In Vitro Percutaneous
`' Absorption:
`Principles , Fundamentals f’and
`Applications
`
`Editors
`
`Robert L. Bronaugh, Ph.D.
`Supervisory Research Pharmacologist
`Division of Toxicological Studies
`U.S. Food and Drug Administration
`Washington, DC.
`
`Howard I. Maibach, MD.
`Professor of Dermatology
`School of Medicine
`
`University of California
`San Francisco, California
`
`CRC Press
`Boca Raton Ann Arbor Boston London
`
`  
`
`

 
`
`MYLAN - EXHIBIT 1026
`
`

`

`
`
`”mamumumr’
`
`/.
`[’RLtR‘t
`
`Library of Congress Cataloging-in-Publication Data
`
`In vitro percutaneous absorption : principles, fundamentals, and
`applications / editors, Robert L. Bronaugh, Howard 1. Maibach.
`p. cm
`Includes bibliographical references.
`Includes index.
`ISBN 0-8493—4748—3
`
`2. Skin absorption--Research--Methodology.
`1. Skin absorption.
`3. Skin-Cultures and culture media.
`I. Bronaugh, Robert L., 1942—
`II. Maibach, Howard I.
`2. Models, Biological.
`[DNLM:
`1. Administration, Cutaneous.
`3. Skim-metabolism.
`4. Skin Absorption——physiology. WR 102 I35]
`QP88.5.I46
`1991
`612.7’91-—dc20
`DNLM/DLC
`for Library of Congress
`
`90-15183
`CIP
`
`This book represents information obtained from authentic and highly regarded sources. Reprinted material is
`quoted with permission, and sources are indicated. A wide variety of references are listed. Every reasonable effort
`has been made to give reliable data and information, but the author and the publisher cannot assume responsibility
`for the validity of all materials or for the consequences of their use.
`
`All rights reserved. This book, or any parts thereof, may not be reproduced in any form without written consent
`from the publisher.
`
`Direct all inquiries to CRC Press, Inc., 2000 Corporate Blvd., N.W., Boca Raton, Florida 33431.
`
`. © 1991 by CRC Press, Inc.
`
`International Standard Book Number 0-8493—4748-3
`
`Library of Congress Card Number 90—15183
`Printed in the United States
`
`

`

`85
`
`E)
`
`Chapter 8 ‘
`
`EFFECTS OF OCCLUSION*
`
`‘ D. Bucks, R. Guy‘9 and H. Maibach
`
`TABLE OF CONTENTS
`
`I.
`
`Introduction ...................................................................... 86
`
`II.
`
`III.
`
`Percutaneous Absorption of p—Phenylenediamine (PPDA) in Guinea Pigs ........ 86
`
`Percutzineous Absorption of Volatile Compounds in Rhesus Monkeys ........... 86
`
`IV.
`
`Percutaneous Absorption of Steroids in Man ..................................... 88
`
`V.
`
`VI.
`
`Percutaneous Absorption of Phenols in Man ..................................... 92
`
`Discussion ....................................................................... 95
`
`References .............................................................................. 1 13
`
`* Sections of this chapter have been adapted from the 2nd edition in this series on Percutaneous Penetration”
`and from the doctoral thesis entitled “Prediction of Percutaneous Absorption”.12
`
`

