In Vitro Activity of NXL104 in Combination with β-Lactams against Klebsiella pneumoniae Isolates Producing KPC Carbapenemases

Klebsiella pneumoniae isolates producing class A KPC carbapenemases (KPC-Kp) are spreading at an alarming rate around the world (8, 10, 11). These isolates are highly resistant to penicillins, cephalosporins, and commercially available β-lactam/β-lactamase inhibitor combinations and show reduced susceptibility to carbapenems. KPC-Kp are also commonly resistant to quinolones, aminoglycosides, and occasionally to colistin (3, 7, 10). Therefore, our antibiotic choices for the treatment of infections due to KPC-Kp isolates are extremely limited. Developing novel β-lactamase inhibitors that are active against different classes of carbapenemases is an important goal (1). NXL104 (Novexel SA, Romainville, France) is a new β-lactamase inhibitor currently in clinical trials (http://clinicaltrials.gov/) and active against class A (e.g., TEM-, SHV-, and CTX-M-types) and class C β-lactamases (2, 9). However, data regarding its in vitro activity in combination with β-lactams against KPC-Kp isolates are very limited (9). In the present work, we analyzed the in vitro activity of NXL104 in combination with different β-lactams against a collection of 42 well-characterized KPC-Kp clinical isolates collected in the United States (6, 7). In a previous analysis, we demonstrated that (i) these strains possessed a complex β-lactamase background (i.e., three or more bla genes per isolate) and that (ii) clavulanate or tazobactam were unable to lower the MICs of β-lactams to susceptibility ranges for these strains (7). MICs for β-lactams and β-lactams plus NXL104 at three different constant concentrations (i.e., 1, 2, and 4 μg/ml) were determined by using the agar dilution method according to Clinical and Laboratory Standards Institute (CLSI) criteria, on cation-adjusted Mueller-Hinton agar (BBL, Becton Dickinson, Sparks, MD) using a Steers replicator (4). We tested piperacillin, cefotaxime, ceftazidime (Sigma Chemical Co.), cefepime, and aztreonam (Bristol-Myers Squibb, Princeton, NJ). NXL104 was a kind gift of Dr. Christine Miossec (Novexel). ATCC strains Escherichia coli 25922, Pseudomonas aeruginosa 27853, and K. pneumoniae 700603 were used as controls. Susceptibility results, including those for the combinations with NXL104, were interpreted according to the CLSI criteria established for the β-lactams when tested alone (5). As shown in Table ​Table1,1, KPC-Kp isolates were very resistant to all noncarbapenem β-lactams tested (overall, MIC90 values were ≥128 μg/ml). In contrast, MICs for the combination of NXL104 at a constant concentration of 4 μg/ml with piperacillin, extended-spectrum cephalosporins, or aztreonam were in the susceptible range for all strains (overall, MIC90 values were ≤2 μg/ml). All KPC-Kp strains were also susceptible to β-lactams plus NXL104 at a constant concentration of 2 μg/ml (overall, MIC90 values were ≤8 μg/ml). Additionally, NXL104 used at a concentration of 1 μg/ml was very effective at lowering MICs when combined with a cephalosporin or aztreonam (Table ​(Table11). TABLE 1. Antimicrobial susceptibility test results for the 42 K. pneumoniae isolates producing the KPC carbapenemase collected in United States In conclusion, we demonstrate that NXL104 can effectively lower the MIC of β-lactams when tested against contemporary KPC-Kp clinical isolates. The combination of NXL104 with extended-spectrum cephalosporins or aztreonam could represent a promising therapeutic strategy to treat infections due to KPC-Kp isolates. Further studies to evaluate the activity of NXL104 in combination with investigational β-lactams should be performed against large collections of gram-negative bacilli producing different classes of carbapenemases.