CELLETS® are pellets or spheres made of microcrystalline cellulose. The size ranges from 100 µm to 1400 µm. Being neutral starter cores, they can be used as carrier system for low-dosed APIs and allow diverse functional coating. See pellet technologies for a detailed description.

CS_sphericity_image_4

Electron microscopy yield perfect imaging data of the MCC pellets’ surfaces. Magnification: 250x, working distance 8.0 mm, voltage: 10 keV.

Available size classes are (click for more information):

  • CELLETS® 100
  • CELLETS® 200
  • CELLETS® 350
  • CELLETS® 500
  • CELLETS® 700
  • CELLETS® 1000

Any size class of CELLETS® have same striking advantages:

  • low friability and extreme hardness
  • insolubility in water
  • high spherictity
  • smooth surface
  • good monodispersity

See case studies to see these starter pellets in action!

Figure 4: Images of a Jelly without (left) and with incorporation of sustained release micropellets (right).

Gliclazide formulations for Sustained Release Delivery

Abstract

Patients with dysphagia may have obstacles to swallow tablets or large multiparticulates. The former dosage form can even not be crushed in case that the tablet exhibits a modified release or taste-masking profile through outer layering. As a solution, so called jelly formulations may be a valuable attempt. Jellies are delivery vehicles incorporating sustained release microparticles for patients with dysphagia. This case study investigates a modified release formulation based on Gliclazide. Gliclazide is used to treat diabetes mellitus type 2. In combination with selected excipients, a jelly-like appearance is composed. Micropellets made of microcrystalline cellulose (Cellets®) are used as API carrier systems.

Goals and Formulation of a Gliclazide drug

The goal is to investigate a revolutionary method for geriatrics with dysphagia or potentially for paediatrics based on jelly-like formulations. The formulation should carry an API such as Gliclazide and show a modified release profile.

Free-standing jellies are formulated by mixing sodium alginate (0.5 % w/v with another polymer, and 1 % w/v w/o polymer), with an aqueous solution of dicalcium phosphate dihydrate (0.1-1 % w/v).

Soft granular jellies are formulated by preparing an aqueous sodium alginate (0.5-2 % w/v) solution with or without the presence of another polymer and by later adding an aqueous calcium chloride solution (0.1-0.3 % w/v).

Figure 1: Image of MCC micropellets (Cellets® 100).

Figure 1: Image of MCC micropellets (Cellets® 100).

MCC micropellets (Cellets® 100, Figure 1) are used as drug carriers. Gliclazide is layered onto the starter beads using a Wurster fluidized bed coater (Mini-Glatt, Glatt GmbH, Germany), so that a 50 % drug loading weight gain was reached. The overall final drug load including the functional layer is 21 % w/w. The composition of the layering suspension is given in Table 1.

Material QTY
Starter pellet: Cellets® 100 100 g
API: Gliclazide 10 % w/w
Aqueous vehicle for API:
  Hypromellose 1 % w/w
  Talc 1.9 % w/w
Coating of API layered pellets:
  Water
  Eudragit® NM 30 D
  Talc
Functional coating:
  Magnesium stearate
  Silicon dioxide

Table 1: Formulation for Gliclazide layered starter pellets: starter pellets, aqueous API layering, release profile coating, functional coating.

Although the formulation contains several coating and layering processes, the processed micropellets stay smooth in surface, show a high sphericity and narrow size distribution.

Size distribution and dissolution profiles of Gliclazide microparticles

Polymer coated micropellets with CL25 (coating level 25 %) are shown in  Figure 2. The yield of polymer coating and the final D50 values of the micropellets are displayed in Table 2.

Figure 2: SEM image of layered Gliclazide sustained release micropellets with a weight gain at 25 % (CL25).

Figure 2: SEM image of layered Gliclazide sustained release micropellets with a weight gain at 25 % (CL25).

Depending on the polymer coating, micropellets show a different Gliclazide release profile as shown in Figure 3: With increasing weight gain, the dynamics of Gliclazide release are slowed down. A comparison to Diamicron SR tablets in a pH 7.4 phosphate buffer, the CL25 formulation results in an adequate release profile.

Micropellet Size D50 [µm] Yield [%]
Starter pellet (Cellets® 100) 160 ± 2.1
Micropellet at CL16 173 ± 3.6 98.4
Micropellet at CL20 185 ± 2.4 99.3
Micropellet at CL25 198 ± 4.3 99.0
Micropellet at CL60 208 ± 6.7 98.7

Table 2: Particle size of the micropellets with and without layering. CL = coating level / weight gain in [%]. The yield for the polymer coatings at respective weight gains.

Figure 3: Gliclazide release from layered micropellets at coating levels 16 % (filled diamond), 20 % (open circles), 25 % (filled squares) and 60 % (filled circles) and the commercial Diamicron SR tablets (open squares) in phosphate buffer pH 7.4.

Figure 3: Gliclazide release from layered micropellets at coating levels 16 % (filled diamond), 20 % (open circles), 25 % (filled squares) and 60 % (filled circles) and the commercial Diamicron SR tablets (open squares) in phosphate buffer pH 7.4.

Incorporation of the Gliclazide microparticles into jellies

The incorporation of sustained release Gliclazide microparticles into the Jellies is realized through mixing the required quantity of microparticles with polymers (sodium alginate or polymer mixture).

