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Ceftaroline, the active metabolite of the prodrug ceftaroline-fosamil, is an advanced-generation cephalosporin with activity against methicillin-resistant Staphylococcus aureus (MRSA). This investigation provides in vitro susceptibility data for ceftaroline against 1,971 S. aureus isolates collected in 2012 from seven countries (26 centers) in the Asia-Pacific region as part of the Assessing Worldwide Antimicrobial Resistance and Evaluation (AWARE) program. Broth microdilution as recommended by the CLSI was used to determine susceptibility. In all, 62% of the isolates studied were MRSA, and the ceftaroline MIC90 for all S. aureus isolates was 2 μg/ml (interpretive criteria: susceptible, ≤1 μg/ml). The overall ceftaroline susceptibility rate for S. aureus was 86.9%, with 100% of methicillin-sensitive S. aureus isolates and 78.8% of MRSA isolates susceptible to this agent. The highest percentages of ceftaroline-nonsusceptible MRSA isolates came from China (47.6%), all of which showed intermediate susceptibility, and Thailand (37.1%), where over half (52.8%) of isolates were resistant to ceftaroline (MIC, 4 μg/ml). Thirty-eight ceftaroline-nonsusceptible isolates (MIC values of 2 to 4 μg/ml) were selected for molecular characterization. Among the isolates analyzed, sequence type 5 (ST-5) was the most common sequence type encountered; however, all isolates analyzed from Thailand were ST-228. Penicillin-binding protein 2a (PBP2a) substitution patterns varied by country, but all isolates from Thailand had the Glu239Lys substitution, and 12 of these also carried an additional Glu447Lys substitution. Ceftaroline-fosamil is a useful addition to the antimicrobial agents that can be used to treat S. aureus infections. However, with the capability of this species to develop resistance to new agents, it is important to recognize and monitor regional differences in trends as they emerge.
The challenges of multidrug-resistant (MDR) Staphylococcus aureus are problematic in many geographic regions. Asia has one of the highest prevalence rates of methicillin-resistant S. aureus (MRSA), including MDR isolates, in the world (1). The relative paucity of therapeutic options has led to significant morbidity and mortality in this region (2). There are antimicrobial agents available for treating infections caused by MDR MRSA infections (e.g., daptomycin, linezolid). However, safety concerns have been raised for some of these compounds, and resistance against these agents has been described, albeit rarely (3,–6). The recently approved agent ceftaroline-fosamil has a good safety profile and is approved by the U.S. Food and Drug Administration (in 2010) and the European Medicines Agency (in 2012) for the treatment of infections caused by S. aureus (7,–10). Current clinical indications include community-acquired pneumonia (CAP) and complicated skin and skin structure infections (cSSSI), with MRSA included among the target pathogens for cSSSI (11,–20). In addition to inhibiting essential penicillin-binding proteins (PBPs), ceftaroline also has a high affinity for PBP2a, which confers resistance to an array of other beta-lactams characteristic of MRSA (21). Ceftaroline's special attribute of having activity against MRSA is key to this drug's contribution as a therapeutic option for treating serious infections caused by S. aureus (22,–26).
With the clinical availability of ceftaroline in some Asia-Pacific countries, it is important to establish a baseline of in vitro activity from the outset and to use this baseline to evaluate trends in resistance should they occur. This study evaluated the in vitro activity of ceftaroline and comparator agents against S. aureus recovered from patients hospitalized in Asia-Pacific countries. In addition, molecular analysis of a subset of ceftaroline-nonsusceptible isolates was performed to determine their PBP2a substitutions and the sequence types associated with this group of isolates.
A total of 1,971 clinical isolates of S. aureus were collected in 2012 from 26 medical centers in Asia-Pacific as part of the Assessing Worldwide Antimicrobial Resistance and Evaluation (AWARE) program. Countries and the number of medical centers included Australia (3), China (9), Japan (3), Philippines (3), South Korea (2), Taiwan (3), and Thailand (3). The majority of isolates were collected from skin and skin structure (1,004 isolates) and respiratory tract (755) specimens. The remainder were from other specimen sources. MICs were determined and interpreted according to Clinical and Laboratory Standards Institute (CLSI) guidelines (27, 28). Quality-control isolates were tested concurrently, and all values were within recommended CLSI quality-control ranges (28).
