International Journal af Pharmaceutics, 36 (1989) 43-50
`Elsevier
`
`IJP 01889
`
`Urea analogues in propyleneglycol as penetration enhancers
`in human skin
`
`A.C. Williams and B.W. Barry
`Postgraduate Studies in Pharmaceutical Technology, The School of Pharmacy, University of Bradford, Bradford (U.K.)
`(Received 20 April 1989)
`(Accepted 10 May 1989)
`
`Key words: Percutaneous absorption; Penetration enhancer; Urea; Urea analogue; Propylene glycol;
`5-Fluorouracil
`
`Summary
`
`Urea, 1-dodecylurea, 1,3-didodecylurea and 1,3-diphenylurea were assessed as skin penetration enhancers for the model penetrant
`5-fluorouracil (S-FU). The permeability coefficient (X,,) was determined for 5-FU applied in saturated aqueous solutions to human
`epidermal membranes. Then each urea was applied as a saturated solution in dimethylisosorbide, light liquid paraffin or propylene
`glycol:
`the solutions were removed and A,, was redetermined; the enhancement ratio (A, after enhancer treatment/K, 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 5-FU 6 times by increasing the diffusivity of the stratum corneum. Thus, the role of propylene glycol as
`a synergistic vehicle for 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-
`bility of the stratum corneum 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 con-
`stituents, disrupting the highly ordered structure
`
`Correspondence: B.W. Barry, Postgraduate Studies in Phar-
`maceutical Technology, The School of Pharmacy, University of
`Bradford, Bradford, BD7 1DP, U.K.
`
`(e.g. Southwell and Barry, 1983; Barry et al., 1984:
`Southwell and Barry, 1984; Goodman and Barry,
`1988; Okamotoet al., 1988). Ideally, a penetration
`enhancer is pharmacologically inert, has a specific,
`immediate yet reversible, action and is 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 5-fluorouracil (5-
`FU), chosen as a model penetrant.
`Urea is a mild keratolytic agent used in the
`treatment of ichthyosis and other hyperkeratotic
`skin conditions. As a 10% cream, it increases the
`water-holding capacity of the stratum corneum by
`100%, and haslittle effect on the epidermal water
`barrier (Grice et al., 1973). The moisturizing and
`
`0378-5173,/89,/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)
`
`Noven Pharmaceuticals, Inc.
`EX2007
`Mylan Tech., Inc. v. Noven Pharma., Inc.
`IPR2018-01119
`
`0001
`
`

