370 JOURNAL OF COSMETIC SCIENCE EXTENDED RELEASE OF LIDOCAINE FROM LINKER-BASED LECITHIN MICROEMULSIONS Jessica S. Yuan and Edgar J. Acosta University of Toronto Department of Chemical Engineering and Applied Chemistry Introduction The goal of transdennal drug delivery is to achieve adequate percutaneous absorption and permeation of active ingredients for local treatment. Microemulsions could improve skin absorption due to their large interfacial area and ultra-low interfacial tension. In our previous study (Yuan et al., 2008), non-toxic lecithin microemulsions were formulated using linker molecules with GRAS or food additive status. These formulations produce a significant increase in the absorption of lidocaine in the skin. In this work, we hypothesize that by topically administrating an active ingredient using the developed linker-based lecithin microemulsion, the active will be safely absorbed into the skin, thus producing an "in-situ" delivery patch for extended release. Potential advantages of this "in-situ" patch include its application on uneven and exposed parts, its low cost, and customizable dose. Traditional drug-in­ adhesive patches can not be used over bums, blisters, painful skin irritations or other skin wounds. Even for normal skin, they often cause skin irritation. The use of sprayable therapeutical microemulsions may represent a significant advance over the traditional patches. Since the microemulsions are transparent, the formulation could be used to treat various skin conditions without affecting the appearance of the patient. The objective of this study was to test the "in-situ" delivery patch hypothesis by evaluating skin absorption and release profile of lidocaine via linker-based microemulsions. Lidocaine-laden microemulsions were applied on pieces of cadaver pig ear skin and in vitro extended release studies were conducted. Moreover, fluorescent microscopy was used to study the permeation and location of a fluorescent dye within the skin strata. Experimental Microemu/sion preparation The method of preparing linker-based lecithin microemulsions and loading lidocaine was described elsewhere (Yuan et al., 2008). The microemulsions contain 4% lecithin, 12% sorbitan monooleate (hydrophobic linker), 1% (for Type II) and 7% (for Type I) sodium caprylate (hydrophilic linker), 3% caprylic acid, 0.9% sodium chloride, H20 and isopropyl myristate (1PM). The Type II formulation was loaded with 5 and 10% \idocaine, while the Type I formulation was loaded with 4 and 8 % lidocaine. In vitro extended release studies The in vitro extended release experiments were conducted in MatTek Permeation Devices (MPD). The pig ear skin was placed in a MPD with the epidermis facing up. A test formulation (0.4 ml) was applied in the donor compartment and was withdrawn after 30 min. The skin surface was blotted dry with an inert paper and was continued for extended release. The receptor compartment was filled with 5 ml of PBS. At predetermined times ( l , 3, 6, 12, 2 4 and 48 h), the receiver solution was withdrawn completely from the receptor compartment and was immediately replaced by fresh PBS solution. At 48 h, the experiment was terminated. Fluorescence Microscopy To visualize the "in-situ" patches, linker-based lecithin microemulsions containing 0.001% Nile red (fluorescent dye) were prepared and were used to conduct in vitro extended release. At predetermined times (1 and 6 h), the skin samples were taken off, blotted dry, and then rinsed twice with PBS. Solutions of 0.001 % Nile red in 1PM was considered as control. The skin samples were cross sectioned and were observed under a fluorescence stereomicroscope. Results and discussion Extended release from "in-situ" patches Fig. l reports the cumulative amount of lidocaine permeated across the skin as a function of time. For the "in­ situ" patches applying the Type II and I microemulsions, they typically show a large increase in the first 24h, followed by a continuous, slow increase up to 48h. Similar to the continuous dosage (Yuan et al., 2008), the one with the Type II fonnulations provided higher permeation amount than their Type I counterparts. When doubling the drug loading in either the Type II or I microemulsions, the lidocaine accumulation in the receiver also doubled. This result is in good agreement with the literature (Dreher et al., 1997) that transdermal drug delivery increases due to higher drug loading in microemulsions. The increased higher drug permeation from the "in-situ" patches is attributed to the larger lidocaine concentration absorbed in the skin after applying the microemulsions with higher drug loading. For example, by applying the Type II formulation with 5% lidocaine, the

