deposits from creams containing 20% (w/w) urea and

1084 Vol 64 No 8

copy 2016 The Pharmaceutical Society of Japan

Chem Pharm Bull 64 1084ndash1091 (2016)

Regular Article

Deposits from Creams Containing 20 (ww) Urea and Suppression of Crystallization (Part 1) Rate of Crystallization from Cream Containing 20 (ww) Urea and Evaluation of the Properties of the Deposit

Norio Gotoadagger Yutaka Moritab and Katsuhide TeradacDagger

a Eisai Co Ltd Honjo Research Section Drug Development Technology Center Customer Joy Department Eisai Japan 2ndash3ndash14 Minami Honjo Saitama 367ndash0048 Japan b Research amp Development Div Elmed Eisai Co Ltd 2ndash3ndash14 Minami Honjo Saitama 367ndash0048 Japan and c Faculty of Pharmaceutical Sciences Toho University 2ndash2ndash1 Miyama Funabashi Chiba 274ndash8510 JapanReceived October 12 2015 accepted April 28 2016

Two creams containing 20 (ww) urea and various emulsifiers a nonionic surfactant (NS) and lecithin (LEC) were prepared and the rate of crystallization following application of the cream and differences in the properties of the deposits were investigated Post-application crystallization was slower with the LEC for-mulation Differences in the crystals obtained from the two formulations and from a 20 aqueous solution of urea were evaluated by powder X-ray diffraction (PXRD) differential scanning calorimetry (DSC) powder X-rayndashDSC (PXRDndashDSC) and Fourier transform infrared spectrophotometry (FT-IR) PXRD and PXRDndashDSC measurements showed that the diffraction patterns of both formulations differed from that of urea TheNS formulation provided diffraction peaks for urea and a urea composite whereas only the urea compositewas evident in the LEC formulation DSC scans of urea showed an endotherm at around 134degC whereas thedeposits from both formulations provided an endotherm 23ndash25degC below that of urea the NS formulation alsoshowed a peak at around 140degC These results indicate a tendency for urea crystallization in the NS formu-lation FT-IR measurements showed that both deposits have a urea-based structure The effects of the LECformulation components on the physical properties of urea were investigated by PXRD and showed that alldiffraction peaks were evenly weakened suggesting that urea tends to be amorphous and that the formula-tion impacts post-application urea crystallization Consequently the amorphous state of urea can be main-tained post-application by optimizing the formulation thereby increasing the clinical efficacy of the cream

Key words urea cream crystallization urea composite amorphous powder X-ray-differential scanning calorimetry (PXRDndashDSC)

Urea helps retain moisture in the stratum corneum and is widely used in external preparations to treat dry skin disor-ders such as keratosis and senile xerosis The softening effect on the stratum corneum generally increases with the urea content of the cream12) Urea is more readily transferred to the stratum corneum in the dissolved (amorphous) state3) and exhibitsthinsphigherthinsp clinicalthinsp efficacythinspwhenthinspcomparedthinsp tothinspcrystallinethinspurea consequently preparations are formulated to suppress crystallization4) and thus maintain easy absorption5) External preparations that crystallize post-application exhibit drasti-cally decreased transfer of urea to the skin6) and may feel taut or gritty and irritate the skin4) Winter conditions (low temperature and humidity) tend to aggravate skin conditions and thus preparations must be compatible with low humidity conditionsthinsp Itthinsp isthinsp difficultthinsp tothinsp addthinsp sufficientthinsp waterthinsp tothinsp maintainthinspthe solubility of the urea in preparations containing ge20 urea yet external preparations must be formulated to ensure that urea does not crystallize post-application

Urea forms inclusion compounds through intermolecular interactions with other molecules and atoms7) and the size and molecular structure of the guest compound greatly affects the

formation of urea composites8ndash10)thinsp Paraffinsthinsp withthinsp sixthinsp orthinsp morethinsphydrocarbons11ndash16) and their derivatives (for example alcohols and carboxylic acids) such as stearic acid17) palmitic acid18) and cetyl alcohol19) as well as fatty acid-based surfactants are commonly used guest molecules in numerous urea exter-nal preparations20ndash22) Therefore urea inclusion compounds are likely formed when urea crystallizes from an external preparation4) suggesting that crystallization is affected by the formulation

Here we compared the post-application deposition states of two 20 (ww) urea preparations containing different emulsi-fiersthinsp (LECthinsp formulationthinsp lecithinthinsp formulationthinspNSthinsp formulationthinspnonionic surfactant formulation)Inthinsp thethinsp LECthinsp formulationthinsp thethinsp emulsifierthinsp consistedthinsp primarilythinsp

ofthinsp thethinsp naturalthinsp emulsifierthinsp lecithinthinsp togetherthinspwiththinsp glycerinthinsp fattythinspacid esters as a nonionic surfactant The main component of the lecithin was phosphatidylcholine a phospholipid contain-ingthinsp twothinsp fattythinsp acidsthinsp Inthinsp thethinsp NSthinsp formulationthinsp thethinsp emulsifierthinspconsisted primarily of the nonionic surfactant polyoxyethylene (50) hydrogenated castor oil (HCO 50) together with othersorbitan fatty acid esters

