International Journal of Pharmaceutics. 36 (I 989) 43—50
`Elsevier
`
`43
`
`UP 01889
`
`Urea analogues in propylene glycol as penetration enhancers
`in human skin
`
`A.C. Williams and B.W. Barry
`Postgraduate Studies in Pharmut'eutrcuf Technou’vgy. The School! of Pharmact', University of Bradford. Bradfirrd (UK)
`(Received 20 April [989)
`{Accepted 10 May 1989]
`
`Key wordr: Percutaneous absorption; Penetration enhancer; Urea; Urea analogue; Propylene glycol:
`S—Fiuorouracil
`
`Summary
`
`Urea. l—dodecyiurca. 1.3-didodecylurea and 1.3-diphenylurea were assessed as skin penetration enhancers for the model penetrant
`5—f1uorouraci1(5-Fl;]. The permeability coefficient (KP) was determined for :‘s-I—‘U applied in saturated aqueous Solutions to human
`epidermal membranes. Then each urea was applied as a saturated solution in dimethyiisosorbide. light liquid paraffin or propylene
`glycol:
`the solutions were removed and KI, was redetermined: the enhancement ratio (Kr after enhancer treatment,«”!€p
`before
`enhancer treatment) measured the accelerant effect. Urea and the vehicles alone were ineffective as enhancers: the urea analogues
`behaved similarly at saturation in any one vehicle; and the analogues were only effective when delivered from propylene glycol,
`enhancing the permeation of S-FU 6 times by increasing the diffusivity of the stratum corneum. Thus. the role of propylene glycol as
`a synergistic vehiclefor penetration enhancers was confirmed.
`
`Introduction
`
`Topical administration of therapeutic agents
`promises many advantages over oral and in-
`travenous administration (Barry, 1983; Guy and
`Hadgraft, 1985). However, the relative impermea—
`biljty of the stratum oorneum offers considerable
`resistance to drug permeation.
`in attempts to re-
`duce reversibly this diffusiOnal barrier. researchers
`have employed penetration enhancers (or accel-
`erants) which interact with stratum corneum cott-
`stitucnts, disrupting the highly ordered structure
`
`Correrpondence: KW. Barry, Postgraduate Studies in Phar-
`maceutical Technology. The School of Pharmacy. Universin of
`Bradford. Bradford. BD'l' 101’. UK.
`
`(cg. Southwell and Barry, 1983: Barry et a]., 1984:
`Southwell and Barry, 1984; Goodman and Barry,
`1988: Okamoto ct al.. 1988). Ideally, a penetration
`enhancer is pharmacologically inert. has a specific.
`immediate yet reversible. action and i5 cosmeti-
`cally acceptable (Barry, 1983; Hadgraft. 1984'.
`Woodford and Barry, 1986). In the present study.
`urea and 3 analogues. dissolved in 3 vehicles, were
`compared for their penetration-enhancing activi-
`ties towards the cytotoxic agent S-fluorouraci] (5-
`FU), chosen as a model penetrant.
`Urea is a mild keratolytic agent used in the
`treatment of ichthyosis and othcr hyperkeratotic
`skin conditions. As a 10% cream. it increases the
`water-holding capacity of the stratum corneum by
`100%, and has little effect on the epidermal water
`barrier (Grice ct al.. 1973}. The moisturizing and
`
`(HTS-5 1?3/89/$UB.5{1 1'" I989 Elsevier Science Publishers B.V. (Biomedical Division)
`
`Noven Pharmaceuticals, Inc.
`EX2007
`
`0001
`
`Mylan Tech, Inc. v. Noven Pharma., Inc.
`IPR2018—00174
`
`

`

`44
`
`o
`I:
`H\N/C\N/H
`rlt
`rt'
`
`Urea
`
`ii
`HxN/CxN/H
`coin" (Clilzlfi
`c'H3
`Ci!3
`1,3‘Dldodocylurea
`
`H
`H30_C—CH,
`|
`|
`OH OH
`rem: Gleol
`y
`
`I”
`
`Pro
`
`o
`ll
`”\N/cxN/H
`icing"
`'li
`Ell-la
`ivooa-cyluru
`
`if
`HEN/CRN/H
`6) ©
`
`1.3-Dlphonylurlo
`
`Haco
`
`H
`
`.
