Activity of doripenem with and without levofloxacin, amikacin, and colistin against Pseudomonas aeruginosa and Acinetobacter baumannii

Activity of doripenem with and without levofloxacin, amikacin, and colistin against Pseudomonas aeruginosa and Acinetobacter baumannii

Available online at www.sciencedirect.com Diagnostic Microbiology and Infectious Disease 67 (2010) 191 – 197 www.elsevier.com/locate/diagmicrobio Ac...

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Available online at www.sciencedirect.com

Diagnostic Microbiology and Infectious Disease 67 (2010) 191 – 197 www.elsevier.com/locate/diagmicrobio

Activity of doripenem with and without levofloxacin, amikacin, and colistin against Pseudomonas aeruginosa and Acinetobacter baumannii Glenn A. Pankucha , Harald Seifertb , Peter C. Appelbauma,⁎ a Department of Pathology, Hershey Medical Center, Hershey, PA 17033, USA Institute for Medical Microbiology, Immunology and Hygiene, University of Cologne, Cologne, Germany Received 11 December 2009; accepted 10 January 2010

b

Abstract At 24 h, sub-MIC doripenem and levofloxacin showed synergy against 21 of 25 Pseudomonas aeruginosa strains, sub-MIC doripenem and amikacin against 22 isolates, and sub-MIC doripenem and colistin against 19 isolates. Of 25 Acinetobacter baumannii strains, sub-MIC doripenem and levofloxacin showed synergy against 11 strains at 24 h, sub-MIC doripenem and amikacin against 24 strains, and sub-MIC doripenem and colistin against all isolates. © 2010 Elsevier Inc. All rights reserved. Keywords: Doripenem; Time–kill synergy; Pseudomonas aeruginosa; Acinetobacter baumannii

Carbapenem resistance occurs increasingly in Pseudomonas aeruginosa and Acinetobacter baumannii (Falagas et al., 2006; Hujer et al., 2006; Kean, 2008; Livermore et al., 2008). Previous studies have demonstrated synergy between β-lactams and quinolones or other compounds against Gram-negative bacilli by time–kill (Bajaksouzian et al., 1997; Cappelletty and Rybak, 1996; Credito et al., 2007; Pankuch et al., 2008). We used synergy time–kill to examine activity of doripenem (Kean, 2008; Pillar et al., 2008; Marti et al., 2009), alone and combined with levofloxacin, amikacin, and colistin, against 50 P. aeruginosa and A. baumannii strains. Bacteria comprised 25 P. aeruginosa strains. Twenty-one were recent clinical specimens (blood, urine, wounds, respiratory tract) at Hershey Medical Center (Hershey, PA) collected from different patients for 2 years identified by standard methodology (Manual of Clinical Microbiology, 2007). Four additional doripenem-resistant (≥256 μg/mL) isolates included 2 VIM-2 β-lactamase–producing isolates from JMI Laboratories (North Liberty, IA). Twenty-five A. baumannii strains were studied; 22 were from similar clinical specimens at the University of Cologne

⁎ Corresponding author. Tel.: +1-717-531-5113; fax: +1-717-531-7953. E-mail address: [email protected] (P.C. Appelbaum). 0732-8893/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.diagmicrobio.2010.01.004

(Cologne, Germany). Species identification was by amplified ribosomal DNA restriction analysis (Vaneechoutte et al., 1995). To exclude copy strains, we included A. baumannii strains on the basis of exhibiting a unique randomly amplified polymorphic DNA profile (Grundmann et al., 1997). Three additional A. baumannii strains with doripenem ≥32 μg/mL comprised 2 strains producing OXA-24 and OXA-51 β-lactamase (JMI Laboratories) and a third strain (Creighton University Medical School, Omaha, NE) whose resistance mechanism was not identified. Doripenem and levofloxacin powder was obtained from the manufacturer, and amikacin and colistin sulfate were obtained from Sigma (St. Louis, MO). MIC determination, synergy time–kill determination, and definition of synergy were based on standard methodology. Counts were taken after 3, 6, 12, and 24 h (Clinical and Laboratory Standards Institute, 2009; Eliopoulos and Moellering, 1991; Pankuch et al., 2008). Fisher exact test was used for comparison of proportions of the killing synergy between groups (Fisher, 1990). A value of P b .05 was considered significant. MIC and synergy data of 25 P. aeruginosa are presented in Table 1. MICs (μg/mL) of drugs alone were doripenem, 0.12 to 512; levofloxacin, 0.5 to 64; amikacin, 2 to 512; and colistin, 0.12 to 32. Doripenem plus levofloxacin, at 3 h, yielded synergy against 5 isolates at sub-MIC doripenem (0.12–1 μg/mL) and levofloxacin (0.12–8 μg/mL); at 6 h,