The deposits were evaluated by powder X-ray diffraction (PXRD) differential scanning calorimetry (DSC) powder X-rayndashDSC (PXRDndashDSC) and Fourier transform infraredspectrophotometry (FT-IR) The post-application depositionstate and the rate of crystallization were found to dependon the formulation thus providing insights into the effect of

thinspTothinspwhomthinspcorrespondencethinspshouldthinspbethinspaddressedthinsp e-mailthinspn-gotohhceisaicojp

daggerthinspthinspPresentthinsp addressthinspEisai Co Ltd Customer Joy Department Eisai Japan 13ndash1 Nishigoken-cho Shinjuku-ku Tokyo 162ndash0812 Japan

DaggerthinspthinspPresentthinspaddressthinspFaculty of Pharmaceutical Sciences Takasaki University of Health and Welfare 60 Nakaorui-machi Takasaki Gunma 370ndash0033 Japan

Vol 64 No 8 (2016) 1085Chem Pharm Bull

cream components on suppressing post-application crystalliza-tion

ExperimentalSamplesModelthinsp formulationsthinsp (OWthinsp (Oilthinsp inthinsp Water)thinsp typethinsp cream)thinsp① NS formulation ②thinspLECthinspformulationTwenty percent urea aqueous solutionModel FormulationsThethinsp compositionsthinsp ofthinsp thethinsp NSthinsp andthinsp LECthinsp formulationsthinsp arethinsp

shown in Table 1Preparation MethodsPreparation of Model FormulationsFormulations were prepared as shown in Fig 1 The aque-

ous phase consisted of urea which is the active pharmaceuti-calthinsp ingredientthinsp glycerinthinsp andthinsp purifiedthinsp waterthinsp andthinsp thethinsp oilthinsp phasethinspconsistedthinspofthinspnon-polarthinspandthinsppolarthinspoilthinspemulsifier23) and cetostea-ryl alcohol The phases were separately dissolved by heating to 80degC with stirring and then the aqueous phase was added to the oil phase and mixed The mixture was stirred using athinsp homomixerthinsp (HM)thinsp vacuumthinsp emulsifierthinsp (PVQ-5UNthinsp MizuhothinspIndustrialthinsp Cothinsp Ltdthinsp Osakathinsp Japan)thinsp withthinsp athinsp rotationalthinsp speedthinspof 3500 rpm and an anchor rotational speed of 70 rpm under vacuum (40 cmHg) Cooling was initiated under stirring HM

stirring was stopped at 40degC and the model formulation was obtained by cooling to 35degC

Preparation of 20 Urea Aqueous SolutionTothinsp 80thinspgthinsp ofthinsp purifiedthinspwaterthinsp 20thinspgthinsp ofthinsp ureathinsp wasthinsp addedthinsp andthinsp dis-

solved at room temperature by stirring with a stirrer for 15 min

Polarizing Microscope Observation by Simple Glass-Surface Coating5) Crystallization on a glass surface was observed with a microscope and crystal growth and growth rate were investigated for the two formulations and the aque-ous urea sample

About 20 mg of sample was placed on a 60times26 mm glass slide The sample was thinly and evenly spread with an oint-ment spatula at an application rate of 32 mgcm2 maintained at 40ndash50 relative humidity 20ndash25degC and crystallization over a 2 h period was clearly observed using a polarizing microscope (Polarizing Microscope Model BH Model PM-10AK Olympus Corporation Tokyo Japan 100timesobjective)thinspPhotos were taken at 15 60 and 120 min

PXRD Measurements The two formulations and 20 urea aqueous solution were applied to glass plates as in Sec-tion ldquoPolarizing Microscope Observation by Simple Glass-Surface Coatingrdquo and maintained at 20ndash25degC 40ndash50 relative humidity for 6 h Samples showing deposits were compared using an X-ray diffractometer (MiniFlex600 Rigaku Corpora-tion Tokyo Japan)

Measurement conditions are shown in Table 2DSC of Deposited Crystals The test samples were pre-

pared as in Section ldquoPolarizing Microscope Observation by Simple Glass-Surface Coatingrdquo and maintained at 20ndash25degC 40ndash50 relative humidity for 6 h Samples showing deposits

