In Vitro Activity of Doripenem against Pseudomonas aeruginosa and Burkholderia cepacia Isolates from both Cystic Fibrosis and Non-Cystic Fibrosis Patients

Chronic lung infection, repeated exacerbations, and progressive deterioration in lung function caused by Pseudomonas aeruginosa and Burkholderia cepacia are a major cause of mortality in cystic fibrosis (CF) patients despite current antimicrobial therapy (1, 5). In approximately 25% of CF patients infected with B. cepacia (2) and 40 to 60% of cases with Pseudomonas pneumonia, the clinical course is ultimately fatal (6). Novel antibiotics may help improve patient survival and reduce healthcare resource utilization. Both the infections caused by P. aeruginosa and those caused by B. cepacia are often treated with difficulty due to emergence of resistance and lack of effective antibiotics. Doripenem is a new carbapenem currently under clinical investigation for treatment of a variety of infections in humans. This study was designed to help assess the in vitro activities of doripenem against B. cepacia and P. aeruginosa isolates from CF patients and non-CF patients.

(A portion of this work was presented at the 104th General Meeting of the American Society for Microbiology, New Orleans, La., 24 to 27 May 2004.)

A total of 194 clinical strains from our frozen stock culture collection were tested. All stock isolates were stored at −70°C in defibrinated sheep blood. They included 160 stock isolates of P. aeruginosa (82 from CF patients and 78 from non-CF sources) and 34 isolates of B. cepacia. CF Pseudomonas isolates were collected from 1996 through 2001, and non-CF Pseudomonas isolates were collected from mid-1999 through mid-2002. B. cepacia isolates were all collected before 2001, and there are no records as to whether or not they were isolated from CF patients or other sources.

In addition, Escherichia coli ATCC 25922 and P. aeruginosa ATCC 27853 were tested for quality control purposes.

All strains were tested by the broth microdilution method recommended by the Clinical Laboratory Standards Institute (formerly NCCLS) (3). Doripenem was compared to a variety of antimicrobial compounds including imipenem, levofloxacin, piperacillin, ceftazidime, aztreonam, tobramycin, and cefepime.

The broth microdilution method outlined in the CLSI approved standard (M7-A6, 2003) (3) was used throughout. Each compound was dissolved and diluted according to CLSI specifications. Stock solutions of each agent were prepared immediately before use. Serial twofold dilutions of from 0.25 to 512 μg/ml were prepared for each drug except cefepime, for which dilutions of from 0.25 to 128 μg/ml were prepared. The 96-well microdilution plate susceptibility test panels were prepared at the Clinical Microbiology Institute and stored in a −70°C freezer until use. CLSI breakpoints were used for all comparative agents (4). No interpretive breakpoints were available for doripenem since there are no FDA- or CLSI-approved breakpoints at this time.

All MICs obtained when testing E. coli ATCC 25922 and P. aeruginosa ATCC 27853 were within the ranges recommended by the CLSI (M100-S13). The doripenem MICs for E. coli ATCC 25922 and P. aeruginosa ATCC 27853 were all ≤0.25 μg/ml (five replicates of each strain).

Table 1 summarizes the MICs (in micrograms per milliliter) of doripenem and comparative drugs against all bacterial isolates tested. Results for each group represented are arranged by geometric mean MIC in ascending order. Doripenem had the lowest geometric mean MIC of all antimicrobials tested against the entire collection of organisms. The doripenem MIC at which 50% of the isolates tested were inhibited (MIC₅₀) was 4- to 32-fold lower than those of the other comparative agents, and the doripenem MIC₉₀ was equal to that of levofloxacin (8 μg/ml) and 2- to 16-fold lower than those of the other agents under study.

Against 34 strains of B. cepacia, doripenem was 1 doubling dilution less active than levofloxacin (doripenem MIC₅₀ of 2 μg/ml versus 1 μg/ml for levofloxacin) but 2- to 32-fold more active than the other antimicrobials tested (Table 1). The MIC₉₀s for doripenem and levofloxacin were both 8 μg/ml (2- to 32-fold lower than those of the other antimicrobials).

As noted earlier, doripenem was the most active agent in terms of overall in vitro potency for all isolates tested, including P. aeruginosa strains isolated from both CF patients (MIC_50/90, 0.25/2 μg/ml) and non-CF patients (MIC_50/90, 0.25/1 μg/ml). The P. aeruginosa strains from CF patients tended to be slightly more resistant to all antimicrobials than were the non-CF isolates. The greatest differences in percent resistance between CF strains and non-CF strains were observed with aztreonam (22% versus 6.4%), piperacillin (13.4% versus 3.8%), and ceftazidime (13.4% versus 3.8%, respectively). The geometric mean doripenem MIC for CF isolates was 0.52 μg/ml versus 0.36 μg/ml for non-CF strains (Table 1). In both instances, the doripenem geometric mean MICs were substantially lower than those of any other antimicrobial compound tested.

Table 2 shows the MIC data for doripenem versus imipenem on 16 P. aeruginosa strains that were either resistant or intermediate to imipenem. Eleven of these strains were from CF patients, and five were from non-CF patients. Eight of the 16 strains (50%) had doripenem MICs that were 2- to 16-fold lower than the imipenem MICs for these strains.

In conclusion, the in vitro activity of doripenem against clinical strains of B. cepacia and P. aeruginosa was greater than that of all other antimicrobials tested. Doripenem was active against most B. cepacia strains (MIC₅₀, 2 μg/ml, and MIC₉₀, 8 μg/ml). Tobramycin was the least effective among those tested, with 76% of isolates being resistant. Strains of P. aeruginosa isolated from CF patients tended to be slightly more resistant to all antimicrobials than were strains recovered from non-CF patients. Of the 16 strains of P. aeruginosa that were resistant or intermediate to imipenem, 50% had doripemen MICs that were 2- to 16-fold lower than those of imipenem. If successfully developed, doripenem may be a useful agent against P. aeruginosa and B. cepacia infections.

• American Society for Microbiology