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Table 1 P. aeruginosa MICs (μg/mL) of drugs alone and results of time–kill studies for synergy (electronic supplement) Strain

MIC (μg/mL) Doripenem

Doripenem + levofloxacin (μg/mL) Levofloxacin

Amikacin

Colistin

3h a

6h

12 h b

4.0

NS

NS

2.0

NS

NS

32

1.0

NS

NS

4.0

16

1.0

NS

NS

0.12

1.0

16

1.0

NS

NS

326

8.0

2.0

32

1.0

NS

327

2.0

8.0

16

2.0

NS

Syn 2.0/0.5 NS

328

4.0

1.0

16

2.0

NS

NS

329

1.0

2.0

64

1.0

NS

330

1.0

2.0

64

1.0

331

0.5

2.0

8.0

0.12

Syn 0.25/1.0 NS

Syn 0.25/1.0 Syn 0.25/1.0 NS

332

1.0

2.0

8.0

0.5

334

1.0

1.0

8.0

1.0

Syn 0.25/0.5 NS

Syn 0.25/0.5 NS

335

4.0

337

2.0

1.0

0.5

Syn 1.0/8.0 NS

338

2.0

1.0

16

1.0

NS

Syn 1.0/8.0 Syn 0.5/0.25 NS

339

0.5

0.5

16

1.0

NS

NS

340

0.5

0.5

16

2.0

341

0.5

8.0

Syn 0.12/0.12 NS

Syn 0.12/0.12 NS

344

0.5

4.0

Syn 0.25/1.0 NS

Syn 0.12/1.0 NS

Syn 1.0/0.5 Syn 0.25/1.0 Syn 0.25/1.0 Syn 0.25/0.5 Syn 0.5/0.5 Syn 0.25/0.25 Syn 1.0/8.0 Syn 0.5/0.25 Syn 1.0/0.25 Syn 0.12/0.12 Syn 0.12/0.25 Syn 0.25/4.0 Syn 0.25/1.0 NS

NS

Syn 64/16 Syn 128/16 Syn 64/16 NS

Syn 128/16 Syn 128/32 Syn 128/16 NS

319

2.0

2.0

320

2.0

1.0

322

4.0

2.0

323

1.0

324

32

16 8.0

16 4.0

8.0 64

0.5 0.5

16

16

379c,d

256

64

512

380c,d

512

64

512

1.0

NS

381c

256

32

512

4.0

NS

382c

256

64

16

2.0

NS

371

2.0

1.0

a No synergy. Doripenem + amikacin (μg/mL) b Synergy ratio. 3h c 6h 12 h 24 h Doripenem MICs, ≥256 μg/mL. d VIM-2 β-lactamase–producing P. aeruginosa isolates.

drugs produced synergy against 11 isolates with sub-MIC doripenem concentrations (0.12–128 μg/mL) and levofloxacin concentrations of 0.12 to 16 μg/mL. At 12 h, drugs showed synergy against 22 strains at sub-MIC doripenem

2.0 32

Syn 0.5/0.5 Syn 0.5/0.25 Syn 1.0/1.0 Syn 0.25/2.0 Syn 0.03/0.5 Syn 2.0/1.0 NS

24 h Syn 0.5/0.5 Syn 0.5/0.5 Syn 1.0/0.5 Syn 0.5/1.0 Syn 0.03/0.5 Syn 2.0/1.0 Syn 0.5/2.0 Syn 1.0/0.25 Syn 0.25/1.0 Syn 0.25/1.0 Syn 0.12/1.0 Syn 0.5/1.0 Syn 0.25/0.25 Syn 1.0/16 Syn 0.5/0.5 Syn 1.0/0.25 Syn 0.25/0.12 Syn 0.12/0.25 Syn 0.25/4.0 Syn 0.25/2.0 NS Syn 128/32 NS NS NS

Doripenem + colistin (μg/mL) 3h

6h

12 h

24 h

concentrations (0.03–128 μg/mL) and levofloxacin concentrations of 0.12 to 32 μg/mL. At 24 h, 21 strains showed synergy with sub-MIC concentrations of doripenem and levofloxacin (0.03–128 and 0.12–32 μg/mL, respectively).

