognised as a dreadful respiratory pathogen among the non-tuberculous mycobacteria (NTM) because of misdiagnosis, prolonged therapy with poor treatment outcomes and a high cost. This pathogen also shows extremely high antimicrobial resistance against current antibiotics, including the anti-tuberculosis agents. Therefore, current chemotherapies require a long curative period and the clinical outcomes are not satisfactory. Thus, there is an urgent need for discovering and developing novel, more effective anti-M. abscessus drugs. In this review, we sum the effectiveness of the current anti-M. abscessus drugs and drug candidates. Furthermore, we describe the shortcomings and difficulties associated with M. abscessus drug discovery and development.

Atypically Modified Carbapenem Antibiotics Display Improved Antimycobacterial Activity in the Absence of β-Lactamase Inhibitors

Commercial carbapenem antibiotics are being used to treat multidrug resistant (MDR) and extensively drug resistant (XDR) tuberculosis. Like other β-lactams, carbapenems are irreversible inhibitors of serine d,d-transpeptidases involved in peptidoglycan biosynthesis. In addition to d,d-transpeptidases, mycobacteria also utilize nonhomologous cysteine l,d-transpeptidases (Ldts) to cross-link the stem peptides of peptidoglycan, and carbapenems form long-lived acyl-enzymes with Ldts. Commercial carbapenems are C2 modifications of a common scaffold. This study describes the synthesis of a series of atypical, C5α modifications of the carbapenem scaffold, microbiological evaluation against Mycobacterium tuberculosis (Mtb) and the nontuberculous mycobacterial species, Mycobacterium abscessus (Mab), as well as acylation of an important mycobacterial target Ldt, Ldt_Mt2. In vitro evaluation of these C5α-modified carbapenems revealed compounds with standalone (i.e., in the absence of a β-lactamase inhibitor) minimum inhibitory concentrations (MICs) superior to meropenem-clavulanate for Mtb, and meropenem-avibactam for Mab. Time-kill kinetics assays showed better killing (2–4 log decrease) of Mtb and Mab with lower concentrations of compound 10a as compared to meropenem. Although susceptibility of clinical isolates to meropenem varied by nearly 100-fold, 10a maintained excellent activity against all Mtb and Mab strains. High resolution mass spectrometry revealed that 10a acylates Ldt_Mt2 at a rate comparable to meropenem, but subsequently undergoes an unprecedented carbapenem fragmentation, leading to an acyl-enzyme with mass of Δm = +86 Da. Rationale for the divergence of the nonhydrolytic fragmentation of the Ldt_Mt2 acyl-enzymes is proposed. The observed activity illustrates the potential of novel atypical carbapenems as prospective candidates for treatment of Mtb and Mab infections.

Tebipenem pivoxil hydrobromide-No PICC, no problem!

Tebipenem pivoxil hydrobromide is a novel orally bioavailable prodrug of tebipenem, a carbapenem antimicrobial, that binds to penicillin-binding proteins, inhibiting the synthesis of the bacterial cell wall. This results in weakening of peptidoglycan, leading to lysis of bacterial cells. Tebipenem displays a broad spectrum of activity against anaerobic, gram-positive, and gram-negative pathogens, including extended-spectrum β-lactamase producing Enterobacterales. In a large phase 3 clinical trial (ADAPT-PO), oral tebipenem pivoxil hydrobromide 600 mg every 8 h was shown to be non-inferior to intravenous ertapenem 1 g every 24 h. Overall response at test of cure was 58.8% [264/449] in the tebipenem pivoxil hydrobromide group compared to 61.6% [258/419] in the ertapenem group for the treatment of complicated urinary tract infections, including acute pyelonephritis. At the test of cure, clinical cure rates were 93.1% and 93.6% and microbiological eradication was 59.5% and 63.5% with tebipenem pivoxil hydrobromide and ertapenem, respectively. The most common adverse reactions associated with tebipenem pivoxil hydrobromide are diarrhea, headache, and nausea. As with other carbapenems, tebipenem pivoxil hydrobromide is expected to have the potential to decrease the seizure threshold and will likely require renal dosage adjustment for patients with altered renal function due to high renal clearance. If approved in the United States, tebipenem pivoxil hydrobromide can serve as a potential oral antimicrobial option to decrease hospital length of stay and prevent hospital admissions due to resistant pathogens.

A review of current and promising nontuberculous mycobacteria antibiotics

Nontuberculous mycobacteria infections are a growing concern, and their incidence has been increasing worldwide in recent years. Current treatments are not necessarily useful because many were initially designed to work against other bacteria, such as Mycobacterium tuberculosis. In addition, inadequate treatment means that resistant strains are increasingly appearing, particularly for Mycobacterium abscessus, one of the most virulent nontuberculous mycobacteria. There is an urgent need to develop new antibiotics specifically directed against these nontuberculous mycobacteria. To help in this fight against the emergence of these pathogens, this review describes the most promising heterocyclic antibiotics under development, with particular attention paid to their structure-activity relationships.

