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Antimicrobial Agents and Chemotherapy, March 2009, p. 1185-1193, Vol. 53, No. 3
0066-4804/09/$08.00+0     doi:10.1128/AAC.01292-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Breakthrough Aspergillus fumigatus and Candida albicans Double Infection during Caspofungin Treatment: Laboratory Characteristics and Implication for Susceptibility Testing

Maiken Cavling Arendrup,^1* Guillermo Garcia-Effron,² Walter Buzina,³ Klaus Leth Mortensen,¹ Nanna Reiter,⁴ Christian Lundin,¹ Henrik Elvang Jensen,⁵ Cornelia Lass-Flörl,⁶ David S. Perlin,² and Brita Bruun⁷

Unit of Mycology and Parasitology, Statens Serum Institut, Copenhagen, Denmark,¹ Public Health Research Institute, New Jersey Medical School, UMDNJ, Newark, New Jersey,² Institute of Hygiene, Microbiology, and Environmental Medicine, Medical University Graz, Graz, Austria,³ Department of Intensive Care Medicine, Roskilde Hospital, Roskilde, Denmark,⁴ Department of Disease Biology, Faculty of Life Sciences, University of Copenhagen, Copenhagen, Denmark,⁵ Department of Hygiene and Medical Microbiology, Innsbruck Medical University, Innsbruck, Austria,⁶ Department of Clinical Microbiology, Hillerød Hospital, Hillerød, Denmark⁷

Received 26 September 2008/ Returned for modification 24 November 2008/ Accepted 11 December 2008

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Caspofungin is used for the treatment of acute invasive candidiasis and as salvage treatment for invasive aspergillosis. We report characteristics of isolates of Candida albicans and Aspergillus fumigatus detected in a patient with breakthrough infection complicating severe gastrointestinal surgery and evaluate the capability of susceptibility methods to identify candin resistance. The susceptibility of C. albicans to caspofungin and anidulafungin was investigated by Etest, microdilution (European Committee on Antibiotic Susceptibility Testing [EUCAST] and CLSI), disk diffusion, agar dilution, and FKS1 sequencing and in a mouse model. Tissue was examined by immunohistochemistry, PCR, and sequencing for the presence of A. fumigatus and resistance mutations. The MICs for the C. albicans isolate were as follows: >32 µg/ml caspofungin and 0.5 µg/ml anidulafungin by Etest, 2 µg/ml caspofungin and 0.125 µg/ml anidulafungin by EUCAST methods, and 1 µg/ml caspofungin and 0.5 µg/ml anidulafungin by CLSI methods. Sequencing of the FKS1 gene revealed a mutation leading to an S645P substitution. Caspofungin and anidulafungin failed to reduce kidney CFU counts in animals inoculated with this isolate (P > 0.05 compared to untreated control animals), while both candins completely sterilized the kidneys in animals infected with a control isolate. Disk diffusion and agar dilution methods clearly separated the two isolates. Immunohistochemistry and sequencing confirmed the presence of A. fumigatus without FSK1 resistance mutations in liver and lung tissues. Breakthrough disseminated aspergillosis and candidiasis developed despite an absence of characteristic FKS1 resistance mutations in the Aspergillus isolates. EUCAST and CLSI methodology did not separate the candin-resistant clinical isolate from the sensitive control isolate as well as did the Etest and agar methods.

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Candins are among the preferred choices for initial treatment of candidemia (35) and are also indicated as salvage treatment for aspergillosis (48). Since caspofungin was introduced on the market in 2001, its use has increased remarkably, e.g., in Denmark, from 3 to 9,000 daily defined doses during the period of 2003 to 2007 (http://www.medstat.dk/MedStatDataViewer.php). This is a consequence of a number of recent fungemia surveys reporting a considerable proportion of cases involving species with reduced susceptibility to fluconazole, such as Candida glabrata and Candida krusei (3, 22, 31, 42, 47). Moreover, in a recent trial comparing fluconazole with anidulafungin, the latter was associated with an improved success rate even in cases involving fluconazole-susceptible species like Candida albicans and Candida tropicalis (20). Resistance to candins has been reported only sporadically for Candida and Aspergillus isolates (4, 5, 19, 24, 25, 28, 36, 37, 39), but with the increased use of candins, the selection pressure has risen, and close monitoring and sensitive susceptibility testing methods have become increasingly important. Recently, the CLSI suggested 2 µg/ml as a tentative susceptibility breakpoint for caspofungin, micafungin, and anidulafungin for Candida spp. However, Candida infections involving isolates with mutations in FKS1, which encodes the candin target, do not necessarily show MICs above this breakpoint, and minimal effective concentration determinations do not always detect Aspergillus isolates with reduced susceptibility when using microdilution tests (4, 19, 24).