`

`86
`
`In Vitro Percutaneous Absorption: Principles, Fundamentals, and Applications
`
`I. INTRODUCTION
`
`Mammalian skin provides a relatively efficient barrier to the ingress of exogenous
`materials and the egress of endogenous compounds, particularly water. Loss of this vital
`function results in death from dehydration; compromised function is associated with com-
`plications seen in several dermatological disorders. Stratum comeum intercellular lipid do-
`mains form a major transport pathway for penetration.1“'“"22 Perturbation of these lamellar
`lipids causes skin permeation resistance to fall and has implicated their crucial role in barrier
`function. Indeed, epidermal sterologenes‘lgs> appears to be modulated by the skin’s barrier
`requirements.31 Despite the fact that the s in is perhaps the most impermeable mammalian
`membrane, it is semipermeable; as such, the topical application of pharmaceutical agents
`has been shown to be a viable route of entry into the systemic circulation as well as an
`obvious choice in the treatment of dermatological ailments. Of the various approaches
`employed to enhance the percutaneous absorption of drugs, occlusion (defined as the com—
`plete impairment of passive transepidermal water loss at the application site) is the simplest
`and most common method in use.
`
`The increased clinical efficacy of topical drugs caused by covering the site of application
`was first documented by Garb.21 Subsequently, Scholtz36 using fluocinolone acetonide, and
`Sulzberger and Witten37 using hydrocortisone, reported enhanced corticoid activity with
`occlusion in the treatment of psoriasis. The enhanced pharmacological effect of topical
`corticosteroids under occlusion was further demonstrated by the vasoconstriction studies of
`McKenzie” and McKenzie and Stoughton.30 Occlusion has also been reported to increase
`the percutaneous absorption of various other topically applied compoundsg‘m'z‘S‘27 However,
`as will be shown below, short term occlusion does not necessarily increase the percutaneous
`absorption of all chemicals.
`
`II. PERCUTANEOUS ABSORPTION OF p-PHENYLENEDIAMINE
`(PPDA) IN GUINEA PIGS
`
`The in vivo percutaneous absorption of PPDA from six occlusive patch test systems was
`investigated by Kim et al.27 The extent of absorption was determined using 14C radiotracer
`methodology. The 14C~PPDA was formulated as 1% PPDA in petrolatum (USP) and applied
`from each test system at a skin surface dose of 2 mg/cmZ. Thus, the amount of PPDA was
`normalized with respect to the surface area of each patch test system (and, hence,
`to the
`surface area of treated skin). A sixfold difference in the level of skin absorption (p < 0.02)
`was found (Table 1).
`The rate of 14C excretion following topical application of the radiolabelled PPDA in the
`various patch test systems is shown in Figure 1. Clearly, the rate and extent of PPDA
`absorption was dependent upon the occlusive patch test system employed. It should be noted
`that a nonocclusive control study was not conducted.
`
`III. PERCUTANEOUS ABSORPTION OF VOLATILE
`l
`COMPOUNDS IN RHESUS MONKEYS
`
`The in vivo percutaneous absorption of two fragrances (safrole and cinnamyl anthranilate)
`and two chemical analogs (cinnamic alcohol and cinnamic acid) were measured under
`nonoccluded and plastic wrap (Saran Wrap®——a chlorinated hydrocarbon polymer) occluded
`conditions by Bronaugh et al.3 The extent of absorption following single dose administration
`was determined using 14C radiotracer methodology. Each compound was applied at a topical
`dose of 4 tug/cm2 from a small volume of acetone. The fragrance materials were well absorbed
`through monkey skin. Plastic wrap occlusion of the application site resulted in large increases
`
`