`

`44
`
`oO
`oO
`li
`il
`HAoN yt By
`hoo
`Chi,
`h
`Urea
`an
`1-Dodecylurea
`f
`i
`KeSe HONEH
`(en, ein:
`© oO
`CHg
`chi,
`1,3-Didodecylurea
`
`1,3-Diphenylurea
`
`ej
`H,C—C—CH,
`|
`|
`OH OH
`Propylene Glycol
`4
`re
`
`H,covy
`
`ss
`
`HY OCH,
`Dimethylisosorbide
`
`Fig.
`
`formulae of the urea analogues and
`1. The structural
`vehicles evaluated as penetration enhancers.
`
`keratolytic effects of urea increase the activity and
`bioavailability of hydrocortisone from Alphaderm
`cream (Barry and Woodford, 1977; Barry, 1983;
`Woodford and Barry, 1984). As a 10% solution in
`propylene glycol
`(PG), urea has no effect on
`naloxoneflux through humancadaverskin (Aungst
`et al., 1986).
`such as
`Established penetration enhancers
`Azone, which contains a Cl2 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).
`Weinvestigated 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 penetration-enhancing activity towards 5-FU
`were: urea, 1-dodecylurea (DDU), 1,3-didodecy-
`lurea (DDDU) and 1,3-diphenylurea (DPU);
`the
`vehicles were dimenthylisosorbide (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 havelittle penetration-enhancing activity (Barry
`et al., 1984; Bennett et al., 1985). It has therefore
`been selected as a standard vehicle for compari-
`
`sons of 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
`cosolvent
`for
`penetration enhancers.
`It has been reported to
`increase the permeation of oestradiol (Mollgaard
`and Hoelgaard, 1983a) and hydrocortisone (Barry
`and Bennett, 1987)
`through excised human ab-
`dominal skin, yet
`is ineffective in promoting the
`topical bioavailability of betamethasone 17-benzo-
`ale as assessed by the occluded vasoconstrictor
`assay (Barry et al., 1984). [t 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 membraneshas no significant effect on 5-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 (e.g. 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 LLP (B.D.H. Chemicals Ltd.) were used as
`supplied. 5-[6-7HJFU (Amersham International
`PLC) was the model permeant, a saturated aque-
`ous solution (10.2 mg/ml at 32 +1°C; Bond and
`Barry, 1988) being prepared with the help of un-
`labelled
`5-FU (Sigma Chemical Company).
`Saturated solutions of urea and the analogues
`were prepared in the 3 vehicles,
`the approximate
`concentrations being evaluated gravimetrically.
`Partition coefficients (octanol/ water) of urea and
`the analogues were calculated by the fragment
`method of Hansch and Leo (1979).
`
`Synthesis
`1-Dodecylurea. 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 106.8—107.5°C
`(Erickson, 1954). The thermal analysis showed the
`product to be approximately 98% pure.
`1,3-Didodecylurea. A mixture of dodecylamine
`(13.0 g, 0.07 mol), urea (2.0 g, 0.03 mol) and
`butan-1-ol (20 ml) was refluxed for 30 h in a fume
`cupboard and cooled over ice and the crystalline
`product wasfiltered 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 103.3-105.5° C (Erickson,
`1954).
`
`Preparation of human epidermal 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 Christophers (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 corneum was fully
`hydrated.
`
`Permeation experiments
`Experiments at 32+1°C used an automated
`diffusion apparatus with 24 stainless-steel diffu-
`sion cells, diffusional area 0.126 cm*, and 0.002%
`aqueous sodium azide receptor solution (Akhteret
`al., 1984). Fully hydrated epidermal membrane
`samples were mounted in the cells and 150 pl
`aliquots of saturated, radiolabelled 5-FU solution
`placed in the donor compartments which were
`covered. 4 ml samples of receptor solution 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 azide solution and replaced with 150 ul
`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 5-FU again monitored for 36 h.
`
`Partitioning experimenis
`The effect of urea analogue/PG formulations
`on the partitioning of 5-FU was investigated. Per-
`meation of 5-FU 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 5-FU in the membrane
`was evaluated by liquid scintillation counting. The
`pseudo-steady-state concentration of 5-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 thetissue.
`
`Results and Discussion
`
`the drug
`Example permeation profiles of
`through the membranebefore 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 (K,,) 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 (E.R.), may be calculated
`
`0003
`
`

