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Lecithin Organogel as Matrix for TransdermalTransport of DrugsH.WILLIMANN, P. WALDE*,P. L. Lusi*,A. G m N i G A * , AND F. STROPPOLO*Received May 21, 1991, from *nH-Zen tnr m, lnsti tu t f i ir Polymere, Ur ive@Mstrasse6, CH-8092, Zurich, Switzerland, and *lnphs nam RicercheS.A. (Zambon Gr oup), Cadempm o, Swrfzerlend. Accepted for publication November 25 1991.

Abstract 0 Organogels obtained by adding small amounts of water to asolution of lecithin n organic solvents were studied as matrices for thetransdermal transport of drugs. Gels obtained from isopropyl palmitateand cyclooctane were used molar ratios of water to lecith in of and 12,respectively). Preliminarily histologicalstudies showed that the gels haveno harmful effect when applied to the skin for prolonged periods. Datarelative to the stability of the organogels with time are also presented.Scopolamine and broxa terol were used as model drugs, and thetransdermal experiments were done with a Franz diffusion cell andhuman skin obtained from plastic surgery. The transport rate of scopo-lamine obtained with the lecithin gels was about one order of magnitudehigher than that obtained with an aqueous solution of the drug at thesame concentration. In contrast, the transport rates of scopolamineobtained with the microemulsion solution prior to gela tion molar ratio ofwater to lecithin, 0 were not differen t from those ob tained with the gel.The same variations in transport ra tes were observed for broxaterol, inwhich case the flux through the skin was directly proportional to theconcentration of drug in the gel. At a concentration of broxaterol of 75mglmL in the donor gel, the flux was 47 pg * h- cm-*. Becausepreliminary results showed that transdermal transport i s successful withamino acids and peptides also, it is concluded that lecithin gels may beefficient vehicles for the transdermal transport of various drugs.

Lecithin organogels are readily obtained by adding aminimal amount of water to a solution of lecithin in organicsolvents.1 Surprisingly high viscosities can be achieved (upto10000 poise) in various organic solvents1.2 addition of waterto the small reverse lecithin micelles present initially bringsabout a monodimensional growth of long cylindrical giantmicelles; above a certain critical concentration of lecithin,these spaghettilike micelles2 build a continuous entanglednetwork, which, despite its dynamic character, has sufficientstructural persistence to yielda high macroscopic viscosity.-Biocompatible liquids, such as isopropyl palmitate (IPP) andother esters of this kind, can be used to form gels, which arethen suitable for pharmaceutical and cosmetic formulations.Lecithin gels are isotropic and thermoreversible. At temper-atures >40 C,hey become liquids with much lower viscos-ity, and high-viscosity gels are again formed by cooling andstirring. These gels are also interesting because of theirability to host various guest molecules. Three types of mole-cules (lipophilic, hydrophilic, and amphoteric), includingenzymes,6can be solubilized in the gels.1.7 The present worktakes advantages of two basic properties of lecithin gels,biocompatibility and ability to solubilize drugs, and exploresthe possibility of using these drug-containing, biocompatiblelecithin gels as a matrix for transdermal transport.Transdermal transport of drugs (i.e., the transport ofpharmacologically active compounds through the skin intothe blood vessels) is currently regarded as an importantalternative to the classical ways of delivering drug, mostnotably peroral and percutaneous delivery by injection. Theadvantages of the transdermal route have been emphasized inthe literature.gl1 The prerequisites of the chemical structure

needed to penetrate through the skin are not yet completelyunderstood. Even if these prerequisites were known, however,chemically modifying all drugs of medicinal interest to makethem transdermally active would not be practical. I t is morereasonable to look for a carrier that interacts with the skinsuch that i t allows various molecules to pass into the skin.In this paper, which follows a short preliminary report onthe subject,12 we present the results of an investigation oflecithin gels as carriers for the transdermal transport ofdrugs. The general properties of lecithin gels (interactionwith human skin, stability with time, and ability to solubilizedrugs); the potential use of lecithin gels for the transdermaltransport of scopolamine (known as a skin-penetrating sub-stance); and the transdermal transport of the new drugbroxaterol [(t)-3-bromo-a-[(tert-butylamino)methyll-5-isoxazolemethanol]bronchodilator agent with selective p2agonist activity,ls are discussed. Broxaterol hydrochloride isactive in the treatment of asthma at oral doses of 0.25-0.5mg,14J5 inhalation doses of 0.2-0.4 mg,16-19 and intravenousdoses of 0.1-0.2 mg.20