A random sample of 38 isolates with a ceftaroline MIC value of 2 or 4 μg/ml, i.e., intermediate or resistant, respectively, was selected from China (9 isolates), Japan (6), Philippines (1), South Korea (4), Taiwan (4), and Thailand (14) for molecular characterization. Whole-genome sequencing was performed to enable determination of sequence types (ST), staphylococcal cassette chromosome mec element (SCCmec) types, and PBP2a substitutions as previously described (29). Seven reference isolates with ceftaroline MIC values of 1 or 2 μg/ml were included for comparisons and quality assurance.
The overall profile of the S. aureus population analysis based on ceftaroline and comparator activities is provided in Table 1. Of the 1,971 isolates 1,220 (61.9%) were MRSA. Resistance to erythromycin, clindamycin, and levofloxacin was 51.9%, 35.7%, and 41.2%, respectively. Resistance to any of the other drugs tested was uncommon. Against this total population of Asian isolates, the ceftaroline MIC90 value was 2 μg/ml. Ceftaroline maintained an overall susceptibility level of 86.9% against these isolates. All ceftaroline-nonsusceptible isolates were MRSA, and the 13.1% nonsusceptible isolates were comprised of 11.6% with MIC values in the intermediate category (MIC of 2 μg/ml) and 1.5% with a ceftaroline MIC of 4 μg/ml, which is the resistant breakpoint.
The cumulative frequency MIC distributions of ceftaroline according to methicillin susceptibility status and country of origin are shown in Table 2. For MSSA isolates, the ceftaroline MIC50 and MIC90 values (0.25 μg/ml) were identical across all countries. By-country variability for ceftaroline MIC90s was observed among the MRSA isolates and ranged from 1 μg/ml (Australia) to 4 μg/ml (Thailand). Of the 30 ceftaroline-resistant isolates (all with an MIC of 4 μg/ml), 28 (93.3%) were from Thailand; the remaining 2 were from Japan and South Korea. Within Thailand, the 28 resistant isolates came from two of the three study sites (one site collected 12 resistant isolates, and the other collected 16 isolates). It was not determined if these 28 isolates were clonally related. The highest percentage of ceftaroline-nonsusceptible isolates (MIC of 2 μg/ml) came from China (162 of 340 [47.6%]). In contrast, 100% of the isolates from Australia were susceptible to ceftaroline.
Analyses of ceftaroline activity against S. aureus samples isolated from the source of infection with the approved indications demonstrated that the overall ceftaroline nonsusceptibility percentage was higher among MRSA isolates from respiratory specimens (32.2%) than those from skin and skin structure specimens (9.1%) (data not shown). Among respiratory specimen isolates, the highest rates of nonsusceptibility were 48.5%, 38.8%, and 36.4% among isolates from China, Thailand, and South Korea, respectively. This same hierarchy of nonsusceptibility was observed among skin and skin structure specimens.
Among the 38 ceftaroline-nonsusceptible isolates selected for molecular characterization, the most common sequence types were ST-5, ST-238, and ST-239 (Table 3). All ceftaroline-nonsusceptible isolates (MICs of 2 and 4 μg/ml) selected for analysis from Thailand were ST-228. Six isolates from three countries had wild-type sequence PBP2a substitutions (defined as that found in S. aureus USA300). The remainder of the isolates analyzed had PBP2a substitutions that varied considerably among countries of origin. All of the molecularly characterized isolates from Thailand (11 isolates inhibited by 4 μg/ml ceftaroline and 3 isolates inhibited by 2 μg/ml) had a Glu239Lys substitution, and all but 2 isolates also had an additional Glu447Lys substitution.
In this study, ceftaroline exhibited potent in vitro activity against clinical isolates of S. aureus from Asia-Pacific countries, with MRSA isolates from the seven countries that were monitored showing varied activity. The overall ceftaroline susceptibility percentage of this 2012 Asia-Pacific collection was 86.9%. This susceptibility rate was slightly lower than that observed in a previous surveillance study done in this region, in which susceptibility was 93.4% (30). However, differences in the medical centers and countries that provided isolates for analysis in these two separate surveillance studies preclude the ability to evaluate any changes in ceftaroline susceptibility trends over time. In comparison to other geographic areas studied, the Asia-Pacific region ceftaroline susceptibility rate found in this study was slightly higher than the 83.6% susceptibility found among isolates collected in Latin America during 2011 (31, 32). In contrast, the 98% ceftaroline susceptibility rate found in previous U.S. surveillance studies, in which the MRSA rate was approximately 50%, was substantially higher than the susceptibility rate found in the current Asia-Pacific study (32,–34). The current Asia-Pacific S. aureus data also demonstrated ceftaroline activity against MRSA (MIC50, 1 μg/ml) similar to that observed in Europe, Turkey, and Israel (35). Based on these findings, it is evident that the level of in vitro activity of ceftaroline against MRSA can vary notably from one geographic region to another.
