Formulation and Evaluation of Controlled Release Microspheres
of Zidovudine
Vinod R, Ashok Kumar P*, Amit S Yadav, Someshwara Rao B and Suresh V Kulkarni
Department
of Pharmaceutics, Sree Siddaganga
College of Pharmacy, B.H.Road, Tumkur-572102,
Karnataka, India.
ABSTRACT
The aim of this study was to formulate and evaluate
microencapsulated controlled release preparations of zidovudine, using Copolymers Eudragit
S 100 and RL 100 (acrylic and methacrylic acid
esters) and Ethyl cellulose as the retardant material. Microspheres were
prepared by solvent evaporation method using an acetone/liquid paraffin system.
Magnesium stearate was used as the droplet stabilizer
and n-hexane was added to harden the microspheres. The prepared microspheres
were characterized for their micromeritic properties,
drug loading, as well by Fourier transform infrared spectroscopy (FTIR) and
scanning electron microscopy (SEM). The in vitro release studies
were performed in pH 7.4, phosphate buffer. The prepared microspheres were
white, free flowing and spherical in shape. The drug-loaded microspheres show
81-93% of entrapment and release was extended more than 10hrs. Stability
studies revealed that polymers used were stable and compatible with the drug
and there is no significant effect on physical characteristics, drug content
and dissolution profile of the microspheres. Scanning electron microscopy study
revealed that the microspheres were spherical with rough surface. The best-fit
release kinetics was achieved with Higuchi’s plot followed by First order and Zero
order. The release of Zidovudine was influenced by
the drug to polymer ratio, particle size & was found to be diffusion
controlled.
KEYWORDS:
Controlled release, Ethyl cellulose, Eudragit S100 and RL 100, Microspheres, Zidovudine.
INTRODUCTION
In
long-term therapy for the treatment of chronic disease conditions, conventional
formulations are required to be administered in multiple doses, and therefore
have several disadvantages1.
Controlled release microsphere formulations are much desirable and preferred
for such therapy because they offer better patient compliance, maintain uniform
drug levels, reduce dose and side effects, and increase safety margin for
high-potency drugs2. Microsphere is one of the multiple unit dosage
forms. In this present study, microspheres were prepared by solvent evaporation
method using Eudragit S 100 and RL 100, Ethyl
cellulose as the retardant material. solvent
evaporation method is the preparation technique that is widely preferred for
the preparation of controlled release microspheres3. Liquid paraffin
and acetone system were used for the preparation of microspheres. Magnesium stearate was used as a droplet stabilizer to prevent
droplet agglomeration in oil phase and n-hexane was added as a non-solvent to
the processing medium to solidify the microspheres. Zidovudine
is a potent antiviral agent used in the treatment of AIDS, either alone or in
combination with other antiviral agents.
Zidovudine has low oral
bioavailability (60%) due to considerable first-pass metabolism and a high
clearance, thus necessitating frequent administration of large doses to
maintain therapeutic drug levels4.
Conventional
formulations of Zidovudine are administered multiple
times a day depending on the dose (300 mg twice daily or 200 mg thrice daily)
due to its short half-life (t1/2
= 0.5 to 3 h)5,6. Treatment of AIDS using conventional formulations
of Zidovudine is found to have many drawbacks such as
adverse side effects due to accumulation of drug in multi-dose therapy, poor
patient compliance and high cost. So controlled release formulations of Zidovudine can overcome some of these problems7.
The effect of the variations of drug/polymer ratio on the preparation of
microspheres and their characteristics were determined and evaluated for entrapment
efficiency, percentage yield value, particle size, surface characteristics of
microspheres and dissolution tests8.
MATERIALS AND
METHODS:
Materials:
Zidovudine and Eudragit was obtained as a gift sample from Strides Arco
Labs (Bangalore, India). Ethyl cellulose was
obtained from Micro Labs (Bangalore,
India). All
other reagents and solvents used were of pharmaceutical or analytical grade.
Methods:
Zidovudine microspheres were
prepared by solvent evaporation technique (Table 1). Ethyl cellulose and Eudragit S100 and RL 100 in different ratios were dissolved
separately in 25 ml of acetone by using a magnetic stirrer (Remi
motors, Mumbai, India). Pure zidovudine
(300 mg previously dissolved in 10 ml methanol) and magnesium stearate (100 mg) were dispersed in the polymer solution.
