1. INTRODUCTION:
A major challenge in the formulation of oral drug delivery is the
low bioavailability of drug candidates exhibiting poor solubility
characteristics. With the onset of high throughput screening in drug discovery,
the number of poorly water-soluble drug candidates has increased significantly
[1]. Many approaches have been developed to improve solubility and to enhance
the dissolution rate of poorly soluble drugs, including both modifications to
the drug substance itself and the creation of specific formulations [2]. Among these techniques, preparation of solid dispersions has been
the most successful strategy because they can produce a solid dosage form in an
amorphous state [3].
Chio and Riegelman
defined the term solid dispersion as “a dispersion of one or more active
ingredients in an inert carrier or matrix of solid state prepared by melting
(fusion), solvent or melting solvent method” [4] . The
improvement of drug dissolution from solid dispersion is attributed to particle
size reduction of the drug molecules, reduction of aggregation, solubilization
effect of the carrier, and specific molecular interactions between the drug and
the polymer [5]. Irbesartan is chemically denoted
as 2-butyl-3-({4-[2-(2H-1,2,3,4-tetrazol-5-yl)phenyl]phenyl}methyl)-1,3 diazaspiro
[4.4]non-1-en-4-one (Fig.
1). Irbesartan
is indicated for the treatment of hypertension. It may also delay progression of diabetic nephropathy and is also
indicated for the reduction of renal disease progression in patients with type
2 diabetes. Irbesartan is also used as a second line agent in the treatment of
congestive heart failure. It is practically
insoluble in water. Being a BCS class II drug, its rate of absorption and
extent of bioavailability are controlled by rate of dissolution in the
gastrointestinal fluids. Hence, improvement in its solubility and dissolution
rate may lead to an enhancement in bioavailability. In this study, inclusion
complexes of Irbesartan were prepared by kneading technique. β-CD and HP
β-CD were used as hydrophilic carriers to enhance dissolution rates and
thus bioavailability. Solubility and dissolution rate of the solid dispersions
were compared with Irbesartan. The physical properties of the prepared solid dispersions
were characterized by FTIR and DSC studies.
Fig.1. Structure of Irbesartan
2. MATERIALS AND METHODS:
2.1 Materials:
Irbesartan was
received as a gift sample from Alembic Ltd, Baroda, India. All the other
materials used in the present work were commercial samples. β- Cyclodextrin (Yarrow Chem
Products, Mumbai), Hydroxy Propyl
β- Cyclodextrin (Yarrow Chem
Products, Mumbai), Hydrochloric acid (Yarrow Chem
Products, Mumbai), Methanol (Merck Limited, Mumbai). All the reagents used were
of analytical grade. Freshly prepared distilled water was used in the work.
2.2 METHODS:
2.2.1 Phase Solubility Studies:
Drug was added
in excess to 20ml of distilled water containing various concentrations (1%, 2%,
3% and 4% w/v) of β-CD and HP β-CD in a series of glass vials. These
mixtures were shaken on a rotary shaker at 37 0C for 48
hours in order to achieve equilibrium solubility. After 48 hours, samples were
filtered using Whatman no. 1 filter paper. The filtrate was suitably diluted
with corresponding polymer carrier solution and analyzed spectrophotometrically
at the wavelength of 230nm using a spectrophotometer (Agilent Cary 60). Phase solubility studies were conducted with
and without the addition of hydrophilic carrier. The solubility experiments
were performed in triplicate.
2.2.2 Preparation of Inclusion Complexes:
By Kneading Method:
The drug and
carrier (β-CD or HP β-CD) were taken in a mortar and were triturated
with a small volume of water: methanol (9:1) to form a wet mass which was then
dried at 45˚c. The dried mass was pulverized and passed through 60 mesh
sieve.
