Effect of
Dispersing Agent on the Characteristics of Eudragit Microspheres
Sethi RK*,
University Department of
Pharmaceutical Sciences,
ABSTRACT
Eudragit RS microspheres containing Indinavir sulphate for oral use
were prepared using two different dispersing agents: aluminium
stearate and magnesium stearate,
by solvent evaporation method. The effects of the type and concentration of the
dispersing agents and the inner phase polymer concentration on the size of
microspheres was studied. The morphology of microspheres was characterized by
scanning electron microscopy. The surface of microspheres prepared with aluminium stearate was smoother
and non-porous. When magnesium stearate was used as
dispersing agents, the particle size of microspheres decreased. Increasing
amounts of this dispersing agent led to the accumulation of their free
particles onto the surfaces of the microspheres. The drug release from the
microspheres was faster with the microspheres from aluminium
stearate based on their hydrophobic structures. The
encapsulation efficiency is more in case of aluminium
stearate in comparison to magnesium stearate. Formulation containing aluminium
stearate shows a more sustained effect than
formulation containing magnesium stearate. This may
due to fact that aluminium stearate
is more hydrophobic in comparison to magnesium stearate.
KEYWORDS: Indinavir sulphate, Dispersing
agent, Eudragit RS 100, controlled release.
INTRODUCTION
The solvent evaporation
method is a commonly employed process for the preparation of polymeric
microspheres. This method involves the emulsification of a solution containing
polymer and drug into another medium in which the drug and polymer cannot be
dissolved by using a suitable dispersing agent.1 There are several
formulation and process parameters that affect microsphere properties and
performance during the preparation of microspheres by solvent evaporation
method. Some of these parameters are the aqueous solubility of drug, the type
and concentration of dispersing agent, the aqueous and organic phases volumes,
the ratio of polymer-drug and solvent, and the stirring rate of emulsion
system.2-4
Dispersing agents have an important role in
the production of microspheres using the solvent evaporation method. These
substances decrease the interfacial tension between the lipophilic
and hydrophilic phases of the emulsion and simplify the formation of the
microspheres.5 During the process of droplet formation in the
solvent evaporation procedure, the gradual removal of solvent from polymer
droplets is accompanied by a corresponding decrease in the volume and increase
in the viscosity of the individual droplets. In particular highly viscous
droplets coalesce much faster than they can redivide.
Droplet coalescence and particle coagulation can usually be overcome by use of
a small amount of a suitable droplet/particle stabilizer (dispersing agent).
The dispersing agent provides a thin protective layer around the droplets and
hence reduces the extent of their collision and coalescence.6Dispersing
agent used can be various polymeric materials,7 proteins5,8,9
or surfactants. Water soluble drugs such as theophylline,
caffeine, salicylic acid and indinavir sulphate could not be entrapped within microspheres when
using an o/w emulsion system.
In order to improve the drug
loading of relatively water-soluble compounds, a w/o emulsion system has been
reported.[1] Dispersing agents used in w/o systems are especially
metallic soap (magnesium stearate, aluminium stearate), sorbitan fatty esters (Spans, Tweens,
Arlacels) and poly-oxyethylene
fatty ethers. [10-13] Esters have also been proposed as dispersing
agents. They have the advantage of low toxicity and biodegradation.
In the present study, two different dispersing agents, aluminium stearate and magnesium stearate, were used to produce Eudragit
microspheres containing Indinavir sulphate
for oral use by the solvent evaporation method. The study aimed to determine
the effects of the type and concentration of the dispersing agents and the
inner phase polymer concentration, six formulation variables, the drug-polymer
variation (1:1, 1:2, 1:3) and different dispersing agent (aluminium
stearate, magnesium stearate).
The response (output) variables examined to characterize the microspheres and
drug release were the size of the microspheres.
MATERIALS AND METHODS:
Indinavir sulphate
and Eudragit RS 100 was obtained as a gift sample
from Cipla Ltd., Mumbai. Magnesium stearate, Aluminium
stearate, Paraffin liquid light, Acetone, Methanol,
n-hexane, Petroleum ether, used were of analytical grade, purchased from
Loba Chem, S. D. Fine-Chem, E-Merck and CDH (Mumbai) respectively.
