Preparation of Ethylcellulose Coated Gelatin Microspheres as a Multiparticulate
Colonic Delivery System for 5-Aminosalicilic Acid
Iranian Journal of
Pharmaceutical Research (2004) 2: 81-86
Received: August 2003
Accepted: July 2004 |
Copyright ? 2004 by School of Pharmacy
Shaheed Beheshti University of Medical Sciences and Health Services |
Original Article
Preparation of Ethylcellulose Coated
Gelatin Microspheres as a Multiparticulate Colonic Delivery
System for 5-Aminosalicilic Acid
Fatemeh Atyabi*, Rudabeh Vahabzadeh and Rassoul Dinarvand
Department of Pharmaceutics, Faculty of
Pharmacy,Tehran University of Medical Sciences, Tehran, Iran.
Abstract
In the long-term management of ulcerative
colitis patients, repeat dosing maybe required. Since 5-ASA is largely
absorbed from the upper intestine, selective delivery of drugs into the
colon may be regarded as a better method of drug delivery with fewer
side effects and a higher efficacy. The aim of this study was to
prepare and evaluate a double coated multiparticulate system for 5-ASA
delivery using gelatin and ethylcellulose as the primary and secondary
polymer respectively. Gelatin microspheres containing 5-aminosalicylic
acid was produced using the solvent evaporation method. Prepared
gelatin microspheres were spherical, free flowing, non-aggregated and
showed no degradation in the acidic medium. Entrapment efficacy of
microspheres was about 50%. Results showed that drug release was fast
and complete and is affected by the amount of core material entrapped.
Gelatin microspheres were then coated by ethylcellulose using a
coacervation phase separation technique. The idea for this approach was
to prepare a delayed drug delivery system, in which, ethylcellulose
protects particles for the first 6 h transit through the
gastrointestinal tract. However, it was shown that this system could
provide a suitable drug release pattern for colonic delivery of active
agents, as 30% of the drug was released from the ethylcellulose-coated
microcapsules within 6 h, while this amount was 90% of the loaded drug
for gelatin microspheres under the same condition.
Keywords: 5-Aminosalicilic
acid; Gelatin; Ethyl cellulose; Colon delivery; Microparticles;
Microspheres; Microencapsulation.
Introduction
There have been considerable researches
in the field of colonic drug delivery for many purposes:
a) Development of new therapeutic agents
for the treatment of colonic diseases has required colon-
specific delivery systems to maximize the effectiveness of
these drugs (1).
b) Introduction of once a day sustained
release formulations has required a better understanding of the
transit of dosage forms through the colon, and of the colonic
absorption of the drug present within them (2).
c) The colon itself is susceptible to
many disease states including constipation, irritable bowel
syndrome and more serious diseases such as Crohn's disease,
ulcerative colitis, Carcinomas and infections. At present these
diseases are often poorly and inefficiently managed either by
oral drugs, which are largely absorbed before they reach the
colon, or by rectal administration, which is less acceptable as
a route of administration and rarely succeeds in delivering
drugs to the ascending colon. 5-ASA is indicated for the
treatment of mild to moderate active ulcerative colitis. In the
long-term management of ulcerative colitis delivering drugs to
the colon may be regarded as a better method of drug delivery
with less side effects and higher efficacy (3).
Using natural polymers such as
polysaccharides as a controlling agent for delivering of drugs
has received much interests in recent years (4, 5).
Biocompatibility and biodegradability of these materials
provide good advantages for this reason. Natural polymers in
fact could obviate toxicity or biodegradability problems (i.e.
formation of localized granulomatous inflammation), possibility
due to the use of synthetic materials. Gelatin in particular
represents a good candidate for the preparation of
microspheres. Indeed gelatin has bioadhesive properties which
allows the production of drug delivery systems for mucosal
delivery i.e. mouth, nasal and gastrointestinal tract. On the
contrary, gelatin dissolves rather rapidly in aqueous
environments, making its use somewhat difficult for the
preparation of long term drug delivery systems (6). This
problem may be resolved by coating the gelatin microspheres,
using a hydrophobic polymer such as ethylcellulose.
The aim of this study was a) to prepare
and characterize gelatin microspheres designed for colonic drug
delivery, and b) to prepare and characterize double layer
microcapsules to overcome the fast dissolution of gelatin.
Experimental
Materials
5-Amino salicylic acid (ASA) was obtained
from Pliva (Zagreb). Sesame Oil was purchased from Aseel,
(UAE). Ethylcellulose was obtained from ICN, (USA). Gelatin,
Tween 20, Hydrochloric acid, potassium dihydrophosphate,
n-hexane, toluene, 2-propanol, sodium hydroxide, and span 80
were purchased from Merck, (Germany).
