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Journal of Clinical Microbiology, August 2008, p. 2620-2629, Vol. 46, No. 8
0095-1137/08/$08.00+0 doi:10.1128/JCM.00566-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Correlation of MIC with Outcome for Candida Species Tested against Caspofungin, Anidulafungin, and Micafungin: Analysis and Proposal for Interpretive MIC Breakpoints
M. A. Pfaller,1*
D. J. Diekema,1
L. Ostrosky-Zeichner,2
J. H. Rex,3
B. D. Alexander,4
D. Andes,5
S. D. Brown,6
V. Chaturvedi,7
M. A. Ghannoum,8
C. C. Knapp,9
D. J. Sheehan,10 and
T. J. Walsh11
University of Iowa, Iowa City, Iowa,1
University of Texas, Houston, Texas,2
AstraZenica, Macclesfield, Cheshire, United Kingdom,3
Duke University, Durham, North Carolina,4
University of Wisconsin, Madison, Wisconsin,5
The Clinical Microbiology Institute, Wilsonville, Oregon,6
New York State Department of Health, Albany, New York,7
Case Western Reserve University, Cleveland, Ohio,8
Trek Diagnostic Systems, Cleveland, Ohio,9
Pfizer, Inc., New York, New York,10
National Cancer Institute, Bethesda, Maryland,11
Received 25 March 2008/
Returned for modification 12 May 2008/
Accepted 15 June 2008

ABSTRACT
The CLSI Antifungal Subcommittee followed the M23-A2 "blueprint"
to develop interpretive MIC breakpoints for anidulafungin, caspofungin,
and micafungin against
Candida species. MICs of

2 µg/ml
for all three echinocandins encompass 98.8 to 100% of all clinical
isolates of
Candida spp. without bisecting any species group
and represent a concentration that is easily maintained throughout
the dosing period. Data from phase III clinical trials demonstrate
that the standard dosing regimens for each of these agents may
be used to treat infections due to
Candida spp. for which MICs
are as high as 2 µg/ml. An MIC predictive of resistance
to these agents cannot be defined based on the data from clinical
trials due to the paucity of isolates for which MICs exceed
2 µg/ml. The clinical data set included only three isolates
from patients treated with an echinocandin (caspofungin) for
which the MICs were >2 µg/ml (two
C. parapsilosis isolates
at 4 µg/ml and one
C. rugosa isolate at 8 µg/ml).
Based on these data, the CLSI subcommittee has decided to recommend
a "susceptible only" breakpoint MIC of

2 µg/ml due to
the lack of echinocandin resistance in the population of
Candida isolates thus far. Isolates for which MICs exceed 2 µg/ml
should be designated "nonsusceptible" (NS). For strains yielding
results suggestive of an NS category, the organism identification
and antimicrobial-susceptibility test results should be confirmed.
Subsequently, the isolates should be submitted to a reference
laboratory that will confirm the results by using a CLSI reference
dilution method.

INTRODUCTION
Members of the echinocandin class of antifungal agents act by
inhibition of the synthesis of 1,3-β-
D-glucan in the fungal
cell wall (
8,
16). All three available echinocandins—anidulafungin,
caspofungin, and micafungin—possess fungicidal activity
against most species of
Candida, including polyene- and azole-resistant
species (
9,
14,
33,
48,
59,
65). All have been approved by the
U.S. Food and Drug Administration (FDA) for the treatment of
esophageal candidiasis and invasive candidiasis, including candidemia
(
15,
30,
35,
43,
45,
56; Mycamine [micafungin] package insert,
2005, Astellas Pharma US, Deerfield, IL; Cancidas [caspofungin]
package insert, 2001, Merck & Co., Whitehouse Station, NJ;
and Eraxis [anidulafungin] package insert, 2006, Pfizer, Inc.,
New York, NY). These agents all provide excellent clinical efficacy
coupled with low toxicity for the treatment of serious candidal
infections.
Collaborative studies conducted by the Clinical and Laboratory Standards Institute (CLSI) Antifungal Subcommittee have resulted in a consensus recommendation for a standardized method for in vitro susceptibility testing of the echinocandins against Candida spp. (11, 41, 49). The broth microdilution (BMD) method employs RPMI 1640 broth medium, incubation at 35°C for 24 h, and an MIC endpoint criterion of prominent reduction in growth (
50% inhibition relative to growth of control). This standardized method provides reliable and reproducible MIC results with good separation of the "wild-type" MIC distribution from isolates of Candida with mutations in the FKS1 gene for which reduced susceptibility to echinocandins has been documented (7, 41, 46, 47, 49).
There is broad experience with testing the echinocandins by using the CLSI BMD method (24, 42, 50, 51, 55, 58-60). In addition to standardized testing methods, the CLSI Antifungal Subcommittee has approved quality control limits for BMD test methods with all three echinocandins (11, 12).
Previously, the CLSI Antifungal Subcommittee used the accumulated microbiological and clinical data to provide a blueprint for the establishment of interpretive breakpoints for antifungal susceptibility testing of fluconazole (52) and voriconazole (53) against species of Candida. The analytical model followed that outlined for all types of antimicrobial susceptibility testing in CLSI document M23-A2 (37). During their June 2007 meeting, the CLSI Antifungal Subcommittee utilized this approach to propose interpretive breakpoints for MIC testing of anidulafungin, caspofungin, and micafungin against Candida species. These analyses are summarized below.