`
`H H 0cm;4
`Dimethvrlleesorblrlo
`
`Fig.
`
`formulae of the urea analogues and
`l. The structural
`vehicles evaluated as penetration enhancers.
`
`keratolytie effects of urea increase the activity and
`bioavailability of hydrocortisone from Alphaderm
`cream (Barry and Woodford, 19'”; Barry. 1983;
`Woodl‘ord and Barry, 1984). As a 10% solution in
`propylene glycol
`(PG). urea has no effect on
`naloxone flux through human cadaver skin (Aungst
`ct al.. 1986).
`such as
`Established penetration enhancers
`Azone, which contains a C12 saturated hydro-
`carbon chain, interact with and disrupt the struc-
`tured lipid environment in the stratum corneum
`(Barry, 1987a and b; Goodman and Barry, 1989).
`We investigated the poSSibility of combining the
`moisturizing and keratolytic properties of urea
`with the disrupting effects of alkyl and aryl groups,
`one or two per molecule. The chemicals thus tested
`for penetrationvenhancing activity towards S-FU
`were: urea,
`l-dodeeylurca (DDU), 1,3-didodecy-
`lurea (DDDU) and 1,3—diphcnylurca [DPU);
`the
`vehicles were dimenthylisosorbidc (DMI),
`light
`liquid paraffin (LLP) and (PG) (Fig. 1).
`DMI. promoted for use in cosmetics, is a solvent
`which is poorly adsorbed by the skin and appears
`to have little penetration—enhancing activity (Barry
`el al., 1984; Bennett et al., 1985). It has therefore
`been selected as a standard vehicle for compari—
`
`sons ot‘ potential penetration enhancers. Light
`liquid paraffin is a widely used lipophilic vehicle
`for topical preparations. It is used as an emollient
`in irritant skin conditions and for the removal of
`
`desquamative crusts. PG is valuable in dermato-
`logical
`formulations and as
`a
`eosolvent
`for
`penetration enhancers.
`It has been reported to
`increase the permeation of oestradiol (Mollgaard
`and l-ioelgaard, 1983a) and hydrocortisonc (Barry
`and Bennett. 1987)
`through excised human ab-
`dominal skin, yet
`is ineffective in promoting the
`topical bioavailability of betamcthasonc 17-bcnzo-
`ate as assessed by the occluded vasoconstrictor
`assay (Barry ct al., 1984). it is also ineffective in
`promoting permeation of metronidazole through
`excised full-thickness human skin {Mollgaard et
`al., 1988) and PG pretreatment of human epider—
`mal membranes has no significant effect on S-FU
`pseudo-steady state permeation (Goodman and
`Barry, 1988). However, when used in combination
`with accelerants such as Azone and oleic acid, PG
`shows a marked synergistic response (cg. Barry
`and Bennett, 1987’; Barry, 1987a; Goodman and
`Barry. 1988).
`
`Materials and Methods
`
`Urea and DPU (Sigma Chemical Company)
`were used as received; DDU and DDDU were
`synthesised (Erickson. 1954). PG (B.D.H. Chem-
`icals Ltd), DMI
`(Aldrich Chemical Company)
`and LL? {B.D.I—l. Chemicals Ltd.) were used as
`supplied. 5-[6-3H]FU (Amersham International
`PLC) was the model permcant, a saturated aque-
`ous solution (10.2 mg/ml at 32 11°C.; Bond and
`Barry, 1988) being prepared with the help of un-
`labelled S-FU (Sigma Chemical Company).
`Saturated solutions of urea and the analogues
`were prepared in the 3 vehicles.
`the approximate
`concentrations being evaluated gravimetrieally.
`Partition coefficients (octanol/ water) of urea and
`the analogues were calculated by the fragment
`method of Hansch and Leo (1979}.
