November 1998
`
`Chem. Pharm. Bull, 46(1 l) 1797--1802 (1998)
`
`1797
`
`Enhancing Effect of N-Acetyl-L-cysteine or 2-Mercaptoethanol on the in
`Vitro Permeation of 5-Fluorouracil or Tolnaftate through the Human Nail
`Plate
`
`Yoichi KOBAYASHIa, Misao MIYAMOTOb, Kenji SUG1BAYASHIa and Yasunori MORIMOTO*’a
`
`Faculty of PharmaceuticaI Sciences, dosai University,’~ 1-1 Keyakidai, Sakado, Saitama 350-0295, Japan and Nissan
`Chemical Co., Ltd./’ 3 7-1 Kanda-Nishiki-cho, Chiyoda-ku, To~o 10l 0054, Japan.
`Received June 1, 1998; accepted August 5, 1998
`
`The enhancing effects of various vehicles on the in vitro permeation of a hydrophilic model drug, 5-fluo-
`rouracil (5-FU), or a lipophilic model drug, tolnaftate (TN), through human nail plates were investigated using a
`modified side-by-side diffusion cell. Tip pieces from the 5th finger-nail, clipped from healthy volunteers, were
`used in this permeation study. The swelling and softening properties of the nail pieces were also measured in
`each vehicle. The weights and stresses of the nail pieces were dramatically changed after immersion in aqueous
`solvents containing N-acetyl-L-cysteine (AC) or 2-mercaptoethanol (ME). However, no significant change in the
`physicochemical properties of the nail pieces was found in the lipophilic vehicles. Thus, the water content in the
`nail plates absorbed from vehicles may relate to their physicochemical properties. Although keratin-softening
`agents and new skin permeation enhancers did not significantly promote 5-FU permeation compared with water
`alone, the flux from solvent systems containing AC or ME was substantially higher. In addition, TN permeation
`from solvents containing AC or ME could be measured, whereas that from other solvents was undetectable.
`When the AC concentration was increased, the 5-FU permeation and the nail weight increased and the stress of
`each nail piece decreased. It is concluded from these experimental results that AC and ME may be useful as en-
`hancers for increasing drug permeation through the human nail plate.
`
`Key words nail; nail permeation; permeation enhancer; acetylcysteine; mereaptoethanol; 5-fluorouracit; tolnaftate
`
`Recent research on fungal diseases has produced a variety
`of new antifungal agents, having a low minimum inhibitory
`concentration (MIC) on Trichophyton rubrum or Trichophy-
`ton mentagrophytes. Trichophytosis is a parasitic mycosis in
`keratinous tissues such as the stratum corneum of skin, hair
`and nails. Parasitic nail mycosis (onychomycosis trichophyt-
`ica) is known as a particularly troublesome disease. Ony-
`chomycosis trichophytica has been treated mainly with oral
`antifungal medication.~’21 This oral therapy, however, some-
`times has severe systemic side-effects, such as liver dysfunc-
`tion,31 which interrupt the therapy. Although a lot of research
`has been published on the effect of clinical topical and sys-
`temic treatments of onychomycosis trichophytica, there are
`few in vitro nail plate penetration studies following topical
`drug exposure. Waiters et al.41 have suggested that the nail
`plate behaves like a hydrophilic gel membrane and that it has
`an additional lipophilic route from penetration data using ho-
`mologous alcohols. They also inferred that solvents such as
`dimethylsulphoxide and isopropyl alcohol, which tend to
`promote drug diffusion through the stratum corneum of skin,
`had little effect on nail plate permeation.5~
`Since few investigations have appeared regarding the en-
`hancement of nail plate permeation, we developed a modified
`side-by-side (2-chamber) diffusion cell to find out a penetra-
`tion-enhancing system.61 The barrier function of the nail
`plate may be closely related to changes in its physicochemi-
`cal properties (swelling and softening). The changes in the
`weight and stress of the nail pieces were then investigated.
