Structure-Activity Relationship of Penem Antibiotic Side Chains Used against Mycobacteria Reveals Highly Active Compounds.

The rise of antibiotic-resistant Mycobacterium tuberculosis and non-tuberculous mycobacterial infections has placed ever-increasing importance on discovering new antibiotics to treat these diseases. Recently, a new penem, T405, was discovered to have strong antimicrobial activity against M. tuberculosis and Mycobacteroides abscessus. Here, a penem library of C2 side-chain variants was synthesized, and their antimicrobial activities were evaluated against M. tuberculosis H37Rv and M. abscessus ATCC 19977. Several new penems with antimicrobial activity stronger than the standard-of-care carbapenem antibiotics were identified with some candidates improving on the activity of the lead compound, T405. Moreover, many candidates showed little or no increase in the minimum inhibitory concentration in the presence of serum compared to the highly protein-bound T405. The penems with the strongest activity identified in this study were then biochemically characterized by reaction with the representative l,d-transpeptidase LdtMt2 and the representative penicillin-binding protein d,d-carboxypeptidase DacB2.

T405, a New Penem, Exhibits In Vivo Efficacy against M. abscessus and Synergy with ß-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 ß-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 ß-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.

Penicillin Binding Proteins and ß-Lactamases of Mycobacterium tuberculosis: Reexamination of the Historical Paradigm.

Penicillin binding proteins (PBPs) have been extensively studied due to their importance to the physiology of bacterial cell wall peptidoglycan and as targets of the most widely used class of antibiotics, the ß-lactams. The existing paradigm asserts that PBPs catalyze the final step of peptidoglycan biosynthesis, and ß-lactams inhibit their activities. According to this paradigm, a distinct enzyme class, ß-lactamases, exists to inactivate ß-lactams. This paradigm has been the basis for how bacterial diseases are treated with ß-lactams. We tested whether this historical view accurately reflects the relationship between ß-lactams and the PBPs and the ß-lactamase, BlaC, of Mycobacterium tuberculosis. BlaC was the major inactivator of the cephalosporin subclass of ß-lactams. However, the PBPs PonA1 and PonA2 inactivated penicillins and carbapenems more effectively than BlaC. These findings demonstrate that select M. tuberculosis PBPs are effective at inactivating several ß-lactams. Lesser-known PBPs, DacB, DacB1, DacB2, and Rv2864c, a putative PBP, were comparably more resistant to inhibition by all ß-lactam subclasses. Additionally, Rv1730c exhibited low affinity to most ß-lactams. Based on these findings, we conclude that in M. tuberculosis, BlaC is not the only source of inactivation of ß-lactams. Therefore, the historical paradigm does not accurately describe the relationship between ß-lactams and M. tuberculosis. IMPORTANCE M. tuberculosis, the causative agent of tuberculosis, kills more humans than any other bacterium. ß-lactams are the most widely used class of antibiotics to treat bacterial infections. Unlike in the historical model that describes the relationship between ß-lactams and M. tuberculosis, we find that M. tuberculosis penicillin binding proteins are able to inactivate select ß-lactams with high efficiency.

Development of a penem antibiotic against Mycobacteroides abscessus.

ß-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 ß-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 ß-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.