Research paper
Tableting and stability evaluation of enteric-coated omeprazole pellets
Murat Tu¨rkog˘lu*, Hakan Varol, Mine C¸elikok
Pharmaceutical Technology Department, Faculty of Pharmacy, Marmara University, Istanbul, Turkey
Received 7 March 2003; accepted in revised form 24 October 2003
Abstract
In this study, fluidized-bed manufactured enteric-coated omeprazole pellets were compressed into tablets. The stability of the pellets and
those of compressed tablets were evaluated for remaining omeprazole and for degradation products under an accelerated stability protocol.
The data were analyzed using the artificial neural network (ANN) and analysis of variance (ANOVA). It was found that enteric-coated
omeprazole pellets could be compressed into quickly disintegrating tablets using microcrystalline cellulose granules as the pressure
absorbing matrix. The ANN, using the multilayer perceptron model, predicted that there was a positive correlation between tablet crushing
strength and microcrystalline cellulose concentration. Microcrystalline cellulose matrix showed a strong plastic deformation and all the
pellets inside the tablet maintained their integrity with no significant change in their surface properties. Omeprazole degradation in acid
medium was mainly dependent on microcrystalline cellulose concentration. A 90-day accelerated stability test in brown glass bottles with a
desiccant showed that all prototype formulations would result in an acceptable stability profile for both remaining omeprazole, and also for
the increase of impurity concentrations.
q 2003 Elsevier B.V. All rights reserved.
Keywords: Pellets; Artificial neural networks; Pellet compression; Omeprazole; Accelerated stability
1. Introduction
Tableting of multiparticulate systems such as pellets and
microspheres is an attractive approach to prepare a single
unit dosage form that will readily disintegrate into its
essential components when exposed to gastro-intestinal
fluid. That type of dosage form will maintain the advantages
of pellets despite being a tablet. There are two points of
interest when compressing a coated particle. The first one is
the effect of excipients, and the other is the composition and
the amount of coating on the particle. In this study, we only
investigated the effect of excipients on coated particles. The
polymer type and the polymer/plasticizer ratio were kept
constant for the optimized enteric-coated product. Maganti
and C¸elik reported that increasing the amount of polymer on
the coated particles reduced their yield strengths and
resulted in compacts with lower tensile strength and higher
elastic recovery, pellets coated with increasing amounts of
coating exhibited relatively more punch velocity depen-
dence [1]. Schmidt et al. found that the most important
factors were the coating polymer and the amount of coating
when enteric-coating integrity was tested for bisacody
pellets in acid medium after compression [2]. Another
report by Schmidt et al. focused on the effect of excipients
on enteric-coated pellets and for approximately 1 mm
pellets, the larger size Avicel granules caused more
deformation of pellets but less damage to the coating in
comparison to Avicel PH 101 powder and it was also
concluded that damage to the coating mainly occurred on
the tablet surface during compression [3]. Beckert and
Lieneweg reported that it was possible to compress enteric-
coated pellets into tablets without significant damage using
Eudragit L30 D-55 at 35% level and propylene glycol at
20% level as preferred plasticizers [4]. Lefranc et al. also
reported similar findings for enteric-coated 5-ASA pellets
[5]. Omeprazole is a member of acid-labile H þ Kþ-
ATPase inhibitors also known as gastric proton pump
inhibitors. Omeprazole is sensitive to heat, humidity, light,
and organic solvents. Discoloration ranging from light beige
to deep purple will occur immediately when omeprazole is
exposed to unfavorable conditions. Stability of omeprazole
for different pH values in solution was reported by Bailey
0939-6411/$ - see front matter q 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ejpb.2003.10.008
European Journal of Pharmaceutics and Biopharmaceutics 57 (2004) 279–286
www.elsevier.com/locate/ejpb
* Corresponding author. Address: Pharmaceutical Technology
Department, Faculty of Pharmacy, Marmara University, 34668
Haydarpas¸a, Istanbul, Turkey. Tel.: þ90-216-418-5029; fax: þ90-216-
345-2952.
E-mail address: turkoglu@marmara.edu.tr (M. Tu¨rkog˘lu).
et al. [6], Stability of commercial omeprazole products from
13 countries was reported by McCallum [7]. An in vitro
evaluation, the degradation products for commercial
omeprazole pellets were reported by Rodrigues [8]. A
detailed list and chemical structure of omeprazole degra-
dation products can be found in the USP or EP. The purpose
of this study was to investigate the possibility of tableting
enteric-coated omeprazole pellets using common tableting
excipients, to study the accelerated stability of the tablets,
and also to model omeprazole degradation based on the
process and formulation factors.
2. Materials and methods
2.1. Materials
Omeprazole containing pellets were obtained by drug
layering on sucrose–starch spheres a HPMC water-soluble
film coating, and a final Eudragit L30 D-55 enteric coating
was applied using a rotary fluidized-bed equipment based on
a previously described method by Turkoglu et al. [9].