`
`synthesis
`J—Dodecylurca. A mixture of dodecylamine
`(18.5 g, 0.10 mol). urea (6.6 g, 0.11 mol) and
`
`0002
`
`

`

`pyridine (200 ml) was refluxed for 4.5 h in a fume
`cupboard, cooled over
`ice and the crystalline
`product filtered off under suction. The crystals
`were washed with water to remove excess urea and
`
`pyridine. The product was recrystallised from
`chloroform to a constant melting point. de-
`termined by differential
`scanning calorimetry
`(Perkin-Elmer 7 Series Thermal Analysis System)
`of 107.2°C. The literature gives t06.8—107.5°C
`(Erickson, 1954). The thermal analysis showed the
`product to be approximately 98% pure.
`{,3-Didodetyiareu. A mixture of dodecylaminc
`[13.0 g, 0.07 mol), urea (2.0 g1 0.03 mol) and
`butan—l —ol (20 ml) was refluxed for 30 h in a fume
`cupboard and cooled over ice and the crystalline
`product was filtered off under suction. Recrystal-
`lisation from acetone gave a constant melting point
`of 104.8°C, and a purity of approximately 92%.
`which is adequate for penetration enhancer stud
`ies. The literature gives lO3.3—105.5°C (Erickson,
`1954).
`
`Preparation of human epidermoi membranes
`Caucasian abdominal skin (male and female,
`70—89 y) obtained post-mortem was stored frozen
`at —-20°C (Harrison et al., 1984). Epidermal
`membranes were prepared by the heat separation
`technique of Kligman and Christophcrs (1963).
`Excess fatty and connective tissues were removed
`from the skin which was then immersed in water
`
`at 60°C for 45 s. The epidermal membrane was
`teased off the underlying dermis and floated on an
`aqueous solution of 0.002% sodium azide for 36 h
`to ensure that
`the stratum eorneum was fully
`hydrated.
`
`Permeation experiments
`Experiments at 32i1°C used an automated
`diffusion apparatus with 24 stainless—steel diffu—
`sion cells, diffusional area 0.126 cm2. and 0.002%
`aqueous sodium azide receptor solution (Akhter et
`3.1., 1984). Fully hydrated epidermal membrane
`samples were mounted in the cells and 150 pl
`aliquots of saturated, radiolabelled S-FU solution
`placed in the donor compartments which were
`covered. 4 ml samples of receptor solutiou were
`collected every 2 h for 36 h,
`to which 10 ml
`
`45
`
`Scintran Cocktail T was added, and the radio-
`labelled drug determined by liquid scintillation
`counting (Packard 460C). The permeant solution
`was washed from the membrane with 0.002%
`
`sodium azidc solution and replaced with 150 pl
`saturated solution of urea or an analogue in one of
`the vehicles. After 12 h the test solution was
`
`washed from the membrane and the permeation of
`radiolabelled S-FU again monitored for 36 h.
`
`Partitioning experiments
`The effect of urea analogue/PG formulations
`on the partitioning of S-FL' was investigated. Per-
`meation of S—FLJ
`through untreated epidermal
`membranes was monitored. and at pseudo steady
`state flux the concentration of the drug in the
`membrane was determined as follows. The epider-
`mal membranes were removed from the diffusion
`
`cells, rinsed with distilled water, blotted dry and
`the diffusional areas were solubilised in 1 ml
`Soluene-350. 10 ml Scintran Cocktail T scintilla-
`
`tion fluid and 0.1 ml glacial acetic acid were
`added and samples stored at room temperature
`overnight to allow chemiluminescence to subside.
`Acidification of the mixture reduces non—radiation
`
`events which may interfere with drug determina-
`tion. The concentration of S-FU in the membrane
`
`was evaluated by liquid scintillation counting. The
`pseudo—steady—state concentration of S-FU in epi-
`dermal membranes after 12 h treatments with a
`
`urea analogue/PG mixture were similarly de-
`termined to illustrate the accelerant effects on
`
`partitioning of the drug into the tissue.-
`
`Results and Discussion
`
`the drug
`Example permeation profiles of
`through the membrane before and after treatment
`with a solution of the urea analogues saturated in
`propylene glycol are given in Fig. 2. Computer-
`aided analysis of these results evaluated the per—
`meability coefficient (KP) of the drug in the mem-
`brane before and after treatment with a penetra-
`tion enhancing solution. A measure of
`the
`penetration—enhancing activity of the agent.
