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J. of Supercritical Fluids 90 (2014) 126–133

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids

j o ur na l ho me page: www.elsev ier .com/ locate /supf lu

ffect of operating parameters on PVP/tadalafil solid dispersionsrepared using supercritical anti-solvent process

unsung Parka,b, Wonkyung Choa,b, Han Kanga,c, Benjamin Byung Jun Leea,c,aek Sun Kima,c, Sung-Joo Hwanga,c,∗

Yonsei Institute of Pharmaceutical Sciences, Yonsei University, 162-1, Songdo-dong, Yeonsu-gu, Incheon 406-840, Republic of KoreaCollege of Pharmacy, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of KoreaCollege of Pharmacy, Yonsei University, 162-1 Songdo-dong, Yeonsu-gu, Incheon 406-840, Republic of Korea

r t i c l e i n f o

rticle history:eceived 11 September 2013eceived in revised form 26 March 2014ccepted 2 April 2014vailable online 13 April 2014

eywords:adalafil

a b s t r a c t

Supercritical anti-solvent (SAS) process was employed to produce tadalafil solid dispersion sub-micronparticles. Three independent variables for the SAS process (temperature, pressure, and drug concentra-tion) were varied in order to investigate the effects on particle size and morphology of PVP/tadalafilsolid dispersion (drug to polymer ratio 1:4). The mean particle size decreased with decreasing tempera-ture (50 → 40 ◦C) and concentration (15 → 5 mg/mL) and increasing pressure (90 → 150 bar). Dependingon the experimental variable, the mean particle size varied from 200 nm to 900 nm, and the dominantexperimental variable was determined to be the drug concentration. Moreover, at a concentration of

peration parameterox–Behnken designupercritical anti-solvent processolid dispersion

15 mg/mL with any other process conditions, tadalafil tended to partially aggregate in crystalline formwith irregular particle shapes. The results of in vitro dissolution experiments showed good correlationwith mean particle size and crystallinity of the SAS-processed particles, in that the highest drug concen-tration showed the least dissolution rate and vice versa. Therefore, among the three variables studied,the drug concentration is the major factor that produces sub-micron particles in the SAS process.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Tadalafil has a chemical structure of (6R-trans)-6-(1,3-enzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino1′,2′:1,6) pyrido-[3,4-b] indole-1,4-dione (Fig. 1), a chemicalormula of C22H19N3O4, and a molecular weight of 389.404 g/mol.adalafil was approved in November of 2005 by the US Foodnd Drug Administration (FDA) to treat ED (impotence), and islassified within the FDA biopharmaceutical classification systemBCS) as a class II drug that has low solubility, high permeability,nd an octanol-water-distribution coefficient (log Poct) of 2.48experimental data). The pKa value of tadalafil is 16.68 [1], indicat-ng that it is a non-ionizable drug over the physiological pH range,

hich implies that neither ionization nor salt formation would beseful solubilization techniques [2].

A solid dispersion technique can be applied to enhance theolubility of poorly water-soluble drugs and achieve faster dissolu-ion [3,4]. Sekiguchi and Obi in 1961 provided the first description

∗ Corresponding author at: 162-1, Songdo-dong, Yeonsu-gu, Incheon 406-840,epublic of Korea. Tel.: +82 32 749 4518; fax: +82 32 749 4105.

E-mail address: sjh11@yonsei.ac.kr (S.-J. Hwang).

ttp://dx.doi.org/10.1016/j.supflu.2014.04.001896-8446/© 2014 Elsevier B.V. All rights reserved.