Table 1 Model Formulations

Component SuppliersAmount

NS formulation LECthinspformulation

Urea JunseithinspChemicalthinspCothinspLtdthinspTokyothinspJapan 200 200Oily substances Various sources 150 150Cetostearyl alcohol (CSA) KokyuthinspAlcoholthinspKogyothinspCothinspLtdthinspChibathinspJapan 40 40Polyoxyethylene (50) hydrogenated castor oil (HCO 50) NikkothinspChemicalsthinspCothinspLtdthinspTokyothinspJapan 10 00Polyoxyethylene sorbitan monostearate NikkothinspChemicalsthinspCothinspLtdthinspTokyothinspJapan 10 00Sorbitan monostearate NikkothinspChemicalsthinspCothinspLtdthinspTokyothinspJapan 04 00Purifiedthinspsoybeanthinsplecithinthinsp(Lecithin) TsujithinspOilmillsthinspCothinspLtdthinspMiethinspJapan 00 08Propylene glycol monostearate NihonthinspEmulusionthinspCothinspLtdthinspTokyothinspJapan 00 08Glyceryl monostearate NikkothinspChemicalsthinspCothinspLtdthinspTokyothinspJapan 00 08Glycerin NOF Corporation Tokyo Japan 100 100Purifiedthinspwater YoshidathinspPharmaceuticalthinspCothinspLtdthinspTokyothinspJapan 486 486

Total 1000 1000

Fig 1 Preparation Method

Table 2 PXRD Measurement Conditions

Parameter Description

X-Ray source CuKαScan mode ContinuousX-Ray tube voltagendashcurrent 40 kVndash15 mAScan axis 2θθScan range (2θ) 20ndash500degScan speed 100degminSamping interval 002degstep

1086 Vol 64 No 8 (2016)Chem Pharm Bull

were compared by DSC using thermal analysis equipment (Thermo Plus DSC8230 Rigaku Corporation)

Measurements were conducted under a stream of nitrogen (40thinspmLmin)thinsp usingthinsp thethinsp followingthinsp conditionsthinsp startingthinsp tempera-ture 20degC ending temperature 150degC rate of temperature increase 5degCmin container closed aluminum (Al) pan

PXRDndashDSC Measurements Test samples were prepared as in Section ldquoPolarizing Microscope Observation by Simple Glass-Surface Coatingrdquo and maintained at 20ndash25degC 40ndash50 relative humidity for 6 h Samples showing deposits were compared by PXRDndashDSC using an automated horizontal X-raythinspdiffractometerthinsp(SmartLabPXRDndashDSCthinspManagethinspRigakuthinspCorporation)

Measurement conditions are shown in Table 3Effect of LEC Formulation Components on Urea Crys-

tallizationthinsp thinspBasedthinsp onthinsp thethinsp LECthinsp formulationthinsp 1thinspgthinsp ofthinsp eachthinspformulation component was blended with urea in order (No 1rarr2rarr3rarr4) and four samples were prepared (Table 4) Each

sample was placed in a crucible mixed melted by heating to 145degC and was allowed to cool naturally Each sample was lightly pulverized in a mortar and evaluated by PXRD as described in Section ldquoPXRD Measurementsrdquo using the condi-tions shown in Table 2

Identification of Deposits by FT-IR Absorption Mea-surementsthinsp thinspNSthinsp andthinsp LECthinsp formulationthinsp testthinsp samplesthinspwerethinsp pre-pared as in Section ldquoPolarizing Microscope Observation by Simple Glass-Surface Coatingrdquo and maintained at 20ndash25degC 40ndash50 relative humidity for 6 h Several milligrams of each test sample was collected and pulverized with an agate mortar and pestle KBr was added and the sample was further mixed and pulverized for about 5 min until homogeneous Disk-shaped samples were formed with a pellet press then FT-IR spectra were measured using a Fourier transform infrared spectrophotometer (Model FT-210 HORIBA Kyoto Japan) and compared with that of ureaMeasurementthinsp conditionsthinspwerethinspmethodthinspKBrthinspwavethinsp numberthinsp

range 400ndash4000 cmminus1 scan number 20 resolution 4 cmminus1

Measurement ResultsPolarizing Microscope Observation of Glass Slides

Coated with Sample The results of polarizing microscope observations are provided in Fig 2 They show that the NS andthinsp LECthinsp formulationsthinsp differthinsp inthinsp theirthinsp crystalthinsp growththinsp patternsthinsp(crystalthinsp habits)thinsp Thethinsp LECthinsp formulationthinsp formedthinsp smallthinsp plate-

Table 3 PXRDndashDSC Measurement Conditions


X-Ray Source CuKαScan MODE ContinuousX-Ray tube voltagendashcurrent 45 kVndash200 mAScan axis 2θθScan range (2θ) 30ndash400degScan speed 80deg minSamping interval 002deg stepStart temperature Room temperature (rt)End temperature 150degCTemperature rising speed 50degCminN2thinspflowthinsprate 50thinspmLminSample vessel Open pan (Al)Detector 1D High-speed detector (DteX Ultra250)

Tablethinsp 4thinsp CombinationsthinspofthinspUreathinspandthinspLECthinspFormulationthinspComponentsthinsp(Mix-ing Ratios Are Shown in Parentheses)

No Sample (mix rate)