`the
`enhancement
`ratio (ER), may be calculated
`
`0003
`
`

`

`4ft
`
`x105
`
`CJMLLATIVE
`
`comicn‘t2
`
`o
`
`to
`
`20
`TIME {h}
`
`30
`
`35
`
`Fig. 2. Example permeation profiles of SmFU through human
`epidermal membranes before and. after treatment with saturated
`solutions of the urea analogues in propylene glycol: squares,
`DIJU; triangles, DPU: circles. DDDU: diamonds, control.
`
`(Goodman and Barry, 1988):
`
`E.R. = Ki, of membrane after application
`of penetration enhancer/
`KP of membrane before application
`of penetration enhancer
`
`The values reported were the mean enhance—
`ment ratios from a minimum of 5 replicates.
`
`The experimental design of determining Kw
`treating the epidermal membrane with penetration
`
`enhancers, and then redetermining KP, allows each
`piece of skin to act as its own control. thereby
`reducing errors due to the biological variability of
`human skin. The conditions for drug delivery were
`maximised with the use of saturated drug solu-
`tions,
`thereby maintaining the permeant at
`its
`maximum thermodynamic activity. The epidermal
`membrane was fully hydrated, a condition which
`enhances the permeation of most penetrants,
`in-
`cluding S-FU (Barry,'1987a; Goodman and Barry,
`1989). This last condition thus provides a stringent
`test of penetration enhancing activity. The use of
`saturated solutions of urea and its analogues al—
`lows a direct comparison of the penetration—en—
`hancing abilities of each agent
`from different
`vehicles as the chemical potential of the penetra-
`tion enhancer is constant (maximal) in all the test
`solutions. The approximate saturated concentra-
`tions of the test agents in the different vehicles,
`
`'I'ABLE 1
`
`The approximate saturated r-wlt‘emmt‘ionr (mg/ m1) of urea and
`the analogues- in the 3 aehicles at room temperature H 9 i l °(,‘;,
`and Ike articulated log partition coefficients (ocranol/ water) for
`urea and the analogues
`
`Urea
`Vehicle
`log P
`
`“3'03“
`LLP
`DMI
`PG
`4.77-
`DDL
`0.3
`3.8
`2.6
`11.7
`DDDU
`0.2
`0.5
`0.7
`2,98
`DPU
`l .0
`1.5
`15
`
`1.6 5.0 I?Urea —2.11
`
`
`
`
`and the calculated log partition coefficient (log P)
`values, are give in Table 1. All the test agents are
`poorly soluble in LLP with urea having the grea—
`test concentration of 1.6 rug/ml. The saturated
`concentrations of DDU and DDDU in DM1 and
`
`PG are similar. thus any differences observed in
`the penetration—enhancing effects of
`these ana—
`logues from the two vehicles is unlikely to be due
`to a large difference in concentrations of the test
`agents.
`A comparison of the permeability coefficients
`of the penetration enhancers from the 3 vehicles is
`given in Table 2. From these results,
`the mean
`control value for the permeability coefficient of
`S-FU in the untreated membrane at 32°C is 2.16
`
`TABLE 2
`
`Mean permeability coefficients of 5—FU through human cadaver
`skirt, with standard error of the mean, before and after treatmen!
`with urea analogues applied from 3 ”entries: :1, before irearmem;
`b, after treatment
`
`
`
`
`Permeability coefficient X 105 (cm/h}
`Urea
`
`”Mingus
`1.1.?
`DMI
`._ Po
`Vehicle
`alone
`
`
`
`3.36i 1.24
`1.84 +0.98
`2.]?1030
`a
`4,18ilfi'i"
`1.73 $0.4?
`2.60:: 0.63
`b
`1.56i0.36
`3.48:1.36
`2.33i03’4
`a
`1.71 i046
`3.50:1,44
`1.5T:0.33
`b
`0.96 $0.25
`0.93:0}?
`3.?4i l .0?
