INTRODUCTION:

Solubility is key determinant in drug liberation and thereby the drug absorption, which is directly related with oral bioavailability of drug or drug products. The majority of the new drugs have poor water solubility which results in difficulty in formulating into effective drug delivery systems. Therefore, enhancement of solubility of these drugs is of prime requisite of pre-formulation studies during drug product development ^[1]. Various approaches have been evidenced to overcome the poor solubility of drug which is a major challenge for formulation scientists worldwide.

These include particle size reduction, freeze drying, use of co-solvents, cyclodextrin inclusion complexes and alteration of aqueous micro-environment ^[2-6]. Solid dispersions is another familiar approach to improve solubility of poorly soluble drugs by employing amphiphilic polymers such as cellulosics, acrylates, polyvinylpyrrolidone (PVP) and its copolymers, and polyethylene oxide (PEO) and its copolymers ^[7-9]. Solid dispersion technology is extensively used method to enhance the dissolution characteristics of insoluble drug molecules. Moreover ease of scalability and conversion into solid dosage forms such as tablets, capsules, taste masking strips etc. make this technology superior to other approaches ^[10].

Telmisartan is an angiotensin II receptor antagonist, used in the prevention and treatment of hypertension. The aqueous solubility of telmisartan is very low i.e. 0.078 mg/ml in physiological pH range. The poor dissolution of drug leads to variable absorption characteristics and suboptimal bioavailability. Thus, improving the dissolution behaviour of telmisartan is of therapeutic importance ^[11,12].

Improvement in dissolution characteristics of telmisartan has been evidenced in literature by formulating liquisolid compacts using propylene glycol, Avicel PH 102, Aerosil 200 and indion 414 ^[13], Pellets using MCC ^[14], liquisolid formulations (converted into free flowing drug powder) containing Poly ethylene glycol 400 and Avicel ®PH 102 ^[15], solid dispersions using Soluplus and PEG 1000 prepared via hot melt extrusion and spray drying technique ^[16].

Neem gum, a natural polymer, is collected from incised trunks of A. indica plant (Family Meliaceae) ^[17]. Neem gum possesses high water retention capacity, sufficient swelling index and binding ability and it is being investigated as tablet binding agent, film coating agent, and mucoadhesive agent ^[18-21]. Furthermore, significant solubilization, wettability and dispersibility effects of purified neem gum have revealed enhancement of solubility and thereby dissolution rate of drugs such as atorvastatin and aceclofenac ^{[22, 23]}.

MATERIALS AND METHODS:

Materials:

Telmisartan was kindly gifted by sequel pharmaceuticals; neem gum was obtained from local market. All other chemicals used were of AR grade.

Methods:

Formulation of various mixtures:

Preparation of solid dispersions (SD) via solvent evaporation technique:

Solid dispersions were prepared by solvent evaporation method. In this method, the pure drug telmisartan and neem gum were mixed in different ratio of (1:1, 1:2, 1:3 and 1:4) in 25 ml of solvent (70% ethanol) in the round bottom flask. The solvent was removed under vacuum in rotary evaporator. The resultant dried solid dispersions were allowed to dry at room temperature. Finally the dried solid dispersions were sieved through 80# mesh and stored in air tight containers at room temperature.

Preparation of co-grinding mixture (CGM):

Accurately weighed pure drug telmisartan and neem gum in drug to polymer ratio (1:3) was placed in the mortar and grinded properly. The mixture was passed through the 80# mesh sieve and stored in desiccator.

Preparation of kneading mixture (KM):

Pure drug telmisartan and neem gum with ratio (1:3) was kept in mortar and grinded thoroughly. 70% w/w ethanol was added in the mixture and placed in oven at 40ºC to evaporate the ethanol resulting in dry film. Final dried film was scratched and passed through the 80# mesh sieve.

Preparation of physical mixture (PM):

Pure drug telmisartan and neem gum with ratio (1:3). Placed the drug and neem gum in the mortar and mix it properly and passed through 80# sieve. The mixture then was stored in an air tight container till further studies.

Preparation of swollen mixture (SM):

Neem gum was hydrated in sufficient distilled water in china dish and drug was mixed into it (drug: polymer ratio of 1:3). China dish was kept in oven till complete drying of mixture. Then the dried mixture and passed through the 80# mesh sieve. The mixture was stored in an air tight container.

Evaluation of various mixtures:

Equilibrium Solubility Studies:

Equilibrium solubility of all the mixtures was determined by dissolving 100mg of different mixtures in 25ml distilled water in volumetric flask for 24hr with intermittent stirring in orbital shaker and then filtered with fine filter paper. Absorbance of various solutions was taken by double beam UV spectrophotometer at 290 nm.