Therapeutic Drug Monitoring in Non-Tuberculosis Mycobacteria Infections

Nontuberculous mycobacteria can cause minimally symptomatic self-limiting infections to progressive and life-threatening disease of multiple organs. Several factors such as increased testing and prevalence have made this an emerging infectious disease. Multiple guidelines have been published to guide therapy, which remains difficult owing to the complexity of therapy, the potential for acquired resistance, the toxicity of treatment, and a high treatment failure rate. Given the long duration of therapy, complex multi-drug treatment regimens, and the risk of drug toxicity, therapeutic drug monitoring is an excellent method to optimize treatment. However, currently, there is little available guidance on therapeutic drug monitoring for this condition. The aim of this review is to provide information on the pharmacokinetic/pharmacodynamic targets for individual drugs used in the treatment of nontuberculous mycobacteria disease. Lacking data from randomized controlled trials, in vitro, in vivo, and clinical data were aggregated to facilitate recommendations for therapeutic drug monitoring to improve efficacy and reduce toxicity.

Pipeline of anti-Mycobacterium abscessus small molecules: Repurposable drugs and promising novel chemical entities

The Mycobacterium abscessus complex is a group of emerging pathogens that are difficult to treat. There are no effective drugs for successful M. abscessus pulmonary infection therapy, and existing drug regimens recommended by the British or the American Thoracic Societies are associated with poor clinical outcomes. Therefore, novel antibacterial drugs are urgently needed to contain this global threat. The current anti-M. abscessus small-molecule drug development process can be enhanced by two parallel strategies-discovery of compounds from new chemical classes and commercial drug repurposing. This review focuses on recent advances in the finding of novel small-molecule agents, and more particularly focuses on the activity, mode of action and structure-activity relationship of promising inhibitors from five different chemical classes-benzimidazoles, indole-2-carboxamides, benzothiazoles, 4-piperidinoles, and oxazolidionones. We further discuss some other interesting small molecules, such as thiacetazone derivatives and benzoboroxoles, that are in the early stages of drug development, and summarize current knowledge about the efficacy of repurposable drugs, such as rifabutin, tedizolid, bedaquiline, and others. We finally review targets of therapeutic interest in M. abscessus that may be worthy of future drug and adjunct therapeutic development.

First Penicillin-Binding Protein Occupancy Patterns for 15 β-Lactams and β-Lactamase Inhibitors in Mycobacterium abscessus

Mycobacterium abscessus causes serious infections that often require over 18 months of antibiotic combination therapy. There is no standard regimen for the treatment of M. abscessus infections, and the multitude of combinations that have been used clinically have had low success rates and high rates of toxicities. With β-lactam antibiotics being safe, double β-lactam and β-lactam/β-lactamase inhibitor combinations are of interest for improving the treatment of M. abscessus infections and minimizing toxicity. However, a mechanistic approach for building these combinations is lacking since little is known about which penicillin-binding protein (PBP) target receptors are inactivated by different β-lactams in M. abscessus We determined the preferred PBP targets of 13 β-lactams and 2 β-lactamase inhibitors in two M. abscessus strains and identified PBP sequences by proteomics. The Bocillin FL binding assay was used to determine the β-lactam concentrations that half-maximally inhibited Bocillin binding (50% inhibitory concentrations [IC₅₀s]). Principal component analysis identified four clusters of PBP occupancy patterns. Carbapenems inactivated all PBPs at low concentrations (0.016 to 0.5 mg/liter) (cluster 1). Cephalosporins (cluster 2) inactivated PonA2, PonA1, and PbpA at low (0.031 to 1 mg/liter) (ceftriaxone and cefotaxime) or intermediate (0.35 to 16 mg/liter) (ceftazidime and cefoxitin) concentrations. Sulbactam, aztreonam, carumonam, mecillinam, and avibactam (cluster 3) inactivated the same PBPs as cephalosporins but required higher concentrations. Other penicillins (cluster 4) specifically targeted PbpA at 2 to 16 mg/liter. Carbapenems, ceftriaxone, and cefotaxime were the most promising β-lactams since they inactivated most or all PBPs at clinically relevant concentrations. These first PBP occupancy patterns in M. abscessus provide a mechanistic foundation for selecting and optimizing safe and effective combination therapies with β-lactams.