G.A. Pankuch et al. / Diagnostic Microbiology and Infectious Disease 67 (2010) 191–197

193

Table 1 (continued) Doripenem + amikacin (μg/mL)

Doripenem + colistin (μg/mL)

3h

6h

12 h

24 h

3h

6h

12 h

24 h

NS

NS

NS

NS

Syn 1.0/4.0 NS

Syn 0.5/4.0 Syn 0.5/2.0 Syn 1.0/8.0 Syn 0.25/4.0 Syn 0.03/8.0 Syn 2.0/16 Syn 0.5/8.0 Syn 1.0/4.0 Syn 0.25/32 Syn 0.25/32 Syn 0.12/4.0 NS

Syn 0.5/1.0 NS

NS

Syn 0.5/4.0 Syn 0.5/2.0 Syn 1.0/8.0 Syn 0.5/8.0 Syn 0.03/4.0 NS

Syn 0.5/1.0 Syn 1.0/0.5 Syn 1.0/0.25 Syn 0.25/0.25 Syn 0.03/0.25 Syn 2.0/0.25 Syn 0.5/0.5 NS

Syn 0.5/1.0 Syn 0.5/0.5 Syn 1.0/0.25 Syn 0.25/0.25 Syn 0.03/0.25 Syn 2.0/0.25 Syn 0.5/0.5 NS

Syn 0.25/0.25 Syn 0.25/0.25 NS

Syn 0.25/0.5 Syn 0.5/0.25 NS NS

Syn 0.25/2.0 Syn 1.0/8.0 Syn 0.5/2.0 Syn 0.5/8.0 Syn 0.12/8.0 Syn 0.12/4.0 Syn 0.12/4.0 Syn 0.12/16 NS

NS

Syn 0.5/0.12 NS

Syn 0.5/1.0 Syn 0.50/0.5 Syn 1.0/0.25 Syn 0.25/0.25 Syn 0.03/0.25 Syn 2.0/0.5 Syn 0.5/0.5 Syn 2.0/0.5 Syn 0.5/0.5 Syn 0.25/0.5 Syn 0.25/0.03 NS

NS

NS

NS

NS

NS

NS`

Syn 2.0/0.5 NS

Syn 2.0/0.5 NS

NS

NS

NS

NS

Syn 0.12/0.25 Syn 0.12/0.5 Syn 0.12/0.12 Syn 0.12/0.12 NS

Syn 0.12/0.25 Syn 0.12/0.5 Syn 0.12/0.12 Syn 0.12/0.12 NS

Syn 0.25/0.25 Syn 0.12/1.0 Syn 0.12/0.12 NS

Syn 64/256 Syn 128/128 Syn 64/256 NS

NS

Syn 64/8.0 NS

Syn 0.12/0.25 Syn 0.12/0.5 Syn 0.12/0.12 Syn 0.12/0.12 Syn 8.0/1.0 Syn 64/16 NS

Syn 128/2.0 Syn 64/0.5

Syn 128/2.0 Syn 64/0.5

Syn 128/2.0

Syn 0.12/8.0 NS

Syn 1.0/4.0 Syn 0.5/1.0 Syn 0.5/8.0 Syn 0.12/8.0 NS

NS

NS

Syn 0.12/16 NS

Syn 0.12/16 NS

Syn 0.5/4.0 Syn 0.5/2.0 Syn 1.0/8.0 Syn 0.25/4.0 Syn 0.03/8.0 Syn 2.0/8.0 Syn 0.5/8.0 Syn 1.0/4.0 Syn 0.25/32 Syn 0.25/32 Syn 0.12/4.0 Syn 0.25/4.0 Syn 0.25/2.0 Syn 1.0/4.0 Syn 0.5/1.0 Syn 0.5/8.0 Syn 0.12/8.0 Syn 0.12/4.0 Syn 0.12/4.0 Syn 0.12/16 NS

NS

Syn 64/128 Syn 128/128 Syn 64/128 NS

Syn 64/128 Syn 128/128 Syn 64/128 NS

NS Syn 1.0/8.0 NS NS

NS NS NS Syn 0.25/2.0 Syn 0.25/2.0 Syn 1.0/4.0 Syn 0.5/1.0 NS

Ns Syn 64/128 NS

Syn 0.25/32 NS Syn 0.25/2.0 NS

When doripenem was combined with amikacin, 8 strains showed synergy at 3 h with sub-MIC concentration of doripenem and amikacin (0.12–64 and 1.0–128 μg/mL, respectively). At 6 h, 16 strains showed synergy with