`a
`4.17zl; 1.15
`2.03:0.96
`2.?2i093
`b
`1.24j; 0.18
`0.53:0.10
`3,28i0.64
`a
`3.49 10.77
`1.25:0.36
`2.571030
`b
`OJTiUDS
`0,?9:U.22
`5.3Di 1.39
`a
`DPU
`
`
`
`0.79:0.0?5.68i3.0]b 2.6?+0.33
`
`Urea
`
`DDU
`
`DDDU
`
`0004
`
`

`

`RATIO
`
`ENHANCEMENT
`
` 3
`
`nnnu LIOUlO
`
`PnRfiFFlN
`
`DIMETHYL-
`ISUSOREIBE
`
`non-
`UDU
`PRDPYLENE
`GL'I'COL
`
`Fig. 3. The mean enhancement ratios of urea and the analogues
`from the 3 vehicles, with S.l:'.M.1 u. urea; V, vehicle alone.
`
`j: 0.35 X 10’ 5 cm/h (n = 75). a value that shows
`good agreement with other published data (Good—
`man and Barry, 1988). The activity of the urea
`analogues are more clearly demonstrated in terms
`of the enhancement ratios,
`the mean values of
`which are shown in Fig. 3. These results show that
`the vehicles alone, and urea saturated in the
`vehicles, produce no significant
`increase in the
`permeability coefficient of S—FU (P = 0.05). Also,
`no significant difference exists in the penetration-
`enhancing activities of the three urea analogues
`delivered from a given vehicle (P=0.05). How-
`ever.
`the choice of vehicle clearly affects the en-
`hancing activity of the test agents. In particular,
`when applied as a saturated solution in propylene
`glycol, the enhancement ratios of the urea ana~
`logues are significantly greater than when applied
`saturated in DMI or LLP (P: 0.05).
`The mechanisms of action of penetration en—
`hancers are becoming clear, and a general theory
`of accelerant activity based on molecular changes
`in the stratum ccrneum has been proposed (Barry,
`1987a; Goodman and Barry, 1989). Based on this
`theory, penetration enhancers may act mainly by
`
`4?
`
`one or more of 3 main mechanisms; disruption of
`the highly ordered lipid structure between the
`corneocytes,
`interaction with intracellular
`pro—
`teins, and partitioning effects.
`a concept
`for—
`malised as the lipid—protein partitioning (LPP)
`theory (Barry, 1989).
`The synergistic effect of PG with a variety of
`penetralion enhancers such as Azonc and oleic
`acid is well documented (Cooper. 1984; Sheth et
`al., 1986; Baldy, 1987a), and studies by Wotten et
`al. (1985) concluded that the glycol is necessary to
`maximise the penetration-enhancing properties of
`Azone. Differential scanning calorimetry studies
`of PG—treated stratum corneum show an alteration
`
`in the intracellular keratin structure. probably due
`to displacement of bound water (Goodman and
`Barry, 1989). This effect reduces drug/skin bind-
`ing,
`thereby enhancing intracellular
`transport.
`However,
`this effect would only be important
`under conditions whereby the intercellular lipid
`structures were not rate-limiting in diffusion, or
`had been disrupted by a penetration enhancer
`(Barry, 1987a). PG permeates the skin in substan-
`tial amounts (Mollgaard anti
`l-loelgaard.
`l983b).
`With urea analogue PG mixtures. the glycol per-
`meating into the skin will enhance partitioning of
`the Lipophjlie accelerants into the stratum corne-
`um. Once in the lipoidal environment, the hydro-
`phobic moieties of the penetration enhancers may
`interact
`to disrupt
`the highly ordered barrier
`structure. Fig. 2 shows a reduction in the lag time
`(L) for S—FU permeation after treatment with
`each urea analogue in PG. The lag time is related
`to the diffusivity (D) of the drug in the membrane
`by:
`
`hi!
`L267)
`
`where h is the membrane thickness (taken as
`
`approximately 3 x 10—3 cm for human ademinal
`stratum corneum}. Thus the diffusivity of S—FU in
`the membrane after treatment with DDU in PG
`
`(mean log time 1.03 h, n = 4} may be calculated
`approximately:
`
`2
`—6
`'11:!
