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Although in vitro skin absorption studies often detect small residues of applied test material in the epidermis/dermis, it is uncertain whether the residue is within the living skin. We studied the dermal absorption of a hair dye hydroxyanthraquinone-aminopropyl methyl morpholinium methosulphate (HAM) in human skin in vivo and in vitro. In vivo, skin (back and scalp) received 0.5% HAM in a commercial formulation at 20microg/cm2 After 0.5 or 48h, skin was tape stripped, followed by cyanoacrylate biopsies (CAB). Sebum from scalp sites was collected for 48h. In vitro, skin was treated with 20mg/cm2 dye for 0.5h, penetration determined after 24h. In vivo, at 0.5h, total recovery (back) was 0.67microg/cm2 (tape strips+CAB). Fluorescence microscopy showed HAM in the hair follicle openings (HFO). At 0.5h, scalp tape strips contained 1.80microg/cm2, HFO 0.82microg/cm2. At 48h, HFO contained 0.21microg/cm2, sebum 0.80microg/cm2. In vivo, skin residues were in the non-living skin and eliminated via desquamation and sebum secretion. In vitro, the SC contained 1.50microg/cm2, epidermis/dermis 0.86microg/cm2, receptor fluid

The objective and quantitative application of tape stripping in pharmaceutics and dermatopharmacokinetics requires the determination of the exact position of each removed tape strip inside the stratum corneum (SC) and/or the determination of the relative SC thickness. In this study, transepidermal water loss (TEWL) and the optical spectroscopic data of the corneocytes were measured simultaneously during the complete removal of the SC by tape stripping. The spectroscopic data quantitatively reflect the amount of corneocytes removed by the individual tape strips, whereas TEWL and 1/TEWL are not sensitive enough to measure the relatively small changes in the SC thickness realized by the removal of the individual strips. The relative SC thickness can be determined directly by the spectroscopic data, while the 1/TEWL values require a second independent method. The results demonstrate the importance of tape stripping characterizing the behaviour of topically applied substances.

Summary Background: The stratum corneum, the outermost layer of the skin, not only represents a barrier, but also a reservoir for topically applied substances. Both properties influence the penetration process, as does the vehicle. It determines the penetration pathway and the retention time of the topically applied substances. Methods: Two commercial products (Temovate Cream and Temovate Emollient), both containing 0.05 % clobetasol propionate, were applied on the inner forearms of healthy volunteers. 20 tape strips were removed from each treated skin area. The horny layer profile was determined measuring the absorption of the corneocytes at 430 nm. The concentration of clobetasol propionate on the individual tapes was then analyzed. Results: We found differences in the reservoir formation depending on the formulation applied, which correlated to the biological response in the form of a blanching effect. Conclusions: The method applied is well-suited to determine the reservoir formation in the stratum corneum for topically applied substances. Zusammenfassung Hintergrund: Die Anwendung von Lokaltherapeutika und kosmetischen Produkten erfolgt mit dem Ziel, das Stratum corneum zu uberwinden, um in bestimmten Schichten der Haut oder systemisch wirksam zu werden. Die auserste Hautschicht besitzt jedoch nicht nur eine Barriere-Funktion, sondern auch eine Reservoir-Funktion. Beide Funktionen beeinflussen den Penetrationsprozes topisch applizierter Substanzen. Die Grundlage, in der eine Substanz appliziert wird, bestimmt den Penetrationsweg sowie die Retentionszeit und beeinflust somit das Penetrationsverhalten der aufgebrachten Substanz. Methodik: Zwei kommerzielle Produkte (Temovate Cream und Temovate Emollient), die jeweils 0,05 % Clobetasolpropionat enthalten, wurden auf den Unterarmen gesunder Probanden appliziert. Von jedem behandelten Hautareal wurden 20 Abrisse entnommen. Das Hornschichtprofil wurde durch die Messung der Extinktion der Korneozyten bei 430 nm bestimmt. Anschliesend wurde die Konzentration von Clobetasolpropionat auf jedem einzelnen Abris analysiert. Ergebnisse: In Abhangigkeit von der Grundlage wurde die Ausbildung eines unterschiedlichen Reservoirs innerhalb des Stratum corneums beobachtet. Dieses korrelierte mit der Auspragung und Verteilung des Blanching-Effektes. Schlusfolgerungen: Unterschiedliche Grundlagen fuhren zur Ausbildung eines unterschiedlichen Reservoirs im Stratum corneum. Die beschriebene Methode ist geeignet, dieses zu analysieren und darzustellen.

