STRUCTURE AND PERMEABILITY OF HUMAN NAIL 381 a function of AC concentration, are shown in Figure 7. The results depicted in Figure 7 are for 2% rniconazole nitrate formulations containing 20% urea and varying amounts of AC, with the donor being replenished every three days. Replenishing the donor compartment at 3- or 7-day intervals did not produce a change in the permeation profiles for rniconazole. The total duration of the study was three weeks. From Figure 7, it is seen that doubling the AC concentration from 5% to 10% results in a 2-2.5-fold increase in the amount of drug permeating through the nail, as well as into it. By dividing the amount of drug in the nail (mg/cm 2) by the nail thickness (cm), these researchers calculated miconazole or itraconazole concentrations in the nail (rng/cm 3) from formulations containing AC and urea. It was reported that the drug concentrations measured in the nail exceeded the MICs of these antifungal compounds. Kobayashi et al. (46) also reported the effect of various enhancers on the in vitro nail permeation of 5-fluorouracil (5-FU) (water-soluble) and tolnaftate (TN) (water- insoluble) from aqueous and lipophilic vehicles. Analysis of the drugs was carried out by HPLC. Aqueous solvent systems containing urea, sodium salicylate, N-acetyl-l-cysteine (AC), 2-mercaptoethanol (ME), or menthol were used, as well as lipophilic systems containing lactic acid, AC, or ME. Nail weights and stresses (measured using a rheom- eter) as a result of these treatments were also reported. Nail weights increased and stresses decreased in water-containing solvent systems. The most dramatic effect on nail weights and stresses was seen with aqueous systems containing AC or ME. No signifi- cant changes in nail swelling and softening properties were observed with the lipophilic systems. Thus, the authors state that the water content of the nail may relate to its physicochernical properties. Permeation studies using 5-FU and TN were carried out in modified side-by-side diffusion cells (area = 0.049 cm2), with temperature maintained at 37øC. Fluxes of 5-FU were 13 and 16 times higher for aqueous vehicles containing AC or ME than for the control vehicle (water), respectively. Lipophilic vehicles with AC or ME resulted in a 6.7 and 8.4 times increase in flux for 5-FU, as compared with the control. For TN, the fluxes were only detectable from systems containing AC or ME. The change in physicochemical properties of the nail (weights and stresses) and the increase in drug permeation observed with AC and ME suggest that these two compounds may be effective penetration enhancers for the nail. The authors postulate that the increased permeation results because of a reduction in the keratin disulfide linkages in the nail by the action of AC or ME. In the same study (46), the effect of AC concentration and reversibility of penetration enhancement by AC was assessed. For this purpose, the permeation of 5-FU in a control vehicle (water) was monitored over a period of seven days (step 1), followed by a washout period of a day. Subsequently, the same nails were treated with 5-FU in vehicles containing AC in concentrations of O. 1-10% for another week (step 2), and the washout procedure (one day) was repeated. Finally, the permeation of 5-FU from the control vehicle (water) was again measured for a week (step 3). Flux ratios (step 2/step 1) were used to investigate the effect of AC concentration, whereas flux ratios (step 3/step 1) were used to assess the reversibility of the treatment. Flux ratios (step 2/step 1) increased with increasing AC concentration. Moreover, flux ratios (step 3/step 1) were very similar to the step 2/step 1 values, indicating low reversibility of the process. The authors spec- ulated that the nail permeability barrier may require more time to recover to its original state.

382 JOURNAL OF COSMETIC SCIENCE TOPICAL TREATMENT OF ONYCHOMYCOSIS: INVESTIGATIONAL STUDIES Topical medication for OM has remained a secondary form of treatment due to the extremely low permeability of the nail plate to antifungal drugs. Thus, these actives are unable to achieve significant concentrations at the site of infection in order to be therapeutically effective. At best, topical medication is effective in the case of the more superficial forms of OM. Despite the challenges associated with topical therapy, direct application of an efficacious antifungal product to the nail appears to be most desirable, since it is noninvasive and free of systemic side effects. Some effort has been made in the last two decades in improving the efficacy of topical products intended to treat OM. Farher and South (47) first reported the use of urea for nonsurgical nail removal. Following this, it was reported that a 20% urea and 10% salicylic acid preparation was successful in achieving painless, nonsurgical avulsion of nails after a two-week occlusive application (48). Atraumatic nail avulsion using 50% potassium iodide or 40% urea has been reported to be effective without damage to the nail organ (49). Several investigational treatments now include the use of urea along with an antimycotic agent for the chemical removal of the infected nail. Urea increases the binding capacity of the nail for water, which results in swelling of the keratin matrix, thus simultaneously enhancing penetration of the active. A typical example of an in- vestigational trial is the application of a 1% bifonazole/40% urea ointment until removal of the infected nail, followed by supplementary treatment with a 1% bifonazole cream for four weeks (50). Improvement in the condition of the nails was observed, provided the patients adhered to the treatment protocol. A review of the recent patent literature also shows that urea is commonly used in concentrations as high as 60-70% (possibly to induce noninvasive onycholysis) in conjunction with an antimycotic agent in various types of topical formulations. In cases where such keratolytic agents or other penetration enhancers are absent, very high drug concentrations are used (up to 30%). Agents capable of interacting with and reducing the disulfide linkages of keratin have recently emerged as a class of enhancers for nail penetration. Van Hoogdalem et aL (51) investigated the in vivo nail penetration of the antimycotic agent oxiconazole from a 1% w/v lotion, and the potential enhancing effect of 15% acetylcysteine (AC) on this penetration. The topical medication was applied twice daily to the fingernails of six healthy subjects for a period of six weeks, and nail clippings were collected every two weeks over the treatment period and until two weeks after completion of the treatment. The levels of the antifungal drug at various depths in the nail clippings were analyzed by a gas-chromatographic method. The maximum levels of oxiconazole nitrate in the absence of AC were observed in the upper layers (0-50 tam) of the nail clippings and varied from 120-1420 ng/mg. Co-delivery of AC statistically prolonged the mean residence time significantly for the drug in the 51-100 tam layer in the ring fingernails from 3.7-4.9 weeks to 4.1-6.4 weeks, implying increased retention in the nail. Mean drug levels in the upper nail layers also increased in the presence of AC relative to the control, thus suggesting that AC is a potential penetration enhancer for nails. Thus, the use of a reducing agent containing a sulfhydryl group (such as AC) appears to be effective in enhancing penetration of antifungal drugs through the nail. CONCLUSIONS Topical management of nail diseases remains a challenge due to the low permeabilities