`c—E—Msxm em/h
`
`0005
`
`

`

`48
`
`Comparing this with the diffusivity of the mem-
`brane prior to treatment (mean lag time 9.65 h.
`17:17) of 1.55X 10'—T CmZ/h. shows a 9.4mfold
`increase in diffusivity after
`treatment with the
`urea analogue. It
`is widely aceeptecl
`that many
`molecules traversing the stratum corneum do so
`by a tortuous interoellular pathway, and the diffu—
`sional pathlength for a molecule has recently been
`speculated to be approximately 350 pm {Guy and
`Hadgraft, 1988). Thus, the value of it used above
`to calculate D may be significantly underesti-
`mated. However, the precise value for diffusional
`pathlength is irrelevant when taking the ratio of
`diffusivities before anti after enhancer treatment,
`as the pathlength is assumed to be constant in
`both cases. This may not be the true situation as
`PG may alter the epidermal membrane thickness.
`Thus the diffusivity values. and their ratios. are
`approximate, but are useful guides to molecular
`cvents within the tissue.
`
`The 9—fold increase in diffusi'vity correlates with
`a mechanism of action whereby the penetration
`enhancer disrupts the lipid structure of the stra-
`tum eorneum. The partition coefficient (P) of the
`drug from its vehicle (aqueous solution) into the
`stratum corneum is related to the membrane diffu-
`
`sivity by:
`
`
`
`Thus. the partition coefficient for the drug may be
`evaluated approximately to give a control pre-
`treatment value of 0.484 and a posttreatment value
`of 0.327. As expected. the presence of PG and the
`urea analogue in the membrane correlates with a
`reduction in partitioning of
`the drug into the
`tissue, by a factor of 0.68. To verify this conclu-
`sion,
`the steady state concentrations of 5-FU in
`epidermal membranes were determined. After
`treatment with DDU in PG the steady state con-
`centration of S-FU in the membrane fell by a
`factor of 0.7l i009 (n=3), which is in good
`agreement with the factor of 0.68 above. There-
`fore. we conclude that
`the experimentally de-
`termined enhancement ratio with DDU in PG is
`
`which give a combined effect: 9,42 X068 = 6.40
`(= ER.)
`A similarly reduced lag time was observed with
`DDDU and DPU. again illustrating increased
`membrane diffusivity and reduced drug parti-
`tioning.
`The calculated log P (octanol/water) values
`for the urea analogues range from approximately
`3 to 11.7, and we expect
`the rank-order of the
`analogues in this system to correlate with log P
`(stratum corneum/ water). Clearly, with PG pre~
`sent in the stratum eorneum, the solvent nature of
`the membrane is altered and hence the partition
`coefficient data are of little value for predicting
`the amounts of the accelerants entering the mem~
`brane. However. once the analogues are in the
`lipid domain of
`the stratum corneum,
`their
`clearance into aqueous receptor solutions will be
`governed by a partitioning mechanism. DDDU
`(log P= 11.7) is not cleared from the skin before
`36 h, whereas DPU {log P=2.98} begins to be
`eliminated approximately 20 h after treatment, as
`suggested by the onset of curvature in the permea—
`tion profile in Fig. 2.
`A similar lipid disruption mechanism has been
`proposed for the action of several accelerants in—
`cluding Azonc and olcic acid (Barry, 1987a; Barry.
`1989; Goodman and Barry. 1989) Following a
`reduction in the barrier function of the stratum
`
`corneum, additional PG may enter the membrane,
`thereby further increasing partitioning of the urea
`analogues. DMl does not penetrate the stratum
`corneum well, does not promote partitioning of
`the test agents into the skin, and is thus a less
`effective vehicle for
`the administration of
`the
`
`penetration enhancers.
`In conclusion, Our data indicate that urea and
`the vehicles alone are ineffective in promoting
`permeation of S-FU through human cadaver skin.