`

`*10°
`
`46
`
`CUMULATIVE
`
`cpm/cem
`
`O
`
`10
`
`20
`TIME (h)
`
`30
`
`36
`
`Fig, 2. Example permeation profiles of 5-FU through human
`epidermal membranesbefore and after treatment with saturated
`solutions of the urea analogues in propylene glycol: squares,
`DDU: triangles, DPU: circles, DDDU; diamonds, control.
`
`(Goodman and Barry, 1988):
`
`E.R. = K, of membraneafter application
`of penetration enhancer/
`K,, of membranebefore application
`of penetration enhancer
`
`TABLE 1
`
`The approximate saturated concentrations (mg/ml) of urea and
`the analogues in the 3 vehicles at room temperature (19 + 1°C),
`and the calculated log partition coefficients (octanol / water) jor
`urea and the analogues
`
`Urea
`Vehicle
`log P
`analogue
`LLP
`DMI
`PG
`
`DDU
`0.3
`3.8
`2.6
`4.77
`DDDU
`0.2
`0.6
`0.7
`7
`DPU
`1.0
`1.6
`15
`2,98
`
`1.6 5.0 afUrea —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 mg/ml. The saturated
`concentrations of DDU and DDDU in DMI 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 vehiclesis
`given in Table 2. From these results,
`the mean
`control value for the permeability coefficient of
`5-FU in the untreated membrane at 32°C is 2.16
`
`TABLE 2
`
`Mean permeability coefficients of 5-FU through human cadaver
`skin, with standard error of the mean, before and after treatment
`with urea analogues applied fram 3 vehicles: a, before treatment;
`5, after treatment
`
`
`
`Urea
`Permeability coefficient X 10° (cm//h)
`LLP
`DMI
`———~PG
`analogue
`
`Vehicle
`alone
`
`The values reported were the mean enhance-
`ment ratios from a minimum of 5 replicates.
`The experimental design of determining K,,
`treating the epidermal membrane with penetration
`enhancers, and then redetermining K,,, 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 5-FU (Barry, 1987a; Goodman and Barry,
`3.36 + 1.24
`1.84 +0.98
`2.17+0.30
`a
`1989). This last condition thus provides a stringent
`4,18 + 1.87
`1.73+0.47
`2.60 + 0.63
`b
`test of penetration enhancingactivity. The use of
`1.5640.36
`3.4841.36
`2.38+0.74
`a
`saturated solutions of urea and its analogues al-
`1.71 +0.46
`3.50 + 1.44
`1.57+0.33
`b
`lows a direct comparison of the penetration-en-
`0.9640.25
`0.9340.27
`3.744107
`a
`hancing abilities of each agent
`from different
`4.1741.15
`2.03+0.96
`-2.72 + 0.93
`b=
`1.24+0.18
`0.584010
`3.284+0.64
`a
`DDDU
`vehicles as the chemical potential of the penetra-
`
`b=2.5740.70 1.25+0.36 3.49 +0.77
`
`tion enhancer is constant (maximal) in all the test
`DPU
`a
`5.30 + 1.39
`0.79 40.22
`0.77 +0.05
`solutions. The approximate saturated concentra-
`55.68 + 3.01 0.79=0.07b=. 2.674+0.33
`
`
`
`tions of the test agents in the different vehicles,
`
`Urea
`
`DDU
`
`
`
`0004
`
`

`

` > 6
`
`44
`
`RATIO
`
`ENHANCEMENT
`
`o
`DIMETHYL-
`ISOSORBIDE
`
`6oa
`3
`oPu
`PROPYLENE
`GLYCOL
`
`LIQUID
`PARAFFIN
`
`Fig. 3. The mean enhancementratios of urea and the analogues
`from the 3 vehicles, with $.E.M.; u, urea; V, vehicle alone.
`
`+ 0.35 * 10 °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 5-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 corneum has been proposed (Barry,
`1987a; Goodman and Barry, 1989). Based on this
`theory, penetration enhancers may act mainly by
`
`47
`
`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
`penetration enhancers such as Azone and oleic
`acid is well documented (Cooper, 1984; Sheth et
`al., 1986; Barry, 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 and Hoelgaard, 1983b).
`With urea analogue PG mixtures, the glycol per-
`meating into the skin will enhance partitioning of
`the lipophilic 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 5-FU permeation after treatment with
`each urea analogue in PG. The lag timeis related
`to the diffusivity (D) of the drug in the membrane
`by:
`
`hh?
`L=<¢D
`
`where # is the membrane thickness (taken as
`approximately 3 X 107? em for human abdominal
`stratum corneum). Thus the diffusivity of 5-FU in
`the membrane after treatment with DDU in PG
`(mean log time 1.03 h,
`1» =4) may becalculated
`approximately:
`
`h*
`D = = = 1.46 107%
`
`—6
`
`z
`em*/h
`
`0005
`
`