Fourier Transform Infrared Spectroscopy:

10 mg of sample was mixed with potassium bromide of equal weight. The mixture is properly grinded using pestle and mortar. Pellets are formed by compressing the mixture by using hydraulic pressure. Transparent pellets formed in this way are scanned. The IR spectra were recorded using KBr pellet over the range of 400 – 4000 cm^-1 using (Perkin Elmer Spectrum 400) FTIR spectrophotometer.

Differential Scanning Calorimetry:

Thermal properties of pure drug telmisartan, neem gum and SD3 were analysed by Differential Scanning Calorimetry (DSC 821^eMettler Toledo, USA). The sample was sealed in aluminium pan and scanned from 30 to 300^ºC at a heating rate of 10^ºC/min in nitrogen atmosphere.

Scanning Electron Microscopy:

Samples were mounted onto the stubs using double sided adhesive tape and then coated with gold palladium alloy (150-200 A^º) using fine coat ion sputter (Joel, fine coat ion sputter, JFC-1100). The samples were subsequently analyzed under the scanning electron microscope for external morphology.

X-Ray Diffraction:

Powder X-Ray Diffraction patterns were traced employing X-ray diffractometer for the samples using Ni filtered Cu (K-α) radiations, a voltage of 45 kV, a current of 40 mA. The samples were analyzed over 2θ range of 0-50^º with scan step size of 0.0170^º (2θ) and scan step time 25 s.

In vitro Dissolution Studies:

In vitro dissolution studies of various mixtures were carried out in 900ml of 0.1 N HCl (pH 1.2) at 37±0.5^oC with the stirrer rotation speed of 100 rpm using USP dissolution apparatus II (paddle stirrer). A 5 ml aliquot of dissolution medium was withdrawn at specified time intervals (5, 15, 30, 45, 60, 90, and 120 min). The samples were suitably diluted and assayed spectrophotometrically at 290 nm. The study was done in triplicate.

Formulation of tablet dosage form using solid dispersions:

Two tablet batches (T1 and T2) containing pure drug and SD3 powder were compressed into tablets (200mg) by direct compression method. Various excipients (Microcrystalline cellulose, crosspovidone, talc and magnesium stearate) were weighed, sieved and mixed sequentially. Crosspovidone was used as superdisintegrant (equivalent to 10% of tablet weight). The tablets formulation batches were evaluated further. The composition of tablet batches is shown in table 1.

Table 1: Composition of tablet batches

Ingredients (mg)	T1 (PD)	T2 (SD3)
Pure drug (Telmisaratan)	20	80
Talc	2	2
Mg stearate	4	4
Crosspovidone	20	20
MCC	154	94
Total Weight (mg)	200	200

Evaluation of tablet dosage form:

Pre-compression parameters:

Various pre-compression parameters such as angle of repose, density and compressibility index were evaluated by standard methods and applying formulas.

Post-compression parameters:

Post compression parameters such as weight variation, friability, hardness and disintegration test were carried out using standard methods and apparatus as per USP.

Wetting Time:

Five circular tissue papers 10cm diameter were placed in a petridish. 10 ml of water containing Eosin, a water soluble dye, was added to petridish. The tablet was carefully placed on the surface of the tablet. Time taken by dye solution to upper surface of tablet is noted as wetting time

Accelerated stability studies:

The stability studies of T2 tablet batch was carried out in stability chamber (REMI SC-10 (Plus) kept at 40ºC and 75% relative humidity conditions and at room temperature conditions for three months. The effects of temperature and time on the physical characteristics of the tablet were evaluated for assessing the stability of the prepared formulations. The tablets were evaluated for their physiochemical parameters (such as hardness, thickness, diameter, friability, in vitro disintegration time and in vitro dissolution) after 15 days, 1 month, 2 months and 3 months

In vitro dissolution studies:

In vitro dissolution of tablet batches T1 and T2 and marketed tablet was carried out in 900ml of 0.1N HCl (1.2 pH) at 37±0.5°C with the stirrer rotation speed of 50rpm using USP II dissolution apparatus. 5ml aliquots were withdrawn at specified intervals of 5, 15, 30, 45, 60, 120 min and suitably diluted and assayed spectrophotometrically at 290nm.