`Urea and sodium salicylate are well known to have a keratin-
`softening effect and we previously reported that a new hy-
`drophilic multicomponent vehicle, consisting of l-menthol,
`ethanol and water (MEW system), markedly enhanced the
`in 71 permeation of morph e and several cardiovascular agentss)
`
`through hairless rat and human skin. We also reported that a
`
`new lipophilic multicomponent vehicle, consisting of L-lactic
`acid, ethanol and isopropyl myristate (LEI system), greatly
`promoted the skin permeation of ketotifen and several other
`drugs.9’1°1 In addition, chloramphenicol permeation from
`lipophilic vehicles or nail lacquers through the human nail
`plate or hoof membrane was compared with that from aque-
`ous systems. ~ l ) Also, a clinical trial showed that N-acetyl-L-
`cysteine (AC) increased oxiconazole levels in the nailJ21
`Therefore, keratin-softening agents (urea and sodium salicy-
`late), new skin permeation enhancers (MEW and LEI sys-
`tems) and agents reducing the disulfide linkages in hard ker-
`atin (AC and 2-mercaptoethanol) were selected as additives
`for evaluation. Tolnaftate (TN) and 5-fluorouracil (5-FU)
`were used as lipophilic and hydrophilic model drugs, respec-
`tively.
`
`Experimental
`Materials TN was supplied by Nissan Chemical Co., Ltd. (Tokyo,
`Japan). 5-FU and IPM were obtained from Tokyo Kasei Kogyo Co., Ltd.
`(Tokyo, Japan). Urea, sodium salicylate, L-lactic acid, ethanol, AC and 2-
`mercaptoethanol (ME) were obtained from Wako Pure Chemical Industries,
`Ltd. (Osaka, Japan). 1-Menthol was supplied by Toko Pharmaceutical Ind.
`Co., Ltd. (Tokyo, Japan). Other reagents were from commercial sources.
`Preparation of the Nail Plate Tip nail pieces (8--25mg) were ob-
`tained from the fingers of healthy volunteers (9 males; mean age 24 years,
`range 21 -32) using nail clippers. Nail pieces, which had been allowed to
`grow for at least one month, were used in this study. Nine or seven nail
`pieces were obtained from the 5th finger of each volunteer, hydrated for a
`day and then used to compare the nail permeation from each solvent system.
`Nail pieces from the 2nd, 3rd and 4th fingers were used to measure physico-
`chemical properties.
`Solubilities and Partition Coefficients 5-FU- or TN-vehicle suspen-
`sions were mixed by a magnetic stirrer at 37 °C. After 24h each suspension
`underwent filtration (Ekicrodisc 3 or 3CR; German Sciences Japan, Ltd.,
`Tokyo). The filtrate was immediately diluted with methanol or acetonitrile to
`obtain samples for analysis. The octanol/vehicle partition coefficient of the
`drugs (Ko~) was defined as the solubility ratio in octanol/vehicle at 37 °C.
`Measurement of Physicochemical Properties of Nail Pieces The
`
`* To whom correspondence should be addressed.
`
`’~, 1998 Pharmaceutical Society of Japan
`
`NII-Electronic Library Service
`
`ARGENTUM EX1024
`
`Page 1
`
`

`

`Vol. 46, No. 11
`
`system (LC-6A, Shimadzu Seisakusho, Kyoto), an UV detector (SPD-6A,
`Shimadzu), a chromatopack (C-R6A, Shimadzu), a system controller (SCL-
`6B, Shimadzu), an auto injector (SIL-6B, Shimadzu) , and a reverse phase
`column (lnertsil ODS 250mmX4.6mm i.d., GL Sciences Inc., Tokyo). The
`mobile phase used tbr measuring TN was water : methanol (3 : 7) at a flow
`rate of 1 ml/min, and detection at ,t.=260nm. The mobile phase for 5-FU
`was 0.l% phosphoric acid:acctonitrile (98 : 2) at a flow rate of 1 ml/min, and
`detection at ,%-270 nm.