MATERIALS AND METHODS
Organisms.
All isolates of
Candida used to generate the MIC distribution
profiles and cross-resistance studies were obtained from the
ARTEMIS Global Antifungal Surveillance Program (
55). A total
of 5,346 isolates of
Candida (15 different species from 91 study
centers) collected from blood and normally sterile body sites
from 2001 through 2006 were sent to the ARTEMIS central reference
laboratory (University of Iowa) for identification and susceptibility
testing by the CLSI BMD method (
11).
In addition to the isolates noted above, all Candida spp. isolated at baseline from subjects with definite infections in phase II and III primary studies of caspofungin (28), anidulafungin (56), and micafungin (30, 45) were identified and tested by using the CLSI BMD MIC method in reference laboratories located at Merck (Rahway, NJ), International Health Management Associates (Schaumberg, IL), and the University of Texas (Houston), respectively. A total of 406 isolates were obtained from caspofungin esophageal and invasive candidiasis clinical trials (28), while 135 isolates were obtained from the anidulafungin-versus-fluconazole candidemia study (56), and 410 isolates were obtained from two micafungin candidemia studies (30, 45). These isolates represented the baseline isolates from subjects eligible for this analysis.
Antifungal susceptibility testing.
All isolates were tested in accordance with the standards in CLSI document M27-A3 (10) using RPMI 1640 medium, an inoculum of from 0.5 x 103 to 2.5 x 103 cells/ml, and incubation at 35°C. MICs were determined visually after 24 h of incubation as the lowest concentration of drug that caused a significant diminution (
50%) of growth below control levels (47, 55).
Quality control.
Quality control was performed on each day of testing by using CLSI-recommended reference strains C. krusei ATCC 6258 and C. parapsilosis ATCC 22019 (12).
Phase II and III clinical trials.
The clinical trial data (patient outcomes and baseline isolates) used in this analysis included the results from four phase II or III studies of esophageal candidiasis treated with caspofungin (4, 28, 60, 62), two phase III studies of invasive candidiasis treated with caspofungin (28, 35, 63), one phase III study of invasive candidiasis treated with anidulafungin (56), and two phase III studies of invasive candidiasis treated with micafungin (30, 45). These were all multi-institutional studies, the details of which are described elsewhere (4, 28, 30, 35, 45, 56, 60, 61, 63). The responses to echinocandin therapy in each study were determined by the investigator at the end of therapy as cured, improved, or failed. A cured or improved response was classified as success, and all other responses as failure.
In vivo correlation.
Clinical outcomes as determined by the investigators at the end of therapy were compared to the relevant echinocandin MIC for each baseline Candida isolate. Where more than one baseline pathogen was identified per patient, the isolate for which the MIC was highest was used.
Development of MIC interpretive breakpoints.
The MIC interpretive breakpoints for the three echinocandins and Candida spp. were developed by taking into account the available microbiologic data, the known resistance mechanisms and their relation to both MICs and in vivo outcomes, pertinent pharmacokinetic (PK) and pharmacodynamic (PD) parameters, and clinical outcome data as described previously for fluconazole (52) and voriconazole (53). The PD indices associated with treatment efficacy for the echinocandins include the area under the concentration-time curve (AUC)/MIC and time to maximum concentration of drug in serum (Cmax)/MIC ratios. The PD target associated with a stasis endpoint for echinocandins is equivalent to AUC/MIC and Cmax/MIC ratios near 10 and 1, respectively (2, 3, 20, 24, 32, 64). Free (not bound to protein) echinocandin concentrations are considered in these estimates. The results of PD studies with most anti-infective drugs have shown that only unbound drug is generally available for microbiologic activity.

