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Antimicrobial Agents and Chemotherapy, December 2002, p. 3797-3801, Vol. 46, No. 12
0066-4804/02/$04.00+0 DOI: 10.1128/AAC.46.12.3797-3801.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Central Research Laboratories, Kaken Pharmaceutical Co., Ltd., Shinomiya, Yamashina-ku, Kyoto-city, Kyoto 607-8042,1 Faculty of Pharmaceutical Sciences of Kinki University, Kowakae, Higashi-Osaka-shi, Osaka 557-0818, Japan2
Received 30 May 2002/ Returned for modification 28 June 2002/ Accepted 19 August 2002
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When terbinafine was orally administered to healthy individuals at a daily dose of 250 mg for 28 days, a concentration 10- to 1,000-fold higher than MICs for dermatophytes was achieved in nails and persisted for up to 4 months after stopping treatment (3). Nevertheless, a long-period treatment over 3 months does not lead to cure rates above 80% for toenails (31), and relapse often occurs after treatment (4, 21, 30). Presumably because oral antifungal drugs cannot remove fungi from nails before nails are turned over, the pace of response of tinea unguium seems to be a function of the nail growth rate. It is likely that the low fungicidal effect for tinea unguium is related to pharmacokinetic problems of oral antifungal drugs in the nails (20). Indeed, many antifungal agents are strongly bound to keratin, which not only reduces their antifungal potency but also may restrict their penetration into the nails (22, 24, 25).
We reported that KP-103, a novel topical triazole antimycotic, is highly effective in treating and preventing relapse in guinea pig models of tinea pedis and tinea corporis (16, 24-26), and its effectiveness is presumably because KP-103 has lower keratin affinity than the existing antifungal drugs and is largely retained as an active form that is not bound to keratin in the horny layer (25).
In the present study, to predict the potency of antifungal activity of KP-103 in the nails, we examined the influence of keratin on its in vitro antidermatophyte activity and its affinity to keratin compared with those of amorolfine and terbinafine. Moreover, we evaluated the therapeutic effect of KP-103 for guinea pigs with tinea unguium and plantar tinea pedis caused by Trichophyton mentagrophytes in comparison with the two reference drugs. Their therapeutic efficacies were assessed using the new method that we recently reported (26), which permits the precise recovery of fungi from the treated tissues by removal of a carryover of the drug remaining in the skin tissues into the culture media.
(This work was presented at the 40th Interscience Conference on Antimicrobial Agents and Chemotherapy, Toronto, Ontario, Canada, 17 to 20 September 2000 [Y. Tatsumi, M. Yokoo, and H. Senda, Abstr. 40th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 384, 2000].)
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Media. Sabouraud dextrose broth (SDB) and Sabouraud dextrose agar (SDA) were purchased from Difco Laboratories, Detroit, Mich. Potato dextrose agar was purchased from Nissui Seiyaku, Tokyo, Japan. The morpholinepropanesulfonic acid (MOPS)-buffered RPMI 1640 agar medium (pH 7.0), which contained liquid RPMI 1640 medium supplemented with L-glutamine (Nissui), 2% sodium bicarbonate, 0.165 M MOPS, and 1.6% agar, was used.
Test organism. Clinical isolates of T. mentagrophytes SM-110 and KD-04 were supplied by S. Fujita, Niigata University School of Medicine, Niigata, Japan, and H. Takahashi, Teikyo University School of Medicine, Tokyo, Japan, respectively. Candida kefyr JCM 1167 was purchased from The Institute of Physical and Chemical Research, Saitama, Japan.
In vitro study. (i) In vitro antidermatophyte activity in the presence of keratin. MICs for T. mentagrophytes KD-04 were determined by the following method. Ten microliters of twofold serial dilutions of the drugs solubilized in dimethyl sulfoxide (DMSO) was dispensed into a tube containing 1 ml of SDB medium containing 50 mg of defatted-keratin powder. Each tube was seeded with 106 microconidia of T. mentagrophytes SM-110 and then incubated at 30°C for 7 days. For reference, the same test was also run in SDB without keratin powder. The MICs were defined as the lowest drug concentrations that inhibited visible growth of the fungi.