`
`Results and Discussion
`Effect of Each Vehicle on the Physicochemical Proper-
`ties of the Nail Plate Table 1 summarizes the composition
`of the various solvent systems and the solubility of 5-FU and
`TN in the solvent systems used in these experiments. The ef-
`fect of each vehicle on the physicochemical properties
`(swelling and softening) of the nail plate was studied to com-
`pare the drug permeation from each vehicle through each
`human nail plate. Water was selected as a control vehicle.
`Figure 4 shows the weight and stress ratios (WEst~p21/
`W[stcp[] and S[Step2]/S[stepl]) 6d after immersion of the nail
`pieces in several solvent systems. In comparison with non-
`treated nail pieces, the nail piece weight ratio of the control
`
`Synchronous motor
`
`Star-head bar
`
`N
`
`Cell adapter
`
`Water jacket
`
`Nail piece
`
`O-Shaped ring
`
`Fig. 2. Schematic Diagram of the Modified Side-by-Side Diffusion Cell
`
`Step 1
`with5-FU
`
`Time (day) ~ 0
`
`I
`
`Step 2
`Step 3
`! i withS-FU I I with5-FU
`II
`II
`
`4
`I
`
`23
`
`7 8
`15 16
`washout
`washout
`Treatment -------I~- I I I t I I
`water water
`0,0.1,0.5, 1,3, 5, or
`10 % AC solution
`
`1798
`
`weights and stresses of the finger nail pieces from healthy volunteers were
`measured before treatment (day 0) (Step 1). Each nail piece was immersed
`in 10ml of each vehicle at 37 °C, and was subsequently measured on day 6
`(Step 2). In the investigation to measure the effect of AC concentration on
`the physicoehemical properties of the nail plate, each nail piece was im-
`mersed in 10 ml water for another 6 days to check the reversibility (Step 3).
`Alter this nail piece was wiped by Kimwipes® and weighed by an electronic
`balance (JL-200, Cbyo Balance Corp.). The bending stress of each nail piece
`was measured by a rheometer (NMR-2000J, Fudo Kogyo Co., Ltd.) with an
`attachment guide (Fig. 1).13) The resulting stress (Sistepz or 31) and weight
`(W[srep2 or 31) product of each treated nail piece and its respective untreated
`nail piece (Srstepll’W[stepll) were used as an index for nail softening and
`swelling (Scstep2 or 3l/Srstepl], W[step2 or 3j/W[slcpl])’
`Permeation Studies A piece of nail plate was sandwiched between 2
`adapters made of polypropylene with an O-shaped ring (effective diffusion
`area, 0.049cm2) and mounted in a side-by-side (2-chamber) diffusion cell
`with a water-jacket connected to a water-bath at 37°C (Fig. 2). The dorsal
`nail plate side was filled with 2.5ml drug suspension in each solution, and
`the ventral nail plate side with the same volume of water or 40% polyethyl-
`ene glycol 400. No preservative was added because the receiver solution was
`clear even at the end of the experiment. The drug permeation was measured
`by sampling the ventral nail plate side solution at predetermined times. The
`experimental period was 7 d (5-FU) or 18 d (TN), because of the low nail
`permeability of the drugs. To overcome the large variation in nail permeabil-
`ities due to individual differences in the barrier properties, two experimental
`schedules were maintained: (i) use of nail pieces from the same finger of the
`same volunteer and (ii) continuous use of the same nail piece. The effect of
`different concentrations of AC (0, 0.1, 0.5, 1, 3, 5, 10%) on the in vitro nail
`permeation of 5-FU was investigated according to the experimental schedule
`shown in Fig. 3.
`Analytical Methods 5-FU and TN were determined by HPLC. The ab-
`solute calibration method was applied for 5-FU. The sample containing TN
`was added to a methanol solution containing dipbenyl phthalate as an inter-
`nal standard. The solution was injected into an HPLC consisting of a pump
`
`Load Cell
`
`Nail piece
`
`Guide
`
`Nail piece
`under st,’ess
`
`Fig. 1.