Sodium alginate is known to form gels in the presence of calcium ions at room temperature. Depending on the formulation, granular jellies (soft and easy to flow) or free-standing jellies (“ready-to-eat”) are formed. Formulations of jellies with and without API layered micropellets are shown in Figure 4. Incorporating the micropellets into the jellies did not cause a visual change in color or appearance. The API was kept inside the jellies. Also physical-chemical properties such as the gel strength, the texture, and the oral transit time in an in-vitro swallowing simulator are remained unchanged.

Figure 4: Images of a Jelly without (left) and with incorporation of sustained release micropellets (right).

Figure 4: Images of a Jelly without (left) and with incorporation of sustained release micropellets (right).

Figure 4: Images of a Jelly without (left) and with incorporation of sustained release micropellets (right).

A release profile of Gliclazide with a coating level of 25 % in a jelly formation is shown in Figure 5. In comparison to a reference release profile of a Diamicron 30 mg SR tablet, the coated micropellets show a competitive behavior as already discussed in Figure 3. After incorporating into the jelly formation, the release profile is decaying. Obviously, the intact and also the fragmented jelly formulation show comparable dynamics. In order to obtain a comparable release profile than with the non-formulated micropellets, a coating level of down to 20 % is required.

Figure 5: Gliclazide release from coated microparticles and in combination with Jellies in a pH 7.4 phosphate buffer. Diamicron 30 mg SR tablet (open triangle), no jelly at CL25 (closed triangle), jelly formulation (intact) incorporated with CL25 (closed circle), jelly formulation (fragmented) incorporated with CL25 (open circle), jelly formulation (intact) with CL20 (open square).

Figure 5: Gliclazide release from coated microparticles and in combination with Jellies in a pH 7.4 phosphate buffer. Diamicron 30 mg SR tablet (open triangle), no jelly at CL25 (closed triangle), jelly formulation (intact) incorporated with CL25 (closed circle), jelly formulation (fragmented) incorporated with CL25 (open circle), jelly formulation (intact) with CL20 (open square).

Summary

Sustained release Gliclazide micropellets with a final particle size D50 of less than 200 µm are successfully formulated with a 99 % production yield and adjustable drug release profiles.

The micropellets are based on Cellets® 100 and present an excellent surface smoothness, high sphericity and narrow size distribution. They were successfully incorporated in jelly formulations. This novel drug delivery platform is a suitable vehicle for the administration of sustained release microparticles. It is a valuable attempt to replace the commonly used thickened fluids for dysphagia patients.

Acknowledgement

Dr. Fang Liu and her team are gratefully acknowledged for serving content for this note.

Fluid Pharma logo

Fluid Pharma Ltd

Contact: Dr. Fang LIU

College Lane, Hatfield, AL10 9AB, UK

Tel: +44 1707 28 4273

+44 796 3230 628

www.fluidpharma.com

References

[1] S. Patel et al., Journal of Pharmaceutical 109 (2020) 2474-2484.

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Figure 2: SEM image of drug loaded and coated starter beads. Particles show a high level of homogeneity in size distribution.

Microparticle coating in Wurster Fluidized Bed – Scalable Production of Oral Sustained-Release Drugs

Abstract

Modified drug release formulations for suspensions are a perfect solution for children and patients with swallowing difficulties. In many cases, these formulations are based on pellets serving as starter beads. In this report, an attempt on microparticle coating by Mohylyuk et al. [1] is described. Herein, small scaled microcrystalline cellulose pellets (Cellets® 90 and Cellets® 100, Table 1) in the size range smaller than 150 µm are used. Through a modified Wurster fluidized bed process, a yield of 99 % was reached.

Starter materials PSD (> 85 %)
Cellets® 90 63-125 μm
Cellets® 100 100-200 µm

Table 1: Size distribution of Cellets® as starter beads in this formulation.

Goals and Formulation

The goal is to investigate a revolutionary platform for sustained-release microencapsulation using the industrial fluidized bed coating technology. Significant challenges of particle cohesion in the process shall be avoided by applying a small quantity of dry powder glidant periodically during the coating process. A highly water-soluble drug, which is metoprolol succinate, is reproducibly microencapsulated on pellet technologies with total pellet sizes of less than 200 µm and a drug release time of 20 hours.

Excipients for extended release profiles

For obtaining a sustained release profile, polymethacrylate-based copolymers, Eudragit RS/RL® 30 D and Eudragit® NM 30 D, were used in combination with a range of anti-tacking agents. The coating onto placebo Cellets® 100 starter beads was performed in a fluidized bed coater with a Wurster insert (Mini-Glatt, Glatt GmbH, Germany) in order to analyze the release profile. Process parameters are shown in Table 2. A small quantity of dry powder glidant was periodically added during processing, so that particle cohesion was eliminated. The optimized excipient composition for the desired release profile is achieved by testing 10 different compositions.

Parameter Value
Inlet air temperature
 Eudragit RS/RL® 30 D 35-40 °C
 Eudragit® NM 30 D 30-35 °C
Product temperature
 Eudragit RS/RL® 30 D 25-30 °C
 Eudragit® NM 30 D 18-20 °C
air flow rate 18 m3/h
Atomization pressure 1.5 bar
Spray rate 1.1-2.4 g/min

Table 2: Process parameter for a fluidized bed coater with a Wurster insert. A sustained release drug layer is coated onto placebo Cellets® 100 starter beads.

