J. Muselík 1; K. Dvořáčková 1; M. Rabišková 1; A. Bartošíková 1; M. Rosendorfová 1; M. Dračková 2; L. Vorlová 2
University of Veterinary and Pharmaceutical Sciences Brno, Faculty of Pharmacy, Department of Pharmaceutics, Czech Republic
1; University of Veterinary and Pharmaceutical Sciences Brno, Faculty of Veterinary Hygiene and Ecology, Institute of Milk Hygiene and Technology, Czech Republic
Čes. slov. Farm., 2009; 58, 155-159
The aim of this study was to develop the method for off-line determination of hard capsules coating thickness. The polymer films studied were Eudragit® L 12.5 and Eudragit® S 12.5 applied on hypromellose capsules surface in a bottom-spray fluid-bed coater equipped with a Wurster column. The coating thickness reference values for calibration were obtained from microscopic analysis. Calibration model was created using partial least squares regression with six factors for Eudragit® L film and five factors for Eudragit® S film. The calibration and validation results led to following parameters: determination coefficients R2 were more than 0.98 and standard error of cross validation was 6.6 μm (coat thickness range was 15–235 μm) for Eudragit® L film and 8.3 μm (coat thickness range was 30–230 μm) for Eudragit® S film, respectively. Obtained results confirmed suitability of near infrared spectroscopy for coating process quality control in pharmaceutical industry.
Key words: film-coating – Eudragit® L and S – hard capsules – coating thickness – near-infrared spectroscopy
Currently, coating of hard capsules plays an important
role in drug administration due to a number of reasons such as the
protection of capsule filling (against moisture, light, oxygen),
easier identification, and especially the possibility of controlled
drug release. A clear advantage is the independence of the capsule
coating process on its contents 1).
The pH-dependent coating is efficient in drug site-specific delivery
to different parts of the gastrointestinal (GI) tract such as
duodenum, ileum or colon. Some coating polymers exhibiting
pH-dependent solubility belong to the anionic copolymers of
methacrylic acid and methylmethacrylate or ethyl acrylate well known
under the brand name Eudragit®.
Eudragit® L 12.5
and Eudragit® S
12.5 are 12.5% (w/w) aqueous isopropyl alcohol solutions containing
Eudragit® L 100
or S 100. Eudragit®
L 12.5 is soluble in a dissolution medium above pH 6.0, thus enabling
an enteric coating to protect drugs against degradation in the
stomach or to prevent irritation of the gastric mucosa. Eudragit®
S 12.5 is soluble in a medium of a higher pH value, i. e. above 7.0
2), thusappropriatefor ileac or colonic drug delivery that
can be used for local treatment of a variety of intestinal diseases
or oral bioavailability improvement of some drugs such as peptides
and proteins 3).
Capsules coated with Eudragit®
S are also prospective for oral vaccines delivery near to the Peyer’s
Traditionally, the film coating thickness on the surface
of dosage forms (tablets, pellets and capsules) has been determined
either by applying a specified amount of the coating material, that
is by measuring the weight gain, or by microscopic analysis of the
dosage form cross-section until the desired end point is achieved.
These off-line methods allow the coating evaluation at the end of the
process or if the coating operation is interrupted. The Food
Drug Administration (FDA) initiated the
control of manufacturing processes by real-time monitoring of
critical manufacturing points. Real-time monitoring has been used in
pharmaceutical unit operations such as drying, blending,
content-uniformity testing, coating etc. 5).
Near-infrared spectroscopy (NIRS) has gained wide acceptance within
the pharmaceutical industry for raw material testing, product quality
control and process monitoring 6).
When compared to traditional methods, NIRS offers many advantages. It
is an instantaneous, non-destructive method, requiring minimal or no
sample preparation 6–8).
Once calibrated, NIRS is simple to operate and is well suited for the
determination of the major components in many types of samples. NIRS
has a possibility of working in the reflectance mode and the use of
fibre probe modules coupled to the spectrophotometer makes it useful
for in-process or at-line monitoring 9).