`
`Measurement of Stress on Nail Pieces
`
`8mm
`
`Table 1.
`
`Solubilities of 5-FU and TN in Each Solution
`
`Fig. 3. Experimental Schedule (ii) for the Nail Plate Permeation Study
`
`Composition of solvent systems
`
`5-FU (rag/rot)
`
`TN (pg/ml)
`
`Water
`MEW (3% 1-menthol-40% ethanol water)
`AEW (3% acetylcysteine-40% ethanol water)
`MEEW (3% mercaptoethanolM0% ethanol-water)
`LEI (1% lactic aci~l 0% ethanol-isopropyl myristate)
`AEI (0.5% acetylcysteine-10% ethanol isopropyl myristate)
`MEEI (3% mercaptoethanol-10% ethanol-isopropyt myristate)
`Urea (8 M urea)
`Na salicylate (40% sodium salicylate)
`
`17.1
`19.6
`20.2
`24.6
`0.228
`0.528
`0.266
`5.11
`38.4
`
`0.391
`373.8
`118.1
`107.1
`29610
`24681
`28009
`0.853
`39.82
`
`NII-Electronic Library Service
`
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`
`

`

`November 1998 1799
`
`a)
`
`Non-treatment
`
`MEW
`
`AEW
`
`MEEW
`
`LEI
`
`AEI
`
`MEEI
`
`Urea
`
`b)
`
`Non-treatment
`
`Water
`
`MEW __
`
`AEW ¯
`
`MEEW
`
`LEI
`
`AEI
`
`MEEI -
`
`Urea "__
`
`J
`~////AV i
`
`Na Salicylate
`
`Na Salieylate
`
`0.0 0.5
`
`1.0 1.5 2.0 2.5
`
`3.0
`
`3.5 4.0
`
`|
`0.4 0.6 0.8 1.0 1.2 1.4 1.6
`
`0.0 0.2
`
`Weight ratio
`
`Stress ratio
`
`Fig. 4.
`
`Weight and Stress Ratio 6 days after Treatment with Several Solvent Systems
`
`Each value represents the mean+-S.E. (n=6).
`
`vehicle (water) was increased by about 0.2points and its
`stress ratio was decreased by about 0.7points. In addition,
`the nail piece weight was increased and its stress was de-
`creased in water-containing solvent systems. Compared with
`the water control, 8 M urea (Urea) and 40% sodium salicylate
`(Na salicylate) increased the nail piece weight ratio by
`0.25--0.47 points and lowered the nail piece stress ratio by
`0.06--0.10 points. In contrast, the MEW system had approx-
`imately the same nail weight and stress ratio as those of the
`control vehicle. AEW (3% acetylcysteine-40% ethanol-
`water) and MEEW (3% mercaptoethanol-40% ethanol-
`water) systems containing agents reducing the disulfide link-
`ages increased the nail weight ratio by 2.45 and 0.45 points,
`respectively. The AEW and MEEW systems decreased the
`nail stress ratio by about 0.915 and 0.926 points, respectively.
`In contrast with the non-treated nail pieces, the LEI system, a
`water free system, had little effect on the weight and stress
`ratio of each nail piece. Although the AEI (0.5% acetylcys-
`teine-10% ethanol-isopropyl myristate) and MEEI (3% mer-
`captoethanol-10% ethanol-isopropyl myristate) systems also
`had little effect on nail swelling and softening, AC and ME in
`aqueous solvent systems had a significant effect. These re-
`sults suggest that water may physically change the nail plate
`keratin. In addition, urea, sodium salicylate, AC and ME in
`aqueous solvent systems may denature the nail keratin,
`whereas this denaturation may not take place in water-fi’ee
`solvents. Urea and ME can be used to extract keratin from
`nails or wool.l<lS~ Therefore, those additives are likely to in-
`crease drug permeation through the human nail plate.