We report a case of breakthrough invasive C. albicans infection and aspergillosis during long-term caspofungin treatment. The C. albicans isolate was characterized by notably raised Etest endpoints, decreased susceptibility determined by disk diffusion and agar dilution, and in vivo resistance to caspofungin and anidulafungin in an animal model but with microdilution MICs not exceeding the suggested CLSI breakpoint of 2 µg/ml. Sequencing demonstrated a point mutation in the FKS1 target gene. Sequencing of liver and lung tissue obtained during autopsy confirmed the presence of Aspergillus fumigatus but revealed no resistance mutations in the FKS1 gene.

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A 59-year-old woman was admitted to a general hospital with a history of 5 days of nonspecific abdominal pain accompanied by sepsis. As a child, she had suffered from polio but was otherwise healthy. Upon admission, an abdominal radiograph showed intraperitoneal gas. Acute laparotomy revealed perforation and necrosis of the sigmoid colon with fecal peritonitis leading to a Hartmann's operation. Small-bowel resection was also necessary due to extensive adhesions. Ileostomy, sigmoid colostomy, and bilateral salpingo-oophorectomy were performed. Postoperatively, the patient was transferred to the intensive care unit (ICU) in septic shock and was treated according to Surviving Sepsis Campaign guidelines (9). Respirator treatment was given during the entire ICU stay, and the patient was placed on hemodialysis due to acute renal failure. Broad-spectrum antibiotics were given and changed according to culture and susceptibility tests. On day 4, empirical treatment with 400 mg/day fluconazole was given for 3 days. Due to the finding of yeast (subsequently identified as C. albicans) in blood cultures, the antifungal treatment was changed to standard doses of caspofungin on day 8. Treatment with caspofungin was continued for the rest of the ICU stay. Relaparotomy, done on day 12, revealed necrosis of the ileostomy and fecal peritonitis. Small-bowel resection was done again (day 23) and also on day 25 because of iliac perforation. On day 31, contrast radiography showed iliac fistula at the site of the former perforation. No further surgical options were considered to be left, and the patient was treated conservatively. An abdominal computed tomography scan on day 33 showed mesenteric edema, liver abscesses, as well as abscesses in relation to the ileostomy. In order to optimize the chances of conservative healing, the patient was transferred to the ICU in another hospital on day 35, where continuous renal replacement therapy was performed. At the new ICU, broad-spectrum antibiotics and caspofungin were continued. As the patient continued to deteriorate on conservative treatment with rising C-reactive protein, leukocyte counts, and lactate, surgical debridement with resection of necrotic tissue, drainage of liver abscesses, rinsing, and VAC treatment were done on days 39, 40, and 41. Fluconazole (400 mg/day) was added due to the growth of caspofungin-resistant C. albicans from a dialysis catheter and catheter urine. Despite all efforts, the patient died with multiorgan failure on day 42.

Autopsy revealed peritonitis with extensive necroses and bleeding, sequelae after the various operations, and multiple abscesses in liver, lungs, retroperitoneum, and left kidney. Histologically, multiple filamentous fungal elements were demonstrated within the pulmonary and hepatic processes. The elements were identified as being A. fumigatus due to positive reactivity only with the monoclonal antibody directed toward Aspergillus spp. and the heterologously absorbed polyclonal antibody directed toward A. fumigatus.