Experimental SectionM a t e r i a l d o y b e a n l e c i t h i n ( E p i k u r o n 200) was from LucasMeyer (Hamburg, Germany); isopropyl palmitate, cyclooctane, 4 2 -hydroxyethy1)piperazine-1-ethanesulfonic cid (HEPES),nd sodium

dihydrogen phosphate were from Fluka (Buchs, Switzerland); scopo-lam ine hydrobromide and polyethylene glycol 4000 (PEG 4000) werefrom Merck (Darmstadt, Germany); pestradiol 17-acetate, pestra-diol diacetate, pestradio1 1 7-valerate, pes trad iol 17-enanthate, andpes trad iol 17-cypionate were from Sigma (St.Louis, M O); acetoni-trile (C hromasolv) was from Riedel-de Ha en (Seelze, Germany); andbroxaterol base, nifedipine, clonidine, an d isosorbide dinitra te werefrom Inp ha na m Ricerche S.A. Campedino, Switzerland).Water wasdeionized and doubly distilled in quartz glass.Hum an s kin w as received from th e Plastic Surg ery Division (Prof.M. Frei) of the University Hospital, Zurich, Switzerland. All skinsam ples were from chest sections of 30-40-year-old female patien ts.After surgery, th e skin (epidermis and dermis) was carefully cleanedwith water and stored at -40 C for a maximum of 150 days. Beforeeach set of transdermal experim ents, the skin sample was thawed atroom tempe rature by soaking in 50mM H EPE S buffer (pH 6 .8 ) or 15min. The remaining fatty layers were carefully removed by forceps,and th e skin samp le was the n cu t into sections of -1 x 1 cm.Method+Prepara twn of Gels-Lecithin gels were prepared atroom temperature by first dissolving lecithin in organic solvent(isopropyl palmitate or cyclooctane) an d th en adding, with magneticstirring, the necessary amount of water to obtain the gel. Theviscosity of the gel depends on the amount of water generallyexpressed as w,, th e molar ra tio of added wat er to lecithin (w,[H,O]/[lecithin]). Dry Epikuron 200 lecithin usually contains-0.5-1 water molecule per lecithin molecule. Fu rth er details on thepreparation of lecithin gels are given elsewhere.1.2Comm ercial lecithins of lower purity do not form gels. High purity ,as udged by thin-laye r chromatography, however, does not mean tha twe are dealing with only one compound chemically. In fact, naturallecithins are mixtur es because of the heterogen eity of the fa tty acidalkyl chain s in positions 1 a n d 2 of the glycerol moiety.

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ICH20HScopolamine

Br

OHBroxaterol

The drug-containing gels were prepared by dissolving th e drug in th elecithin dissolved in organic solvent an d then adding wate r to inducegelation, asdescribed ear lier. The viecosity of the final gel formulationmay be reduced, depending on the amount of solubilized drug.ransdermalInVitro Studies-The passage of th e solubilized drug sfrom the lecithin gel through hum an s kin was studied with home-made, glass, vertical F ranz diffusion cells.11-21 n th ese cells, th e sk insamples were sandwiched between the upp er donor compartment andthe lower acceptor compartment. The circular are a of skin th at w asin contact with t he two compartm ents was 0.64 cm2. The acceptorcompartment contained 5% PEG 4000 in 50 mM HEP ES buffer (pH6.8). PEG is often used in th e acceptor solution,a main ly to increasethe solubility of organic compounds. The cells were therm ostated a t35 C in a n incubator (Reacti-Therm; Pierce, Wageningen, Th e Neth -erlands), and the acceptor solution was stirred w ith a magnetic st irrerat 400 rpm. Us ually, 0.4 mL of gel conta ining the dru g was placed in