Preparation of gelatin core microspheres
Drug loaded microspheres were produced as
follows. Gelatin was dissolved in water to produce 5%, 7% and
8% W/V solutions. The dispersion of drug into the polymeric
solution was aided by sonication for 30 min. The aqueous
solution of the drug in gelatin was stirred for further 30 min
at 200 rpm. To produce an emulsion of aqueous gelatin solution
containing drug molecules in the second oil phase, the aqueous
solution was dispersed in sesame oil using an overhead paddle
and left stirring for 45 min. When microspheres appeared in the
solution (detected by optical microscope), then the temperature
of the medium was brought down to 5°C by placing the beaker
in an ice bath while 100 ml 2-propanol was added gradually to
the system along with and stirring at 1000 rpm for a further 20
min. Microspheres containing the drug particles were formed
due to coacervation induced by lowering the temperature
of the medium. Subsequently the excess toluene was added to the
system to facilitate separation of the prepared microspheres as
well as the removal of sesame oil. Prepared microspheres were
then collected by filtration, washed with acetone and dried at
room temperature. Microspheres with drug to polymer ratios of
20:80, 30:70 and 50:50 were obtained at this stage.
Microspheres coating method
Drug loaded gelatin microspheres were
used as a core material for the preparation of double-coated
system. A coacervation phase separation method was applied for
this step. A known amount of the microspheres having particle
size of 100-250 µm was dispersed in an in Ethyl Acetate
(25ml) solution containing ethyl cellulose (50, 100, 150 mg)
and containing 0.02% W/V span 80. This mixture was
agitated for 5 min at 400 rpm. Subsequently 50 ml n-hexane (as
the non-solvent ) was poured into the polymeric solution
containing the core material with the rate of 1 ml/min. The
medium was stirred for 60 min to complete the process of
microparticles coating. Coated microspheres were then washed
with an excess of n-hexane, filtered and dried at room
temperature.
Microsphere morphology and particle size
determination
The morphology of gelatin microspheres
was evaluated by optical microscopy (Prior, England) as well as
the scanning electron microscopy (SEM) (Stereoscan 360, Leica
Cambridge, UK).
Particle size range and distribution of
microspheres were determined using standard sieves.
Drug content and efficacy measurement
To determine drug entrapment within the
microspheres, 50 mg of microspheres was dissolved in 100 ml of
HCl (0.1 N). After complete dissolution of gelatin, the amount
of drug was quantified using a spectrophotometric method at 302
nm in the presence of a blank prepared from microspheres
containing all materials except the drug. Drug loading was
determined as the percentage of the amount of the drug obtained
to the applied amount.
Efficacy of the microspheres preparation
method was determined by dividing the amount of the prepared
microspheres to the initial amount of the applied material.
Drug entrapment within the coated
microspheres was determined by dissolving 50mg of microspheres
in 100ml of methylene chloride and HCl. Solute was extracted
from the medium by the removal of methylene chloride. The
amount of drug was measured spectrophotometrically.
Drug release studies
Profiles of drug release from the
prepared microspheres were studied using a USP (Apparatus I)
dissolution tester. 100 mg of microspheres was incubated in 500
ml phosphate buffer (pH =7) containing 0.02% W/V tween 20 to
aid wetting. The media were agitated at 100 rpm, while
maintaining the temperature at 37°C. 5 ml samples were
withdrawn from the dissolution medium at regular time intervals
and replaced with fresh medium. Concentration of the withdrawn
samples was measured spectrophotometrically as mentioned above.
In order to investigate the influence of
EC on the release profiles of microspheres, two different
dissolution test methods were set in hydrochloric acid (as the
gastric medium) for 2 h and then Phosphate buffer as intestinal
medium for 22 hours.
Results and discussion
Development of microparticulate drug
delivery systems using a combination of polymers has
significant advantages over the homogenous polymeric systems
(7). Through these systems by the selection of an appropriate
combination of core and coat polymers, a microparticulate
system for simultaneous entrapment of hydrophilic and
hydrophobic drugs is achievable. Indeed, the drug could be
entrapped in the core material using the proper characteristics
of the core polymer while its disadvantages is improved by the
desirable properties of the coating material.
