Katrin Jess

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. 

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. 

Chitosan is of pharmaceutical interest because of its positive attributes with respect to toxicity, biocompatibility and bioavailability.

  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.

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.

				  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.

  For the production of cryopellets, Hofmann

 used a chitosan solution (chitosan dispersion) that had been added to a cooled nonsolvent.

Another approach to prepare heterogeneous pellets is to coat cores with a chitosan solution, thus obtaining a release-controlling polymer film.

  In addition, it is possible to produce homogeneous pellets using extrusion/spheronization. For that purpose, Tapia

. dissolved chitosan in diluted acetic acid and then added this as granulation liquid to the powder mixture.

  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

 (using a screw extruder) achieved an increase of the chitosan fraction to 16% in the pellet matrix.

A further increase of the chitosan fraction in the pellets was obtained by Goskonda 

by an extrusion/spheronization technique.

				  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 

, 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.

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. 

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: 			

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. 

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 

 As a granulation liquid, 0.2 N acetic acid was used as determined in previous studies.

 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. 

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 (

) of the extrudate was calculated according to Equation 1, where 

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. 

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 

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). 

The porosity (ε) of the pellets was calculated using Equation 2, with ρ

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. 

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. 

For this part of the investigation, a friability tester was used.

) 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 (

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. 

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. 

Katrin Jess

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