Activity of Meropenem-Vaborbactam against Pseudomonas aeruginosa and Acinetobacter baumannii in a Neutropenic Mouse Thigh Infection Model

We have evaluated the activity of meropenem-vaborbactam against clinical isolates of Pseudomonas aeruginosa and Acinetobacter baumannii in a neutropenic mouse thigh infection model. Data show that meropenem-vaborbactam regimens equivalent to 3-h infusions every 8 h with 2 g meropenem and 2 g vaborbactam produced bacterial killing against strains with MICs of 2 to 16 mg/liter and suggests that this combination may have utility in the treatment of infections caused by P. aeruginosa and A. baumannii.

ABSTRACT

We have evaluated the activity of meropenem-vaborbactam against clinical isolates of Pseudomonas aeruginosa and Acinetobacter baumannii in a neutropenic mouse thigh infection model. Data show that meropenem-vaborbactam regimens equivalent to 3-h infusions every 8 h with 2 g meropenem and 2 g vaborbactam produced bacterial killing against strains with MICs of 2 to 16 mg/liter and suggests that this combination may have utility in the treatment of infections caused by P. aeruginosa and A. baumannii.

INTRODUCTION

The World Health Organization recently designated carbapenem-resistant A. baumannii and carbapenem-resistant P. aeruginosa as critical priority pathogens (1), and the Centers for Disease Control and Prevention designated both multidrug resistant (MDR) A. baumannii and MDR P. aeruginosa as serious public health threats requiring monitoring and prevention strategies for effective patient management (2). These opportunistic pathogens can be the causative agents for bacteremia, pneumonia, urinary tract infections, meningitis, and surgical site infections (3). Given the limited treatment options for infections due to these organisms, new agents or improvements on older agents are needed urgently.

Vaborbactam is a new boronic acid containing a beta-lactamase inhibitor that restores the activity of carbapenems against organisms expressing multiple class A and class C beta-lactamases (4). Vabomere (meropenem-vaborbactam) was recently approved by the U.S. FDA for the treatment of complicated urinary tract infections, including pyelonephritis, due to susceptible organisms. A randomized, multinational, open-label trial showing superior outcomes for suspected or documented carbapenem-resistant Enterobacteriaceae infections with meropenem-vaborbactam treatment compared to best available therapy was also recently completed (5).

The in vitro activity of meropenem-vaborbactam against P. aeruginosa and A. baumannii has been shown previously (6, 7). Here, we have evaluated the activity of meropenem-vaborbactam against these pathogens in a neutropenic mouse thigh infection model.

For both susceptibility and animal studies, meropenem (as meropenem for injection) was purchased from commercial sources, and vaborbactam was manufactured by The Medicines Company. Meropenem was prepared as described in the prescribing information. Vaborbactam was dissolved in water, and the pH was adjusted to 7.5 by the addition of NaOH. Meropenem-vaborbactam was prepared by admixing the solutions of meropenem and vaborbactam.

Six P. aeruginosa clinical isolates and three A. baumannii clinical isolates were used in these studies. MICs were determined for meropenem alone and in combination with a fixed concentration of 8 mg/liter of vaborbactam, using the broth microdilution method described by the Clinical and Laboratory Standards Institute (8). The MIC values are shown in Table 1. The MIC values for meropenem and meropenem-vaborbactam were similar for all isolates tested.

TABLE 1.

MICs for the strains used in these studies

Species or strain	Meropenem MIC (mg/liter)
	Alone	With 8 mg/liter vaborbactam
P. aeruginosa
PAM3210	2	2
PAM3353	4	2
PAM3377	4	4
PAM3312	8	8
PAM3232	8	8
PAM3304	16	8
A. baumannii
AB1022	2	2
AB1157	4	4
AB1137	8	8

The approved dosage regimen for meropenem-vaborbactam is administration of 4 g (2 g meropenem and 2 g vaborbactam) every 8 h by 3-h infusion (9). This dose and dosage regimen produce plasma exposures that are predicted to treat organisms with MICs as high as 8 mg/liter (10).

We have previously shown the activity of meropenem-vaborbactam in mouse thigh, lung, and pyelonephritis models of infection due to carbapenem-resistant, Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae (11, 12). In addition, we have shown a pharmacokinetic profile in the mouse that produces exposures equivalent to those of the approved human dosage regimen (9, 11, 13).