Preparation of microspheres:
In the
present study Indinavir microspheres were prepared by
solvent evaporation techniques using Eudragit RS100.
Microspheres were prepared by solvent evaporation method using acetone and
liquid paraffin. During the preparation, amount of drug was kept constant;
polymer concentration was changed and using different dispersing agent
(magnesium stearate, aluminium
stearate) were used. The drug: polymer ratio were
kept constant [1:1, 1:2, 1:3]. The amount of liquid paraffin, n-hexane,
dispersing agent and acetone were kept constant. The influence of formulation
factors e.g. stirring speed, polymer: drug ratio, amount of droplet stabilizer
i.e. magnesium stearate, aluminium
stearate etc. on particle size, encapsulation
efficiency and in-vitro release profile of the microspheres were investigated.
Measurement of micromeritic
properties:14
The flow properties of prepared microspheres were
investigated by measuring the bulk density, tapped density, Carr’s index and
packing factor. The bulk and tapped densities were measured in a 10 ml
graduated measuring cylinder. The sample contained in the measuring cylinder
was tapped mechanically by means of constant velocity rotating cam. The initial
bulk volume and final tapped volume were noted from which, their respective
densities were calculated.
Particle size analysis:14
Microspheres
were separated into different size fractions by sieving for 10 minutes using a
mechanical shaker (Cuprit Electrical Co. India)
containing standard sieves # 16, # 24, # 30,
# 44 and # 60 and mean particle sizes of microspheres were calculated.
Drug content and encapsulation efficacy of eudragit microspheres:
50 mg of
formulations was dissolved in 50 ml of distilled water. The samples were
assayed for drug content by UV-
spectrophotometer (Shimadzu UV-1700) at λmax
259.0 nm and the drug content was calculated.
Microsphere morphology:
The
external and internal morphology of microspheres were studied by scanning
electron microscopy (Joel, SEM Model JSM - 6400,
Fourier Transform Infrared Spectroscope
[FTIR]:
The IR Spectra of Indinavir
was recorded by using (JASIO Model No.410). Drug sample was prepared in KBr disks [2mg sample in 200mg KBr].
The scanning range was 400-4000cm-1 and the resolution was 2cm-1
Differential
Scanning Calorimeter [DSC]:
The
DSC analysis of pure drug, drug-loaded microspheres were carried out using
Perkin Elmer,
In vitro drug release study:
The drug release rate from eudragit
microspheres were carried out using USP -dissolution apparatus Type 2 (VEEGO,
VDA-6D). A weight of eudragit microspheres
corresponding to 100 mg of drug was filled into a capsule and placed in basket.
Dissolution media was 500 ml distilled water maintained at 37 ± 0.50 C
and stirred at 100 rpm. Samples (1 ml) were withdrawn at suitable interval of
time and volume was adjusted. It was then assayed spectrophotometrically at
259.0 nm.
RESULTS AND DISCUSSION:
As Concentration of aluminium stearate increased from
50-200mg, the arithmetic diameter of the formulations decreased. Keeping
polymer to drug ratio at the same level, low concentration (50mg) of aluminium stearate produced
larger shape microsphere, where as higher concentration of aluminium
stearate produced smaller particle. The desire size
microspheres were produced at a concentration kept of 150mg.
The effect of dispersing
agent concentration on particle size has been related to the droplet
stabilization preventing droplet coalescence and the formation larger droplets.
Reduction of particle size at higher aluminium stearate concentration may be due to the accelerated
dispersion of microsphere in the system. When the drug : polymer ratio was 1:1,
there was formation of microspheres with small and irregular size. As the
polymer concentration was increased, viscosity of the solution increased,
resulting in large particles. Therefore mean particle size also increased.
The flow properties of microspheres are tabulated in
Table 1. The flow property is represented in terms of compressibility Index.