Gelatin is an interesting biomaterial for
drug delivery. Degradation of gelatin by natural microorganisms
existing in the colon makes it suitable for delivering drugs to
the colon. However, its use in oral administration is
restricted due to its fast dissolution in the upper part of the
gastrointestinal tract and limited capacity for controlling the
release of drugs. To overcome these limitations a
multiparticulate controlled release system consisting of a
hydrophilic gelatin core entrapped within a hydrophobic polymer
(ethyl cellulose) was prepared. The purpose of this study was
to present an approach for the preparation of gelatin
microspheres suitable for oral application. These microcapsules
were prepared by different amounts of gelatin and various
ratios of core and coat materials. 5-ASA was used as the model
drug to investigate the ability of the system for entrapment
and controlling drug release in the simulated colon medium.
Production of microspheres under
different conditions was investigated. From these experiments
it was possible to encapsulate various gelatin concentrations
(5, 7, 8%), leading to the formation of particles with
different drug to polymer ratio (20:80, 30:70, 50:50).
Microscopic observations (Figure 1)
showed that all the dried gelatin microspheres were spherical,
free flowing and non-aggregated. The microparticles were stable
at low pH values. The morphology of microspheres (as assessed
by SEM) did not change significantly following incubation,
either in an acidic or neutral medium (figure 2), supporting
the suitability of the system for colonic delivery. The size of
the microspheres varied between 50 to 400 mm, while 92% of the
particles had a size range between 50 to 250 mm. However, about 60% of
the microspheres fall within the 100 to 250 mm size range. These
results were not much different for the three microsphere
preparation conditions applied. Figure 3 shows the particle
size distribution of the prepared microspheres.
The entrapment efficiencies and
preparation efficacy of different prepared samples consisting
various ratios of drug to polymer are shown in table 1. These
results show that the preparation efficacy of microspheres was
very high irrespective of the processing condition. This is
approximately 88% for gelatin microspheres. The drug entrapment
was also good in all samples, being about 52% for the primary
microspheres. Further more, this amount did not differ for the
coated microspheres, showing negligible drug loss during the
second reaction.
Figure 4 Shows the pattern of drug
release from gelatin microspheres with different drug to
polymer ratios in phosphate buffer medium. In another attempt
microspheres were placed in the acidic medium for 2 h then the
microspheres were transferred to the second medium, which
contained phosphate buffer solution for better simulation of
the gastrointestinal transit. As swelling is the main factor
influencing drug release from the microspheres prepared by
polysaccharides, leaving this system in the acid medium for 2 h
prior to exposure to buffer medium affects drug release. Figure
5 represents behavior of the gelatin microspheres in this
condition. Due to the high water uptake of the microspheres,
drug release pattern is fast and nearly complete within 6 h,
while a large amount of drug is released in the acidic medium
before coming into contact with the phosphate buffer medium.
These results also show the influence of
core/coat ratio on the in vitro behavior of the microspheres. The higher
the amount of core material, the faster the in-vitro release
rate of the drug. This core to coat dependence of the drug
release behavior could be logically explained by the
hydrophilicity of gelatin. The probable drug release mechanisms
from the gelatin microspheres involve the following processes;
i) water penetration into the microspheres, ii) gelatin
swelling/gelling and dissolution of the drug and iii) diffusion
of the active compound through the gelatin hydrogels.
Therefore, the drug release rate would be controlled by the
extend and rate of water absorption/swelling of the gelatin
included within the microparticles, and the rate of diffusion
of drug out of the gel. Gelatin microspheres swell very rapidly
and then form a gel-like barrier, which facilitates drug
release.
To delay and hinder drug release from
microspheres and overcome a fast drug release, ethylcellulose
was used to coat the gelatin microspheres.
Ethylcellulose coated microcapsules are
spherical, with smooth surfaces. Figure 6 shows the scanning
electron microscopic image of these microcapsules. Particle
size distribution of the coated microspheres is shown in figure
3. since particles having a size range of 100 to 250 µm
were chosen as core for EC coating, about 80% of the prepared
microcapsules still have this particle size range. Drug release
pattern from the microspheres were studied under the condition
mentioned above. As can be seen in figure 7, the drug release
pattern has been modified in comparison with the gelatin
microspheres. While 90% of the loaded drug was released from
the gelatin microspheres in 6 h, about 30% of drug was released
under the same condition from ethylcellulose-coated
microcapsules. Figure 8 compares the pattern of drug release
from gelatin microspheres and ethylcellulose-coated
microcapsules.
Conclusion
As conclusion, it could be said that a
drug delivery system prepared by combination of a hydrophobic
polymer and a polysaccharide has the capability to be applied
as a colonic delivery system. Achieving this potential needs
further researches in this area.
Acknowledgement
This study was financially supported by a
research grant from the Tehran University of Medical Sciences.
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