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Susceptibility testing. Susceptibilities of the index isolate (C. albicans R) and of a control isolate randomly chosen among clinical blood isolates (C. albicans C) to anidulafungin and caspofungin were determined by Etest (AB Biodisk, Solna, Sweden) using RPMI 1640 medium with 2% glucose agar (SSI Diagnostika, Hillerød, Danmark) according to the manufacturer's recommendations. Susceptibility testing was also performed according to European Committee on Antibiotic Susceptibility Testing (EUCAST) discussion document E7.1 (7, 8) and according to CLSI (formerly National Committee for Clinical Laboratory Standards) M27-A2 methodology (29). Drugs used were as follows: dimethyl sulfoxide (DMSO) (catalog no. D8779; Sigma-Aldrich), fluconazole (10,000 µg/ml in water) (Pfizer A/S, Ballerup, Denmark), amphotericin B (5,000 µg/ml in DMSO) (catalog number A2411; Sigma-Aldrich, Vallensbæk Strand, Denmark), caspofungin (5,000 µg/ml in DMSO) (Merck, Sharp & Dohme, Glostrup, Denmark), anidulafungin (5,000 µg/ml in DMSO) (Pfizer A/S, Ballerup, Denmark), itraconazole (5,000 µg/ml in DMSO) (Janssen-Cilag, Birkerød, Denmark), and voriconazole (5,000 µg/ml in DMSO) (Pfizer A/S, Ballerup, Denmark). Microtiter plates were read spectrophotometrically at 490 nm. The MIC was defined as the lowest drug dilution giving 50% growth inhibition. C. krusei ATCC 6862 was included as a control in each run.

For disk diffusion testing, we used 90-mm plates containing RPMI 1640 medium with 2% glucose agar and Mueller-Hinton agar (both from SSI Diagnostika, Hillerød, Danmark). Inoculation was done by swabbing the agar in three directions with a swab soaked in a yeast suspension of 1 x 10⁶ to 5 x 10⁶ cells/ml. Disks containing 1, 5, or 25 µg of drug were prepared by placing 20 µl of a suitable drug concentration on sterile 6-mm-diameter paper disks (Struers, Denmark) and subsequently placing the disks on the inoculated plates.

Agar dilution testing was done using Sabouraud agar containing 0, 0.5, 1, and 2 µg/ml of caspofungin or 0, 0.003, 0.125, and 0.5 µg/ml anidulafungin, respectively, in 1-ml volumes in four-well multidish plates (1.9 cm²/well) (Nunc; Thermo Fisher Scientific, Roskilde, Denmark). The three Candida isolates (C. albicans R and C. albicans S and C) were subcultured for 2 days on CHROMagar (SSI Diagnostika, Hillerød, Denmark). Suspensions containing 1 x 10⁶ to 5 x 10⁶ CFU/ml were prepared in sterile water and diluted 1,500 times, and 5 µl (approximately 3 to 15 cells) was spread onto each of the agar surfaces.

Candidiasis animal model. A total of 56 NMRI mice (weight, 26 to 30 g) (Harlan Scandinavia, Allerød, Denmark) were kept with free access to food and water. On day 0, mice were inoculated by intravenous injection with a suspension of C. albicans (1 x 10⁵ CFU in 200 µl) using a 25-gauge syringe. Eight groups of seven mice were challenged with either C. albicans isolate R or C. Mice were treated by intraperitoneal injection on days 1 to 3 with 0.5 ml of either caspofungin (6 mg/kg of body weight), anidulafungin (12 mg/kg), or saline intraperitoneally (control mice) or with 0.7 ml of fluconazole (50 mg/kg). Doses were chosen by multiplying the normal maintenance human daily dose (50 mg caspofungin, 100 mg anidulafungin, and 400 mg fluconazole) by the mouse/human body surface ratio. Mice were sacrificed by cervical dislocation on day 4. Kidneys were aseptically removed, weighed, and placed in pairs into 1,000 µl of sterile saline. All organs were stored at –80°C before homogenization with a homogenizer (RW 16 Basic; IKA Labortechnik, Bie & Berntsen, Denmark). CFU determinations were performed by the spot technique by plating 20-µl spots of 10-fold dilutions. The results were expressed as the log₁₀ of the number of CFU/ml kidney tissue homogenate. The lower limit of detectable CFU was 50 CFU/ml. The experiments were approved by the Danish Animal Experimentation Committee under the Ministry of Justice (approval no. 2004/561-835).

