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Antimicrobial Agents and Chemotherapy, September 2007, p. 3317-3321, Vol. 51, No. 9
0066-4804/07/$08.00+0 doi:10.1128/AAC.01185-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Unit of Mycology and Parasitology, Statens Serum Institut, Copenhagen, Denmark,1 Department of Pharmacy, Martin-Luther-Universitaet, Halle-Wittenberg, Germany,2 National Centre for Antimicrobials and Infection Control, Statens Serum Institut, Copenhagen, Denmark,3 Department of Plastic Surgery Odense University Hospital, Odense, Denmark,4 Department of Dermatology, Bispebjerg University Hospital, Copenhagen, Denmark5
Received 22 September 2006/ Returned for modification 28 January 2007/ Accepted 13 June 2007
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Oral griseofulvin has long been the standard treatment of tinea capitis; however, this agent has been withdrawn from the market in several countries in Europe. Alternative antifungal agents have been suggested: itraconazole, which is not universally licensed for the treatment of children (7, 10); fluconazole, which in most comparative studies appears to be less effective when M. canis is involved and which is not approved for tinea capitis in the United States (8); and finally terbinafine, which again is less efficient in Microsporum than in Trichophyton infections (6). Although there is no standardized method for susceptibility testing of dermatophytes, studies using modifications of the Clinical and Laboratory Standards Institute (CLSI; formerly the National Committee for Clinical Laboratory Standards) methodology have shown voriconazole to be active against dermatophytes, including Microsporum species, with MICs in the range of 0.01 to 2 µg/ml (5, 13). We therefore sought to investigate the pharmacokinetics and in vivo efficacy of voriconazole in a guinea pig model of M. canis dermatophytosis. Skin biopsy specimens, whole blood, and samples obtained by microdialysis (from infected and uninfected skin) were collected in order to measure the total concentrations of voriconazole and the unbound fraction to correlate this with in vivo efficacy.
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Preparation of inoculum. A clinical isolate of M. canis was incubated at 25°C on Sabouraud glucose agar (SAB) with cycloheximide and chloramphenicol (SSI Diagnostica, Hillerød, Denmark) for 3 weeks. The fungal cells were harvested by floating the colonies with 12 ml of sterile water mixed with 2 drops of phosphate-buffered saline (pH 7.4) containing 0.01% Tween 20 (SSI Diagnostica), followed by gentle agitation with a single-use sterile pipette. Macroconidia and large hyphal elements were removed by filtration (Nylon Net Filters, pore size 11 µm; Millipore, Carrigtwohill, Ireland); the number of microconidia was counted in a hemacytometer and adjusted to approximately 8 x 105 conidia/ml. The viability of the conidia was confirmed by growth on SAB.
Antifungal susceptibility testing. The MIC was determined according to the CLSI M38-A method procedure with the following modifications. The inoculum used was 105 CFU/ml, and the incubation was done at 25°C for 10 days (11). Determinations were performed in triplicates, and the MIC was determined to be 0.25 µg/ml.
Anesthesia. Guinea pigs were anesthetized by intramuscular administration of a combination anesthetic containing Zoletil 50 Vet (Zolazepam [125 mg] and Tiletamin [125 mg]; Virbac S.A., Carros, France), Xylazin Vet (20 mg/ml; Intervet International B.V., Boxmeer, The Netherlands), and Torbugesic Vet (Butorphanol [10 mg/ml]; Fort Dodge Veterinaria S.A., vall de Bianya, Spain). The guinea pigs were anesthetized before inoculation, voriconazole dosing, and specimen collection. Neutral Vaseline ointment was used to prevent cornea damage.
Infection model. Sixteen guinea pigs (n = 8 in the voriconazole group and n = 8 in the untreated control group) were clipped and shaved with a single-use manual razor (Zorrik twin blades; Super-Max., Ltd., Feltham, United Kingdom) at five areas at the flanks and back to prepare the inoculation. Shaving traumatizes the skin and makes it more susceptible to infection (17). Portions (50 µl) of fungal suspension (8 x 105 conidia/ml) were used to inoculate four of the five ring areas on each guinea pig. The areas were margined by a Vaseline ring and measured 1.5 cm in diameter. The guinea pigs were evaluated clinically on days 3, 7, 10, 14, and 17 by the same person. The clinical evaluation consisted of a semiquantitative score where the inoculated areas were evaluated and compared to the uninfected ring area on the back of the same animal. The redness score was graduated as follows: 0, normal; 1, pink; 2, red; and 3, violet. The lesion score was graduated as follows: 0, normal; 1, papule; 2, skin scales; 3, single layer of skin scales and ulcers; and 4, multiple layers skin scales and ulcers. Examples of clinical scores are shown in Fig. 1. Areas that had been used for prior specimen sampling were excluded for subsequent clinical evaluation to avoid bias by redness and ulcers due to prior scraping. Mycological examinations were performed on days 3, 7, 14, and 17. The specimens were taken according to routine standards for obtaining material for dermatomycological examinations using a sterile sharp spoon for scraping and forceps for the collection of hair (12). Sampling was done after disinfection with 70% isopropyl alcohol and 0.5% chlorohexidine (OY Teampac AB, Helsinki, Finland) in order to remove superficial residual fungal elements from the previously applied inoculum. One of the four inoculation ring areas per animal was used on each occasion of sampling. The entire ring area (diameter of 1.5 cm) was scraped; however, the amount of material varied depending on the severity of the lesion (amount of hair, scales, and crust). In the case of no visual lesion, 20 to 30 hairs were obtained and the skin scraped. The specimens were placed between two glass slides and wrapped in paper. Half of the material was subsequently used for direct microscopy and the other half for culture. Direct examination was performed in 20% KOH (SSI Diagnostica) by using a light (phase) microscope. Microscopy was positive when hyphae, arthroconidia, and/or conidia were detected. Cultures were incubated on SAB with cycloheximide and chloramphenicol (SSI Diagnostica) for 28 days and evaluated weekly. M. canis is considered a true pathogen, and a positive culture is significant for the diagnosis of dermatophytosis. The numbers of colonies (i.e., CFU) per specimen were noted, and identification to the species level was done according to the method of Campell et al. (2). Lactophenol Cottonblue-Phenol+Ethanol (SSI Diagnostica) was used for coloring the material before microscopy. Negative controls of hair were used to test the possibility of contaminating the specimens in the laboratory.