Drug coating

For drug coating, Cellets® 90 were layered with a suspension of metoprolol succinate in a composition as shown in Table 3.

Material Concentration (w/w)
Metoprolol succinate 22.8 %
Hypromellose 0.6 %
talc (Pharma M) 4.0 %
Deionized water 72.6 %

Table 3: Composition of metoprolol succinate suspension for drug layering onto Cellets® 90.

The metoprolol succinate-loaded Cellets® 90 microparticles were successfully coated with the Eudragit® NM 30 D based aqueous dispersion, achieving a high product yield of 99 % and a final particle size of less than 200 µm (D50 value).

Figure 1: Size distribution of Cellets® 90 as uncoated (empty squares), drug loaded (filled diamonds) and drug loaded and coated (filled circles) particles.

Figure 1: Size distribution of Cellets® 90 as uncoated (empty squares), drug loaded (filled diamonds) and drug loaded and coated (filled circles) particles.

The API loaded and coated starter beads are of high sphericity and show a homogeneous and narrow size distribution, which is shown as a SEM (scanning electron microscope) image in Figure 2.

In dissolution tests, an extended release time of up to 20 hours is obtained and can still be varied by the composition of excipients (Figure 3).

Figure 2: SEM image of drug loaded and coated starter beads. Microparticles show a high level of homogeneity in size distribution.

Figure 2: SEM image of drug loaded and coated starter beads. Microparticles show a high level of homogeneity in size distribution.

Figure 3: Drug release profiles of three batches of metoprolol succinate loaded and coated Cellets. An extended release of 20 hours is obtained.

Figure 3: Drug release profiles of three batches of metoprolol succinate loaded and coated Cellets. An extended release of 20 hours is obtained.

Summary

This case study is a short abstract of the publication on microparticle coating by Mohylyuk et al. [1], highlighting the proof of concept for reproducible microencapsulation of a highly water-soluble drug by applying a small quantity of dry powder glidant periodically during Wurster fluidized bed coating. The challenge of particle cohesion in the “down flow” zone was eliminated and a high product yields up to 99% was achieved.

Coated microparticles are in size of less than 200 μm and show a 20 hours sustained drug release profile. These conditions allow the usage in liquid suspensions. Furthermore, the applied technology is scalable. In conclusion, this displays a sustained-release dosage solution, which is suitable for paediatrics and geriatrics with swallowing difficulties.

Acknowledgement

Dr. Fang Liu and her team are gratefully acknowledged for serving content and data for this note.

Fluid Pharma logo

Fluid Pharma Ltd

Contact: Dr. Fang LIU

College Lane, Hatfield, AL10 9AB, UK

Tel: +44 1707 28 4273

+44 796 3230 628

www.fluidpharma.com

References

[1] V. Mohylyuk et al., AAPS PharmSciTech (2020) 21:3

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Coating uniformity of hot-melt coated particles Figure 2 (pure)

Coating uniformity of hot-melt coated particles

Abstract

Coating uniformity is a critical parameter in coating processes in novel pharmaceutical formulations. Speaking about pellet technology, coating and layering are the main methods for implementing drug functionalities, such as modified release of the active, taste-masking properties and further more. Coating uniformity guaranties not only upholding functionalities of the formulation, but also prevent risks such as dose dumping.

This application note is based on a publication of Wörthmann et al. [1] and focuses on selected aspects which are related to starter cores.

Cellets 1000, magnification 100x

Figure 1: Microscopic image of Cellets® 1000, magnification 100x.

Materials and techniques

Coating was applied on highly spherical starter cores Cellets® 1000 (Figure 1). The pellets have a relatively narrow size distribution with a mean particle size of d­­­­50 = 1197 μm, a standard deviation of σ = 113 μm, and particle density of 1.4 g/cm3. For analyzing the coating uniformity, stearin (54 % stearic acid and palmitic acid) and hydrogenated palm oil were used. For the hot-melt coating experiments a lab-scale Wurster fluidized bed was used. The overspray rate was estimated to 8 % (w/w). Processed particles were analyzed by image analysis (Figure 2) and micro-computed-tomography (μCT) (Figure 3). 2D and 3D software analysis were further conducted for the evaluation of the sphere dimension, layer thickness and coating uniformity.

Figure 2 shows a wax-coated particle, where the coating thickness varies and delamination is clearly visible (Figure 3). Small pores and fractions of the coating layer area are obvious.

Coating uniformity of hot-melt coated particles Figure 1

Figure 2: Images of coated pellets are used for a stepwise evaluation of the particle shell thickness. A: original image; B: segmented coating layer. Further software calculation steps are not shown here.

These undesired artefacts result from imperfect parameters, such as spreading mechanism, temperature fluctuations, viscosity, or drop size.

The coating layer thickness is analyzed for three particles of the same batch (Figure 4) using 5 % (w/w) stearin at a spraying rate of 1 g/min. The layer thickness varies between approximately 2 µm to 30 µm. A mean coating thickness is found between 12 µm and 16 µm.

Coating uniformity of hot-melt coated particles Figure 2

Figure 3: Portion of a micro-computed-tomography image of a wax-coated particle showing.

