The extrusion and spheronization of chitosan

Abstract

Extrusion/spheronization is a common pellet production technique. Typically, microcrystalline cellulose (MCC) is used for this purpose, but this has distinct disadvantages, such as lack of disintegration and a limited capability for controlled drug release. A promising alternative extrusion filler and binder is chitosan, a natural, cationic polymer, which is widely available.

In this study, the effect of the degree of deacetylation of different chitosan grades on process and pellet properties was investigated. Effects on the drug release (budesonide was used as the model drug) from chitosan pellets were observed.

It was found that the degree of deacetylation of chitosan markedly influences the process capability of chitosan and, as a result, the pellet-forming properties. The higher the degree of deacetylation, the more stable the process and the better the resulting pellet properties (measured as pellet size, aspect ratio, crushing strength and friability). Oscillatory rheological measurements of the extrudate indicated that a higher degree of deacetylation also influenced the viscoelastic properties of the wet mass. A greater degree of deacetylation clearly showed a higher phase angle and, accordingly, a higher proportion of the viscous modulus.

These findings correlated well with better extrudability as a result of a higher degree of deacetylation. Drug release was found to be less influenced by the degree of deacetylation. In all cases, a linear drug release from the chitosan pellets was observed. In conclusion, the study underlined that chitosan offers an excellent alternative to MCC for extrusion/spheronization, and allows for a controlled drug release with zero order release kinetics.

Introduction

Chitosan (Figure 1) is produced by N-deacetylation of chitin, the second most common natural polymer. This process leads to a cationic polymer that is characterized by pH-dependent solubility in water. In contrast to many other high molecular weight polymers of anionic character, chitosan is only soluble in acidic aqueous solutions.

Figure 1

Chitosan is of pharmaceutical interest because of its positive attributes with respect to toxicity, biocompatibility and bioavailability.¹ In addition to wound-healing effects, chitosan is also said to have fat and cholesterol-binding properties, as well as antiulcerogenic effects. As a result, chitosan has been used for numerous delivery systems, especially in the form of tablets, microparticles, pellets and beads for oral application.^2–6

The intention of pellet production is the development of a multiparticulate delivery system, in particular, sustained release formulations for oral application because of their biopharmaceutical advantages compared with monolithic dosage forms.

Gastric residence time of monoliths, such as nondisintegrating tablets, is highly variable and depends on stomach volume, size and density of chime and particle size. It is also well-accepted that particles smaller than 2–3 mm (e.g., pellets) rapidly pass the pylorus with constant rate.⁷ In addition, gastrointestinal (GI) irritations are reduced by a rapid and homogeneous distribution of pellets in the intestine. A burst effect has so far not been noticed with multiparticulate systems.

From a technological view, pellets can be favourably used instead of monolithic systems in case incompatible drugs have to be processed. They can be pelletized separately and mixed later, or pellets with different release mechanisms can be mixed individually to give a new modified release profile. Until now, MCC, despite intensive research on alternative materials, is mostly used as a universal binder and filler for extrusion/spheronization. Chitosan represents an alternative filler and binder, and may be a substitute for MCC. Compared with MCC, it is also of natural origin and biodegradable, and so has been widely used in the pharmaceutical and cosmetic industries. Furthermore, chitosan can be degraded by the colonic microflora and could, therefore, be particularly useful for colon-targeted delivery of drugs incorporated into enteric-coated pellets. In addition, it is assumed that the drug release from chitosan pellets differs from that of MCC pellets, so it offers new opportunities for drug delivery.

Up until now, several techniques have been used to prepare chitosan pellets. A method to produce chitosan beads is the precipitation in an acetic medium by adding a salt-containing solution.⁸ For the production of cryopellets, Hofmann et al. used a chitosan solution (chitosan dispersion) that had been added to a cooled nonsolvent.⁹

Another approach to prepare heterogeneous pellets is to coat cores with a chitosan solution, thus obtaining a release-controlling polymer film.¹⁰ In addition, it is possible to produce homogeneous pellets using extrusion/spheronization. For that purpose, Tapia et al. dissolved chitosan in diluted acetic acid and then added this as granulation liquid to the powder mixture.¹¹ The wet mass was passed through a ram extruder and spheronized into pellets. The chitosan fraction in the obtained pellets was as low as 2–3% w/w. Chatchawalsaisin et al. (using a ram extruder) and Santos et al. (using a screw extruder) achieved an increase of the chitosan fraction to 16% in the pellet matrix.^12,13