Development of a penem antibiotic against Mycobacteroides abscessus

β-lactams are the most widely used antibiotic class to treat bacterial infections in humans. Mycobacteroides abscessus is an emerging pulmonary pathogen resistant to most antibiotics, including penicillins and cephalosporins. With no current FDA-approved treatment and cure rates <50%, there is a pressing need for effective therapies. Here we report T405, a new β-lactam of the penem subclass that exhibits potent activity against M. abscessus and a panel of drug-resistant strains isolated from cystic fibrosis patients. Additionally, in combination with the β-lactamase inhibitor avibactam, the rate of spontaneous resistance of M. abscessus to T405 approached the limit of detection. Lastly, we show the favorable pharmacokinetic profile of T405 in mice and the absence of toxicity at elevated dosage, which support the clinical potential of this compound.

Batchelder et al. report a new penem class antibiotic, T405, which exhibits potent activity against M. abscessus and clinical isolates from cystic fibrosis patients. The development of resistance to T405 is inhibited with the addition of a β-lactamase inhibitor, avibactam. Its clinical potential is further demonstrated by T405 displaying a favourable pharmacokinetic profile in mice with an absence of toxicity.

Experience of Ceftazidime/avibactam in a UK tertiary cardiopulmonary specialist center

OBJECTIVES

Antimicrobial resistance is a major threat to public health. New drugs such as Ceftazidime/avibactam have been developed for the treatment of Multi-Drug resistant (MDR) pathogens. Susceptibility can be variable and inappropriate use can add a financial strain on the National Health Service (NHS). There is a pressing need to ensure these new and invaluable antimicrobials are preserved and used effectively.

METHODS

We undertook a retrospective observational study to assess the use of Ceftazidime/avibactam and evaluated prescribing against applied standards.

RESULTS

Between August 2017 and January 2019, 28 patients received 31 courses of Ceftazidime/avibactam. Prescribing according to the approved indications was observed for 68% of prescriptions (p < 0.0001). Duration of therapy was often prolonged but improved with Antimicrobial stewardship interventions. We observed 56% susceptibility (15/27 isolates) of MDR organisms (Pseudomonas, Klebsiella, Burkholderia, Enterobacter aerogenes, Achromobacter). We also report first in vivo experience to treat pulmonary disease caused by Non-tuberculous mycobacteria (NTM). Ceftazidime/avibactam was well tolerated, with no evidence of development of resistance at 6-months follow-up.

CONCLUSIONS

Our study showed that Antimicrobial stewardship interventions led to a more appropriate use of Ceftazidime/avibactam (as measured by duration of therapy), preserving it as a treatment option for MDR infections.

Insights into the l,d-Transpeptidases and d,d-Carboxypeptidase of Mycobacterium abscessus: Ceftaroline, Imipenem, and Novel Diazabicyclooctane Inhibitors

Mycobacterium abscessus is a highly drug-resistant nontuberculous mycobacterium (NTM). Efforts to discover new treatments for M. abscessus infections are accelerating, with a focus on cell wall synthesis proteins (M. abscessus l,d-transpeptidases 1 to 5 [LdtMab1 to LdtMab5] and d,d-carboxypeptidase) that are targeted by β-lactam antibiotics. A challenge to this approach is the presence of chromosomally encoded β-lactamase (BlaMab). Using a mechanism-based approach, we found that a novel ceftaroline-imipenem combination effectively lowered the MICs of M. abscessus isolates (MIC50 ≤ 0.25 μg/ml; MIC90 ≤ 0.5 μg/ml). Combining ceftaroline and imipenem with a β-lactamase inhibitor, i.e., relebactam or avibactam, demonstrated only a modest effect on susceptibility compared to each of the β-lactams alone. In steady-state kinetic assays, BlaMab exhibited a lower Ki app (0.30 ± 0.03 μM for avibactam and 136 ± 14 μM for relebactam) and a higher acylation rate for avibactam (k2/K = 3.4 × 105 ± 0.4 × 105 M-1 s-1 for avibactam and 6 × 102 ± 0.6 × 102 M-1 s-1 for relebactam). The kcat/Km was nearly 10-fold lower for ceftaroline fosamil (0.007 ± 0.001 μM-1 s-1) than for imipenem (0.056 ± 0.006 μM-1 s-1). Timed mass spectrometry captured complexes of avibactam and BlaMab, LdtMab1, LdtMab2, LdtMab4, and d,d-carboxypeptidase, whereas relebactam bound only BlaMab, LdtMab1, and LdtMab2 Interestingly, LdtMab1, LdtMab2, LdtMab4, LdtMab5, and d,d-carboxypeptidase bound only to imipenem when incubated with imipenem and ceftaroline fosamil. We next determined the binding constants of imipenem and ceftaroline fosamil for LdtMab1, LdtMab2, LdtMab4, and LdtMab5 and showed that imipenem bound >100-fold more avidly than ceftaroline fosamil to LdtMab1 and LdtMab2 (e.g., Ki app or Km of LdtMab1 = 0.01 ± 0.01 μM for imipenem versus 0.73 ± 0.08 μM for ceftaroline fosamil). Molecular modeling indicates that LdtMab2 readily accommodates imipenem, but the active site must widen to ≥8 Å for ceftaroline to enter. Our analysis demonstrates that ceftaroline and imipenem binding to multiple targets (l,d-transpeptidases and d,d-carboxypeptidase) and provides a mechanistic rationale for the effectiveness of this dual β-lactam combination in M. abscessus infections.