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FIG. 1. Examples of clinical scores: A, redness score (RS) pink and lesion score (LS) normal; B, RS pink, LS skin scales; C, RS pink, LS one layer skin scales and ulcers; D, RS red, LS multiple skin scales and ulcers.
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Microdialysis procedure. The three first numbers of the treated guinea pigs were included in the pharmacokinetic part of the study day 14. The animals underwent surgery in general anesthesia. Skin channels were made in the dermis by penetrating uninfected (normal) and inoculated skin by a cannula with diameters of 1.2 and 1.65 mm (BD Microlance; BD Drogheda, Ireland). CMA 70 microdialysis brain catheters with an outer diameter of 0.9 mm (CMA/Microdialysis, Solna, Sweden) were placed in the channels and CMA 107 microdialysis infusion pumps were connected. The positions of the catheters were controlled by visual inspection of the dermal channel by autopsy. The tissue was allowed to recover for 30 min after the insertion trauma before starting the experiment. Flow rates were set to 1 µl/min, and isotonic NaCl solution was chosen as the perfusate. Prior to the voriconazole administration, in vivo relative recovery of voriconazole was attained in every guinea pig. This was done according to the retrodialysis method as previously published (19) in order to calibrate the probe. For this, two premanufactured concentrations (5 and 10 µg/ml) of voriconazole (intravenous solution, purchased from Pfizer) were added to the perfusate. The in vivo relative recovery was calculated as follows: RR (%) = [1 – (Cdialysate/Cperfusate)] x 100, where Cdialysate is the outlet concentration (µg/ml) and Cperfusate is the inlet concentration of voriconazole (µg/ml). The unbound tissue concentration was defined as Ctissue = 100 x Cdialysate x RR–1.
Time zero was defined as the end of the administration of voriconazole (20 mg/kg/day, the twelfth dose). Microdialysates were collected every 30 min over a period of 3 h.
Blood samples and skin biopsy specimens. Blood samples were obtained from the eye vein and collected hourly beginning 1 h after voriconazole administration. Full-thickness 3-mm skin punch biopsy specimens (Produkte für die Medizin AG, Cologne, Germany) were collected from the margin of the inoculated area hourly.
Voriconazole concentration determinations in serum, skin biopsy, and interstitial fluid samples. (i) Chemicals. Voriconazole for high-performance liquid chromatography (HPLC) analyses was provided by Pfizer Global Research and Development (Sandwich Kent, USA) and was used under isocratic conditions with a flow rate of 1 ml/min. Acetonitrile (HPLC gradient grade) was purchased from Roth (Karlsruhe, Germany), ammonia (30%) was from Gruessing (Filsum, Germany), and ammonium dihydrogen phosphate was from Fluka Chemie AG (Neu-Ulm, Germany).
(ii) HPLC. HPLC experiments were performed by using an HPLC system (JASCO, Gross-Umstadt, Germany) with UV detection at 254 nm. Samples were separated on a LiChrospher-100 RP-18 (5 µm, 125-by-4-mm column with an integrated precolumn (Merck, Darmstadt, Germany) as the stationary phase. The mobile phase consisted of an acetonitrile-ammonium phosphate buffer (pH 6.0; 0.04 M; 46:54 [vol/vol]) and was used under isocratic conditions with a flow rate of 1 ml/min. The lower and upper limits of quantification of voriconazole were 0.2 and 10 µg/ml, respectively.