In these studies, female Swiss Webster mice (5 to 6 weeks of age) were obtained from Envigo Laboratories (Livermore, CA). Animals were provided food and water ad libitum in accordance with National Institutes of Health guidelines for the care and use of laboratory animals (14), and all studies using animals were performed under protocols approved by an institutional animal care and use committee (IACUC). Mice were rendered neutropenic by the administration of 150 mg/kg cyclophosphamide (Baxter, IL) by the intraperitoneal route, 4 days and 1 day prior to the start of the study. Two hours prior to the initiation of treatment, thighs (both thighs of two mice per group) were infected by an intramuscular injection of 0.1 ml of inoculum (∼10⁷ CFU/ml) while under isoflurane anesthesia (5% isoflurane in oxygen running at 4 liter/min). Treatment regimens were designed to produce exposures equivalent to those produced by the approved dosage in humans (9, 11, 13). Briefly, 300 mg/kg meropenem alone was administered every 2 h over a 24-h period. This dosage regimen produces a meropenem exposure equivalent to 2 g meropenem administered every 8 h by 3-h infusion in humans (11). Doses of 300 mg/kg meropenem and 50 mg/kg vaborbactam (meropenem-vaborbactam) were administered every 2 h over a 24 h period. This dosage regimen in mice produces an exposure equivalent to 2 g meropenem and 2 g vaborbactam administered every 8 h by 3-h infusion in humans (11). All doses were administered by intraperitoneal injection. For each strain, untreated mice were euthanized prior to the start of treatment to determine baseline bacterial counts and at the end of the treatment period to confirm bacterial growth. All treated groups were euthanized 2 h following the last dose, and thighs were removed aseptically and homogenized in ice cold saline. The homogenized thighs were serially diluted 10-fold and plated on Mueller-Hinton agar. Plates were incubated overnight at 37˚C, and then colonies were counted. Differences in the colony counts were analyzed using a two-tailed t test (GraphPad Prism, version 6). A P value of <0.05 was considered statistically significant.

The bacterial counts for P. aeruginosa PAM3210, PAM3312, and PAM3332 are presented in Fig. 1. For all three strains, both meropenem alone and meropenem-vaborbactam produced 2 to 3 log CFU/thigh of bacterial killing. The reduction in bacterial counts for meropenem alone or meropenem-vaborbactam were very similar for these clinical isolates.

FIG 1.

Activity of meropenem and meropenem-vaborbactam against P. aeruginosa PAM3210, P. aeruginosa PAM3312, and PAM3232 in a neutropenic mouse thigh infection model.

The bacterial counts for P. aeruginosa PAM3353, PAM3377, and PAM3304 are shown in Fig. 2. For PAM3353, PAM3377, and PAM3304, treatment with meropenem alone produced 1.43, 0.77, and 0.60 log CFU/thigh reductions in bacterial counts against each strain, respectively. Treatment with meropenem-vaborbactam produced 2.68, 1.73, and 1.57 log CFU/thigh reductions in bacterial counts for PAM3353, PAM3377, and PAM3304, respectively. The reductions in bacterial counts with meropenem-vaborbactam were significantly greater than those produced by meropenem alone (P < 0.05).

FIG 2.

Activity of meropenem and meropenem-vaborbactam against P. aeruginosa PAM3353, P. aeruginosa PAM3377, and P. aeruginosa PAM3304 in a neutropenic mouse thigh infection model.

The reduction in bacterial counts for A. baumannii AB1157, AB1137, and AB1022 after treatment with meropenem alone were 4.32, 2.32, and 3.79 log CFU/thigh, respectively (Fig. 3). Treatment with meropenem-vaborbactam produced bacterial killing nearly identical to meropenem alone with bacterial reductions of 4.32, 2.33, and 3.52 log CFU/thigh for AB1157, AB1137, and AB1022, respectively (Fig. 3).

FIG 3.

Activity of meropenem and meropenem-vaborbactam against A. baumannii AB1022, A. baumannii AB1157, and A. baumannii AB1137 in a neutropenic mouse thigh infection model.

In these studies, only 2 of the organisms tested (P. aeruginosa PAM3210 and A. baumannii AB1022) were considered susceptible to meropenem (MIC ≤ 2 mg/liter; see reference 15). However, meropenem exposures in mice simulating 2 g every 8 h by 3-h infusion in humans produced at least 1 log CFU/thigh of bacterial killing against 7 of the 9 strains, despite MICs of 2 to 16 mg/liter. The ability to produce bacterial killing in these “nonsusceptible” isolates is likely due to the increased time above the MIC that the higher dose and prolonged infusion provide (16, 17).

In contrast to meropenem alone, meropenem-vaborbactam produced over 1 log CFU/thigh of bacterial killing against all nine strains tested. For P. aeruginosa strains PAM3353, PAM3377, and PAM3304, meropenem-vaborbactam produced approximately 1 log CFU/thigh more bacterial killing than meropenem alone, and this difference was statistically significant. This increased bacterial killing was observed despite the in vitro MICs being the same for both agents, and it suggests that these strains may contain an inducible beta-lactamase that is inhibited by vaborbactam. For meropenem-vaborbactam, a breakpoint has not been established for P. aeruginosa or A. baumannii, but based on exposures in phase 3 studies and target attainment analysis, meropenem-vaborbactam would be expected to be effective against organisms with MICs as high as 8 mg/liter (18).

Overall, these results demonstrate that both meropenem and meropenem-vaborbactam have excellent activity in this model against infections caused by P. aeruginosa and A. baumannii when administered at doses equivalent to 2 g every 8 h by 3-h infusion. The combination of meropenem and vaborbactam does produce additional bacterial killing in some strains, despite the same MIC. Given this additional activity, further investigation is warranted.