Nontuberculous mycobacteria in solid organ transplant

PURPOSE OF REVIEW

Nontuberculous mycobacteria (NTM) are emerging pathogens of concern especially in solid organ transplant candidates and recipients. This review aims to address diagnostic challenges, new and emerging treatment options, and infection prevention.

RECENT FINDINGS

The incidence of NTM infections in transplant candidates and recipients is rising. The infection prevalence of these environmental pathogens varies geographically by species with a coastal predominance. Although existing guidelines from the American Thoracic Society, Infectious Diseases Society of America, and British Thoracic Society provide recommendations for diagnosis and management, they do not fully address the subtle nuances and challenges faced in managing infections in immunocompromised transplant recipients. Evolving data on new therapeutic agents and their use in combination therapy will help individualize treatment regimens while limiting adverse effects and improving compliance. Use of combination β-lactams, avibactam, tedizolid, clofazimine, bedaquiline, liposomal amikacin, and ciprofloxacin for commonly isolated species such as Mycobacterium abscessus and Mycobacterium avium complex have proven effective.

SUMMARY

Further studies are needed to determine the incidence of NTM infection in a prospective, multicentric manner and evaluate the most promising synergistic treatment combinations in transplant recipients.

The Many Hosts of Mycobacteria 8 (MHM8): A conference report

Mycobacteria are important causes of disease in human and animal hosts. Diseases caused by mycobacteria include leprosy, tuberculosis (TB), nontuberculous mycobacteria (NTM) infections and Buruli Ulcer. To better understand and treat mycobacterial disease, clinicians, veterinarians and scientists use a range of discipline-specific approaches to conduct basic and applied research, including conducting epidemiological surveys, patient studies, wildlife sampling, animal models, genetic studies and computational simulations. To foster the exchange of knowledge and collaboration across disciplines, the Many Hosts of Mycobacteria (MHM) conference series brings together clinical, veterinary and basic scientists who are dedicated to advancing mycobacterial disease research. Started in 2007, the MHM series recently held its 8th conference at the Albert Einstein College of Medicine (Bronx, NY). Here, we review the diseases discussed at MHM8 and summarize the presentations on research advances in leprosy, NTM and Buruli Ulcer, human and animal TB, mycobacterial disease comorbidities, mycobacterial genetics and 'omics, and animal models. A mouse models workshop, which was held immediately after MHM8, is also summarized. In addition to being a resource for those who were unable to attend MHM8, we anticipate this review will provide a benchmark to gauge the progress of future research concerning mycobacteria and their many hosts.

A mouse model of pulmonary Mycobacteroides abscessus infection

There is no preclinical mouse model to investigate pulmonary Mycobacteroides abscessus (formerly Mycobacterium abscessus) infection in an immunocompetent mouse strain, especially in the context of antibiotic testing and regimen development. We developed a mouse model of pulmonary M. abscessus infection using the aerosolized route of infection that leads to an increase in bacterial burden post- implantation and develops pathology as a result. In this mouse model, treatment with corticosteroid allows for initial proliferation and sustained M. abscessus pulmonary infection and permits evaluation of efficacies of antibiotics. Administration of corticosteroids that permitted higher levels of bacterial burden in the lungs were more likely to have pathology. Treatment of mice with antibiotics administered intranasally or subcutaneously significantly reduced lung M. abscessus burden. In addition to the reference strain, independent clinical isolates of M. abscessus also readily establish infection and proliferate in the lungs of mice in this model.

Looking beyond Typical Treatments for Atypical Mycobacteria

The genus Mycobacterium comprises not only the deadliest of bacterial pathogens, Mycobacterium tuberculosis, but several other pathogenic species, including M. avium and M. abscessus. The incidence of infections caused by atypical or nontuberculous mycobacteria (NTM) has been steadily increasing, and is associated with a panoply of diseases, including pulmonary, soft-tissue, or disseminated infections. The treatment for NTM disease is particularly challenging, due to its long duration, to variability in bacterial susceptibility profiles, and to the lack of evidence-based guidelines. Treatment usually consists of a combination of at least three drugs taken from months to years, often leading to severe secondary effects and a high chance of relapse. Therefore, new treatment approaches are clearly needed. In this review, we identify the main limitations of current treatments and discuss different alternatives that have been put forward in recent years, with an emphasis on less conventional therapeutics, such as antimicrobial peptides, bacteriophages, iron chelators, or host-directed therapies. We also review new forms of the use of old drugs, including the repurposing of non-antibacterial molecules and the incorporation of antimicrobials into ionic liquids. We aim to stimulate advancements in testing these therapies in relevant models, in order to provide clinicians and patients with useful new tools with which to treat these devastating diseases.