1 Urea2 Urea+Cetostearylthinspalcoholthinsp(1thinspthinsp1)3 Urea+Cetostearyl alcohol+Lecithinthinsp(1thinspthinsp1thinspthinsp1)4 Urea+Cetostearyl alcohol+Lecithin+Glycerinthinsp(1thinspthinsp1thinspthinsp1thinspthinsp1)

Figthinsp 2thinsp ObservationthinspofthinspCrystallizationthinspfromthinspthethinspModelthinspFormulationsthinsp(MeasurementthinspEnvironmentthinsp40ndash50thinspRelativethinspHumiditythinsp20ndash25degC)


shaped crystals whereas the NS formulation and the 20 urea aqueous solution both formed needle-shaped crystals with similar crystal habits

Needle crystals were observed in the NS formulation after 15 min crystals covered the entire glass slide after 60 then remained unchanged up to 120 min

In contrast a small number of plate crystals were present in thethinsp LECthinsp formulationthinsp afterthinsp 15thinspminthinsp thethinsp intensitythinsp ofthinsp thethinsp polar-ized section remained weak at 60 and 120 min compared with the NS formulation showing that crystal growth was slow ThethinspsmallthinsplightthinspspotsthinspobservedthinspearlythinspinthinspthethinspprocessthinspinthinspthethinspLECthinspformulation are liquid crystal emulsion particlesThesethinsp resultsthinsp confirmedthinsp thatthinsp thethinspquantitythinspofthinspcrystalthinspdeposi-

tionthinsp overthinsp timethinspwasthinsp smallestthinsp forthinsp thethinspLECthinsp formulationthinsp andthinsp canthinspbethinsp seenthinsp onthinsp visualthinsp inspectionthinsp tothinsp increasethinsp inthinsp thethinsp orderthinsp ofthinsp LECthinspformulationltNS formulationlturea aqueous solution and that the rate of crystal deposition varied with the ingredientsInthinsp thethinsp LECthinsp formulationthinsp thethinsp smallthinsp pointsthinsp ofthinsp lightthinsp observedthinsp

in the initial stages are attributable to liquid-crystal regions thatthinsp formedthinsp inthinsp thethinsp emulsifiedthinsp particlesthinsp onthinsp thethinsp lossthinsp ofthinsp waterthinspcontent immediately following application rather than to changes in the base accompanied by polarization

The 20 urea aqueous solution provided results similar to that of the NS formulation after 15 min but the crystals were larger

PXRD Results PXRD measurements on deposits from the three sample types are shown in Fig 3 A sharp X-ray dif-fraction peak for urea was observed at 2θ=222deg

CrystalsthinspobtainedthinspfromthinspthethinspNSthinspandthinspLECthinspformulationsthinsphadthinspathinspmain diffraction peak () at 2θ=215deg which is slightly lower than that for urea and thus the diffraction patterns differed from that of urea A difference was also observed between thethinsp NSthinsp formulationthinsp andthinsp thethinsp LECthinsp formulationthinsp Thethinsp depositthinspfrom the NS formulation showed a diffraction peak () at 2θ=222deg the same as that for urea in addition to the diffrac-tion peak at 2θ=215deg whereas the 2θ=222deg peak was not observedthinspforthinspthethinspdepositthinspfromthinspthethinspLECthinspformulation

DSC Results for Deposits DSC results for the three types of sample are shown in Fig 4 The deposit from the NS for-

Fig 3 PXRD Pattern of Samples (a b c)(a)thinspUreathinspaqueousthinspsolutionthinsp(b)thinspLECthinspformulationthinsp(c)thinspNSthinspformulation

Figthinsp 4thinsp DSCthinspCurvesthinspofthinspLECthinspFormulasthinspNSthinspFormulasthinspandthinspUreathinspAqueousthinspSolution (Close Pan Heating Rate 50degCmin)


mulation provided endothermic peaks at 1108 and 1392degC whereasthinsp thesethinsp peaksthinsp werethinsp absentthinsp fromthinsp thethinsp scanthinsp ofthinsp thethinsp LECthinspformulation deposit but an endothermic peak at 1123degC was obtained These melting temperatures are approximately 250degCthinsp lowerthinsp (NSthinsp formulation)thinsp andthinsp 235degCthinsp lowerthinsp (LECthinsp for-mulation) than that of urea (1358degC) The results indicate that the deposit from the NS formulation contains both a urea compositethinsp andthinsp ureathinsp inthinsp contrastthinsp tothinsp thethinsp depositthinsp fromthinsp thethinsp LECthinspformulation

PXRDndashDSC Results PXRDndashDSC results and the X-ray diffraction patterns before and after the main endothermic peak temperature of the three sample types are shown in

