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International Journal
of Pharmaceutical Sciences and Drug Research ISSN: 0975-248X | DOI Prefix: 10.25004 | CODEN (USA): IJPSPP

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Volume No.: 9 (2017) Issue No: 1

International Journal of Pharmaceutical Sciences and Drug Research 2017; 9(1): 1-9 http://dx.doi.org/10.25004/IJPSDR.2017.090101
Research Article

Development and in vitro Characterization of Gastro Retentive Raft Forming Stavudine Tablets

Mahendar Rupavath*, K. S. K Rao Patnaik

 

Department of Pharmaceutics, University College of Technology, Osmania University, Hyderabad-500007,

Telangana, India

 

*Corresponding author: Mr. Mahendar Rupavath

 Address: Department of Pharmaceutics, University College of Technology, Osmania University, Hyderabad-500007, Telangana, India

 E-mail *: mahi1.rupavath@gmail.com

Relevant conflicts of interest/financial disclosures: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Received: 01 January, 2017; Revised: 15 January, 2017; Accepted: 19 January, 2017; Published: 27 January, 2017

Copyright © 2017 Mahendar Rupavath et al. This is an open access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License which allows others to remix, tweak, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

ABSTRACT

The objective of the present investigation was to identify a suitable raft forming agent and to develop raft forming stavudine matrix tablets using different rate controlling natural, semi-synthetic and synthetic polymers to achieve prolonged gastric residence time, leading to an increase in drug bioavailability and patient compliance. Various raft forming agents were used in preliminary screening. Raft forming floating tablets were developed using pullulan gum as natural rate controlling polymer, and directly compressible grades of hydroxypropyl methylcellulose (Benecel K4M DC) as semi synthetic, and Carbopol 71G as synthetic rate controlling polymers respectively and optimum concentrations of sodium-bicarbonate as gas generating agent to generate optimum buoyancy by direct compression method. Raft forming tablets were evaluated for weight variation, thickness, hardness, friability, drug content, in vitro drug release, floating buoyancy and raft strength. Drug-excipients compatibility study showed no interaction between drug and excipients. Raft forming tablets showed satisfactory results when evaluated for weight variation, thickness, hardness, friability, drug content, and raft strength. The optimized formulation was selected based on physicochemical characteristics and in vitro drug dissolution characteristics. Further, the optimized formulation was evaluated for in vivo radiographic studies by incorporating BaSO4 as radio opaque substance. Optimized formulation showed controlled and prolonged drug release profiles while floating and raft formation over the dissolution medium. Diffusion followed by erosion with raft forming drug release mechanism was observed for the formulation, indicating that dissolution media diffusion and polymer erosion played an essential role in drug release. In vivo radiographic studies revealed that the raft forming formulations remained in the stomach for 240 ± 30 min in rabbits and indicated that gastric retention time was increased by the floating and raft forming principle, which was considered and desirable for absorption window drugs.

Keywords: Raft forming floating tablets, Stavudine, Benecel K4M DC, Carbopol 71G, Floating lag time, in vivo radiographic studies.

INTRODUCTION

Oral administration is the most versatile, convenient and commonly employed route of drug delivery for systemic action. Indeed, for controlled release system, oral route of administration has received more attention and success because gastro intestinal physiology offers more flexibility in dosage form design than other routes. [1] Oral controlled release dosage forms have been developed for the past three to four decades due to their considerable therapeutic advantages and applications. The high level of patient compliance in taking oral dosage forms is due to the ease of administration and handling of these forms. Using current technology, oral delivery for 24 h or more is possible for many drugs; however, the substance must be well absorbed throughout the gastrointestinal tract (GIT). A significant obstacle may arise if there is a narrow therapeutic window for drug absorption in the GIT, if the drug is poorly soluble in the intestine or acts locally in the stomach or a stability problem exists in gastrointestinal fluids. Thus, the real issue in the development of oral controlled release dosage forms is not just to prolong the delivery of the drugs for more than 12 h, but to prolong the presence of the dosage forms in the stomach or somewhere in the upper part of intestine until all of the drug is released over the desired period of time. [2] Gastro retentive drug delivery offers various potential advantages for drug with poor bioavailability due their absorption is restricted to the upper gastrointestinal tract (GIT) and they can be delivered efficiently thereby maximizing their absorption and enhancing absolute bioavailability.

