insights into the roles of carrier microstructure

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Research Paper

Insights into the roles of carrier microstructure in adhesive/carrier-based

dry powder inhalation mixtures: Carrier porosity and fine particle

content

Ahmed O. Shalash a, Abdulla M. Molokhia a, Mustafa M.A. Elsayed b,,1

a European Egyptian Pharmaceutical Industries, Alexandria, Egyptb Department of Pharmaceutics, Faculty of Pharmacy, Alexandria University, Alexandria, Egypt

a r t i c l e i n f o

Article history:

Received 24 May 2015

Revised 6 August 2015

Accepted in revised form 7 August 2015

Available online 11 August 2015

Keywords:

Dry powder inhaler

Carrier

PorosityPore size distribution

Surface roughness

Surface rugosity

Fines

Mercury intrusion porosimetry

Air permeabilityFluidization

Aerolizer

Fluticasone propionate

a b s t r a c t

To gain insights into complex interactions in carrier-based dry powder inhalation mixtures, we studiedthe relationships between the carrier microstructural characteristics and performance. We used mercuryintrusion porosimetry to measure the microstructural characteristics and to also derive the air perme-

ability of eight carriers. We evaluated the performances of inhalation mixtures of each of these carriers

and fluticasone propionate after aerosolization from an Aerolizer. We did not observe a simple relation-ship between the carrier total porosity and the performance. Classification of the porosity according topore size, however, provided interesting insights. The carrier nanoporosity, which refers to pores smaller

than micronized drug particles, has a positive influence on the performance. Nanopores reduce the carriereffective contact area and the magnitude of interparticulate adhesion forces in inhalation mixtures. The

carrier microporosity, which refers to pores similar in size to drug particles, also has a positive influenceon the performance. During mixing, micropores increase the effectiveness of frictional and press-on

forces, which are responsible for breaking up of cohesive drug agglomerates and for distribution of drugparticles over the carrier surface. On the other hand, the carrier macroporosity, which refers to pores lar-

ger than drug particles, apparently has a negative influence on the performance. This influence is likelymediated via the effects of macropores on the powder bed tensile strength and fluidization behavior. Theair permeability better represents these effects. The inhalation mixture performance improved as the car-

rier air permeability decreased. Interestingly, as the carrier fine particle content increased, the carriermicroporosity increased and the carrier air permeability decreased. This proposes a new mechanism

for the positive effect of fine excipient materials on the performance of carrier-based inhalation mixtures.Fine excipient materials apparently adhere to the surface of coarse carrier particles creating projections

and micropores, which increase the effectiveness of mixing. The data also support the mechanism ofpowder fluidization enforcement by fine excipient materials. The current study clearly demonstrates that

the microporosity and the air permeability are key dry powder inhalation carrier performance determi-nants. Mercury intrusion porosimetry is a useful tool in the dry powder inhalation field; it successfully

allowed resolution of carrier pores which contribute differently to the performance.2015 Published by Elsevier B.V.

1. Introduction

Complex interactions in adhesive/carrier-based dry powderinhalation (DPI) mixtures are till date not fully understood. This

is mainly underlain by the large number and variety of factorswhich come into play. The use of analytical techniques that maynot quantify these factors to the relevant degree of detail also

contributes to the poor understanding. The effects of the carriersurface porosity/roughness on the performance are among theinteractions in this domain that are open till date. In carrier-based dry powder inhalation mixtures micronized drug particles

with aerodynamic size of 15 lm, i.e. respirable, are distributedover coarse carrier particles. Coarse carrier particles improve flowproperties of cohesive drug particles.

The carrier surface porosity/roughness influences the

aerodynamic performance of carrier-based inhalation mixtures[18]. Despite extensive investigations, reported observationsare inconsistent and the influence is not yet fully understood. Tothe best of the current knowledge, the influence of the carrier

http://dx.doi.org/10.1016/j.ejpb.2015.08.006

0939-6411/2015 Published by Elsevier B.V.

Corresponding author at: Department of Pharmaceutics, Faculty of Pharmacy,Alexandria University, El-Khartoum Square, El-Azarita, Alexandria 21521, Egypt.

E-mail address: [email protected](M.M.A. Elsayed).1 Mustafa M.A. Elsayed, Ph.D., is the principal investigator.

European Journal of Pharmaceutics and Biopharmaceutics 96 (2015) 291303

Contents lists available at ScienceDirect

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surface porosity/roughness depends on the size of pores/disconti-nuities in comparison with the size of drug particles [2,9]. Pores

and discontinuities larger than drug particles increase thedrug-carrying capacity and thus promote drug emission from theinhalation device[2,4]. These large pores, however, provide shelterfor drug particle from drag separation forces during aerosolization,thus hinder drug detachment/dispersion from carrier particles, and

thus often decrease the drug respirable fraction [2,4].This negativeeffect does not apply when dry powder inhalation devices that relyon inertial separation forces are used [8]. It is noteworthy thatlarge pores provide shelter for drug particle also from fictional

and press-on forces during mixing; this would have a positiveinfluence on the performance. Prediction of the net effect of largepores is thus not always straightforward. On the other hand,microprojections and pores smaller than drug particles increase

the drug respirable fraction, probably by reducing the effectivedrug-carrier contact area[2,10].

Microscopic studies highlighted the influence of pore size. How-ever, the surface porosity/roughness of dry powder inhalation car-

riers is most often quantified by air permeametry[1,3,4]and BETgas adsorption[2,5,11], which are in this regard limited. One canderive specific surface areas, but not pore size distributions, from

air permeametry measurements. Moreover, specific surface areasderived from air permeametry reflect only large pores anddiscontinuities. They do not reflect fine and deep pores which donot contribute to air permeability. This explains why several airpermeametry studies have suggested carrier surface roughness

has a negative impact on the aerodynamic performance[1,3]. Thismay be indeed true if only large pores and discontinuities are con-sidered. On the other hand, BET gas adsorption provides specificsurface areas which include fine pores with diameters down to

0.3 nm. Fine pores may dominate specific surface areas derivedfrom BET gas adsorption. This may explain the outcome of a BETgas adsorption study [7] which suggested carrier surface roughnesshas a positive impact on the aerodynamic performance. BET gas

adsorption provides pore size distributions. Such distributions,

however, cover a limited pore diameter range from 0.3 to300 nm. This range is far below the size range of micronized drugparticles used in dry powder inhalation. Pores over this distribu-

tion contribute similarly to drug-carrier particle interactions ininhalation mixtures. Such distribution is thus of little value fordry powder inhalation carriers. Laser profilometry [12], imageanalysis [6], and atomic force microscopy [5,6] have been used

for topographic assessment of dry powder inhalation carriers.These techniques are of limited applicability in routine analysisof bulk materials.

