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Antimicrobial Agents and Chemotherapy, May 2009, p. 2171-2175, Vol. 53, No. 5
0066-4804/09/$08.00+0     doi:10.1128/AAC.00129-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Potency and Bactericidal Activity of Iclaprim against Recent Clinical Gram-Positive Isolates

Helio S. Sader,^1,2* Thomas R. Fritsche,^1, and Ronald N. Jones^1,3

JMI Laboratories, North Liberty, Iowa,¹ Universidade Federal de São Paulo, São Paulo, Brazil,² Tufts University School of Medicine, Boston, Massachusetts³

Received 28 January 2009/ Returned for modification 18 February 2009/ Accepted 5 March 2009

Top ABSTRACT INTRODUCTION REFERENCES

 
The in vitro activity of iclaprim, a novel diaminopyrimidine derivative, was evaluated against 5,937 recent gram-positive clinical isolates collected in the United States and Europe. Iclaprim demonstrated potent activity against Staphylococcus aureus (including methicillin-resistant S. aureus [MRSA]), beta-hemolytic Streptococcus spp., and Enterococcus faecalis strains tested. In addition, iclaprim exhibited bactericidal activity against all S. aureus strains tested, including MRSA.

Top ABSTRACT INTRODUCTION REFERENCES

 
Staphylococcus aureus strains, including methicillin-resistant S. aureus (MRSA) strains, beta-hemolytic streptococci (most commonly Streptococcus pyogenes and Streptococcus agalactiae), and Enterococcus spp. are the principal gram-positive pathogens responsible for complicated skin and skin-structure infections (cSSSI). The increasing prevalence of MRSA in hospital and community settings (5, 20), as well as the potential for resistance or emergence of resistance during therapy to drugs such as vancomycin (1, 19), linezolid (9), and daptomycin (12), underscores the urgent need for additional well-differentiated therapeutic agents (17). Iclaprim is a new-generation diaminopyrimidine that potently and selectively inhibits bacterial dihydrofolate reductase (DHFR) (11, 18, 21). It was designed by a rational drug design approach based on structural information available on trimethoprim (TMP), the best known compound of the diaminopyrimidine class which, either alone or in its synergistic 1:19 combination with sulfamethoxazole, has been widely used in medical practice for over four decades. Iclaprim has been shown to have very potent activity against gram-positive bacteria that are susceptible to TMP. Iclaprim inhibits bacterial DHFR in a similar manner to TMP but possesses higher affinity due to increased hydrophilic interactions between iclaprim and DHFR (15). Thus, iclaprim may retain activity against some TMP-resistant isolates, but it would be lower than that against TMP-susceptible strains.

Unlike TMP, the spectrum of activity of iclaprim is more focused against gram-positive pathogens, including MRSA strains. Iclaprim has been shown to be rapidly bactericidal against these strains and to possess a low potential for resistance development when used on its own, without the synergistic combination of a sulfonamide agent (8). For these reasons, iclaprim is under development as a monotherapy, and an intravenous formulation of iclaprim has completed two phase 3 trials for the treatment of cSSSI caused by gram-positive pathogens (11, 16). In addition, an oral formulation of iclaprim as a step-down therapy for patients with cSSSI is ongoing (16).

We evaluated the potency and bactericidal activity of iclaprim against a large collection of contemporary gram-positive isolates from hospitalized patients in the United States (26 centers), the European Union (22 centers), Israel (1 center), and Turkey (1 center) from 2004 to 2006. In total, 5,937 clinical isolates representative of predominant gram-positive pathogens in cSSSI were tested. All organisms were collected from skin and soft tissue, bloodstream, and respiratory clinical specimens. The numbers of individual strains for each species tested are shown in Tables 1 and 2. Susceptibility testing by broth microdilution, including the appropriate quality controls, was performed according to the documents CLSI M7-A7 (2) and CLSI M100-S18 (3). Antimicrobial agents tested included iclaprim (Arpida Ltd., Reinach, Switzerland) and comparators. Minimum bactericidal concentration (MBC) values were determined for 101 randomly selected strains. MBC experiments were performed by plating the broth from wells from at least five log₂ dilutions from the MIC (13, 14) onto growth medium. The lowest concentration of the antimicrobial that killed 99.9% of the starting test inoculum was defined as the MBC endpoint. Cidality was defined as an MBC/MIC ratio of 4. In all cases, the thymidine content of the test medium was assessed to ensure that no artifactual inhibition of iclaprim activity occurred (2, 6).