Gastro retentive dosage forms can remain in the gastric region for long periods and hence significantly prolong the gastric retention time (GRT) of drugs. Over the last few decades, several gastro retentive drug delivery approaches being designed and developed, including high density (sinking) systems, low density (floating) systems, mucoadhesive systems, unfoldable, extendible, or swellable systems, superporous hydrogel systems [3], magnetic systems. [4] Among these, the floating dosage form has been used most commonly. This technology is suitable for drugs with an absorption window in the stomach or in the upper part of the small intestine, drugs acting locally in the stomach, and for drugs that are poorly soluble or unstable in the intestinal fluid. The floating systems include single, multiple, and raft forming systems.

A raft-forming formulation requires sodium or potassium bicarbonate; in the presence of gastric acid, the bicarbonate is converted to carbon dioxide, which becomes entrapped within the gel precipitate, converting it into foam, which floats on the surface of the gastric contents. The formulation components provide a relatively pH-neutral barrier. [5-6] Calcium carbonate can be used as a raft-strengthening agent. It releases calcium ions, which react with alginate and form an insoluble gel. [7-8] Different polymers, especially various polysaccharides, have been used in different research works in the pharmaceutical research fraternity. Sodium alginates, alginic acid, and pectin are the most commonly used raft-forming agents. [9] Other polysaccharides are also being used, which include guar gum, locust bean gum, carrageenan, pectin and isapgol. [5-6, 9] Alginate rafts may be formed in liquid products by the action of gastric fluid on a soluble alginate to form an insoluble gel of alginic acid. They may also be formed by the interaction of soluble alginate with metal ions released by acid from an insoluble antacid such as calcium carbonate. The simultaneous action of gastric acid on a bicarbonate salt produces carbon dioxide, which should ideally be trapped inside the alginate gel to aid buoyancy of the raft. [10]

Acquired Immuno deficiency Syndrome (AIDS), which threatens to cause a great plague in the present generation, was first identified in California in 1981. AIDS is a disease in which the body's immune system breaks down and is unable to fight off infections caused by human immuno deficiency virus (HIV). Stavudine is a dideoxy nucleoside analog that inhibits reverse transcriptase and has in vitro activity against HIV. Stavudine is absorbed rapidly following oral administration producing peak plasma concentrations within 1hr and with a reported bioavailability of about 86%. Stavudine has a very short halflife of 11.5 hours, thus necessitating frequent administration to maintain constant therapeutic drug levels. [11-12] Formulation of sustained release effervescent floating tablets of stavudine improves patient compliance and minimizes the dose-related side effects. Based on the above physicochemical and biopharmaceutical properties, Stavudine was selected as a drug candidate for developing floating drug delivery systems to reduce the severity of toxicity and also to improve patient compliance. The aim of the present investigation was to develop floating matrix raft forming tablets of stavudine to achieve prolong gastric residence time, leading to an increase in drug bioavailability and patient compliance by utilizing hydroxypropyl methylcellulose (Benecel K4M) and Carbopol 71G as directly compressible grades of synthetic polymers, pullulan gum as natural rate controlling polymers and optimum amounts of sodium-bicarbonate and calcium carbonate as gas generating and raft gel strengthening agents respectively in suitable ratios to generate optimum buoyancy.

MATERIALS & METHODS

Materials

Stavudine and Pullulan gum were received as generous gift samples from Aurobindo Pharma Ltd, Hyderabad, India. Hydroxypropylmethylcellulose (Benecel K4M) received as generous gift sample from Ashland Inc., Carbopol 71G received as generous gift sample from Lubrizol and all other excipients were purchased from Kanwarlal Industries, India. All other chemicals used were of analytical grade.

Methods

Drug–excipients compatibility study

Fourier transform infrared Spectroscopy (FT-IR) and Differential Scanning Calorimetry (DSC) study

Fourier transform infrared (FT-IR) Spectroscopy was used to study the physical and chemical interaction between the drug and excipients used. FT-IR spectra of pure drug and optimized raft forming floating matrix tablet were recorded using KBr mixing method on FT-IR Spectrophotometer (FT-IR-1700, Shimadzu, Tokyo, Japan). DSC was used to study physical and chemical interaction between the drug and excipients used. DSC spectra of pure drug and drug composite mixture were recorded on differential scanning calorimeter (DSC-60, Shimadzu, Tokyo, Japan).

Pre-compression parameters of granules

The flow properties of granules (before compression) were characterized in terms of bulk density, tapped density, Hausner’s ratio, Carr’s index and angle of repose.