For the evaluation and quantification of follicular penetration processes, the knowledge of variations of hair follicle parameters in different body sites is basic. Characteristics of follicle sizes and potential follicular reservoir were determined in cyanoacrylate skin surface biopsies, taken from seven different skin areas (lateral forehead, back, thorax, upper arm, forearm, thigh, and calf region). The highest hair follicle density and percentage of follicular orifices on the skin surface and infundibular surface were found on the forehead, whereas the highest average size of the follicular orifices was measured in the calf region. The highest infundibular volume and therefore a potential follicular reservoir was calculated for the forehead and for the calf region, although the calf region showed the lowest hair follicle density. The calculated follicular volume of these two skin areas was as high as the estimated reservoir of the stratum corneum. The lowest values for every other parameter were found on the forearm. The present investigation clearly contradicts former hypothesis that the amount of appendages of the total skin surface represents not more than 0.1%. Every body region disposes its own hair follicle characteristics, which, in the future, should lead us to a differential evaluation of skin penetration processes and a completely different understanding of penetration of topically applied drugs and cosmetics.

Hair follicles have been assigned an increasingly significant role in dermatopharmacology. Previous studies have shown that the penetration of substances into the follicle depends on its activity. Telogen follicles that were not producing sebum were closed for follicular penetration. The present study uses optical coherence tomography (OCT) to show that the orifices of inactive follicles are blocked by plugs formed of corneocytes. The follicular penetration of a fluorescent dye was analyzed by cyanoacrylate surface biopsies in combination with laser scanning microscopy. Surface exfoliation and cyanoacrylate surface peels were used as a pre-treatment to remove the corneocyte plugs from the follicular orifices and therefore to open up closed follicles. It was shown that both procedures could increase the number of open hair follicles significantly. The present experiments show that OCT in combination with laser scanning microscopy is an excellent non-invasive in vivo method for the investigation of hair follicle properties and follicular penetration processes.

Tape stripping is a well-known method to study the barrier function of the stratum corneum (SC) and penetration processes of topically applied substances into the horny layer. The quantification of the removed corneocytes for each tape strip is the prerequisite for these studies. The pseudo-absorption of the corneocytes was proposed as a measure for the quantification of the removed corneocyte aggregates. In this study, the pseudo-absorption of the corneocytes in the visible range (430 nm) is compared with the protein absorption in the UV range (278 nm) and an absorption at 652 nm obtained after staining of the SC proteins with Trypan blue. Both the protein absorption and the absorption measured after staining correlate well with the pseudo-absorption measured at 430 nm (R2 = 0.92 ± 0.04 and R2 = 0.95 ± 0.04, respectively).

This article gives the results of a study whose aim was to compare the compartmental distribution and absorption of 5 UV filters, in vitro, by fresh human skin, after exposure times of 30 min and 16 h. These UV filters from BASF (octyl methoxycinnamate, benzophenone 4, benzophenone 3, octyl triazone and octocrylene) were incorporated separately in a simple oil-in-water emulsion. The composition of the emulsions was designed in order to obtain a sun protection factor of 5. Therefore the UV filters were introduced into the emulsions at different concentrations. We show that the affinity for each skin level [stratum corneum (SC), viable epidermis, dermis and receptor fluid] is different according to the test substance used. Some substances accumulated in the SC, whereas others passed through the skin very quickly and were quantified in the receptor fluid. The stripping technique allowed us to see that more than 94% of the chemical compound in the SC was in the first eight tapes. The problem of individual values below the limit of detection was raised, a correlation between the two exposure times was found (y = 1.702x – 0.105; R = 0.94) and a classification of products according to their affinity for the SC was determined.

Appropriate evaluation of sunscreens is required to provide better knowledge of their safety and efficacy. One of the most important elements of this evaluation is the assessment of percutaneous absorption. In vitro methods are largely used for such assessments, and the accuracy of the measurements generated with these methods depends on the use of a proper methodology. This study was designed to evaluate an in vitro protocol for investigating the percutaneous absorption of two sunscreens under standardized experimental conditions. Octyl methoxycinnamate and benzophenone 4 were each incorporated in a typical oil-in-water emulsion and tested separately. Salicylic acid was tested as a reference compound. In vitro percutaneous absorption was evaluated using two species, the pig and human, and two models, full-thickness and split-thickness skin. The reproducibility of study results was evaluated by comparing the data generated by two industrial laboratories, L’Oréal and Hoffmann-La Roche. The correlation of quantitative data between pig skin and human skin was very good, and the split-thickness skin model seemed to be more appropriate for measuring the absorption of sunscreens. Results obtained for salicylic acid demonstrated the relevance of the protocol in terms of prediction of in vivo percutaneous absorption. Finally, the comparison of pig skin data between the two laboratories demonstrated a good correlation and underlined the need for a standardized in vitro procedure.