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TABLE 1. Activity of iclaprim and comparator agents tested against Staphylococcus aureus isolates from the United States and the European Union

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TABLE 2. Iclaprim, trimethoprim, and trimethoprim-sulfamethoxazole MIC distributions for MSSA, MRSA, beta-hemolytic streptococcal strains, and enterococci evaluated by reference methods

Numbers of strains and activity summaries of all drugs tested are shown in Table 1. Whereas the vast majority of methicillin-sensitive S. aureus (MSSA; 1,513 strains) strains were susceptible to most of the compounds tested in this study, a large proportion of MRSA strains were resistant to erythromycin (84.3%), clindamycin (47.0%), and both quinolones tested (83.1% and 82.4% for ciprofloxacin and levofloxacin, respectively) (Table 1). Iclaprim was highly active against both MSSA and MRSA (MIC₅₀/MIC₉₀, 0.06/0.12 µg/ml for both) (Tables 1 and 2), with 98.7% of MSSA strains and 94.6% of MRSA strains being inhibited at a MIC of 2 µg/ml (Table 2). For group A streptococci (GAS), resistance rates to erythromycin, clindamycin, and tetracycline were higher in the European Union isolates (26.1%, 5.9%, and 14.2%, respectively) than those isolated in the United States (6.0%, 0.7%, and 5.0%, respectively; data not shown). Iclaprim inhibited 100% of GAS at an MIC of 0.12 µg/ml and an MIC₅₀/MIC₉₀ of 0.015/0.03 µg/ml (Tables 1 and 2). After penicillin (MIC₅₀/MIC₉₀, 0.06/0.06 µg/ml) (Table 1), iclaprim was the most active among all the antibacterial agents tested. Unlike GAS, group B streptococci (GBS) from the United States showed higher rates of resistance to erythromycin (39.2% versus 12.7%), clindamycin (14.7% versus 5.9%), and tetracycline (82.4% versus 73.5%) than those isolated in the European Union (data not shown). Iclaprim was also highly potent against GBS (MIC₅₀/MIC₉₀, 0.12/0.25 µg/ml; MIC₁₀₀, 0.5 µg/ml) (Tables 1 and 2), and its activity was not affected by the organisms' resistance to erythromycin, clindamycin, or tetracycline. As expected, E. faecalis isolates were generally more susceptible than Enterococcus faecium isolates to the drugs tested (Tables 1 and 2), and resistance rates for E. faecalis did not vary much between the United States and European Union isolates (data not shown). Resistance rates were generally high among E. faecium isolates and differed significantly between those from the United States and European Union. Most notably, resistance to vancomycin was 70.8% among isolates in the United States compared to 14.8% among non-United States isolates. Iclaprim demonstrated the typical bimodal activity of its class against enterococci (Table 2). However, it showed high potency (MIC₅₀/MIC₉₀, 0.015/4 µg/ml) (Table 1) against E. faecalis, with approximately 72% and 97% of the isolates being inhibited at MICs of 2 and 4 µg/ml, respectively (Table 2). Iclaprim was also active (MIC₅₀/MIC₉₀, 2/>8 µg/ml) against E. faecium (Table 1), and its activity was not affected by resistance to vancomycin; approximately 58% and 75% of the isolates were inhibited at iclaprim MICs of 2 and 4 µg/ml, respectively (Table 2).

Iclaprim was bactericidal against the S. aureus strains tested (Table 3). Against MRSA, iclaprim demonstrated MBC/MIC ratios of 4 for 100% of strains compared to 60% of strains for vancomycin. Against MSSA, iclaprim and vancomycin demonstrated MBC/MIC ratios of 4 for 86% of strains (Table 3). Iclaprim exhibited bactericidal activity against 45.0% of GAS and 65.0% of GBS, compared to ratios of 32 for vancomycin for most GAS and 100.0% of GBS tested (Table 3). Against enterococci, MBC/MIC ratios of 4 were seen in two out of five E. faecium strains and 3 out of 15 E. faecalis strains for iclaprim, whereas all the enterococcal strains tested exhibited MBC/MIC ratios of 32 for vancomycin (Table 3). The bactericidal activity of iclaprim when tested against staphylococci corroborates the results of previous studies (8, 10). In contrast, the fact that iclaprim demonstrated higher bactericidal activity against staphylococci than against streptococci and enterococci has not been previously reported and warrants further evaluation.

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TABLE 3. MBC/MIC ratios for iclaprim and vancomycin against 101 organisms

Results from this study support previously reported nonclinical susceptibility data (4, 7, 21) and demonstrate the potent bactericidal activity of iclaprim in vitro against both MSSA and MRSA. This finding, together with the high activity of iclaprim against beta-hemolytic streptococci and enterococcal species, confirms iclaprim as an important addition to the existing panel of therapies for the treatment of cSSSI caused by gram-positive organisms, including MRSA.

 
This study was funded by Arpida AG (Reinach, Switzerland).

 
* Corresponding author. Mailing address: JMI Laboratories, 345 Beaver Kreek Centre, Suite A, North Liberty, IA 52317. Phone: (319) 665-3370. Fax: (319) 665-3371. E-mail: helio-sader{at}jmilabs.com

Published ahead of print on 16 March 2009.

Present address: Marshfield Clinic, 1000 N. Oak Ave., Marshfield, WI 54449.

Top ABSTRACT INTRODUCTION REFERENCES

 

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Antimicrobial Agents and Chemotherapy, May 2009, p. 2171-2175, Vol. 53, No. 5
0066-4804/09/$08.00+0     doi:10.1128/AAC.00129-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

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