Spray freeze drying to produce a stable Δ9-tetrahydrocannabinol containing inulin-based solid dispersion powder suitable for inhalation

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Chapter 7 Spray freeze drying to produce a stable ∆9-tetrahydrocannabinol containing inulin-based solid dispersion powder suitable for inhalation Dirk Jan van Drooge, Wouter L.J. Hinrichs, Bastiaan H.J. Dickhoff, Marco N.A. Elli, Marinella R.Visser, Gerrit S. Zijlstra and Henderik W. Frijlink Department of Pharmaceutical Technology and Biopharmacy, Groningen University Institute for Drug Exploration (GUIDE), Groningen, The Netherlands Published in Eur. J. of Pharm. Sci., 2005, 26(2), 231-240 Keywords: spray freeze drying, solid dispersion, sugar glass, stabilization, inhalation, ∆9-tetrahydrocannabinol

Chapter 7 Spray freeze drying to produce inhalable THC solid disperion powder

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7.1. Abstract The purpose of this study is to investigate whether spray freeze drying produces an inhalable solid dispersion powder in which ∆9-tetrahydrocannabinol (THC) is stabilised. Solutions of THC and inulin in a mixture of tertiary butanol (TBA) and water were spray freeze dried. Drug loads varied from 4 to 30 wt-%. Various powder characteristics of the materials were determined. Stability of THC was determined and compared with freeze dried material. The powders, dispersed with an inhaler based on air classifier technology, were subjected to laser diffraction analysis and cascade impactor analysis. Highly porous particles having large specific surface areas (about 90 m2/g) were produced. At high drug loads, THC was more effectively stabilised by spray freeze drying than by freeze drying. Higher cooling rates during spray freeze drying result in improved incorporation. Fine particle fractions of up to 50% were generated indicating suitability for inhalation. It was concluded that spray freeze drying from a water-TBA mixture is a suitable process to include lipophilic drugs (THC) in inulin glass matrices. High cooling rates during the freezing process result in effective stabilization of THC. The powders can be dispersed into aerosols with a particle size appropriate for inhalation.

7.2. Introduction ∆9-tetrahydrocannabinol (THC, Dronabinol) is the main pharmacologically active constituent of the cannabis sativa. THC can be used for the treatment of various diseases [22]. Although so far the only registered indications are the relief of chemotherapy-related nausea and vomiting and the enhancement of appetite [192]. THC is also found to have analgesic, anti-inflammatory, anxiolytic, broncho-dilative, hypotensive, spasmolytic and intraocular pressure reducing activity [22, 192-194]. In spite of the promising pharmacological profile and the increasing clinical interest in THC, no suitable dosage forms have been marketed so far. This is due several problems that can be encountered during THC dosage form development. First of all, THC is a very sticky resin, which makes it difficult to process. Secondly, THC is a labile molecule; it is readily oxidised upon contact with air and degrades in aqueous solutions especially at low pH [196-198]. Thirdly, THC is poorly soluble in aqueous solutions (only 0.77 mg/l in 0.15 M NaCl), hence bioavailability is dissolution rate limited after oral absorption [200]. Despite these problems, few attempts have been made so far. The soft gelatine capsule, marketed in the USA as Marinol®, contains sesame oil in which THC is dissolved. However, this capsule has a limited shelf life and shows a low and irregular bioavailability [23, 202, 203]. Poor solubility in combination with

