局部皮膚製劑之生體可用率定義為：藥物的有效成份進入作用部位之速率和吸收之程度，其目標為在作用部位達到最大的治療效果，而全身吸收率為最小，以避免不必要的副作用及潛在的毒性。局部皮膚製劑的生體可用率仍有必要建立與臨床試驗之相關性，但相較於耗時耗財之臨床試驗，發展體外體內相關性評估方法做為替代方案以更符合經濟、效率等原則。體內皮膚微透析與角質層撕法是目前美國FDA認為較能觀察在實際作用部位中藥物藥動狀態的評估方式，而體外體內相關性（IVIVC）雖很不易取得一致性，但在評估皮膚製劑之生體可用率上可提供實驗相關數據，以建立日後選擇評估方法時更有效率的依據。
本研究利用體外流動滲透裝置、體內皮膚微透析及體內皮膚層撕法評估Ibuprofen 和Felbinac兩種藥物，三種不同製劑型式: 貼布(patch)、乳膏(cream)、凝膠(gel) 的體外穿皮速率、體內皮下吸收量及角質層吸收量之間的相關性。
結果顯示，全部劑型在體外穿皮速率、體內皮下吸收量、體內角質層吸收量中，Ibuprofen皆以10% 凝膠（R1）為最大，以5% patch C為最小；Felbinac皆以5% 油性貼片patch H為最大，以0.5% 油性對照品 （R4）為最小。兩種藥物以不同劑型分類比較（水性、油性貼布、乳膏、凝膠），在三種評估方法中的排序大致一致。由 Spearman相關係數的分析中，在全部劑型中，兩種藥品經由皮膚微透析法測得之體內皮下吸收量相對於層撕法測得之體內角質層吸收量的相關性皆比其他方法強（Ibuprofen：r=0.900, P＜0.05；Felbinac：r=0.929, P＜0.01）。相同貼布劑型中，則由皮膚微透析法測得之皮下吸收量與體外滲透法測得之穿皮速率的相關性較強（Ibuprofen：r=1.000, P＜0.01；Felbinac：r=1.000, P＜0.01）。
本研究結果顯示皮膚微透析法與其他兩種方法比較皆呈現不錯的相關性，皮膚微透析法可在不同部位、細胞外液空間、內生物質和外生物質同時取樣，提供藥物動力學連續時間的資訊；在臨床上，亦可應用於病變或受破壞的皮膚以偵測藥物的治療動態；對於日後局部皮膚製劑的評估方法，提供了更多的選擇。針對治療部位主要在角質層的藥物，角質層層撕法仍是適合的評估方法。然而，如何降低皮膚微透析法個體內與個體間的變異性，及提高對於親脂性、蛋白質結合率高的藥物與透析液間之親合性，仍有待進一步突破。

For a topically dermal formulation, bioavailability (BA) is defined as the rate and extent to which the drug is absorbed from the formulation and becomes available at the site of action. It is essential for both the (trans)-dermal delivery of pharmacological active drugs and from a undesirable side-effects or toxicological point of view.
In vitro drug penetration studies using flow-through diffusion cells have been extensively used in the past, and there is a pertinent need to establish a correlation between results from in vitro penetration studies of topical drugs and in vivo data obtained by other methodology. FDA has been receptive to microdialysis and tape stripping data as an addition to in vivo BA experiments. It has been a difficult challenge to obtain an in vitro–in vivo correlation for a topically applied drug product.
The purpose of this study was to assess the BA of Ibuprofen and felbinac topical formulation by evaluating the in vitro skin flux, in vivo dermal absorption and in vivo Stratum corneum (SC) absorption determined by in vitro flow-through diffusion cell, in vivo microdialysis and tape stripping methodologies, respectively, and to evaluate the correlation between in vitro and in vivo methodologies.
The results demonstrated that, for both Ibuprofen and Felbinac, good rank-order correlations are found between in vitro skin flux, in vivo dermal absorption and in vivo SC absorption data. Of all formulations for skin flux, dermal absorption and SC absorption, Ibuprofen of 10% gel is the highest and 5% patch C is the lowest. For Felbinac, 5% oil patch H is the highest and 0.5% oil patch R4 is the lowest. By Spearman analysis, the correlation between the data obtained from the two in vivo methods was stronger than the data obtained from in vitro diffusion cell and the in vivo tape stripping methods (Ibuprofen：r=0.900, P＜0.05；Felbinac：r=0.929, P＜0.01). There was also stronger correlations between the in vivo dermal concentration and in vitro skin flux (Ibuprofen：r=1.000, P＜0.01；Felbinac：r=1.000, P＜0.01) within the same type of patch formulations.
In summary, our results showed that dermal concentrations of Ibuprofen and Felbinac can be correlated to SC absorption or skin flux. In the case of substances meant to their effect correlated in the SC, the tape stripping method will often be more suitable. The microdialysis technique offers a unique opportunity for real-time chronological sampling in the target tissue. It allows continuous monitoring of the rate and extent of drug penetration into the skin. The in vivo sampling of endogenous and exogenous substances in the extracellular fluid is possible. Further, the method may be used when the skin’s barrier is disrupted and has been shown to be applicable to measurements on diseased skin. However, there are still several areas that need to be solved, particularly for lipophilic and highly protein-bound drugs, which do not distribute significantly into the aqueous perfusate in the microdialysis probe.