(ii) Affinity to keratin. The affinities of the test drugs to keratin powder were determined by a slight modification of the method of Uchida et al. (29). A 100-µl sample of the drug solution prepared in DMSO at a concentration of 1 mg/ml was dispensed into 9.9 ml of saline containing 5% defatted-keratin powder. After shaking at 37°C for 1 h (75 rpm), the mixture was centrifuged at 3,000 x g for 5 min, and two 100-µl portions of the supernatant were taken to determine the rate of binding of drug to keratin. The precipitate (the drug-bound keratin) was washed 10 times by shaking (75 rpm) in 10 ml of saline at 37°C for 10 min. After each washing, the mixture was then centrifuged at 3,000 x g for 5 min, and two 100-µl samples were taken from the supernatant to determine the rate of release of the drug from keratin.
To determine the drug concentration in the sample solution using bioassay, standard curves were constructed. All drugs tested were dissolved in DMSO at a concentration of 1 mg/ml, serially diluted twofold with DMSO, and then diluted 1:100 with saline. Bioassay was performed in MOPS-buffered RPMI 1640 agar medium containing C. kefyr JCM 1167 (2 x 104 cells/ml) for KP-103 and SDA and potato dextrose agar containing T. mentagrophytes KD-04 (1 x 104 conidia/ml) for amorolfine and terbinafine, respectively. Assay plates were prepared in duplicate. Wells (8 mm in diameter) were cut in the agar. Aliquots (100 µl) of samples and standards were dispensed into the wells and incubated at 30°C for 3 days. Diameters of zones of inhibition were measured with a vernier caliper to the nearest 0.01 mm. Standard curves relating the zone diameter to the concentration of the drugs were prepared on a semilogarithmic graph and were linear over the range of 0.039 to 10 µg/ml for KP-103 and terbinafine and 0.156 to 10 µg/ml for amorolfine, with
2 being >0.99.
In vivo efficacy. (i) Animals. Male Hartley strain guinea pigs weighing 370 to 420 g were used in the study. The experiments were performed with groups of five animals.
(ii) Preparation of inocula. Arthrospores of T. mentagrophytes SM-110 were prepared by the method described previously (24-26). The arthrospores were collected and suspended in saline containing 0.05% Tween 80. The suspension was homogenized with a glass homogenizer, filtered through gauze, and then adjusted to give a concentration of 108 spores/ml by counting with a hemocytometer.
(iii) Production of tinea unguium and tinea pedis. Guinea pigs were infected by a slight modification of the method described previously (25, 26). In brief, two paper disks (4 by 8 mm; AA disk; Whatman Japan KK) were immersed in the fungal suspension, applied between the toes of the hind paw (between the second and third toes and between the third and fourth toes) with a foam pad (Reston self-adhering foam pads [catalog no. 1560]; 3M Co.), and fixed with an adhesive elastic tape (Elastopore; Nichiban, Tokyo, Japan) (day 0 postinfection). The disks were removed on day 21 postinfection.
(iv) Histological examination of nail tissue. Nails were taken from infected animals on day 60 postinfection. The nails were fixed in 10% (vol/vol) buffered neutral formalin solution, decalcified in 10% (vol/vol) buffered formic acid for 1 month, fixed again in the formalin solution, and embedded in paraffin. Thin paraffin sections of the nails were examined by light microscopy after staining with periodic acid-Schiff stain.