the donor compartmen t onto the skin, and the concentration of thedru g in 5-20-pL aliquot8 of the acceptor solution (as reported inFigures 2-4 as determ ined by high-performance liquid chromatog-raphy (Series 4 system; Perkin-Elmer, Norwalk, CT; equipped withan LC 90 UV detector). For both scopolamine and broxaterol, a HibarLichrosorb CN column (5 -p n mean particle size, 250-mm leng th,4-mm id .) from E. Merck (Darmstadt, Germany) was used. Elutionwas performed w ith CH,CN:sodium phosphate buffer (70:30,20 mM,pH 4) at a flow rate of 1 mu min . The UV detec tor was set a t 259 and217 nm , for scopolamine and broxaterol, respectively. Quantificationswere performed by comparing peak heights with calibration curvesobtained with known amounts of drugs under identical analyticalconditions.Histology-Thin sections of th e skin were prepared and thenstained w ith eosine B at the Dermatology Department of the Uni-versity Hospital in Ziirich under the guidance of Dr. L. Bruckner.Results and Discuss ion

General Propertiesof Lecithin GelsasMatrices forTransdermal Transport-The gel preparations used in this workwere made with soybean lecithin (Epikuron 200) containing atleast 95 phosphatidylcholine.22Isopropyl palmitate was cho-sen as a representative biocompatible solvent, and cyclooctanewas chosen because it permits gels to be made at much largervalues ofw,. Thew, values for gels with isopropyl palmitate andcyclooctane are 3 and 12, respedively.1 (The w, value of 1 2 isdifferent from the value of 7 reported earlier for soybean lecithinfrom Sigma.) The dynamic shear viscosities at 25 C and 0.5rad/s for gels with isopropyl palmitate and cyclooctane are 170and 3260 poise, respectively.1

Lecithin gels kept at constant temperature are indefinitelystable in closed vials, without change of color or appearance. Forexample, the absorption spect rum between 250 and 500 nm ofa 120mMsoybean lecithin solution in cyclohexane (w, 3)doesnot change within a t least 1month. Even in open vialsstored atroom temperature, most of the lecithin gels remain stable for atleast 30 days. For example, if one follows the Fourier trans-formed infrared spectrum of a 200 mM lecithin solution inisopropyl palmitate (w, 2.5) during this time, no significantchanges are observed between 3000 and 4000 cm-l, the OHstretching region of the spectrum (data not shown).This obser-vation also indicates that the lecithin gel does not absorb asignificant amount of humidity from the air during storage. Thesame is true for gels containing solubilized guest molecules,such as vitamin A palmitate or nifedipine.The solubility of drugs in gels tested to date has beensatisfactory. Actually, the presence of lecithin in the organicsolution often brings about an increase of solubility withrespect to that observed in the nea t solvent. Thus, broxaterolcan be solubilized in isopropyl palmitate up to only 11mg/mL.In isopropyl palmitate with 200 mM lecithin, however, thesolubility can be brought up readily to 75 mg/mL. Likewise,the solubility of the drug nifedipine in lecithin gels isconsiderably higher than the solubility of the drug in eitherwater or isopropyl palmitate (data not shown). Likewise, weobtained good solubility of the following compounds in leci-thin gels: pestradiol 17-acetate, pestradiol diacetate, @s-tradiol 17-valerate, pest radio l 17-enanthate, pest radio l 17-cypionate, isosorbide dinitrate, and clonidine. The solubilityof several amino acids and peptides is described elsewhere.6Another important question preliminary to pharmaceuti-cal applications is whether, and to what extent, lecithin gelsare harmful to human skin. To partly clarify this point, wecarried out a light microscopic investigation of human skinbefore and after treatment with either isopropyl palmitatealone or with 200 mhl soybean lecithin-isopropyl palmitategel (wo 3). No significant alterations of the skin wereapparent after 3 days of application of either isopropylpalmitate solvent alone or with the lecithin gel in isopropylpalmitate (Figure 1 . n particular, the stratum corneum wasstill intact after gel treatment. There was no differencebetween the skin samples treated with gel (Figure 1B) orsolvent alone (data not shown) and the control samplestreated with physiological NaCl solution (Figure 1C). In allcases, including the control, only an increase in vacuole sizeof the cells in the spinous layer of the epidermis was observedduring the course of the transport experiments.Transdermal Transport of Scopolamine-Scopolaminewas used as a reference substance, because its transportthrough skin and skin models has been studied to a greatextent.Sl1 A transdermal device for scopolamine has beencommercialized (developed by Aha and marketed by Ciba-Geigy under the name Transderm-Scop or Scopoderm 'ITS).With this device, a small dose applied behind the ear permitsthe transdermal transport of scopolamine with beneficialeffects against motion-induced sickness.8Four transdermal transport curves were obtained from sco-polamine with four different matrices in the donor compartment(Figure 2). The comparison between an aqueous solutionan d alecithin gel is shown in Figure 2A, and the transport rate for twodifferent gels, an isopropyl palmitate microemulsion solution(the ecithin organic solutionat w, 0; i.e., priorto gelation)andcommercial scopolamine plaster (Scopoderm?Ts from Ciba-Geigy)are shown in Figure 2B. The comparison in Figure 2B ismade only for comparison with a commercial preparation withregard to the flux intensity. It is not meant to be a comparisonof therapeutic efficiency (for which many other parametersshould be taken into account, which is not the purpose of thepresent paper). The results in Figure 2B do, however, show that