Compressibility value of all microspheres was 1-17, which indicates excellent
flow ability of microspheres compared to original drug crystals. Also the
microspheres were found a higher packability compared
to that of original drug crystals. The improvements of flow properties suggest
that the microspheres can be easily handled.
TABLE 1: MICROMERITIC
PROPERTIES OF INDINAVIR SULPHATE MICROSPHERES
|
Formulations |
Mean particle size (mm) |
Flow properties |
|
|
% Compressibility |
Packing factor |
||
|
FM1 |
340.12 ± 5.56 |
7.6± 0.33 |
1.08 ± 0.06 |
|
FM2 |
415.85 ± 5.70 |
15.78 ± 0.25 |
1.18 ± 0.02 |
|
FM3 |
464.61 ± 4.86 |
16.66 ± 0.52 |
1.2 ± 0.01 |
|
FA1 |
437.64 ± 3.68 |
11.11± 0.26 |
1.12± 0.03 |
|
FA2 |
520.85 ± 5.13 |
3.22± 0.48 |
1.03± 0.04 |
|
FA3 |
611.0 ± 3.72 |
5.71±0.54 |
1.06±0.02 |
|
Pure drug |
--- |
39.96± 0.11 |
1.66 ± 0.02 |
Each observation is the mean ± S.D. of three determinations
TABLE 2: CHARACTERISTIC OF INDINAVIR SULPHATE
MICROSPHERES.
|
Formulation |
% Yield |
% Drug content |
% Drug entrapped |
|
FM1 |
58.0± 0.503 |
12.37 ± 0.093 |
24.75 ± 0.185 |
|
FM2 |
72.7± 0.569 |
16.76 ± 0.246 |
33.53 ± 0.491 |
|
FM3 |
75.6± 0.864 |
21.18 ± 0.221 |
42.37 ± 0.443 |
|
FA1 |
93.1± 0.449 |
38.35 ± 0.113 |
76.71 ± 0.226 |
|
FA2 |
97.7 ± 0.614 |
43.26 ± 0.177 |
86.53 ± 0.350 |
|
FA3 |
99.2 ±0.457 |
45.44 ± 0.235 |
90.88 ± 0.120 |
Each observation is the mean ± S.D. of three determinations.
TABLE.3 : VARIOUS PARAMETERS OF THE MODEL EQUATIONS OF
THE IN- VITRO RELEASE KINETICS
|
Formulation Code |
Zero order |
First order |
Higuchi Model |
Hixon Crowell Model |
||
|
r2 |
t1/2 |
r2 |
r2 |
Slope |
r2 |
|
|
FM1 |
0.9883 |
0.72 |
0.9708 |
0.9991 |
63.146 |
0.9921 |
|
FM2 |
0.9709 |
0.21 |
0.9822 |
0.994 |
56.414 |
0.993 |
|
FM3 |
0.9587 |
0.49 |
0.9473 |
0.9647 |
51.402 |
0.9608 |
|
FA1 |
0.9792 |
1.35 |
0.9212 |
0.9856 |
52.874 |
0.9636 |
|
FA2 |
0.9567 |
1.36 |
0.9626 |
0.9927 |
51.655 |
0.992 |
|
FA3 |
0.9851 |
1.63 |
0.9433 |
0.9988 |
48.767 |
0.9871 |
r2 = correlation co-efficient K0, K1,
KH, KHC are the rate constants for Zero
Order, First Order, Higuchi Model, Hixon-Crowell
Model
Figure.1: SEM Photographs indicate Eudragit
RS 100 Loaded Indinavir Microspheres (FA) before
dissolution (A, B & C)
It was observed that when the speed of stirrer was kept
below 500rpm, there was formation of clumps or aggregate mass. This could be
due to the reason that stirring was not enough to disperse the inner phase in
the total mass. Therefore particles settled down at the bottom of vessel. The
speed of stirrer at 700-1000rpm caused high turbulence which ultimately
resulted in smaller particle. Spherical and desire sized microspheres were
found at stirring 500-700rpm. The similar effects were also observed in case of
magnesium stearate.