Spray freeze drying to produce inhalable THC solid disperion powder Chapter 7

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extensive first pass metabolism explains why the bioavailability of THC after oral administration of the soft gelatine capsule is only 6-20% [22, 23, 203, 204] Moreover, the maximum effect is produced after 1 to 6 hours [22, 23]. To produce a rapid onset of action and to circumvent first pass metabolism a sublingual tablet was developed [175]. However, no in vivo data are available yet. Rectal administration can be a suitable alternative as well since it partially circumvents first pass metabolism. However, the results are ambiguous: Mattes et al. [203] reported a 30-fold increase in bioavailability compared to Marinol® in three females. Using the pro-drug THC-hemisuccinate ester resulted in a bioavailability of 13.5% [205] or even 67% [206], whereas in some cases THC was not absorbed at all [202]. Pulmonary administration circumvents first pass metabolism and THC is directly absorbed into the systemic blood circulation. Furthermore, due to a large alveolar surface and the thin epithelium, THC is rapidly absorbed. In many studies, pulmonary THC administration was investigated by smoking a cigarette containing the raw plant material [22, 23, 153, 204, 207, 208], resulting in a bioavailability of 10-35% related to the amount of THC released from the cigarette. In some studies a large difference was discerned between experienced and non experienced users [207, 208]. However, smoking requires elevated temperatures to vaporise and extract the THC. This extraction is far from complete: only 68% of the initial THC is released by smoking [204]. Furthermore, it is assumed that about 30% of the released THC is destroyed by pyrolysis and in fact only about 16 to 19% enters the main stream smoke [209]. In these studies the but was analysed and the bioavailability was related to the amount of THC that actually was released from the cigarette. However, to calculate the real bioavailability, the absorbed amount of THC should be related to the dose originally present in the cigarette before lighting and is in fact lower. More recently, an aqueous THC inhalation solution containing various solubilisers was tested in vivo [210]. A bioavailability, again related to the amount released from the dosage form, of 28.5% (0.4-60.6%) was found. However, the inhalation procedure took a long time (20-25min) and a lot of irritation in the throat and upper respiratory tract was reported, most likely caused by the presence of surfactants in the solution. Therefore, in this study a surfactant free dry powder is developed for THC inhalation. It is known that dry powder inhalation involves a procedure of only a few seconds and results in reproducible and high bioavailability [211, 212]. In previous papers [174, 175] a platform technology was presented that enabled the incorporation of lipophilic drugs in stable sugar based solid dispersions by lyophilization. The oligo-fructose inulin was shown to be an excellent stabilising carrier due to its high glass transition temperature [174]. THC was dissolved in tertiary butyl alcohol (TBA) and inulin was dissolved in water. The mixture of these solutions was lyophilised, yielding a dry product. It was shown that incorporation in inulin glasses prevented degradation of THC [175]. This study had a two-fold aim. Firstly, we investigated whether spray freeze drying is a suitable process to produce a solid dispersion in which THC is stabilised. Spray freeze drying is chosen, since it combines several advantages:


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1.) Spraying yields spherical particles of controllable size 2.) Fast cooling of the small droplets minimizes the risk of phase separation

during freezing 3.) Lyophilization avoids heat and air induced degradation or phase separation

of the lipophilic drug and hydrophilic carrier during drying [213]. These advantages make spray freeze drying a potentially attractive process to incorporate labile drugs in a stabilizing carrier [136, 214]. The resulting powders may also be applied to accelerate aqueous dissolution [179] resulting in a high and reproducible bioavailability of lipophilic drugs [135, 215]. Secondly, we investigated whether the powder obtained from a spray freeze drying process is suitable for dispersion into an aerosol with a particle size appropriate for pulmonary administration. Such a dry powder inhalation system would enable low temperature THC inhalation without using lengthy inhalation procedures or having to smoke.

7.3. Materials Inulin, type TEX!803, having a number average degree of polymerization (DP) of 23, was provided by Sensus, Roosendaal, The Netherlands. ∆9-tetrahydrocannabinol was a gift of Unimed Pharmaceuticals Inc., Marietta, USA. Anthrone reagent was supplied by Fluka Chemie GmbH, Steinheim, Germany. Sulfuric Acid 95-97% (for analyis) was supplied by Merck KGaA, Darmstadt, Germany. All other materials were of analytical grade.