`Effect of Each Vehicle on Drug Permeation through the
`Nail Plate The steady-state flux (J) of drugs through the
`human nail plate can be represented by Fick’s first law of dif-
`fusion, as follows:
`
`J=D. K,,,,. CJL
`
`(1)
`
`where D, KN/,,, Cv and L are the diffusion coefficient in the
`human nail plate, the human nail plate/vehicle partition coef-
`ficient, the concentration in the donor vehicle of the drug and
`the thickness of the human nail plate, respectively. The per-
`meability coefficient (P) is represented by
`
`Table 2. Effect of Several Solvent Systems on the Flux of 5-FU and TN
`through the Human Nail Plate
`
`Solvent system
`
`5-FU (pg/cm2/h)
`
`TN (yg/cm2/h)
`
`Water
`MEW
`AEW
`MEEW
`LEI
`AEI
`MEEI
`Urea
`Na salicylate
`
`17.4+-4.3
`16.3_+3.3
`
`228.8_+52.3
`275.1 4-125.8
`20.5_+5.2
`116.4_+6.8
`146.8--_23
`3.7_+0.2
`6.6_+0.3
`
`N,D.~)
`N.D.
`0.137-+0.080
`0.058+0.017
`N.D.
`0.053 -+0.009
`0.223_+0.032
`N.D.
`N.D.
`
`a) Not determined. Each value represents the mean+-S.E. (n=3).
`
`P= (D. KN,~)!L
`
`(2)
`
`In the present study, both TN and 5-FU were suspended in
`all the donor vehicles. Therefore, their thermodynamic activ-
`ities in each donor vehicle are at a maximum and the same
`value. In the case of equal diffusion coefficients or equal ac-
`tivity coefficients, it is thought that the permeation fluxes of
`the drugs from each vehicle are the same. On the other hand,
`the permeability coefficient reflects the activity coefficient of
`the drugs in the human nail plate and the change in their sol-
`ubility in vehicle. Thus, it is possible to evaluate drug diffu-
`sion in the membrane and drug partition to the membrane.
`Table 2 shows the steady-state flux of TN and 5-FU from
`several solvent systems through the human nail plate. The 5-
`FU flux from the MEW system was not increased, in com-
`parison with that from the water (control) vehicle. In spite of
`other changes in the physicochemical properties of the nail
`plate, aqueous solvent systems containing keratin-softening
`agents (urea and Na salicylate) decreased the 5-FU flux. The
`LEI system (a lipophilic system) slightly enhanced the per-
`meation of 5-FU through the human nail plate. It has been
`reported that the dissociation of benzoic acid and pyridine
`leads to a reduction in their penetration rate through hoof
`membrane.16) When an acidic drug, 5-FU (pK, = 8.0, 13.0), is
`suspended in aqueous solutions of keratin-softening agents
`(urea: pH=7.2, Na salicylate: pH=6.2), the pH in the nail
`plate would be shifted to a more basic value, compared with
`
`NII-Electronic Library Service
`
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`

`

`1800 Vol. 46, No. I 1
`
`a) 5-FU
`
`b) TN
`
`AEI
`MEEI ¯
`
`LEI
`I
`
`-3
`
`-4
`
`-5
`
`-6
`
`-7
`
`ta
`
`AEW
`
`MEEW
`
`Water
`
`MEW ~ Urea¯
`
`-5
`
`-6
`
`-7
`
`-8
`
`MEEI
`
`-9
`
`AEI
`
`AEW
`
`MEEW
`
`Na sallcylate
`¯
`
`i -
`-1
`
`-8
`-2
`
`/ i ¯
`0
`1
`
`i
`2
`
`-10
`-1
`
`¯
`
`i ¯
`
`i ¯
`
`o
`
`1
`
`i
`
`2
`
`Log Kov
`
`Log Kov
`
`Fig. 5. Relationship between log K~ and log P of 5-FU and TN
`
`Each value represents the mean+S.E. (n :3).