NS Syn 0.25/0.25 Syn 0.03/0.25 Syn 2.0/0.25 NS NS NS Syn 0.25/0.25 NS NS

Syn 128/0.25 NS Syn 64/1.0

Syn 8.0/1.0 Syn 128/16 NS

Syn 64/1.0

doripenem (0.03–128 μg/mL) and amikacin (1.0– 128 μg/mL). After 12 h, 23 strains showed synergy with doripenem and amikacin (0.03–128 and 1.0–128 μg/mL, respectively). After 24 h, 22 strains showed synergy with

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Table 2 A. baumannii MICs (μg/mL) of drugs alone and results of time–kill studies for synergy (electronic supplement) Strain

MIC (μg/mL) Doripenem

Doripenem + levofloxacin (μg/mL) Levofloxacin

Amikacin

882

1.0

0.25

8.0

883

0.25

0.25

4.0

884

4.0

885

16

32 32

8.0 16

Colistin

6h

12 h

24 h

NS

NS

NS

NS

NS

NS

Synb 0.5/0.12 NS

2.0

NS

NS

NS

NS

8

NS

NS

Syn 4.0/8.0 NS

Syn 4.0/16 NS

Syn 0.25/0.12 NS

NS NS

Syn 0.25/0.06 NS

Syn 0.25/0.12 NS Syn 0.25/0.25 NS

1.0 32

3h a

886

0.5

0.5

4.0

2.0

NS

NS

887

0.5

0.25

8.0

8.0

NS

NS

888

0.5

0.25

4.0

8.0

NS

NS

889

0.5

0.25

4.0

NS

NS

890

1.0

0.25

4.0

NS

NS

891

1.0

0.5

8.0

32

NS

NS

4.0

16

NS

NS

16 1.0

893

0.5

0.12

4.0

1.0

NS

NS

Syn 0.5/0.12 Syn 8.0/16 NS

894

1.0

2.0

8.0

1.0

NS

NS

NS

895

1.0

4.0

2.0

NS

NS

NS

896

2.0

8.0

8.0

0.5

NS

897

0.5

0.25

2.0

2.0

NS

Syn 0.5/4.0 NS

898

4.0

8.0

0.5

NS

NS

900

0.5

8.0

1.0

NS

NS

901

8.0

8.0

NS

NS

902

1.0

0.25

8.0

2.0

NS

NS

903

2.0

0.5

8.0

1.0

NS

NS

Syn 0.5/4.0 Syn 0.25/0.06 Syn 2.0/8.0 Syn 0.12/0.12 Syn 2.0/16 Syn 0.25/0.06 NS

904

2.0

1.0

8.0

NS

NS

NS

NS

NS

NS

NS

NS

892

16

32

16

16 0.25 32

16

932c

128

32

256

933c

32

32

1024

934

32

16

8.0

a No synergy. Doripenem + amikacin (μg/mL) b Synergy ratio. 3 hc 6h 12 h 24 h Strains producing OXA-24 and OXA-51 β-lactamase.

doripenem (0.03–128 μg/mL) and amikacin (2–256 μg/mL). Doripenem plus colistin, at 3 h, showed synergy against 11 strains at doripenem concentration of 0.03 to 128 μg/mL and colistin of 0.12 to 1.0 μg/mL; at 6 h,17 stains showed synergy

32 0.5 16 8.0

Syn 0.5/0.25 Syn 64/16 Syn 8.0/16 Syn 16/8.0

NS Syn 0.25/1.0 NS Syn 0.5/4.0 NS NS Syn 0.25/0.6 Syn 2.0/16 NS NS Syn 1.0/0.5 NS Syn 8.0/16 Syn 16/8.0

Doripenem + colistin (μg/mL) 3h

6h

12 h

24 h

with doripenem (0.03–128 μg/mL) and colistin (0.12– 8.0 μg/mL). At 12 h, 18 strains showed synergy at doripenem concentrations of 0.03 to 128 μg/mL and colistin concentrations of 0.12 to 16 μg/mL. After 24 h, 19 strains showed

G.A. Pankuch et al. / Diagnostic Microbiology and Infectious Disease 67 (2010) 191–197

195

Table 2 (continued) Doripenem + amikacin (μg/mL)

Doripenem + colistin (μg/mL)