7.4. Methods

7.4.1. Preparation of spray freeze dried powder To produce a spray freeze dried powder, an aqueous inulin solution of various concentrations and a 10-mg/ml THC in TBA solution were prepared (table 1). Subsequently these solutions were mixed at a volume ratio water/TBA of 6/4. In a previous study, it was shown that this volume ratio is required to obtain a clear and homogeneous solution for at least 10 minutes in case of the highest inulin concentration [174]. The solution containing both THC and inulin was sprayed with the 0.5-mm two fluid nozzle. The liquid feed rate was 10.5 ml/min and the atomising air flow was set at 400 ln/h (i.e the equivalent of 400 litres of air of 1 atm and 0°C). The outlet of the nozzle was positioned about 10 cm above liquid nitrogen. Hot water (about 90°C) was pumped through the jacket of the nozzle in order to avoid freezing of the solution inside the nozzle. The resulting suspension (frozen droplets of the solution in liquid nitrogen) was transferred into the freeze dryer (Christ, model Alpha 2-4 lyophilizer, Salm and Kipp, Breukelen, The Netherlands). Vacuum was applied as soon as all nitrogen was evaporated. During the first 24 hours the pressure was set at 0.220 mbar and the shelf temperature at


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-35°C. (condensor temperature -53°C). During the second 24 hours, the shelf temperature was gradually raised to 20°C while the pressure was decreased to 0.05 mbar. After removing the samples from the freeze drier, they were stored over silicagel in a vacuum desiccator at room temperature for at least 1 day.

Table 1: Composition of the different mixtures used to produce the solid dispersions.

Before mixing After mixing After spray freeze drying

inulin in water

(mg/ml)

THC in TBA

(mg/ml)

inulin in water/TBA

(mg/ml)

THC in water/TBA

(mg/ml)

solid material (mg/ml)

Drug load (%w/w)

160 10.0 96.0 4.00 100 4.0

76.7 10.0 46.0 4.00 50.0 8.0

48.8 10.0 29.3 4.00 33.3 12

35.0 10.0 21.0 4.00 25.0 16

26.7 10.0 16.0 4.00 20.0 20

15.6 10.0 9.33 4.00 13.3 30 As can be seen in table 1, the drug load was varied by spray freeze drying solutions of various inulin concentrations while keeping the THC concentrations constant. When solid dispersions were prepared by freeze drying (for comparison) a previously described freeze drying procedure was followed [175]. This procedure uses the same instrument settings as applied during drying of spray freeze dried material.

7.4.2. Estimation of porosity after spray freeze drying The porosity (ε) after spray freeze drying was measured according to the following procedure. Inulin was dissolved in water/TBA mixtures of 6/4 v/v. The inulin concentration (c) was varied from 13.3 mg/ml up to 100 mg/ml. These solutions were slowly pumped through a tube to generate equally sized droplets. The volume of the generated droplets (Vdrop) was determined by counting the number of drops necessary to fill a volume of 5.00 ml. The droplets were frozen by dropping them into liquid nitrogen. The frozen solution spheres were photographed by a digital camera together with a ruler for calibration. Sigma Scan Pro 5.0 (Jandel Scientific, Erkrath, Germany) was used to determine the cross sectional area of the frozen droplets. Subsequently, the diameter was calculated. The diameter of the spray freeze dried particles (dp) was determined according to the same procedure. The


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porosity was calculated with the following equation:

3p6

1inulin

drop3p6

1

d

cVd

πρ

πε

−=

For the density of inulin, ρinulin, 1.534 g/cm3 was taken [129].

7.4.3. Scanning electron microscopy (SEM) Double sided adhesive tape was placed on an aluminium specimen holder upon which a small amount of powder was deposited. The particles were coated with approximately 10-20 nm gold/palladium, using a sputter coater (Balzer AG, type 120B, Balzers, Liechtenstein). Scans were performed using a JEOL scanning electron microscope (JEOL, type JSM-6301F, Japan) at an acceleration voltage of 1.5 kV. All micrographs were taken at a magnification of 2000.