2EEMCO (The European Group for Efficacy Measurements on Cosmetics and Other Topical Products) was created in 1994. It is a group of independent experts from private and public areas from different Member States of the European Union and with competence in the field of instrumental assessment of cosmetic products. EEMCO prepares overviews of existing evaluation methods and analyses the advantages and limitations of the existing techniques to provide general guidance to both the experimenter and the inspector. 3 This document represents the agreed report of the participants as individual scientists.

OBJECTIVES: The aim of this study was to assess the cardiovascular effects of BAY y 5959, a calcium promoter modulating myocardial calcium channels, in the presence or absence of congestive heart failure. BACKGROUND: There is still a clinical need for short-term administration of intravenous positive inotropes. BAY y 5959 was developed as a new approach to increase myocardial performance by selectively enhancing calcium influx in the myocytes. METHODS: Forty-one patients (21 without and 20 with congestive heart failure) were studied in an open label, dose-ranging study. Hemodynamic variables (including left ventricular [LV] angiography) and plasma samples were obtained at baseline and after 20 min of intravenous infusion of BAY y 5959 at doses ranging from 0.25 to 4.5 microg/kg body weight per min. RESULTS: In both study groups, BAY y 5959 produced dose-dependent increases in the indexes of inotropic state, without affecting isovolumetric relaxation rate. The magnitude of the response was comparable in patients with or without heart failure (average 38% increase in maximal first derivative of LV pressure [dP/dt max] at plasma levels of 100 microg/liter). BAY y 5959 also induced mild but statistically significant bradycardia and significantly decreased end-systolic volume while producing a leftward shift of the pressure-volume loop. Mean aortic pressure was unaffected at doses up to 3.0 microg/kg per min, and cardiac index improved in patients with heart failure at doses of 2.0 microg/kg per min (+23%, p < 0.05). However, at a dose of 4.5 microg/kg per min, mean aortic pressure and LV systolic wall stress increased, suggesting systemic vasoconstriction. The QT interval was also prolonged significantly at most doses. CONCLUSIONS: BAY y 5959 exhibits positive inotropic effects in patients with and without heart failure. The optimal response--combining bradycardia, reduced preload and improved cardiac output--appeared to be achieved at a dose of approximately 2.0 microg/kg per min. The impact of QT prolongation with regard to potential antiarrhythmic or proarrhythmic effects is unclear at this time.

The efficacy of the penetration of microparticles into the human skin depends on the size and the type of the formulation with which they are topically applied. Microparticles with a diameter of >1 microm barely penetrate into the human skin. They are located on the skin surface and form a film which, for instance, can be used for camouflage or protection against UV radiation in sunscreens. While the penetration of the microparticles in the lipid layers of the stratum corneum is limited, they penetrate efficiently into the hair follicles up to a depth >2 mm, providing their diameter is

The knowledge of penetration pathways in and through the skin is a prerequisite for the development and optimization of topically applied drugs and cosmetics. Skin penetration has been assumed to occur via diffusion though the lipid domains of the stratum corneum. Recent studies show that the skin appendages, especially the hair follicles, play an important role in skin penetration processes. Topically applied substances cannot enter all follicles. We discuss the reasons for the phenomenon of open and closed hair follicles.

Serum amyloid A protein (SAA) concentrations were monitored in 12 consecutive liver transplant recipients until the 70th postoperative day. Fourteen rejection episodes were identified histologically in 42 liver biopsies of the grafts. Of 12 rejections 8 (66.7%) were characterized by pronounced simultaneous increases in SAA concentrations in plasma, the mean peak value being 16.94 +/- 8.82 mg/dl (range 4.58-28.55 mg/dl) compared with a mean normal value of 0.98 +/- 0.42 mg/dl in healthy controls. Of 42 biopsies 28 did not show histological evidence of graft rejection. Of 25 negative biopsies 24 (96.0%) were not accompanied by a parallel SAA increase in plasma. These findings demonstrate that measurements of SAA concentrations may provide a valuable noninvasive aid in identifying acute liver allograft rejection in humans.