of solid dispersion when they formed a eutectic mixture of thepoorly water-soluble drug, sulfathiazole, with the water-solublecarrier, urea [5]. The first report of solid dispersion prepared bya solvent method was described by Tachibana and Nakamura in1965; they dissolved �-carotene and polyvinylpyrrolidone in chlo-roform and then obtained solid dispersion particles after dryingthe organic solvent [6]. Since then, the solid dispersion methodhas been continuously improved and is now easier to produceand more applicable compared to other methods. In addition,solid dispersion can be connected to other solubilization meth-ods to improve solubility. Therefore, solid dispersion may be usedto improve the solubility of poorly water-soluble drugs, such astadalafil. Moreover, there are several technologies used to formu-late solid dispersions, such as the simple hot melt method [7] andthe solvent method, which includes solvent evaporation [8,9], spraydrying [10,11], and supercritical anti-solvent processes [9,12,13].Among these methods, the supercritical anti-solvent (SAS) pro-cess is the most appropriate method since it has many advantages,including the production of solid dispersion sub-micronparticles,

use of thermo-labile compounds, and low levels of remaining toxicorganic solvent [14,15]. Despite the many advantages of the SASprocess, there are relatively few studies that investigate its char-acteristics [16], and the SAS process conditions that are required

itical Fluids 90 (2014) 126–133 127

ts

dpcfi(mw

2

2

fPfw(fM(

J. Park et al. / J. of Supercr

o obtain sub-micron particles are generally missing or shown by aingle experimental condition [17,18].

Therefore, the purpose of this study is to use the Box–Behnkenesign to determine patterns and relationships between the SASrocessing parameters (temperature, pressure, and drug con-entration) that could affect tadalafil solid dispersion particleormation and provide an enhanced dissolution rate. Variousn vitro characterizations, such as powder X-ray diffractionPXRD), differential scanning calorimetry (DSC), scanning electron

icroscopy (SEM), particle size analysis, and dissolution study,ere investigated.

. Materials and methods

.1. Materials

Tadalafil (MSN Laboratories Ltd., India) was kindly giftedrom Dongkuk Pharmaceutical (Seoul, Republic of Korea).olyvinylpyrrolidone (Kollidon® 30, PVP) was kindly giftedrom BASF (Ludwigshafen, Germany), and carbon dioxide (CO2)ith a high purity of 99.9% was supplied by Yuseong Gas Co. Ltd.

Incheon, Republic of Korea). Acetonitrile (ACN) was purchasedrom Burdick & Jackson Laboratories (HPLC grade, MI, USA).

ethanol (MeOH), ethanol (EtOH), 2-propanol (POH), acetoneAC), methylene chloride (MC), and dimethyl sulfoxide (DMSO)

Fig. 2. Schematic diagram

Fig. 1. Chemical structure of tadalafil.

were purchased from OCI Company Ltd. (Seoul, Republic of Korea).All other chemicals were of reagent grade.

2.2. Experimental design used for SAS process

The SAS process was performed using previously described

experimental equipment [19] and is shown in Fig. 2. Since the tra-ditional design, which changes only 1 variable at a time, is difficultand time-consuming, a Box–Behnken design with 3 factors, 3 lev-els, and 15 runs was selected to investigate the effects of process

of the SAS process.

128 J. Park et al. / J. of Supercritical F

Table 1Values of independent variables and fixed experimental factors.

Independent variables Levels

−1 0 1

X1 = Temperature (◦C) 40 45 50X2 = Pressure (bar) 90 120 150X3 = Concentration (mg/mL) 5 10 15

Fd

pTfssesbflhvmc

ppcv

ssMmse

Y

wit

2

(DsuStvtoEtaao

2

v

ixed values: Y1 = Total volume of organic solvent (20 mL). Y2 = Flow rate of carbonioxide (CO2) (50 g/min). Y3 = Flow rate of the drug solution (1 mL/min).

arameters on the mean particle size of tadalafil solid dispersions.he process parameters that were chosen as the three factors are asollows: pressure (X1), temperature (X2), and concentration of drugolution (X3), which are independent variables. However, there areeveral other factors can be controlled, which were excluded in thisxperiment, such as volume of organic solvent, CO2 flow rate, drugolution flow rate. The volume of organic solvent had the issue ofeing homogeneous one phase if it was used in large amount. CO2ow rate and the drug solution flow rate give effect to each other,owever, to control these factors, experimental design would beery confusing and might not show major effect on the particle for-ation in SAS process. Therefore, only pressure, temperature and

oncentration of drug solution were varied.The levels of these three parameters were determined from our

ast experiences. The experimental design consisted of a set ofoints lying at the mid-point of each edge and 3 replicates of theenter point from the multi-dimensional cube. The independentariables and fixed values are listed in Table 1.