(iii) Sample preparation. Microdialysates, retrodialysis, and probe calibration solution samples were transferred from the microdialysis microvials into safe-lock tubes. After a simple one-step dilution of microdialysates with acetonitrile at the ratio 40 to 60 (vol/vol) and vortexing (Microshaker Type 326; Premed, Warsaw, Poland), a volume of 20 µl was injected into the HPLC system. Whole-blood samples were prepared by a two-in-one step preparation procedure. Precipitation of proteins and dilution were performed by mixing a 50-µl aliquot with 75 µl of acetonitrile. The mixtures were vortexed and centrifuged at 13,500 x g for 15 min (Eppendorf Centrifuge 5417R; Eppendorf AG, Hamburg, Germany). Then, 30 µl of the clear supernatant was injected into the HPLC system. Biopsy samples were extracted with acetonitrile-ammonium phosphate buffer (0.04 M, 60:40 [vol/vol]) for 6 h at room temperature under continuous shaking. Five freeze-thaw cycles with liquid nitrogen were performed during the first hour in order to force extraction of the skin. After the extraction procedure the samples were centrifuged at 13,500 x g for 10 min, and 35 µl of the supernatant was injected into the HPLC system.
Statistics. Statistical analysis was performed by using PC SAS version 9.1.3 (SAS Institute, Inc., Cary, NC). A Wilcoxon two-sample test was used to compare the VCZ group to the control group with respect to clinical scores and colony counts per assessment time. P values of <0.05 were considered significant.
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FIG. 2. Clinical score in eight voriconazole-treated and eight untreated animals, respectively. (A) Lesion score (0, normal; 1, papule; 2, skin scales; 3, single layer of skin scales and ulcers; 4, multiple layers of skin scales and ulcers). (B) Redness score (0, normal; 1, pink; 2, red; 3, violet). The results are indicated as the means of the inoculated areas of all of the animals in each of the groups. Areas previously used for mycological sampling were excluded. Days 3 and 7 consist of four areas per animal, days 10 and 14 consist of three areas per animal, and day 17 consists of two areas per animal. Scores were compared between the two groups for each observation day, and the P values (Wilcoxon test) are shown above the columns.
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TABLE 1. Microscopy and culture results from the voriconazole-treated animals and an untreated positive control group over time
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FIG. 3. Voriconazole concentrations in interstitial fluid (microdialysis) and blood (A) and in skin biopsy specimens (B) after the twelth dose (20 mg/kg/day). NS, normal skin; IN, inoculated skin. The results are presented as means ± the SD.
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TABLE 2. Voriconazole concentrations in whole blood, microdialysates, and skin biopsy samples over the whole sampling time (3 h)
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Voriconazole has been demonstrated to possess in vitro activity against dermatophytes (5, 13), with MICs for Microsporum spp. in general (M. canis, M. gypseum, M. cookei, M. distortum, M. audouinii, and M. nanum) of between 0.01 and 2 µg/ml (5, 13) and for M. canis specifically in the range of 0.01 to 0.5 µg/ml. In the present study we found an average concentration of non-protein-bound voriconazole in microdialysate fluid of 1.4 µg/ml (mean Cmax = 1.7 µg/ml; range, 1.1 to 2.0 µg/ml) in the 3-h observation period after dosing the twelfth day of treatment. The concentration of voriconazole in the microdialysate was four times lower than in blood but still above the MIC of the M. canis isolate (0.25 µg/ml). Furthermore, the concentration in skin biopsy specimens was notably high, as illustrated by a mean concentration of 18.3 µg/g. Although these concentrations cannot be directly compared, the data demonstrate an accumulation in the skin that cannot be explained by protein binding solely. Such accumulation has been described for other compounds in this drug class (4, 18). Itraconazole and fluconazole have been shown to accumulate in stratum corneum of the skin and fluconazole has been shown to be incorporated into the keratin structures of epidermal adnexa, where it can be detected months after the cessation of the treatment (4, 18, 20). This accumulation of itraconazole and fluconazole is thought to contribute to the eradication of the dermatophyte infection and to the fact that pulse therapy including weeks without treatment is effective. Future studies are warranted to elucidate the nature and time course of the voriconazole accumulation in the skin.
We did not find any difference between the voriconazole concentrations measured in microdialysates from normal and infected skin areas. This was probably due to the fact that the infection was almost healed at this time. Thus, we cannot rule out that an ongoing infection with inflammation may influence voriconazole concentration in interstitial fluid. A possible difference might have been found, if the pharmacokinetic experiment had been performed at an earlier stage of the infection. Furthermore, the fact that the concentrations did not vary considerably during the sample period may be explained by the multiple dosing previously.
The levels of voriconazole were still increasing 3 h after dosing in whole-blood and skin biopsy specimens, indicating that the peak concentration was not reached yet. This is in concordance with a study by Roffey et al. (16), who showed an increasing voriconazole blood concentration in guinea pigs during an 8-h period after a single oral dosing. The plasma concentration typically peaks earlier than 2 h postadministration in humans (15), and this discrepancy may at least in part be due to decreased gastrointestinal absorption during anesthesia.
In conclusion, we showed here the efficacy of oral voriconazole for treating dermatophytosis caused by M. canis in a guinea pig model. Furthermore, HPLC measurements of voriconazole concentrations in whole blood, microdialysates from skin, and especially skin biopsy specimens were in the range or higher than the MICs previously published for M. canis. These findings indicate that voriconazole may be a future alternative to griseofulvin for the treatment of tinea capitis in humans.
Published ahead of print on 18 June 2007. ![]()
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