Coated pellets were sieved and a sieve fraction between 425
and 710 mm was used for further tablet manufacturing.
Some common tablet excipients such as microcrystalline
cellulose NF (Avicel PH 102, FMC, USA), pregelatinized
starch (Starch 1500, Colorcon, USA), PEG 6000 (Merck,
Germany), sodium carboxymethyl starch (Primojel,
Generichem, USA) were used to design different tablet
formulas containing 150 mg enteric-coated pellets corres-
ponding to 20 mg omeprazole per tablet. Magnesium
stearate was used as a lubricant in all formulations.
2.2. Wet granulation for excipients
To match the particle size of the excipients with the
pellets a wet granulation process was carried out using
Avicel and Starch 1500. Water was used as the binder and
granules were dried overnight in a conventional oven at
45 8C, then the final product was sieved to collect granules
with a size between 425 and 710 mm. The mixtures of
enteric-coated pellets and the other excipients were formed
in a laboratory size V-blender.
2.3. Tableting
Normal convex, 10 mm tablets with a target weight of
450 mg were compressed using a single-punch instrumented
tablet press (Korsch EKO, Germany). Compression and
ejection forces were monitored and recorded continuously
using PC-based software (National Instruments, MAX,
USA) and two force sensors (ICB Model 203B for upper
punch and Model 201B03 for lower punch, PCB Piezo-
tronics, New York, USA). Compression of tablets in an
industrial type machine was performed using a 24-station
rotary tablet press (GEA Courtoy R 190 FT, Belgium).
2.4. Tests for tablets
Tablet properties such as crushing strength (Holland
C50, UK), friability, weight uniformity were checked and a
gastro-resistance study was performed using the apparatus II
of the USP 24 at 50 rev./min, in 0.1 N HCl at 37 8C for 2 h.
A dissolution study (Sotax AT 7 Smart, Switzerland) was
performed in pH 6.8 USP buffer for omeprazole release. A
10 ml sample was withdrawn from each dissolution beaker
and then 2 ml of 0.25 N NaOH solution was added. The
samples were filtered through a 45 mm filter and injected
into the HPLC system as described in Table 1 assay method.
2.5. Stability testing protocol
The stabilities of five formulations and those of enteric-
coated omeprazole pellets were studied under the acceler-
ated conditions in 40 ^ 2 8C and at 75% ^ 5 RH in a
humidiy cabinet (Binder 240, Germany), tablets and pellets
were stored in brown glass bottles with a dessicant
(S series), and without dessicant (F series) using rubber
stoppers, and also in open containers (P series). During
the stability test, the remaining omeprazole and the
degradation products in tablets were followed at the first,
second, and the third months using a stability-indicating
HPLC method for omeprazole. Only the S series was
analyzed for omeprazole and degradation products. A visual
inspection was carried out for discoloration of tablets or
pellets every day at the first week, and then once a week
until the third month for all series (S, F, and P). A list of
omeprazole impurities that were studied during the
accelerated stability were: impurity A, 5-methoxy-1H-
benzimidazole-2-thiol; impurity B, 5-methoxy-2-[[(4-meth-
oxy-3,5-dimethypyridin-2-yl)methyl]-sulphanyl]-1H-benzi-
midazole; impurity C, 5-methoxy-2-[[(4-methoxy-3,
5-dimethypyridin-2-yl 1-oxide)methyl]sulphinyl]-1H-ben-
zimidazole; impurity D, 5-methoxy-2-[[(4-methoxy-3,
5-dimethypyridin-2-yl)methyl]-sulphonyl]-1H-benzimida-
zole, 1-oxide; impurity G, 5-methoxy-2-[[(4-methoxy-3,
5-dimethypyridin-2-yl)methyl]-sulphonyl]-1H-benzimida-
zole.
Table 1
Summary of HPLC method for omeprazole assay and impurities
Assay method Stability-indicating method
Column Nova Pak C18 Zorbax SB-PHENYL
150 £ 3.9 mm 250 £ 4.6 mm, 5 mm
Flow rate 1.0 ml/min 1.2 ml/min
Injection volume 20 ml 30 ml
Column temperature 35 8C 35 8C
Wavelength 280 nm 280 nm
Mobile phasea Buffer:ACN Buffer:ACN
a pH 6.0 USP phosphate buffer:acetonitrile (72:28).