Angle of repose (θ) was determined by using a funnel whose tip was fixed at a constant height (h) of 2.0 cm from the horizontal surface. The granules were passed separately through the funnel until the tip of the conical pile touches the tip of the funnel. The radius of the base of the conical pile is measured as r (cm). It was calculated with the formula:

θ = tan-1(h/r)

The previously weighed granules were collected into a graduated measuring cylinder and the initial (or bulk) volume was noted. It was placed in the tapped density tester USP (Electrolab, Mumbai, India) and subjected to constant tapping at a rate of 100 drops/min. It was recorded as the final tapped volume. Carr’s index and Hausner’s ratio were calculated with the following formulae:

 % Carr’s index = Tapped density – Poured density

   Tapped density

 

       Hausner’s ratio   =     Tapped density

Poured density

Development of Stavudine gastro retentive floating raft forming tablets

Stavudine gastro retentive floating raft forming matrix tablets were developed by direct compression method. Accurately weighed quantities (Table 1) of stavudine, anhydrous lactose, HPMC K4M (Benecel K4M PH DC), carbopol 71G, pullulan gum, sodium bicarbonate and calcium carbonate were sifted through # 30 mesh to get uniform size particles, then they were transferred into a suitable blender (Erwika) and blended for 10 minutes at 230 rpm. Extra granular material talc was sifted through mesh # 40 and added as a glidant to the blend and blended for 5 minutes. Resulting extra-granular added blend was lubricated with magnesium stearate and compressed into tablets using a 16-station punching machine (Rimek, India).

Characterization of gastro retentive floating raft forming tablets

The prepared gastro retentive floating raft forming matrix tablets were evaluated for weight variation by sartorious balance, hardness was measured by a hardness tester (Erweka tester, Germany), thickness was measured using a verniercaliperse (Mitutoyo Corporation, Japan) and friability was determined using a Roche friabilator (Germany). The drug content in each formulation was determined by triturating 20 tablets and a quantity of powder equivalent to the mass of one tablet was transferred into a 100-mL volumetric flask. To this, 50 mL of 0.1N HCl was added and then the solution was subjected to sonication for about 1h. The solution was made up to the mark with 0.1N HCl, filtered and suitable dilutions were prepared with 0.1N HCl. The drug content was estimated by recording absorbance at 266 nm by using a UV-Visible spectrophotometer (ELICO, India).

In vitro buoyancy studies

The in vitro buoyancy was determined by the floating lag time. The tablets were placed in a 250 mL beaker containing 0.1N HCl. The time required for the tablet to rise to the surface for floating was determined as the floating lag time and further floating duration of all tablets was determined by visual observation.

Measurement of raft thickness

The raft-forming mixture used in the formulation (1g) was added to 10 mL HCl at 37°C in a 25 mL pyrex cylinder. The raft was allowed to form for 10 min without agitation. The thickness of the raft was measured at three places around the cylinder. Three measurements were made for each raft and three rafts were studied from each formulation. Multiple repeats showed that the reproducibility was within a coefficient of variation of approximately 5%, although the uneven physical characteristics of the rafts rendered specific quantitative assessment extremely difficult.

Raft strength

Rafts were formed by dropping a gastro retentive raft forming tablet to 100 mL of 0.1M HCl, maintained at 37ºC in a 250 mL glass beaker with inclusion of a wire probe. Each raft was formed around an L-shaped stainless steel wire probe held upright in the beaker throughout the whole period (around 30 min) of raft development. After 30 min of raft development, the beaker was placed on the table of a TAXT Plus Texture Analyzer (Stable Micro Systems, UK), the wire probe was hooked onto the Texture Analyzer arm and pulled vertically up through the raft at a rate of 5 mm/s. The force (g) required to pull the wire probe up through the raft, was recorded by the Texture Analyzer. [12]

Raft volume and raft weight

Rafts were formed and developed for 30 min in glass beakers, but without the inclusion of a wire probe. Each beaker used for raft formation was pre-weighed (W1). The position to which the top of each raft reached was marked on the outside of the beaker. The total weight of the beaker and contents was obtained after raft development (W2). The raft was then removed from the beaker by carefully decanting off the sub-natant liquid and tipping the raft into a pre-tared plastic weighing boat. This was left to stand for 30s, excess sub-natant liquid was drained off and the raft was weighed (W3).

Remaining liquid was removed from the inside of the beaker with a paper towel and it was then refilled with water to the marked position and weighed (W4).

