BackgroundNXL104 is a novel-structure β-lactamase inhibitor with potent activity against both class A and class C enzymes. Among the class A carbapenemases, KPC-type enzymes are now spreading rapidly and KPC-related carbapenemase resistance is an emerging phenomenon of great clinical importance. The activity of NXL104 against KPC β-lactamases was examined.MethodsEnzymatic activity of purified recombinant KPC-2 was measured with nitrocefin as reporter substrate and inhibition by NXL104 was measured by determination of IC50 values. Antimicrobial susceptibility testing of various β-lactams combined with a fixed concentration of NXL104 at 4 mg/L against strains producing KPC enzymes was performed by the broth microdilution method.ResultsNXL104 was a potent inhibitor of KPC-2 with an IC50 of 38 nM. NXL104 restored the antimicrobial activity of ceftazidime, ceftriaxone, imipenem and piperacillin against Enterobacteriaceae strains producing KPC-2 or KPC-3. MIC values of ceftazidime against KPC producers were reduced by up to 1000-fold by combination with NXL104.ConclusionsNXL104 inhibitory activity is unique in terms of spectrum, encompassing class A extended-spectrum β-lactamases, class C enzymes and class A carbapenemases. Given the limited therapeutic options available for infections caused by multiresistant Enterobacteriaceae isolates, NXL104 β-lactamase inhibitor is a promising agent to be used in combination with a β-lactam to protect its antibacterial activity.

The ␤-lactamase inhibitor avibactam (NXL104) displays potent inhibition of both class A and C enzymes. The in vitro antibacterial activity of the combination ceftazidime-avibactam was evaluated against a clinical panel of Pseudomonas aeruginosa isolates. Avibactam offered efficient protection from hydrolysis since 94% of isolates were susceptible to ceftazidime when combined with 4 g/ml avibactam, compared with 65% susceptible to ceftazidime alone. Ceftazidime-avibactam also demonstrated better antipseudomonal activity than imipenem (82% susceptibility), a common reference treatment. Pseudomonas aeruginosa is a major opportunistic pathogen frequently involved in hospital-acquired infections that may be resistant to many classes of antimicrobial agents. The expression of the naturally occurring chromosomal AmpC ␤-lactamase, as well as acquired extended-spectrum ␤-lactamases (ESBLs), strongly limits the susceptibility of P. aeruginosa isolates to penicillins, such as piperacillin, and cephalosporins, such as ceftazidime. In addition, most of these enzymes are only poorly inhibited by the ␤-lactamase inhibitors clavulanic acid and tazobactam. Resistance to imipenem is also common and is frequently associated with the loss of functional OprD outer membrane protein and, less frequently, with production of metallo-␤-lactamases or specific carbapenemase class A enzymes, such as GES or KPC variants (3). Infections caused by P. aeruginosa represent a serious therapeutic challenge, with significant morbidity and mortality (8) and high rates of resistance to ␤-lactams, aminoglycosides, and fluoroquinolones. Due to its ability to acquire and coregulate multiple resistance mechanisms, multidrug resistance is relatively common in P. aeruginosa. While carbapenems are considered the treatment of choice for severe P. aeruginosa infections, colistin is currently the only antibacterial agent active in vitro against the most highly resistant strains (6).Avibactam is a novel non-␤-lactam inhibitor of ␤-lactamases that is currently in clinical development with both ceftazidime and ceftaroline. It displays a broad-spectrum inhibitory profile against clinically relevant enzymes belonging to Ambler classes A and C (1, 7). Avibactam has virtually no intrinsic antibacterial activity, but it efficiently protects ␤-lactams from hydrolysis in a variety of class A-and class C-producing strains, including ESBL and KPC producers, as well as AmpC-overexpressing strains, many of which are poorly inhibited by clavulanic acid and tazobactam. Avibactam was shown to restore ceftazidime activity against a series of selected P. aeruginosa strains with identified ␤-lactamases (4). The aim of this study was to evaluate the in vitro antibacterial activity of the combination ceftazidime-avibactam compared with those of ceftazidime, imipenem, aztreonam, and piperacillin, alone or in combination with the commercially available inhibitors clavulanate, sulbactam, and tazobactam, against a panel of unselected, clinically isolated P. aeruginosa strains, in order...