`

`87
`
`TABLE 1
`
`Percutaneous Absorption of PPDA from Patch
`Test Systems“
`
`Patch test system
`
`mg PPDA
`in chamber
`
`Mean % dose
`absorbed (SD)
`
`Hill Top chamber
`Teflon (control)
`Small Finn chamber
`Large Finn chamber
`AL—Test chamber
`Small Firm chamber with paper disc insert
`
`40
`16
`16
`24
`20
`16
`
`53 (21)
`49 (9)
`30 (9)
`23 (7)
`8
`(1)
`34 (20)
`
`Note: The rate of 1“C excretion following topical application of the radiolabeled
`PPDA in the various patch test systems is shown in Figure 1. Clearly,
`the rate and extent of PPDA absorption was dependent upon the occlusive
`patch test system employed. It should be noted that a nonocclusive control
`study was not conducted.
`
`“
`
`2 mg/mm2 PPDA for 48 h on the dorsal mid—lumbar region of the guinea
`pig.
`
`Data from Kim, H. 0., Wester, R. C., McMaster, J. R., Bucks, D. A. W.,
`and Maibach, H. I., Contact Dermatitis, 17, 178, 1987.
`
`1.2
`
`1.0
`
`0.4
`
`0.2
`
`%Dose/Hour
`
`0.8
`+ SMALL FINN
`+ LARGE FINN
`4} ALTEST 0.6
`
`{r HTC
`
`+ TEFLON
`+ SM FINNWPAPER
`
`
`
`
`0
`
`20
`
`40
`
`60
`
`80
`
`100
`
`120
`
`Midpoint (Hrs.)
`
`In viva percutaneous absorption of PPDA (2 mg/mmz) following a
`FIGURE 1.
`48 h exposure on the dorsal lumbar region of guinea pigs (Redrawn from Kim, H.
`O. , Wester, R. C., McMaster, J. R., Bucks, D. A. W., and Maibach, H. 1., Contact
`Dermatitis, 17, 178, 1987.)
`
`l
`1
`
`in absorption (see Table 2). The authors also presented in vitro data documenting the
`significant increase in percutaneous absorption of these chemicals under occluded compared
`to nonoccluded conditions.
`
`Investigation of the effect of occlusion on the percutaneous absorption of six additional
`volatile compounds (benzyl acetate, benzamide, benzoin, benzophenone, benzyl benzoate,
`and benzyl alcohol) was conducted using the same in vivo methodology. These studies
`included occlusion of the site of application with a glass cylinder (secured to the skin by
`
`

`

`88
`
`In Vitro Percutaneous Absorption: Principles, Fundamentals, and Applications
`
`TABLE 2
`
`In Vivo Percutaneous Absorption of
`Fragrances in Monkeys
`
`% Dose absorbed“
`
`Nonprotected
`
`Plastic wrap occlusion
`
`Cinnamyl anthranilate
`Safrole
`Cinnamic alcohol
`Cinnamic acid
`
`(4.6)
`26.1
`(1.6)
`4.1
`25.4 (4.4)
`38.6 (16.6)
`
`39.0 (5.6)
`13.3
`(4.6)
`74.6 (14.4)
`83.9 (5.4)
`
`Note: 24-h exposure at 4 tug/cm2 prior to soap and water washing.
`
`a
`
`Single dose application; values corrected for incomplete renal elim-
`ination. Mean i SD (N = 4).
`
`Data from Bronaugh, R. L., Stewart, R. F., Wester, R. C., Bucks, D.,
`and Maibach, H. I., Fd. Chem. Toxicol., 23, 111, 1985.
`
`TABLE 3
`
`In Vivo Percutaneous Absorption of Benzyl Derivatives in Monkeys
`
`% Dose absorbedB
`
`Nonprotected
`
`Plastic wrap occlusion
`
`Glass chamber occlusion
`
`Log Ko/w
`
`Benzamide
`Benzyl alcohol
`Benzoin
`Benzyl acetate
`Benzophenone
`Benzyl benzoate
`
`47 (14)
`32 (9)
`49 (6)
`35 (19)
`44 (15)
`57 (21)
`
`(8)
`85
`56 (29)
`43 (12)
`17
`(5)
`69 (12)
`71
`(9)
`
`73 (20)
`80 (15)
`77 (4)
`79 (15)
`69 (10)
`65 (20)
`
`0.64
`0.87
`1.35
`1.96
`3.18
`3.97
`
`Note: 24-h exposure at 4 gig/cm2 prior to soap and water washing.
`
`“
`
`Single dose application; values corrected for incomplete renal elimination. Mean i SD (N = 4).
`
`Data from Bronaugh, R. L., Wester, R. C., Bucks, D. A. W., and Maibach, H. 1., Fd. Chem. Toxicol., 28, 369,
`1990.
`
`silicone glue) capped with Parafilm, occlusion with plastic wrap, and nonprotected condi-
`tions.4 As shown in Table 3, occlusion, in general, enhances the percutaneous absorption
`of these compounds. However, differences in percutaneous absorption were observed be-
`tween plastic wrap and “glass chamber” occlusive conditions. The absorption of benzyl
`acetate was lower under plastic wrap compared to the nonprotected condition, whereas glass
`chamber occlusion resulted in the greatest bioavailability. This discrepancy might be due to
`compound; sequestration by the plastic wrap.
`
`IV. PERCUTANEOUS ABSORPTION OF STEROIDS IN MAN
`
`The earliest attempt to correlate the increased pharmacological effect of hydrocortisone
`under occlusive conditions with the pharmacokinetics of absorption was reported by Feld—
`mann and Maibach.18 In this study, the rate and extent of ”C—label excretion into the urine
`following topical application of ”C—hydrocortisone to the ventral forearm of normal human
`volunteers were measured. Radiolabeled hydrocortisone (75 pg) was applied in acetone
`solution (1000 pl) as a surface deposit over 13 cm2 of skin. The authors estimated that this
`
`