The aim of the current study was to gain further insights into

the influence of the carrier microstructure on the performance ofcarrier-based dry powder inhalation mixtures. To this end, we usedmercury intrusion porosimetry to study carrier microstructural

properties, such as the pore volume distribution and the surfaceroughness. Mercury intrusion porosimetry allows determinationof pore size distributions over a broad pore diameter range from0.003 to more than 300 lm, i.e. five orders of magnitudes broad.This allows resolution of carrier pores on a scale relevant to thesize of micronized drug particles. It is noteworthy that the porositymeasured by mercury intrusion porosimetry includes interparticu-late spaces and is not limited to surface pores. To our knowledge,

the use of mercury intrusion porosimetry for quantification ofthe porosity of dry powder inhalation carriers has not been earlierconsidered in the literature. Eight materials, with different chemi-cal nature or crystalline structure, were tested as carriers. These

were hydroxypropyl-b-cyclodextrin (CD), dextrose anhydrous(DA), dextrose monohydrate (DM), lactose anhydrous (LA), lactose

monohydrate (LM), mannitol (MN), xylitol (XL), and sucrose (SU).These materials are widely available in pharmaceutical quality.

The variety allowed us to also test the roles of carrier chemicalcomposition and crystalline/polymorphic form. After processing,the carriers also differed in their contents of fines (D< 10lm),but their coarse components were of almost the same size. Flutica-sone propionate, a hydrophobic adhesive drug, was used as modeldrug. The performances of the inhalation mixtures were assessedafter aerosolization from an Aerolizer. Separation forces gener-

ated in an Aerolizer during inhalation are mainly lift and dragforces.

2. Materials and methods

2.1. Materials

Hydroxypropyl-b-cyclodextrin (Kleptose HPB), dextrose

monohydrate (Roferose SF;Dmean= 50 lm), mannitol (Pearlitol50 C;Dmean= 50lm), and xylitol (Xylisorb 300; Dmean= 300 lm)were from Roquette, Lestrem, France. Dextrose anhydrous wasfrom SunTin MediPharma Co. Ltd., Hong Kong, China. Lactose anhy-

drous (Lactopress anhydrous 265; b-lactose anhydrous;D50< 150 lm; anhydrous lactose is crystallized by rapid drying of

a solution of lactose at high temperature; crystals are then milledand sieved to the required particle size distribution) was from

Borculo Domo Ingredients, Zwolle, The Netherlands. Lactosemonohydrate (Lactohale LH200, milled a-lactose monohydrate;D50= 50100lm) was from Friesland Foods Domo, Zwolle, TheNetherlands. Sucrose was from Daqahlia Sugar Co., Cairo, Egypt.

Fluticasone propionate (superfine micronized grade; D90< 10lm)was from Jayco Chemical Industries, Maharashtra, India. All otherreagents were of analytical grade.

2.2. Preparation of the carriers

We prepared carriers with similar, narrow size-distributions

from the carrier original materials by sieving. We sieved each car-

rier original material through a stack of 150, 106, and 75 lm ana-lytical sieves (Retsch GmbH, Germany) and collected the fractionsieved between the 75 lm and the 106 lm sieves. To minimizethe content of fines smaller than 75 lm, we placed the collectedfraction below a 75 lm sieve and aerated it for 2 min at 2-barpressure by Schlick 970 S75 coating gun (Dsen-Schlick GmbH,

Germany), placed perpendicularly 5 cm above the sieve. Thisprocedure removes only loose fines. We stored the preparedcarriers in polyethylene bags at 20 C and 45% RH.

2.3. Preparation of the inhalation mixtures

Before mixing, we screened/sieved the carriers and the drugfluticasone propionate, FPthrough a 250 lm sieve to break up

and remove agglomerates. We then prepared 1% FP inhalation mix-tures (2-g each) in test tubes using the sandwich method. We firstvortexed each mixture for one minute. We then added three 4-mm316 L stainless steel balls to each mixture and vortexed it again forone minute. In order to take the effects of the mixing process oncarrier characteristics into consideration, we similarly processed

blank carrier samples before further characterization. We storedthe inhalation mixtures and the blank carrier samples in polyethy-lene containers at 20 C and 45% RH for at least one week to allowfor mechanical relaxation.

2.4. Characterization of the carriers

2.4.1. Crystallinity

We measured the crystallinities of the carriers and the drug bydifferential scanning calorimetry (DSC) using a PerkinElmer DSC 6

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differential scanning calorimeter (PerkinElmer Inc., USA). In eachmeasurement an approximately 4-mg carrier sample was heated

in an aluminum pan under nitrogen purge from 30 C to 300 C(to 400 C for CD) at a heating rate of 10 C min1.

2.4.2. Loss on drying

We determined the losses on drying of the carriers using a Met-tler Toledo HR73 halogen moisture analyzer (Mettler Toledo,Switzerland). 1-g sample of each carrier was dried at 105 C for15 min (sufficient to reach constant weight). The loss on drying

represents the amount of volatile matter of any kind that is drivenoff under the conditions specified.

2.4.3. Shape analysis

We performed the shape analyses of the carriers using a Micro-vision LW1135C-GT1 image analysis system, which is comprised ofa microscope with 4 objective lens, a camera, and EllixTM version7.6.2 software (Microvision Instruments, France). In each measure-

ment we covered a small carrier amount (2 mg) with dime-thicone and homogenously distributed it on a microscope slide.We mounted the slide on the microscope and took images of at

least 100 randomly selected particles with diameters near the car-rier D 50, determined by laser diffraction particle size analysis (cf.Table 2). We calculated the aspect (elongation) ratio, RA, and thecircularity,C, defined as follows:

RA LW

1

C 4pAP2

; 2

whereL is the length, Wthe width,Pthe perimeter, andA the pro-

jected surface area of the particle.

2.4.4. Particle size distribution

We measured the particle size distributions of the carriers and

the drug using a Malvern Mastersizer 2000 laser diffraction parti-cle size analyzer, operating with HeNe laser (k= 632.8 nm) asred light source and equipped with a Hydro 2000MU wet sampledispersion unit (Malvern Instruments Ltd., United Kingdom). We

used acetone and isopropyl alcohol as the dispersing media forCD and all the other carriers, respectively. For fluticasone propi-onate we used 10/90 w/w ethanol/water mixture containing 0.2%polysorbate 80 and saturated with the drug as the dispersing

medium. Each measurement was set to collect data for 412 s(1000 snap/s). Measurements were conducted in triplicates.

