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Antimicrobial Agents and Chemotherapy, July 2006, p. 2583-2586, Vol. 50, No. 7
0066-4804/06/$08.00+0     doi:10.1128/AAC.01432-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Activity of Retapamulin (SB-275833), a Novel Pleuromutilin, against Selected Resistant Gram-Positive Cocci

JMI Laboratories, North Liberty, Iowa 52317,¹ Tufts University School of Medicine, Boston, Massachusetts 02111²

Received 7 November 2005/ Returned for modification 25 January 2006/ Accepted 12 April 2006

	ABSTRACT

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Retapamulin (SB-275833), the first pleuromutilin to be developed for human topical use, was tested against a selected population of staphylococci and ß-hemolytic streptococci. The MIC₉₀ results for retapamulin were 0.12 µg/ml for Staphylococcus aureus and 0.03 µg/ml for Streptococcus pyogenes; no cross-resistance was observed for organism subsets resistant to oxacillin, erythromycin, or mupirocin.

	TEXT

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A class of antimicrobial agents that has remained largely undeveloped for human clinical use is the pleuromutilins (5). These antimicrobials are derivatives of the naturally occurring pleuromutilin produced by Pleurotus mutilus, an edible mushroom. The pleuromutilin class has a unique mode of action, which involves inhibition of bacterial protein synthesis by binding to the prokaryotic ribosome (5, 14). Agents in this class have been described as binding to domain V of 23S rRNA, thereby blocking peptide formation directly by interfering with substrate binding (13). This binding site is unique compared to other ribosomally targeted inhibitors. Pleuromutilins have no target-specific cross-resistance to other antibacterials, which makes them appealing for human clinical development (1). However, mutations in the genes encoding 23S rRNA have led to reduced susceptibility to tiamulin (11). In veterinary practice, tiamulin and valnemulin (two semisynthetic pleuromutilin analogs) are used for the control and treatment of serious infections in swine (1, 5). Tiamulin has exceptional activity (MIC, 1 µg/ml) against anaerobic bacteria, Mycoplasma spp., and intestinal spirochetes (1, 5, 6). Tiamulin has also shown potent activity against some staphylococci from human sources (MIC₅₀, 0.5 µg/ml) (5).

Retapamulin (formerly called SB-275833) (Fig. 1) is a novel pleuromutilin antimicrobial being developed for topical treatment of skin infections. According to a recent global surveillance report (4), the most common bacteria found in skin and soft tissue infections (SSTI) include the gram-positive organisms Staphylococcus aureus (55.2%), ß-hemolytic streptococci (5.0%), and coagulase-negative staphylococci (CoNS) (4.9%). Escalating numbers of these bacteria causing SSTI have become resistant to the leading topical antimicrobials used in human clinical practice. Mupirocin, a topical agent, has shown resistance rates ranging from 1.3% in Latin America to 8.7% in Europe among S. aureus isolates (3). Mupirocin resistance has increased in CoNS, ranging from 12.7% in Europe to 38.8% in the United States (3). Deshpande et al. showed that mupirocin resistance is higher among oxacillin-resistant staphylococcal strains than oxacillin-susceptible organisms (S. aureus, 4.6 to 17.8%; CoNS, 14.0 to 43.1%) (3).

View larger version (7K): [in this window] [in a new window]   FIG. 1. Chemical structure of retapamulin, or mutilin 14-(exo-8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-sulfanyl-acetate.

A total of 604 recent (2002 to the present) clinical isolates were identified and confirmed by a reference laboratory (JMI Laboratories, North Liberty, IA), using colonial characteristics on standard media, rapid tests (catalase test, latex agglutination kits, coagulase test, pyrrolidonyl arylamidase, etc.), the use of an automated identification system (Vitek; bioMerieux, Hazelwood, MO), and other tests, as necessary. The list of tested isolates included S. aureus (281 strains), CoNS (232 strains), and Streptococcus pyogenes (91 strains). The results were analyzed according to susceptibility pattern subgroups, with the number of strains varying from 56 to 212 for each group.