`
`a)
`
`b)
`
`0,4
`
`0.3
`
`0.2
`
`o.1
`
`.9
`
`I.
`
`t
`
`5
`
`4
`
`3
`
`2
`
`z=
`
`1’
`
`¯
`
`0
`
`0
`
`i , i -
`i . i ¯
`i ¯
`i
`2 4 6 8 10 12
`AC conc. (%)
`
`0.0
`
`¯
`
`i , p , i , i ¯
`i ¯
`i
`2 4 6 8 10 12
`AC conc. (%)
`
`Fig. 6. Weight and Stress Ratio 6 d after Treatment with a Variety of AC Concentrations
`
`O: Step2!Stepl, A: Step3iStepl. Each value represents the mean_+S.E. (n=6).
`
`that of the control vehicle (pH=4.7). In contrast, the pH
`would be lowered by L-lactic acid in the LEI system. There-
`fore, a pH change in the nail plate can account for these ob-
`servations. A combination with urea effectively increased bi-
`fonazole penetration into the nail plate in clinical trials.]7/If
`the penetrant is a basic drug, such as bifonazole, then urea
`and sodium salicylate may exhibit increased permeation
`through the nail plate.
`On the other hand, aqueous solvent systems (AEW or
`MEEW systems) containing AC or ME had about 13 and 16
`times higher 5-FU fluxes than those obtained in the control
`vehicle. In addition, lipophilic systems (AEI and MEEI sys-
`tems) containing AC or ME had about 6.7 and 8.4 times
`higher 5-FU fluxes than in the control, respectively. Only the
`permeation of TN from AEW, MEEW, AEI and MEEI sys-
`tems could be measured, while that from other solvent sys-
`tems was undetectable. It was assumed that TN had a low
`nail permeation because of its high molecular weight and low
`solubility in water, compared with 5-FU. The TN flux levels,
`through the human nail plate from several solvent systems
`containing AC or ME, exhibited the following pattern:
`AEI<MEEW<AEW<MEEI. AC and ME also increased the
`permeation flux of TN through the nail plate.
`
`The octanol/vehicle partition coefficient (Ko~) has been
`used extensively as an index of biological membrane!vehicle
`partition. We thus examined the relationship between the log-
`arithm of the octanol/vehicle partition coefficient tbr TN and
`5-FU and the logarithm of the permeability coefficient (Fig.
`5). TN showed high permeability coefficients from aqueous
`solvent systems, whereas 5-FU had high values from
`lipophilic systems. Solvent systems containing AC or ME in-
`creased the permeability of 5-FU and TN, but they were not
`dependent on log Kov. The lipid content in the human nail
`plate is much lower than that in the stratum cornetan of
`skin.Is’j9) Lipophilic additives such as I-menthol and iso-
`propyl myristate, which increase the skin permeation of
`drugs, had no effect on drug permeation through the nail
`plate. Ethanol was not able to promote drug diffusion
`through the nail plate, which agrees with the findings of Wal-
`ters et al.5) The nail plate of a healthy volunteer is usually hy-
`drated and the water vapor loss normally amounts to about
`1.6 mg!cm2/h.2°) If the nail plate has swelled as is the case in
`vitro, a hydrophilic drug such as 5-FU would require a
`lipophilic vehicle to retain the high thermodynamic activity
`of the drug. Consequently, aqueous solvent systems are
`found to be best for lipophilic drugs like TN.
`
`NII-Electronic Library Service
`
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`

`

`November 1998
`
`Effect of AC Concentration The effect of different con-
`centrations of AC on the physicochemical properties of nail
`pieces and the nail plate permeation of 5-FU (Step 2/Step 1)
`were investigated in more detail. The reversibility of any
`changes was also tested (Step 3!Step 1). The flux ratio for
`Step 2/Step 1 was used as an index for the enhancing effect
`of AC, and the flux ratio for Step 3/Step 1 was used as an
`index for the disappearance of the enhancing effect of AC.