M. Tu¨rkog˘lu et al. / European Journal of Pharmaceutics and Biopharmaceutics 57 (2004) 279–286280
2.6. Data analysis
For modeling the data to determine the effects of
compression pressure and studied excipients on omeprazole
degradation and tablet properties, artificial neural network
software (STATISTICA Neural Networks, Release 4.0,
USA) was used. The details of the artificial neural network
(ANN) methodology can be found in the literature [10]. A
three-way analysis of variance (ANOVA) was performed
(SPSS 10.01 for Windows) as an example of traditional data
analysis method and the results were compared with the
ANN. Historical data were used for model forming
including the five batches evaluated in accelerated stability
study. The best network was reported based on the
regression ratio, correlation coefficient, and the minimized
error. As independent factors, tablet compression force,
Avicel and Starch 1500 concentrations were used. The
dependent variables included tablet crushing force and
percent omeprazole degradation. The linear (LNN), multi-
layer perceptron (MLP), radial basis function (RBF),
generalized regression (Genetic, GRNN) networks were
considered. Three-dimensional response surfaces were
constructed based on the model predicted values.
2.7. Analytical procedures and HPLC conditions
Two HPLC procedures were used. The first one was the
assay procedure for omeprazole and the second method was
the stability-indicating method for omeprazole. Both
methods were fully validated before their routine use.
Both HPLC procedures used the external standard method,
and the area under the peak values were used for
calculations. The validation tests included: system suit-
ability, accuracy, reproducibility, linearity and ruggedness.
The conditions are summarized in Table 1.
3. Results and discussion
3.1. Tablet properties
The five tablet formulations studied (F1, F2, F3, F4 and F5)
contained 150 mg of enteric-coated pellets that corre-
sponded to 20 mg omeprazole in a 455 mg final tablet
formula. Table 2 summarizes the composition of the
formulas, and Table 3 shows the mechanical properties.
Table 3 also includes additional data for an optimized batch
(F4) that was compressed using an industrial size rotary
tablet press to compare the results with the single station
tablet press. When physical tablet properties were con-
sidered, F4 was found to be the best formulation with an
average crushing strength value of 8.7 kg force and a
friability value of less than 0.5%. F2, F3 and F5 showed
friability values more than 1% USP limit. Disintegration
times for all formulations were less than 5 min.
3.2. Percent dissolution and gastro-resistance
One of the most important properties of a modified release
item is its resistance against gastric conditions. It is required
that no more than 10% drug degradation would occur after
2 h in 0.1 N HCl solution. All formulations except F5
complied with the condition. The best gastro-resistance was
obtained with F4 as 5% omeprazole degradation after 2 h
using USP 24 Apparatus 2 at 50 rev./min and 37 8C. The free
pellets showed no more than 1% degradation for omeprazole.
Therefore the difference can be attributed to the tableting
process and the effects of excipients. Percent dissolution data
for tablets did not differ from free pellets since all
formulations disintegrated freely in less than 5 min and at
the 15th min, all the batches released at least 80%
omeprazole and then drug release continued much more
slowly. Fig. 1 shows the release profiles of five formulations
without being exposed to HCl for 2 h and Fig. 2 shows the
gastro-resistance of the formulations after being exposed to
0.1 N HCl solution for 2 h.
3.3. Scanning electron microscopy
Several scanning electron microscopy (SEM) photos
were taken to obtain a visual assessment of the pellets under
compression The SEM photos were taken by breaking a
tablet in half. Fig. 3a shows a single enteric-coated pellet
700 mm in diameter protecting the integrity of the acrylate
coating and embedded in the microcrystalline cellulose
Table 2
Prototype tablet formulations
Formulation F1 F2 F3 F4 F5
Pelleta 150 150 150 150 150
Avicel PH 102 150 100 150 270 150
Starch 1500 70 50 120 2 2
Primogel 30 60 30 30 30
PEG 6000 50 90 2 2 120
Lubricant (%) 0.1 0.1 0.1 0.1 0.1
Average tablet weight: 455 mg ^ 5%.
a Enteric-coated pellets containing 20 mg omeprazole per 150 mg pellet.
Table 3
Some tablet properties
Formulation F1 F2 F3 F4 F4
Rotary Press F5
Tablet thickness (mm) 5.92 5.79 5.52 5.63 4.69 5.73
Crushing
strength (N)
63 54 52 90 85 65
Friability (%) 1.23 20 8 0.34 0.1 2.0
Disintegration
time (min)
3 2.5 2 3 1.5 2
F4
Rotary Press, batch size 11.000 tablets (Courtoy R 190 FT, 24-station
rotary tablet press).
M. Tu¨rkog˘lu et al. / European Journal of Pharmaceutics and Biopharmaceutics 57 (2004) 279–286 281
matrix. Fig. 3b shows a cross-section of a tablet (F4)
containing individual pellets scattered inside a supporting
matrix. One can observe the matrix deformation corre-
sponding to a single pellet and there was no visible damage
to the pellets. However, only damage to the enteric coating
occured on the surface of the tablets. Granulation of the
excipients to match the pellet size had a crucial role in
absorbing the compression pressure. The Avicel matrix
showed a strong plastic deformation and all the pellets
inside a tablet maintained their shape with no significant
change in their surface properties. We also agree that
properly adjusting the tablet shape, having the smallest
surface area/volume ratio, would result in better pellet
protection as previously reported by Wagner et al. [3].