Phylogenetic and Biochemical Analyses of Mycobacterial l,d-Transpeptidases Reveal a Distinct Enzyme Class That Is Preferentially Acylated by Meropenem

The genomes of diverse mycobacterial species encode multiple proteins with the canonical l,d-transpeptidase (Ldt) sequence motif. The reason for this apparent redundancy is not well understood, but evidence suggests paralogous Ldts may serve niche roles in maintaining and/or remodeling mycobacterial peptidoglycan. We examined 323 mycobacterial Ldts and determined these enzymes cluster into six clades. We identified a variably represented yet distinct Ldt class (class 6) containing Mycobacterium smegmatis (Msm) LdtF and built a homology model of Msm LdtF toward elucidating class 6 structural and functional differences. We report class 6 Ldts have structurally divergent catalytic domains containing a 10-residue insertion near the active site and additionally determined that meropenem preferentially acylates LdtF. Our data demonstrate an evolutionary basis for mycobacterial Ldt multiplicity that lends support to the idea that paralogous Ldts serve nonredundant roles in vivo and suggests class 6 Ldts can be selectively targeted by specific carbapenem antibiotics.

New β-Lactamase Inhibitors Nacubactam and Zidebactam Improve the In Vitro Activity of β-Lactam Antibiotics against Mycobacterium abscessus Complex Clinical Isolates

The new diazabicyclooctane-based β-lactamase inhibitors avibactam and relebactam improve the in vitro activity of β-lactam antibiotics against bacteria of the Mycobacterium abscessus complex (MABC). Here, we evaluated the in vitro activities of two newer diazabicyclooctane-based β-lactamase inhibitors in clinical development, nacubactam and zidebactam, with β-lactams against clinical isolates of MABC. Both inhibitors lowered the MICs of their partner β-lactams, meropenem (8-fold) and cefepime (2-fold), respectively, and those of other β-lactams, similar to prior results with avibactam and relebactam.

Synergistic Efficacy of β-Lactam Combinations against Mycobacterium abscessus Pulmonary Infection in Mice

Mycobacterium abscessus is an emerging pathogen capable of causing invasive pulmonary infections in patients with chronic lung diseases. These infections are difficult to treat, necessitating prolonged multidrug therapy, which is further complicated by extensive intrinsic and acquired resistance exhibited by clinical M. abscessus isolates. Therefore, development of novel treatment regimens effective against drug-resistant strains is crucial. Prior studies have demonstrated synergistic efficacy of several β-lactams against M. abscessus in vitro; however, these combinations have never been tested in an animal model of M. abscessus pulmonary disease. We utilized a recently developed murine system of sustained M. abscessus lung infection delivered via an aerosol route to test the bactericidal efficacy of four novel dual β-lactam combinations and one β-lactam/β-lactamase inhibitor combination. All five of the novel combinations exhibited synergy and resulted in at least 6-log₁₀ reductions in bacterial burden in the lungs of mice at 4 weeks compared to untreated controls (P = 0.038).

Non-tuberculous mycobacterial pulmonary disease

Nontuberculous mycobacterial pulmonary disease (NTM-PD) is a challenging infection which is becoming increasingly prevalent, particularly in the elderly, for reasons which are unknown. While underlying lung disease is a well-established risk factor for NTM-PD, it may also occur in apparently healthy individuals. No single common genetic or immunological defect has been identified in this group, and it is likely that multiple pathways contribute towards host susceptibility to NTM-PD which further interact with environmental and microbiological factors leading to the development of disease.The diagnosis of NTM-PD relies on the integration of clinical, radiological and microbiological results. The clinical course of NTM-PD is heterogeneous, with some patients remaining stable without the need for treatment and others developing refractory disease associated with considerable mortality and morbidity. Treatment regimens are based on the identity of the isolated species, drug sensitivity testing (for some agents) and the severity of disease. Multiple antibiotics are typically required for prolonged periods of time and treatment is frequently poorly tolerated. Surgery may be beneficial in selected cases. In some circumstances cure may not be attainable and there is a pressing need for better regimens to treat refractory and drug-resistant NTM-PD.This review summarises current knowledge on the epidemiology, aetiology and diagnosis of NTM-PD and discusses the treatment of two of the most clinically significant species, the M. avium and M. abscessus complexes, with a focus on refractory disease and novel therapies.