We used the Mie theory of light scattering for the data analysisto ensure accurate measurement of fine particles (D< 25lm), forwhich the assumptions of the Fraunhofer approximation are inva-lid (the Fraunhofer assumptions are invalid for particles smaller

than 40 k [13]). Use of the Mie theory requires knowledge of boththe dispersing medium and the particle refractive indices. For the

dispersing media we used literature values of refractive indicesatk = 589 nm: These were 1.3588 for acetone, 1.3776 for isopropylalcohol, and 1.3395 for the hydroethanolic dispersing medium offluticasone propionate[14,15]. The expected differences between

the refractive indices of the dispersing media at k= 589 nm andat k= 632.8 nm had negligible effects on size measurements. Forthe carriers we measured the refractive indices using the followingprocedure. For each carrier we prepared solutions of different con-

centrations in deionized water. We measured the refractive indicesof these solutions using an Abbe refractometer and daylight as alight source. For each of the carriers the refractive index was a lin-ear function of the concentration with R2 > 0.999. Extrapolating the

measured refractive indices to 100% concentration provided thecarrier refractive index. The refractive indices were accordingly1.462 for CD, 1.511 for DA, 1.494 for DM, 1.531 for LA, 1.521 forLM, 1.521 for MN, 1.551 for XL, and 1.544 for SU. The expected dif-

ferences between the particle refractive indices measured usingwhite light and those at k = 632.8 nm had negligible effects on sizemeasurements. The measured refractive indices moreover alwaysled to the best fits of scattering data, i.e. the lowest residuals.

The imaginary refractive indices (absorption) were assigned thevalues leading to best agreements between the calculated andthe measured data on the extinction channel. These were 0.01for CD, 0.001 for DA and DM, 0.005 for LA and LM, 0.001 for MN,0.02 for XL, and 0 for SU. For fluticasone propionate the real refrac-

tive index was set to 1.55 and the imaginary refractive index to0.005.

2.4.5. Pore size distribution, surface roughness, and permeability

We measured the pore size distributions of the carriers by mer-cury intrusion porosimetry using a Micromeritics Poresizer 9320(Micromeritics Instrument Corporation, USA). We placed anapproximately 0.4-g sample of the carrier into a 5-ml penetrome-

ter (penetrometer constant = 22.07 ll/pF). The penetrometer wasthen assembled, evacuated, and filled in a horizontal position withmercury under vacuum. The applied pressure was increased in astepwise fashion from 1.17 to 30,000 psi. At each step the volume

of intruded mercury was recorded after 10-second equilibration.The pore diameter, D, is related to the applied pressure, P, bymeans of the Washburn equation:

D 4cP cos h; 3

Table 1

The crystallinities, the losses on drying, and the shape parameters of the carriers.

Carrier Crystallinity Loss on drying

[%W]

Shape analysis

(optical microscopy)

Aspect ratio, RA Circularity, C

CD Amorphous 11.41 0 .40 1.52 0.23 0.56 0 .09

DA Crystalline 0.58 0 .09 1.45 0.17 0.54 0 .10DM Crystalline 9.23 0 .09 1.56 0.31 0.55 0 .12

LA Crystalline 0.55 0 .05 1.51 0.27 0.54 0 .12

LM Crystalline 0.94 0.20a 1.75 0.40 0.62 0.07

MN Crystalline 0.36 0 .03 1.96 0.68 0.54 0 .10

XL Crystalline 0.30 0 .02 1.72 0.53 0.58 0 .10

SU Crystalline 0.45 0 .13 1.45 0.32 0.64 0 .09

a The conventional drying process does not remove lactose monohydrate water

of crystallization (cf. Section3.1.1andFig. 1).

Table 2

The particle size distributions of the carriers.a

Carrier DV,Mean

[lm]

DV,Mode

[lm]

DV,10

[lm]

DV,50

[lm]

DV,90

[lm]

% Fines

(D< 10.0 lm)

CD 79.91 77.59 32.12 72.80 138.87 3.03

DA 72.69 74.70 26.23 67.03 127.42 2.92

DM 73.67 75.08 28.38 68.06 128.24 3.72

LA 61.35 76.04 9.03 55.40 122.72 11.22

LM 72.93 79.39 12.31 64.09 144.05 8.21

MN 61.42 59.26 16.43 52.30 119.23 5.64

XL 81.31 76.95 43.81 75.68 127.39 0.31

SU 71.19 77.10 19.58 65.28 130.18 4.56

FP 7.20 3.86 1.97 4.49 14.63 83.17FPb 8.46 4.53 2.31 5.27 17.19 78.49

a The data represent volume-weighed size distributions, which are referred to by

the subscript V. The given values are means of at least triplicate measurements. The

relative standard deviations of the median diameters, DV,50, were always smaller

than 1.5%.b Aerodynamic diameters calculated using the relation: Daerodynamic= D(q/v)

0.5.

The particles were assumed to be spherical, i.e. the dynamic shape factor, v, equals1. Fluticasone propionate density, q, is 1.38g cm3 [22].

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where c is the mercury surface tension (set to 485 dyne/cm) and h isthe contact angle of mercury on the sample (set to 130).

We calculated the surface roughness, SR, which we defined as

SR SSAMIPSSAPSD

: 4

SSAPSD is the specific surface area estimated from the particle size

distribution assuming spherical particle shape. For calculation ofSSAPSD we used literature values of densities: These were

1.533 g mL1 for CD, 1.562 g mL1 for DA [14], 1.540g mL1 forDM [16], 1.59 g mL1 for LA [14,16], 1.547 g mL1 for LM [14],1.514 g mL1 for MN [16], 1.52 g mL1 for XL[16], and 1.581 g mL1

for SU[14]. SSAMIP is the specific surface area estimated from themercury intrusion porosimetry data.

We also calculated the air permeabilities of the carriers from

the mercury intrusion porosimetry data. A plot of log P versuslog V, where P is the applied pressure and V is the volume ofintruded mercury, approximates a hyperbola and can be expressedmathematically by[17]:

log P log Pd G

logV logV1 : 5

Pdis the displacement pressure, obtained by extrapolation toV 0.V1

is the volume of the intruded mercury at infinite pressure, i.e.the total interconnected pore volume.Gis a pore geometrical factorthat reflects pore sorting and interconnection. We determinedV

1,

Pd, and G values for each carrier by nonlinear fitting (LevenbergMarquardt) of the mercury intrusion porosimetry data to Eq. (5).We excluded the data points with P> 200 psia, which correspond

to pores with D< 0.904lm. These pores do not contribute to theair permeability and their exclusion allows more robust identifica-tion of the pore geometrical factor. We then calculated the air per-meability via the procedure suggested by Swanson [18].

Accordingly, the apex of the hyperbola is indicative of intrusion ofthe major connected pore space which dominates fluid flow withmercury. Swanson[18]suggested identification of the apex point

by the intersection of the hyperbola with a 45 line passing throughthe origin of the hyperbolic axes. We alternatively calculated thevolume of intruded mercury, VA, and the applied pressure, PA, atthe apex point from the hyperbolic function using the relations:

logVA logV1 ffiffiffiffiG

p 6

log PA log Pd ffiffiffiffiG

p 7

We then calculated the air permeability from the correlation sug-gested by Swanson[18]:

Ka 399 VA 100Vbulk

1PA

1:691: 8

Kais the air permeability in mD.PAis in psia.Vbulkis the bulk pow-der volume.