7.4.4. Laser diffraction The particle size distribution was measured with a Sympatec HELOS compact KA laser diffraction apparatus (Sympatec GmbH, Clausthal-Zellerfeld, Germany). The powder was dispersed using a RODOS dry powder dispenser at 0.5 bar or using an inhaler adapter (INHALER) both from Sympatec GmbH, Clausthal-Zellerfeld, Germany, in combination with a test inhaler based on air classifier technology at 60 l/min for 3 seconds [216, 217]. A 100 mm lens was used and calculations were based on the Fraunhofer theory. All data given are the mean of at least four measurements.

7.4.5. Differential Scanning Calorimetry Thermal behaviour of the spray freeze dried powders was determined by temperature modulated differential scanning calorimetry (TMDSC) on a differential scanning calorimeter (DSC2920, TA Instruments, Gent, Belgium). A modulation amplitude of 0.318°C, a modulation period of 60 seconds and a heating rate of 2°C/min was used. Calibration was performed with indium. Standard aluminium sample pans were used. During measurement, the sample cell was purged with nitrogen at a flow rate of 35 ml/min. Before scanning, the sample pan was heated at 2°C/min to 50°C to remove all residual moisture. Subsequently, the sample was cooled to -20°C and then scanned up to 180°C. The glass transition temperature (Tg) was defined as the inflection point of the change in specific heat in the reversing signal.

7.4.6. BET analysis A 5-point nitrogen adsorption isotherm at 77 K was measured with a Tristar surface analyser Micromeritics Instrument Corporation, Norcross (GA), USA. The


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BET theory [218] was used to calculate the surface area. Duplicate analyses were performed with all spray freeze dried powders taken from a vacuum desiccator. For every drug load two different batches were analysed.

7.4.7. Stability study To investigate the degradation of pure THC, 20 ml glass vials were charged with 70 µl of a solution of THC in methanol containing 2.52 mg THC. They were left overnight under a flow of dry nitrogen to allow for methanol evaporation. The resulting thin layers of THC spread over the bottom of the vials (4.5 cm2). Spray freeze dried and freeze dried material containing THC were weighed in vials. All samples were stored in climate chambers of 20°C/45%RH and 60°C/8%RH. Samples (n=3) were taken at different time intervals and analysed by means of HPLC using a previously described method [175]. Briefly, samples were extracted with methanol. A Waters 717+ autosampler was used to inject 50 µl of supernatant on a precolumn (HPLC precolum inserts, µBondapak C18 Guardpak) followed by a Chrompack Nucleosil 100 C18 column (4.6x250 mm). Absorbance at 214 nm was measured with a UV detector (Shimadzu SPD-M6A). Chromatograms and peak areas were analysed with an integrator (waters 741 Data Module) and Kromasystem 2000 software. The flow rate of the eluens (methanol/water 92/8 (v/v) and 0.25 ml concentrated sulphuric acid per litre eluens) was set at 1.0 ml/min. THC retention time was about 7.5 min. In every series of HPLC-runs some calibration samples were included.

7.4.8. Cascade Impactor Analysis In-vitro deposition of the powder formulations was tested with a multi-stage liquid impinger (MSLI) of the Astra type (Erweka, Heusenstamm, Germany). A flow rate of 60 l/min was used for 3 seconds according to the procedure described by the European Pharmacopeia 4th Ed. 2002. A mixture of water and ethanol (90%v/v water) was used as solvent since the use of pure water resulted in inhomogeneous solutions and improper rinsing due to the low aqueous solubility of THC. Each impactor stage was filled with 20 ml of solvent. In the final stage a dry glass filter (Gelman Sciences, type A/E, Michigan, USA) was used for the retention of particles that passed the fourth stage. A previously described test inhaler based on air classifier technology [217] was used under controlled ambient conditions (20°C/50%RH). In each experiment 10 inhalations were performed and 2-3 mg (exactly weighed) per inhalation was used. All powders used were pre-equilibrated in a climate chamber at 20°C and 45%RH. Two independently produced spray freeze dried batches were analysed. The real dose deposition was defined as the weight fraction powder relative to the total mass of the powder inserted in the inhaler during the cascade impactor analysis and was calculated from the drug load and the inulin concentration. The inulin concentration on each of the different stages was analysed using the Anthrone assay [173]. Samples of 1.00 ml were mixed with 2.00 ml Anthrone reagent 0.1%w/v in concentrated sulphuric acid. Due