2008 ANNUAL SCIENTIFIC SEMINAR 371 lidocaine abso T tion in the skin was E 1000 490±19 µg/cm , while that for the � one with I 0% lidocaine doubled to � I 022±59 µg/cm2• j BOO During the extended release ,i experiments, most of the drug i 600 : absorbed in the skin was released. i Fig. 2 illustrates this observation by � plotting the fraction of lidocaine � rele ed as a function of time. In 1 200 the first 24 h, the drug release from I 0 • � • Type II -10% �Typtll-5% • A • Type I • 8% __.,_Typel-4% the "in-situ" patches applying the � Type I microemulsions is obviously u slower than those applying the 0 6 12 18 24 30 36 42 4i Tlme,h Type II formulations. Both of them Fig I. In vitro release profiles of lidocaine follow a first-order release with from "in-situ" patches via Type II and Type I different time constants of 0.04 h" 1 microemulsions with different drug loading. (for Type 11) and 0.03 h" 1 (for Type J). After 24 h, the Type I started to approach the fraction of lidocaine released of the Type II and eventually reached same. Microscopic observations Figures Ja-f show the location of the fluorescent dye (Nile Red) in the "in-situ" patches after applying the formulations of 1PM (Ja­ b), Type II (Jc-d) and J (Je-f) microemulsions at different time intervals. After 1 h (Fig. Ja, c and e), all formulations showed a deposition of Nile red in the superficial level of stratum comeum (SC) and a permeation in SC, the epidermis and dermis. In comparison to the 1PM system, both Type II and I microemulsions produced higher absorption of Nile red in the superficial layer and higher permeation in deeper skin layers. Especially, the Type II formulation had the highest deposition on the uppermost skin layer because its external phase is oil which contains more hydrophobic Nile red. As expected, the deposition in superficial level of SC clearly decreased after 6 h treatment (Fig. Jb, d and f). Because of the skin absorption, the microemulsions penetrated deeper layers and was able to diffuse uniformly further in the dermis to a great extent after 6 h application time. In contrast, the 1PM formulation did not 1.0 0.8 l Io.a I! ! j 0.4 · 0 ,, ::J 0.2 -�•Typell-10% -+-Type II - 5% ·A•Typel-8% -+-Type 1-4% 0.0 .,.: ---��-- r·-T--r-, 0 6 12 18 24 30 36 42 4i Tlme,h Fig 2. Fraction of lidocaine released from "in-situ" patches via Type II and I microemulsions with different drug loading �-- ... show the increase permeation of Nile red in deeper skin layers as the time of treatment increases. . ' ' - Conclusion We observed the extended release of lidocaine from the "in- (e) Type I, 1 h (f) Type I, Sh Fig. 3. Penetmtion of nile red into pig ear skin from (a) 1PM afu.."'1' lh, (b) 1PM after 6h, (c) Type TI situ" patches via linker-based lecithin microemulsions. After microemulsion after lh, (d) Type n microemulsion applying the microemulsions for a period of time, the drug was after 6h, (c) Type I microcmulsion after lh, (l) Type I absorbed in the skin which acted as drug reservoir and provided microemulsion oiler 6h extended release of the drug over 24 hours, with more than 99% released. Microscopic observations proved the drug uptake by the upper SC layer from the application of the linker microemulsions, and then the penetration into the deeper skin layers. In conclusion, linker microemulsions can act as "in-situ" delivery patches for extended release of active ingredients. Referenc Yuan et al., Linker-based lecithin microemulsions for transdermal delivery of lidocaine. International Journal of Pharmaceutics. 2008. 349:130-143. Dreher et al., Interaction of a lecithin microemulsion gel with human stratum comeum and its effect on transdennal Transport. 1997. Journal of Controlled Release, 45 (2): 131-140.