The barrier properties of human epidermis grafted for 1–3 months onto nude mice are compared with normal human skin. Beside penetration studies with tritiated water and measurements of transepidermal water loss (TEWL), we analyzed the epidermal lipids by high-performance thin layer chromatography and evaluated the ultrastructure of the intercorneocyte lipid arrangement by freeze fracture electron microscopy (FFEM). The permeability of human skin for tritiated water and the TEWL exhibit no significant changes after grafting onto nude mice. FFEM analysis showed that grafted epidermis has the same morphological pattern as normal human epidermis. Regular desmosomes and lamellar lipid structures are present. Grafting did not qualitatively affect the lipid composition of human epidermis. Ceramides which contribute largely to the barrier function, have the same distribution profile.

Percutaneous absorption occurs after passive diffusion through the different layers of the skin and its appendages. Thereafter, a resorption process into the cutaneous microcirculation brings the compounds into the systemic circulation. The objective of this in vivo study in the hairless rat was to compare the percutaneous absorption of two steroids on normal and appendage-free (scar) skin and to show if differences in absorption result only from the lack of hair follicles and sebaceous glands and/or from a modification of local blood flow. Percutaneous absorption was evaluated with estradiol and progesterone after 30 min, 2 and 6 h. Except after 30 min, the reservoir function of the stratum corneum of scar skin was approximately twice as high as in normal skin. Eighty to ninety percent of the estradiol and progesterone found in the stratum corneum were located in its superficial layers. Inversely, whatever the application time was, the concentrations of both steroids in the epidermis and dermis were significantly higher in normal skin than in scar skin with maximal difference between about 40 and 400 µm, the area of sebaceous gland localization. Cutaneous blood flow in full-thickness skin, assessed by the thallium-201 method, was globally identical in normal and in scar skin. In scar skin, at the level of papillary dermis, a decrease in blood flow due to the thicker viable epidermis and the flat dermoepidermal junction has been shown without implying an accumulation of drug in the epidermis and superficial dermis. Under these conditions, our results clearly demonstrate that the nutritional blood flow does not interfere with the percutaneous absorption of estradiol and progesterone in normal and scar skin. Thus, they confirm the significant contribution of hair follicles and sebaceous glands to drug penetration into the skin and subsequently the systemic circulation.

A reversed-phase high-performance liquid chromatographic (HPLC) method that allows the sensitive and selective quantification of a novel 4-quinolone (BAY y 3118, I) in biological fluids is described. After sample dilution with 0.05 M phosphoric acid (plasma) or 0.1 M phosphate buffer pH 7.5 (urine), samples can be directly injected into the HPLC system. Prior to fluorescence detection, I is decomposed to fluorescence compound(s) by post-column derivatization utilizing either photolysis (Beam Boost reaction unit) or a combination of thermolysis and photolysis (laboratory-made post-column reactor). Compared with fluorescence detection alone, derivatization increases the signal intensity (about 80-fold) and the selectivity of the detection significantly. Concentrations down to 0.01 mg/l could be quantified in biological fluids. Only thermolysis was not able to decompose I to fluorescence products. Investigations on the stability of I in plasma and urine demonstrate good stability under the different conditions tested. The method was applied to human plasma and urine samples from a subject after a single oral dose of 100 mg of I.

Topical retinoids are therapeutically active at low pharmaceutical concentrations, i.e. in essence lack of penetration is not a problem. Thus studies on the cutaneous pharmacokinetics focus on the distribution pattern in the skin, the penetration pathways, the cutaneous metabolism, the systemic body load and the mode of excretion. Two basically different pathways have to be taken into account: the transepidermal pathway, which gives rise to a typical epidermal/dermal concentration gradient and which is certainly part of the mode of action in disorders of keratinization, and the transfollicular pathway, which sheds some light on the activity in follicular disorders like acne. The cutaneous part of the pharmacokinetics of retinoids depends largely on their chemical structure and thus differs. However, common features are relatively high cutaneous concentrations, whereas the spillover to the systemic compartment appears to be slow and small, giving rise to low or hardly detectable bodyloads. Concerning pharmacokinetics topical application of retinoids presents major advantages over systemic medication. They can be targeted to the intended site of action, furthermore systemic toxicity is of much lesser concern.