Sequencing. C. albicans R and C were cultured on Sabouraud agar for 24 h at 37°C. For DNA extraction, the PrepMan Ultra kit (Applied Biosystems, Foster City, CA) was used. A loopful of cells was suspended in 200 µl of PrepMan Ultra sample preparation reagent in a 1.5-ml microcentrifuge tube. The samples were vortexed for 30 s and incubated at 100°C for 10 min. Thereafter, the samples were centrifuged for 3 min at a relative centrifugal force of 16,000, and the supernatants were transferred into a 1.5-ml Eppendorf tube and stored at –20°C until use. For PCR of the FKS1 region at positions 1717 to 2135, the primer pair consisting of GSC1F and GSC1R was used (19); for the FKS1 region at positions 4054 to 4267, the newly designed forward primer 5'-ATTGCTCCTGCCGTTGATTG-3' and recently described reverse primer 5'-GGTCAAATCAGTGAAAACCG-3' (24) were used. The conditions for amplification were an initial denaturation step at 94°C followed by 30 cycles of denaturation at 94°C for 30 s, annealing at 54°C for 1 min, and extension at 74°C for 1.5 min and a final extension step at 70°C for 4 min as described previously (12). The resulting amplicons were purified using an Invisorb PCRapid kit (Invitek, Berlin, Germany). Cycle sequencing was performed in both directions with the same primers as those used for PCR using the BigDye Terminator cycle sequencing kit (Applied Biosystems). Purification of the cycle sequencing products was carried out with a solution containing 2.5 µl 125 mM EDTA and 25 µl 96% ethanol. The pellets were dried for 15 min in a drying chamber and dissolved in 15 µl HiDi (Applied Biosystems), and the sequences were generated using the 3130 Genetic Analyzer automated capillary DNA sequencer (Applied Biosystems). The resulting nucleic acid sequences were assembled and transcribed into the amino acid sequences using the internet freeware Nucleic Acid to Amino Acid Translation (http://www.biochem.ucl.ac.uk/cgi-bin/mcdonald/cgina2aa.pl).

Molecular studies of Aspergillus using paraffin-embedded tissue biopsies. DNA isolation was done with the Q-Biogene FastDNA kit (Irvine, CA) according to the manufacturer's instructions. A previous step of paraffin elimination was performed by incubating the paraffin-embedded biopsies in 10 ml of Citrosolv (Thermo Fisher Scientific, Austria) at room temperature for 4 h. Molecular identification of Aspergillus was performed by sequencing of the 5.8S RNA gene and the adjacent internal transcribed spacer 1 (ITS1) and ITS2 as described previously by White et al. (49). ITS sequences from A. fumigatus ATCC 13073, A. fumigatus 293, and A. fumigatus CNM-CM-237 (27) were used as controls. Sequence analysis was performed by comparing the DNA sequences with those of the control strains included in this study and with the sequences obtained from the GenBank database.

FKS1 sequencing. The A. fumigatus FKS1 gene (GenBank accession no. AFU79728) was sequenced between nucleotides 1875 and 4318 by Sanger methodology using a Beckman Coulter CEQ 8000 genetic analysis system. The putative FKS1 gene from Aspergillus lentulus was sequenced between nucleotides 1880 and 2300 and between nucleotides 3900 and 4300 (nucleotides equivalent to A. fumigatus FKS1 hot spots 1 and 2). A. fumigatus ATCC 13073, A. fumigatus 293, A. fumigatus CNM-CM-237 (2), A. fumigatus EMFR-S678P (46), A. lentulus CNM-CM-3583, A. lentulus CNM-CM-3599, and A. lentulus CNM-CM-4420 (1) were used as control strains.

Immunohistochemistry. Tissue sections were mounted onto adhesive slides (Superfrost Plus; Menzel-Glaser, Germany) and kept at 4°C until processed. As primary reagents for immunostaining, two monoclonal antibodies that reacted specifically with antigens of Aspergillus spp. and the Mucorales group (MCA1276 and MCA2577; Serotec, Oxford, United Kingdom) were used together with genus- and species-specific rabbit polyclonal antibodies directed against Candida spp., A. fumigatus, Aspergillus flavus, Aspergillus niger, Geotrichum candidum, Fusarium solani, and Scedosporium apiospermum (17, 18). All polyclonal antibodies were absorbed heterologously according to procedures described previously by Okuda et al. (32) and Jensen et al. (16). The Power-Vision⁺ Poly-HRP histostaining kit detection system (Immunovision Technologies Co., Brisbane, CA) was applied for visualization of immunoreactivity (21). The immunoreaction was followed by incubation for 6 min in amino-ethyl carbazol solution (Immunovision). The sections were counterstained in Harris hematoxylin for 10 s before reading. Experimentally infected murine tissues with reference fungi were used as positive controls (17).