Figure.2: SEM Photographs indicates Eudragit
RS100 Indinavir Microspheres (FA) after dissolution
(A, B & C).
The effect of dispersing agent had profound effect on
yield value, entrapment efficiency and dissolution profile. In case of
preparation prepared with aluminium stearate the yield value was found to be in between range
93-99% but in case of magnesium stearate the values
lies between 58-76%. This may be due to the fact that at the particular
temperature (room temperature) of preparation the solubility of magnesium stearate was more in comparison to aluminium
stearate. The same effect was found in case of mean
particle size.The encapsulation efficiency is more in
case of aluminium stearate
in comparison to magnesium stearate.
Wave number (Cm-1)
Figure.3 : IR Spectras of pure Indinavir (A), Eudragits RS 100 loaded
microspheres[FM](B), Eudragits RS 100
loaded microspheres[FA](C), Eudragit RS 100 loaded Blank Microspheres(D).
SEM study suggests spherical shape of particles. The
study of drug loaded microspheres had shown the presence of drug particle on
surface as shown in Figure 1 and 2. The spherical nature and size of the
microspheres does not change after dissolution.
IR spectra of pure Indinavir
and blank microspheres of RS100, drug loaded microspheres of Eudragit RS100 were shown in Figure 3. Drug spectrum shows
prominent peaks at 3371.9cm-1, 2974.4cm-1, 1678.3cm-1,
and 1658.2cm-1 corresponding to OH stretching, C–H stretching, C=H
stretching and C=O stretching. Eudragit RS100 loaded Indinavir microspheres show similar peaks that
corresponding to IR Spectra of pure drug. The IR study suggests the stable
nature of drug during encapsulation process. This was further supported by DSC
studies.
The drug could be either dispersed in
crystalline or amorphous form or dissolved in the polymeric matrix during
process of preparation of microspheres. Also any abrupt or drastic change in
the thermal behaviour of either the drug or polymer
may indicate a possible drug- polymer interaction.
The thermal curves of pure Indinavir and Eudragit RS100
loaded Indinavir microspheres were presented in
Figure 4.The thermal behaviour of the pure Indinavir shows endotherms at 1600C,
corresponding to its melting point (1570C to 1680C). The
thermal behaviour of Eudragit
RS100 loaded microspheres was observed at 1600C but with loss of it
sharp appearance. It appears that there is a significant reduction of drug crystallinity in the polymer matrix.
Release mechanism of Indinavir
from various formulations was finding out by comparing their respective
correlation co-efficient, as shown in Figure 5 and Table 3. The various release
kinetics were carried out i.e. Zero order, first order, Higuchi and Hixon Crowell models. It was suggested that mechanism of
drug release from microspheres was on diffusion controlled as well as
dissolution controlled in both cases. Formulations were found to release the
drug by Higuchi kinetic model and the effect of dispersing agent was evident on
the release behaviour from the formulation.
Figure.4 : DSC Curve of pure drug (Indinavir), DSC Curve of Eudragit RS 100 loaded microspheres of Indinavir (FA). DSC Curve of Eudragit
RS 100 loaded microspheres of Indinavir (FM)
Figure.5 : Cumulative % release of Indinavir from different formulations
Formulation containing aluminium
stearate shows a more sustained effect than
formulation containing magnesium stearate. This may
due to fact that aluminium stearate
is more hydrophobic in comparison to magnesium stearate.
CONCLUSION:
The present study was aimed
to compare the performance of two types of dispersing agent.Aluminium
stearate (hydrophobic) and magnesium stearate (less hydrophobic) which have different ampiphillic structure led to formation of microsphere by
different ways, which ultimately affected different parameter like drug
entrapment efficacy, shape, yield and dissolution profile.
At last it may conclude that aluminium stearate is a better
dispersing agent than magnesium stearate.
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Received on 03.11.2009
Accepted on 01.01.2010
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Research Journal of Pharmaceutical Dosage
Forms and Technology. 2(1): Jan. –Feb. 2010, 67-71