Coating uniformity of hot-melt coated particles Figure 3

Figure 4: Relative frequency of the coating layer-thickness of three particle shells from the same batch using 5 % (w/w) stearin at a spraying rate of 1 g/min. Mean thicknesses: particle I (blue): 15.5 μm, particle II (red): 12.4 μm, and particle III (grey): 15.6 μm.

In terms of customer safety and of compliance aspects, not only statistical information about the layer thickness are of interest. In case of inhomogeneous layers, taste-masking functionalities or even uncontrolled dose dumping might occur. In this context, a single-particle analysis is mandatory. 3D µCT is a powerful tool, which is complementary to existing methods, such as laser imaging methods, 2D analysis or thickness estimations. The analyzed mean thickness deviates by approximately 13 % among these methods (Figure 5).

Coating uniformity of hot-melt coated particles Figure 4

Figure 5: Mean layer-thicknesses measured using different methods. Relative standard deviation: 13 %.

Summary

Microcrystalline cellulose pellets (Cellets®) are used to study coating uniformity. 3D μCT can be a powerful tool to assess the quality of the final product coating and facilitates the selection of an appropriate combination of core particles and coating material. 3D visualization methods allow a critical single-particle analysis with a resolution of up to 2 µm. Furthermore, the determination of the particle’s uncoated surface area can be specified.

Acknowledgement

Prof. Heiko Briesen, Mario Wörthmann (Technical University Munich) and team are gratefully acknowledged for serving content for this note.

Research was financially supported by the Ministry of Economics and Energy (BMWi) and FEI (Germany) via project AiF 19970 N. Equipment funded by Deutsche Forschungsgemeinschaft (DFG, Germany) 198187031.

References

[1] B.M. Woerthmann, J.A. Lindner, T. Kovacevic, P. Pergam, F. Schmid, H. Briesen, Powder Technology 378 (2021) 51–59

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Fig. 3: Dissolution as a function of time. Black: ASD layered pellets (FB). Red: ASD pellets from direct pelletization (SB). Blue: physical mixture.

Amorphous solid dispersions layered pellets

Abstract

Amorphous solid dispersions layered pellets solve a problem of poorly water soluble drugs. Speaking about oral drug formulations, drug carrier solutions based on starter cores are suitable for several drug classes and open new opportunities for modified drug release profiles. Layering and coating techniques, such as Wurster fluid bed process at different batch sizes, are well established.

However, an increasing number of poorly water soluble drugs challenges modern formulations. A novel approach improving the solubility of those drugs is to formulate them as amorphous solid dispersions (ASD) with a suitable polymer candidate [1]. In this study, Nifedipine was used as a model drug. Nifedipine manages angina, high blood pressure, Raynaud’s phenomenon, and premature labor [2].

Formulation & techniques

ASD formulations can be performed by hot-melt extrusion or spray drying technique. Both techniques have disadvantages such that hot-melt extrusion cannot be employed for temperature-sensitive drugs [3], and spray drying needs a further compaction step not to result in fine powder with poor flowability, broad particle size distribution and high sensitivity to electrostatic charge. Therefore, a further compaction step is required to obtain a freely flowable product [4].

In this context, two techniques for the preparation of ASDs are compared: A 6”-Wurster fluid bed with Type-C bottom plate (Glatt, Germany) and spouted bed (ProCell5™ with Zig-Zag-sifter, Glatt, Germany) are used.

A: GF3™ (fluidized bed); B: ProCell5™ (spouted bed)

Fig. 1: A: 6”-Wurster fluid bed; B: ProCell5™ spouted bed.

The formulation contains the drug and a stabilizing co-polymer (Kollidon®, KVA64, BASF, Germany). Nifedipine and Kollidon are mixed resulting in a drug load of 40 % (w/w) and dissolved in Acetone (30 % w/w solid content).

Parameter FB SB
Spray rate [g/min] 20 20-35
Product temp. [°C] 50-60 50-60
Process gas temp. [°C] 65 80
Process air flow [m³/h] 180-200 65-120
Spraying nozzle diameter [mm] 1.2 1.2
Spraying pressure [bar] 2.0 0.5

Table 1: Manufacturing parameters for fluid bed (FB) and spouted bed (SB).

In the fluid bed process, microcrystalline pellets (Cellets® 500, IPC Dresden, Germany) were layered with the spraying solution such that a drug load of 21.8 % (w/w) is reached. In the spouted bed process, fine powder is generated by spray drying, further agglomeration and layering. An overview on the process parameters is given in Table 1.

Dissolution Tests

Dissolution tests were conducted in a PBS buffer at pH 6.8 and 37 °C (± 0.5 °C). A physical mixture of Nifedipine and KVA64 (40 % w/w drug load) is used as reference.

Results

In the following, results from both experiments, which are amorphous solid dispersions layered pellets (fluid bed) and ASD pellets from direct pelletization (spouted bed) are compared.

Flowability and particle size

ASD layered pellets show a better sphericity, higher level of monodispersity and better flowability properties than the ASD pellets from direct pelletization (Figure 2). Nonetheless, it has to be pointed out that both techniques result in a high particle quality for capsule filling. Analysis data is shown in Table 2.

Parameter FB SB
D­10 [µm] 824 ± 23 559 ± 28
D50 [µm] 943 ± 13 732 ± 50
D90 [µm] 1091 ± 11 1374 ± 410
Bulk density [g/L] 427 280
Flowability [s/100g] 12.1 16.2

Table 2: Analysis of ASD layered pellets (FB) and ASD pellets from direct compaction (SB).