3h

6h

12 h

24 h

3h

6h

12 h

24 h

NS

NS

NS

NS

Syn 0.25/2.0 NS

NS

NS

Syn 0.5/2.0 NS

NS

NS

NS

NS

NS

Syn 1.0/0.5 NS

NS

Syn 4.0/8.0 NS

NS

NS

Syn 0.12/2.0 NS

Syn 0.12/2.0 NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

Syn 0.12/4.0 NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

Syn 0.25/1.0 NS

Syn 0.25/1.0 NS

NS

Syn 0.12/0.5 Syn 1.0/2.0 Syn 0.12/2.0 Syn 2.0/4.0 NS

Syn 1.0/2.0 Syn 4.0/4.0 Syn 0.12/2.0 Syn 0.12/2.0 Syn 0.12/2.0 Syn 0.12/1.0 Syn 0.25/1.0 Syn 0.25/2.0 Syn 4.0/2.0 Syn 0.25/2.0 Syn 0.25/2.0 Syn 0.25/1.0 Syn 0.5/2.0 Syn 0.25/0.5 Syn 1.0/2.0 Syn 0.12/2.0 Syn 2.0/8.0 Syn 0.25/2.0 Syn 0.5/2.0 Syn 0.5/2.0 Syn 32/64 Syn 8.0/512 Syn 8.0/4.0

NS

NS

Syn 1.0/2.0 Syn 4.0/4.0 Syn 0.12/1.0 Syn 0.12/2.0 Syn 0.12/1.0 Syn 0.12/1.0 Syn 0.25/1.0 Syn 0.25/2.0 Syn 4.0/2.0 Syn 0.25/2.0 Syn 0.25/2.0 Syn 0.25/1.0 Syn 0.5/2.0 Syn 0.12/1.0 Syn 1.0/2.0 Syn 0.12/2.0 Syn 2.0/4.0 Syn 0.25/2.0 Syn 0.5/2.0 Syn 0.5/2.0 Syn 32/64 Syn 16/256 Syn 8.0/4.0

Syn 0.25/0.5 Syn 0.06/8.0 Syn 1.0/0.5 Syn 4.0/2.0 Syn 0.12/1.0 Syn 0.25/2.0 Syn 0.12/4.0 Syn 0.12/4.0 Syn 0.25/0.5 Syn 0.25/8.0 NS

Syn 0.12/0.25 NS

Syn 0.25/0.5 Syn 0.25/1.0 Syn 1.0/0.12 Syn 0.12/0.5 Syn 1.0/0.12 Syn 0.12/0.25 NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

Syn 8.0/2.0

NS

Syn 0.5/0.25 Syn 0.06/16 Syn 1.0/0.5 Syn 4.0/2.0 Syn 0.12/1.0 Syn 0.12/4.0 Syn 0.12/4.0 Syn 0.12/4.0 Syn 0.25/0.5 Syn 0.25/8.0 Syn 4.0/8.0 Syn 0.12/0.5 Syn 0.25/0.25 Syn 0.25/0.5 Syn 0.5/0.12 Syn 0.12/1.0 Syn 1.0/0.12 Syn 0.12/0.5 Syn 2.0/2.0 Syn 0.25/0.5 Syn 0.5/0.25 Syn 0.5/8.0 Syn 32/0.25 Syn 8.0/4.0 Syn 8.0/2.0

NS Syn 0.12/2.0 NS NS NS NS

Syn 1.0/2.0 NS

NS

NS

NS

NS

NS

Syn 8.0/2.0

synergy with concentrations of doripenem of 0.03 to 128 μg/mL and colistin of 0.03 to 16 μg/mL. There were 4 P. aeruginosa strains with doripenem MICs N256 μg/mL. Synergy occurred at 6 to 24 h, albeit with high

NS NS NS NS

Syn 0.12/0.5 Syn 0.25/0.5 Syn 0.25/1.0 Syn 0.5/0.12 Syn 0.12/1.0 Syn 1.0/0.12 Syn 0.12/0.5 Syn 2.0/2.0 Syn 0.25/0.5 Syn 0.5/0.25 Syn 0.5/8.0 Syn 32/0.25 NS Syn 8.0/2.0

MICs, for 3 of these strains with doripenem–amikacin and doripenem–colistin combinations. Fig. 1 depicts the number of P. aeruginosa strains showing synergy. All 3 doripenem combinations produced synergistic killing of ≥72% strains