Figsthinsp 5ndash8thinsp (ureathinsp Figthinsp 5thinspNSthinsp formulationthinsp Figsthinsp 6thinsp 7thinsp andthinspLEC-formulationthinsp Figthinsp 8)thinsp Anthinsp endothermicthinsp peakthinsp correspondingthinsp tothinspmelting was observed at about 135degC for urea the diffraction peak arising from the crystals (2θ=222deg) disappeared and a halo due to amorphous urea appeared (Fig 5)ThethinspDSCthinspprofilethinspofthinsp thethinspNSthinsp formulationthinspdepositthinsp showedthinsp en-

dothermic peaks at about 110degC and 140degC but no diffraction peak was observed by PXRDndashDSC (Fig 6) a diffraction peak (uarrthinsp 2θ=222deg) due to urea was observed at room tempera-ture but disappeared above 75degC The main diffraction peak (2θ=215deg) disappeared upon melting of the sample at about 110degC and a halo arising from amorphous urea appeared

Fig 5 PXRDndashDSC Curves for Urea (Open Pan Heating Rate 50degCmin)

Fig 6 PXRDndashDSC Curves of NS Formulation (Open Pan Heating Rate 50degCmin)


Inthinsp contrastthinsp thethinsp depositthinsp fromthinsp thethinsp LECthinsp formulationthinsp showedthinspthe endothermic peak due to melting at about 110degC and no endothermic change at about 140degC consistent with the DSC profilethinsp (Figthinsp 7)thinsp Thethinsp diffractionthinsp peakthinsp (2θ=215deg) disappeared during melting at about 110degC and a halo due to amorphous urea appeared (Fig 8)

Effect of Formulation Components on Urea Crystals in LEC Formulation The effect of formulation components was investigated by PXRD to determine why urea crystallizes morethinspslowlythinsp inthinsp thethinspLECthinspformulationthinspwhenthinspcomparedthinspwiththinsp thethinspNS formulation

Figure 9 shows that the diffraction peak due to urea de-creasedthinsp significantlythinsp followingthinsp thethinsp additionthinsp ofthinsp cetostearylthinspalcohol (CSA) and a new diffraction peak likely due to a urea composite61719) was observed at 2θ=215deg The addition of lecithin suppressed both peaks and no new diffraction peak was generated The addition of glycerin essentially eliminated the peak due to urea and a halo became dominant showing that the urea was amorphous

Thus post-application crystallization can be suppressed by judiciousthinspchoicethinspofthinspformulationthinspcomponents

Identification of the Deposits by IR Spectroscopy The IR spectrum of urea (deposits from urea aqueous solution) in Fig 10 (a) shows two peaks (3444 cmminus1 and 3347 cmminus1) due to NndashHthinspstretchingthinspvibrationsthinsp(rangethinsp3300ndash3500thinspcmminus1) and a peak due to CndashO stretching vibrations in the range 1000ndash1200 cmminus1

The spectrum of the NS formulation deposit was similar to that of urea and showed two peaks due to NndashH stretching vibrations in the range 3300ndash3500 cmminus1 but the peaks were broadened suggesting that the environment of the urea NndashH groups was different when compared to that of urea in aque-ous solution The CndashO stretching vibration at 1000ndash1200 cmminus1 was also broadened indicating that the C=O double bond had transformed to an ether (CndashOndashC) and that a urea composite had formedThethinsp spectrumthinsp ofthinsp thethinsp LECthinsp formulationthinsp depositthinsp wasthinsp dif-

ferent from that of urea and very different from that of the NS formulation The two peaks due to urea in the range 3300ndash3500 cmminus1 broadened into one peak and the physical environment of the NndashH groups was apparently different from that in urea and the NS formulation In addition the peak due to the CndashO stretching vibrations of urea at 1153 cmminus1 (∆) weakened and a lower frequency peak appeared at 1043 cmminus1 () again indicating the transformation of the C=O to CndashOndashC and the formation of a urea composite

The observed differences in the IR spectra of the NS and LECthinsp formulationsthinsp indicatedthinsp thatthinsp thethinsp ureathinsp inthinsp thethinsp formulationsthinspexisted in a state that was distinct from that of urea aloneThethinspdepositthinsp fromthinsp thethinspLECthinspformulationthinsp showedthinsponethinsppeakthinsp inthinsp

the range 3300ndash3500 cmminus1 whereas the NS formulation and urea showed two peaks suggesting a structural difference be-tweenthinspthethinspLECthinspformulationthinspandthinspthethinspotherthinsptwothinspsamples

Fig 7 Changes in the NS Formulation PXRD Pattern with Increasing Temperature (Open Pan Heating Rate 50degCmin)

Figthinsp 8thinsp PXRDndashDSCthinspCurvesthinspofthinspLECthinspFormulationthinsp(OpenthinspPanthinspHeatingthinspRatethinsp50degCminthinsp)


Discussion and ConclusionThisthinsp studythinsp (1)thinsp confirmedthinsp thethinsp slowthinsp ratethinsp ofthinsp ureathinsp crystalliza-

tionthinsp inthinsp anthinsp LECthinsp formulationthinsp whenthinsp comparedthinsp withthinsp anthinsp NSthinsp for-mulation and urea aqueous solution under winter-like low hu-miditythinspconditionsthinsp(humiditythinsp40ndash50thinsptemperaturethinsp20ndash25degC)