SEM images of processed pellets. A: ASD layered pellets based on Cellets® (FB)

Fig. 2a: SEM images of processed pellets. A: ASD layered pellets based on Cellets® (FB)

SEM images of processed pellets. B: ASD pellets from direct palletization (SB)

Fig. 2b: SEM images of processed pellets. B: ASD pellets from direct pelletization (SB)

Dissolution profiles

Independent from the processing technique, pellets achieved an approximately factor 2 higher end concentration than the physical mixture. Pellets obtained from the fluid bed process showed a clear supersaturation phase after 1 hour and a generally higher dissolution rate than pellets obtained from the spouted bed process. Contrarily, the dissolution rate of the latter pellets approaches the supersaturation phase more continuously after 3 hours.

Fig. 3: Dissolution as a function of time. Black: ASD layered pellets (FB). Red: ASD pellets from direct pelletization (SB). Blue: physical mixture.

Fig. 3: Dissolution as a function of time. Black: ASD layered pellets (FB). Red: ASD pellets from direct pelletization (SB). Blue: physical mixture.

Summary

Both techniques, fluid bed and spouted bed as well, can be employed for manufacturing amorphous solid dispersions with good flow properties and dissolution profiles. Both techniques can be scaled up to pilot and production scale for batch or continuous manufacture of freely flowable ASDs. Cellets® serve stable and reliable cores for this venture.

Acknowledgement

We gratefully acknowledge Dr. Annette Grave and Dr. Norbert Pöllinger (Glatt Pharmaceutical Services, Germany), and Prof. Karl G. Wagner and Marius Neuwirth (University Bonn, Germany).

References

[1] T. Vasconcelos, B. Sarmento, and P. Costa, Drug Discovery Today, 12(23): 1068-1075 (2007)

[2] “Nifedipine”. The American Society of Health-System Pharmacists. Retrieved: Sept 17, 2019.

[3] J. Breitenbach, European Journal of Pharmaceutics and Biopharmaceutics, (54)2: 107-117 (2002)

[4] I. Weuts et al., Journal of Pharmaceutical Sciences, (100)1: 260-274 (2011)

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Figure-3Ingredientpharm
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Impact of the particle size on powder behavior in a Wurster fluid-bed process

Abstract

Cellets are inert starter cores made of microcrystalline cellulose (MCC). They play an important role in new formulations of solid dosage forms. As a carrier system for actives, the chemical inertness and surface smoothness are crucial parameters. Additionally, high level of robustness and sphericity simplify formulations and technical processes, such as fluidized bed technologies for coating and layering. In a joint study between the University of Hertfordshire and Freeman Technology (a Micromeritics company), the effect of pellets’ size on the behavior in a Wurster process is explained. Wurster fluid bed coating of Cellets with particle size larger than 400 µm is unproblematic. However, decreasing the particle size begins to complicate the coating process. So, powder rheology was used to compare Cellets with different particle sizes in terms of their effect on the powder flow in the Wurster fluid bed coater. For deeper knowledge, we strongly recommend reading investigations by V. Mohylyuk et al. [1]

Materials

Cellets® 90, 100, 200 and 350 (D50-size from 94 µm to 424 μm, Ingredientpharm, Switzerland). MCC powder Avicel® PH-102 (supplied by IMCD UK Ltd., UK) is included in the investigations, as it is widely used in industry and can be used by readers for comparison with other studies.

Wurster fluid-bed

Wurster process is a bottom-spray method, employed as a coating technology, for layering powder-like particles in a fluidized bed system (Figure 1). The process can be separated in different zones of mass flow, such as the down-flow zone or the horizontal transport zone. The flowability in these zones is crucial for homogeneous and efficient coating of the particles.

wurster_500x500

Figure 1: Wurster process is a bottom-spray method for layering powder-like particles in a fluidized bed system.

Hereby, the size of particles might play an important role. The narrow size distribution of MCC pellets is shown in Figure 2 and Figure 3. Measured data is presented in Table 1.

Figure-2

Volume weightened size distribution of Cellets 90 (red, diamonds), Cellets 100 (orange, triangles), Cellets 200 (blue, circles), Cellets 350 (green, squares).

The compact Cellets with fair sphericity, zero friability and a high level of surface smoothness show a fair mass flow rate which is almost independent of particle size at given experimental conditions and was determined by the gravitational funnel method. The reference MCC powder did not flow through the orifice.

Excipient PSD
[µm]		 Span
[µm]		 Flow rate
[g/s]
Avicel PH-102 115a 1.85 No flow
Cellets 90 94b 0.44 1.76
Cellets 100 163b 0.27 2.06
Cellets 200 270b 0.34 1.89
Cellets 350 424b 0.22 1.83

Table 1: Particle size distribution (D50) value and span by laser diffraction (a) and digital microscopy (b), and the mass flow rate by gravitational funnel method (5 mm diameter orifice) for the investigated excipients.

Impact of the Cellets’ size

The impact of the Cellets’ size on bulk powder behavior can only be estimated by screening additional parameters. In addition to the mass flow rate, standard pharmacopoeia methods such as bulk/tapped density were initially employed for the characterization of the powder’s properties. This was extended to rotating drum measurements providing the dynamic angle of repose and dynamic cohesivity index. Via powder rheology the conditioned bulk density, basic flowability energy, specific energy, pressure drop, permeability and compressibility (Figure 4) were obtained [1].