A further increase of the chitosan fraction in the pellets was obtained by Goskonda et al. by an extrusion/spheronization technique.¹⁴ For this purpose they used chitosan with different molecular weights and, for the extrusion process, mixed them with colloidal MCC — a spray-dried mixture of 89% MCC and 11% carboxymethylcellulose sodium. However, the amount of chitosan in the obtained pellets did not exceed 40% of the total solid content. In contrast to the study of Goskonda et al., Steckel and Mindermann-Nogly produced MCC pellets with a maximum chitosan fraction of 50% with an extrusion/spheronization technique using demineralized water as the granulation liquid. Substitution of the demineralized water through diluted acetic acid (as the granulation liquid) enabled the process to be performed with pure chitosan. This made the production of pellets with good morphological and mechanical properties without using any further additives possible.¹⁵

The chitosan used in those studies had a degree of deacetylation greater than 95% and could be handled without any problem. However, subsequent batches of the same specified quality showed inadequate plasticity for the extrusion process. Minor differences in the degree of deacetylation were expected to cause these process differences. Therefore, the aim of this study is to investigate the influence of the degree of deacetylation of chitosan on the production of pure chitosan pellets, as well as on the physicochemical pellet and release properties of the obtained pellets.

Materials

Chitosan (Chitoclear FG 95; Primex, Iceland) with a specified degree of deacetylation (DD) of >95% was used as matrix forming material. Different degrees of deacetylation, as determined by the manufacturer, were used to study the influence of the degree of deacetylation on the extrusion/spheronization properties. The following batches were used:

• TM 1977 (DD=99.9%)

• TM 1762 (DD=98%)

• TM 2262 (DD=90%)

• TM 1763 (DD=95%).

Concentrated acetic acid (Merck, Germany) and demineralized water were mixed and used as the granulation fluid for the extrusion step. The used drug, budesonide, was a gift from AstraZeneca (Germany). All other reagents used (Merck) and solvents were of analytical grade. The water used was either of demineralized or double-distilled quality, depending on the intended use.

Methods

Preparation of the pellets. Initially, the drug/chitosan mixtures were blended for 5 min in a Turbula T10 (W.A. Bachhofen AG, Switzerland), deagglomerated by the use of a sieve (250 μm [Retsch GmbH, Germany]) and blended again for 25 min in a Turbula blender. The intermediate sieving step was included to obtain a homogeneous distribution of the drug substance, which is present in a very low concentration. Afterwards, the powder mixture was fed by means of a screw feeder (K-Tron Schweiz AG, Switzerland) to the extruder at a rate of 15 g/min. The extrusion of the different powder mixtures was performed by a power consumption-controlled twin screw extruder (8 ZE 25; Berstorff, Germany) at a fixed power consumption level of 120 W. A detailed description of the functional principle is given by Lindner et al.¹⁶ As a granulation liquid, 0.2 N acetic acid was used as determined in previous studies.⁶

400–600 g of the extrudate was collected per batch and spheronized in a spheronizer (Type S-320; Nica, Sweden) with a cross-hatched plate at 800 rpm for 5 min.

The obtained pellets were then dried in a fluid-bed dryer (TR2; Glatt GmbH, Germany) for 30 min at 55 °C.

Analytical methods.

Moisture content. Three samples of approximately 8 g were taken during the extrusion and dried at 75 °C during 36 h in a hot air oven (Heraeus T6; Kendro, Germany). The moisture content (MC) of the extrudate was calculated according to Equation 1, where m_w describes the wet mass and m_d is the dried mass:

Extrudate viscosity. A sample of approximately 8–10 g was collected during the extrusion process and measured using a plate–plate viscosimeter (Bohlin Instruments, UK) during a fixed period of 120 s with constant split amplitude. The calculation of the phase angle and the proportion of elastic to viscous parts was performed with standard computer software (Bohlin Instruments). Measurements of each batch was performed in triplicate.

Image analysis. The fine content was removed by using a 500 μm sieve. The shape and size of the pellets >500 μm were determined individually by an image analysis system (Leco 2001; Leco, Germany). The average Feret diameter was calculated indirectly from the (directly calculated) middle Feret diameter as the mean of eight measured Feret diameters from eight different angles of each single pellet. At least 500 pellets/batch were determined. For the evaluation of the pellet shape, the aspect ratio was determined. A detailed description of the test conditions used for pellet analysis is given by Lindner et al.¹⁷

Scanning electron microscopy. To evaluate the shape and surface characteristics, photographs of pellet samples from the different batches were taken by using a scanning electron microscope (Phillips XL120; Phillips, The Netherlands). Pellets were fixed on an aluminium stub with conductive double-sided adhesive tape (Leit-Tabs; Plano GmbH, Germany) and coated with gold in an argon atmosphere (50 Pa) at 50 mA for 50 s (Sputter Coater; Bal-Tec AG, Liechtenstein).