`The urea analogues are equally effective from a
`given vehicle, but are more effective when applied
`in PG compared with application from DMI or
`LLP. No correlation was found between the log
`partition coefficients
`(octanol/water) and en-
`hancement ratios of the urea analogues, and the
`intervehicle variations in the enhancement ratios is
`
`composed of an increase in diffusivity of the mem-
`brane and a decrease in partitioning of the drug,
`
`not due to solubility differences. The results sup-
`port the LPP theory for accelerant activity with
`
`0006
`
`

`

`the analogues disrupting the lipid packing in the
`stratum corneum thereby increasing the mem-
`brane diffusivity to S-FU.
`
`Acknowledgements
`
`The authors thank the Science and Engineering
`Research Council
`for a studentship for A.C.W..
`and Dr.
`.1.V. Greenhill
`for assistance with the
`
`organic synthesis.
`
`References
`
`Akhter. 5A.. Bennett. S.L., Waller. LL. and Barry. B.W.. An
`automated diffusion apparatus for studying skin penetra-
`tion.
`lot. J. Pharm, 21 (1984) 17—26.
`Aungst, 13.1., Rogers, NJ. and Shelter. E., Enhancement of
`naloxone penetration through human skin in vitro using
`fatty acids,
`fatty alcohols, surfactants, sulphoxides and
`amides. Int, J. ”term. 33 [1936) 225—234.
`Barry, B.W.. Properties that influence percutaneous absorp—
`tion. In Dermatological Formulations; Permmneous Absorp~
`tion. Dekkcr, New York. 1933, pp. 1271 -233.
`Barry, B.W., Mode of action of penetration enhancers in
`human skin. J. Control. Release, 6 (19873) 85—97.
`Barry, B.W.. Transderrnal drug delivery.
`In Drug Delivery
`Systems; Fundamentals and Techniques. P. Johnson and
`1.6. Lloyd—Jones (Eds). VCU, LIV... 1987b.
`Barry, B.W.. Action of skin penetration enhancers — the lipid
`protein partitioning theory. far. J’. Cosmet. Sci. (1989) in
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`Barry. B.W. and Bennett, S.1.., Effect of penetration enhancers
`on the permeation of mannitol, hydrocortisonc and pro-
`gesterone through human skin. J. Pharm. Pharmaml, 39
`(1937) 5357546.
`Barry, B.W. and Woodford. R.. Vasoconstrictor activities and
`bioavailabilities of seven proprietary corticosteroid creams
`assessed using a non-occluded multiple dosing regimen:
`clinical implications, Br. J. Dermatol, 97 {1917) 555—560.
`Barry. B.W., Sou lhwell, D. and Woodford, R.. Optimisation of
`bioavailability of topical steroids: penetration enhancers
`under occlusion, J. Invest. Dermatol, 82 (1984) 49— 52.
`Bennett. S.L., Barry. KW. and Woodford, R.. Optimisation of
`bioavailability of topical steroids: non—occluded penetra-
`tion enhancers under thermodynamic control.
`.l. lerm.
`Pharmacol. 3'? (1985) 298 304.
`Bond. J.R, and Barry. B.W.. Hairless mouse skin is limited as a
`model for assessing the effects of penetration enhancers in
`human skin. J, liroest‘. Der-moral, 90 (1933) 810—313.
`Cooper. F..R.. Increased skin permeability for lipophilic mole—
`cules. J. Pharm. Sci, 313 (1984) 1153 1156.
`
`49
`
`Erickson. 1.0.. Reactions of long chain amines. Il. Reactions
`With urea. J. Am. Chem. Soc. ‘lo (1954) 3917—3978.
`Goodman. M. and Barry, B.W., Action of penetration en-
`hancers on human skin as assessed by the permeation of
`model drugs S-E'Iuorouracil and estradiol.
`1. Infinite dose
`technique. I. Incest. Dermatol, 91 (1988) 323—3211.
`Goodman. M. and Barry, B.W., Action of penetration en—
`hancers on human stratum corneum as assessed by dif—
`ferential scanning calorimetry, In Permmnmus Absorption.
`R..L. Bronaugh and 11.1. Maibach {Eds}. 2nd edn.. Dekker.
`New York. 1989. in press.