By picking the compressibility of Cellets at an applied force of 10 kPa normal stress, two key points need to be mentioned: (a) smaller particle size induces a higher rate of compressibility; (b) Cellets are less compressible than the reference MCC powder.

These findings are part of the open question on powder flow in a Wurster process. It is expected, that Cellets with a lower compressibility will result in better flow behavior in the fluidized bed.

Figure-3

Microscopic imaging of Cellets 100 (top left), 200 (top right), 350 (bottom left) with 1 mm scale bar and 100x magnification. Bottom right: surface of Cellets 350 in 1000x magnification.

Figure-4

Compressibility at 10 kPa normal stress on Cellets with varying particle size (D50) and Avicel® PH-102.

Summary

The flow of Cellets’ through a Wurster fluid-bed coater is likely to show improved performance as the Cellets’ particle size increases. Among others, a lower compressibility demonstrates a rheological behavior which is superior to MCC.

References

[1] Mohylyuk V, Styliari ID, Novykov D, Pikett R, Dattani R. Assessment of the effect of Cellets’ particle size on the flow in a Wurster fluid-bed coater via powder rheology. J D Deliv Sci Tec. 2019; 54: 101320, doi: 10.1016/j.jddst.2019.101320.

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Pellets and micropellets can be further processed into a wide rangeglatt.com
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The path to the perfect sphere

The renaissance of micropellets is promoting innovative technologies

In recent years, formulations based on pellets and micropellets have been the trend. New technologies make it possible to circumvent property rights for active ingredients and are therefore very popular with pharmaceutical customers. But which technologies are the most important?

Pellets are the jack-of-alltrades of solid dosage forms. Positioned somewhere between powder and granulate, they make bitter medicine more palatable and can even awaken a child’s instinct to play when the dosage forms are imaginative enough. One well-known example is the Xstraw, a plastic tube shaped like a drinking straw which is filled with pellets of active ingredient, through which children or elderly people can take in the medicine with water. Pellets in tablets are also making a splash – hybrids which combine all the advantages of both dosage forms. The pioneers in the development of these formulations, known as Multiple Unit Pellet Systems (or MUPS for short), was Astra Zeneca in 1999. Their move to embed the proton pump inhibitor Omeprazole in micropellets and then compress these pellets into immediate release tablets was an award-winning one at the time. The development of MUPS and Xstraw symbolizes the impetus pellets have fueled in recent years.

Klaus N. Möller, Head of Business Development at Glatt in Binzen / Germany, explains: “New excipients, coating materials and sophisticated processes allow us to extend the patent protection period and to make the dosage form more attractive.“

The number of patents registered yearly for pellet-based formulations has increased exponentially and is set to continue. According to research performed by IMS Health, the market for OSD (Oral Solid Dosage Forms) is growing by 6 to 8 percent every year. The number of drugs approved by the FDA also reflect this trend: in 2015, more than half were solid products.

Pellets, as defined by pharmacy guru Prof. Peter Kleinebudde are “an isometric agglomerate of powder particles in an approximate spherical or cylindrical form”, and are a task for perfectionists. The smoother and rounder the pellets, the better they are at fulfilling their purpose. The equipment manufacturer Glatt and their specialists from Pharmaceutical Services have been actively ursuing the subject for years.

There are two fundamental ways of making active ingredient pellets: direct pelletization, in which the powdered active ingredient and excipient combine in a matrix, and active ingredient layering, in which uses side spray or Wurster technology to apply the active ingredient to a starter core of sugar or microcrystalline cellulose.

A case for the specialists

One interesting process variant for matrix pellets is the extrusion of wet granulate in a basket extruder and subsequent rounding in a spheronizer. Möller elucidates: “Continuous wet granulation, followed by extrusion, spheronization and drying now make it possible to perform continuous processes”. Active ingredient pellets made like this can then be covered with a functional coating, be continuously mixed with excipients and be directly compressed into a MUPS tablet. The challenge is to avoid separation of the ingredients and destruction of the tablets during pressing.

Glatt, whose portfolio comprises all granulation and pellet manufacturing techniques, has spent recent years developing additional ways of “fine tuning” the pellet process and has opened up a range of new, interesting possibilities for the lifecycle management of active ingredients.

Pellets and micropellets can be further processed into a wide range

Pellets and micropellets can be further processed into a wide range

Applying the final touches

But what differentiates the manufacturing of granulates from the manufacturing of pellets? From a pharmaceutical point of view, both processes are closely related and are only separated by the form of the particle, since the ideal shape for pellets is a sphere. There are also definite commonalities in procedure. As Möller explains: “The fluidized bed can be used for both granulation and pelletization. This is why we configure fluidized bed machines on request to be multipurpose installations which then allow the continuous manufacturing of pellets. The individual process modules for direct pelletization with rotor technology, for layering active ingredient and for pellet coating with Wurster technology or the simple drying of wet granulates can be added as necessary. Wurster technology has been used in practice for many years: it is a fluidized bed technique in which starter cores or active ingredient pellets are sprayed with a insists. Möller says: “This method is robust and, because the process is so stable, it’s generally the most popular way to process pellets.”