Various models, including Linear, 2FI, Quadratic, and Cubic, wereimultaneously fitted to the data for a response of mean particleize d(50) using Design Expert software (version 7.0, Stat-Ease Inc.,inneapolis, MN, USA). The adequacy and goodness of fit of eachodel were tested using analysis of variance (ANOVA). The regres-

ion coefficients were calculated based on the quadratic polynomialquation for the experimental data:

= A0 + A1X1 + A2X2 + A3X3 + A4X1X2 + A5X1X3 + A6X2X3

+ A7X21 + A8X2

2 + A9X23 (1)

here Y is the predicted response, A0 is the constant term of anntercept, A1–A3 are the linear coefficients, A4–A6 are the interac-ion terms, and A7–A9 are the quadratic coefficients.

.3. High performance liquid chromatography (HPLC) analysis

HPLC analysis was performed with a Waters HPLC systemMilford, MA, USA) composed of a Waters 996 Photodiode Arrayetector and a Waters 2695 Separations Module. Tadalafil was

eparated at a temperature of 40 ◦C using C18 reverse phase col-mn Inertsil® ODS-3, 150 mm × 4.6 mm, 5-�m particles from GLciences Inc. (Tokyo, Japan). The mobile phase consisted of acetoni-rile:water 50:50 (v:v) at flow rate of 1 mL/min, and the injectionolume was set at 10 �L in order to produce adequate UV responseso detect tadalafil. The detector was operated at a wavelengthf 283.6 nm (�max for tadalafil). The data were integrated usingmpower software version 5.0 (Empower Pro, Waters Corpora-ion, Milford, MA, USA). Stock solution of tadalafil was prepared byccurately weighing 25 mg of tadalafil in 100 mL EtOH to furnish

solution containing 250 �g/mL. The serial dilutions were carriedut using EtOH to obtain the concentration ranges required.

.4. Saturation solubility study using various organic solvents

Saturation solubility of tadalafil was determined in various sol-ents only to compare to solubilizing ability of solvents, such as

luids 90 (2014) 126–133

distilled water, EtOH, MeOH, POH, ACN, AC, MC, and 1:1 mix-ture of EtOH:MC (EMC), at 37 ± 0.1 ◦C. Tadalafil solubility at theexperimental temperatures such as 40, 45 and 50 ◦C, temperatureswere high enough for the organic solvents to evaporate. Therefore,saturation solubility was performed at 37 ◦C which is the pre-sumed body temperature. Excess drug was added to the solvents,and these samples were sonicated for 30 min. Then, the sampleswere placed in a shaking water bath (60 rpm) for 24 h, which waspreviously determined to be an adequate time for equilibration.Suitable aliquots were withdrawn after 24 h and filtered using a 0.2-�m membrane syringe filter. The filtrate was diluted with mobilephase, and the concentration of tadalafil was determined by HPLC.

2.5. Investigation of crystallinity using powder X-ray diffraction(PXRD) and differential scanning calorimetry (DSC)

PXRD patterns were recorded on a powder X-ray diffraction sys-tem (Smart Lab®, Rigaku, Japan) with Cu K� radiation. Sampleswere run over the most informative range of 2�: 3–30◦. The stepscan mode was performed using a step size of 0.02◦ at a rate of4◦/min. DSC measurements were carried out using a DSC S-650(Scinco Co. Ltd., Republic of Korea). Samples of 3–4 mg were accu-rately weighed and sealed in aluminum pans. Measurements wereperformed over 25–350 ◦C at a heating rate of 20 ◦C/min. An emptypan was used for reference; indium, tin, and zinc were used tocalibrate baseline, temperature, and enthalpy at a heating rate of20 ◦C/min. A nitrogen flow rate of 40 mL/min was used for each DSCrun to purge.