(v) Drugs and treatment. KP-103, amorolfine, or terbinafine was dissolved in a mixture of polyethylene glycol 400-ethanol (75:25, vol/vol) (1, 17, 24-26) at 1% for topical application. Terbinafine hydrochloride tablets (Lamisil) were purchased from Novartis Pharma KK and suspended in coconut oil with a mortar, and the suspension was dispensed into capsules for oral administration. A 1% solution (0.1 ml) of KP-103, amorolfine, or terbinafine was topically applied to the nails and whole sole of the foot of guinea pigs once daily, or a capsule of terbinafine was orally administered to guinea pigs at a once-daily dose of 40 mg/kg of body weight. Each treatment was started on day 60 postinfection and continued for 30 consecutive days. The control group of animals was infected and received vehicle therapy.
(vi) Evaluation of therapeutic efficacy. The therapeutic efficacy for tinea unguium was evaluated by the method reported previously (26) with a slight modification. Two days after the last treatment, all animals were sacrificed and nails were taken from the treated feet, which were wiped with a cotton swab containing 70% ethanol. The length of each nail was measured with a vernier caliper. The nail sample was minced with scissors, homogenized in phosphate-buffered saline (PBS) (pH 7.4) containing 2% trypsin, and digested at 37°C for 1 h. After centrifugation, the precipitate was washed with 10 ml of PBS three times by centrifugation at 3,000 x g for 10 min to remove trypsin and the remaining drug and was suspended in 4 ml of sterilized water. The suspension was dialyzed with a cellulose membrane tube in distilled water at 4°C for 14 days to eliminate completely the drugs in the nails. The suspension was centrifuged at 3,000 x g for 10 min, and the precipitate was suspended in 1 ml of PBS. One hundred microliters of the sample and serial 10-fold dilutions thereof were spread onto an SDA plate containing 1 g of cycloheximide, 100 mg of gentamicin, 50 mg of chloramphenicol, and 50 mg of flucytosine, per liter, to select the fungi. The plates were incubated for 10 days at 30°C, and fungal colonies were counted. The CFU were enumerated, and the log10 of CFU in the nails per foot was calculated (assay limit, 10 CFU per foot). Nail specimens yielding more than one fungal colony were regarded as fungus positive. The therapeutic efficacy against plantar tinea pedis was evaluated by the same method reported previously (26), on the basis of fungus-positive feet (percentage) and the log10 of CFU in the treated plantar skin per foot (assay limit, 20 CFU per foot).
(vii) Statistical analysis. Nail and skin samples with negative culture results were considered to contain 10 and 20 CFU, respectively, for numerical and statistical purposes. The frequency of fungus-positive nails or plantar skin per foot was analyzed by Fisher's exact probability test. The log10 of CFU in nails and plantar skin and the length of the nails were analyzed by the Kruskal-Wallis test (Tukey-type comparison test). P values of less than 0.05 were regarded as significant.
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TABLE 1. Influences of keratin powder on the antifungal activities of KP-103 and reference antifungal drugs against T. mentagrophytes SM-110a
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FIG. 1. Release of KP-103 or two reference antifungal drugs from the drug-preloaded keratin powder. The drug-bound keratin was washed in saline at 37°C for 10 min with shaking (75 rpm), and this procedure was repeated 10 times.
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FIG. 2. Histological examination of the nails of guinea pig hind paws, 60 days after T. mentagrophytes SM-110 inoculation. The fungi invaded the nail plate. Bar = 40 µm.
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TABLE 2. Therapeutic efficacies of KP-103 and reference drugs in a guinea pig model of tinea unguiuma
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TABLE 3. Therapeutic efficacies of KP-103 and reference drugs in a guinea pig model of tinea pedisa
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In the present study, we successfully eliminated drugs from all treated nails using the new technique with trypsin treatment and the dialysis procedure of treated tissues that we reported previously (26), with resultant detection of infecting fungi in all treated nails. The dialysis period of 14 days was necessary for removing all drugs tested from the treated nails, which was longer than the 3 days used for plantar skin tissues previously (26). This is presumably because antifungal agents have a high binding affinity to keratin (24, 28) and more accumulated in the nails than in the skin tissues. It is suggested that the method is useful for evaluating the therapeutic efficacy of antifungal agents against the tinea unguium model of animals because it permits the complete recovery of viable fungi from all drug-treated nails, whereas the conventional method does not recover fungi because of the drug carryover effects. Moreover, the results obtained by the new method in the present study correlated well with clinical knowledge that tinea unguium responds more poorly to antifungal chemotherapy than does tinea pedis (13, 23).