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to the enthalpy of mixing, the sample was heated to its boiling point. The boiling mixture was then cooled to room temperature. After 45 minutes the sample was vortexed and 200 µl of sample was analysed in a plate reader (Benchmark Platereader, Bio-Rad, Hercules, USA) at 630 nm. In every assay two 11 point calibration curves of the appropriate spray freeze dried powder in the appropriate medium was established. In each of the experiments the recovery was above 90%.

7.5. Results and Discussion

7.5.1. Characteristics of spray freeze dried powder The spray freeze dried solid dispersions appeared as a white powder with a low bulk density ranging from about 20 to 85 mg/cm3 and a very high bulk porosity ranging from 94% to 99% depending on the total solid concentration in the solution. Furthermore, the powder easily swirled up, which is a first indication of its applicability for inhalation. The SEM pictures of the different powders are shown in figure 1. They showed spherical particles with a high porosity and a rough surface in all cases. The surface texture does not change when the drug load increased but somewhat more broken particles were observed at the highest drug load indicating high fragility. Due to handling problems, the porosity of the spray freeze dried particles could not be measured directly. However, an estimation could be performed with larger spheres. The effect of solute concentration on droplet formation, freezing and particle size after drying was investigated. The results are given in table 2.

Table 2: Size (relative to droplet size) and porosity of particles during spray freezing process (n = 15-20, ± S.D.)

inulin conc. (mg/ml) 100 50.0 25.0 13.3

droplet size (%) 100 ± 0.2 100 ± 0.1 100 ± 0.5 100 ± 0.7

frozen droplet size (%) 102 ± 5.1 104 ± 2.4 103 ± 2.1 104 ± 2.6

particle size (%) 84.1 ± 2.4 79.7 ± 2.8 78.2 ± 2.7 66.9 ± 3.0

porosity of particle (%) 89.0 ± 0.24 93.6 ± 0.13 96.6 ± 0.09 97.1 ± 0.08

density of particle (mg/cm3) 169 ± 3.63 99.4 ± 2.06 52.6 ± 1.37 44.6 ± 1.26 The droplet sizes were 3.45 mm and independent of inulin concentration. After freezing a small increase in diameter was observed, indicating that a water/TBA solution containing inulin expands slightly upon freezing. The expansion was irrespective of inulin concentration. Furthermore, as can be expected, spray freeze drying of lower concentrated solutions yielded particles of higher porosities. However, after lyophilization of the frozen solution spheres, all particles were significantly smaller. During drying, particle diameters decreased to 84.1% of the


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droplet size for the most concentrated solution and even more (79.7-66.9%) for particles with lower inulin concentrations. This implies that particles prepared from low concentrated solutions shrink more during drying which is caused by their lower strength. The porosities are measured in larger particles and represent estimations for the smaller particles. However, the trends in particle shrinkage is comparable.

Figure 1: Representative SEM pictures of spray freeze dried

solid dispersions with drug loads of 4, 8, 12, 16, 20 and 30 wt-% designated A, B, C, D, E and F, respectively

The geometric volume median diameter (x50) of all THC containing powders was analysed with laser diffraction using two different dispersion methods. Firstly, the materials were dispersed with the RODOS disperser (Sympatec GmbH, Clausthal-