`

`89
`
`was equivalent to a sparing application of a 0.5% hydrocortisone topical preparation (5.8
`ug/cmz). The site of application was either nonprotected or occluded with plastic wrap (Saran
`Wrap®). When the skin was unprotected, the dosing site was washed 24 h post application.
`On the other hand, when the skin was occluded,
`the plastic wrap remained in place for
`96 h (4 (1) post application before the application site was washed. The % of the applied
`dose excreted into the urine, corrected for incomplete renal elimination, was (mean 1* SD)
`0.46 i 0.20 and 5.9 i- 3.5 under nonprotected and occluded conditions, respectively (see
`Tables 4, 5, and Figure 2). These numbers differ from those in the original paper which
`were calculated incorrectly. A paired t test of the results indicates a significant difference
`(p = 0.01) in cumulative absorption of hydrocortisone between two exposure conditions.
`Quantitatively,
`the occlusive condition employed increased the cumulative absorption of
`hydrocortisone by about an order of magnitude. However, note that the occlusive system
`retained the drug in contact with the skin for 96 h compared to the 24 h exposure period
`under nonprotected conditions.
`Guy et a1.26 investigated the effect of occlusion on the percutaneous absorption of steroids
`in viva following single and multiple application. The extent of absorption of four steroids
`(progesterone, testosterone, estradiol, and hydrocortisone), using radiotracer elimination into
`the urine following topical application to the ventral forearm of male volunteers, was reported.
`The chemical dose was 4 p.g/cm2 and application area 2.5 cm2. The l4C—labeled chemicals
`were applied in 20 pl acetone. In the occlusive studies, after evaporation of the vehicle,
`the site of application was covered with a plastic (polyethylene—vinyl acetate copolymer)
`chamber.34 In all cases, after 24 h, the site of application was washed with soap and water
`using a standardized procedure.5 In the occlusive studies, the administration site was then
`recovered with a new chamber. An essentially identical protocol was also performed fol—
`lowing a multiple dosing regimen.6 Daily topical doses of three of the steroids (testosterone,
`estradiol, and hydrocortisone) were administered over a 14 d period. The first and eighth
`doses were 14C-labeled and urinary excretion of radiolabel was followed. As above, the 24 h
`washing procedure was performed daily and a new chamber applied. Occlusive chambers
`and washes were collected and assayed for residual surface chemical. The results of this
`study are in Table 6. Steroid percutaneous absorption as a function of penetrant octanol—
`water partition coefficient (KO/w) is shown in Figure 3. The studies indicate that:
`
`1.
`
`2.
`
`3.
`
`4.
`
`The single-dose measurements of the percutaneous absorption of hydrocortisone, es-
`tradiol, and testosterone are predictive of percutaneous absorption following a com—
`parable multiple dose regimen (see chapter on the effect of repetitive application),
`under both occluded and nonoccluded conditions.
`
`Occlusion significantly (p < 0.05) increased the percutaneous absorption of estradiol,
`testosterone, and progesterone, but not that of hydrocortisone.
`Percutaneous absorption increases with increasing Ko/w up to testosterone but declines
`for progesterone, under occluded and nonoccluded conditions.
`The occlusive procedure generally permits excellent dose accountability (Table 7).
`
`l T
`
`he percutaneous absorption of these same four steroids under “protected” (i.e,, cov—
`ered, but nonocclusive) conditions has also been measured in vivog'10 using the same meth-
`odology. The data obtained from these later experiments permitted the effect of occlusion
`to be rigorously assessed (since complete mass balance of the applied dose was possible).
`With the exception of hydrocortisone (Table 8), occlusion significantly increased the per-
`cutaneous absorption (p < 0.01) of the steroids. These results were in excellent agreement
`with the comparable nonprotected studies described above. As stated before, excellent dose
`accountability was reported (Table 9).
`To investigate the apparent discrepancy between the effect of plastic wrap occlusion18
`
`