2.2.3 Preparation of Physical mixtures:
Required amounts
of Irbesartan and the carriers (β-CD, HP β-CD) in different ratios of
% w/w (drug: carrier) were thoroughly mixed in a mortar and pestle in order to
obtain a homogenous mixture. The resulting mixture was passed through 60 mesh
sieve. The powder was stored in a screw cap vial at room temperature until use.
Table 1: Composition of Solid
dispersion formulations of Irbesartan
|
Formulation Code |
Polymer used |
Drug to Polymer ratio |
Method of preparation |
|
I 1 |
β CD |
1:0.125 |
Physical Mixing |
|
I 2 |
1:0.5 |
||
|
I 3 |
1:1 |
||
|
I 4 |
β CD |
1:0.125 |
Kneading Method |
|
I 5 |
1:0.5 |
||
|
I 6 |
1:1 |
||
|
I 7 |
HP β CD |
1:0.125 |
Physical Mixing |
|
I 8 |
1:0.5 |
||
|
I 9 |
1:1 |
||
|
I 10 |
HP β CD |
1:0.125 |
Kneading Method |
|
I 11 |
1:0.5 |
||
|
I 12 |
1:1 |
2.2.4 In vitro
Dissolution Study:
Dissolution
studies of Irbesartan in powder form and from its inclusion complexes was
studied using Electro lab which is an 8 station dissolution rate test apparatus
with a paddle stirrer. These studies were conducted in 900 ml of simulated
gastric medium (0.1N HCl of pH≈1.2) maintained
at a temperature of 37±0.5 0C at 50rpm speed. 50mg of Irbesartan or
its solid dispersion equivalent to 50mg of Irbesartan was added to the
dissolution medium. At predetermined sampling intervals, 5ml of dissolution
medium was withdrawn, filtered through Whatman filter paper. The withdrawn
volume was replenished immediately with the same volume of the prewar med (37 0C)
dissolution medium in order to maintain a constant volume throughout the test.
The filtered samples were analyzed spectrophotometrically at 230 nm.
Dissolution experiments were conducted in triplicate (n=3). Thus obtained
dissolution profiles are compared using model dependent approaches, where the
release data can be fitted to different kinetic models including
zero order, first order, Higuchi matrix, Peppas-Korsmeyer
and Hixson Crowell
2.2.5 Characterization of
Inclusion Complexes:
2.2.5.1 Fourier Transform
Infra-Red Spectroscopy (FTIR):
Fourier-transform infrared (FT-IR) spectra were obtained by using an FT-IR spectrometer (Schimazdu) by potassium bromide (KBr)
pellet method. The samples (pure drug or solid dispersions) were previously
ground and mixed thoroughly with potassium bromide, an infrared transparent
matrix, at 1:5 (Sample: KBr) ratio, respectively. The
KBr discs were prepared by compressing the powders at
a pressure of 5 tons for 5 minutes in a hydraulic press. Scans were obtained at
a resolution of 2 cm-1, from 4000 to 400 cm-1.
2.2.5.2 Differential Scanning
Calorimetry (DSC):
Measurements were performed on a DSC- 6100 (Seiko
Instruments, Japan) with a thermal analyzer. All accurately weighed samples
(about 2 mg of Irbesartan or its equivalent) were placed in sealed aluminum
pans, before heating under nitrogen flow (20 mL/min)
at a scanning rate of 100C min-1 from 50 to 3000C.
An empty aluminum pan was used as reference [6].
2.2.6 Dissolution Data
Analysis:
2.2.6.1 Phase Solubility
Studies:
The value of apparent stability constant, Ks, between
drug-carrier combinations were computed from the phase solubility profiles, as
described below
Ks = 
(1)
The Gibbs free energy values provide the information
whether the reaction condition is favorable or unfavorable for drug solubilization in the aqueous carrier solution. Negative
Gibbs free energy values indicate favorable conditions. The Gibbs free energy
of transfer (ΔGtr0) of modafinil
from pure water to the aqueous solution of carriers was calculated using
following Eq.