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was proportional to the concentration (inset of Figure 3).Comparing the microemulsions to a clearly more viscoussystem(as btainedat w, 3 shows that the transport rate doesnot change significantly (Figure 4). Although cyclooctane s notinteresting for physiological application, it is a solvent thatpermits one to study the dependence on w, in a much broaderrange (Figure 4B). In fact, gels were obtained at w, 12 in thiscase Again, the results in Figure 4B indicate that the differ-ences between the transport rates of the microemulsion and thegels are within experimental errors.

Comparing Figures 4A and 4B shows that the efficiency oftransdermal transport is lower for cyclooctane; that is, theinfluenceof the solvent on the transport rate maybe significant.Conclusions

Lecithin organogels can be prepared easily and rapidly andcan be obtained with biocompatible components. They arestable for a long time; can incorporate sizeable amounts ofquite different chemicals as guest molecules; and seem,therefore, to fulfill the conditions necessary for cosmetic andpharmacological applications. These gels also are transpar-ent, a property allowing the use of spectroscopic methods todetect possible structural changes of the guest molecules and,possibly, the kinetic mechanism involved.In addition to scopolamine and broxaterol reported here,preliminary studies in our laboratory show that various othersubstances, such as estradiol, amino acids, and peptides, canbe transported transdermally via lecithin gels. These results

tl 0 O 0 i

Xm 5001

0 -

TO2m

A

0 20 40 60 80Time h)

6 -1 B

450

Time h)Figure Transport of broxaterol through human sk n n vitro. A)Soybean lecithin (200mMHsopropyl palmitate,w, 0 (curve ; 0 or3 (curve2;0 . ith an initial broxaterol concentration of 40 mglmL. 6)Soybean lecithin(200mM)-cyclooctane,w, 0 (curve3;0 r 12 curve4;B), with an initial broxaterol concentration of 20 mg/mL. The corre-sponding flux value for cyclooctaneis 9.5 p h- cm-2,compared with17.3 9 h- for isopropyl palmitate (see inset o Figure 3).

suggest that there are no great restrictions on the chemicalstructure of the drug.More studies should be carried out to clarify whether and towhat extent the chemical structure really affects the trans-port rate. The hydrophobicity, the molecular weight, and thecharge density are the obvious parameters to investigate first.The gel components are also important, and systematicstudies on the effect of the solvent, the concentration oflecithin, and the amount of water are probably also important(water as gel inducer can be substituted for other substances,such as glycerol and other low-molecular-weight, hydrogen-bonding liquids).Only preliminary speculations can be offered at this pointon the mechanism of the transport. Perhaps, lecithin gelsslightly disorganize the structure ofthe skin, and thus, permitthe permeation of various substances. The stratum corneumcontains regularly arranged layers of lipids (see classicalstudies by Elias and Brown=). I t is possible that the proposeddisorganization is due to interaction between these lipids andthe phospholipids of the gel.