RESULTS AND DISCUSSION
Interpretive breakpoints for caspofungin against Candida species and MIC distribution profile.
The MIC
90 and the percentage of isolates for which caspofungin
MICs were 2 µg/ml or less are shown in Table
1. These
results were all determined in a single reference laboratory
using CLSI-recommended BMD methods. This large data set represents
recent (2001 to 2006), clinically important (blood and normally
sterile site) isolates from 91 different medical centers throughout
the world. The overall MIC
90 for caspofungin was 0.25 µg/ml,
and 99.9% of the 5,346 isolates were inhibited by

2 µg/ml
of caspofungin.
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TABLE 1. Comparative in vitro activity of three echinocandin antifungal agents against bloodstream isolates of Candida speciesa
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The caspofungin MIC
90 for
Candida krusei (0.25 µg/ml),
C. parapsilosis (1 µg/ml), and
C. guilliermondii (1 µg/ml)
was considerably higher than that observed for the three common
species,
C. albicans (0.06 µg/ml),
C. glabrata (0.06 µg/ml),
and
C. tropicalis (0.06 µg/ml). The mechanism for this
intrinsic reduced susceptibility appears to be a direct reflection
of amino acid polymorphisms within the
FKS1 "hot spot" regions
of the respective species (
22,
47). Nevertheless, 95.1% of
C. guilliermondii, 99.9% of
C. parapsilosis, and 100% of
C. krusei isolates were inhibited by

2 µg/ml of caspofungin, a concentration
that is attained throughout the dosing interval at standard
doses (70-mg loading dose and 50-mg daily dose) of caspofungin
(
8). As noted previously (
50,
51,
54), 100% of fluconazole-resistant
isolates of
Candida spp. were inhibited by