`

`48
`
`Comparing this with the diffusivity of the mem-
`brane prior to treatment (mean lag time 9.65 h,
`n=17) of 1.55107’ cm’/h, shows a 9.4-fold
`increase in diffusivity after
`treatment with the
`urea analogue. It
`is widely accepted that many
`molecules traversing the stratum corneum do so
`by a tortuousintercellular pathway, and the diffu-
`sional pathlength for a molecule has recently been
`speculated to be approximately 350 wm (Guy and
`Hadgraft, 1988). Thus, the value of A 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 and after enhancer treatment,
`as the pathlength is assumed to be constant in
`both cases. This may not be the true situation as
`PG mayalter the epidermal membranethickness.
`Thus the diffusivity values, and their ratios, are
`approximate, but are useful guides to molecular
`events within the tissue.
`The 9-fold increase in diffusivity correlates with
`a mechanism of action whereby the penetration
`enhancer disrupts the lipid structure of the stra-
`tum corneum. Thepartition coefficient (P) of the
`drug from its vehicle (aqueous solution) into the
`stratum corneum is related to the membranediffu-
`
`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 5-FU in the membrane fell by a
`factor of 0.71+0.09 (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
`composed of an increase in diffusivity of the mem-
`brane and a decrease in partitioning of the drug,
`
`which give a combinedeffect: 9.42 x 0.68 = 6.40
`(= E.R.).
`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 corneum, 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 Azone and oleic 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. DMI 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 5-FU through human cadaverskin.
`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 ratiosis
`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 5-FU.
`
`Acknowledgements
`
`The authors thank the Science and Engineering
`Research Council
`for a studentship for A.C.W..,
`and Dr. J.V. Greenhill
`for assistance with the
`organic synthesis.
`
`References
`
`Akhter, S.A., Bennett, 5.L., Waller, I.L. and Barry, B.W., An
`automated diffusion apparatus for studying skin penetra-
`tion. Int. J. Pharm., 21 (1984) 17-26.
`Aungst, B.J., Rogers, N.J. and Shefter, E., Enhancement of
`naloxone penetration through human skin in vitro using
`fatty acids,
`fatty alcohols, surfactants, sulphoxides and
`amides, Jnr. J. Pharm., 33 (1986) 225-234.
`Barry, B.W., Properties that influence percutaneous absorp-
`tion. In Dermatological Formulations; Percutaneous Absorp-
`tion, Dekker, New York, 1983, pp. 127-233.
`Barry, B.W., Mede of action of penetration enhancers in
`human skin. J. Control. Release, 6 (1987a) 85-97.
`Barry, B.W., Transdermal drug delivery.
`In Drug Delivery
`Systems; Fundamentals and Techniques. P. Johnson and
`J.G. Lloyd-Jones (Eds.), VCU, U.K., 1987b.
`Barry, B.W., Action of skin penetration enhancers — the lipid
`protein partitioning theory. Int. J. Cosmet. Sci. (1989) in
`press.
`Barry, B.W. and Bennett, S.L., Effect of penetration enhancers
`on the permeation of mannitol, hydrocortisone and pro-
`gesterone through human skin. J. Pharm. Pharmacol., 39
`(1987) 535-546.
`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. Dermarol., 97 (1977) 555-560,
`Barry, B.W., Southwell, 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, B.W. and Woodford, R., Optimisation of
`bioavailability of topical steroids: non-occluded penctra-
`tion enhancers under thermodynamic control. J. Pharm.
`Pharmacol., 37 (1985) 298-304.
`Bond, J.R. and Barry, B.W.,. Hairless mouse skinis limited as a
`model for assessing the effects of penetration enhancers in