It has been shown that NIRS can be successfully used to
predict the required coating amount sprayed on tablets 7,
10) and pellets 11)
surfaces or to evaluate the plastic primary packing thickness 12).
For quantitative NIR spectroscopy, it is crucial to propose a
calibration model. The objective of this study was to develop new
methods based on near-infrared spectroscopy as an alternative for
rapid, non-destructive and reproducible determination of coating
thickness of two different polymer films Eudragit®
L (EL) and Eudragit®
S (ES) applied as isopropyl alcohol solutions on hard hypromellose
Caffeine as the model drug (Jilin Province Shulan
Synthetic Pharmaceutical Co., Ltd., Shulan City, China) in a dose of
100 mg together with the soluble filler α lactose
monohydrate q. s. (Cerapharm, Vienna, Austria) were filled into hard
HPMC capsules of size 4 – Vcaps®
(Capsugel, Colmar, France). Eudragit®
L 12.5 or Eudragit®
S 12.5 (Evonik Degussa, Darmstadt, Germany) were used as the coating
materials. Polyethylene glycol (PEG) 6000 (Sigma-Aldrich, Prague,
Czech Republic) was added into the Eudragit®
coating solution as the plasticizer. All materials were of Ph.
capsules filling process and evaluation of prepared capsules
Hypromellose hard capsules were filled in a manual
filling machine with a caffeine and lactose monohydrate mixture which
was prepared by homogenization of 10.0 g of the model substance
and 2.3 g of lactose monohydrate to fill 100 pieces of hard capsules.
Weight uniformity of capsules and caffeine content were evaluated
according to Ph. Eur. 5.
Two hundred filled capsules were subsequently coated in
a Wurster-M 100 coater (Medipo ZT, s. r. o., Czech Republic) using
either a 66.67 g Eudragit®
12.5 isopropyl alcohol solution (S or L) with a mixture of 5.05 g
of a 33% water solution of PEG 6000 and 28.28 g of isopropyl alcohol
to reach the solid content of polymer 8.3%, according to the
producer’s recommendation. Subsequently, within the coating
process, the samples of 20 hard capsules with an increasing coating
amount were withdrawn for the subsequent evaluation. Overall, seven
batches for EL and eight batches for ES were prepared, and the
experiment was carried out twice.
determination of the layer thickness
The coating thickness was determined by optical analysis
using an optical microscope (DN 45, Lambda, Prague, Czech Republic)
connected to a CCD camera (Alphaphot, Nikon, Tokyo, Japan) and
operated by an Ia32 software. Twenty different positions of 3
randomly selected coated hard capsules from each prepared batch were
measured. The results were expressed as the mean thickness and
standard deviation for each tested capsule separately.
Near infrared diffusion reflectance spectra were
acquired with a FT-NIR Nicolet Antaris (Thermo Electron Corporation,
USA) spectrophotometer equipped with an integrating sphere. Solid
samples were placed into the sampling cup and the spectra were
recorded in the 10000–4000
range. A total of 60 scans were collected from both sides of each of
the capsules and the average spectrum was stored. A total time
measurement of one capsule was about 2 minutes. The NIR-data
were analysed using a TQ Analyst software version 184.108.40.2069 (Thermo
Nicolet Corporation, Inc., USA).
RESULTS AND DISCUSSION
Prior to any quantitative analysis, the NIR spectrometer
requires calibration based on primary measurements of the
characteristic being measured, since NIRS does not give an absolute
value of this characteristic 13).
The calibration process basically involves selection of a
representative calibration sample set, spectra acquisition and
determination of reference values, multivariate modelling and
validation of the model by cross validation, set validation or
external validation 6).
During the first stage, a calibration experiment was
performed. This involved collecting a set of reference values of
calibration samples. The calibration set included 21 coated capsules
for an EL film (3 capsules from each batch of 7 produced) with a
polymer film thickness in the range of approximately 15–235 μm
and 24 coated capsules for ES film (3 capsules from each batch of 8
produced) with the polymer film thickness in the range of
approximately 30–230 μm.