2 µg/ml (MIC
90,
0.25 µg/ml) of caspofungin (Table
2). These data, including
the species distribution rank-order (Table
1), are highly representative
of those published in numerous in vitro studies (
21,
42,
54).
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TABLE 2. In vitro activity of three echinocandin antifungal agents against fluconazole-resistant isolates of Candida speciesa
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Relationship between resistance mechanisms, MICs, and in vivo response.
The mechanisms of resistance to caspofungin include (i) specific
"hot spot" mutations in the
FKS1 gene (which encodes essential
components of the glucan synthesis enzyme complex) and (ii)
overexpression of Sbe2p, a Golgi protein involved in transport
of cell wall components (
7,
46,
47). Among these mechanisms,
only the
FKS1 mutations have been implicated in clinical resistance
(
47). Unlike the azole class drugs, drug efflux transporters
do not appear to be a factor in the resistance of
Candida spp.
to caspofungin or other members of the echinocandin class (
5,
39,
47).
Clinical isolates of C. albicans displaying elevated MICs for caspofungin have been shown to contain FKS1 mutations (Table 3) (46, 47). Furthermore, these strains showed a decreased sensitivity for inhibition of glucan synthase by caspofungin and reduced echinocandin efficacy in animal models (Table 3). It is notable that such mutations have only been observed in resistant strains (46, 47).
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TABLE 3. Resistance properties associated with caspofungin and clinical isolates of Candida albicans from single patientsa
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FKS1 mutations conferring resistance to caspofungin and other
echinocandins have been identified in several
C. albicans strains
from patients (
6,
31,
34,
46), as well as in two
C. glabrata strains (
10,
17) and in two strains of
C. krusei (
25,
27,
46)
isolated from patients refractory to therapy (Table
4). These
and other case reports, for which studies to document mutations
were not performed (Table
4), provide compelling examples of
the relationship between high or increasing caspofungin MICs
and a poor clinical outcome. In each of these instances, progressive
resistance to caspofungin, as well as to other echinocandins,
was observed (Table
4). Notably, caspofungin MICs for strains
of
Candida with documented
FKS1 gene mutations and for other
published resistant strains generally show values of from 4
µg/ml to more than 8 µg/ml (Table
4). Furthermore,
in four instances, resistance to caspofungin was confirmed in
an animal model (
26,
27,
46).
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TABLE 4. Published cases of Candida sp. infections associated with increased MICs of echinocandins as determined by the CLSI reference methoda
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Cross-resistance among echinocandins and between echinocandins and fluconazole.
It is now well established that cross-resistance between caspofungin
and fluconazole does not exist (
39,
54). Caspofungin and the
other echinocandins are poor substrates for most multidrug efflux
transporters, and the results of studies involving fluconazole-resistant
strains of
C. albicans expressing high levels of CDR1, CDR3,
and/or MDR1 demonstrated full susceptibility to caspofungin
(
5). Furthermore, 100% of 315 clinical isolates of fluconazole-resistant
Candida spp. were susceptible to caspofungin at an MIC of 2
µg/ml or less (MIC
90, 0.25 µg/ml) (Table
2).
Given the mechanism of action that is shared among the echinocandins, it is not surprising that they demonstrate a similar spectrum and potency (47). Scatterplots of anidulafungin (Fig. 1a) and micafungin MICs (Fig. 1b) versus caspofungin MICs show a high degree of correlation (R = 0.85 and 0.84, respectively). The essential agreement (MIC ± 2 dilutions) for the comparisons is striking at 93% for anidulafungin versus caspofungin and 97% for micafungin versus caspofungin. These findings support the observations of Balashov et al., Park et al., and Perlin (7, 46, 47) indicating that among FKS1 mutants expressing resistance to caspofungin, the strains are cross resistant to micafungin and anidulafungin. The strength of these relationships is modified somewhat by the distinctly rare occurrence of clinical isolates for which the MICs of caspofungin, micafungin, and anidulafungin exceed 4 µg/ml (55).
In vitro correlation of in vivo data.
A total of 406 patients enrolled in phase II/III clinical trials
for treatment of esophagitis (292 patients) and invasive candidiasis
(114 patients) were infected with
Candida spp., received caspofungin
therapy, and were characterized as treatment successes or failures
at the end of therapy by the site investigators (Table
5). The
overall species and MIC distribution was comparable to that
shown in Table
1. No significant differences in clinical response
to caspofungin therapy were noted among the various species
of
Candida (
28), and so for purposes of this analysis, the results
for all species were merged.
Previously, Kartsonis et al. (
28) concluded that there was no
relationship between caspofungin MIC results and clinical outcome
for either infection type. Indeed, there is no apparent difference