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. 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.

Select β-Lactam Combinations Exhibit Synergy against Mycobacterium abscessus In Vitro

Mycobacterium abscessus is a nontuberculous mycobacterium that causes invasive pulmonary infections in patients with structural lung disease. M. abscessus is intrinsically resistant to several classes of antibiotics, and an increasing number of strains isolated from patients exhibit resistance to most antibiotics considered for treatment of infections by this mycobacterium. Therefore, there is an unmet need for new regimens with improved efficacy to treat this disease. Synthesis of the essential cell wall peptidoglycan in M. abscessus is achieved via two enzyme classes, l,d- and d,d-transpeptidases, with each class preferentially inhibited by different subclasses of β-lactam antibiotics. We hypothesized that a combination of two β-lactams that comprehensively inhibit the two enzyme classes will exhibit synergy in killing M. abscessus Paired combinations of antibiotics tested for in vitro synergy against M. abscessus included dual β-lactams, a β-lactam and a β-lactamase inhibitor, and a β-lactam and a rifamycin. Of the initial 206 combinations screened, 24 pairs exhibited synergy. A total of 13/24 pairs were combinations of two β-lactams, and 12/24 pairs brought the MICs of both drugs to within the therapeutic range. Additionally, synergistic drug pairs significantly reduced the frequency of selection of spontaneous resistant mutants. These novel combinations of currently available antibiotics may offer viable immediate treatment options against highly-resistant M. abscessus infections.

Compromised longevity due to Mycobacterium abscessus pulmonary disease in lungs scarred by tuberculosis

Structural lung diseases or scarring related to prior infections such as tuberculosis (TB) are risk factors for the development of invasive nontuberculous mycobacterial (NTM) pulmonary infections, such as Mycobacterium abscessus. M. abscessus is intrinsically resistant to many antibiotics and in vitro susceptibility correlates poorly with clinical response, especially in pulmonary disease. Treatment is often difficult due to the lack of effective antibiotic regimens. We present a case of a 56-year-old male previously treated for TB, with presumed exacerbation, who was diagnosed after much delay with pulmonary M. abscessus disease and subsequently failed initial treatment with an empirical antibiotic regimen. When placed on a synergistic combination regimen that included amikacin, linezolid, clarithromycin, ethambutol and faropenem, the patient showed a favourable response and was culture-negative for over 12 months when the treatment was stopped as per American Thoracic Society (ATS) recommendations. Unfortunately, he developed recurrent symptoms and died 9 months after stopping treatment, following an acute exacerbation of fever and respiratory failure.

In Vitro Activity of the New β-Lactamase Inhibitors Relebactam and Vaborbactam in Combination with β-Lactams against Mycobacterium abscessus Complex Clinical Isolates

Pulmonary disease due to infection with Mycobacterium abscessus complex (MABC) is notoriously difficult to treat, in large part due to the intrinsic resistance of MABC strains to most antibiotics, including β-lactams. MABC organisms express a broad-spectrum β-lactamase that is resistant to traditional β-lactam-based β-lactamase inhibitors but inhibited by a newer non-β-lactam-based β-lactamase inhibitor, avibactam. Consequently, the susceptibility of MABC members to some β-lactams is increased in the presence of avibactam. Therefore, we hypothesized that two new non-β-lactam-based β-lactamase inhibitors, relebactam and vaborbactam, would also increase the susceptibility of MABC organisms to β-lactams. The objective of the present study was to evaluate the in vitro activity of various marketed β-lactams alone and in combination with either relebactam or vaborbactam against multidrug-resistant MABC clinical isolates. Our data demonstrate that both β-lactamase inhibitors significantly improved the anti-MABC activity of many carbapenems (including imipenem and meropenem) and cephalosporins (including cefepime, ceftaroline, and cefuroxime). As a meropenem-vaborbactam combination is now marketed and an imipenem-relebactam combination is currently in phase III trials, these fixed combinations may become the β-lactams of choice for the treatment of MABC infections. Furthermore, given the evolving interest in dual β-lactam regimens, our results identify select cephalosporins, such as cefuroxime, with superior activity in the presence of a β-lactamase inhibitor that are deserving of further evaluation in combination with these carbapenem-β-lactamase inhibitor products.