`human skin. J. Invest. Dermatol., 90 (1988) 810-813.
`Cooper, E.R., Increased skin permeability for lipophilic mole-
`cules. J. Pharm. Sci., 73 (1984) 1153-1156.
`
`49
`
`Erickson, J.G., Reactions of long chain amines. II. Reactions
`with urea. J. dm. Chem. Soe., 76 (1954) 3977-3978.
`Goodman, M. and Barry, B.W., Action of penetration en-
`hancers on human skin as assessed by the permeation of
`model drugs 5-fluorouracil and estradiol.
`I. Infinite dose
`technique. J. Invest. Dermatol, 91 (1988) 323-327.
`Goodman, M. and Barry, B.W., Action of penetration en-
`hancers on human stratum corneum as assessed by dif-
`ferential scanning calorimetry. In Percutaneous Absorption,
`R.L. Bronaugh and H.1. Maibach (Eds.), 2nd edn., Dekker,
`New York, 1989, in press.
`Grice, K., Sattar, H. and Baker, H.. Urea and retinoic acid in
`ichthyosis and their effect on transepidermal water loss and
`water holding capacity of stratum corneum. Acta Der-
`matovener., 53 (1973) 114-118.
`the
`Guy, R.H. and Hadgraft, J., Transdermal drug delivery:
`ground rules are emerging. Pharm. Inr., 6 (1985) 112-116.
`Guy, R.H. and Hadgraft, J.. Physicochemical aspects of per-
`cutaneous penetration and its enhancement. Pharm. Res..5
`(1988) 753-758.
`Hadegraft, J., Penetration enhancers in percutaneous absorp-
`tion. Pharm. Int., 5 (1984) 252-254,
`Hansch, C. and Leo, A., Substitutent Constants for Correlation
`Analysis in Chemistry and Biology, Wiley, New York, 1979.
`Harrison, $.M., Barry, B.W. and Dugard, P.H., Effects of
`freezing on human skin permeability. J. Pharm. Pharmacol...
`36 (1984) 261-262.
`Kligman, A.M. and Christophers, E. Preparation of isolated
`sheets of human stratum corneum. Arch. Dermatol., 88
`(1963) 70-73.
`Mollgaard, B. and Hoelgaard, A., Permeation of estradiol
`through skin - effect of vehicles. nt. J. Pharm., 15 (1983a)
`185-197.
`
`Mollgaard, B. and Hoelgaard, A., Vehicle effect on topical
`drug delivery. Il. Concurrent skin transport of drugs and
`vehicle
`components. Acta Pharm. Suec., 20 (1983b)
`443-450.
`Mollgaard, B., Hoelgaard, A. and Baker, E., Vehicle effect on
`topical drug delivery -— effect of N-methyl-pyrrolidone,
`polar lipids and Azone on percutaneous drug transport.
`Proc.
`Int. Symp. Control. Rel. Bioact. Mater., 15 (1988)
`209-210.
`Okamoto, H., Hashida, M. and Sezaki, H., Structure—activity
`relationship of 1-alkyl- or 1-alkenylazacycloalkanone de-
`Tivatives as percutaneous penetration enhancers. J. Pharm.
`Sei., TT (1988) 418-424,
`Sheth, N.V., Freeman, D.J., Higuchi, W.I. and Spruance, S.L..
`The influence of Azone, propylene glycol and polyethylene
`glycol on in vitro skin penetration of trifluorothymidine.
`Int. J, Pharm., 28 (1986) 201-209,
`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-alcohols and caffeine. J.
`Invest. Dermatol, 80
`(1983) 507-514.
`.
`Southwell, D. and Barry, B.W., Penetration enhancement in
`human skin; effect of 2-pyrrolidone, dimethylformamide
`
`0007
`
`

`

`50
`
`and increased hydrationonfinite dose permeation of aspirin
`and caffeine. Jnt. J. Pharm., 22 (1984) 291-298,
`Woodford, R. and Barry, B.W., Alphaderm cream (1% hydro-
`cortisone plus 10% urea):
`investigation of vasoconstrictor
`activity, bioavailability and application regimens in human
`volunteers. Curr. Ther. Res., 35 (1984) 759--767.
`Woodford, R. and Barry, B.W., Penetration enhancers and the
`
`percutaneous absorption of drugs: an update. J. Toxicol.
`Cut. Ocul. Toxicol, 5 (1986) 165-175.
`Wotton, P.K., Mollgaard, B., Hadgraft, J. and Hoelgaard, A.,
`Vehicle effect on topical drug delivery. IIL Effect of Azone
`on the cutaneous permeation of metronidazole and pro-
`pylene glycol. Jnr. J. Pharm., 24 (1985) 19-26.
`
`0008
`
`