2.6. Particle size and morphology analysis

The mean particle size of each sample was measured by dynamiclight scattering (DLS) using an electrophoretic light scattering spec-trometer (ELS-Z, Otsuka Electronics, Tokyo, Japan). Samples weredispersed in silicon oil (KF-96-10CS, Shin-Etsu Chem. Co. Ltd.,Tokyo, Japan) and sonicated for 30 s at 120 W (Branson UltrasonicsCo., Danbury, CT, USA). Scanning electron microscopy (SEM; JSM-7000F, Jeol Ltd., Japan) was employed for morphological analysis ofthe resultant particles.

2.7. Powder dissolution study

Powder dissolution studies were performed according to theUSP XXXVI paddle method using the Distek dissolution system2500 (Distek Inc., North Brunswick, NJ, USA). The stirring speedwas 50 rpm, and the temperature was maintained at 37 ± 0.1 ◦C.Each test was carried out in 1000 mL of distilled water. Accuratelyweighed samples containing equivalent amounts of tadalafil (5 mg)were placed in the dissolution medium, and 5-mL aliquot sam-ples were withdrawn at given time intervals (10, 20, 30, 45, 60, 90,and 120 min) and filtered using a 0.45-�m membrane syringe fil-ter. At each sampling time, an equal volume of the test mediumwas replaced. Filtered samples were appropriately diluted withEtOH and analyzed for drug concentration by HPLC. Each test wasperformed in triplicate (n = 3) and the calculated mean values ofcumulative drug release were used to plot the release curve.

3. Results and discussion

The saturated solubility of tadalafil in various solvents is sum-marized in Table 2. Raw tadalafil is practically insoluble in distilledwater (3.21 �g/mL). However, organic solvents showed much

higher solubility in the range of approximately 300- to 3000-fold.According to the equilibrium solubility study in various organic sol-vents at 37 ◦C, the highest tadalafil concentration (25 mg/mL) wasachieved when EtOH and MC were mixed in 1:1 volume ratio (EMC)

J. Park et al. / J. of Supercritical Fluids 90 (2014) 126–133 129

Table 2Maximum solubility of tadalafil in various solvents at 37 ◦C.

Solvents Watera (�g/mL) MeOHa (mg/mL) EtOHa (mg/mL) POHa (mg/mL) ACNa (mg/mL) ACa (mg/mL) MCa (mg/mL) EMCb (mg/mL)

Concentration 3.21 2.05 1.64 1.04 8.06 10.53 10.05 ≤25.00

M MC, m

wawaTlaHracuop

ttd

Fct

shown in Fig. 3. All of the SAS-processed solid dispersion sam-ples showed similar DSC patterns with no endothermic peak, while

eOH, methanol; EtOH, ethanol; POH, 2-propanol; ACN, acetonitrile; AC, acetone;

a An excess amount of tadalafil could be added in one step into the solvent.b Tadalafil was added in stages until recrystallization was observed.

hich is estimated concentration value. When excess tadalafil wasdded to the EMC solution, a cotton-like suspension developed,hich is different solution of the other organic solvents that had

clear solution with some amount of tadalafil on the bottom.herefore, tadalafil was added in stages to determine the EMC equi-ibrium concentration for tadalafil. The tadalafil-EMC solution wasbsolutely clear until the tadalafil concentration reached 24 mg/mL.owever, as soon as the concentration exceeded 25 mg/mL, it

ecrystallized and became a cotton-like suspension. Therefore, wessume that the critical concentration of tadalafil required to staylear in EMC is approximately 25 mg/mL. As a result, we decided tose EMC for further experiments because it has a tadalafil solubilityf approximately 25 mg/mL, which will provide easy control of therocess parameters of drug concentration.