However, the interpretation of relative ceftaroline activity across regions, or patient types, or any other parameter based on percent susceptibility alone should be viewed with caution. As shown in Table 2, the MIC distributions were very similar across countries, and the nonsusceptible populations were entirely comprised of strains with ceftaroline MICs only 1 or 2 dilutions above the susceptible breakpoint of 1 μg/ml. These slight differences in MIC distributions more likely reflect differences in PBP2a substitutions associated with certain lineages than they do the acquisition of any overt resistance mechanism. Several recent studies in which the documented mechanism among isolates with higher ceftaroline MICs involved substitutions in PBP2a support this likelihood (21, 36, 37, 38). The most common substitutions appear to be located in the non-penicillin-binding domain that produces low-level (intermediate) resistance, and molecular analysis for selected isolates demonstrated that most, but not all, isolates with ceftaroline MICs of 2 μg/ml carried such substitutions (37). This association was confirmed in the current study. Additional substitutions in the penicillin-binding region have resulted in MIC values of >2 μg/ml, which was observed for isolates from Japan, South Korea, Thailand, Spain, Switzerland, and Greece (36,–38). In the present study, the majority of isolates with an MIC value of 4 μg/ml were from Thailand, and all of those that were analyzed had a Glu447Lys substitution in the penicillin-binding region. This substitution was not seen in any of the other ceftaroline-nonsusceptible isolates characterized in this study. However, the association of these substitutions with higher ceftaroline MICs has been described previously (21, 22, 36). These findings suggest a clonal relationship among the isolates from Thailand, with MICs of 4 μg/ml, but further studies would be needed to establish this relationship. It is of interest that in the countries with the highest ceftaroline nonsusceptibility rates (China and Thailand), ceftaroline had not been approved at the time of the study. While at least one in vitro study suggested that certain beta-lactams (e.g., ceftobiprole) could select for mutants with higher ceftaroline MICs, there are no reports of this happening in the clinical environment (22). In any case, without the availability of usage data and with insufficient year-to-year data for each region, it is difficult to conclude the reasons for differences in regional ceftaroline susceptibility rates and whether there have been any changes over time. The different susceptibility rates may very well be a reflection of the most common PBP2a substitution “clones” most prevalent in a given area pre-ceftaroline usage. Certain substitutions, such as those found in isolates from Thailand, would result in reduced affinity for ceftaroline and thus higher MICs relative to other PBP2a substitutions that have a lesser impact on ceftaroline activity. Ongoing surveillance and further molecular characterization of such isolates will be important to help understand what trends may or may not be emerging.
In consideration of the underlying mechanisms behind higher ceftaroline MICs, the clustering and proximity of ceftaroline MICs between susceptible and nonsusceptible populations may be more a reflection of where the interpretive breakpoints are set than of any emerging resistance (38). Discerning the true clinical relevance between a strain that is susceptible to ceftaroline with an MIC of 1 μg/ml and one that is intermediate (MIC, 2 μg/ml) or resistant (MIC, 4 μg/ml) could be quite difficult. Nonetheless, the in vitro data provided in this study indicate that ceftaroline-fosamil provides a strong treatment option for infections caused by S. aureus, including MRSA. However, because S. aureus has a proven propensity to develop resistance to many antimicrobial agents, including beta-lactams, ongoing regional and global surveillance initiatives are needed to monitor any changes in susceptibility trends over the time that ceftaroline clinical use expands.
We gratefully acknowledge the contributions of the clinical trial investigators, laboratory personnel, and all members of the AWARE program.
This study at International Health Management Associates, Inc. (IHMA) was supported by AstraZeneca Pharmaceuticals L.P., which also included compensation fees for services in relation to preparing the manuscript. None of the IHMA authors (D.J.B., D.F.S., D.J.H., and S.K.B.) has a personal financial interest in the sponsor of this paper (AstraZeneca Pharmaceuticals).
All authors provided analysis input and have read and approved the final manuscript.
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.