Abrams K. And Harvell J.D. Effect of organic solvents on in vitro human skin water barrier function. J Invest Dermatol. 101(4): 609-13.1993.
Benfeldt E., Hansen S.H., Volund A., Menne T., Shah V.P. Bioequivalence of topical formulations in humans: evaluation by dermal microdialysis sampling and the dermatopharmacokinetic method. J Invest Dermatol. 127(1):170-8. 2007.

Borsadia S., Ghanem A. H., Set Y., Higuch W. I., Flynn G. L., Behl C. R., Shah V. P. Factors to be considered in the evaluation of bioavailability and bioequivalence of topical formulations. Skin Pharmacol. 5:129–145. 1992.

Boutsiouki P., Thompson J.P., Clough G.F. Effects of local blood flow on the percutaneous absorption of the organophosphorus compound malathion: a microdialysis study in man. Arch Toxicol 75: 321–328. 2001.

Chaurasia C.S., Muller M., Bashaw E.D., Benfeldt E., Bolinder J., .Bullock R., Bungay P.M., De Lange E.C., Derendorf H., Elmquist W.F., Hammarlund-Udenaes M., Joukhadar C., Kellogg D.L. Jr, Lunte C.E., Nordstrom C.H., Rollema H., Sawchuk R.J., Cheung B.W., Shah V.P., Stahle L., Ungerstedt U., Welty D.F., Yeo H. AAPS-FDA workshop white paper: microdialysis principles, application and regulatory perspectives. Pharm Res 24: 1014–1025. 2007.

Escobar-Chavez J.J., Merino-Sanjuan V., Lopez-Cervantes M., Urban-Morlan Z., Pinon-Segundo E., Quintanar-Guerrero D., Ganem-Quintanar A. The tape-stripping technique as a method for drug quantification in skin. J Pgarm Pharm Sci 11:104-130. 2008.

FDA: Guidance for industry: topical dermatological drug product NDAs and ANDAs – in vivo bioavailability, bioequivalence, in vitro release, and associated studies. Draft guidance, June 1998. Silver Spring, US Department of Health and Human Services, Food and Drug Administration, Center for Drug Evaluation and Research, 1998.

FDA, Nonsterile Semisolid Dosage Forms Scale-Up and Postapproval Changes: Chemistry, Manufacturing, and Controls; In Vitro Release Testing and In Vivo Bioequivalence Documentation, http://www.fda.gov/cder/guidance.htm, 1997.

Godin B. and Touitou E. Transdermal skin delivery: Predictions for humans from in vivo, ex vivo and animal models. Advanced Drug Delivery Reviews 59: 1152–1161. 2007.

Herkenne C., Alberti I., Naik A., Kalia Y. N., Franc¸ois-Xavier Mathy, Ve´ronique Pre´ at, Guy1 R. H. In vivo methods for the assessment of topical drug bioavailability. Pharmaceutical Research 25(1): 87-103.2008.

Holmgaard R., Nielsen J.B., Benfeldt E. Microdialysis sampling for investigations of bioavailability and bioequivalence of topically administered drugs: current state and future perspectives. Skin Pharmacol Physiol. 23: 225–243. 2010.

Joukhadar C. and Mu¨ller M. Microdialysis : Current Applications in Clinical Pharmacokinetic Studies and its Potential Role in the Future. Clin Pharmacokinet 44 (9): 895-913. 2005.

Klimowicz A., Farfal S., Bielecka-Grzela S. Evaluation of skin penetration of topically applied drugs in humans by cutaneous microdialysis: acyclovir vs salicylic acid. J Clin Pharm Ther 32: 143–148. 2007.

Kreilgaard M. Dermal pharmacokinetics of microemulsion formulations determined by in vivo microdialysis. Pharm Res 18: 367–373. 2001.

Kreilgaard M. Influence of microemulsions on cutaneous drug delivery. Adv Drug Deliv Rev 2002; 54(suppl 1):S77–S98.

Kreilgaard M. A ssessment of cutaneous drug delivery using microdialysis. Advanced Drug Delivery Reviews 54 Suppl. 1 S99–S121. 2002.

Lonnroth P., Jansson P.A., Smith U. A microdialysis method allowing characterization of intercellular water space in humans. Am J Physiol 253:E228–E231. 1987.

Marks R. and Dykes P. Plasma and cutaneous drug levels after topical application of piroxicam gel: a study in healthy volunteers. Skin Pharmacol. 7:340–344. 1994.

Navidi W., Hutchinson A., N’Dri-Stempfer B., Bunge A. Determining bioequivalence of topical dermatological drug products by tape-stripping. J Pharmacokinet Pharmacodyn. 2008.