RESULTS:

Evaluation of various mixtures:

Equilibrium solubility studies:

Equilibrium solubility of pure drug in distilled water was found to be very low i.e. 11.9 µg/ml. Mixing of drug with neem gum in solid dispersion leads to significant increase in drug solubility (19.3 µg/ml in SD1, 37.85 µg/ml in SD2, 67.36 µg/ml in SD3 and 43.74 µg/ml in SD4). Therefore, other mixtures PM, KM, CGM and SM were formulated in the ratio of 1:3. Also the equilibrium solubility of drug in KM, CGM, SM was observed to be increased (59.66 µg/ml, 25.7 µg/ml and 49.6 µg/ml respectively) (Figure 1). No change in solubility of drug in PM (16.2 µg/ml) was observed. Discussion: The increase in drug solubility may be due to increased wettability by the gum and reduced particle size during formulation of mixtures. SD3 showed maximum solubility of drug. Higher ratio of neem gum (SD4) further led to decrease in the solubility of drug, which may be due to increased viscosity of the mixture due to higher amounts of the neem gum. Kneading mixture can be suitable alternate to solid dispersions as seen from solubility results and no change on solubility in physical mixture.

FT-IR Spectroscopy:

FTIR was recorded to evaluate the molecular states of pure drug, excipients and all mixtures. The spectra were scanned over 400 to 4000cm^-1. FTIR spectrum of Telmisartan exhibits characteristic peaks at 3649 cm⁻¹ (OH bond), 3059 cm⁻¹ (O-H free), 1696 cm⁻¹ (amines and oximes), and the peak at 1460 cm⁻¹ (CH₃ bent), 653 cm⁻¹ (alkenes). FTIR spectra of neem gum showed some peaks at 1243 cm⁻¹ (C-H bending), 1420 cm⁻¹ (C=C Bending), 2934 cm⁻¹ (O-H Stretching), 1607 cm⁻¹ (Aromatic C=C Stretching ), 3283 cm⁻¹ (N-H Stretching), 1735 cm⁻¹ (C=O Stretching). FTIR spectra of various mixtures showed presence of almost all characteristic peaks of drug (3649cm^-1, 3059cm^-1, 1696 cm^-1, 1460 cm^-1, 653 cm^-1) which indicates no chemical interaction between the drug and gum. The Overlay of FTIR of drug, neem gum and solid dispersion batches is shown in Figure 2.

Figure 1: Equilibrium solubility graph for various mixtures

Figure 2: Overlay of FTIR of PD, SD1, SD2, SD3, SD4 and NG

X-Ray Diffraction studies (X-RD):

The presence or absence of crystallinity of drug and polymer was determined by X- Ray Diffraction studies by comparing some representative peak height in the diffraction pattern of the solid dispersions with that of pure drug. The X-RD pattern of drug, polymer and various mixtures are shown in the figure. XRD patterns of pure drug telmisartan showed sharp peaks of the diffraction angle of 2Ө at 6.8254, 14.2755, 15.0953, 18.3704, 21.4170, 22.3619, 25.0680 with peak intensities of 100, 47.17, 16.92, 13.45, 10.96, 26.92, 12.86 and the area of 2755.22, 1485.34, 599.52, 592.25, 431.52, 953.59, 455.66 respectively. The mixtures showed minor shifts in drug peaks along with reduced intensities and areas in comparison to that of pure drug. The results suggested conversion of crystalline state of drug to more soluble amorphous state. The overlay diagram of X-RD pattern of drug and other mixtures is shown in Figure 3.

Differential Scanning Calorimetry:

DSC thermogram of pure drug telmisartan showed sharp endothermic peak at 267.43ºC with enthalpy of fusion -79.08J/g corresponding to its melting point, which indicates its crystalline nature. DSC thermo gram of neem gum showed small endotherm at 68.35°C with enthalpy of fusion 203.78J/g. The DSC thermo gram of SD3 showed endothermic peak at 283.85°C with enthalpy of fusion -10.99 J/g. Slight shift in endothermic peak of drug alongwith significantly decreased intensity was observed in DSC thermo gram of SD3. The results revealed the conversion of crystalline state to amorphous state of drug in SD3 mixture. Figure 4 shows the overlay diagram of DSC of drug, neem gum and SD3 batch.

Formulation of tablets:

Solid dispersion SD3 was compressed into 200 mg tablets by direct compression method. SD powder equivalent to 20mg of telmisartan (SD3) was used in T2 batch and pure drug 20 mg into T1 batch. Talc, Magnesium stearate, Avicel 102 and crosspovidone were mixed uniformly and active blend was sieved through #80.

Evaluation of tablets:

Pre-compression parameters:

The flow properties of active blends were evaluated. Density (bulk and tapped), angle of repose, Carr’s index and Hausner’s ratio were calculated. The results revealed good flow characteristics of both the blends (Table 2).