Statistics. CFU kidney burden levels among the various treatment groups were compared using the Mann-Whitney test.

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Mycology. C. albicans cells were cultured on numerous occasions, and mold was isolated from three separate tracheal suctions (Table 1). The mold was not further characterized, as the patient was dying or dead at the time of detection. The urine and catheter tip C. albicans isolates obtained on days 35 and 38 were resistant to caspofungin by Etest, in contrast to the earlier blood culture isolate (day 4) (Fig. 1). The urine isolate (C. albicans R) was further tested by EUCAST and CLSI microdilution (Table 2). Both tests confirmed the raised MICs of caspofungin but not to levels exceeding the suggested susceptible CLSI breakpoint of 2 µg/ml. The murine candidiasis model showed no significant reduction in kidney burden for animals inoculated with C. albicans R and treated with caspofungin or anidulafungin (P values of 0.053 and 0.3374, respectively) (Fig. 2). In contrast, caspofungin and anidulafungin reduced the fungal burden in the animals inoculated with the control isolate (named C. albicans C) to below the detection level. For both isolates, fluconazole resulted in a significant reduction in kidney burden (P values of 0.0006 and 0.0252 for C. albicans R and control isolates, respectively).

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TABLE 1. Microbiological specimens with growth of fungi during admission at hospital A day on days 0 to 35 and hospital B on days 35 to 42

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FIG. 1. Etest susceptibility endpoints for the day 4 candin-susceptible C. albicans S isolate, the day 38 candin-resistant C. albicans R isolate, and a susceptible unrelated control C. albicans C isolate, respectively, to caspofungin (CAS) and anidulafungin (Anid). From left to right, data for C. albicans S and caspofungin Etest, C. albicans S and anidulafungin Etest, C. albicans R and caspofungin Etest, C. albicans R and anidulafungin Etest, C. albicans S and caspofungin Etest, and C. albicans C and anidulafungin Etest are shown. The MICs for each isolate and antifungal are indicated below the photograph.

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TABLE 2. Susceptibility results by EUCAST, Etest, and agar dilution

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FIG. 2. In vivo susceptibility of C. albicans R and C to anidulafungin, caspofungin, and fluconazole in a murine invasive candidiasis model. Kidney burden on day 4 is shown for mice challenged with C. albicans R (solid marks) and C. albicans C (open marks) and subsequently treated with caspofungin (circles), anidulafungin (squares), fluconazole (triangles), or glucose (control group) (diamonds).

In order to investigate the discriminatory potential of alternative methods for susceptibility testing, disk diffusion and agar dilution susceptibility testings were undertaken. Disk diffusion was performed with 1-, 5-, and 25-µg disks on two different agars and read after 24 and 48 h. For all combinations, the zones of the C. albicans R isolate were smaller than those for the control isolate, as illustrated by a mean difference in zone diameter of 46.2% (range, 33.7% to 58.4%) after 24 h and with the control isolate as a comparator (Table 3). Agar dilution was performed using caspofungin concentrations of 0.5, 1, and 2 µg/ml and anidulafungin concentrations of 0.003, 0.125, and 0.5 µg/ml. While C. albicans R grew on all the candin agars, no growth was observed for the control isolate, with the exception of agar containing 0.003 µg/ml anidulafungin (Table 2 and Fig. 3).

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TABLE 3. Zone diameters obtained under different conditions for the caspofungin-resistant isolate and the caspofungin-susceptible isolatea

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FIG. 3. Agar dilution susceptibility testing. Repeated testing of C. albicans S (a), C. albicans R (b), and the control C. albicans C isolate (c) to anidulafungin and caspofungin. Concentrations (µg/ml) of the antifungals are indicated next to the wells.

Molecular characterization. In order to characterize the underlying candin resistance mechanism, the hot spot regions of FKS1 spanning the prominent S645 codon (36) were amplified and sequenced. The translated sequences corresponding to amino acids 641 to 648 of C. albicans Fks1p (19) were aligned and showed a site-specific mutation (at nucleic acid position 1933) (Fig. 4), resulting in a change of hydrophilic serine to hydrophobic proline. A silent heterozygous mutation was found in strain R at nucleotide 4230. Immune histochemistry using anti-Aspergillus fumigatus monoclonal antibodies was positive for liver and lung sections obtained postmortem (Fig. 5). PCR and sequencing of the tissue demonstrated that both biopsy samples contained A. fumigatus DNA. One hundred percent homology was found between the biopsy ITS amplicon and control A. fumigatus strains, and FKS1 sequencing demonstrated no mutations at the two equivalent C. albicans hot spot regions associated with echinocandin resistance (Table 4) (46).