`Grice. K.. Saltar. 1-1. and Baker, H.. Urea and retinoic acid in
`ichthyosis and their effect on transepidermal water loss and
`water holding, capacity of stratum corneum. Acra Der—
`maloeerten, 53 [1973} 114—118.
`the
`Guy. RH. and Hudgraft. 1.. Transdcrmal drug delivery:
`ground rules are emerging. Pharm.
`lm., 6 (1985] 112—116.
`Guy. R.H. and Hadgraft, J.. Physicochemical aspects of pure
`cutaneous penetration and its enhancement. Pl’lfl't‘flf. R81. 5
`(1983} 'l53--'l38,
`Hadgraft, J.. Penetration enhancers in percutaneous absorp—
`tion. Pliorm. .fflL. 5 [1984) 252—254.
`Ilansch, C. and Leo. A“ Substilmert! Constants for Correlation
`Attribute in Chemistry and Biology, Wiley. New York, 1979.
`Ilarrison, S.M., Barry, 11W. and Dugard, P.H.. Effects of
`freezing on human skin permeability. J. Pltamt. Monaural.
`36 (1984} 261—262.
`Kligman, AM. and Christophe-rs. F.. Preparation of isolated
`Sheets of human stratum corneum. Arch. Dermatol. 88
`{1963) 70—73.
`Mollgaart‘l, B. and Hoelgaard. A., Permeation of estradiol
`through skin — effect of vehicles. Int. J. Pharm.. 15 (198321)
`185 197.
`
`Mollgaard. B. and Hoelgaard. A. Vehicle effect on topical
`drug delivery. If. Concurrent skin transport of drugs and
`vehicle
`components. Arm l’lmrm. Street,
`20 (1983b)
`443- 450.
`Mollgaard, 3., lioclgaard. A. and Baker, E,, Vehicle effect on
`topical drug delivery — effect of N—mcthyl-pyrrolidone.
`polar lipids and Aarone rm percutaneons drug transport.
`Proc.
`lm. Symp. Control. Rel. Blond. Mater" 15 (1933)
`209—210.
`Okamoto. H.. Hashjda. M. and Sczaki. H.. Structure—activity
`relationship of lwalkyl— or l—alkenylazacycloalkanone dc—
`rivatives as percutaneous penetration enhancers.
`.l. Pltorm.
`Sci, 77 {1988) 418—424.
`Sheth, N.V., Freeman, D.J.. Hignchi. WJ. and Spruance. S.L..
`The influence of Aaone, propylene glycol and polyethylene
`glycol on in vitro skin penetration of trifluorothymidinc.
`Int. J. Pharm, 28 {1986) 201-309.
`Southwell. D. and Barry. B.W.. Penetration enhancers for
`human skin: mode of action of 2—pyrrolidone and dimethyl—
`formamide on partition and diffusion of model compounds-
`water, n—alcohois and caffeine.
`J'.
`Invest. Dermatol, 80
`.
`(1983) 50'? 514.
`Southwcll. D. and Barry. B.W.. Penetration enhancement in
`human skin; effect of 2--pyrrolidone, dimetl’tylforrnamide
`
`0007
`
`

`

`50
`
`and increased hydration on Finite dose permeation of aspirin
`and caffeine. Int. J'. Phflf'fll. 22 (1984) 291—298.
`Woodford. R. and Barry‘ B.W., Alphaderm cream (1% hydro-
`cortisone plus 10% urea):
`investigation of vasoconstrielor
`atlivil)’. bioavailahilily and application regimens in human
`voiunteers. Curr. Tiler. Rm, 35 (1984) 759—367.
`Wnodfnrd. R. and Barry. B.W.. Penetration enhancers and the
`
`percutaneous absorption of drugs: an update. 1. Tbxiroi.
`Cm. Owl. Toxicoi. S (1986) 165—1115.
`Walton, P.K.. Muiigaard, B_, Hadgral't, J. and Hoelgaard, A,
`Vehicle effect on lopieai drug delivery. II[. Effect of Azone
`on the culaneous permeation of metronidazole and pro-
`pylene glycol. 15:. J. Pharm. 24 (1985) 19—26.
`
`0008
`
`