The resulting dispersion was then poured into 1000 ml beaker, containing the
mixture of 200ml liquid paraffin light and 30 ml n-hexane while stirring (Remi motors, Mumbai,
India).
Stirring (at 500-700 rpm) was continued for 4 hours, until acetone is
evaporated completely. After evaporation of acetone, the microspheres formed
were filtered using Whatman no.1 filter paper. The residue was washed for 4-5
times with 50 ml petroleum ether. Microspheres were dried at room temperature
for 24 hrs. The dried microspheres were then weighed and the yield of
microsphere preparations was calculated using the following formula8.
Measurement of micromeritic
properties of microspheres(Table 2):
The flow properties of prepared microspheres
were investigated by measuring the bulk density, tapped density and Carr’s
index. 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 calculated9.
Carr’s
index =
[(Tapped
density-bulk density) / Tapped density] X 100
Drug
Entrapment Efficiency:
About
10mg of accurately weighed drug-loaded microspheres were added to 100ml of
Phosphate buffer, of pH 7.4. The resulting mixture was shaken in a mechanical
shaker for 24hr. The solution was filtered with a 0.45μm pore size filter
and 1ml of the solution was appropriately diluted to 10ml using phosphate
buffer, pH 7.4, and analyzed spectrophotometrically at 272nm using UV-Visible
spectrophotometer (Labindia, Mumbai, India).
The percentage entrapment of microsphere was calculated by the formula.
Scanning electron microscopy (SEM):
A
scanning electron microscope was used to characterize the surface morphology of
the microspheres10. Dried microspheres were mounted onto stubs by
using double-sided adhesive tape. The microspheres were coated with gold and
observed under scanning electron microscope (Joel, JSM-5600 LV, Japan) for
surface characteristics.
Fourier
Transform Infrared Spectroscopy (FTIR):
FTIR
spectroscopy was used to study drug-polymer compatibility. The spectra were
recorded for pure drug and drug-polymer mixtures using FTIR spectrophotometer
(FTIR-8400 S, Shimadzu, Japan) with KBr
pellets. The scanning range was 600-4000cm-1.
Drug Release Study:
The
in vitro dissolution studies were carried out in 500 ml of phosphate
buffer, pH 7.4, maintained at 37 ± 0.5°C and 100 rpm by using USP basket type
dissolution test apparatus (Electrolabs, Mumbai, India)
under sink conditions. Accurately weighed samples of microspheres (approx. 50
mg of drug) were added to the dissolution medium and at preset time intervals,
2 ml aliquots were withdrawn and replaced with an equal volume of fresh
dissolution medium. After suitable dilution, the samples were analyzed
spectrophotometrically at 266 nm. The concentration of zidovudine
in test samples was obtained and calculated using a regression equation of the
calibration curve. The dissolution studies were carried out in triplicate and
the mean values were plotted as percentage cumulative release versus time11.
Drug release
kinetics:
To study the release kinetics, data obtained
from in vitro drug release studies
were plotted in various kinetic models: zero order (Equation 1) as cumulative
amount of drug release vs time, first order (Equation
2) as log cumulative percentage of drug remaining vs
time, and Higuchi’s model (Equation 3) as cumulative percentage of drug
released vs square root of time.
C=K0
t………………………………………….……………..
(1)
Where, K0
is the zero order rate constant expressed in units of concentration/time
and t is the time in hours. A graph
of concentration vs time would yield a straight line
with a slope equal to K0
and intercept the origin of the axes12.
LogC = log C0 -
Kt/2.303………………..…………………..
(2)
Where C0
is the intial concentration of drug, k is the first order constant, and t is the time13.
Q = kt˝ …………………………..…………………………..
(3)
Where, k
is the constant reflecting the design variables of the system and t is the time in hours. Hence, drug
release rate is proportional to the reciprocal of the square root of time14.
Mechanism
of Drug release:
To evaluate the mechanism of drug release from
zidovudine controlled release microspheres data for
drug release were plotted in Korsmeyer et al’s
equation (Equation 4) as log cumulative percentage of drug release vs log time and the exponent n was calculated through the
slope of the straight line.