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FIG. 4. Sequence alignment corresponding to nucleotides 1837 to 2136 of the FKS1 region (amino acids 613 to 712 of Fks1p) of C. albicans R (resistant) and C (control). The mutated site is indicated by a box.

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FIG. 5. Immunohistochemical staining demonstrating the presence of Aspergillus hyphae in liver tissue. (The fungal elements are stained with anti-Aspergillus antibodies as described in Materials and Methods and appear red.)

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TABLE 4. Sequencing of FKS1 hot spot regions 1 (nucleotides 2023 to 2049) and 2 (nucleotides 4156 to 4183) from A. fumigatus DNA extracted from biopsies and from control strainsa

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We report here a case of disseminated aspergillosis and the emergence of a caspofungin-resistant C. albicans isolate from an abdominal surgical patient during 40 days of caspofungin treatment. Breakthrough Aspergillus infections in allogeneic hematopoietic stem cell transplant recipients on caspofungin therapy have been reported (25), but to our best knowledge, this is the first reported case in a nonneutropenic patient on caspofungin treatment. ICU patients with underlying chronic obstructive pulmonary disease constitute a new and rising risk group for aspergillosis (26), but disseminated aspergillosis in abdominal surgery patients is extremely rare (14, 38). In our case, only one of the three airway samples yielding growth of mold was positive before the patient died, and thus, the diagnosis was first established at autopsy. Immunohistochemistry and sequencing of the DNA extracted from liver and lung tissue confirmed the presence of A. fumigatus in both tissues. There were no mutations in the FKS1 hot spot regions. Moreover, it was confirmed by FKS1 sequencing that the sample did not contain A. lentulus DNA (A. fumigatus-related species with reduced echinocandin susceptibility) (46). It thus appears that the patient developed invasive aspergillosis with a wild-type A. fumigatus isolate; however, we cannot rule out that the isolate had other resistance mechanisms, as the isolate was not available for susceptibility testing. An isolate with reduced susceptibility due to an upregulation of the target enzyme level was recently reported (4).

Echinocandin resistance in C. albicans has been associated with mutations in two hot spot regions of FKS1, which encode the target and major subunit of glucan synthase (24, 39, 46). In our case, a point mutation in the FKS1 gene leading to a S645P amino acid substitution was found. Caspofungin resistance was clearly shown by Etest, disk diffusion, and agar dilution, and the CLSI and EUCAST microdilution reference methods also demonstrated elevated MICs. However, none of the microdilution MICs were above the suggested CLSI susceptibility breakpoint of 2 µg/ml for the candins (44). For anidulafungin, the microdilution MICs were remarkably low, 0.06 to 0.25 µg/ml using EUCAST and 0.25 to 0.5 µg/ml using CLSI methodology, and the Etest MIC was also below the suggested breakpoint.