2.5. Evaluation of the inhalation mixtures

2.5.1. Assay and content uniformity

We randomly took one 100 mg and six 25 mg samples of eachmixture and dissolved the samples in 50% w/w ethanol in deion-

ized water. We determined the fluticasone propionate concentra-tions in these solutions using the spectrophotometric methoddescribed in Section2.6.

2.5.2. Aerodynamic evaluation (in vitro deposition)

We filled the inhalation mixtures into size 3 hard gelatin cap-

sules (Capsugel, France). Each capsule contained 25 2 mg inhala-tion mixture, corresponding to 250 lg fluticasone propionate. We

stored the capsules at 20 C and 45% RH for at least 3 days to allowfor mechanical relaxation and electrostatic charge dissipation.

We conducted the aerodynamic evaluation (in vitro deposition)

using a Next Generation Impactor (Copley Scientific, United King-dom). The United States Pharmacopeia[19]suggests using a flowrate which produces a pressure drop of 4 kPa (40.8 cm H2O) over

the inhalation device for a duration which draws 4 L of air fromthe mouthpiece of the inhaler. If the flow rate required to produce4 kPa pressure drop over the inhalation device is greater than100 L/min, a flow rate of 100 L/min should be used. The Aerolizerhas a specific resistance of 0.055 cm H2O

0.5 min/L[20]. A flow rate

of 116 L/min is accordingly required to produce the 4 kPa pressuredrop over the Aerolizer. However, we chose to adjust the flowrate to 60 L/min for two reasons. First, a flow rate of 100 L/min isnot attainable by all asthma and chronic obstructive pulmonary

disease patients. Bronsky et al.[21]assessed the peak inspiratoryflow rate through the Aerolizer in asthma patients. 27% and 9%of the adult patients had peak inspiratory flow rates


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cumulative (undersize) aerodynamic particle size distribution bymass. The geometric standard deviation (GSD) was calculated as

the square root of the ratio of the 84.13th percentile to the15.87th percentile of the distribution.

2.6. Analytical method for quantification of fluticasone propionate

We determined the concentrations of fluticasone propionate inthe assay, the content uniformity, and the in vitro depositionsamples spectrophotometrically using a Shimadzu UV-2450

double-beam UVVis spectrophotometer, equipped with ShimadzuUVProbe version 2.10 software (Shimadzu Corporation, Japan). Theanalytical signal was the UV absorption spectrum first-orderderivative at k= 252 nm. The first-order derivative was used to

eliminate interference from some carriers. The method wasvalidated for linearity, accuracy, precision, specificity, limit ofdetection, and limit of quantification. The method was linear overthe concentration range of 230 lg/mL withR2 = 0.9993. The limitof detection was 0.37 lg/mL and the limit of quantification was1.11 lg/mL.

2.7. Data analysis

We used OriginPro 2015 (OriginLab Corporation, USA) andMathcad version 15.0 (Parametric Technology Corporation, USA)

for mathematical data analysis.

3. Results

3.1. Characterization of the carriers

3.1.1. Crystallinity

The differential scanning calorimetry (DSC) measurements

(Fig. 1) suggest that CD was amorphous while all the other carriers

were crystalline.Fig.1provides more details.

3.1.2. Shape analysis

Optical micrographs of the carriers are shown in Fig. 2. Thecalculated shape parameters of the carriers are listed in Table 1.The carriers were slightly elongated, with aspect (elongation)ratios, RA, between 1.45 and 1.96. They had similarly irregular

shapes, with circularities, C, between 0.54 and 0.64 (a sphericalparticle has a circularity of 1, cf. Eq.(2)). LM particles had the char-acteristic tomahawk, prismatic, or pyramidal shape. MN particleswere the most elongated and had characteristic surface corruga-

tions. LA particles exhibited remarkable surface roughness, whichresulted from adherence of fine carrier particles (cf. Section 3.1.3andTable 2). On the other side, XL particle surfaces appeared very

smooth, reflecting its low fine particle content.

3.1.3. Particle size distribution

The particle size distributions of the carriers and the drug are

shown inFig. 2andTable 2. Of the size distribution descriptors

Fig. 1. The differential scanning calorimetry thermograms of the carriers. LW refers to loss of water/dehydration. M refers to melting. DC refers to decomposition. The

temperature provided is endothermtemperature range, temperature at peak maximum (denoted by P), or temperature at endotherm onset (denotedby O). CD decomposition

started at 306 C (endotherm not shown). Sealed aluminum pans were used. For DM, sealing avoids escape of water of crystallization and transformation of dextrose

monohydrate to dextrose anhydrous during the measurement. The sharp melting peak at 82 C rather than a broad water evaporationpeak confirms water escapeduring the

measurement was avoided. DM thusapparently comprised a mixture of dextrose monohydrateand dextrose anhydrous crystals.The heat flowscales of the MN and XL panels

are different from those of the other panels.

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tabulated, we find the mode diameter and the content of fine par-

ticles withD< 10lm most useful for dry powder inhalation carri-ers. Coarse and fine carrier components contribute to theaerodynamic performance of an adhesive inhalation mixture viadifferent mechanisms. The use of the mode diameter and the con-

tent of fine particles thus help distinguish between these contribu-tions. The mode diameter reflects the size of the main, coarse

carrier particles and is not affected by the content of theperformance-modulating, fine carrier particles. In contrast, the

median and the mean diameters provide an averaged overview.All the carriers except MN were similar in size, with mode diame-ters,DV,Mode, in the range of 74.7079.39 lm. MN was smaller withmode diameter of 59.26 lm. The carriers differed in their contentsof fine particles withD< 10lm. The contents of fine particles ran-ged from 0.31% for XL to 11.22% for LA. Fluticasone propionate hada median diameter, DV,50, of 4.49 lm. We also estimated the aero-dynamic particle size distribution of fluticasone propionate from

the laser-diffraction measurements as described in Table 2. Thisprovides just guidance and roughly highlights the theoretical max-imum fine particle fraction. Accordingly, fluticasone propionatehad a median aerodynamic diameter of 5.27 lm, a fine particle

fraction (FPF8.06) of 70.56%, and a respirable particle fraction(RPF4.46) of 40.95% (by volume).

3.1.4. Pore size distribution, surface roughness, and permeability

The pore size distributions of the carriers are provided inFig. 3andTable 3. We classified pores into three classes. The first classincludes pores with diameters smaller than 1.00 lm, i.e. poressmaller than drug particles. The cumulative pore volume of this

class will be referred to as nanoporosity. The second class includespores with diameters between 1.00lm and 8.06 lm, i.e. pores

similar in size to micronized drug particles. The cumulative porevolume of this class will be referred to as microporosity. The third

class includes pores with diameters larger than 8.06 lm, i.e. poreslarger than drug particles. The cumulative pore volume of this classwill be referred to as macroporosity. Table 3 also provides otherparameters which we derived from the mercury intrusion

porosimetry data. These are the porosity, the specific surface area,the surface roughness, and the air permeability.

