Journal of Dermatological Science (2007) 46, 150—152

www.intl.elsevierhealth.com/journals/jods

LETTER TO THE EDITOR

Iontophoretic permselective property ofhuman nail

KEYWORDSElectroosmosis; Nail; Keratin; Griseofulvin; Iontophoresis

Delivery of therapeutically effective level of drugsto nail stratums is a significant task in treating naildisorders. Although oral drug delivery is somewhatsuccessful in treating the nail disorders, side effectsare severe due to considerable high doses. Topicalmonotherapy is considered less efficient in treatingnail disorders such as onychomycosis due to poortrans-nail bioavailability of drugs [1]. The ability ofkeratolytic and thiolytic agents to enhance pene-tration is limited by several factors [1,2]. Jameset al. reported iontophoresis enhanced transport ofprednisolone sodium phosphate across the thumbnail [3,4]. We found that iontophoresis couldenhance the transport of salicylic acid due to elec-trorepulsion [2]. Electroosmosis is the convectivesolvent flow mechanism that occurs during ionto-phoresis. Understanding the importance of therelative contributions of electrorepulsion and con-vective solvent flow to trans-nail iontophoretictransport is essential for the optimization of drugdelivery. Here we report the permselective natureof nail plate (keratin pI of �5) and its potentialapplication in trans-nail drug delivery [5,6].

All the chemicals were purchased from Sigma—Aldrich (St. Louis, MO, USA). In vitro transport stu-dies were carried out using Franz diffusion cells(Logan Instruments Ltd., Somerset, NJ) fitted witha custommade Teflon nail holder [4]. The whole nailtips were obtained from the fingers of healthy maleand female adults (20—30 years, 10 volunteers; IRBprotocol # 1001) after obtaining their consent. Thenail plates having a thickness of about 400 � 20 mmonly were used for the studies. The nail plates weretreated as discussed previously and mounted on thediffusion cell (area 0.25 cm2) [2]. A 0.5 mA/cm2

constant dc was applied using Iomed Phoresor II dose

0923-1811/$30.00 # 2007 Japanese Society for Investigative Dermadoi:10.1016/j.jdermsci.2006.12.010

controller through Ag/AgCl electrodes. A 500 ml ofglucose (10 mM) solution or griseofulvin (of0.03 mM) solution prepared in buffer was placedin the donor compartment. 0.5 mL samples weredrawn from the receiver compartment (5 mL) at 2 hintervals for 24 h. The amount of glucose trans-ported was measured by a procedure described byRaabo and Terkildsen [7] and griseofulvin by HPLC[8]. Passive transport experiments were run in par-allel with iontophoretic experiments wherevernecessary for comparison. The concentration ofchloride ion after the transport experiments wasmeasured in the donor and the receiver compart-ment buffers to ensure that the buffers containedsufficient chloride ions till the end of the experi-ment. Control experiments were also carried out byplacing plain buffers in both the compartments andapplying iontophoresis over a period of 10 h toensure no endogenous glucose leaches during thestudy.

The flux was calculated from the slope of thelinear part of the cumulative transport-time profileusing GraphPad Prism 3.03 software. The P-value<0.05 (ANOVA) was considered statistically signifi-cant. The data points in the graphs are an average ofn = 6 � S.D.

Passive flux of glucose did not differ significantlyat different pH conditions. No endogenous glucoseappears to leach into the buffer during iontophor-esis.

At pH 7, the total transport of glucose due toanodal iontophoresis was 12.42 � 1.3 nmol/(cm2 h);whereas at pH < 5, it was less than the passive fluxof glucose. The pH dependent transport due tocathodal iontophoresis followed an opposite trend.In the case of cathodal iontophoresis, the transportwas many folds higher at pH < 5 as compared tothat at any pH > 5. The cathodal iontophoretictransport flux of glucose at physiological pHwas 1.12 � 0.21 nmol/(cm2 h). At pH 5, the anodaland cathodal transports of glucose did not differsignificantly from the passive flux.

These results clearly indicate that the nail plateexhibits iontophoretic permselectivity similar to

tology. Published by Elsevier Ireland Ltd. All rights reserved.