Mt/ Mα=ktn……………………………………………...…
(4)
Where, Mt/
Mα are the fractional solute
release, t is the release time, K is
a kinetic constant characteristic of the drug/polymer system, and n is an exponent that characterires the mechanism of release of tracers15. For controlled release
microspheres, if the exponent n
=0.45, then the drug release mechanism is fickian
diffusion, and if 0.45 < n <
0.89, then it is non-fickian or anomalous diffusion.
An exponent value of 0.89 is indicative of case-II transport or typical zero-order
release.
Stability
study:16
Stability study was carried out on the
optimized formulation. The formulation was wrapped in aluminium
foil and then placed in an amber colored bottle. It was stored at 40 + 20
C, 75% + 6% relative
humidity for 6 months. Microspheres were evaluated for in vitro drug release
after Two, Four and Six months. Result obtained was compared with the data
obtained for zero times at room temperature and humidity (Temperature 28 +
20 Cand humidity 42% + 2% )
RESULTS
AND DISCUSSION:
The flow properties are expressed in terms of
Carr’s index. The results of Carr’s index (%) ranged from 8.09% to 14.05%. The
lowest compressibility index is 5 to 15% which indicates excellent flow
properties. The drug entrapment efficacy of all the formulations was in the
range of 81 to 93%. The highest drug percentage entrapment was observed in
formulation F1 (92.67%). The results of the dissolution studies for
formulations F1, F2, F3, F4, F5 and F6 are shown in the figure-1. The cumulative percentage drug release for F1, F2,
F3, F4, F5 and F6 was (97.39%, 87.54%, 95.08%, 92.59%, 82.76% and 73.18%) at
the end of 10hrs respectively. Among all the formulation F1 shows highest drug
release (97.39%), where as the drug release from formulation F5 and F6 was slow,
this shows that ethyl cellulose is permeable. The release rate of Eudragit was extended by adding ethyl cellulose in
combination. The data clearly indicate the drug release can effectively be
controlled by varying the polymer and its ratio.
The regression coefficients obtained for
first order kinetics were found to be higher (0.949 to 0.995), when compared
with those of zero order kinetics (0.947 to 0.978), indicating that drug
released from all formulation followed first order kinetics (Table-3). To
evaluate drug release mechanism from the microspheres, plots of cumulative
percentage release vs square root of time as per
Higuchi’s equation were constructed. These plots were found to be linear with
all the formulations (R2: 0.974 to 0.995) indicating that the drug
release from the microspheres was diffusion controlled. To confirm the
diffusion mechanism the data were fit into korsmeyer
et al’s equation. All the formulation shows good linearity (R2:
0.988 to 0.999), with the slope (n)
values 0.5644 to 0.7416, anomalous (non-Fickian)
diffusion (0.45 < n < 0.89) is
the dominant mechanism of drug release with all the formulations.
Scanning electron
microscopy
SEM of Formulation F1
SEM of Formulation F6
Scanning electron microscopy indicates that
the microspheres are spherical in shape and with rough surface. FTIR studies of
Zidovudine shows prominent peaks at 3463,3022,1694cm-1
due to the presence of O-H stretching, C-H aromatic stretching, C=O stretching.
Stability studies revealed that polymers used were stable and compitable with the drug and the formulations were stable.
CONCLUSION:
Results of the present study demonstrated
that combination of both hydrophilic and hydrophobic polymers could be
successfully employed for preparing microspheres by using the solvent
evaporation method. The yields and entrapment efficiency were high for all
formulations. SEM studies show that the particles were spherical with rough
surface. The microspheres were found to be effective in sustaining the drug release
more than 10hrs. Drug release was diffusion controlled and followed first order
kinetics. Stability studies revealed that there was no significant change in
drug content and dissolution profile of microspheres. FTIR studies revealed
that there was no shift in peaks, indicating there is no interaction between zidovudine and other ingredients used. Controlled release
without initial peak level achieved with these formulations may reduce dose
frequency and side effects as well as improved patient compliance.
ACKNOWLEDGEMENT:
The authors are thankful to the Management, Sree Siddaganga College of
Pharmacy, for providing necessary facilities to carryout this work.
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