An Update in Antimicrobial Therapies and Infection Prevention in Pediatric Lung Transplant Recipients

Lung transplantation can offer life-prolonging therapy to children with otherwise terminal end-stage lung disease. However, infectious complications, like those experienced by their adult counterparts, are a significant cause of morbidity and mortality. These include bacteria, viruses, and fungi that infect the patient pretransplant and those that may be acquired from the donor or by the recipient in the months to years posttransplant. An understanding of the approach to the management of each potential infecting organism is required to ensure optimal outcomes. In particular, emphasis on aggressive preoperative management of infections in pediatric patients with cystic fibrosis is important. These include multidrug-resistant Gram-negative bacteria, fungi, and Mycobacterium abscessus, the posttransplant outcome of which depends on optimal pretransplant management, including vaccination and other preventive, antibiotic-sparing strategies. Similarly, increasing the transplant donor pool to meet rising transplant demands is an issue of critical importance. Expanded-criteria donors-those at increased risk of blood-borne viruses in particular-are increasingly being considered and transplants undertaken to meet the rising demand. There is growing evidence in the adult pool that these transplants are safe and associated with comparable outcomes. Pediatric transplanters are therefore likely to be presented with increased-risk donors for their patients. Finally, numerous novel antibiotic-sparing therapeutic approaches are on the horizon to help combat infections that currently compromise transplant outcomes.

Mycobacterium abscessus: Shapeshifter of the Mycobacterial World

In this review we will focus on unique aspects of Mycobacterium abscessus (MABS) which we feel earn it the designation of "shapeshifter of the mycobacterial world." We will review its emergence as a distinct species, the recognition and description of MABS subspecies which are only now being clearly defined in terms of pathogenicity, its ability to exist in different forms favoring a saprophytic lifestyle or one more suitable to invasion of mammalian hosts, as well as current challenges in terms of antimicrobial therapy and future directions for research. One can see in the various phases of MABS, a species transitioning from a free living saprophyte to a host-adapted pathogen.

Mycobacterium abscessus and β-Lactams: Emerging Insights and Potential Opportunities

β-lactams, the most widely used class of antibiotics, are well-tolerated, and their molecular mechanisms of action against many bacteria are well-documented. Mycobacterium abscessus (Mab) is a highly drug-resistant rapidly-growing nontuberculous mycobacteria (NTM). Only in recent years have we started to gain insight into the unique relationship between β-lactams and their targets in Mab. In this mini-review, we summarize recent findings that have begun to unravel the molecular basis for overall efficacy of β-lactams against Mab and discuss emerging evidence that indicates that we have yet to harness the full potential of this antibiotic class to treat Mab infections.

The Role of Antibiotic-Target-Modifying and Antibiotic-Modifying Enzymes in Mycobacterium abscessus Drug Resistance

The incidence and prevalence of non-tuberculous mycobacterial (NTM) infections have been increasing worldwide and lately led to an emerging public health problem. Among rapidly growing NTM, Mycobacterium abscessus is the most pathogenic and drug resistant opportunistic germ, responsible for disease manifestations ranging from “curable” skin infections to only “manageable” pulmonary disease. Challenges in M. abscessus treatment stem from the bacteria’s high-level innate resistance and comprise long, costly and non-standardized administration of antimicrobial agents, poor treatment outcomes often related to adverse effects and drug toxicities, and high relapse rates. Drug resistance in M. abscessus is conferred by an assortment of mechanisms. Clinically acquired drug resistance is normally conferred by mutations in the target genes. Intrinsic resistance is attributed to low permeability of M. abscessus cell envelope as well as to (multi)drug export systems. However, expression of numerous enzymes by M. abscessus, which can modify either the drug-target or the drug itself, is the key factor for the pathogen’s phenomenal resistance to most classes of antibiotics used for treatment of other moderate to severe infectious diseases, like macrolides, aminoglycosides, rifamycins, β-lactams and tetracyclines. In 2009, when M. abscessus genome sequence became available, several research groups worldwide started studying M. abscessus antibiotic resistance mechanisms. At first, lack of tools for M. abscessus genetic manipulation severely delayed research endeavors. Nevertheless, the last 5 years, significant progress has been made towards the development of conditional expression and homologous recombination systems for M. abscessus. As a result of recent research efforts, an erythromycin ribosome methyltransferase, two aminoglycoside acetyltransferases, an aminoglycoside phosphotransferase, a rifamycin ADP-ribosyltransferase, a β-lactamase and a monooxygenase were identified to frame the complex and multifaceted intrinsic resistome of M. abscessus, which clearly contributes to complications in treatment of this highly resistant pathogen. Better knowledge of the underlying mechanisms of drug resistance in M. abscessus could improve selection of more effective chemotherapeutic regimen and promote development of novel antimicrobials which can overwhelm the existing resistance mechanisms. This article reviews the currently elucidated molecular mechanisms of antibiotic resistance in M. abscessus, with a focus on its drug-target-modifying and drug-modifying enzymes.