It also showed that the crystal habits in their deposition differedthinsp withthinsp thethinsp LECthinsp formulationthinsp formingthinsp smallthinsp plate-likethinspcrystals and both the NS formulation and the urea aqueous so-lution forming needle-like crystals The deposits also differed in their properties

Polarizing microscope observation of glass slides individu-ally coated with the three samples showed numerous needle crystals in the NS formulation and the urea aqueous solution depositsthinsp afterthinsp 15thinspminthinsp whereasthinsp thethinsp LECthinsp formulationthinsp providedthinspfar fewer but plate-like crystals Even after 120 min the in-tensitythinspofthinspthethinsppolarizedthinspsectionthinspofthinspthethinspLECthinspformulationthinspdepositthinspwas weak compared with the NS formulation and crystal

Fig 9 Changes in the Urea PXRD Patterns with Sequential Addition ofthinspLECthinspFormulationthinspComponentsNothinsp 1thinsp Ureathinsp Nothinsp 2thinsp Urea+Cetostearylthinsp alcoholthinsp (1thinspthinsp1)thinsp Nothinsp 3thinsp Urea+Cetostearyl

alcohol+Lecithinthinsp (1thinspthinsp1thinspthinsp1)thinsp Nothinsp 4thinsp Urea+Cetostearyl alcohol+Lecithin+Glycerin (1thinspthinsp1thinspthinsp1thinspthinsp1)

Figthinsp 10thinsp InfraredthinspAbsorptionthinspSpectrathinspofthinspUreathinspAqueousthinspSolutionthinspandthinspthethinspNSthinspandthinspLECthinspFormulationthinspDeposits(a)thinspUreathinspaqueousthinspsolutionthinsp(b)thinspNSthinspformulationthinsp(c)thinspLECthinspformulation


growth remained slow These differences (crystal growth rate crystal habit and intensity of the polarized section) are likely duethinsp tothinsp thethinspcombinationthinspofthinsp emulsifierthinspandthinspureathinsp inthinsp thethinsp formula-tion4) Slow urea crystal growth helps maintain a stable crystal interface and the formation of a regular hexahedron with ideal equivalent faces whereas fast urea crystal growth results in an unstable crystal interface and dendritic growth49)

From these results it was inferred that the ingredients par-ticularlythinsp thethinspemulsifierthinsp inthinsp thethinspLECthinspformulationthinspwhichthinspformedthinspsmall plate-like crystals tended to decrease the deposition rate induce a characteristic crystal habit and lower the de-position quantity in comparison with both the NS formulation and the aqueous urea solution with their differing ingredients and formation of needle-like crystals

Urea forms urea composites (inclusion compounds etc) with many linear hydrocarbons and their derivatives Urea easily arranges along the crystal c-axis by adopting a heli-cal structure through NndashHhellipO hydrogen bonding to form a hexagonal cylinder-type tunnel known as hexagonal prisms89) suggesting that the urea inclusion compound forms hexagonal crystals41719)

Inclusion compounds vary widely depending on the guest724ndash26) Studies on urea inclusion compounds have fo-cused on inclusion compounds formed by linear hydrocarbons and their derivatives such as long-chain alcohols17182728) for example the urea-cetyl alcohol inclusion compounds reported in 197919) and urea-stearic acid and urea-palmitic acid inclu-sion compounds reported in 1996 and 1997 repectively1718) The DSC PXRD and PXRDndashDSC results obtained here are discussed in reference to these earlier reports The NS and LECthinsp formulationsthinsp providedthinsp athinsp peakthinsp atthinsp aroundthinsp 2θ=215deg sug-gesting the formation of an inclusion compound (urea compos-ite) comprising a long-chain alcohol (CSA) and urea PXRDndashDSC scans of the deposit from the NS formulation showed an endothermic peak and diffraction peak due to urea indicating thethinsp presencethinsp ofthinsp ureathinsp andthinsp athinsp ureathinsp compositethinspwhereasthinsp thethinsp LECthinspformulation deposit showed only the peak due to the urea compositethinspThethinspIRthinspspectrathinspofthinspthethinspthreethinspsamplesthinspconfirmedthinspthatthinspthethinsp depositsthinsp fromthinsp thethinsp LECthinsp andthinsp NSthinsp formulationsthinsp containedthinspurea-based structures although other peaks were also present