3.4. Stability of tablets
Stability studies were continued up to 90 days in a
stability chamber at 40 8C and 75% RH free pellets were
used as the control along with F1, F2, F3, and F4. Fig. 4
shows the remaining omeprazole vs time relative to the
initial assay. None of the batches showed significant
changes based on the ICH conditions such as a 5% potency
Fig. 4. Remaining omeprazole after 90 days in storage in tightly closed
brown glass bottles with a dessicant.
Fig. 3. (a) A single enteric-coated pellet inside the tablet matrix (pellet
diameter, 650 mm). (b) Pellet distribution in the tablet matrix. Empty pellet
nests (plastic deformation) and intact pellets inside the tablet are visible (F4).
Fig. 1. Percent omeprazole release from tablets in phosphate buffer (pH 6.8)
(USP 24 Paddle apparatus, 37 8C and 100 rev./min, acid phase was omitted).
Fig. 2. Results of gastro-resistance test for tablets (2 h in 0.1 N HCl) (HPLC
assay based on remaining omeprazole in tablets after 2 h).
M. Tu¨rkog˘lu et al. / European Journal of Pharmaceutics and Biopharmaceutics 57 (2004) 279–286282
change from the initial assay or a specific degradant
concentration exceeding the limits. All five tablet formu-
lations showed acceptable stability in rubber-capped,
brown glass bottles with a dessicant capsule. Fig. 5a–c
shows the amount of omeprazole degradation products
based on a stability-indicating HPLC assay at the initial,
30th and 90th day for the batches SF1, SF2, SF3, SF4, SF5,
and for the free pellets. The ‘S’ series were the ones that
contained a dessicant capsule inside the brown bottles.
Initially all the impurities were identical with the free
pellets and the total amount was less than 0.05%. At
30 days, Imp B increased to 0.2% for all batches regardless
of the formulation, however, it did not increase further
even at 90 days. The 30 day graph showed some increase
in Imp C for SF4 and SF5, and some increase in Imp D for
free pellets. As can be seen in Fig. 5c, that is a summary of
the 90th day assay, all five degradation products were
visible for all the batches. Interestingly, Imp A did not
form significantly in SF1, SF2, and SF3, but occurred in
SF4, SF5, and free pellets. This difference may be attributed
to the presence of Starch 1500 in F1, F2, and F3. Also SF2
showed the smallest concentration of Imp D among all the
batches.
The visual assessment of discoloration of tablet surfaces
was also carried out up to 90 days. Fig. 6 shows the results.
At the 90th day SF1, SF2, SF3, SF4, SF5, and free pellets
maintained white to light beige color as also expected from
their HPLC assay. SF1, SF2, SF4, and SF5 did not show any
discoloration until the 63rd day. However, the pellets in SF3
started discoloration on the 35th day. Free pellets ‘S’ series
showed the first sign of discoloration at the 63rd day exactly
the same as the other formulas.
The formulations without a dessicant (F1–F5) and the
pellets appeared as light beige to brownish at the 90th day,
F1 and F2 being significantly darker. F1, F4 and F5 showed
first sign of discoloration at the 21st day, F2 and F3 at the
14th day. Free pellets ‘F’ series showed the first sign of
discoloration at the 21st day. In this series of tablets, F1 and
F2 showed inferior stability to the other formulas including
the free pellets. Since all other factors were fixed, this faster
discoloration was attributed to PEG 6000 in the formula.
Hence, we concluded that PEG 6000 should have been
excluded from an optimized formula containing omepra-
zole. In open containers, discoloration for all formulas was
observed starting from the second day and reached a peak as
dark brown at the third day.
Fig. 5. (a) The initial impurity profile of formulations and the pellets based on the stability-indicating HPLC assay. (b) The 30th day impurity profile of
formulations and the pellets based on the stability-indicating HPLC assay (40 8C, 75% RH). (c) Impurity profiles of formulations after 90 days in accelerated
stability (40 8C, 75% RH) using the stability-indicating HPLC assay.
M. Tu¨rkog˘lu et al. / European Journal of Pharmaceutics and Biopharmaceutics 57 (2004) 279–286 283
3.5. ANN and ANOVA evaluation
One of the purposes of this study was to model the tablet
crushing strength and percent omeprazole degradation in
0.1 N HCl which was used as an indicator of enteric coat
damage based on the applied compression force, micro-
crystalline cellulose and pregelatinized starch concen-
tration. Table 4 summarizes the experimental design
which was a 20-experiment set containing th