Fifteen experiments were performed to investigate the effects of

hree process parameters, temperature, pressure, and drug concen-ration, on the responses of the mean particle size and particle sizeistribution. Moreover, the drug to polymer ratio was fixed at 1:4 in

ig. 3. DSC curves of tadalafil raw material, representative of SAS processed in drugoncentration of 5 mg/mL and 10 mg/mL (tadalafil to PVP ratio; 1:4), SAS processedadalafil, and SAS processed PVP.

ethylene chloride; EMC, ethanol and methylene chloride 1:1 (v/v) mixture.

order to maximize the effect of polymer solubilization and expect-ing complete amorphous form. After 15 runs, the crystallinity ofthe SAS-processed solid dispersion sample was determined usingDSC and PXRD. The yield of the particle was 69 ± 8%. This SAS pro-cess is closed system, however, the particles were precipitated notonly onto the filter paper, but also all over the surface of the cham-ber. Therefore, its yield is not high enough as it can be expected.However, drug loading efficiency of each run was about 98% thattadalafil was homogeneously distributed in the solid dispersionparticle.

DSC curves of tadalafil raw material, representative samplesof the SAS-processed solid dispersion process, SAS-processedtadalafil (drug only), and SAS-processed PVP (polymer only) are

SAS-processed tadalafil showed the same endothermic peak asthe raw material. Therefore, we used the representative figure of

Fig. 4. PXRD patterns of tadalafil raw material, representative of SAS processedin drug concentration of 5 mg/mL and 10 mg/mL (tadalafil to PVP ratio; 1:4), SASprocessed tadalafil, and SAS processed PVP.

130 J. Park et al. / J. of Supercritical Fluids 90 (2014) 126–133

Table 3Box–Behnken experimental parameters and results.

Run Factors Response

Temperature (◦C) Pressure (bar) Concentration (mg/mL) d(50) (nm) SPAN

1 45 90 5 2612 40 150 10 2353 45 120 10 3524 40 120 5 4465 50 120 5 3926 45 150 5 2117 45 120 10 4348 50 150 10 5929 40 90 10 40310 40 120 15 67911 45 150 15 70912 45 120 10 38213 50 90 10 49314 50 120 15 64315 45 90 15 891

Range 40–50 90–150 5–15

d d(10)(50) .

Suwtsmmnpttr1sawftMsldpda(ob

wat(ri

ahimfsta

of these parameters were not significant. As shown in Fig. 8,three-dimensional response surface plots were created to estimatethe effects of the independent variables on mean particle size atlow drug concentration (5 mg/mL), relatively higher pressure, and

(50) – mean particle size was calculated as the intensity distribution SPAN = d(90)−2×d

AS processed sample in Fig. 3, not showing all of the DSC fig-res for all SAS-processed samples. A sharp endothermic peakas observed at 305 ◦C for the melting point of the drug, while

he SAS-processed sample had no endothermic peak and insteadhowed a hollow hump around 130 ◦C. This lack of an endother-ic peak indicates that the tadalafil molecules were dispersedolecularly but irregularly within the amorphous polymer chain

etwork and therefore existed in the amorphous form, while SAS-rocessed tadalafil did not exist in the amorphous form. However,he PXRD results showed 2 patterns that depended on drug concen-ration (Figs. 4 and 5). As illustrated in Fig. 4, the PXRD patterns ofaw tadalafil showed numerous distinct peaks at 2� of 7.3◦, 14.5◦,8.5◦, 21.7◦, 24.2◦, and 25.0◦. On the other hand, SAS-processedolid dispersion samples with concentrations between 5 mg/mLnd 10 mg/mL and other process conditions as described aboveere characterized by the complete absence of any peak. There-

ore, these results correlated with those of DSC, confirming thatadalafil was completely in amorphous form in these samples.

oreover, SAS-processed tadalafil also showed distinctive peaksimilar to tadalafil raw material, again demonstrating good corre-ation between PXRD and DSC. However, in SAS-processed solidispersion samples with a concentration of 15 mg/mL and otherrocess conditions as described above, the crystalline region asetected by PXRD showed tiny peaks at 2� of 14.5◦, 18.5◦, 21.7◦,nd 24.2◦, which are the distinct peaks of tadalafil crystalline formFig. 5). The reason for this difference from DSC is that the amountf crystalline material was below the detection limit of DSC, orecause of the thermal effect.