Porosity. The porosity (ε) of the pellets was calculated using Equation 2, with ρ_e as true density and ρ_a as apparent density:

The true density was determined by means of a helium gas pycnometer (Pycnomatic ATC; ThermoFinnigan, USA) and the apparent density was measured using a mercury intrusion porosimeter (Pascal 140; Carlo Erba Instruments, Italy). All measurements were performed in triplicate.

Crushing strength. For the investigation of crushing strength, the pellets were initially conditioned in a climate chamber with a relative humidity of 55% during 24 h to reduce the influence of changing ambient humidity. Afterwards, 50 pellets/batch from the sieve fraction 900–1120 μm were analysed with a texture analyser TAXT2 (Stable Micro Systems, UK). Briefly, a single pellet was centred below the punch. Punch movement down onto the pellet was standardized to 1 mm/s. After contact, the strain was 50% of the height while measuring the force. The arithmetic mean of these 50 measurements was used as the crushing strength in the study.

Friability. For this part of the investigation, a friability tester was used.¹⁸ Samples of 8.000 g (m₁) pellets with a size >710 μm were taken and then fluidized by an air stream of 450 L/min for 16 min. The removed pellets were weighed after the test (m₂) and the friability (F) calculated according to Equation 3.

Drug release. The drug release of budesonide-containing pellets was determined by the liberation cell equipment Dissotest CE 6 (Sotax, Switzerland). The drug release medium phosphate buffer pH 7.4 (Ph. Eur.) was degassed in an ultrasonic bath. A pellet mass corresponding to a single dose of 3 mg budesonide was filled into each of the release cells. The release medium was then pumped through the cells at a flow rate of 8 mL/min. During the drug release test, samples were collected out of the discharged medium in discontinuous intervals of 1 min. 100 μL of the samples were taken for HPLC analysis. The HPLC system consisted of a Gynkotek High Precision Pump Model 300 (Gynkotek, Germany), a Kontron HPLC Autosampler 360, a Kontron HPLC Detector 430 (Kontron Instruments, Italy) and LiChrospher 100 RP18 columns (5 μm; 125 mm; [Merck]). The peak integration (wavelength 246 nm) was performed using a computer controlled software (Data System 450; Kontron Instruments). A mobile phase of acetonitrile/phosphate buffer pH 3.0 (Ph. Eur.) mixture (55:45) was used. The amount of drug was calculated using an external standard. Three samples per batch were determined in duplicate by HPLC.

Results and discussion

Preparation of chitosan pellets. The extrusion of chitosan with a degree of deacetylation of >99% led to a stable process at a fixed power consumption level of 120 W (Table 1). Other chitosan properties made it necessary to adjust the power consumption to a higher level. The extrusion of chitosan with a degree of deacetylation of 90% at a power consumption level of 120 W resulted in a lack of granulation liquid in the wet powder mass.

Table 1 Extrusion parameters for the different chitosan batches.

Consequently, the mass accumulated in front of the die plate and blocked the holes; therefore, the extrusion had to be stopped. As can be seen in Table 1, indicated by the low standard deviation (SD) of the power consumption, a stable extrusion process was only possible at a 99.9% degree of deacetylation. With a decreasing degree of deacetylation of the used chitosan, the extrusion process became unstable.

Chitosan, with a 90% degree of deacetylation could not be extruded anymore. The process is unstable because of incomplete wetting and feeding of the wet mass, leading to a blockage of the die plate at irregular intervals. Accordingly, the pressure at the die plate, as well as the power consumption, increased.

In case the pressure exceeded the binding strength of the extrudate mass, the die holes were pressed free by the accumulated mass in front of the plate and so a drug-containing batch was not produced with this chitosan batch.

The process parameters of the produced budesonide-containing batches compare favourably to the pure chitosan batches. The influence of the low budesonide fraction in the powder mixture on the extrusion process was, therefore, excluded (Table 1).