Brian W. Barry f, Charan R. Behl ‘, Dale P. Conners g, C. Carnot Evans ‘, Gordon L. Flynn ‘, Thomas J. Franz h, Eugene H. Gans i, William I. Higuchi d, Nicholas Kail e, Gerald G. Krueger j, James Leyden k, Howard I. Maibach ‘, A. Waseem Malick b, Sargio Nacht m, Shirley Ng “, Carl C. Peck a, Lynn K. Pershing j, Russell 0. Potts O, Boyd J. Poulsen p, Hans Schaefer e, Robert C. Scott q, Joel A. Seqeira r, Vinod P. Shah ‘, Dinesh Sharma ‘, Jerome P. Skelly a and Maw-Sheng Wu ’ ” Center for Drug Et,aluation und Research, FDA, RocktYlle, MD 20857 (U.S.A. ), ’ Pharmar~Mtic~i R&D, Hoffmann-La Roche, Inc., 340 Kingsland Street, Nutley, NJ 07110 (U.S.A. ), ’ College of Pharmacy, The iJnic,er.Tit‘y of Michigan, Ann Arbor, MI 48109-1065 (U.S.A.). ” College of Pharmacy, The lJnir,ersity of Utah, Salt Lake City, UT 84112 (U.S.A.), ” CIRD Galderma, Sophia-Antipolis, 06565 Valbonne Cedex (France), f School of Pharmacy. Unicaersity of Bradford, Bradford 807 IDP (U.K.), g Dit’ision of Clinical Pharmacology, Uniformed Seri>ices UniL,ersity of the Health Sciences, Bethesda, MD 20814 (U.S.A.), ” Deportment of Derm~tolo~, Unit~ersiq of Arkansas for Medical Sciences, Lirtle Rock, AR 72205 (U.S.A.I. ’ Hastings Associate, R&D, 5 Fnirriew Drire, Wesrport, CT 06~~~ iU.X.4. ), ’ Di&ion of Dermatolo~, Department of i~edi~ine, Health Sciences &enter3 &zit~ersity of Utah, Salt Lake City, UT 84112 (U.S.A.), k Unil~ersity of Pennsylr,ania, 3400 Spruce Street, Molaney Building, PhiladeIphia, PA 19104 (U.S.A.), ’ Department qf Dermatology, Unic,ersity of California at San Fruncisco, San Francisco, CA 94143 (U.S.A.), n’ R&D Department, Advance Polymer Systems, 3696 Hal,en AL,enue, Redwood City, CA 94063 (U.S.A.), n Ortho Pharmaceutical Corp., Route 202. P.O. Box 300, Raritan. NJ 08869-0602 (U.S.A.), ” Cygnus Therapeutic Systems, 400 Penobscor Dr., Redwood City. CA 94063 (U.S.A.), p Syntex Research, 3401 HillrYew Ar,enue, Palo Alto, CA 94304 (U.S.A.), 4 Acute Toxicity Section, Imperini Chemical Industry PLC, Central Toxicity Laboratory, Alderley Park, NR, Mu~~les~eld

In order to improve the therapeutic index of adapalene, a new drug under development for the treatment of acne, site-specific delivery to the hair follicles using 50:50 poly(DL-lactic-co-glycolic acid) microspheres as particulate carriers was investigated in vitro and in vivo. The percutaneous penetration pathway of the microspheres was shown to be dependent on their mean diameter. Thus, after topical application onto hairless rat or human skin, adapalene-loaded microspheres (5-µm diameter) were specifically targeted to the follicular ducts and did not penetrate via the stratum corneum. The in vitro release of adapalene from the microspheres into artificial sebum at 37°C was controlled and faster than the in vivo sebum excretion in humans. Aiming to reduce either the applied dose of drug or the frequency of administration, different formulations of adapalene-loaded microspheres were evaluated in vivo in the rhino mouse model. A dose-related comedolytic activity of topical formulations of adapalene-loaded microspheres was observed in this model. Furthermore, by applying a site-specific drug delivery system (0.1% adapalene) every other day or by administering a 10-fold less concentrated targeted formulation (0.01%) every day, a pharmacological activity equivalent to a daily application of an aqueous gel containing drug crystals (0.1% adapalene) was observed. Since an aqueous gel containing 10% adapalene-loaded microspheres was not irritating in a rabbit skin irritancy test, this formulation was applied onto forearms of human volunteers. Site-specific drug delivery was further evidenced by follicular biopsy. These results support the view that follicular drug targeting using 5-µm polymeric microspheres may represent a promising therapeutic approach for the treatment of pathologies associated with pilosebaceous units.