3.2. Evaluation of the inhalation mixtures

3.2.1. Assay and content uniformity

The nominal drug content was 250 lg per 25 mg inhalationmixture or per capsule. The inhalation mixtures exhibited mean

drug recoveries between 96.8% and 108.3% and satisfactory unifor-mities with RSD6 5.2%.

Fig. 2. The particle size distributions and optical micrographs of the carriers.



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3.2.2. Aerodynamic evaluation (in vitro deposition)

The aerodynamic performances of the tested mixtures are

described in Table 4. The different tested carries led to similar drug

emission. The emitted fraction ranged between 91% and 94% of the

recovered dose. Drug dispersibility, however, considerably varied

among the different inhalation mixtures. The fine particle fraction,

Fig. 3. The pore size distributions of the carriers. The vertical dashed lines refer to diameters of 1.00 and 8.06 lm; they serve as borderlines between the three pore sizeclasses.

Table 3

Microstructural characteristics of the carriers.

Carrier Cumulative pore volume [mL g1]a Porosity [%V] Specific surfacearea [m2 g1]b

S. roughness Air permeability [mD]

VTotal VD=0.0071lm VD=18.06lm VD>8.06lm SSAPSD SSAMIP

CD 1.1934 0.2057 0.0533 0.9167 64.65 0.082 56.585 690.061 6796.6

DA 0.7423 0.0879 0.0701 0.5738 50.71 0.088 29.894 339.705 2113.8

DM 1.0257 0.0999 0.0339 0.8812 61.45 0.091 30.551 335.725 5551.0

LA 0.7008 0.0931 0.1603 0.4415 50.88 0.392 18.809 47.982 975.6

LM 0.6307 0.0615 0.1033 0.4583 49.69 0.149 21.166 142.054 1636.3

MN 0.8901 0.0933 0.0685 0.7151 56.02 0.132 33.017 250.129 2792.2

XL 0.8453 0.0767 0.0138 0.7461 58.74 0.059 26.468 448.610 8932.9

SU 0.7619 0.0845 0.0519 0.6166 56.42 0.105 27.720 264.000 3643.3

a In addition to thetotal pore volume, VTotal, we classified pores into three classes. The first class includes poreswith diameters smaller than 1.00 lm, i.e. poressmallerthandrug particles. This class will be referred to as nanoporosity. The second class includes pores with diameters between 1.00 lm and 8.06 lm, i.e. pore similar in size tomicronized drugparticles. Thisclass will be referred to as microporosity. The thirdclass includes poreswith diameterslarger than8.06 lm, i.e. pore largerthan drug particles.This class will be referred to as macroporosity.

b SSAPSDis the specific surface area estimated from the particle size distribution assuming spherical particle shape. SSAMIP is the specific surface area estimated from the

mercury intrusion porosimetry data.

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FPF8.06,ED, ranged between 4.37% and 21.23% and the respirableparticle fraction, RPF4.46, ED, ranged between 2.78% and 16.00% ofthe emitted dose.

4. Discussion

4.1. Microstructural determinants of the performance of carrier-based

inhalation mixtures

4.1.1. The carrier porosity

Fig. 4 shows the relationship between the carrier total porevolume (upper panel) or the carrier surface roughness (lowerpanel, Eq.(4)) and the performance of the carrier-based inhalation

mixture. The performance is expressed in terms of FPF8.06, ED, thefraction of fine particles with aerodynamic diameter smaller than8.06 lm. Presuming that the carrier chemical nature and crys-tallinity do not contribute to the performance, the data presented

in Fig. 4 suggest there is no simple relationship between the carrier

total porosity and the inhalation mixture performance.Fig. 5shows the relationship between the carrier pore volume

classified according to the pore size relative to the size of micro-

nized drug particles and the performance of the carrier-basedinhalation mixture. Nanopores, i.e. pores smaller than drug parti-cles, reduce the carrier effective contact area and increase the dis-tance between drug and carrier particles. Nanopores thus reduce

the magnitude of interparticulate adhesion forces in an inhalationmixture. The upper panel ofFig. 5 shows that all the carriers inves-tigated in the current study except CD had similar nanoporositiesin the range of 0.07670.0999 mL g1. The nanoporosity of CD

(0.2057 mL g1) was 23-fold higher than those of the othercarriers, probably due to its amorphous nature. The upper panelofFig. 5does not suggest an exceptional performance of CD. This

will, however, be reconsidered later.The middle panel ofFig. 5shows the relationship between the

carrier microporosity, i.e. pores similar in size to micronized drugparticles, and the inhalation mixture aerodynamic performance.

The FPF8.06,ED increased linearly with the cumulative volume ofmicropores up to a cumulative volume of about 0.10 mL/g. Thiscorresponds to approximately 14 times the drug particle volumeper 1 g carrier in the inhalation mixtures. The FPF8.06, EDthereafter

almost remained constant. A positive contribution of the carriermicropores to the performance is not expected to take place duringaerosolization. In contrast, micropores are therein expected toincrease the drug-carrier effective contact area and to provide shel-

ter for drug particles from drag separation forces. The positive con-tribution of the micropores to the performance thus most probably

took place during mixing. Micropores, with similar diameters tomicronized drug particles, can therein increase the effectiveness

of frictional andpress-on forces, which are responsible for breakingup (de-agglomeration) of cohesive drug agglomerates and for dis-

tribution of drug particles over the carrier surface. The perfor-mance of the MN-based mixture was lower than that expectedfrom the microporosity-performance relationship suggested bythe data of the other carriers. MN was different in size and shape

from the other carriers. The mode diameter, representing the sizeof the coarse carrier component, was smaller for MN than for the

other carriers (cf.Table 2andFig. 2). MN was also the most elon-gated with characteristic surface corrugations (cf. Table 1 and

Table 4

The aerodynamic performances of the inhalation mixtures. a

Carrier Recovery, RCD [lg] Retention, RTF [%] Emission, EF [%] FPF8.06,ED[%] RPF4.46,ED[%] MMAD [lm] GSD [lm]

CD 226.92 7.37 8.94 2.35 91.06 2.35 13.80 0.88 10.85 0.62 3.22 0.21 2.88 0.15

DA 208.23 0.65 6.25 0.90 93.75 0.90 18.45 0.69 10.82 0.83 5.23 0.39 3.14 0.11DM 230.52 4.32 5.81 0.23 94.19 0.23 8.61 0.64 6.20 0.29 4.39 0.31 3.27 0.32

LA 220.18 3.05 7.08 0.85 92.92 0.85 20.12 1.89 13.46 1.76 4.18 0.28 2.81 0.09

LM 230.93 22.53 8.59 0.22 91.41 0.22 21.23 2.06 16.00 1.83 3.50 0.20 2.65 0.02

MN 209.30 3.04 6.45 1.23 93.55 1.23 10.41 0.17 6.38 0.17 5.41 0.19 3.33 0.05XL 229.52 12.09 8.37 1.92 91.63 1.92 4.37 0.26 2.78 0.19 5.59 0.32 3.29 0.28

SU 221.01 3.89 7.47 1.99 92.53 1.99 12.97 0.18 8.60 0.30 4.53 0.64 2.67 0.10

a RCD is the recovered dose,i.e. thetotaldrug amount collectedfrom the capsule shell, the inhalation device, the induction port, themouth piece adapter, thepreseparator,

andall thestages of the impactor. RTF is theretainedfraction, i.e. theratioof theamount of drug retained in thecapsule shell andthe inhalation deviceto therecovered dose.