Pershing L.K., Silver B.S., Krueger G.G., Shah V.P., Skelley J.P. Feasibility of measuring the bioavailability of topical betamethasone dipropionate in commercial formulations using drug content in skin and a skin blanching bioassay. Pharm Res 9:45–51. 1992.

Pershing L. K., Bakhtian S., Poncelet G., Corlett J. L., and Shah V. P. Comparison of skin stripping, in vitro release, and skin blanching response methods to measure dose response and similarity of triamcinolone acetonide cream strengths from two manufactured sources. J. Pharm. Sci. 91:1312–1323. 2002a.

Pershing L. K., Corlett J. L., Nelson J. L. Comparison of dermatopharmacokinetic vs. clinicial efficacy methods for bioequivalence assessment of moconazole nitrate vaginal cream, 2% in humans. Pharm. Res. 19:270-7. 2002b.

Pershing L. K., Nelson J. L., Corlett J. L., Shrivastava S. P., Hare D. B., Shah V. P. Assessment of dermatopharmacokinetic approach in the bioequivalence determination of topical gel products. J. Am. Acad. Dermatol. 48:740–751. 2003.

Piacquadio D. and Kligman A. The critical role of the vehicle to therapeutic efficacy and patient compliance. J Am Acad Dermatol 39(2 Pt 3): S67-73.1998.
Plock N. and Kloft C. Microdialysis—theoretical background and recent implementation in applied life-sciences. European Journal of Pharmaceutical Sciences 25: 1–24. 2005.

Ross J.H. and Dong M.H. Conservatism in pesticide exposure assessment. Regul Toxicol Pharmacol 31(1): 53-8. 2000.
Schnetz E. and Fartasch M. Microdialysis for the evaluation of penetration through the human skin barrier — a promising tool for future research? European Journal of Pharmaceutical Sciences 12: 165–174. 2001.

Schmelz M. Luz O., Averbeck B., Bickel A. Plasma extravasation and neuropeptide release in human skin as measured by intradermal microdialysis. Neurosci Lett 230: 117–120. 1997.

Schrolnberger C., Brunner M., Mayer B. X., Eichler H. G., Mu¨ller M. Application of the minimal trauma tissue biopsy to transdermal clinical pharmacokinetic studies. Journal of Controlled Release 75:297–306. 2001.

Shah V.P., Flynn G.L., Yacobi A., Maibach H.I., Bon C., Fleischer N.M., Franz T.J., Kaplan S.A., Kawamoto J., Lesko L.J., Marty J.P., Pershing L.K., Schaefer H., Sequeira J.A., Shrivastava S.P., Wilkin J., Williams R.L. Bioequivalence of topical dermatological dosage forms – methods of evaluation of bioequivalence. Pharm Res 15: 167–171. 1998.

Shah V. P. Progress in Methodologies for Evaluating Bioequivalence of Topical Formulations. Am J Clin Dermatol 2 (5): 275-280. 2001.

Shah V. P. IV–IVC for topically applied preparations–a critical evaluation. European Journal of Pharmaceutics and Biopharmaceutics 60: 309–314. 2005.

Shinkai N., Korenaga K., Takizawa H., Mizu H., Yamauchi H. Percutaneous penetration of felbinac after application of transdermal patches: relationship with pharmacological effects in rats. J Pharm Pharmacol 60: 71–76. 2008.

Stahle L., Arner P., Ungerstedt U. Drug distribution studies with microdialysis. III. Extracellular concentration of caffeine in adipose tissue in man. Life Sci 49: 1853–1858. 1991.

Suh H., Jun H.W., Dzimianski M.T., Lu G.W. Pharmacokinetic and local tissue disposition studies of naproxen following topical and systemic administration in dogs and rats. Biopharm Drug Dispos 18:623-33. 1997.

Trommer H. and Neubert R.H.H. Overcoming the Stratum Corneum: The Modulation of Skin Penetration. Skin Pharmacol Physiol 19:106–121. 2006.

Tegeder I., Muth-Selbach U., Lötsch J., Rüsing G., Oelkers R., Brune K. Meller S., Kelm G. R., Sörgel F., Geisslinger G. Application of microdialysis for the determination of muscle and subcutaneous tissue concentrations after oral and topical ibuprofen administration. Clin Pharmacol Ther 65:357-68. 1999.

Uppoor V. R. S., Regulatory perspectives on in vitro (dissolution) / in vivo (bioavailability) correlations. Journal of Controlled Release 72: 127–132. 2001.

Wiedersberg S., Leopold C. S. Guy R. H. Bioavailability and bioequivalence of topical glucocorticoids. European Journal of Pharmaceutics and Biopharmaceutics 68: 453–466. 2008.

Wang Y. S. L. and Sawchuk R. J. Microdialysis calibration using retrodialysis and zero-net flux: application to a study of the distribution of zidovudine to rabbit cerebrospinal fluid and thalamus. Pharm. Res. 10:1411-1419. 1993.

陳恕誼, 利用體外釋出、皮膚層撕法及皮膚藥物力學比較外用
Tretinoin 製劑. 國立成功大學, 台南. 2005.