`Figure 6 shows the weight and stress ratio of nail pieces on
`day 6 (Step 2/Step l) and day 12 (Step 3/Step 1) at each con-
`centration of AC. The weight and stress ratio of nail pieces
`increased and decreased, respectively, with increasing AC
`concentration. In high concentrations of AC, the weight ratio
`of nail pieces increased marginally. A marked effect on the
`stress ratio of nail pieces was found at low concentrations of
`AC. Although low reversibility was observed for the physico-
`chemical changes in nail pieces, the weight and stress ratio of
`the nail, were not similar to those of the control (water).
`Figm’e 7 shows the permeation flux ratio of 5-FU through
`the nail plate from vehicles containing different concentra-
`tions of AC, for each step. The flux ratio of 5-FU for Step
`2/Step 1 increased with an increase in AC concentration. The
`Step 3/Step 1 ratios were similar to the Step 2/Step 1 ratios,
`although they were a little lower than the Step 2/Step 1 ratios,
`especially at high concentrations, suggesting low reversibility
`
`20
`
`15
`
`10
`
`@
`
`i
`
`¯
`
`0 .
`0
`
`i
`2
`
`i . i
`i . i -
`i ¯
`4 6 8 10 12
`AC cone. (%)
`
`1801
`
`of the enhancing effect by AC. A long period may be neces-
`sary until the barrier function of the nail plate recovers. In
`3% AC, water flux was about 3 times higher than that of the
`control (data not shown), showing that AC increased drug
`diffusion across the nail plate.
`Figure 8 shows the relationships between the nail weight
`or stress ratio and the flux ratio of 5-FU. The flux ratio in-
`creased with an increase in the weight ratio and a decrease in
`the stress ratio. This suggests that the penetration-enhancing
`effect of 5-FU by AC is closely depending on the swelling
`and softening of the nail pieces. A marked swelling and soft-
`ening of the nail plate takes place, presumably due to cleav-
`age of S S bonds in the nail plate. As a result, the barrier to
`drug permeation across the nail plate is low.
`A lot of cystines, having stable disulfide linkages, are pre-
`sent in human keratinized nail plate.2u Since cysteine has a
`reducing action on cystine in the human nail plate, AC may
`cleave the S-S bonds in keratin. Cleavage of these bonds
`may be associated with the increase in weight and decrease
`in stress of the nail pieces. In addition, drug permeation
`through the human nail plate may also be enhanced as a re-
`sult of increased diffusion through the affected keratinized
`nail plate.
`
`Conclusion
`The penetration of 5-FU and TN through the human nail
`plate was studied in vitro. A penetration-enhancing effect
`was observed in all solvent systems containing AC or ME.
`The permeation flux of 5-FU was directly proportional to the
`AC concentration. However, the reversibility of the nail plate
`barrier function was insignificant. The penetration-enhancing
`effect by AC was closely dependent on the swelling and soft-
`ening of the nail plate. AC and ME cleave the S-S bond in
`the hard keratin in human nail plate. Such additives are thus
`able to enhance drug permeation through the human nail
`plate.
`
`Acknowledgments The authors wish to thank volunteers at Josai Uni-
`versity tor supplying nail pieces.
`
`Fig. 7. Enhancing Effect of Various AC Concentrations on the Nail Plate
`Permeation of 5-FU
`0: Step2/Stepl, A: Step3/Stepl. Each value represents the mean-+S.E. (n=3).
`
`References
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`
`a)
`
`20
`
`15
`
`lO
`
`o
`
`b)
`
`20
`
`15
`
`.i
`
`$
`
`,
`
`i , i i
`2 3 4
`Weight ratio
`
`0,0
`
`o.’1
`
`0.4
`
`Stress ratio
`
`Fig. 8. Relationship between the Weight Ratio (a) or the Stress Ratio (b) and the Flux Ratio of 5-FU
`
`O: Step2/Stepl, A: Step3/Stepl. Each value represents the mean±S.E. (n=3 6).
`
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`Vol. 46, No. 11
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