References and Notes1 Scarta zzini, R.; Luisi, P. L. J.Phys. Chem. 1988, 2, 29-833.2. Luisi, P. L.;Scartazzini, R.; Haering, G.; Schurtenberger, P.Colloid Polym. Sci . 1990,268, 56-374.3. Schurtenber er, P.; Scartazzini, R.; Ma d, L. J. ; L eser, M. E.;Luiei, P. L. f Phys. Chem. 1990,94,36 i-3701.4. Sch urtenb erger, P.; Scartazz ini, R.; Luisi, P. L. Rheol. Acta 1989,28,372-381.5. Ott , A.; Urbach, W.; Lan evin, D.; Schurtenberger, P.; Scar-tazzini, R.; Luisi, P. L. Phys.: Condens. Matter 1990, 2,5907-5912.6.Scartazzini, R.; uisi P. L. Biocatalysis 1990,3, 77-380.7. Nastruzzi, C.; Colombo, L.; Willima nn, H.; Luisi, P. L., unpub-lished results.8. Transdermal Controlled S stemic Medications; Drugs and thePharmaceutical Sciences; C i i e n , Y. W., Ed.; Marcel Dekker: NewYork, 1987;Vol. 31, p 1-22.9.Dr Delivery Systems: Fundamentals and Techniques; Johnson,P.;U&loyd4ones, J. G., Eds.; Ellis Honvood: Chichester, U.K.,1987; p 200-223.10. Dermal and Transdermal Absorption; Brandau, R.; Lippold,B. H., Eds.; Wissenschaftliche Verlagsgesellschaft m bH: Stu tt-

gar t , Germany, 1982; p 15 170.11 Tmnsderm al Delivery of Drugs donieus, A. F.; Berner, B.,Eds.; CRC: Boca Raton, FL , 1987; 01. I, pp 101-116.12. Willimann, H.; uisi, P. L. Biochem. Biophys. Res. Commun.13. Chia rino, D.; Fantucci, M.; Carenzi, A.; Della Bella, D.; Frigeni,14. Chet ta , A.; Garavaldi, G.; Cuomo, A.; Gu rrier i, G.; Olivieri, D.15. Rampulla, C.; Corsico, R.; Majani, U.; Lodola, E. Respiration16. Robuschi, M.; imone, P.; Vag hi, A.; Ven tresca, G. P.; Bianco, S.17. Simone, P.; Bor ia, M.; Torre, L.; Ventresca, G. P. Eur. J.Clin.18. Robuschi, M.; aghi, A., Ga mb aro , G.; Refini, M.; Bianco, S.Curr.19. U fda hl, ,C. G.; Sigvaldasson, A.; Skoogh, B.-E.; Svedmyr, N.20. Chiravall i , E.; Rimoldi, R. Lo th Contro l TBC e le Malattie21. Franz, T. J C u m . Probl. Dermatol. 1978, 7, 58-68.22. Lucas Meyer (Hambu rg, Germany ), product information.23. Elias, P. M.; Brown, B. E. J nvest. Dermatol. 1979,73,339-348.

1991,177, 97-900.V.; Sale R. I Farmaco Ed. Sc. 1986,41,440-453.Respimtion 1988,53, 20-224.1985,47,299-302.Znt. J Clin . Pharm. Ther. Toxicol. 1987,25, 01-104.Pharmacol. 1998,39, 6-68.Ther. Res . 1988, 3, 725-733.Resprmtron 1984,46, 04.Polmonari Sociali 1986, 6, 70.

AcknowledgmentsWe thank P rof. M. Frei (Department of Plastic Surgery, UniversityHospital , Zurich) for the skin Sam les and Dr. L. Bruckner (Depart-me nt of Dermatolo Un iversity hos pital, Zilrich) for he r histolog-ical analysis of the%in preparations and for her general interest inthe work. We ar e also grateful to Prof. H. P. Merkle (Department ofPharmacology, ETH -Zentrum, Zilrich) for his comments and sugges-tions.

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