A number of studies have similarly demonstrated caspofungin MICs not exceeding 2 µg/ml despite resistance mechanisms in C. albicans (5, 10, 11, 19, 24), while others have found more pronounced MIC elevations in resistant isolates (15, 28, 36). Susceptibility testing by the CLSI method has developed over time. Initially, 48 h of incubation and a stringent 80 to 95% inhibition endpoint were recommended when testing azoles, amphotericin B, and flucytosine. Now, 24 h of incubation and a less stringent 50% endpoint are recommended, due to the earlier and more reproducible endpoints, and the candins have been included. When caspofungin MICs for clinical isolates of Candida species reported in the literature are compared, a considerable variation is observed, which impairs the establishment of epidemiological cutoff values that correctly define the wild-type populations (Table 5) (2, 6, 23, 33, 40, 41, 43, 45). These differences may at least in part be explained by changing methodology, but a lack of stability of the caspofungin pure substance may also, at least in theory, play a role. Head-to-head comparisons of microdilution MICs for the three candins have demonstrated that anidulafungin and micafungin MICs are 1 log₂ step (40) to 4 log₂ steps (33, 34) lower than the caspofungin MICs in RPMI medium. In agreement with this finding, caspofungin endpoints were higher than anidulafungin MICs for both C. albicans isolates by microdilution and by Etest in this study. Three observations indicate that this in vitro difference in the MIC range does not translate into better activity: firstly, when susceptibility testing is performed in the presence of serum, higher and nearly equivalent MICs are yielded (30, 34); secondly, if equivalent doses are compared in an animal model, no superiority is seen for anidulafungin and micafungin (34); and thirdly, in the present case, not only caspofungin but also anidulafungin failed to reduce the kidney burden in the animal model using doses equivalent to the human doses despite the fact that anidulafungin MICs were lower than caspofungin MICs. The CLSI breakpoint for candin susceptibility was established by taking into account analyses of mechanisms of resistance, the MIC population distribution, parameters associated with success in pharmakodynamic models, and results of clinical efficacy studies (44). As no significant differences in clinical response were noted among the various species, results for all species were merged, and a susceptibility breakpoint of 2 µg/ml was found to encompass the vast majority of isolates and not to bisect the population of Candida parapsilosis. However, it was noted that isolates of C. albicans and C. glabrata with MICs of 1 to 2 µg/ml are clearly outside the wild-type population and that further studies and experience with such isolates are warranted. The data reported here suggest that the establishment of species-specific candin breakpoints may be necessary and also that different breakpoints for caspofungin and anidulafungin may be needed, as has been described recently for an analysis of inhibition of glucan synthase and MIC in echinocandin-resistant strains of C. albicans (13). Moreover, the findings in this and others studies showing that the Etest separates the isolates with and without resistance mutations better clearly illustrate that further research is needed in order to define the optimal noncommercial reference methodology for susceptibility testing of candins.

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TABLE 5. Caspofungin MIC90s indicated by species as reported in studies using the M27-A3 CLSI method approved for candin susceptibility testing and in studies using earlier standards

In conclusion, we here present a rare case of breakthrough double infection with A. fumigatus and caspofungin-resistant C. albicans in a patient with long-term caspofungin treatment. This case illustrates the diverse challenges in diagnosing fungal infections, performing susceptibility testing, and choosing optimal antifungal treatment.

 
We thank the Pathology Department of Hillerød Hospital for providing tissue specimens and Jytte Mark Andersen, Frederikke Rosenborg Petersen, Lydia Viekjær, and Birgit Brandt for excellent technical assistance at Statens Serum Institut. We thank Pfizer for providing voriconazole and anidulafungin pure substance and the anidulafungin Etest, Merck for providing caspofungin pure substance, and Schering Plough for providing posaconazole pure substance.

The study was not financially supported by any pharmaceutical company. However, it was supported by NIH grant 1R01AI069397-01 to D.S.P.

M.C.A. has been a consultant for Astellas, Pfizer, and SpePharm; has been an invited speaker for Astellas, Cephalon, Merck Sharp & Dohme, Pfizer, Schering-Plough, and Swedish Orphan; and has received research funding, although not for this particular study, from Pfizer. C.L.-F. has been a consultant for Pfizer and Schering Plough; has been an invited speaker for Pfizer, Gilead, Schering Plough, and Merck Sharp & Dohme; and has received research fundings from Pfizer, Gilead, Merch Sharp & Dohme, and Schering Plough. D.S.P. is a shareholder in Merck, has acted as a consultant for Merck, Pfizer, and Astellas, is an advisory board member for Merck, Pfizer, Astellas, and Myconostica (U.S. patent application 07763-O69WO1); has received research funding, although not for this particular study, from Merck, Pfizer, Astellas, and Myconostica; and has been an invited speaker for Merck, Pfizer, Astellas, and Myconostica. There are no conflicts of interest for B.B., W.B., G.G.-E., K.L.M., N.R., C.L., and H.E.J.

 
* Corresponding author. Mailing address: Unit of Mycology and Parasitology (43/117), Statens Serum Institut, Artillerivej 5, DK-2300 Copenhagen S, Denmark. Phone: 45 22 63 27 85. Fax: 45 3268 8180. E-mail: mad{at}ssi.dk

Published ahead of print on 22 December 2008.

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Antimicrobial Agents and Chemotherapy, March 2009, p. 1185-1193, Vol. 53, No. 3
0066-4804/09/$08.00+0     doi:10.1128/AAC.01292-08
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