Infections caused by hard-to-treat methicillin-resistant Staphylococcus aureus (MRSA) are a serious global public-health concern, as MRSA has become broadly resistant to many classes of antibiotics. We disclose herein the discovery of a new class of non-β-lactam antibiotics, the oxadiazoles, which inhibit penicillin-binding protein 2a (PBP2a) of MRSA. The oxadiazoles show bactericidal activity against vancomycin- and linezolid-resistant MRSA and other Gram-positive bacterial strains, in vivo efficacy in a mouse model of infection, and have 100% oral bioavailability.

In the 80 years since their discovery the β-lactam antibiotics have progressed through structural generations, each in response to the progressive evolution of bacterial resistance mechanisms. The generational progression was driven by the ingenious, but largely empirical, manipulation of structure by medicinal chemists. Nonetheless, the true creative force in these efforts was Nature, and as the discovery of new β-lactams from Nature has atrophied while at the same time multi-resistant and opportunistic bacterial pathogens have burgeoned, the time for empirical drug discovery has passed. We concisely summarize recent developments with respect to bacterial resistance, the identity of the new β-lactams, and the emerging non-empirical strategies that will ensure that this incredible class of antibiotics has a future.

A major mechanism of bacterial resistance to ␤-lactam antibiotics (penicillins, cephalosporins, carbapenems, etc.) is the production of ␤-lactamases. A handful of class A ␤-lactamases have been discovered that have acquired the ability to turn over carbapenem antibiotics. This is a disconcerting development, as carbapenems are often considered last resort antibiotics in the treatment of difficult infections. The GES family of ␤-lactamases constitutes a group of extended spectrum resistance enzymes that hydrolyze penicillins and cephalosporins avidly. A single amino acid substitution at position 170 has expanded the breadth of activity to include carbapenems. The basis for this expansion of activity is investigated in this first report of detailed steady-state and pre-steady-state kinetics of carbapenem hydrolysis, performed with a class A carbapenemase. Monitoring the turnover of imipenem (a carbapenem) by GES-1 (Gly-170) revealed the acylation step as rate-limiting. GES-2 (Asn-170) has an enhanced rate of acylation, compared with GES-1, and no longer has a single rate-determining step. Both the acylation and deacylation steps are of equal magnitude. GES-5 (Ser-170) exhibits an enhancement of the rate constant for acylation by a remarkable 5000-fold, whereby the enzyme acylation event is no longer rate-limiting. This carbapenemase exhibits k cat /K m of 3 ؋ 10 5 M ؊1 s ؊1 , which is sufficient for manifestation of resistance against imipenem.

The structure-activity relationship (SAR) for the newly discovered oxadiazole class of antibiotics is described with evaluation of 120 derivatives of the lead structure. This class of antibiotics was discovered by in silico docking and scoring against the crystal structure of a penicillin-binding protein. They impair cell-wall biosynthesis and exhibit activities against the Gram-positive bacterium Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA) and vancomycin-resistant and linezolid-resistant S. aureus. 5-(1H-Indol-5-yl)-3-(4-(4-(trifluoromethyl)phenoxy)phenyl)-1,2,4-oxadiazole (antibiotic 75b) was efficacious in a mouse model of MRSA infection, exhibiting a long half-life, a high volume of distribution, and low clearance. This antibiotic is bactericidal and is orally bioavailable in mice. This class of antibiotics holds great promise in recourse against infections by MRSA.

The β‐lactam classes of antibacterials are preeminent in the treatment of bacterial infection due to their unparalleled clinical efficacy and clinical safety. Following the discovery of the penicillins, successive β‐lactam drug discovery has added the cephalosporin, penem cephamycin, clavulanate, monobactam, nocardicin, and carbapenem subclasses. The driving force behind much of this era of discovery is the staggering ability of pathogenic bacteria to adapt previous generations of the β‐lactam by the acquisition and expression of resistance mechanisms. Although many factors contribute to β‐lactam resistance, alterations to the molecular targets of the β‐lactams (the penicillin binding proteins) and the use of enzymes (the β‐lactamases) capable of the hydrolytic deactivation of the β‐lactams are paramount. This review traces the historical development of β‐lactam drug discovery, with emphasis on the most recent progress in the medicinal chemistry, biochemistry, and microbiology of the β‐lactams leading to the discovery of new generation β‐lactam antibacterials effective against the Gram‐negative and ‐positive bacterial pathogens of current medical concern.