INTRODUCTION:

Azilsartan is one of the widely used antihypertensive drug and is also having poor water – solubility. It is a selective AT1subtype angiotensin II receptor antagonist. It lowers blood pressure by blocking the action of angiotensin II at the AT1 receptor, a hormone that contracts blood vessels and reduces water excretion through the kidneys.¹ Azilsartan is angiotensin II type 1 receptor blocker discovered in 2011. But till date it is used in form of prodrug (in salt form) azilsartan medoxomil which has been approved in both the United state and Europe. In 2012, the parent compound azilsartan was approved in Japan for the treatment of hypertension. But it has solubility problem which leads to low bioavailability, hence it is necessary to improve solubility of azilsartan.^2,3,4

Self-micro emulsifying drug delivery systems (SMEDDS) are isotropic mixtures of drug, lipids, and surfactants, usually with one or more hydrophilic co-solvents or emulsifiers.^5,6 Upon mild agitation followed by dilution with aqueous media, these systems can form fine (oil in water) emulsion instantaneously.^7,8,9 SMEDDS are normally prepared either as liquids or encapsulated in soft gelatin capsules, which have some shortcomings especially in the manufacturing process, leading to high production costs. Moreover, these dosage forms may be inconvenient to use and incompatibility problems with the shells of the soft gelatin are usual. Incorporation of a liquid self-emulsifying formulation into a solid dosage form may combine the advantages of SMEDDS with those of a solid dosage form and overcome the disadvantages of liquid formulations described above.^10,11

Hence objective of present study was to develop self emulsifying tablet of azilsartan to enhance solubility, dissolution and hence oral bioavailability of azilsartan.

Figure 1 Chemical structure of azilsartan¹²

MATERIALS AND METHODS:

Materials:

Azilsartan was procured from DRVL Mumbai, India. Transcutol P were procured from Gattefosse India Pvt. Ltd., Mumbai and ethyl oleate, Tween 80 and sodium starch glycolate were obtained from Loba Chemie Pvt. Ltd., Mumbai. Neusilin US2 was gifted by Fuji chemicals Japan. All other chemicals used were of analytical grade.

Methods:

Construction of pseudo-ternary phase diagram:

From previous solubility study of azilsartan, ethyl oleate, Tween 80 and Transcutol P were selected as oil, surfactant, and co-surfactant respectively. Micro-emulsion region was predicted by constructing pseudo-ternary phase diagram containing a different proportion of surfactant: co-surfactant (Km value 1:1, 2:1 and 3:1), oil and water. In brief Smix and oil were mixed at a ratio of 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2 and 9:1 in a pre-weighed test tube. To the resultant mixtures, double distilled water was added drop-wise until the first sign of turbidity observed, in order to identify the endpoint. After equilibrium; if the system became clear then the water addition was continued.^13,14

Preparation of azilsartan loaded liquid SMEDDS:

Three batches namely AZS-L1, AZS-L2 and AZS-L3 were selected from pseudo ternary phase diagram of Km=3 as it showed maximum micro emulsion region. Composition of azilsartan loaded liquid SMEDDS is shown in Table 1. In brief azilsartan (40 mg/10gm) was placed in a glass vial. To this ethyl oleate was added and warmed on a water bath. In this oily mixture Tween 80 and Transcutol P in the proportion of 3:1 was added. Then the components were mixed by gentle stirring and vortex mixing at 37ºC until azilsartan was completely dissolved. Then the mixture was sealed in a glass vial and stored at room temperature until used.^7,15,16

Preparation of azilsartan loaded S-SMEDDS:

All three batches of azilsartan loaded liquid SMEDDS were converted to solid form (AZS-S1, AZS-S2 and AZS-S3) by adsorption technique using Neusilin US2 as a solid carrier. In brief; liquid SMEDDS was added drop wise over Neusilin US2 contained in broad porcelain dish. After each addition, mixture was homogenized using glass rod to ensure uniform distribution of formulation.^13,16,17,18

Evaluation of azilsartan loaded S-SMEDDS:

Prepared azilsartan loaded S-SMEDDS were evaluated for various micromeritic properties such as angle of repose, bulk and tapped density, compressibility index and Hausner ratio and drug content.^19,20,21

Formulation of immediate release tablet of azilsartan loaded S-SMEDDS:

Azilsartan loaded S-SMEDDS powder equivalent to 40 mg of azilsartan were blended with super disintegrant such as sodium starch glycolate for 2min. 0.5% magnesium stearate was added as a lubricant and powder blend were directly compressed in to tablet using KBr prees (Techno search Instrument) with 8 mm punch.^22,23

Evaluation of immediate release tablet of azilsartan loaded S-SMEDDS:

Tablets were evaluated for various post compression properties such as; hardness, thickness, friability, weight variation, drug content, disintegration time, etc.^24, ^{25, 26} Tablets were reconstituted in to water and centrifuged at 10000 rpm for 10 min. to remove undissolved components of tablet and following reconstitution properties were evaluated.