The reference values of the polymer film thickness of coated capsules
were determined using microscopic analysis and are listed in Tables 1
Since the NIR spectra consist of uncharacteristic,
highly overlapping, broad, low absorption bands containing chemical
and physical information of all sample components 6),
their widespread use was somewhat hindered until multivariate
calibration methods for withdrawing their analytically relevant
information were developed. To perform quantitative NIR analysis,
mathematical and statistical methods (i.e. chemometrics) are required
to extract “relevant” information and reduce “irrelevant”
information, i.e. interfering parameters. Principal component
regression (PCR) and partial least-squares regression (PLS) are the
two multivariate calibration methods most frequently used in this
In this study, the calibration model was constructed
using the PLS algorithm, assigning the reference value of the coating
thickness to each spectrum and entering these data into the TQ
Analyst software. The range of wavenumbers suitable for the
calculation of the calibration model was evaluated on the basis of
the observation of intensity changes in the absorption bands in the
spectrum of capsules with different amounts of the applied polymer
material (Fig. 1).
Calibration process using the PLS algorithm allows to
find some extent of correlation for each wavenumber between the
changes in the coating thickness and the changes in the intensity of
absorption. The selected wavenumber regions were 9388–7475 cm-1
and 6194–5400 cm-1
for the EL film, and 8894–7185 cm-1
and 6133–5700 cm-1
region for the ES film, respectively.
calibration models characteristic
The qualities of the developed models were checked by
comparison of the reference values to the predicted values, and
calculating, among others, the standard error of calibration (SE),
the number of outliers and the number of PLS factors. The correlation
equations in the form were found to be
Y = 0.458 + 0.9962 ×X (r
= 0.9981, SE = 4.0) for EL film, 
Y = 1.327 + 0.9891 ×X (r
= 0.9945, SE = 6.7) for ES film, 
where Y means the NIRS predicted thickness (μm)
and X denotes the reference coating thickness values (μm).
Chauvenet test was applied to determine the statistical significance
of the outliers. No outliers were found in our calibration sets. PLS
found the directions of greatest variability comparing both spectral
and target property information with the new axes, called PLS
components or PLS factors 6).
The optimal number of factors used to build the calibration model was
found to be six for the EL film and five for the ES film, indicating
a low potential risk of overfitting the model, e.g. modelling the
Validation was performed by calculating the standard
error of prediction (SEP) of five validation capsules over the
thickness range of 60–200 μm
for the EL film and seven validation capsules over the thickness
range of 60–200 μm
for the ES film. The standard error of prediction was 7.5 μm
for the EL film and 5.3 μm
for the ES film.
The same samples from the calibration set were used to
construct the validation model using cross validation 15).
Cross validation (leave-one-out) was performed excluding one of the
standards and calibration of the remaining data was constructed as a
new model that was used for the quantification of the excluded
standard. A simple regression analysis was performed to assess the
linearity of the developed models (Fig. 2) and the standard error of
cross validation (SECV) was calculated. The observed correlation
equation describing the relationship between the PLS algorithm
predicted coating thickness and a reference thickness
Y = 0.913 + 0.9921 ×X (r
= 0.9949, SECV = 6.6) for EL film, 
Y = 2.723 + 0.9796 ×X (r
= 0.9916, SECV = 8.3) for ES film, [4)
where Y is the NIRS predicted thickness (μm)
and X is the reference coating thickness (μm).
A reliable and fast NIRS method was developed for
off-line determination of the thickness of the polymer film applied
to the hard capsules surface. The obtained results clearly
demonstrated the feasibility of the NIRS methods for the
determination of coating thickness of hard capsules. The connection
of a NIR spectrophotometer and a coating equipment makes it possible
to use this method as a part of the at-line quality control procedure
in pharmaceutical industry.
This work was supported by IGA
VFU 136/2008/ FaF and IGA Ministry of Healthcare of Czech Republic NS
Jan Muselík, Ph.D.
of Pharmaceutics, University of Veterinary and Pharmaceutical
1–3, 612 42 Brno, Czech Republic
Received 22 Juny 2009
14 July 2009
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