136

Zellerfeld, Germany) at a relatively low pressure of 0.5 bar in order to minimize the dispersion forces during the measurement. Secondly, the powders were dispersed by means of the test inhaler at 60 l/min for 3 seconds in order to measure the geometric particle size that actually leaves the inhaler. These test conditions correspond with the conditions during cascade impactor analysis. With RODOS measurements it was found that the geometric volume median diameter of all powders except for the 30wt-% drug load more or less corresponded with estimations from SEM pictures (figure 2). At a drug load of 30wt-%, the particle size seems smaller. Apparently, their higher porosity, makes the particles so fragile that the relatively low dispersion forces generated with the RODOS are already large enough to de-agglomerate and break up these powders. Much larger dispersion forces that are generated when the powders are dispersed with the test inhaler. This resulted in a reproducible decreased particles size (figure 2). In this case, also less porous and less fragile particles (lower drug loads) are de-agglomerated and broken. Apparently, they are fragile enough to allow for disruption by the applied dispersion forces. Disruption may be advantageous to obtain a high peripheral deposition during inhalation. The BET specific surface areas of all powders ranged from about 70 to 110 m2/g. These very high specific surface area’s are in accordance with previously reported data on spray freeze dried materials [136, 219].

0

2

4

6

8

10

12

4 8 12 16 20 30Drug load (wt-% THC)

x 50 (

m)

Figure 2: Volume median diameters (x50) of spray freeze dried powder, determined by laserdiffraction with: RODOS dispersion at 0.5 bar (shaded colums) and with test inhaler dispersion at 60 l/min for 3 seconds (open colums) (n ≥ 4, error bars

represent standard deviations).

Finally, the powders were characterised by temperature modulated differential scanning calorimetry (TMDSC). Typical examples of the reversing heat flow are presented in figure 3. In table 3, the glass transition temperatures (Tg’s) of THC, amorphous inulin and the different solid dispersions are presented. As reported before, THC remains also above its Tg in the amorphous state since it resists crystallization [104]. A Tg of 9.3°C was observed for the pure THC. The inulin type used in this study has a Tg of 155°C. The results show that incorporation of THC in inulin glasses does not affect the Tg of inulin (figure 3). Apparently, the


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glassy inulin carrier is not plasticized by the incorporated THC. These observations indicate that the nature of inulin and THC is very different (hydrophilic versus lipophilic) and that no interactions are present. A similar phenomenon was seen when the lipophilic model drug diazepam was included in the sugar (data not shown). Only at the highest drug load a Tg of THC could be discerned as indicated by the arrow in figure 3. This indicates that at this drug load either THC molecules are homogeneously dispersed in the inulin but form a percolating system or that THC is not dispersed homogeneously throughout the inulin carrier anymore. In either case THC molecules are neighbouring resulting in a Tg of pure THC. Future experiments should reveal more information regarding the mode of incorporation.

Rev

Hea

t Flo

w (W

/g)

20 40 60 80 100 120 140 160Temperature (°C)

Exo Up Universal V2.6D TA Instruments

0.02

(W/g

)

1

2

3

4

0

Rev

Hea

t Flo

w (W

/g)

20 40 60 80 100 120 140 160Temperature (°C)

Exo Up Universal V2.6D TA Instruments

0.02

(W/g

)

1

2

3

4

0

Figure 3: Reversing heat flow in TMDSC scans. 1: pure THC, 2: physical mixture of inulin and THC (4wt-% THC), 3: solid dispersion showing no Tg of THC (4wt-% THC), 4: solid dispersion in which a Tg of THC could be discerned (30wt-% THC)

Table 3: Glass transition temperatures found in solid dispersions with various drug loads. All mixtures were prepared by spray freeze drying. (n = 3, ± S.D.)

drug load (wt-%) 1st Tg (°C) 2nd Tg (°C)

0 - 155 ± 0.6

4 not observed 156 ± 0.7





30 8.7 154 ± 1.4

100 9.3 ± 1.0 -


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7.5.2. Stability of THC in the spray freeze dried powder The spray freeze dried solid dispersions containing THC appeared as a white powder that showed, in contrast to for example spray dried materials, no change in colour in time, which is a first indication of effective stabilization of the labile THC by the inulin glass [175]. The results of a more thorough investigation on the stabilization of THC is shown in figures 4A and 4B. The THC content in spray freeze dried powders containing initially 4 or 8wt-% THC is plotted as a function of time. It was found that pure THC degrades completely within about 50 days when exposed to air of 20°C/45%RH (figure 4A). However, when THC is incorporated in the glassy inulin matrix, about 70% be recovered after 390 days. When the more stressful storage condition of 60°C/8%RH is chosen, pure THC degraded completely within 15 days (figure 4B). Again the glassy inulin matrix decelerated THC degradation. No differences in degradation rate were observed between the 4 and 8% drug load. Apparently, for both drug loads THC was effectively shielded from its environment by a matrix of inulin and thereby highly stabilised.