In Figure 2 the moisture content of the different chitosan and chitosan/budesonide batches is shown. It can be seen that the moisture content increases with a decreasing degree of deacetylation — caused by the functional principle of the extruder. Through the mechanism of negative feedback, it met with increasing resistance at the die plate. This led to a higher power increase, which, in turn, increased the volume of granulation fluid needed to return to the fixed power level again.

Figure 2

Accordingly, the less stable extrusion process observed with a lower degree of deacetylation leads to a higher moisture content in the obtained extrudate. A 99.9% degree of deacetylation enabled an extrusion at 120 W and led to an extrudate of sufficient plasticity for the following spheronization. However, the moisture content is small enough to prevent agglomeration of the extrudate. This could be attributed to the fact that the chitosan particles at the used acid concentration and the adequate moisture content of the mass are partly dissolved on the surface and form a gel-like mass. A higher moisture content would lead to an extrudate that could not be spheronized.⁶ With a decreasing degree of deacetylation of the chitosan and increasing moisture content, the gel formation becomes stronger. This would result in the obtained extrudates badly breaking and, therefore, would not become 'rounded' in subsequent spheronizations. Elongated and broken extrudates would result, as our experiment demonstrated.

The presence of budesonide did not affect the moisture content compared with the drug-free extrudates (Figure 2).

The viscoelastic properties of the extrudates were measured by oscillatory viscosimetry to support the assumption that a higher moisture content is related to more elasticity (Table 2). Generally, drug-containing and drug-free batches behaved similarly. The higher the degree of deacetylation, the higher the phase angle of the wetted chitosan extrudate. In line with this finding, the elastic properties of the extrudate increase, which is expressed in an increasing ratio of elastic to plastic modules (Table 2). Because of the higher proportion of elastic behaviour, the extrudate cannot be broken and rounded adequately.

Table 2 Viscosity data of the different chitosan extrudates.

Morphological characteristics. Orifices of the die plate have a diameter of 1 mm. Therefore, the Feret diameter of the produced pellets is expected to be in the range 0.9–1.1 mm. This characteristic could only be achieved for the chitosan pellets produced from chitosan with a 99.9% degree of deacetylation (Figure 3). Furthermore, it is shown that the average Feret diameter increases with a decreasing degree of deacetylation of the used chitosan from 990 μm/1114 μm (DD 99.9%; drug-free/drug-containing) to 2227 μm/2507 μm (DD 95%; drug-free/drug-containing). In addition, the pellet size distribution becomes broader with a decreasing degree of deacetylation as indicated by means of the SD.

Figure 3

On one hand, this can be caused by the widening of the extrudate strings after they are pressed under pressure through the die plate. On the other hand, it can also be explained by the different viscoelastic properties of the extrudate. Only the pellets produced with chitosan having a 99.9% degree of deacetylation had an adequate plasticity of the extrudate, resulting in a regular breakage of the extrudate during spheronization.

Considering the aspect ratio of the pellets (Figure 4), the same trend becomes visible: the aspect ratio increases with a decreasing degree of deacetylation from 1.18/1.11 for the pellets consisting of chitosan (DD 99.9%; drug-free/drug-containing) to 2.92/3.61, by using a chitosan with a 95% degree of deacetylation (drug-free/drug-containing). Because of the higher elasticity of the extrudates with a decreasing degree of deacetylation, the extrudate strings break irregularly, resulting in the predominant formation of sticks. Only the pellets based on chitosan with a 99.9% degree of deacetylation could be characterized as round or rather nearly round (i.e., showed typical pellet shape) because only in this environment was the extrudate plasticity adequate to initiate a regular string break and sufficient rounding.

Figure 4

Figure 5 shows scanning electron miscroscopy (SEM) photographs of the chitosan pellets under the described conditions. The photographs taken at a higher magnification reveal the surface structure of chitosan pellets, which results from the typical particle shape of the chitosan raw material.

Figure 5

This supports the theory that the chitosan is partially dissolved on the surface of the particles by the diluted acetic acid. The particle shape of the chitosan is not changed during the extrusion process as it is for MCC.¹⁵ The surface structure changes with a decreasing degree of deacetylation because of the moisture loss during the drying process. Pores and larger holes on the surface of the pellets consisting of chitosan with a 90% degree of deacetylation can be seen as a result of the swelling because of the high extrudate moisture content.

Considering the porosity (Table 3) of the pellets, porosity is generally at a low level (in the range of 1–3%) for all degrees of deacetylation. Pore formation because of swelling/shrinking and the subsequent drying step as observed for MCC pellets could not be observed.