Determination of MICs was performed by reference M7-A6 methods in the broth microdilution format (8). Antimicrobials used for comparison included tiamulin (veterinary pleuromutilin), mupirocin (topical agent used for skin infections) (10), penicillin (active against gram-positive organisms), oxacillin (active against gram-positive organisms), erythromycin (topical, oral, and parenteral agent used for SSTI), clindamycin (topical, oral, and parenteral agent used for SSTI), ofloxacin (oral agent indicated for treatment of SSTI), gentamicin (topical and parenteral agent indicated for treatment of SSTI), cephalothin (a cephalosporin used as a drug class susceptibility comparator), bacitracin (topical agent used for SSTI as a component of triple antibiotic ointment [polymyxin B, neomycin, and bacitracin]), and linezolid (oral and parenteral oxazolidinone indicated for treatment of SSTI caused by S. aureus or Streptococcus pyogenes). The highest MIC used for each dilution series for the topical agents represents a 1:100 dilution of the approved topical formation. Retapamulin is insoluble in water but is soluble in dimethyl sulfoxide and methanol. Broth microdilution panels were prepared by TREK Diagnostics (Cleveland, OH) with cation-adjusted Mueller-Hinton medium and were used to test the Staphylococcus spp. For processing of Streptococcus spp., lysed horse blood (2 to 5%) was added to the medium (8). Quality control (QC) testing was performed based on a prior multilaboratory study of retapamulin (12). The following QC organisms and target retapamulin ranges were tested concurrently: S. aureus ATCC 29213 (QC range, 0.06 to 0.25 µg/ml) and Streptococcus pneumoniae ATCC 49619 (QC range, 0.06 to 0.5 µg/ml) (12). QC ranges for comparison agents were those recommended in Clinical and Laboratory Standards Institute documents (2, 7, 9, 12). Susceptibility breakpoint criteria for retapamulin or other pleuromutilins have yet to be established for these monitored species.

Retapamulin activity was analyzed with three organism groups and eight subsets of susceptibility phenotype (Table 1). S. aureus subset results for retapamulin showed consistent MIC_50/90 values of 0.06/0.12 µg/ml. Mupirocin-, oxacillin-, and erythromycin-resistant strains of S. aureus did not have differing retapamulin MIC_50/90 results. Retapamulin had the lowest MIC₅₀ (0.12 µg/ml) for mupirocin-resistant S. aureus among the tested comparator agents. Other topical agents performed poorly against resistant S. aureus, with the following MIC₉₀ values: bacitracin, >4 µg/ml; clindamycin, >100 µg/ml; erythromycin, >200 µg/ml; and mupirocin, >256 µg/ml. The only other pleuromutilin tested (tiamulin) showed MIC_50/90 results of 0.5/1 µg/ml for all S. aureus strains, which is an eightfold increase in MIC results compared to the retapamulin MIC_50/90 values.

One subset of erythromycin-resistant ß-hemolytic streptococci (S. pyogenes) was evaluated and displayed retapamulin MIC_50/90 values of 0.03/0.03 µg/ml. Retapamulin showed excellent activity against these isolates, with only two requiring a MIC of 0.06 µg/ml. Against S. pyogenes, retapamulin was at least fourfold more potent than it was versus S. aureus.

Overall, the retapamulin MIC population distribution showed a mode at 0.06 µg/ml and MIC_50/90 results of 0.06/0.12 µg/ml. Other tested agents with potent activities were tiamulin (MIC₉₀ range, 0.25 to 1 µg/ml), linezolid (MIC₉₀ range, 1 to 2 µg/ml; 100.0% of organisms were susceptible), and gentamicin (MIC₉₀ range, 0.25 to 30 µg/ml; 72.6 to 100.0% of staphylococci were susceptible). Tiamulin had its highest MIC at 2 µg/ml, observed among staphylococcal isolates. Mupirocin resistance rates were elevated among erythromycin-resistant and oxacillin-resistant S. aureus subsets (43.9 and 45.9%, respectively) and for the oxacillin-resistant CoNS subset (65.3%).

Retapamulin demonstrated potent in vitro activity (MIC_50/90, 0.06/0.12 µg/ml) and a broad spectrum against several antimicrobial-resistant organism subsets of S. aureus, CoNS, and ß-hemolytic streptococci. Resistance to erythromycin, mupirocin, or oxacillin did not adversely affect the MICs of the pleuromutilins. Compared to tiamulin, retapamulin appears to be uniformly eightfold more potent. While this antimicrobial class has not been developed previously for human application, the use of related antimicrobial agents in veterinary medicine has been well established (6, 9). Given the declining efficacy of contemporary approved topical agents (mupirocin), the development of novel agents lacking cross-resistance to other antimicrobial classes appears warranted.

	ACKNOWLEDGMENTS

 
This study was sponsored by GlaxoSmithKline.

	FOOTNOTES

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

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