0

20

40

60

80

100

120

140

0 60 120 180 240 300 360 420time (days)

THC

con

tent

(% o

f dos

e)

A

0

20

40

60

80

100

120

0 30 60 90 120 150 180time (days)

THC

con

tent

(% o

f dos

e)

B

Figure 4: THC content as a function of storage time in spray freeze dried powders

with 4 and 8wt-% THC and in pure THC samples. A: storage at 20°C/45%RH; shaded squares: pure THC, open squares: 4wt-%, solid squares 8wt-%. B: storage at 60°C/8%RH; shaded squares: pure THC, open squares: 4wt-%; solid squares:

8wt-%.

To investigate the effect of drug load on THC stabilization in the solid dispersions in more detail, spray freeze dried powders of a wide range in drug loads were evaluated. To investigate the effect of freezing rate on THC stabilization, solid dispersions produced by freeze drying instead of spray freeze drying were subjected to a stability study. It appeared that at 20°C/45%RH all spray freeze dried powders effectively stabilised the THC even up to a drug load of 30 wt-% (figure 5). All spray freeze dried powders contained over 85% of the original THC content after storage for 3.5 months. However, when freeze dried cakes were exposed to the same environment, significantly more THC was degraded even though the storage was only 1.5 months. The higher stability of spray freeze dried material is likely to be caused by the higher cooling rate occurring during spray freeze drying. During spray freeze drying the surface area available for heat transfer from the solution is much larger. As a result the droplets are instantaneously frozen. On the other hand during freeze drying glass vials


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decelerate the heat transfer. Fast freezing reduces the time available for phase separation of either inulin or THC in the yet unfrozen solution and thereby reduces the risk of inadequate incorporation of THC. It can be concluded that spray freeze drying is the optimal process for the production of solid dispersions, not only because particles are easily obtained but also because THC is strongly stabilised for all drug loads evaluated.

0

20

40

60

80

100

120

0 4 8 12 16 20 24 28 32Drug load (wt-%)

non-

degr

aded

TH

C (%

)

Figure 5: Stability at 20°C/45%RH of THC in solid dispersions as a function of drug load. Given are the recoveries of THC (Black squares: spray freeze dried

powders after 3.5 months; white diamonds: freeze dried material after 1.5 months, standard deviations all ≤ 15% )

As can be seen from figure 5, especially at high drug loads spray freeze drying results in more stable solid dispersions. Apparently, the higher cooling rate is more important when solid dispersions with high drug loads are produced. This can be explained by the two following mechanisms. The first mechanism is obvious: when the drug load increases less inulin is available to incorporate the drug. Consequently, with increasing drug load the risks of inadequate inclusion of the drug increases resulting in decreased stability. The second mechanism is less obvious and has to be found in the events occurring during freezing. Solid dispersions with increasing THC contents were prepared from solutions in which the inulin concentrations were lowered while keeping the THC concentration the same (see table 1). Therefore, the stability of the solutions to be dried increases with increasing drug load. Based on this consideration the stability should actually increase instead of decrease. However, during freezing the solution is strongly concentrated by solvent crystallization through which the instability of the yet unfrozen part of the solution is increased. Therefore, no matter what the initial concentrations of inulin and THC in the solution were, the system will always be highly vulnerable to phase separation during cooling until it is vitrified by passing the glass transition temperature of the freeze concentrated fraction (Tg’). As solid dispersions with a high drug load were prepared from solutions with a low concentration of solids (see table 1) the amount of solvent that will crystallize increases. Moreover, solvent crystallization is an exothermic event, thus counteracting the reduction of sample temperature. As a result, for with a low concentration of solids, i.e. high drug loads, it will take more time to reach


140

temperatures below the Tg’ and to vitrify the system. Consequently, the risks and/or degree of phase separation will be increased.