The results of this study indicate that the ingredients par-ticularlythinsp thethinsp lecithinthinspandthinspotherthinspemulsifiersthinsp inthinsp thethinspLECthinspformu-lation suppress crystallization of urea thereby substantially lengthening the maintenance of its amorphous state and thus loweringthinsp thethinsp ratethinsp ofthinsp crystalthinsp depositionthinsp Basedthinsp onthinsp thesethinsp find-ings it may therefore be expected to provide relatively long periods of suppressed crystallization and thus a dissolved state of urea contents even under winter conditions which facili-tates transfer of the urea to the skin on applicationItthinsp isthinspdifficultthinsp tothinsp includethinspsufficientthinspwaterthinsp inthinspathinspcreamthinspcontain-

ing 20 or more (ww) urea because of restrictions imposed by the composition of the formulation Thus urea crystals are deposited on the skin over time often resulting in skin irrita-tion an unpleasant feeling and reduced transfer of urea to the stratum corneum4) It is therefore important to suppress urea

crystalthinspgrowththinspfollowingthinspapplicationthinspbythinspdevisingthinspjudiciousthinspfor-mulationsthinspofthinsp 20thinspureathinsppreparationsthinsp tothinsp increasethinsp theirthinsp efficacythinspparticularly under low humidity conditions

Acknowledgment We are grateful to Mr Yutaka Koyama at Rigaku Corporation for providing technical support

Conflict of Interestthinsp thinspThethinsp authorsthinsp declarethinsp nothinsp conflictthinsp ofthinspinterest

References and Notes 1) Kligman A M Acta Derm Venereol 37 155ndash159 (1957) 2) Fukahori M Seki M Akatsu S Sakurai H Yakuzaigaku 56

23ndash31 (1996) 3) Allenby A C Creasen N H Edginton J A G Fletcher J A

Schock C Br J Dermatol 81 (Suppl 4) 47ndash55 (1969) 4) Matumaru H Tsuchiya S Yamaguchi K Sugiyama S Yaku-

zaigaku 37 57 (1977) 5) Ohno H Iwahara R Tagami H Hifukakiyou 85 419ndash428 (1990) 6) Higuchi T J Pharm Sci 52 1145ndash1149 (1963) 7) Takemoto K Sonoda N ldquoInclusion Compoundsrdquo Vol 2 ed by

Atwoodthinsp Jthinsp Lthinsp Daviesthinsp Jthinsp Ethinsp Dthinsp MacNicolthinsp Dthinsp Dthinsp Academicthinsp PressthinspLondonthinsp1984thinspppthinsp47ndash67

8) George A R Harris K D M J Mol Graph 13 138ndash141 190 (1995)

9) Harris K D M J Mol Struct 374 241ndash250 (1996)10) Harris K D M Thomas J M J Chem Soc Faraday Trans 86

2985ndash2996 (1990)11) Knight H B Anal Chem 24 1331 (1952)12) McAdie H G Can J Chem 40 2195 (1962)13) Marik K PhD Thesis Faculty of Natural Sciences Charles Uni-

versity Prague 197214) Marik K Smolkova E Chromatographia 6 420ndash426 (1973)15) Marik K Smolkova E J Chromatogr A 91 303ndash312 (1974)16) Smolkovaacute-Keulemansovaacute E Chromatographia 11 70ndash73 (1978)17) Kuhnert-Brandstatter M Burger A J Therm Anal 50 559ndash567

(1997)18) Kuhnert-Brandstatter M Burger A Pharmazie 51 288 (1996)19)thinsp Smolkovaacutethinsp Ethinsp Feltlthinsp Lthinsp Všetečkathinsp JthinspChromatographia 12 147ndash149

(1979)20) Rattner H Arch Derm 48 47 (1943)21) Wohlrab W Dermatol Mon Schr 174 94 (1988)22)thinsp PopethinspFthinspMthinspReesthinsp JthinspKthinspWellsthinspRthinspSthinspLewisthinspKthinspGthinspBr J Dermatol

86 291ndash296 (1972)23)thinsp Emulsifiersthinsp inthinsp NSthinsp formulationthinsp consistedthinsp polyoxyethylenethinsp (50)thinsp hy-

drogenated castor oil polyoxyethylene (POE) (20) sorbitan mono-stearatethinsp andthinsp sorbitanthinspmonostearatethinsp emulsifiersthinsp inthinspLECthinsp formulationthinspconsistedthinsp ofthinsp purifiedthinsp soybeanthinsp lecithinthinsp propylenethinsp glycolthinsp monostea-rate and glyceryl monostearate

24) Welberry T R Mayo S C J Appl Cryst 29 353ndash364 (1996)25) Kuhnert-Brandstaetter M Burger A Pharmazie 51 288ndash292