EF is theemittedfraction, i.e. theratioof theamount of drug emitted from the device(i.e. collectedfromthe induction port, themouth piece adapter, thepreseparator, andall

the stages of the impactor) to the recovered dose. The fine particle fraction, FPF 8.06,ED, is the ratio of the amount of drug with aerodynamic diameter smaller than 8.06 lm tothe emitted dose. The respirable particle fraction, RPF4.46,ED, is the ratio of the amount of drug with aerodynamic diameter smaller than 4.46 lm to the emitted dose. Fivecapsules were used in each experiment. Experiments were conducted in triplicates, i.e. N=3, except for SU where N=2.

Fig. 4. The relationship between the carrier total porosity (upper panel) or the

carrier surface roughness (lower panel) and the performance of the carrier-based

inhalation mixture. The performance is expressed in terms of the fine particle

fraction, FPF8.06,ED, defined as the ratio of the amount of drug with aerodynamic

diameter smaller than 8.06lm to the emitted dose. Linear regression analysis leadstoR2 = 0.2728 for the FPF8.06,ED vs. the total pore volume data (upper panel) and

R2 = 0.2398 for the FPF8.06,EDvs. the surface roughness data (lower panel). The dataof MN and CD, which exhibited exceptional performances, are presented as open

circles.



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Fig. 2). These two differences are probably responsible for theexceptional performance of MN.

The relationship between the carrier macroporosity, i.e. poreslarger than drug particles, and the inhalation mixture aerodynamic

performance (the lower panel ofFig. 5) is similar to the relation-ship between the carrier total pore volume and the performance(Fig. 4). This is expected since macropores constituted 6090% ofthe total pore volume. It is noteworthy that the macroporosity here

(for the particle size of the studied carriers) includes most of theinterparticulate spaces. The carrier macroporosity apparently has

a negative influence on the performance. Two mechanisms canexplain this negative influence. First, macropores provide shelter

for drug particles from drag and lift separation forces during

aerosolization. Second, the carrier macroporosity influences itstensile strength, cohesivity, and fluidization behavior. These prop-erties are well represented by the powder air permeability. This isdiscussed in the following section. Remarkably, the performance of

the CD-based mixture was higher than that expected from themacroporosity-performance relationship suggested by the data ofthe other carriers. Although CD had the largest macroporosity,

the CD-based inhalation mixture exhibited intermediate perfor-mance (cf.Fig. 5lower panel). CD is here the only amorphous car-rier and it had the highest nanoporosity (cf. Fig. 5 upper panel).Amorphous domains are associated with high surface energy and

high interparticulate adhesion forces. This cannot explain thebetter-than-expected performance of CD. The nanoporosity is thuslikely responsible for this exceptional performance.Fig. 6providesa schematic illustration of the effects of the carrier porosity on the

performance of carrier-based inhalation mixtures.

4.1.2. The carrier air permeability

We also derived the carrier air permeability from the mercuryintrusion porosimetry data. The differences in the air permeability,

which reflects powder bed tensile strength, cohesivity, and

fluidization behavior, were not associated with differences in thedrug emission from the inhalation device (cf. Table 4). However, arelationship between the air permeability and the drug dispersion

was observed. As shown in Fig. 7, the performance of the inhalationmixture improved as the carrier air permeability decreased, i.e.resistance to air flow increased. Fluidization does not take placeuntil air flow provided by the patient reaches a minimum threshold

velocity. This minimum threshold velocity increases with the resis-tance to air flow. Increasedresistance to airflow thus leads to stron-ger aerodynamic (drag and impaction) dispersion forces within drypowder inhalation devices during aerosolization [11,23,24]. The

effect in a dry powder inhalation device may nevertheless be smal-ler than that expected from the air permeability measurementssince the amount of powder material in an inhalation dose is very

small as compared with a powder bed tested in a permeabilitymeasurement. For capsule-based inhalers, one should also considerthat a very cohesive, very poorly flowing inhalation powder may beretained in the capsule[25]. Similar inverse relationships between

the air permeability, measured by Blaines apparatus or a powderrheometer, and the aerodynamic performance of inhalationmixtures have been early reported [11,2224]. This suggests thatthe air permeability derived from mercury intrusion porosimetry

data and that measured by Blaines apparatus or a powderrheometer provide similar information about fluidization of drypowder inhaler formulations. Although the air permeability isderived mainly from the macroporosity data, it better correlates

with the performance (cf.Fig. 5lower panel andFig. 7).Again, MN and CD exhibited exceptional performances. The per-

formance of the MN-based mixture was lower and the perfor-mance of the CD-based mixture was higher than that expected

Fig. 5. The relationship between the carrier porosity and the performance of the

carrier-based inhalation mixture. The performance is expressed in terms of the fine

particle fraction, FPF8.06,ED, defined as is the ratio of the amount of drug with

aerodynamic diameter smaller than 8.06 lm to the emitted dose. Pores are hereclassified according to their sizes into three classes. The effect of each class is

separately presented. The data of MN and CD, which exhibited exceptional

performances, are presented as open circles. The figure demonstrates there is arelationship between the carrier microporosity and the performance. Nonlinear

fitting of the FPF8.06,ED vs. the cumulative micropore volume data, excluding MN

data, to the sigmoidal Boltzmann function y= Lf+ [(Li Lf)/(1+ exp (k(xx0)))]gave Lf= 20.8542,Li= 3.2333,k= 73.3502, andx0= 0.0474 withadjusted R

2 = 0.984;

this is represented by the dashed line. Linear regression analysis of the FPF8.06,EDvs.

the cumulative micropore volume data for cumulative micropore volume smallerthan 0.1 mL/g and excluding the MN data led to R2 = 0.9935.

Fig. 6. A schematic illustration of the effects of the carrier porosity on theperformance of carrier-based inhalation mixtures. Light-grey particles represent

carrier particles. Dark-grey particles represent drug particles.

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from the air permeability-performance relationship suggested bythe data of the other carriers. As discussed earlier, the inferiorperformance of the MN-based mixture is probably due to the smal-

ler size and/or elongated shape of MN, as compared to the othercarriers. The superior performance of the CD-basedmixture is mostlikely due to its high nanoporosity.