7.5.3. In vitro deposition behaviour of the spray freeze dried powders

The particle sizes, reported as the geometric volume median diameter in the white bars in figure 2, indicated that the particles produced with spray freeze drying are rather large for an application in pulmonary drug delivery. Generally particles between 1 and 5 µm having a density of approximately 1 mg/cm3 are considered suitable for inhalation [220, 221]. After dispersion with the inhaler adapter, the geometric particle size of the powders measured with laser diffraction was about this size. However, the size limit refers to the aerodynamic diameter daero, which is determined by the geometric diameter dgeo, the density of the particle ρp (estimations are given in table 2), and the reference density ρr (the density of water taken as 1000 mg/cm3) The shape factor χ equals 1 for spherical particles and is larger than 1 for non-spherical particles. The aerodynamic diameter can be calculated according to the following equation [221, 222].

χρρ⋅

⋅=r

pgeoaero dd

Since the particles in this study are extremely porous, i.e. ρp is very small, the aerodynamic diameter will be substantially smaller than the geometric diameter. When the particles are assumed to be spherical, the aerodynamic diameter will be approximately 40-20% of the geometrical diameter depending on the porosity and density of the particles. Therefore, it was interesting to subject the powders to cascade impactor analysis, which gives the aerodynamic diameter. Moreover, the outcome of cascade impactor analysis is generally considered to indicate the suitability for inhalation in vivo. The air classifier type inhaler was used in the cascade impactor analysis because laser diffraction analysis showed small particles leaving the inhaler, caused by the strong dispersion forces generated by this type of inhaler [217]. As can be seen in figure 6, all spray freeze dried powders showed high fine particle fractions. The fine particle fraction (FPF), here defined as the sum of the 3rd, 4th and the filter stage relative to the total dose, was very high for all powders. This implies that all powders showed excellent inhalation behaviour since the FPF is assumed to represent alveolar deposition during in vivo inhalation. All powders showed similar inhalation behaviour except for the powder with 4% drug load. For unknown reasons, this powder showed a high retention in the inhaler and an FPF of only 35%. However, all other materials showed less inhaler retention and a fine particle fraction of 40-50%. These results indicate that spray freeze dried powders in combination with an air classifier based inhaler are promising for pulmonary


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delivery. Moreover, these in vitro inhalation simulations were performed with unformulated material: only spray freeze dried powder was used without any additional excipients or formulation techniques that could further improve the aerosolization behaviour.

0

10

20

30

40

50

Inhaler Inductionport

1 2 3 4 Filter 3+4+filter

depo

sitio

n (%

of r

eal d

ose)

Figure 6: Cascade impactor results obtained with spray freeze dried powders

having different drug loads. (cross hatched = 4%THC, dotted = 8%THC; black = 12%THC, grey= 16%THC, white= 20%THC) (duplicate of two independently

produced batches, error bars indicate highest and lowest value)

7.6. Conclusions Spray freeze drying using a water-TBA mixture as solvent was found to be a suitable process to produce a solid dispersion powder that contains THC up to 30%-wt incorporated in a glassy matrix of inulin. The spray freeze dried products thus obtained appear as fluffy materials that consist of porous particles. THC in these solid dispersions is effectively stabilised for all drug loads tested. When high drug loads are considered, the stability is significantly better than that of comparable freeze dried solid dispersions. The improved stability of the spray freeze dried products is explained by the higher cooling rate resulting in more effective incorporation of THC. Furthermore, the spray freeze drying process yielded products that were suitable for inhalation. Dispersed with an air classifier type inhaler, the different powders generated aerosols with aerodynamic particle size distributions that are suitable for pulmonary administration.

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Spray freeze drying to produce a stable Δ9-tetrahydrocannabinol containing inulin-based solid dispersion powder suitable for inhalation