International audience; BACKGROUND: Current conventional vaccination approaches do not induce potent CD8 T-cell responses for fighting mostly variable viral diseases such as influenza, avian influenza viruses or HIV. Following our recent study on vaccine penetration by targeting of vaccine to human hair follicular ducts surrounded by Langerhans cells, we tested in the first randomized Phase-Ia trial based on hair follicle penetration (namely transcutaneous route) the induction of virus-specific CD8 T cell responses. METHODS AND FINDINGS: We chose the inactivated influenza vaccine - a conventional licensed tetanus/influenza (TETAGRIP) vaccine - to compare the safety and immunogenicity of transcutaneous (TC) versus IM immunization in two randomized controlled, multi-center Phase I trials including 24 healthy-volunteers and 12 HIV-infected patients. Vaccination was performed by application of inactivated influenza vaccine according to a standard protocol allowing the opening of the hair duct for the TC route or needle-injection for the IM route. We demonstrated that the safety of the two routes was similar. We showed the superiority of TC application, but not the IM route, to induce a significant increase in influenza-specific CD8 cytokine-producing cells in healthy-volunteers and in HIV-infected patients. However, these routes did not differ significantly for the induction of influenza-specific CD4 responses, and neutralizing antibodies were induced only by the IM route. The CD8 cell response is thus the major immune response observed after TC vaccination. CONCLUSIONS: This Phase Ia clinical trial (Manon05) testing an anti-influenza vaccine demonstrated that vaccines designed for antibody induction by the IM route, generate vaccine-specific CD8 T cells when administered transcutaneously. These results underline the necessity of adapting vaccination strategies to control complex infectious diseases when CD8 cellular responses are crucial. Our work opens up a key area for the development of preventive and therapeutic vaccines for diseases in which CD8 cells play a crucial role. TRIAL REGISTRATION: Clinicaltrials.gov NCT00261001.

This report derived from the dermatological workshop discusses the problems and issues in the development and optimization of topical therapeutic drug products. It provides a clear understanding and differentiation between transdermal and dermal products. The report also discusses the bioavailability/bioequivalence issues for topical therapeutic products.

The importance of the transappendageal route on percutaneous absorption was assessed in the hairless rat. Skin permeation of two steroids, hydrocortisone and testosterone, was evaluated in vivo on normal and artificially damaged skin in which follicles and sebaceous glands disappeared during healing. The test compounds were applied for periods of 0.5,2 and 6 h. Thereafter, the stratum corneum reservoir function, the epidermal and dermal distribution profiles, and systemic absorption were determined for both molecules. The results presented here show that the reservoir function of the stratum corneum of appendage-free (scar) skin is more pronounced than that of normal skin, whereas the concentration appearing in the epidermis and dermis was greater in normal skin. Moreover, sebaceous glands probably contribute to the penetration of hydrocortisone and testosterone. We show that the relative importance of the skin appendages depends on the intrinsic physical properties of the molecules tested, and the time of application.

The report differentiates between dermatological and transdermal products, identifies and discusses the principles involved in the development and optimization of topical drug products. The regulatory concerns in the evaluation of topical dermatological products with respect to bioequivalence and quality control are also discussed.

This workshop, ‘In Vivo Percutaneous Penetration/Absorption’ was held in Washington, DC, on May 1–3, 1989. The first workshop in this series, ‘In Vitro Percutaneous Penetration’, took place in November 1986 (the report of this earlier meeting was published in Pharmaceutical Research , 4 (1987) 265–267). The objectives of the workshop were to review the relevant literature and to address in detail: (1) In vivo percutaneous penetration/absorption methodology; (2) The characteristics of dosage forms designed for application to the skin; (3) Critical factors controlling in vivo drug transport into and across the skin; (4) The use of models in the assessment and evaluation of in vivo percutaneous penetration/absorption; and (5) Bioavailability/bioequivalence considerations for topical drug products. Scientific knowledge and technology are rapidly evolving in the topical and transdermal drug products area. This report focuses on the methodologies available for the measurement of percutaneous penetration in vivo; each scientific approach is discussed briefly followed by advantages and disadvantages of the methodology.