`

`90
`
`In Vitro Percutaneous Absorption: Principles, Fundamentals, and Applications
`
`900389
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`

`92
`
`In Vitro Percutaneous Absorption: Principles, Fundamentals, and Applications
`
`'5‘ OCCLUDED
`'9‘ NON-PROTECTED
`
`%DOSE/HR
`
`0
`
`48
`
`96
`
`144
`
`192
`
`240
`
`HOURS
`
`Percutaneous absorption of hydrocortisone in man. Human
`FIGURE 2.
`96 h occluded versus 24-h nonprotected exposure of hydrocoitisone at 4 ug/
`cm2 prior to soap and water washing. Occlusion was with plastic wrap. (Data
`from Feldmann, R. J. and Maibach, H. 1., Arch Dermatol., 91, 661, 1965.)
`
`TABLE 6
`
`Percutaneous Absorption of Steroids in Man
`
`Mean % applied dose
`absorbed (:t SD)
`
`Nonprotected
`
`Occlusion
`
`2 : 2a
`
`3 i-
`3 i— 1
`
`11 i 5“
`
`10 i 2
`11 i 5
`
`13 i 3a
`
`21 i 6
`20 i 7
`
`4 i 2
`
`4 i
`3 i
`
`27 i 6
`
`38 i 8
`22 t 7
`
`46 t 15
`
`51 i 10
`50 i 9
`
`11 i 63
`
`33 i 9
`
`Hydrocortisone
`Single application
`Multiple application:
`1st Dose
`8th Dose
`Estradiol
`
`Single application
`Multiple application:
`lst Dose
`8th Dose
`Testosterone
`Single application
`Multiple application:
`lst Dose
`8th Dose
`Progesterone
`Single application
`
`“ Data from Feldmann, R. and Maibach, H. 1., J. Invest.
`
`i
`
`Dermatol., 52, 89, 1969.
`
`and that of the plastic chamber on hydrocortisone absorption,26 we repeated the measurements
`of penetration using plastic wrap (Saran Wrap®) with the experimental protocol of Guy et
`a1.26 Under these circumstances,- we found no difference between plastic wrap and plastic
`chamber occlusion on the percutaneous absorption of hydrocortisone (Table 10).
`
`V. PERCUTANEOUS ABSORPTION OF PHENOLS IN MAN
`
`We subsequently investigated the effect of occlusion on the in vivo percutaneous ab—
`sorption of phenols following single dose application. The occlusive and protective Chamber
`
`