in outcome at each of the MICs in what could be considered a
"wild-type" MIC distribution (Tables
1 and
5). Unfortunately,
the data set includes only three isolates from patients treated
with caspofungin for which the caspofungin MICs were >2 µg/ml
(two
C. parapsilosis isolates at 4 µg/ml and one
C. rugosa isolate at 8 µg/ml). Thus, the clinical data contain too
few results for patients infected with isolates with reduced
susceptibility to caspofungin to arrive at any firm conclusion
regarding the relationship between elevated caspofungin MICs
and clinical outcome. The data simply show that clinical failures
are distributed evenly across the susceptible wild-type population
of infecting isolates. These failures are likely due to factors
other than the "drug-bug" interaction. Such limitations of clinical
trial data have been noted previously (
57). Overall, these data
define the susceptible population of
Candida species as those
for which caspofungin MICs are 2 µg/ml or less.
Development of caspofungin MIC interpretive breakpoints.
Given the MIC distribution shown in Tables 1, 2, and 5 and the clinical relationship between MIC and efficacy, what are the possible breakpoints for BMD MIC testing of caspofungin against Candida? Regarding the category of susceptible, breakpoints at
1 µg/ml and
2 µg/ml were considered. The MIC distribution profile obtained with the optimal BMD method for over 5,000 clinical isolates indicates that 99.9% of all clinical isolates of Candida spp. are inhibited by
2 µg/ml of caspofungin (Table 1). In light of this MIC distribution, it is notable that caspofungin MICs for strains of Candida with documented FKS1 gene mutations (7, 46, 47) and for the published resistant strains (6, 13, 17, 25, 27, 29, 34, 36, 62) were all >2 µg/ml and were usually
8 µg/ml (Tables 3 and 4). It is known that such strains respond poorly to echinocandin treatment in animals and humans and contain a glucan synthesis enzyme complex that is less sensitive to inhibition by caspofungin than that of wild-type strains, further confirming their status as caspofungin-resistant strains (47). Such strains are rarely encountered clinically (0.1% of 5,346 clinical isolates); however, when observed they appear to exhibit a class-specific resistance profile (47, 55).
Pertinent PK data for caspofungin include a peak serum concentration of approximately 10 µg/ml and a sustained concentration of >1 µg/ml (total drug concentrations) throughout the dosing interval following a loading dose of 70 mg and a daily dosing regimen of 50 mg (8, 16). The AUC is approximately 120 mg·h/liter (total drug concentration).
PD investigations of caspofungin against Candida have been performed, and both in vitro and in vivo models have demonstrated a correlation between drug dose, organism MIC, and outcome (1, 18, 19, 32). Caspofungin exhibits concentration-dependent killing that is optimized at a peak-to-MIC ratio of
4:1 and produces a prolonged (>12 h) postantifungal effect (17, 18, 31). Louie et al. (32) have noted the importance of the total drug exposure (AUC) for determining caspofungin efficacy in a murine infection model of invasive candidiasis. A formal examination of the target AUC/MIC has not been undertaken with caspofungin. The study of Louie et al. (32) employed a single strain of C. albicans and found that the AUC/MIC ratio associated with a stasis endpoint was near 20. Although the echinocandins are significantly bound to human serum proteins, the impact of protein binding on echinocandin activity remains under study, and data generated in animals with yet-again different patterns of protein binding must be interpreted cautiously. In vitro, the agents bind to different human serum proteins and the in vitro impact of this binding varies by agent (40, 44), with caspofungin least affected. As a consequence, the in vivo PD estimates were weighted less heavily in the committee's analysis of the data.
Taken together, the MIC distribution and the PK/PD data would support a caspofungin MIC of either
1 µg/ml or
2 µg/ml as predictive of efficacy. A caspofungin MIC of
2 µg/ml encompasses 99.9% of all clinical isolates of Candida spp. without bisecting any species group. While extensive PD target studies have not been undertaken with caspofungin, the PK of the drug (70-mg loading dose and 50-mg maintenance dose) would produce concentrations above 1 µg/ml (total drug concentration) throughout the treatment period (8, 16). The ability of caspofungin to successfully treat infections due to isolates for which the MIC is as high as 2 µg/ml is strongly supported by the data from clinical trials, as shown in Table 5.
Due to the paucity of isolates for which the caspofungin MIC was elevated (>2 µg/ml), an MIC predictive of resistance cannot be defined based on data from clinical trials. The fact that FKS1 mutants and isolates of Candida spp. in case reports of caspofungin failures (Table 4) generally show MICs of 4 µg/ml to more than 8 µg/ml suggests that the rare isolates for which the MICs exceed 2 µg/ml may not respond optimally to treatment with caspofungin (54). Nevertheless, the consensus of the CLSI Antifungal Subcommittee was that although the data were sufficient to support a susceptible breakpoint of
2 µg/ml, additional data were needed before a resistant breakpoint could be established. Given this reasoning, the subcommittee has recommended that isolates for which the caspofungin MIC is