Depending on the composition of the tablets, processing can last anywhere between eight and ten hours. The knack is knowing how to optimize the efficiency and times of the production process. Additionally, Möller recommends timely expert assistance during the development of the pellet formulation and the production process: “Right from the beginning, it will help to avoid mistakes and to keep an eye on process times and manufacturing costs”.

Micropellets and more

Glatt’s development team demonstrated how to refine an established process with the fluidized bed agglomeration technique known as MicroPx. The trick is to use the Conti process, which was conceived in Pharmaceutical Services’ laboratories in Binzen: first, the active ingredient/excipient liquid is spray-dried to a very fine product dust in a fluidized bed and agglomerated into tiny primary particles. The micropellets then build up, layer by layer, until the desired size is reached. The heart of this technology is a zigzag classifier which continuously ejects particles of sufficient size from the process, while simultaneously allowing smaller particles to reenter the process chamber where they continue to grow. Möller explains that the result of this method are high dosage active ingredient pellets in the size range of 100 to 400 μm with a narrow particle size distribution and content uniformity of a consistent 90 to 95 percent. This means that one significant limitation of former times is now no longer an issue: for many years, the volume of a pellet- filled capsule was larger — and therefore much harder to swallow — than the equivalent tablet with the same dose and composition. The use of microencapsulation, which changes bitter-tasting active ingredients into tasteless microparticles, means the taste is much improved now, too. Micropellets can be also pressed into tablets or MUPS tablets which already begin disintegration in the mouth. But the reason pharmaceutical companies find the MicroPx process so exciting is that it makes completely new formulations possible and therefore allows the legal circumvention of property rights. The technology experts have long known the secret to the perfect pellet, too, an answer provided by Complex Perfect Spheres Technology (CPS). CPS is a souped-up rotor process for fluidized bed machines that uses direct pelletization to yield functionalized pellets and micropellets which are perfectly round and smooth. Unlike classic rotor technology, the modified technique uses a tapered rotating disc which allows the movement of particles to be directed and pelletization to be performed to a defined endpoint. The results are perfectly spherical pellets of exactly defined sizes of between 100 and 1500 μm and extremely narrow size distribution. This is how Glatt’s own Cellets of microcrystalline cellulose are created, which are used as starter cores for pellets and in the Wurster process, for example — thus completing the formulation cycle.

Author

Klaus Möller, Head of Business Development Glatt Process Technology Pharma

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Some light on sugar and microcrystalline cellulose pellets

Abstract

Microcrystalline cellulose pellets (MCC) and sugar are well-known materials in pellet technology. Pellet technology describes the drug load onto starter pellets for controlled release formulations by Wurster process or others. Inert pellets are made of microcrystalline cellulose, while water soluble pellets are composed of sugar. Both material classes show desirable characteristics, such as a narrow particle size distribution, sphericity, surface smoothness. Also the batch-to-batch reproducibility and robustness of starter cores is high. A comparison does not seem to be that easy …

Starter cores in the micron range

Respecting the final application, the initial size of starter pellets defines the final size of the drug loaded pellet. In case of several layers of API and excipients, the initial size is factorized by the layering process. Pellet sizes in a range from 200 µm to 700 µm are frequently used (Table 1). We will focus on three size classes within this range and compare MCC pellets with those made of sugar.

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Figure 1: MCC pellets (here: Cellets® 200) are shown with good sphericity and striking surface smoothness.

Small-sized pellets starting at 200 µm

Small-sized pellets with sizes starting at 200 µm and larger, exhibit a comparably large surface-to-volume ratio. This can be beneficial in some applications. For example, taste-masking of bitter API is accessible.

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Figure 2: Sugar pellets (here: 50/70 mesh) are shown with moderate sphericity and reduced surface smoothness.

Figure 1 displays a microscopic image of MCC Cellets® 200 and Figure 2 displays the image of sugar pellets in 50/70 mesh, respectively. It is obvious, that for small-sized pellets, the sphericity and surface smoothness of MCC pellets is superior.

Size MCC Sugar
small Cellets® 200 50/70 mesh
Medium Cellets® 350 40/50 mesh
large Cellets® 500 25/30 mesh

Table 1: Size definition of MCC and sugar pellets.

Mid-sized pellets up to 500 µm

This class of pellets is frequently used for multi-layer coating technologies. Easy processing and reliable batch-to-batch control are positive aspects. Exemplary application is a hydrocortisone formulation for peadiatrics. Again, Figure 3 (MCC pellets) and Figure 4 (sugar pellets) show advantages in surface properties for the MCC material.

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Figure 3: MCC pellets (Cellets® 350) are shown.

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Figure 4: Sugar pellets (40/50 mesh) are shown

Large-sized pellets above 500 µm

In some applications, larger pellet sizes are requested. Let’s have short excurse into straws which can contain larger pellets in dry state. Upon use by sucking liquid through the straw, the API coating dissolves immediately while the pellet remains in the straw by simple filters.

In this size range the striking advantages of MCC pellets are not of immediate importance, but still visible.

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Figure 5: MCC pellet above 500 µm (Cellets® 500).

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Figure 6: Sugar pellet above 500 µm (25/30 mesh).

Summary

Microcrystalline cellulose pellets (Cellets®) show superior surface and sphericity properties compared to sugar pellets. In case of non-dissolving applications, MCC pellets are first choice. As sugar pellets exhibit strong dissolution in water, there is still a fair application range for them.