`

`93
`
`ES
`
`43‘ OCCLUDED
`+ NON-PROTECTED
`
`ABSORBED
`°/oDOSE
`
`Log Ko/w
`
`Percutaneous absorption of 4 steroids (HC = hydrocortisone, E8
`FIGURE 3.
`= estradiol, TS = testosterone, PG = progesterone) in man as a function of
`penetrant octanol/water partition coefficient. Exposure period 24-h at
`4 pg/cm2 prior to soap and water washing. (Redrawn from Guy et al., in Skin
`Pharmacakinetics, Shroot, B. and Schaefer, H., Eds., Karger, Basel, 1987,
`70.)
`
`TABLE 7
`
`Accountability of Applied Dose in Occluded Studies”
`
`Observed PA
`
`% Removed from skin
`
`Total % dose
`
`Hydrocortisone
`Single doseb
`lstMDc
`8thMDd
`Estradiol
`Single doseb
`lst MDc
`8thMDd
`Testosterone
`Single doseb
`lstMDc
`8th MDd
`Progesterone
`Single doseb
`
`4 t 2
`4i1
`3:1
`
`27 i 6
`38 i 8
`22i7
`
`46 i 15
`51 1 10
`50 i 9
`
`33 i 9
`
`64 i 5
`82i5
`78:2
`
`60 t 12
`62 i
`59:8
`
`44 i 7
`48 i 9
`42 i 9
`
`47 i 10
`
`68 i 4
`85:4
`81i3
`
`87 i 13
`100 i 4
`81 i6
`
`90 t 8
`99 i 4
`92 i 17
`
`80 i 6
`
`Note: Mean (in % applied dose) : SD
`
`“
`
`24-h exposure at 4 pig/cm2 prior to soap and water washing. Occlusion was
`with a plastic (Hilltop) chamber.
`Single dose study.
`1’
`First dose of a l4-d multiple—dose study.
`c
`d Eighth dose of a 14~d multiple-dose study.
`
`Data from Bucks, D. A. W., et a1., Unpublished observations.
`
`

`

`94
`
`In Vitro Percutaneous Absorption: Principles, Fundamentals, and Applications
`
`TABLE 8
`
`Percutaneous Absorption of Steroids in Man
`Single Dose Application for 24 h @ 4 pLg/cm2
`
`Mean % dose absorbed (1- SD; N 2 5)
`Protected“
`Occluded"
`
`Hydrocortisone
`Estradiol
`Testosterone
`Progesterone
`
`4 i- 2
`3 i 1
`18 i- 9
`13 i 6
`
`“ Ventilated plastic chamber.
`b Occlusive plastic chamber.
`
`4 i 2
`27 t 6
`46 i 15
`33 t 9
`
`Data from Guy, R. H., Bucks, D. A. W., McMaster, J. R., Vil-
`laflor, D. A., Roskos, K. V., Hinz, R. S., and Maibach, H. 1.,
`in Skin Pharmacokinetics, Shroot, B. and Schaefer, H., Eds, Kar-
`ger, Basel, 70, 1987.; Bucks, D. A. W., Maibach, H. I., and Guy,
`R. H.,
`in Percutaneous Absorption, V01. 2, Bronaugh, R. and
`Maibach, H., Eds, Marcel Dekker, New York, 1989, 77.; and,
`Bucks, D. A. W., McMaster, J. R., Maibach, H. 1., and Guy, R.
`H., J. Invest. Dermatol, 90, 29, 1988.
`
`TABLE 9
`
`Accountability of Applied Dose in Protected Studies using
`Ventilated Plastic Chambers“
`
`Observed PA
`
`% Removed from skin
`
`Total % dose
`
`Hydroconisone
`Estradiol
`Testosterone
`Progesterone
`
`4 i 2
`3 t 1
`18 i 9
`13 i 6
`
`85 i 6
`96 i 1
`77 i 8
`82 i 7
`
`89 t 6
`100 i 1
`96 i: 2
`96 i 3
`
`Note: Mean % dose (1 SD, N 2 5).
`
`a
`
`Single dose application for 24 h at 4 ng/cm2
`
`Data from Bucks et al.9-"’
`
`TABLE 10
`
`Percutaneous Absorption of
`Hydrocortisone in Man
`
`% Dose absorbed“
`
`1
`
`Plastic wrap occlusion
`Plastic Chamber occlusionc
`“Protected” conditiond
`
`4.7 (2.1)b
`4.0 (2.4)
`4.4 (1.7)
`
`a
`
`Single dose application for 24 h at 4 ng/cmz; values
`corrected for incomplete renal elimination.
`b Mean : SD (N = 6)
`° Guy et 31.26
`d Bucks et al.9'10
`
`