2 µg/ml should be considered susceptible and that isolates for which the MIC is greater than 2 µg/ml should be considered "nonsusceptible." The latter isolates should be subjected to repeat testing and referred to an appropriate reference laboratory for confirmation. It is anticipated that as experience with these uncommon isolates grows, the CLSI Antifungal Subcommittee will ultimately be able to establish a resistant MIC breakpoint.
Interpretive breakpoints for anidulafungin and micafungin against species of Candida.
Although the accumulated in vitro and clinical data to support MIC breakpoints for anidulafungin and micafungin are somewhat less than those used for caspofungin, a parallel logic to that used for caspofungin was employed. This was based in large part on shared mechanisms of action and resistance, a similar MIC distribution profile, cross-resistance data, and the results of clinical trials with each agent.
As shown for caspofungin, the MIC distribution profiles for anidulafungin and micafungin were bimodal, with 98.8% (anidulafungin) to 100% (micafungin) of 5,346 isolates of Candida inhibited by
2 µg/ml of each agent (Table 1). Low MICs for anidulafungin and micafungin were observed with C. albicans, C. glabrata, and C. tropicalis (modal MICs of 0.015 to 0.03 µg/ml), whereas the MICs for both agents were higher for C. parapsilosis and C. guilliermondii (modal MICs of 1 to 2 µg/ml).
As noted previously, cross-resistance was not observed between fluconazole and either anidulafungin or micafungin (Table 2). Cross-resistance was observed between both of these agents and caspofungin (Fig. 1a and 1b) and also between each other (Fig. 1c). The essential agreement between anidulafungin and micafungin was 92% (Fig. 1c). The categorical agreement between anidulafungin and caspofungin (Fig. 1a), calculated using the susceptible breakpoint of
2 µg/ml and caspofungin as the reference result, was 98.1% with 1.1% very major errors (false susceptible) and 0.1% major errors (false resistant). Likewise, the categorical agreement between micafungin and caspofungin (Fig. 1b) was 99.9%, with only 0.1% major errors.
Additional evidence for cross-resistance among all three echinocandins comes from studies of FKS1 mutants, both laboratory-derived and clinical isolates (Table 4) (47). Balashov et al., Park et al., and Perlin (7, 46, 47) have shown that the FKS1 modification mechanism broadly encompasses the class of echinocandin drugs. Strains of Candida found to contain FKS1 mutations displayed highly elevated MICs for caspofungin, anidulafungin, and micafungin (Table 4) (47).
The results of PK/PD studies for both anidulafungin and micafungin reveal a Cmax of approximately 10 µg/ml and trough concentrations of 1 to 2 µg/ml (8, 16). Both agents exhibit concentration-dependent killing and a prolonged (12 to 24 h) postantifungal effect (19, 20). The AUC (total drug concentration) for anidulafungin (200-mg loading dose and 100-mg maintenance dose) is 112 mg·h/liter, and that for micafungin (100-mg daily dose) is 126 mg·h/liter (8, 16). More-extensive animal model PD target investigation has been undertaken with these echinocandins (2, 3). Similar to caspofungin, the PD indices associated with efficacy for these agents were the AUC/MIC and Cmax/MIC ratios (2, 3, 23, 24, 32). A stasis endpoint in an in vivo model of invasive C. albicans and C. glabrata infection for both anidulafungin and micafungin was achieved at an AUC/MIC ratio of 10 to 20 when free-drug concentrations were considered. The PK of these compounds in patients would be expected to meet and exceed this target for these species (2, 3). This target would not be achieved for the MIC distribution commonly observed with C. parapsilosis (Table 1). However, the impact of the higher MICs observed with C. parapsilosis on this PD target has not yet been examined in these models. As discussed above in the section on caspofungin, pending questions regarding echinocandins and binding to human serum proteins led the committee to weight these data less heavily.
The relationship between MIC and clinical outcome for invasive candidiasis, anidulafungin, and micafungin is shown in Table 6. Importantly, no isolates for which MICs were greater than 2 µg/ml for either agent were observed in the respective clinical trials. The MIC distributions for both anidulafungin and micafungin and isolates from the clinical trials were consistent with those of survey data (Table 1) and define the "susceptible" population. The clinical response to each agent was similar irrespective of the MIC, and there were too few isolates (none) with elevated MICs to make any conclusion regarding resistance. Notably, of the seven isolates of C. parapsilosis for which micafungin MICs were 2 µg/ml, five (71%) were treated successfully (overall response of C. parapsilosis to micafungin was 74%) (Table 6).
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TABLE 6. Relationship between MIC and outcome for treatment of invasive candidiasis with andidulafungin and micafungina
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As seen with caspofungin, the MIC distribution, cross-resistance
and resistance mechanism study results, and PK/PD data support
anidulafungin and micafungin MICs of