The mean particle size and SPAN value of unprocessed tadalafilere 26.64 �m and 1.36, respectively, with definite needle shape

nd irregular length and width (Fig. 6). SAS-processed particles, onhe other hand, had spherical shapes with smaller particle sizesFig. 7). The results of the mean particle size and SPAN values fromandomized runs in PVP-tadalafil solid dispersion are summarizedn Table 3.

As summarized in Table 4, the linear model was selected as suitable statistical model for the mean particle size because itad the smallest PRESS value. The model showed a statistically

nsignificant lack of fit. The coefficient estimate and standardizedain effect (SME) values in the form of a polynomial equation

or the responses are listed in Table 5, and drug concentrationhowed the largest SME value, indicating that it is the main factorhat significantly impacts the designated responses. Temperaturend pressure also affected particle formation; however, the effects

Fig. 5. PXRD patterns of tadalafil raw material and SAS processed in drug concen-tration of 15 mg/mL (tadalafil to PVP ratio; 1:4).

J. Park et al. / J. of Supercritical Fluids 90 (2014) 126–133 131

ld

maiafu

Table 4Summary of the results for (a) model analysis, (b) lack of fit, and (c) R-square analysisfor measured responses.

Mean particle size

Sum of squares p > F

(a) Model analysisMean vs. Total 3.381 × 106

Linear vs. Mean 3.525 × 105 0.00512FI vs. Linear 22432.87 0.7555Quadratic vs. 2FI 54155.12 0.4818Cubic vs. Quadratic 91145.38 0.0542Residual 3447.25 –Total 3.905 × 106 –

(b) Lack of fitLinear 1.677 × 105 0.08752FI 1.453 × 105 0.0679Quadratic 91145.38 0.0542Cubic 0.000 –Pure error 3447.25 –

Adjusted R-square PRESS

(c) R-square analysisLinear 0.5839 3.41 × 105

2FI 0.5029 6.856 × 105

6

F1

F

Fig. 6. SEM image of unprocessed tadalafil particles (100×).

ower temperature; smaller particles with a narrower particle sizeistribution were formed.

The formation of crystals from solutions during the anti-solventethod involves three steps: (1) supersaturation, (2) nucleation,

nd (3) crystal growth. Supersaturation occurs when the solubil-

ty of a compound in a solvent exceeds the saturation solubility,nd the compound starts to crystallize. Nucleation refers to theormation of nuclei within a homogeneous, supersaturated liq-id phase. Crystal growth is deposition of the precipitate onto

ig. 7. Representative SEM images of tadalafil-PVP (1:4) particle in various process con50 bar, 10 mg/mL; (b) 45 ◦C, 150 bar, 5 mg/mL; and (c) 45 ◦C, 90 bar, 15 mg/mL (20,000×)

ig. 8. Three-dimensional response surface plots showing effects on mean particle size b

Quadratic 0.4942 1.466 × 10Cubic 0.9539

the nuclei, which can occur only after nuclei are formed and mayattributed to one or more mechanisms of Ostwald-ripening or

agglomeration [20]. Therefore, particles precipitated after reachingsaturation in the case of low drug concentration, while particlesprecipitated immediately in relatively high drug concentrations,

ditions (used solvent – 1:1 mixture of ethanol and methylene chloride) (a) 40 ◦C,.

y variables (a) concentration and pressure and (b) temperature and concentration.

132 J. Park et al. / J. of Supercritical Fluids 90 (2014) 126–133

Table 5Standardized effects of experimental parameters on PVP-tadalafil nanoparticlesgenerated by the SAS process.