ΔGtr0 = -2.303 RT log S0/Ss (2)
Where So/Ss = the ratio of molar solubility of drug in
aqueous solutions of carriers to that of the pure water.
2.2.6.2 In vitro Dissolution Data:
Dissolution studies of the drug in powder form, solid
dispersions of drug and carrier can be carried out using dissolution apparatus.
Thus obtained dissolution profiles are compared by analysis of variance (ANOVA)
based, model-independent and model dependent approaches [7]. ANOVA methods
detect statistically significant differences between dissolution profiles.
Model-independent approaches are based on the ratio of area under the
dissolution curve (dissolution efficiency) or on mean dissolution time [8, 9].
Relative performance of different concentrations of carriers in solid
dispersions can be found by computing percent Dissolution Efficiency
(%DE).
(3)
Where, y is the drug percent dissolved at time t
In model-dependant approaches, release data can be
fitted to different kinetic models including zero order (Eq. 4),
first order (Eq. 5), Higuchi matrix (Eq.6 ), Peppas-Korsmeyer
(Eq .7 ) and Hixson Crowell (Eq. 8) [10].
(4)
(5)
(6)
(7)
(8)
Where, R and UR are the released and
unreleased percentages, respectively, at time t; k1, k2,
k3, k4 and k5
are the rate constants of zero order, first
order, Higuchi matrix, Peppas-Korsmeyer, and
Hixson- Crowell model, respectively.
3.
RESULTS AND DISCUSSION:
3.1 Phase Solubility Studies:
Phase-solubility
diagrams showed a linear increase of drug solubility with an increase of the
concentration of carrier. This has been attributed to the probable formation of
weak soluble complexes. On the other hand, the enhancement of the drug
solubility in the aqueous carrier solution could be equally well explained by
the co solvent effect of the carrier. It has been found that hydrophilic
carriers mainly interact with drug molecules by electrostatic bonds
(ion-to-ion, ion-to-dipole, and dipole-to-dipole bonds), even though other
types of forces, such as van der Waals forces and
hydrogen bonds, can frequently play a role in the drug-carrier interaction. The
drug solubility increased linearly with increasing polymer concentration
indicative of the AL type of solubility phase diagram. An indication
of the process of transfer of drugs from pure water to the aqueous solutions of
carriers was obtained from the values of Gibbs free energy change. The obtained
values of Gibbs free energy provide the information regarding the increased
solubility of drug in the presence of carrier. ΔGtr0 values
were all negative for carrier at various concentrations, indicating the
spontaneous nature of drug’s solubilization, and it
decreased with an increase in its concentration, demonstrating that the
reaction became more favorable as the concentration of carrier increased.
Table 2: Thermodynamic Parameters of the Solubility Process of
Irbesartan in Different Carrier-Water
Solutions at 37 ºC
|
% (w/v) of carrier in water |
Gibb’s Free Energy
∆Gtr0 (J/mol) |
|
|
β-CD |
HP
β-CD |
|
|
1 % |
-3282 |
-3314 |
|
2 % |
-3384 |
-3476 |
|
3 % |
-3705 |
-3821 |
|
4 % |
-3756 |
-3926 |
3.2 Dissolution Studies:
The dissolution curves are shown in the Fig. According to these results, all inclusion complexes
exhibited higher rates of dissolution values than Irbesartan pure drug and
corresponding physical mixtures, indicating amorphization,
increased wettability and dispersibility
and particle size reduction of drug in formulations. Simple physical mixtures of the drug with the hydrophilic
polymer increased the solubility of drug to some extent but formulation of
inclusion complexes by kneading technique further improved the dissolution rate
of the drug. Pure drug showed around 53% dissolution over a period of
60minutes, while its inclusion complex enhanced the dissolution rate up to 86%.
Irbesartan: HP β-CD (1:1) ratio prepared by kneading technique showed
highest dissolution rate. In vitro release data of drug best fitted to Korsemeyer-Peppas model with n value of 0.729 and hence
exhibits non fickian diffusion.