1 µg/ml or

2 µg/ml
as predictive of efficacy. Anidulafungin and micafungin MICs
of

2 µg/ml encompass 98.8 to 100% of all clinical isolates
of
Candida spp. without bisecting any species group and represent
a concentration that is easily maintained throughout the dosing
period. As shown in the data from the clinical trials (Table
6), standard dosing regimens for anidulafungin (200-mg loading
dose and 100-mg maintenance dose) and micafungin (100 mg daily)
may be used to treat infections due to
Candida species for which
MICs are as high as 2 µg/ml. An MIC predictive of resistance
cannot be defined based on the data from clinical trials.
Recommendations for echinocandin MIC breakpoints.
As done previously (52, 53) the CLSI Antifungal Subcommittee followed a "blueprint" to develop interpretive breakpoints for caspofungin, anidulafungin, and micafungin. The process took into account mechanisms of resistance, analysis of the MIC population distribution, consideration of cross-resistance patterns, analysis of parameters associated with success in PD models of infection, and the results of clinical efficacy studies.
Given the overall in vitro and clinical comparability of these agents, it was decided to utilize the same susceptible breakpoint for all three agents. The CLSI Antifungal Subcommittee decided to recommend a "susceptible only" breakpoint of
2 µg/ml, due to the lack of echinocandin resistance in the population of Candida isolates thus far. Although a lower breakpoint would encompass virtually all strains of C. albicans, C. glabrata, and C. tropicalis, a susceptible breakpoint of
2 µg/ml was deemed necessary to avoid bisecting the population of C. parapsilosis, a common species that responds clinically to echinocandin therapy despite elevated MICs. Isolates of C. albicans and C. glabrata for which echinocandin MICs are 2 µg/ml, although considered susceptible, are clearly outside of the normal wild-type distribution of echinocandin MICs for these species. Indeed, Garcia-Effron et al. (22) have shown that isolates of C. albicans and C. glabrata with this "reduced susceptibility" phenotype contain substitutions in the conserved distal proline in Fks1p hot spot 1 that are analogous to those occurring naturally in C. parapsilosis. Impaired glucan synthase enzyme kinetics in these strains suggest that such mutations may result in a fitness cost to the cell. This decrease in fitness, coupled with the excellent PK of the echinocandins, likely contributes to the ability of these agents to effectively treat infections due to Candida species for which the MICs are as high as 2 µg/ml (Tables 5 and 6). Regardless, isolates with this unusual phenotype warrant further study, and although they may respond clinically to echinocandin treatment, they could pose problems under conditions of decreased drug penetration.
For strains yielding results suggestive of a "nonsusceptible" category (>2 µg/ml), organism identification and antimicrobial susceptibility test results should be confirmed. Subsequently, the isolates should be saved and submitted to a reference laboratory that will confirm the results by using a CLSI reference dilution method (37, 38). These isolates should be designated "nonsusceptible." This approach is consistent with that used for antibacterial testing of agents for which resistance is rare or unknown (38).
Balashov et al., Park et al., and Perlin (7, 46, 47) have clearly shown that the Fks1p modification system broadly encompasses the entire class of echinocandin drugs. A 16- to 128-fold change in MIC relative to the MIC of a fully susceptible wild-type strain is consistently observed for all three echinocandins when tested against a strain with FKS1 mutations (47). The MICs for caspofungin and micafungin tend to be somewhat higher than those determined for anidulafungin in such strains (47). This may result in a strain with an FKS1 mutation being classified as nonsusceptible to caspofungin and micafungin but as susceptible to anidulafungin. The clinical significance of such differences remains to be determined; however, the more-conservative approach would be to consider those isolates tested as nonsusceptible to one of the echinocandins to be nonsusceptible to the other agents in the class. Presently, caspofungin results predict those of either anidulafungin or micafungin with an absolute categorical agreement of >98%. For the time being, the susceptibility results for one echinocandin may be considered to be predictive of those for the other two agents in the class.
The so-called "paradoxical effect" refers to the growth of echinocandin-susceptible organisms at highly elevated drug concentrations far in excess of the MIC. Paradoxical growth is not related to FKS1 mutations or modification of the echinocandin sensitivity of the glucan synthase enzyme complex nor to its upregulation in the presence of drug (47). It most likely represents an adaptive stress response and is more of a laboratory-related phenomenon. The relevance of this effect to patient care has not been demonstrated. As such, paradoxical growth should be ignored in determining echinocandin MICs.
It is anticipated that the susceptible and nonsusceptible categories will be further defined through additional study of isolates that are identified during postmarket surveillance efforts for the three echinocandins. This will include detailed characterization of "high-MIC" or nonsusceptible isolates, with a goal of identifying those strains expressing true echinocandin resistance.

ACKNOWLEDGMENTS
Beverly Ringenberg provided excellent support in the preparation
of the manuscript.
This work was supported in part by grants from Astellas, Merck, and Pfizer.

FOOTNOTES
* Corresponding author. Mailing address: Department of Pathology, 200 Hawkins Drive, University of Iowa, Iowa City, IA 52242-1009. Phone: (319) 356-8615. Fax: (319) 356-4916. E-mail:
michael-pfaller{at}uiowa.edu 
Published ahead of print on 25 June 2008. 

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Journal of Clinical Microbiology, August 2008, p. 2620-2629, Vol. 46, No. 8
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