% Transmittance:

1 ml of sample was diluted to 100 ml distilled water and observed for any turbidity and % transmittance was measured at 650 nm using UV–Vis spectrophotometer (Jasco V-530, Japan) against distilled water as a blank.²⁷

Globule size and zeta potential:

1 ml of reconstituted and centrifuged sample was diluted to 10 times with distilled water and average droplet size, polydispersity index and zeta potential was determined by lesser light scattering technique using Malvern Zetasizer.^17,
27

In- vitro drug release study:

In-vitro dissolution study was carried out in United States Pharmacopeia (USP) Type-II dissolution test apparatus. The dissolution medium used was 900 ml of phosphate buffer pH 7.8 maintained at 37.5±0.5°C. The paddle speed was kept constant at 100 rpm. Samples of 5 ml were withdrawn at specific time interval of 5min. The withdrawn samples were analyzed by UV spectroscopy at the wavelength maxima of 249. The same amount of fresh phosphate buffer pH 7.8 was used to replace at each time as amount withdrawn for respective dissolution media.²⁹

Differential scanning calorimetry (DSC):

Physical state of azilsartan in self emulsifying tablet was characterized using differential scanning calorimeter. Thermograms of azilsartan, Neusilin US2 and S-SMEDDS blend were obtained using differential scanning calorimeter. (Mettler Toledo)^{13, 30}

X-ray diffraction (XRD):

To verify the physical state of azilsartan in pure state and the changes in the crystallinity in S-SMEDDS, the XRD study was carried out by using X ray diffractometer. XRD of azilsartan, Neusilin US2 and S-SMEDDS blend was carried out using X-ray diffractometer (Fillips Analytical).³⁰

Fourier-transform infrared spectroscopy (FTIR):

FTIR spectrum of azilsartan, Neusilin US2 and S-SMEDDS blend was taken using Agilent Cary 630 FTIR spectrometer to study compatibility study between drug and solid carrier.^{17, 30}

RESULTS AND DISCUSSION:

Construction of pseudo-ternary phase diagram:

Pseudo ternary phase diagram was constructed to predict micro emulsion region. Constructed diagrams are shown in Figure 2. It gives appropriate approach to determine the concentration of oil, surfactant: co-surfactant and water so that transparent, monophasic low viscous micro emulsion can be formed. Phase diagram of Km value 3 showed maximum micro emulsion region. Hence it was selected for the development of liquid SMEDDS of azilsartan. Three points on micro emulsion region were selected for the formulation of three batches of liquid SMEDDS. The selected composition of liquid SMEDDS is shown in Table 1.

Table 2: Micromeritic properties and drug content of azilsartan loaded S-SMEDDS

Formulation	Angle of repose (degree)	Bulk density (g/ml)	Tapped density (g/ml)	Compressibility index (%)	Hausner’s ratio	Drug content (%)
AZS-S1	17.25±1.02	0.482±0.04	0.650±0.02	25.84±1.02	1.35±0.06	93.26±1.58
AZS-S2	16.48±1.63	0.495±0.02	0.633±0.04	21.80±1.25	1.28±0.05	95.63±2.05
AZS-S3	15.65±2.04	0.502±0.03	0.602±0.06	16.61±1.10	1.20±0.02	98.54±1.02

All value represents mean ± SD (n=3)

Table 3: Post compression parameters and reconstitution properties of azilsartan loaded S-SMEDDS

Formulation	Hardness (Kg/cm²)	Thickness (mm)	Friability (%)	Weight variation	Disintegration Time (min)	Drug content (%)	Reconstitution properties
Formulation	Hardness (Kg/cm²)	Thickness (mm)	Friability (%)	Weight variation	Disintegration Time (min)	Drug content (%)	% T	Globule Size (nm)	Zeta Potential (mV)
AZS-S1	4.5±0.58	1.21±0.03	0.56±0.04	Passes	1.25±0.08	92.42±0.52	92.7±0.38	230	-13.5
AZS-S2	4.2±0.63	1.23±0.01	0.58±0.02	Passes	1.22±0.06	93.34±1.05	95.4±0.27	213	-27.5
AZS-S3	4.5±0.28	1.23±0.02	0.55±0.03	Passes	1.20±0.03	97.27±1.03	97.5 ±0.34	210	-37

All value represents mean ± SD (n=3), %T: % Transmittance

Evaluation of azilsartan loaded S-SMEDDS:

Results of micromeritic properties and drug content of azilsartan loaded S-SMEDDS are shown in Table 2. Angle of repose of all S-SMEDDS formulations was found to be in between 15.65±2.04 indicates that S-SMEDDS have excellent flow properties. Bulk density and tapped density of all S-SMEDDS formulations was found to be in the range of 0.482±0.04 g/ml to 0.502±0.03 g/ml and 0.602±0.06 g/ml to 0.650±0.02 g/ml respectively. Compressibility index and Hausner’s ratio of all formulations were found to be in the range of 16.61±1.10 % to 25.84±1.02 % and 1.20±0.02 to 1.35±0.06 respectively. Drug content and spray drying process yield was found to be 93.26±1.58 % to 98.54±1.02 %. Results of this study indicate that, all formulations of S-SMEDDS have good flow properties with excellent drug content.

