In this review, we summarize the current “state of the art” of carbapenem antibiotics and their role in our antimicrobial armamentarium. Among the β-lactams currently available, carbapenems are unique because they are relatively resistant to hydrolysis by most β-lactamases, in some cases act as “slow substrates” or inhibitors of β-lactamases, and still target penicillin binding proteins. This “value-added feature” of inhibiting β-lactamases serves as a major rationale for expansion of this class of β-lactams. We describe the initial discovery and development of the carbapenem family of β-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.

Based upon knowledge of the hydrolytic profile of major β-lactamases found in Gram-negative bacteria, we tested the efficacy of the combination of ceftazidime-avibactam (CAZ-AVI) with aztreonam (ATM) against carbapenem-resistant enteric bacteria possessing metallo-β-lactamases (MBLs). Disk diffusion and agar-based antimicrobial susceptibility testing were initially performed to determine the in vitro efficacy of a unique combination of CAZ-AVI and ATM against 21 representative Enterobacteriaceae isolates with a complex molecular background that included bla _IMP , bla _NDM , bla _OXA-48 , bla _CTX-M , bla _AmpC , and combinations thereof. Time-kill assays were conducted, and the in vivo efficacy of this combination was assessed in a murine neutropenic thigh infection model. By disk diffusion assay, all 21 isolates were resistant to CAZ-AVI alone, and 19/21 were resistant to ATM. The in vitro activity of CAZ-AVI in combination with ATM against diverse Enterobacteriaceae possessing MBLs was demonstrated in 17/21 isolates, where the zone of inhibition was ≥21 mm. All isolates demonstrated a reduction in CAZ-AVI agar dilution MICs with the addition of ATM. At 2 h, time-kill assays demonstrated a ≥4-log ₁₀ -CFU decrease for all groups that had CAZ-AVI with ATM (8 μg/ml) added, compared to the group treated with CAZ-AVI alone. In the murine neutropenic thigh infection model, an almost 4-log ₁₀ -CFU reduction was noted at 24 h for CAZ-AVI (32 mg/kg every 8 h [q8h]) plus ATM (32 mg/kg q8h) versus CAZ-AVI (32 mg/kg q8h) alone. The data presented herein require us to carefully consider this new therapeutic combination to treat infections caused by MBL-producing Enterobacteriaceae .

Two areas of research have recently converged to highlight important roles for Mn²⁺ in pathogenesis: the recognition that both bacterial Nramp homologs and members of LraI family of proteins are Mn²⁺ transporters. Their mutation is associated with decreased virulence of various bacterial species. Thus, Mn²⁺ appears to be essential for bacterial virulence. This review describes what is currently known about Mn²⁺ transport in prokaryotes and how prokaryotic Mn²⁺ transport is regulated. Some of the phenotypes that arise when microorganisms lack Mn²⁺ are then discussed, with an emphasis on those phenotypes involving pathogenesis. The concluding section describes possible enzymatic roles for Mn²⁺ that might help explain why Mn²⁺ is necessary for virulence.

As the incidence of Gram-negative bacterial infections for which few effective treatments remain increases, so does the contribution of drug-hydrolyzing β-lactamase enzymes to this serious clinical problem. This review highlights recent advances in β-lactamase inhibitors and focuses on agents with novel mechanisms of action against a wide range of enzymes. To this end, we review the β-lactamase inhibitors currently in clinical trials, select agents still in preclinical development, and older therapeutic approaches that are being revisited. Particular emphasis is placed on the activity of compounds at the forefront of the developmental pipeline, including the diazabicyclooctane inhibitors (avibactam and MK-7655) and the boronate RPX7009. With its novel reversible mechanism, avibactam stands to be the first new β-lactamase inhibitor brought into clinical use in the past 2 decades. Our discussion includes the importance of selecting the appropriate partner β-lactam and dosing regimens for these promising agents. This “renaissance” of β-lactamase inhibitors offers new hope in a world plagued by multidrug-resistant (MDR) Gram-negative bacteria.

As resistance determinants, KPC β-lactamases demonstrate a wide substrate spectrum that includes carbapenems, oxyimino-cephalosporins, and cephamycins. In addition, clinical strains harboring KPC-type β-lactamases are often identified as resistant to standard β-lactam-β-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 β-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 sp ² -hybridized C ₃ carboxylate and a bicyclic R1 side chain (dihydropyrazolo[1,5-c][1,3]thiazole [penem 1] and dihydropyrazolo[5,1-c][1,4]thiazine [penem 2]) are potent inhibitors: K _m of penem 1, 0.06 ± 0.01 μM, and K _m of penem 2, 0.006 ± 0.001 μM. 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 μM ⁻¹ s ⁻¹ ). We conclude that KPC-2 is unique among class A β-lactamases in being able to readily hydrolyze clavulanic acid, sulbactam, and tazobactam. In contrast, penem-type β-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 β-lactamase inhibitors.

Introduction: Antimicrobial resistance in Gram-negative pathogens is a significant threat to global health. β-Lactams (BL) are one of the safest and most-prescribed classes of antibiotics on the market today. The acquisition of β-lactamases, especially those which hydrolyze carbapenems, is eroding the efficacy of BLs for the treatment of serious infections. During the past decade, significant advances were made in the development of novel BL-β-lactamase inhibitor (BLI) combinations to target β-lactamase-mediated resistant Gram-negatives. Areas covered: The latest progress in 20 different approved, developing, and preclinical BL-BLI combinations to target serine β-lactamases produced by Gram-negatives are reviewed based on primary literature, conference abstracts (when available), and US clinical trial searches within the last 5 years. The majority of the compounds that are discussed are being evaluated as part of a BL-BLI combination. Expert opinion: The current trajectory in BLI development is promising; however, a significant challenge resides in the selection of an appropriate BL partner as well as the development of resistance linked to the BL partner. In addition, dosing regimens for these BL-BLI combinations need to be critically evaluated. A revolution in bacterial diagnostics is essential to aid clinicians in the appropriate selection of novel BL-BLI combinations for the treatment of serious infections.