Teicoplanin - Tigecycline Combination Shows Synergy Against Mycobacterium abscessus

Lung disease caused by non-tuberculous mycobacteria (NTM), relatives of Mycobacterium tuberculosis, is increasing. M. abscessus is the most prevalent rapid growing NTM. This environmental pathogen is intrinsically resistant to most commonly used antibiotics, including anti-tuberculosis drugs. Current therapies take years to achieve cure, if cure if achieved. Thus, there is an urgent medical need to identify new, more efficacious treatments. Here, we explore the possibility of repurposing antibiotics developed for other indications. We asked whether novel two-drug combinations of clinically used antibiotics can be identified that show synergistic activity against this mycobacterium. An in vitro checkerboard titration assay was employed to test 180 dual combinations of 41 drugs against the clinical isolate M. abscessus Bamboo. The most attractive novel combination was further profiled against reference strains representing three sub-species (M. abscessus subsp. abscessus, massiliense and bolletii) and a collection of clinical isolates. This resulted in the identification of a novel synergistic antibiotic pair active against the M. abscessus complex: the glycopeptide teicoplanin with the glycylcycline tigecycline showed inhibitory activity at 2–3 μM (teicoplanin) and 1–2 μM (tigecycline). This novel combination can now be tested in M. abscessus animal models of infection and/or patients.

NTM drug discovery: status, gaps and the way forward

Incidence of pulmonary diseases caused by non-tuberculous mycobacteria (NTM), relatives of Mycobacterium tuberculosis, is increasing at an alarming rate, surpassing tuberculosis in many countries. Current chemotherapies require long treatment times and the clinical outcomes are often disappointing. There is an urgent medical need to discover and develop new, more-efficacious anti-NTM drugs. In this review, we summarize the current status of NTM drug development, and highlight knowledge gaps and scientific obstacles in NTM drug discovery. We propose strategies to reduce biological uncertainties and to begin to populate a NTM drug pipeline with attractive leads and drug candidates.

Activities of Dual Combinations of Antibiotics Against Multidrug-Resistant Nontuberculous Mycobacteria Recovered from Patients with Cystic Fibrosis

Patients with cystic fibrosis (CF) are at risk for recurrent pulmonary infections due to increased viscosity of airway secretions, leading to persistent colonization with pathogenic bacteria, including nontuberculous mycobacteria (NTM). Extensive antibiotic use for treatment of infections has led to increasing antimicrobial resistance, which is a significant barrier to the treatment of NTMs. We examined the in vitro activity of several antibiotics against a selection of the most drug-resistant clinical isolates of Mycobacterium abscessus, Mycobacterium chelonae, and Mycobacterium avium complex recovered from CF patients at our institution, as well as paired combinations of antibiotics against a subset of M. abscessus strains, to determine whether they exhibit synergy in inhibiting bacterial growth. Most isolates displayed resistance to at least six of the nine antibiotics tested for which phenotypic interpretation is available, and elevated minimum inhibitory concentrations (MICs) were observed for many of the other drugs. The major exception was clofazimine, which had relatively low MICs for most isolates across all species. When synergy testing was performed by using paired combinations of drugs, clofazamine and clarithromycin exhibited 100% synergy for all combinations tested, as did amikacin, with the exception of one isolate. These results suggest that synergistic antibiotic combinations are capable of overcoming drug resistance in vitro, and laboratories might consider implementation of synergy testing in multidrug-resistant (MDR)-NTM organisms to guide treatment decisions in the setting of extensive antimicrobial resistance.

Mycobacterium abscessus l,d-Transpeptidases Are Susceptible to Inactivation by Carbapenems and Cephalosporins but Not Penicillins

As a growing number of clinical isolates of Mycobacterium abscessus are resistant to most antibiotics, new treatment options that are effective against these drug-resistant strains are desperately needed. The majority of the linkages in the cell wall peptidoglycan of M. abscessus are synthesized by nonclassical transpeptidases, namely, the l,d-transpeptidases. Emerging evidence suggests that these enzymes represent a new molecular vulnerability in this pathogen. Recent studies have demonstrated that inhibition of these enzymes by the carbapenem class of β-lactams determines their activity against Mycobacterium tuberculosis Here, we studied the interactions of β-lactams with two l,d-transpeptidases in M. abscessus, namely, Ldt_Mab1 and Ldt_Mab2, and found that both the carbapenem and cephalosporin, but not penicillin, subclasses of β-lactams inhibit these enzymes. Contrary to the commonly held belief that combination therapy with β-lactams is redundant, doripenem and cefdinir exhibit synergy against both pansusceptible M. abscessus and clinical isolates that are resistant to most antibiotics, which suggests that dual-β-lactam therapy has potential for the treatment of M. abscessus Finally, we solved the first crystal structure of an M. abscessus l,d-transpeptidase, Ldt_Mab2, and using substitutions of critical amino acids in the catalytic site and computational simulations, we describe the key molecular interactions between this enzyme and β-lactams, which provide an insight into the molecular basis for the relative efficacy of different β-lactams against M. abscessus.