Novel Bis-Indole Agents Active Against Multidrug-Resistant Acinetobacter baumannii

Abstract

The in vitro activity of five novel Microbiotix bis-indole agents (MBXs) against 30 multidrug-resistant (MDR) A. baumannii (including 18 resistant to carbapenems) was evaluated. Overall, MIC₉₀s ranged from 1-8 μg/ml, whereas those for imipenem were > 64 μg/ml. MBX 1196 was the most potent (MIC₉₀ 1 μg/ml). MBXs are compounds that are highly effective against MDR A. baumannii.

Acinetobacter baumannii are commonly associated with serious hospital-acquired infections, particularly in severely ill, immune-compromised hosts (Perez, Hujer et al. 2007; Munoz-Price and Weinstein 2008). Resistance, both intrinsic and acquired, to nearly all classes of antibiotics significantly limits treatment options, making A. baumannii one of the most difficult multidrug-resistant (MDR) organisms to treat (Dijkshoorn, Nemec et al. 2007). Unfortunately, there are few active agents available for treating serious infections due to MDR Acinetobacter infections, with carbapenems being one of the “last line” treatment agents available. At the present time, the prevalence of carbapenem-resistant A. baumannii isolates is increasing (Perez, Hujer et al. 2007; Munoz-Price and Weinstein 2008). Therefore, our therapeutic choices for the treatment of the infections due to A. baumannii isolates are limited. Developing new antibiotics that are active against these life-threatening pathogens is an important goal.

Bis-imidazolinylindole (or bis-indole) agents are members of a promising new drug class (Panchal, Ulrich et al. 2009). These are diarylamidine derivatives that target DNA synthesis, providing a broad-spectrum antibacterial activity. Currently, investigations are underway to establish the mechanism of bacterial killing. The minor structural differences between the compounds correlate with differences in their effects on bacterial macromolecular synthesis and mechanisms of resistance. Several of these agents were shown to be effective in treating mice lethally infected with Bacillus anthracis, Staphylococcus aureus, or Yersinia pestis (Butler, Williams et al. 2008; Panchal, Ulrich et al. 2009).

This study evaluated five novel bis-indole agents, MBX 1158, MBX 1162, MBX 1196, MBX 1337, and MBX 1339 (Microbiotix, Inc, Worcester, MA) (Figure 1) against recently collected A. baumannii isolates, more than half of which were carbapenem-, ciprofloxacin-, piperacillin/tazobactam-, cefepime-, and amikacin-resistant. These A. baumannii isolates were collected from Iraqi and United States sources (Hujer, Hujer et al. 2006), and were tested by broth microdilution using current procedures outlined by the Clinical and Laboratory Standards Institute (CLSI) (CLSI 2009a). We define “MDR” as strains exhibiting resistance to representatives of three or more of the following classes of antibiotics: quinolones (ciprofloxacin), extended-spectrum cephalosporins (ceftazidime and cefepime), β-lactam/ β-lactamase inhibitor combinations (ampicillin-sulbactam), aminoglycosides (amikacin and tobramycin), and carbapenems (imipenem and meropenem).

Figure 1.

Chemical structures of MBX compounds.

These MBX agents were tested at concentrations of 0.03 to 32 μg/ml; meropenem at 0.25 to 8 μg/ml, and imipenem at 0.5 to 64 μg/ml. Carbapenem-resistant strains were defined as isolates with meropenem and/or imipenem MICs of ≥ 8 μg/ml according to CLSI criteria (CLSI 2009b).

Thirty MDR A. baumannii strains were assayed, including 18 carbapenem-resistant isolates. Unimodal MIC distributions were obtained for the five MBX agents and bimodal distributions for the two carbapenems tested (data not shown). MIC ranges, MIC₅₀ and MIC₉₀ values are shown in Table 1. With MIC₉₀ values of 1-8 μg/ml, the MBX compounds had superior in vitro potency to either meropenem or imipenem (MIC₉₀ values of > 8 μg/ml). MBX 1196 was the most potent agent tested, with MIC₅₀ of 0.25 μg/ml, MIC₉₀ of 1 μg/ml, and MIC range of 0.06-1 μg/ml. The MBX compounds had similar activity against carbapenem-resistant isolates to that against all isolates, with MIC₅₀ values varying by up to one doubling dilution and MIC₉₀ values being identical for the two groups (Table 1).

Table 1.

Results for all isolates (n=30) and for carbapenem-nonsusceptible A. baumannii isolates (n=18), defined as meropenem and/or imipenem MICs ≥ 8 μg/ml (CLSI 2009b).

	MIC values (μg/ml)
	All isolates (n=30)			Carbapenem-resistant isolates (n=18)
Agent	Range	MIC50	MIC90	Range	MIC50	MIC90
MBX 1158	0.5-4	1	2	1-4	1	2
MBX 1162	0.12-4	0.5	2	0.5-2	0.5	2
MBX 1196	0.06-1	0.25	1	0.12-1	0.5	1
MBX 1337	0.25-4	1	2	0.5-4	0.5	2
MBX 1339	0.5-8	2	8	1-8	4	8
Meropenem	0.5 - >8	>8	>8	>8	>8	>8
Imipenem	≤0.5->64	4	>64	4->64	32	>64

Our results show that the five MBX agents tested had potent in vitro activity against the MDR A. baumannii isolates tested, with MBX 1196 being the most potent, regardless of carbapenem susceptibility. Previous work reported with MBX 1162 showed a range of activity against A. baumannii isolates similar to that found in the present study, as well as good activity against a wide variety of Gram-positive and Gram-negative bacteria, including other non-fermenters such as Pseudomonas aeruginosa and Burkholderia cepacia (Butler, Williams et al. 2008; Panchal, Ulrich et al. 2009). MBX 1162 was also effective and well-tolerated in a mouse model of Staphylococcus aureus bacteremia (Butler, Williams et al. 2008). Should the toxicities and pharmacokinetic and pharmacodynamic parameters of these agents prove favorable, they are potentially useful and needed additions to the antimicrobial armamentarium and should undergo further development for clinical use.