4.2. Effects of the carrier chemical/crystalline structure

Lactose monohydrate is the most commonly used dry powderinhalation carrier. Its incompatibility with some drugs, such asformoterol[26]and peptides, raised the need for investigation ofother materials as dry powder inhalation carriers. Alternative

materials, such as mannitol, sorbitol, xylitol, and trehalose, have

been earlier investigated as dry powder inhalation carriers[27,28]. We herein tested eight carrier materials. The correlationspresented inFigs. 5and7 suggest the chemical composition and

the crystalline/polymorphic form of the carriers played a minorrole, if any, in the tested inhalation mixtures.

4.3. The role of fine excipient materials in carrier-based dry powder

inhalation mixtures

Addition of a fine excipient material to a coarse carrier has a

well-recognized, positive influence on the aerodynamic perfor-

mance of carrier-based inhalation mixtures [3,5,2938]. The drugfine particle fraction (FPF) increases with the concentration of fineexcipient material up to a certain threshold value. With further

increase in the concentration of fine excipient material, the FPFremains constant or decreases [29,31,34,35,38]. In the currentstudy we followed a different approach to investigate the effectof fine particle content on the aerodynamic performance ofcarrier-based inhalation mixtures. Instead of adding different con-

centrations of fine excipient material to a carrier, we tested severalcarrier materials with different fine particle content. Fig. 8showsthat there is no simple relationship between the content of fineparticles and the aerodynamic performance. The same conclusion

was reached by Pitchayajittipong et al. [24], who similarly testedthe functionality of anhydrous lactose and lactose monohydrategrades as dry powder inhalation carriers. This suggests the magni-

tude of the effect of fine particle content is dependent on other car-rier properties. Different mechanisms of action for the influence offine excipient material on the aerodynamic performance have beenproposed. Among these mechanisms, the active site theory, the

agglomerate theory, and the fluidization enforcement mechanismhave been most considered.

The active site theory suggests fine excipient particles compet-

itively bind to high energy or active sites on coarse carrier parti-cles, thereby leaving passive (low adhesion) sites for drug-carrieradhesion. Several studies tested this hypothesis [2931,33]. Theresults were contradictory and could not provide a consistent sup-port to the active site theory. In agreement with the active site the-

ory, Zeng et al. [33] has shown that a mixture prepared by blendingof a coarse carrier with a fine excipient material prior to drug addi-tion is superior to a mixture prepared by blending of a coarse car-rier with drug prior to fine excipient material addition. Several

other studies have shown, however, that improvement of the aero-dynamic performance by the addition of a fine excipient material isindependent of the blending order [29,31] and that effects ofblending order, if any, diminish as blending time is increased

[30]. One should note that a mixing order effect does not form con-

clusive evidence in favor of the active sites theory; other proposedmechanisms of action reasonably account for mixing order effects[39]. Measurements of the effect of fine excipient material addition

on carrier surface adhesion forces were also inconsistent, withsome suggesting it decreases [5,35] and others suggesting itincreases drug-carrier adhesion forces[12,29].

The agglomerate theory suggests the drug particles redistribute

between coarse carrier surfaces and fine excipient particlesduring blending. This produces drug-fine excipient mixedagglomerates [31,32,36,37], from which drug is more easilydetached than from coarse carrier surfaces [36,37]. Agglomerates

are subject to stronger drag/deagglomeration forces duringaerosolization[40].

The Fluidization enforcement mechanism is a new term

which we herein introduce for the effect of fine particles on pow-der bed fluidization. Accordingly, fine particles increase cohesiveinter-particulate forces within a powder bed, thereby increasingits tensile strength. This affects powder bed fluidization and

aerosolization behavior and leads to higher aerodynamic (dragand impaction) dispersion forces within inhalation devices (cf. Sec-tion4.1.2)[22,23,41].

Other mechanisms have been also speculated. Fine excipient

particles may not only compete with drug particles but also actas carrier particles, thereby reducing the payload of coarse carrierparticles[9]. Fine excipient particles slightly larger than drug par-ticles buffer press-on forces during mixing [9,39]. Fine excipient

materials affect the performance of carrier-based inhalation mix-tures likely via a combination of different mechanisms. The bal-

ance between the different mechanisms is controlled by othervariables. We herein propose a new mechanism which contributes

Fig. 7. The relationship between the carrier air permeability, derived from the

mercury intrusion porosimetry measurements, and the performance of the carrier-

based inhalation mixture. The performance is expressed in terms of the fine particlefraction, FPF8.06,ED, defined as is the ratio of the amount of drug with aerodynamic

diameter smaller than 8.06lm to the emitted dose. The data of MN and CD, whichexhibitedexceptional performances, are presented as opencircles. Linear regression

analysis of the FPF8.06,EDvs. the carrier air permeability data excluding CD and MN

data led toR2 = 0.9435. This is represented by the dashed line.

Fig. 8. The relationship between the carrier fine (D< 10lm) particle content andthe performance of the carrier-based inhalation mixture. The performance is

expressed in terms of the fine particle fraction, FPF8.06,ED, defined as is the ratio of

the amount of drug with aerodynamic diameter smaller than 8.06lm to theemitted dose.



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to, and may dominate, the overall effect. Our data also support thefluidization enforcement mechanism.

4.3.1. Effect of fine excipient materials on the carrier porosity

Fig. 9explores the effects of the carrier fine particle content onthe porosity. Carrier fines clearly did not considerably contribute to

the nanoporosity or the macroporosity. However, the increaseof the fine particle content above 3% (volume) was associated withlinear increase of the microporosity. CD and DA had highermicroporosities than that expected from this linear relationship.

They apparently had higher intrinsic microporosities than the

other carriers, i.e. the coarse carrier particles originally had highermicroporosity. This finding proposes a new mechanism for thepositive effect of fine excipient materials on the performance. Fine

excipient materials apparently adhere to the surface of coarse car-rier particles creating projections and micropores, which increasethe effectiveness of mixing (c.f. Section4.1.1). Excipient fines wereearlier observed to increase carrier surface roughness[5,7,10]; the

size and the role of the created surface pores/irregularities were,however, not earlier resolved.

4.3.2. Effect of fine excipient materials on the carrier air permeability

Increase of the carrier fine particle content was associated with

decrease of the carrier air permeability, estimated from themercury intrusion porosimetry data (Fig. 10). This decrease of air

permeability, i.e. increase of resistance to air flow, results fromfilling of inter-coarse-particle spaces by fine particles. The

dependence of the permeability on the packing properties of apowder bed is thoroughly described by Carman [42].Appendix A

proposes a simple phenomenological model to describe the effectof the fine particle content on the air permeability. For the carrierswe tested, the powder air permeability drops in a sigmoidal orquasi-exponential fashion as the fine particle content increases.