Letter to the Editor 151

Fig. 2 pH dependent trans-nail cumulative transport ofgriseofulvin by anodal iontophoresis at current density of0.5 mA/cm2.

human skin [6]. Nail at pH < pI carries a net positivecharge and at pH > pI a net negative charge. AtpH > 5, the anodal iontophoretic transport was highdue to the net negative charge on the nail platewhich attracts the cations. The cation transport isassociated with the convective water flow in theanode to cathode direction, which in turn is respon-sible for the enhanced glucose transport observed inthis case. The reason for the decrease in the anodaliontophoretic transport of glucose at pH < 5 is mostlikely due to the reversal of net charge present onthe nail plate at lower pH. When the net charge onthe nail plate turns positive, the convective waterflow occurs in the opposite direction from cathodeto anode. This hinders the transport of glucose fromdonor compartment to receiver compartment.

In other words, the cathodal transport is facili-tated by pH < pI and is affected at pH > pI. How-ever, at pH = pI the effect of the net charge on thenail plate is relatively less apparent. Therefore theanodal and cathodal iontophoretic transport of glu-cose did not differ significantly at pH 5 in the presentexperiments (Fig. 1).

Griseofulvin is a sparingly water-soluble anti-fungal drug with log P 2.0 [8]. Therefore its abilityto permeate into hydrophilic keratinous nail plateis limited. It is electroneutral and the extent ofbinding of griseofulvin to keratin (33 � 3.1%) wasindependent of pH of the medium (unpublisheddata). The passive transport of the drug did notvary as a function of pH. The anodal iontophoretictransport was favored by pH 7 and the cathodaltransport by pH 3 buffers (Fig. 2). At pH 5, theiontophoretic transport was not significantly dif-ferent from passive transport. The transport ofgriseofulvin could be enhanced �8-fold by ionto-phoresis.

Fig. 1 pH dependent trans-nail transport (flux) of glu-cose by anodal and cathodal iontophoresis at currentdensity was 0.5 mA/cm2.

Water solubility being one of the criteria for drugpermeation across the nail, antifungal drugs thatare poorly water-soluble does not appreciate sig-nificant penetration across nail plate. Provided asuitable iontophoretic device could be designed andthe electrical protocols are optimized, the transportof not only the ionic drugs, but also uncharged drugscould be enhanced across the nail stratums.

References

[1] Murdan S. Drug delivery to the nail following topical applica-tion. Int J Pharm 2002;236:1—26.

[2] Murthy SN, Wiskirschen D, Bowers CP. Iontophoretic deliveryacross human nail. J Pharm Sci 2007;96(2):305—11.

[3] Kassan DG, Lynch AM, Steller MJ. Physical enhancement ofdermatologic drug delivery: iontophoresis and phonophor-esis. J Am Acad Dermatol 1996;34:657—66.

[4] James MP, Graham RM, English J. Percutaneous iontophoresisof prednisolone–—a pharmacokinetic study. Clin Exp Dermatol1986;11:54—61.

[5] Walters KA, Flynn GL. Penetration of the human nail plate:the effects of vehicle pH on the permeation of miconazole. JPharm Pharmacol 1985;37:498—9.

[6] Diego M, Guy RH, Delagado-Charro BM. Characterization ofthe iontophoretic permselectivity properties of human andpig skin. J Control Rel 2001;70:213—7.

[7] Raabo E, Terkildsen TC. On the enzymatic determination ofblood glucose. Scand J Clin Lab Invest 1960;12:402—7.

[8] Balakrishnan A, Rege B, Amidon GL, Polli JE. Surfactant-mediated dissolution: contributions of solubility enhance-ment and relatively low micelle diffusivity. J Pharm Sci2004;93:2061—75.

S. Narasimha Murthy*Department of Pharmaceutics,The University of Mississippi,

Oxford, MS 38677, United States

152 Letter to the Editor

Daniel C. WaddellDepartment of Chemistry and Biochemistry,Ohio Northern University, Ada, OH 45810,

United States

H.N. ShivakumarDepartment of Pharmaceutics,

KLE Society’s College of Pharmacy,Bangalore 560 010, India

A. BalajiDepartment of Pharmaceutics,

Dayananda Sagar College of Pharmacy,Bangalore 560 078, India

Christopher Paul BowersDepartment of Chemistry and Biochemistry,

Ohio Northern University,Ada, OH 45810, United States

*Corresponding author. Tel.: +1 662 915 5164;fax: + 1 662 915 1177

E-mail address: murthy@olemiss.edu(S. N. Murthy)

6 October 2006