Mean particle size

Estimatedcoefficient

p-Value aStandardizedmain effect

A0 474.79 0.0051 14.740A1 44.55 0.3342 1.010A2 −37.55 0.4127 −0.851A3 201.65 0.0008 4.572

A1, temperature; A2, pressure; A3, concentration.

b

pastttrwvttpsm(

poTbwDfTtaaatJ

d

wigrlmastwph

sT

a Standardized main effects (SMEs) were calculated by dividing the main effecty the standard error of the main effect.

roducing large, irregular particles, as shown in Fig. 7. Moreover,t higher concentrations, there are more nuclei formed with higherurface areas; therefore, the particle growth intersects with nuclea-ion. Moreover, Ostwald-ripening might be occurring in ordero form thermodynamically favored large primary particles. Forhese reasons, and in accordance with previous studies [20–23],elatively high drug concentrations (15 mg/mL) showed particlesith relatively large mean diameters (micron-range), large SPAN

alues, and irregular shapes, while at low drug concentrations,he mean particle size decreased and the particle size distribu-ion narrowed. This was confirmed by SEM images (Fig. 7). Inarticular, the non-spherical particles are only observed in theamples with the highest drug concentration (15 mg/mL); theseight be the crystalline forms that were determined by PXRD

Fig. 5).In addition to the drug concentration, the operation tem-

erature and pressure also affected particle formation underur experimental conditions, though these effects were minor.he obtained results correspond to different regimes of contactetween CO2 and the drug solution. All temperatures and pressuresere above the one-phase region for the organic solvent, EMC.ecreasing particle size was observed with increasing pressure

rom 90 to 150 bar and decreasing the temperature from 50 to 40 ◦C.hese temperature and pressure relationships can be explained byhe density of the CO2, which changes according to temperaturend pressure. When pressure was fixed, the solution merged with

much denser CO2 phase at 40 ◦C than 50 ◦C, and when temper-ture was fixed, CO2 was denser at higher pressure. The effect ofhese parameters on the initial droplet size can be calculated usingajuja’s equation [24].

32 = 0.17(

�

�G

)0.45( 1UGL

)0.9(1 + L

G

)0.5(dnozzle)0.55

+ 0.0015

[(�L)2

(��L)

]0.5

(dnozzle)0.55(

1 + L

G

)(2)

here � is interfacial tension between the liquid and gas (N/m), �Gs gas density (kg/m3), UGL is velocity difference (UG − UL) betweenas velocity (UG) and liquid velocity (UL) (m/s), L/G is the massatio of liquid flow to gas flow, dnozzle is nozzle diameter, �L isiquid viscosity (Pa s), �L is liquid density (kg/m3), and d32 is the

ean diameter of the droplets. If we assume that one agglomer-ted particle is created from each of the initial droplet, its particleize is dominantly affected by the initial droplet size. According tohe equation, the factor (�/�G) decreased with increasing density,hich resulted in smaller droplet size (d32) and therefore smallerarticle size [25]. As a result, smaller particles can be obtained with

igher density.

The results of the in vitro powder dissolution study (Fig. 9)howed good correlation between particle size and crystallinity.here was faster and higher drug release with smaller particle size,

Fig. 9. Powder dissolution study of 15 runs samples (n = 3).

and slower and lower release with larger particle size. In particular,the samples produced with 5 mg/mL drug concentration showeddrug release around 80%, while raw material showed less than1% drug release in distilled water. Additionally, all samples with15 mg/mL concentration showed less than 50% of the dissolutionrate in 120 min, no matter what other process conditions were

applied. That is due not only to the particle size but also to thecrystalline region in the samples, which was confirmed by PXRDand SEM images.

itical F

4

mctAhofaadot

A

fj

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

J. Park et al. / J. of Supercr

. Conclusions

The mean particle sizes of PVP/tadalafil solid dispersions wereeasured as a function of pressure, temperature, and drug con-

entration. The mean particle size decreased with decreasingemperature and concentration and with increasing pressure.mong the variables, drug concentration was the major factor thatad the most significant effect on the mean particle size. More-ver, when the drug concentration was 15 mg/mL, a crystallineorm of tadalafil with irregular, micron-sized particles was obtainednd resulted in low dissolution rate. Therefore, to obtain the mostppropriate tadalafil solid dispersion using the SAS process, therug concentration must be controlled to low levels in order tobtain smaller particle sizes with narrower particle size distribu-ions and to form completely amorphous solid dispersions.

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/.supflu.2014.04.001.

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