Since dermatophytes parasitize the keratinized tissues of the horny layer of the epidermis, hair, and nails, the therapeutic efficacy of applied antifungal agents depends on not only their in vitro antifungal activities but also their pharmacokinetic properties in the keratinized tissues (27). KP-103 was 8- and 32-fold, respectively, less active than amorolfine or terbinafine against T. mentagrophytes SM-110 in SDB medium. However, KP-103 was more effective than the reference drugs for inhibiting nail collapse or reducing the fungal burden of the nails. These results suggest that KP-103 shows a better pharmacokinetics in the nails than the reference drugs.
To predict the pharmacokinetics of KP-103 in the nails, we examined the influence of keratin on its antifungal activity and its affinity to keratin compared with those of amorolfine and terbinafine. Terbinafine and amorolfine showed a greatly decreased antifungal activity in the presence of 5% keratin. This was because the reference drugs have extremely high rates of binding to keratin of more than 90%. Uchida et al. (29) also reported that terbinafine showed a high rate of keratin binding, 88.4%, when incubated in a buffer with 10% keratin. These results suggest that these reference drugs will be further inactivated in the nails, because about 90% of the constituent of nails is keratin. In contrast, the antifungal activity of KP-103 was not affected by keratin because of its lower affinity to keratin, suggesting that KP-103 largely exists in the nails as an active form that is not bound to keratin. Antifungal agents bound to keratin need to be released readily from it to effectively diffuse into the deeper nail plate and sterilize infected nails. Even though KP-103 was bound to keratin suspended in saline, it was readily released from keratin by washing with saline. Its release from keratin was higher than those of amorolfine and terbinafine. It is likely that KP-103 diffuses in the nails without being trapped by keratin.
Topical amorolfine and topical or oral terbinafine were not effective in reducing the fungal burden of the nails, although the reference drugs reduced or eradicated the fungal burden in plantar skin tissues. These results suggest that the two reference drugs were possibly inactivated by keratin in the nails, although they were absorbed into the nails as in plantar skins. On the other hand, topical KP-103 was effective for both reducing the fungal burden of nails and eradicating infection in plantar skins. The effectiveness of KP-103 for tinea unguium is presumably due to its high activity and good penetration into the nails, which are achieved by its lower keratin affinity.
It was reported that orally administered terbinafine is effective for the treatment of human tinea unguium (3, 4, 12, 30, 31). However, in the present study, oral terbinafine was not effective for the tinea unguium in the guinea pig model. This discrepancy is possibly due to the fact that the treatment period of 30 days for guinea pigs was shorter than that necessary to show therapeutic effect against human tinea unguium, which is at least 3 months (4, 30, 31). It was reported that a concentration of terbinafine (0.1 to 1.53 µg/ml) higher than its MIC (0.001 to 0.02 µg/ml) for dermatophytes was achieved in the nails when it was orally administered to patients with tinea unguium at a daily dose of 250 mg for 28 days (2). In the present study, since guinea pigs were orally treated with terbinafine for 30 days at a daily dose of 40 mg/kg, which is about 10-fold higher than that used in humans, a high concentration of terbinafine would be achieved in the nails. However, terbinafine failed to reduce the fungal burden in the affected nails. This finding also suggests that terbinafine would be inactivated by its strong binding to keratin in the nails.
In conclusion, KP-103 is a very promising antifungal candidate in the treatment of human tinea unguium because topically applied KP-103 exhibits a better therapeutic effect than oral terbinafine and possibly allows shorter periods of treatment, which improves patient compliance and yields no risk of systemic adverse effects and drug interactions, unlike oral antifungal drugs.
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