Avibactam is a novel non-␤-lactam ␤-lactamase inhibitor that is currently undergoing phase 3 clinical trials in combination with ceftazidime. Ceftazidime is hydrolyzed by a broad range of ␤-lactamases, but avibactam is able to inhibit the majority of these enzymes. The studies described here attempt to provide insight into the amount of avibactam required to suppress bacterial growth in an environment where the concentrations of both agents are varying as they would when administered to humans. Following the simulation of a single intravenous dose of the drug, ceftazidime alone had no effect on any test organism, but a ceftazidime-avibactam combination resulted in rapid killing of all of the strains, with growth suppressed for the 8 h of the study. For seven of eight strains, this was achieved with a 1-g-250-mg profile, but a 2-g-500-mg profile was necessary to completely suppress a high-level-AmpC-producing isolate. When ceftazidime was infused continuously for 24 h with a single bolus dose of avibactam, rapid killing of all of the strains was again observed, with growth suppressed for 10 to >24 h. Regrowth appeared to commence once the avibactam concentration dropped below a critical concentration of approximately 0.3 g/ml. In a third series of studies, ceftazidime was administered every 8 h for 24 h with avibactam administered at fixed concentrations for short periods during each ceftazidime dose profile. Simulating a 1-g dose of ceftazidime, an avibactam pulse of >0.25 and <0.5 g/ml was required to suppress growth for 24 h. A vibactam, formerly NXL104 or AVE1330A, is the first of a new class of non-␤-lactam ␤-lactamase inhibitors, referred to as diazabicyclooctanes (1). It displays a broad spectrum of inhibitory activity against both class A and class C ␤-lactamases, including Klebsiella pneumoniae carbapenemase (KPC) enzymes (2), the AmpC ␤-lactamase of Pseudomonas aeruginosa (3), and extendedspectrum ␤-lactamases such as TEM, SHV, and CTX-M variants (4, 5). In studies with isolated enzymes, avibactam inactivates ␤-lactamases at low 50% inhibitory concentrations and with low turnover numbers (6, 7). It also inhibits some class D ␤-lactamases, OXA-48, for example (8). Avibactam has little intrinsic antibacterial activity but efficiently protects ␤-lactams from ␤-lactamase-catalyzed hydrolysis in a range of members of the family Enterobacteriaceae and in P. aeruginosa (3-5, 9). That a combination of ceftazidime and avibactam protects against bacterial infections by ␤-lactamase-producing bacteria has been demonstrated in animal models (10, 11), and two successful phase 2 human studies have been reported (12,13).Like the pharmacodynamic (PD) indices of other cephalosporins (14) and ␤-lactams generally (15), that of ceftazidime is the time during which its free (non-protein-bound) concentration exceeds the MIC for the infecting pathogen (fTϾMIC) (14-19). However, little is known about ␤-lactam-␤-lactamase-inhibitor pharmacokinetic (PK)-PD relationships (20)(21)(22), which is a prerequisite for the optimum design...

The novel ␤-lactamase inhibitor avibactam is a potent inhibitor of class A, class C, and some class D enzymes. The in vitro antibacterial activity of the ceftazidime-avibactam combination was determined for a collection of Enterobacteriaceae clinical isolates; this collection was enriched for resistant strains, including strains with characterized serine ␤-lactamases. The inhibitor was added either at fixed weight ratios to ceftazidime or at fixed concentrations, with the latter type of combination consistently resulting in greater potentiation of antibacterial activity. In the presence of 4 g/ml of avibactam, the ceftazidime MIC 50 and MIC 90 (0.25 and 2 g/ml, respectively) were both below the CLSI breakpoint for ceftazidime. Further comparisons with reference antimicrobial agents were performed using this fixed inhibitor concentration. Against most ceftazidime-susceptible and -nonsusceptible isolates, the addition of avibactam resulted in a significant increase in ceftazidime activity, with MICs generally reduced 256-fold for extended-spectrum ␤-lactamase (ESBL) producers, 8-to 32-fold for CTX-M producers, and >128-fold for KPC producers. Overall, MICs of a ceftazidime-avibactam combination were significantly lower than those of the comparators piperacillin-tazobactam, cefotaxime, ceftriaxone, and cefepime and similar or superior to those of imipenem.A ntimicrobial resistance is a growing problem among Gramnegative pathogens worldwide. The ␤-lactams, which have been among the most widely used of antimicrobial agents, have increasingly been compromised by resistance, primarily through the expression of ␤-lactamases in Gram-negative organisms. Of particular concern are enzymes able to target the expanded-spectrum ␤-lactams, including the AmpC enzymes (Ambler class C cephalosporinases), the so-called extended-spectrum ␤-lactamases (ESBLs), and the carbapenemases.The addition of a ␤-lactamase inhibitor to cephalosporins, monobactams, or penicillins is a valuable alternative to carbapenems for the treatment of infections with the most commonly isolated Gram-negative pathogens, which usually express one or several Ambler class A or C ␤-lactamases. However, the inhibitors currently used in the clinical setting (clavulanate, tazobactam, and sulbactam) have an inhibition spectrum that only partially covers the clinically relevant enzymes.Avibactam is the first of a new class of non-␤-lactam ␤-lactamase inhibitors referred to as diazabicyclooctanes (1). Avibactam displays potent, broad-spectrum inhibition of Ambler class A and class C ␤-lactamases, as well as some class D enzymes. More significantly, avibactam shows effective inhibition of clinically important enzymes, such as ESBLs (including the CTX-M subclass), KPC, AmpC, and some OXA enzymes. The inhibitor has little intrinsic antibacterial activity but efficiently protects ␤-lactams from hydrolysis in bacterial strains producing ␤-lactamases (2-6). The pharmacokinetic profile of avibactam is similar to that of ceftazidime (1), and a combination of 2 g ceftazidime a...

Induction of ampC β-lactamase expression can often compromise antibiotic treatment and is triggered by several β-lactams (such as cefoxitin and imipenem) and by the β-lactamase inhibitor clavulanic acid. The novel β-lactamase inhibitor avibactam (NXL104) is a potent inhibitor of both class A and class C enzymes. The potential of avibactam for induction of ampC expression in Enterobacter cloacae was investigated by ampC messenger ribonucleic acid quantitation. Cefoxitin and clavulanic acid were confirmed as ampC inducers, whereas avibactam was found to exert no effect on ampC expression. Thus, avibactam is unlikely to diminish the activity of any partner β-lactam antibiotic against AmpC-producing organisms.