Table 2: Pre Compression parameters of T1 and T2 batch

Parameter	Tablet batch T1	Tablet batch T2
Bulk Density(mg/ml)	0.463±0.02	0.478±0.03
Tapped Density(mg/ml)	0.512±0.043	0.534±0.019
Carr’s Index	9.57	10.4
Hausner’s Ratio	1.10	1.11
Angle of Repose	23.56±1.25	22.47±1.29

Post-compression Parameters:

The tablet batches were evaluated for weight variation, hardness, friability, disintegration time and wetting time. Both the batches passed weight variation test as % weight variation was within pharmacopoeial (±7.5%) limits. Hardness in the range of 2-4 kg/cm² and friability values below 1% of all tablets of both the batches indicated sufficient mechanical strength. The wetting and disintegration time values of less than 1 min suggested better in vitro disintegration characteristics (Table 3).

Table 3: Physicochemical Characterization of prepared tablet T1 and T2 batch

Parameter	Tablet batch (T1)	Tablet batch (T2)
Thickness (mm)	3.56± 0.12	3.48± 0.18
Diameter (mm)	12.11±0.34	11.95± 0.45
Weight variation	Pass	Pass
Hardness(kg/cm²)	2.8±0.3	3.3±0.3
Friability (%)	0.85±0.05	0.88±0.06
Disintegration time (sec)	28±2	40±3
Wetting time(sec)	25±2	34±2

In vitro drug release from tablet batches T1, T2 and marketed tablet (Telma 20) was evaluated. T2 batch containing SD3 indicated 100% drug release in 45 min, while T1 batch containing pure drug in tablets shows almost 75% in 2 h. Marketed tablet which is uncoated IR tablet shows significantly different release as compared to T2 (p<0.05; t-test) i.e. 94% in 2h. Maximum dissolution characteristics (100% drug release in 60 min) were achieved in T2 as drug is present as SD (solubilized amorphous form) and addition of crosspovidone leads to fast disintegration (65% in 15 min) (Figure 8). The optimized formulation T2 was further selected for stability studies.

Figure 8: Comparative in vitro release profile of T1, T2 and marketed tablet

Accelerated Stability Studies:

Stability studies were performed for T2 batch as per as ICH guidelines accelerated conditions (40 °C and 75% RH) for 90 days. The physicochemical changes were observed and noted during 15 days, 30 days, 60 days and 90 days. No significant change was seen in tablet during storage for 90 days (Table 4). No significant difference in drug release (F2>50) was observed from the tablet batch T2 after stability testing (Figure 9).

Figure 9: Comparative in vitro release profile of T2 batch before and after stability testing

CONCLUSION:

Current study revealed the potential of natural carrier neem gum as a solubility enhancer for BCS class II drug. Solvent evaporation method for solid dispersions and kneading method proved significant improvement of solubility and further in vitro dissolution of drug in drug to polymer ratio of 1:3. Neem gum exhibited no interaction with drug during FTIR studies. Crystalline pure drug telmisartan was partially converted into amorphous form during mixing with neem gum as indicated by X-RD and DSC studies. Mixing with neem gum lead to increased wetting of drug as well as reduced particles size of drug particles may be responsible for enhanced dissolution characteristics. Successful tableting of SD3 powder proved suitability of neem gum as a potential natural carrier for enhanced dissolution.

CONFLICTS OF INTEREST:

The authors declared no conflicts of interest.

ACKNOWLEDGEMENTS:

The authors are thankful to Dr. Madhu Chitkara, Vice Chancellor, Chitkara University; Dr. Ashok Chitkara, Chancellor, Chitkara University; Dr. Sandeep Arora, Director, Chitkara College of Pharmacy for providing necessary facilities and support.

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Received on 04.04.2019 Modified on 28.04.2019

Research J. Pharm. and Tech 2019; 12(9):4387-4393 .

DOI: 10.5958/0974-360X.2019.00754.6

Parameter → Time ￬	Weight variation (mg)	Tablet thickness (mm)	Tablet diameter (mm)	Hardness (kg/cm²)	Friability %	Disintegration time (sec)
15 days	197.52	3.3±0.04	12.05± 0.12	2.85±0.11	0.81±0.23	40±2
30 days	198.23	3.25±0.03	12.06±0.29	2.9±0.23	0.85±0.21	39±3
60 days	199.87	3.24±0.05	12.11±0.67	2.87±0.13	0.86±0.12	39±3
90 days	200.14	3.25±0.04	12.10±1.45	2.95±0.17	0.85±0.22	41±2