In this review, we summarize the current "state of the art" of carbapenem antibiotics and their role in our antimicrobial armamentarium. Among the ␤-lactams currently available, carbapenems are unique because they are relatively resistant to hydrolysis by most ␤-lactamases, in some cases act as "slow substrates" or inhibitors of ␤-lactamases, and still target penicillin binding proteins. This "value-added feature" of inhibiting ␤-lactamases serves as a major rationale for expansion of this class of ␤-lactams. We describe the initial discovery and development of the carbapenem family of ␤-lactams. Of the early carbapenems evaluated, thienamycin demonstrated the greatest antimicrobial activity and became the parent compound for all subsequent carbapenems. To date, more than 80 compounds with mostly improved antimicrobial properties, compared to those of thienamycin, are described in the literature. We also highlight important features of the carbapenems that are presently in clinical use: imipenem-cilastatin, meropenem, ertapenem, doripenem, panipenem-betamipron, and biapenem. In closing, we emphasize some major challenges and urge the medicinal chemist to continue development of these versatile and potent compounds, as they have served us well for more than 3 decades.Carbapenems play a critically important role in our antibiotic armamentarium. Of the many hundreds of different ␤-lactams, carbapenems possess the broadest spectrum of activity and greatest potency against Gram-positive and Gram-negative bacteria. As a result, they are often used as "last-line agents" or "antibiotics of last resort" when patients with infections become gravely ill or are suspected of harboring resistant bacteria (23,(174)(175)(176)229). Unfortunately, the recent emergence of multidrug-resistant (MDR) pathogens seriously threatens this class of lifesaving drugs (189). Several recent studies clearly show that resistance to carbapenems is increasing throughout the world (35,64,73,123,151,155,173,200). Despite this menacing trend, our understanding of how to best use these agents is undergoing a renaissance, especially concerning their role with regard to ␤-lactamase inhibition. In this context, we view the number, type, and diversity of carbapenems as compelling reasons to explore these compounds for new insights into drug development.

As resistance determinants, KPC ␤-lactamases demonstrate a wide substrate spectrum that includes carbapenems, oxyimino-cephalosporins, and cephamycins. In addition, clinical strains harboring KPC-type ␤-lactamases are often identified as resistant to standard ␤-lactam-␤-lactamase inhibitor combinations in susceptibility testing. The KPC-2 carbapenemase presents a significant clinical challenge, as the mechanistic bases for KPC-2-associated phenotypes remain elusive. Here, we demonstrate resistance by KPC-2 to ␤-lactamase inhibitors by determining that clavulanic acid, sulbactam, and tazobactam are hydrolyzed by KPC-2 with partition ratios (k cat /k inact ratios, where k inact is the rate constant of enzyme inactivation) of 2,500, 1,000, and 500, respectively. Methylidene penems that contain an We also demonstrate that penems 1 and 2 are mechanism-based inactivators, having partition ratios (k cat /k inact ratios) of 250 and 50, respectively. To understand the mechanism of inhibition by these penems, we generated molecular representations of both inhibitors in the active site of KPC-2. These models (i) suggest that penem 1 and penem 2 interact differently with active site residues, with the carbonyl of penem 2 being positioned outside the oxyanion hole and in a less favorable position for hydrolysis than that of penem 1, and (ii) support the kinetic observations that penem 2 is the better inhibitor (k inact /K m ‫؍‬ 6.5 ؎ 0.6 M ؊1 s ؊1 ). We conclude that KPC-2 is unique among class A ␤-lactamases in being able to readily hydrolyze clavulanic acid, sulbactam, and tazobactam. In contrast, penem-type ␤-lactamase inhibitors, by exhibiting unique active site chemistry, may serve as an important scaffold for future development and offer an attractive alternative to our current ␤-lactamase inhibitors.In Klebsiella pneumoniae, ␤-lactam resistance is mediated predominantly by class A SHV, TEM, and CTX-M ␤-lactamases (7, 35). Single amino acid substitutions in the SHV and TEM ␤-lactamases can drastically alter the substrate profiles of the enzymes and confer resistance to extended-spectrum cephalosporins and ␤-lactamase inhibitors (5,12,34,36). ␤-Lactamases with altered substrate profiles (i.e., extended-spectrum or inhibitor-resistant ␤-lactamases) have significantly challenged the clinician's approach to the treatment of serious infectious diseases (36). Thus, the search for effective mechanism-based inhibitors of novel ␤-lactamases merits significant effort (8,9,32).First identified in K. pneumoniae, KPC class A ␤-lactamases threaten the use of all current ␤-lactam antibiotics (57). These ␤-lactamase enzymes are present in an increasing number of bacterial genera, becoming the major carbapenemase expressed by Gram-negative pathogens (e.g., Enterobacter spp., Escherichia coli, Citrobacter freundii, Pseudomonas spp., Serratia marcescens, Proteus mirabilis, and Salmonella enterica) in the United States (3,10,11,16,17,25,37,45,49,53,59). Moreover, KPC ␤-lactamases are becoming geographically widespread (having been detecte...