One can speculate that small quantities of fine particles adhereonto the surfaces of the main, coarse carrier particles and littleaffect the powder pore structure and resistance to air flow. Above

certain fine particle content threshold, fine particles progressivelyfill the inter-coarse-particle spaces and decrease the powder airpermeability. Our data thus support the fluidization enforcementmechanism (cf. Section4.3). In the dry powder inhalation field,

Blaines apparatus and the Freeman FT4 powder rheometer areoften used for the measurement of the powder air permeability.Such measurements [22,23,34] produced similar observations tothat shown inFig. 10.This suggests estimation of the air perme-

ability from mercury intrusion porosimetry data using the proce-dure described in Section 2.4.5 is similarly useful to thesetechniques.

5. Conclusions

We explored the relationships between the carrier microstruc-

tural properties and the performance of carrier-based inhalationmixtures. Our results suggest that the microporosity and the air

permeability are key performance determinants of dry powderinhalation carriers. The nanoporosity is of importance for amor-

phous carriers. The current study, however, could not providedetailed information about the relative contribution of each ofthese characteristics to the performance since they varied simulta-neously. This is the subject of an ongoing study. Mercury intrusion

porosimetry is a valuable tool in the field of dry powder inhalation.It successfully resolves carrier pores which differently contributeto the performance. The ability to derive powder air permeabilityfrom mercury intrusion porosimetry data is an additional

advantage.

Acknowledgments

The study was funded by a Global Fellowship Award from theUnited States Pharmacopeial Convention (USP). The authors would

Fig. 9. The relationship between the carrier fine (D< 10lm) particle content andthe carrier porosity. Pores are classified, according to their sizes, into three classes.

Linear regression analysis of the cumulative micropore volume vs. the carrier fineparticle content data for fine particle content larger than 3.5% led to R2 = 0.9951.

Fig. 10. The relationship between the carrier fine (D< 10lm) particle content andthe carrier air permeability, derived from the mercury intrusion porosimetry data.Nonlinear fitting of the data, excluding the outlying DA data, to the sigmoidal

Boltzmann function y= Lf+ [(Li Lf)/(1+ exp (k(xx0)))] gave Lf= 1.2433,

Li= 9.2950,k= 0.9064, andx0= 3.8355 with adjustedR2 = 0.993; this is represented

by the dashed line. Nonlinear fitting of the data, excluding the outlying DA data, to

the exponential function y = a bx gave a = 10.0187 and b = 0.8249 with adjusted

R2 = 0.941; this is represented by the dotted line.

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like to acknowledge the support of Prof. Dr. Nawal M. Khalafallah,Department of Pharmaceutics, Alexandria University, Alexandria,Egypt.

Appendix A. A phenomenological model of the effect of the fine

particle content on the powder air permeability

In a phenomenological approach, we attempted to describe thepowder air permeability vs. the fine particle content data by the

exponential functiony= a bx. The intercept represents the powderair permeability at zero fine particle content, i.e. for powdercomprising only coarse carrier particles. The base represents themagnitude of the effect of the fine particle content on the powder

air permeability. The intercept, a, and the base, b, mainly reflect thepacking properties of the coarse particles. As the particle sizeincreases or the particle shape becomes more regular and less

interlocking, the inter-particle spaces enlarge, the intercept, a,increases, and the base, b, decreases. Fitting our data (Fig. 10),excluding the outlying DA data, to the exponential function gavea= 10.0181 and b= 0.8249 with adjusted R2 = 0.941. Cordts and

Steckel [34] measured the air permeabilities of binary blends ofthe coarse lactose grade Respitose SV003 (D50= 56.76lm;DMV-Fonterra, Vehgel, The Netherlands) and increased amountsof the fine lactose grade Lactohale LH300 (D50= 3.25lm;Friesland Foods Domo, Zwolle, The Netherlands) by a FreemanFT4 powder rheometer. Fitting the data reported by Cordts andSteckel[34], assuming that the coarse lactose component Respi-tose SV003 (D10= 14.13lm) originally had fine particle contentof 7.0%, gave a= 5.2559 andb= 0.9582 with adjustedR2 = 0.965.The intercept, a, is smaller and the base, b, is larger than thosederived from our data. This well reflects the different sizes of thecoarse carrier particles. The coarse lactose grade which Cordts and

Steckel used (Respitose SV003) had D90= 93.04lm. This issmaller than the D90 values of all the carriers herein tested(cf.Table 2). The phenomenological model lends itself for furtherdevelopment and use.

Appendix B. Effect of coating of the collection surfaces of the

Next Generation Impactor stages on the in vitro deposition of

fluticasone propionate from the test inhalation mixtures

In preliminary in vitro deposition experiments, we coated the

collection surfaces of the stages of the Next Generation Impactorbefore each experiment with 1% v/v glycerol solution in methanol.

Coating may be necessary to minimize particle bounce and re-entrainment. The experiments were otherwise conducted as

described in Section2.5.2. We confirmed glycerol does not inter-fere with the analytical method. Table 5 shows the results fortwo selected inhalation mixtures. It demonstrates coating did not

affect the results.

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[4] K. Iida, Y. Inagaki, H. Todo, H. Okamoto, K. Danjo, H. Luenberger, Effects ofsurface processing of lactose carrier particles on dry powder inhalationproperties of salbutamol sulfate, Chem. Pharm. Bull. 52 (2004) 938942,http://dx.doi.org/10.1248/cpb.52.938 .

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Table 5

Effect of coating of the collection surfaces of the Next Generation Impactor stages on the in vitro deposition of fluticasone propionate from two inhalation mixtures.

Stage Amount collected [lg/capsule]

Inhalation mixture 1 Inhalation Mixture 2

With coating No coating p-valuea With coating No coating p-valuea

Capsules 7.44 0.34 7.84 1.65 0.720 9.36 0.85 10.10 3.16 0.728

Device 7.64 0.10 7.77 0.44 0.654 8.14 1.26 9.08 2.23 0.567

IP+ MPAb 44.61 0.56 45.54 3.07 0.653 30.14 5.63 29.69 11.33 0.955Preseperator 105.30 0.93 105.61 1.98 0.826 166.44 0.47 166.13 1.42 0.746

Stage 1 11.96 0.16 12.28 0.09 0.049 5.35 0.08 5.34 0.23 0.913Stage 2 13.49 0.04 13.62 0.22 0.408 3.30 0.08 3.32 0.14 0.846

Stage 3 10.75 0.49 11.11 0.87 0.573 1.72 0.06 1.77 0.10 0.549

Stage 4 8.34 0.56 8.29 1.18 0.951 1.51 0.07 1.52 0.14 0.947

Stages 58 8.55 1.48 8.13 1.43 0.744 2.79 0.33 2.57 0.75 0.680

a Data were compared using unpaired, two-tailed Studentst-test. Thep-values suggest coating did not significantly affect the in vitro deposition of fluticasone propionate

from the two mixtures.b IP is the induction port. MPA is the mouth piece adapter.

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