`

`95
`
`methodology described by Bucks et al.7'10 was utilized. Nine l4C ring labelled para-substituted
`phenols (4-aminophenol, 4-acetamidophenol, 4-propionylamidophenol, phenol, 4-Cyano-
`phenol, 4~nitrophenol, 4—iodophenol, 4—heptyloxyphenol and 4—pentyloxyphenol) were used.
`As in the earlier steroid studies, the site of application was the ventral forearm of male
`volunteers and the area of application 2.5 cm2. Penetrants were applied in 20 u] ethanol
`(95%). The chemical dose was 2 to 4 pig/cm? After vehicle evaporation, the application
`site was covered with either an occlusive or protective device. After 24 h, the patch was
`removed and the site washed with a standardized procedure.5 The application site was then
`recovered with a new chamber of the same type. Urine was collected for seven days. On
`the seventh day: (1) the second chamber was removed, (2) the dosing site was washed with
`the same procedure, and (3) the upper layers of stratum comeum from the application site
`were removed by cellophane tape stripping. Urine, chambers, washes, and skin tape strips
`were collected and assayed for radiolabel. Percutaneous absorption of each compound under
`protected and occluded conditions is presented in Tables 11 through 19 and Figures 4 through
`12. Phenol percutaneous absorption as a function of the penetrant octanol-water partition
`coefficient (KO/w) is shown in Figure 13. Phenol percutaneous absorption is summarized
`in Table 20. The methodology permitted excellent dose accountability (Tables 21 and 22).
`The studies indicate that:
`
`1.
`
`2.
`
`3.
`
`Occlusion significantly increased (unpaired t test, p < 0.05) the penetration of phenol,
`heptyloxyphenol and pentyloxyphenol.
`Occlusion did not enhance the absorption of aminophenol, 4—acetamidophenol, pro—
`pionylamidophenol, cyanophenol, nitrophenol, and iodophenol.
`The methodology employed again permitted excellent dose accountability.
`
`VI. DISCUSSION
`
`A predominant effect of occlusion is to increase hydration of the stratum comeum,
`thereby swelling the corneocytes, and promoting the uptake of water into intercellular lipid
`domains. The normal water content of stratum comeum is 5 to 15%, a value which can be
`increased up to 50% by occlusion.“33 Upon removal of a plastic occlusive dressing after
`24 h of contact, transepidermal water loss values are increased by an order of magnitude;10
`the elevated rate then returns rapidly (~ 15 minutes) to normal with extraneous water
`dissipation. With occlusion, skin temperature generally increases from 32°C to as much as
`37°C.28 Faergemann et al.17 showed that occlusion: (1) increases the transepidermal flux of
`chloride and carbon dioxide, (2) increases microbial counts on skin, and (3) increases the
`surface pH of skin from a preoccluded value of 5.6 to 6.7. Anhidrosis results from occlu—
`sion.32'23 Plastic chamber occlusion can also cause skin irritation (personal observation).
`Occlusion-induced increases in mitotic rate of skin and epidermal thickening have been
`documented by Fisher and Maibach.20
`With respect to percutaneous absorption, occlusion (or a protective cover) prevents loss
`of the surface deposited chemical by friction and/or exfoliation; bioavailability may, thereby,
`be increased. However, comparison of the data in Tables 6 and 8, for the percutaneous
`absorption of steroids under nonprotected and protected conditions, shows clearly that the
`potential increase in bioavailability from protection of the site of application does not explain
`the increase in steroid absorption under occluded conditions.
`Occlusion does not necessarily increase percutaneous absorption. Hydrocortisone ab-
`sorption under occluded conditions was not enhanced in single dose or multiple dose ap-
`plication studies (Table 23).
`This lack of penetration enhancement under occluded conditions has been observed with
`certain para~substituted phenols. However, a trend of occlusion-induced absorption enhance-
`
`

`

`96
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`In Vitro Percutaneous Absorption: Principles, Fundamentals, and Applications
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`4:HQQH
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`In Vitro Percutaneous Absorption: Principles, Fundamentals, and Applications
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`100
`
`In Vitro Percutaneous Absorption: Principles, Fundamentals, and Applications
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`In Vitro Percutaneous Absorption: Principles, Fundamentals, and Applications
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