(1996)26) Yutronic N Manriquez V Jara P Witke O Merchan J Gonzalez

G J Chem Soc Perkin Trans 2 2000 1757ndash1760 (2000)27) Schlenk W Justus Liebigs Ann Chem 565 204ndash240 (1949)28) Schlenck W Jr ldquoOrganische Einschuszligverbindugen in Fortschritte

der chemischen Forschungrdquo Vol 2 Springer Verlag Berlin Got-tingen Heidelberg 1951 p 92


cream components on suppressing post-application crystalliza-tion

ExperimentalSamplesModelthinsp formulationsthinsp (OWthinsp (Oilthinsp inthinsp Water)thinsp typethinsp cream)thinsp① NS formulation ②thinspLECthinspformulationTwenty percent urea aqueous solutionModel FormulationsThethinsp compositionsthinsp ofthinsp thethinsp NSthinsp andthinsp LECthinsp formulationsthinsp arethinsp

shown in Table 1Preparation MethodsPreparation of Model FormulationsFormulations were prepared as shown in Fig 1 The aque-

ous phase consisted of urea which is the active pharmaceuti-calthinsp ingredientthinsp glycerinthinsp andthinsp purifiedthinsp waterthinsp andthinsp thethinsp oilthinsp phasethinspconsistedthinspofthinspnon-polarthinspandthinsppolarthinspoilthinspemulsifier23) and cetostea-ryl alcohol The phases were separately dissolved by heating to 80degC with stirring and then the aqueous phase was added to the oil phase and mixed The mixture was stirred using athinsp homomixerthinsp (HM)thinsp vacuumthinsp emulsifierthinsp (PVQ-5UNthinsp MizuhothinspIndustrialthinsp Cothinsp Ltdthinsp Osakathinsp Japan)thinsp withthinsp athinsp rotationalthinsp speedthinspof 3500 rpm and an anchor rotational speed of 70 rpm under vacuum (40 cmHg) Cooling was initiated under stirring HM

stirring was stopped at 40degC and the model formulation was obtained by cooling to 35degC

Preparation of 20 Urea Aqueous SolutionTothinsp 80thinspgthinsp ofthinsp purifiedthinspwaterthinsp 20thinspgthinsp ofthinsp ureathinsp wasthinsp addedthinsp andthinsp dis-

solved at room temperature by stirring with a stirrer for 15 min

Polarizing Microscope Observation by Simple Glass-Surface Coating5) Crystallization on a glass surface was observed with a microscope and crystal growth and growth rate were investigated for the two formulations and the aque-ous urea sample

About 20 mg of sample was placed on a 60times26 mm glass slide The sample was thinly and evenly spread with an oint-ment spatula at an application rate of 32 mgcm2 maintained at 40ndash50 relative humidity 20ndash25degC and crystallization over a 2 h period was clearly observed using a polarizing microscope (Polarizing Microscope Model BH Model PM-10AK Olympus Corporation Tokyo Japan 100timesobjective)thinspPhotos were taken at 15 60 and 120 min

PXRD Measurements The two formulations and 20 urea aqueous solution were applied to glass plates as in Sec-tion ldquoPolarizing Microscope Observation by Simple Glass-Surface Coatingrdquo and maintained at 20ndash25degC 40ndash50 relative humidity for 6 h Samples showing deposits were compared using an X-ray diffractometer (MiniFlex600 Rigaku Corpora-tion Tokyo Japan)

Measurement conditions are shown in Table 2DSC of Deposited Crystals The test samples were pre-

pared as in Section ldquoPolarizing Microscope Observation by Simple Glass-Surface Coatingrdquo and maintained at 20ndash25degC 40ndash50 relative humidity for 6 h Samples showing deposits

Table 1 Model Formulations

Component SuppliersAmount

NS formulation LECthinspformulation

Urea JunseithinspChemicalthinspCothinspLtdthinspTokyothinspJapan 200 200Oily substances Various sources 150 150Cetostearyl alcohol (CSA) KokyuthinspAlcoholthinspKogyothinspCothinspLtdthinspChibathinspJapan 40 40Polyoxyethylene (50) hydrogenated castor oil (HCO 50) NikkothinspChemicalsthinspCothinspLtdthinspTokyothinspJapan 10 00Polyoxyethylene sorbitan monostearate NikkothinspChemicalsthinspCothinspLtdthinspTokyothinspJapan 10 00Sorbitan monostearate NikkothinspChemicalsthinspCothinspLtdthinspTokyothinspJapan 04 00Purifiedthinspsoybeanthinsplecithinthinsp(Lecithin) TsujithinspOilmillsthinspCothinspLtdthinspMiethinspJapan 00 08Propylene glycol monostearate NihonthinspEmulusionthinspCothinspLtdthinspTokyothinspJapan 00 08Glyceryl monostearate NikkothinspChemicalsthinspCothinspLtdthinspTokyothinspJapan 00 08Glycerin NOF Corporation Tokyo Japan 100 100Purifiedthinspwater YoshidathinspPharmaceuticalthinspCothinspLtdthinspTokyothinspJapan 486 486

Total 1000 1000

Fig 1 Preparation Method

Table 2 PXRD Measurement Conditions


X-Ray source CuKαScan mode ContinuousX-Ray tube voltagendashcurrent 40 kVndash15 mAScan axis 2θθScan range (2θ) 20ndash500degScan speed 100degminSamping interval 002degstep





















































































deposits from creams containing 20% (w/w) urea and

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