Table 3 Porosity, friability and crushing strength of the different chitosan pellets.

Mechanical properties.

Friability. Important pellet parameters, besides the morphological characteristics, are their mechanical properties. Pellets with low friability (<0.5) are obtained when chitosan with a 99.9% degree of deacetylation is used. The SEM photographs of the corresponding pellets show a relative even surface compared with those produced with chitosan having a lower degree of deacetylation. Because of the minor swelling with an increasing degree of deacetylation, extrudates and pellets made of chitosan with a 99.9% degree of deacetylation exhibit good interparticulate adhesive strength, which is reflected in the low friability. With a decreasing degree of deacetylation, the pellet friability increases to more than 1%. Particle packing in the pellet matrix appears to be loose, allowing single particles to be released from the surface more easily. These findings are also supported by the SEM photographs. Based on their morphological and mechanical properties, chitosan pellets with a <99.9% degree of deacetylation do not qualify for further processing.

Crushing strength. The same mechanisms that affect the shape and size of the pellets also have an impact on the crushing strength. However, the crushing strength of all pellets was found to be in a range that is considered sufficient for further processing (Table 3). Pellets made from chitosan with a low degree of deacetylation exhibited a relatively high crushing strength, although the friability was highest for this batch.

Drug release in phosphate buffer pH 7.4 (Ph. Eur.). To elucidate the influence of the degree of deacetylation on the drug release properties of chitosan pellets, budesonide was included as a model drug and compared with the release properties of the marketed product, Entocort (AstraZeneca).

As the chitosan matrix is not soluble in phosphate buffer pH 7.4, but swellable under a low volume expansion, we would expect that drug release from the matrix is diffusion-controlled — comparable to an MCC matrix. As can be seen from Figure 6, sustained release of budesonide was found. Comparing the release curves with Entocort, it is clear that the drug release from the chitosan pellets with any degree of deacetylation is slower than for Entocort. In addition, drug release was not completed after 9 h and was found to be in the range of 53–58% of the loaded dose, regardless of the degree of deacetylation.

Figure 6

Compared with the differences of the chitosan pellets, with respect to processability as well as morphological and mechanical properties, the drug release hardly shows differences between the chitosan pellets of different degrees of deacetylation of the used chitosan. In all cases, swelling of the chitosan matrix can be observed, resulting in a sustained drug release (Figure 6). Looking at the release patterns more closely, the chitosan pellets with a lower degree of deacetylation had the lowest release rate despite a higher porosity compared with pellets that were produced from chitosan having a 99.9% degree of deacetylation. It is assumed that the more intensive swelling of chitosan with a decreasing degree of deacetylation blocks the micropores in the pellets. Linear fitting of the release data revealed that both chitosan and Entocort pellets show a drug release following zero order kinetics (Figure 6).

While the budesonide is constantly released by passive diffusion over the ethylcellulose film in case of the Entocort, the budesonide release from the chitosan pellets was expected to be matrix-controlled. However, fitting the data to HiguchiB4s matrix model (square root of time [Figure 7]) shows that the release cannot be described with the mechanism expected for a non-erodible matrix.

Figure 7

The definite drug release mechanism from chitosan pellets still appears to be unclear. Because the chitosan matrix is insoluble but swellable in the release medium, a constant movement of the gel front into the core can be observed. This increases the diffusion coefficient in the water layer of the swollen chitosan pellet. Although the thickness of the diffusion layer increases with time, the reduced viscosity allows for a constant drug release and, therefore, can best be fit according to zero order kinetics.

Key points

Conclusions

As described by Steckel et al., it is possible to produce pure chitosan pellets with a tight particle size distribution, low friability and high crushing strength by extrusion/spheronization.¹⁵ However, the process was found to be highly dependent on the degree of deacetylation of the used chitosan, because this has a strong influence on the production and resulting properties of the chitosan pellets. Only chitosan with a degree of deacetylation >99% appeared suitable for extrusion/spheronization, which could be linked to the differences in the viscous and elastic properties of the wetted chitosan mass. The drug release from the chitosan pellets was found to be linear over time.

Katrin Jess is a licensed pharmacist and PhD student at the University of Kiel (Germany).

Hartwig Steckel is a licensed pharmacist, specializing in the field of pharmaceutical technology. He has many years of experience, both in academia and industry. He is head of pharmaceutical development at Intendis GmbH (Germany), a BayerScheringPharma company.

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