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at 12 and 24 h. The number of strains killed by different combinations at 3, 6, 12, and 24 h did not statistically differ. MIC and synergy time–kill data of 25 A. baumannii strains are presented in Table 2. MICs (μg/mL) of drugs alone were doripenem, 0.25–128; levofloxacin, 0.12–32; amikacin, 2-1024; and colistin, 0.5 to 32. Doripenem plus levofloxacin did not yield synergy at 3 h. One strain showed synergy at 6 h, at sub-MIC concentrations of doripenem and levofloxacin (0.5 and 4.0 μg/mL, respectively). At 12 h, 15 strains showed synergy with sub-MIC concentrations of doripenem (0.12–64 μg/mL) and levofloxacin (0.06– 16 μg/mL). At 24 h, 11 strains showed synergy at subMIC doripenem and levofloxacin concentrations (0.25–

Fig. 2. Time–kill synergy studies of doripenem combined with levofloxacin, amikacin, and colistin against 25 isolates of A. baumannii. In each pair of bars, the one on the left represents no synergy and the one on the right represents synergy. ⁎Significantly higher number of organisms showing synergy with doripenem/amikacin and doripenem/colistin at 12 and 24 h when compared to doripenem/levofloxacin (P b .05).

Fig. 1. Time–kill synergy studies of doripenem combined with levofloxacin, amikacin, and colistin against 25 isolates of P. aeruginosa. In each pair of bars, the one on the left represents no synergy and the one on the right represents synergy.

16 and 0.06–16 μg/mL, respectively). When doripenem was combined with amikacin, 3 strains showed synergy at 3 h with sub-MIC concentration of doripenem (0.12– 0.25 μg/mL) and amikacin (1.0–2.0 μg/mL). At 6 h, 9 strains showed synergy at sub-MIC concentrations of doripenem and amikacin (0.12–8.0 and 0.5–8.0 μg/mL, respectively). After 12 h, 24 strains showed synergy at subMIC doripenem and amikacin concentrations (0.12–32 and 1.0–256 μg/mL, respectively). After 24 h, 24 strains showed synergy with sub-MIC doripenem (0.12–32 μg/mL) and amikacin (0.5–512 μg/mL). When doripenem was combined

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with colistin, 2 strains showed synergy at 3 h at sub-MIC doripenem (0.12–8.0 μg/mL) and colistin (0.25–2.0 μg/mL). At 6 h, 8 strains showed synergy with sub-MIC concentrations of doripenem (0.12–1.0 μg/mL) and colistin (0.12– 4.0 μg/mL). After 12 h, 23 strains showed synergy with subMIC doripenem and colistin concentrations (0.06–32 and 0.12–8 μg/mL, respectively). After 24 h, all strains showed synergy with sub-MIC doripenem (0.06–32 μg/mL) and colistin (0.12–16 μg/mL). For 3 A. baumannii strains, doripenem MICs were ≥32 μg/mL; synergy was found with doripenem combined with amikacin and colistin at sub-MIC concentrations. Against A. baumannii (Fig. 2), doripenem plus levofloxacin was synergistic against 15 strains at 12 h, but it declined by 24 h. Doripenem–amikacin and doripenem–colistin showed increased synergy compared to doripenem–levofloxacin at 12 and 24 h (P b .05). For 6 P. aeruginosa and 8 A. baumannii strains resistant to doripenem and levofloxacin, in vitro synergy was not clinically significant. Use of combinations in strains with lower MICs is potentially useful in allowing lower doses of toxic drugs, preventing resistance development, and sometimes lowering MICs in synergistic combinations from intermediate resistant to susceptible levels. Hilliard et al. (2009) showed in a mouse model that doripenem in combination with levofloxacin, amikacin, and colistin against 3 P. aeruginosa strains showed better recovery rates than drugs used alone. Doripenem plus levofloxacin produced the greatest percentage of mouse survival. In a prospective clinical study, combination therapy was used to treat 4 transplant patients with multiresistant Acinetobacter pneumonia (Shields et al., 2009). Patients received doripenem (3 patients) or meropenem (1 patient) + intravenous and inhaled colistin. One patient receiving doripenem + colistin failed treatment and died. The remaining 3 patients were cured and survived. More clinical studies are needed to test our in vitro hypothesis. Acknowledgment This study was supported by grant from Johnson and Johnson Pharmaceutical Research and Development (Raritan, NJ). The authors thank Ronald N. Jones and Kenneth S. Thomson for providing carbapenem-resistant organisms. References Bajaksouzian S, Visalli MA, Jacobs MR, Appelbaum PC (1997) Activities of levofloxacin, ofloxacin, and ciprofloxacin, alone and in combination with amikacin, against Acinetobacters determined by checkerboard and time–kill studies. Antimicrob Agents Chemother 41: 1073–1076.

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