Evaluation of immediate release tablet of azilsartan loaded S-SMEDDS:

Results of post compression parameters of immediate release tablet of azilsartan loaded S-SMEDDS are shown in Table 3. Results showed that, all post compression parameters was found to be within acceptable limit with very good disintegration time and drug content.

Reconstitution properties of immediate release tablet of azilsartan loaded S-SMEDDS:

Results of reconstitution properties of immediate release tablet of azilsartan loaded S-SMEDDS such as % Transmittance, globule size and zeta potential are shown in Table 3 and Figure 3 and 4. % Transmittance of all formulations was found in between 92.7±0.38 to 97.5 ±0.34. This indicates that reconstituted S-SMEDDS are clear and no turbid. Globule size was found to be in the range of 210 to 230 nm and zeta potential was found to be in the range of -37 to -13.5 mV. It indicates that, as concentration of surfactant and co-surfactant increases, globule size get decreases.

Figure 3 Globule size of reconstituted tablet of batch AZS-S3

Figure 4 Zeta potential of reconstituted tablet of batch AZS-S3

In- vitro drug release study:

Cumulative percent drug release of all batches of immediate release tablet of azilsartan loaded S-SMEDDS and plain azilsartan in phosphate buffer pH 7.8 are shown in Figure 4. S-SMEDDS immediate release tablet showed that 92.48±3.71 to 99.79±2.63 % of drug release within 30 min. While plain azilsartan showed only 39.27 ± 3.86 % drug release after 30 min. It indicates that, drug releases from S-SMEDDS tablet formulation was found to be significantly higher as compared to plain azilsartan.

Figure 5 Cumulative % drug release of immediate release tablet of azilsartan loaded S-SMEDDS

Differential scanning calorimetry (DSC):

DSC curves of Azilsartan, Neusilin US2 and S-SMEDDS of batch AZS-S3 are shown in Figure 6. It shows sharp endothermic peak at 209.20^oC. The S-SMEDDS exhibit retained small endothermic peak of azilsartan at 212.55 and and it may be due to solubilization of azilsartan in SMEDDS.³¹

Figure 6 DSC thermogram of A: Azilsartan, B: Neusilin US2 and C: S-SMEDDS of batch AZS-S3

X-ray diffraction (XRD):

The powder X-ray diffractogram of Azilsartan, Neusilin US2 and S-SMEDDS of batch AZS-S3 are shown in Figure 7. The powder X-ray diffractogram of pure azilsartan showed numerous distinctive peaks at that indicated a high diffractogram. The height of the characteristic peaks of azilsartan was remarkably reduced in case of diffractogram of S-SMEDDS at low concentrations may have converted to a metastable amorphous form or may have dissolved in the matrix system, or may exist in a microcrystalline form in the matrix system.²⁰

Figure 7 XRD of A: Azilsartan, B: Neusilin US2 and C: S-SMEDDS of batch AZS-S3

Fourier-transform infrared spectroscopy (FTIR):

FTIR spectrum of azilsartan, Neusilin US2 and S-SMEDDS of batch AZS-S3 are shown in Figure 8. Deformation of characteristic peaks of azilsartan in S-SMEDDS may be due to the formation of the amorphous state of azilsartan.

Figure 8 FTIR spectrum of A: Azilsartan, B: Neusilin US2, C: S-SMEDDS of batch AZS-S3

CONCLUSION:

Study concluded that, liquid SMEDDS can effectively be converted into free flowing S-SMEDDS by adsorption technique using Neusilin US2 as a solid carrier. Prepared S-SMEDDS can be converted in to immediate release tablet by addition of super disintegrant. From reconstitution properties of immediate release tablet of azilsartan loaded S-SMEDDS, it can be concluded that, transparent micro emulsion can efficiently produced with globule size in micron range. Cumulative % drug release was found to be much higher than that of plain azilsartan. It may be due to an increase in solubility of the drug. From results of compatibility study using DSC, XRD and FTIR it can be concluded that, azilsartan is compatible with solid carrier such as Neusilin US 2. Hence study concluded that solubility, dissolution and hence bioavailability of poorly water-soluble drug like azilsartan can be enhanced by formulating self micro emulsifying immediate release tablet.

ACKNOWLEDGEMENT:

Authors are very thankful to DRDL, USV Private Ltd. Mumbai for providing gift sample of azilsartan. The authors are also thankful to Fuji Chemicals, Japan for providing gift sample of Neusilin US2.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

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Received on 17.06.2019 Modified on 21.07.2019

Research J. Pharm. and Tech. 2020; 13(1): 197-202.

DOI: 10.5958/0974-360X.2020.00040.2

Formulation	Azilsartan (mg/10g)	% Composition (w/w)
Formulation	Azilsartan (mg/10g)	Ethyl oleate	Tween 80	Transcutol P	Smix	Water
AZS-L1	40	10	30	10	40	50
AZS-L2	40	10	37.5	12.5	50	40
AZS-L3	40	10	45	15	60	30