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Sphericity and size distribution of µm-sized microcrystalline pellets

Abstract

Microcrystalline Cellulose (MCC) pellets represent a chemically inert class of active pharmaceutical ingredients (API) carriers. A narrow particle size distribution (PSD) maximizes control over content uniformity. In this case study, we will focus on measuring the particle size distribution and on sphericity.

Pellets for oral drug forms

MCC pellets are used as starter beads for API loading. Low or high drug dose loading is technically feasible. These pellets are made of pure MCC and provide a robust platform for delivery of one or multiple APIs. Certain processing technologies for pellet coating allow these starter beads to be compatible with soluble or insoluble APIs, e.g. by Wurster bottom spray [1,2] or Rotor dry powder layering technology [3]. Coated pellets can be filled into capsules, or compacted into multiple-unit pellet system (MUPS) tablets [4], where a tight PSD maximizes control over content uniformity.

Particle size distribution maximizes the control over content uniformity in applications of complex oral dosing forms. Speaking about uniform or monodisperse particles, these information always point to general information of the particular system, not of the individual particle itself. Therefore, PSD is a globular measure allowing simple, easy and fast analysis of the particulate matter. Major key information from a PSD measure are the so called D-values. A Dx value represents a dimension, where a ratio of X particles is smaller. For reasons of simplicity weighted functions, such as number, radius or volume, are not. Extending these metrics to D10 and D90 additionally informs about the width of the entire size distribution (Figure 1).

CS_sphericity_image_1

Particle size distribution of two different particle systems with identical median dimension D50. Blue: wide PSD, green: narrow PSD. Dotted lines are guides to the eyes.

Figure 1: Particle size distribution of two different particle systems with identical median dimension D50. Blue: wide PSD, green: narrow PSD. Dotted lines are guides to the eyes.

Dimensions of pellets

In this study, imaging technology (Horiba, Camsizer) was employed for the size analysis. Representatively, more than 50 charges of Cellets® 100 and Cellets® 500  (Figures 2-3) have been analyzed for the D10, D50 and D90 values.

CS_sphericity_image_2

D10 (red), D50 (green) and D90 (blue) value for several Cellets® 100 charges. Solid lines are measures, dashed lines represent the averaged value of all charges. The standard deviation is below 10 %.

Figure 2: D­10 (red), D50 (green) and D90 (blue) value for several Cellets® 100 charges. Solid lines are measures, dashed lines represent the averaged value of all charges. The standard deviation is below 10 %.

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D10 (red), D50 (green) and D90 (blue) value for several Cellets® 500 charges. Solid lines are measures, dashed lines represent the averaged value of all charges. The standard deviation is below 4 %.

Figure 3: D10 (red), D50 (green) and D90 (blue) value for several Cellets® 500 charges. Solid lines are measures, dashed lines represent the averaged value of all charges. The standard deviation is below 4 %.

The results show only slight variations in the PSD between the charges. The standard deviation is smaller than 4 % (Cellets® 500) and smaller than 10 % (Cellets® 100) which confirms a high reproducibility in production (Table 1). Both values are remarkably good for technical spheres. Furthermore, none of the charges was out of specifications and fit into the desired size distribution between 500 µm and 710 µm easily. The close gap between D­10 and D90 clearly identify an excellent monodispersity.

Standard deviation Cellets 100 Cellets 500
of D­10 8.28 % 3.97 %
of D50 7.12 % 3.52 %
of D90 4.68 % 3.11 %

Table 1: Standard deviation for D­10, D­50 and D­90 values Cellets® 100 and Cellets® 500 charges.

CS_sphericity_image_4

Electron microscopy yield perfect imaging data of the MCC pellets’ surfaces. Magnification: 250x, working distance 8.0 mm, voltage: 10 keV.

Figure 4: Electron microscopy yield perfect imaging data of the MCC pellets’ surfaces. Magnification: 250x, working distance 8.0 mm, voltage: 10 keV.

Perfect sphericity? – Yes!

For a more detailed shape analysis, electron microscopy yield perfect imaging data of the MCC pellets’ surfaces (Figure 4). Additionally, MCC pellets have a distinguishing friability.

Summary

Microcrystalline Cellulose (MCC) pellets show excellent chemically inertness, high degree of sphericity, narrow size distribution and high reproducibility in production. These properties make Cellets® becoming one of the first choice for inert API carriers. We have proven these excellent properties for Cellets® 100 and Cellets® 500. The obtained results are representative for other size classes ranging from 100 µm to 1400 µm.

Acknowledgement

We acknowledge IPC Process-Center (Dresden, Germany) for the analytics, and Fraunhofer IFAM (Dresden, Germany) for recording the electron microscopic pictures.

References

[1] H. R. Norouzi, International Journal of Pharmaceutics, Volume 590 (2020) 119931

[2] D. Jones, Developing Solid Oral Dosage Forms, Pharmaceutical Theory And Practice (2009) 807-825

[3] M. Ahtola, Dry powder layering of high viscosity polymers using a fluidized bed rotor granulator, Master thesis, U of Helsinki (2014)

[4] S. Abdul, A. Chandewar, S. Jaiswal, Journal of Controlled Release, Volume 147(1) (2010) 2-16

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