The emergence of Klebsiella pneumoniae carbapenemases (KPCs), β-lactamases that inactivate “last-line” antibiotics such as imipenem, represents a major challenge to contemporary antibiotic therapies. The combination of ceftazidime (CAZ) and avibactam (AVI), a potent β-lactamase inhibitor, represents an attempt to overcome this formidable threat and to restore the efficacy of the antibiotic against Gram-negative bacteria bearing KPCs. CAZ-AVI-resistant clinical strains expressing KPC variants with substitutions in the Ω-loop are emerging. We engineered 19 KPC-2 variants bearing targeted mutations at amino acid residue Ambler position 179 in Escherichia coli and identified a unique antibiotic resistance phenotype. We focus particularly on the CAZ-AVI resistance of the clinically relevant Asp179Asn variant. Although this variant demonstrated less hydrolytic activity, we demonstrated that there was a prolonged period during which an acyl-enzyme intermediate was present. Using mass spectrometry and transient kinetic analysis, we demonstrated that Asp179Asn “traps” β-lactams, preferentially binding β-lactams longer than AVI owing to a decreased rate of deacylation. Molecular dynamics simulations predict that (i) the Asp179Asn variant confers more flexibility to the Ω-loop and expands the active site significantly; (ii) the catalytic nucleophile, S70, is shifted more than 1.5 Å and rotated more than 90°, altering the hydrogen bond networks; and (iii) E166 is displaced by 2 Å when complexed with ceftazidime. These analyses explain the increased hydrolytic profile of KPC-2 and suggest that the Asp179Asn substitution results in an alternative complex mechanism leading to CAZ-AVI resistance. The future design of novel β-lactams and β-lactamase inhibitors must consider the mechanistic basis of resistance of this and other threatening carbapenemases.

Impeding, as well as reducing, the burden of antimicrobial resistance in Gram-negative pathogens is an urgent public health endeavor. Our current antibiotic armamentarium is dwindling, while major resistance determinants (e.g., extended-spectrum β-lactamases [ESBLs]) continue to evolve and disseminate around the world. One approach to attack this problem is to develop novel therapies that will protect our current agents. AAI101 is a novel penicillanic acid sulfone β-lactamase inhibitor similar in structure to tazobactam, with one important difference. AAI101 possesses a strategically placed methyl group that gives the inhibitor a net neutral charge, enhancing bacterial cell penetration. AAI101 paired with cefepime, also a zwitterion, is in phase III of clinical development for the treatment of serious Gram-negative infections. Here, AAI101 was found to restore the activity of cefepime against class A ESBLs (e.g., CTX-M-15) and demonstrated increased potency compared to that of piperacillin-tazobactam when tested against an established isogenic panel. The enzymological properties of AAI101 further revealed that AAI101 possessed a unique mechanism of β-lactamase inhibition compared to that of tazobactam. Additionally, upon reaction with AAI101, CTX-M-15 was modified to an inactive state. Notably, the in vivo efficacy of cefepime-AAI101 was demonstrated using a mouse septicemia model, indicating the ability of AAI101 to bolster significantly the therapeutic efficacy of cefepime in vivo. The combination of AAI101 with cefepime represents a potential carbapenem-sparing treatment regimen for infections suspected to be caused by Enterobacteriaceae expressing ESBLs.