The volume of each raft was then calculated from the following formula:

Raft volume = (W4 −W1) − (W2 −W1 −W3)

Where raft volume is measured in mL and all weights are measured in g. The formula assumes that the density of the sub-natant liquid is the same as that of water. [13]

In vitro drug release studies

Dissolution studies on each formulation were performed in a calibrated eight station dissolution testing apparatus (TDT-08T, Electrolab, India) equipped with paddles (USP apparatus type II method) employing 900 mL of 0.1N HCl as dissolution medium. The paddles were operated at 75 rpm to simulate gastric peristaltic movement and the temperature was maintained at 37 ± 2°C throughout the experiment. Samples were withdrawn at regular time intervals for 16 hours and replenished with equal volume of fresh dissolution medium to maintain the constant volume and sink conditions throughout the experiment. Samples withdrawn at pre-defined time intervals were diluted appropriately and the amount of drug released was estimated by UV-Visible double beam spectrophotometer (ELICO, India) at 266 nm. To analyse the mechanism of drug release studies from the obtained dissolution data, various kinetic model calculations based on the equations of Zero-order, First-order, Higuchi and Korsmeyer Peppas were applied to analyze the drug release mechanism and pattern. [14-16]

Tablets for in vivo radiographic studies

Tablets of 4.3 ± 0.2 mm thickness and of 400 ± 3% weight were prepared. To make the tablet X-ray opaque, incorporation of BaSO4 was necessary. [17] For this purpose, 40 mg of the drug was replaced with BaSO4 and all other ingredients were kept constant. The tablets were characterized for hardness, floating lag time and floating duration.

In vivo radiographic studies

The study was conducted on rabbits, weighing between 2.5–3.5 kg. The tablets prepared for radiography were administered orally. During the study, rabbits were not allowed to eat but water was available ad libitum. After ingestion of optimized placebo floating tablets containing barium sulphate, the rabbits were exposed to X-ray photography in the abdominal region. The X-ray photographs were taken at 1.0, 2.0, 4.0, and 8.0 h after administration of the tablets. The mean gastric residence time was calculated.

Stability Study

To determine the stability study of the gastro retentive raft forming floating matrix tablets of stavudine were packed in 40 cc Heavy weight HDPE bottle and stored at 40 ± 2°C and 75% ± 5% RH for a period of six months as per the ICH guidelines. The tablets were withdrawn at a period of 1, 3 and 6 months and evaluated for content uniformity and dissolution study. [18] The differences in parameters from floating tablets were evaluated using unpaired t-test. In t-test, a probability value of p < 0.05 was considered to be statistically significant.

RESULT & DISCUSSION

Gastro retentive raft forming floating matrix tablets of stavudine were developed to increase the gastric retention time of the drug, so that they can be retained in stomach for longer time and help in controlled release of drug up to 20 h. The raft forming floating matrix tablets were prepared using directly compressible cellulose derivative HPMC K4M (Benecel K4M PH DC), and polycarbophil derivative Carbopol 71G and Pullulan gum as natural polymer for rate controlling drug delivery. Benecel K4M PH DC, Carbopol 71G and Pullulan gum is known to be beneficial in improving the buoyancy characteristics and drug release characteristics. When in a combination of gas generating agent (sodium bicarbonate) and raft gel strengthening agent (calcium carbonate) improved in vitro and in vivo buoyancy characteristics were observed. The talc and magnesium stearate were employed for their glidant and lubricant property. Raft forming preparation includes raft forming agent which forms floating raft on contact with gastric fluid, and gas generating agent like sodium bicarbonate. Here calcium carbonate also used for raft strengthening agent.

Drug–excipients compatibility study

The compatibility evaluations were performed by Fourier transform infrared spectroscopy, and Differential scanning calorimetry. IR spectroscopic studies indicated that there are no drug excipients interactions in the optimized formulation. By compared FT-IR spectra of stavudine with FT-IR of optimized formulation, it was observed that there was no physical and chemical interaction between stavudine and other excipients during the formulation process, because all the principle peaks of pure drug were still there in the FT-IR spectra of the optimized formulations (Figure 1a-d).

The DSC thermogram of Stavudine showed sharp endothermic peak at 170.1°C (Figure 2a). The DSC thermograms of optimized formulations of Stavudine with Benecel K4M PH DC, Carbopol 71G and Pollulan gum in formulations showed sharp endothermic peaks for Stavudine at the temperatures similar to that of the peak of Stavudine alone (Figure 2b-e). This indicated that there were no drug excipient interactions in the formulations. So we can conclude that there is no chemical interaction between drug & excipients. Studies implied that the selected polymers and drug were compatible with each other.

Table 1: Stavudine unit composition of raft forming gastro retentive dosage forms

Ingredients

Formulations (mg/unit)

F1

F2

F3

F4

F5

F6

F7

F8

F9

F10

F11

F12

F13

F14

Stavudine

40

40

40

40

40

40

40

40

40

40

40

40

40

40

Anhydrous lactose

102

82

62

102

82

62

102

82

62

102

82

62

62

62

Sodium alginate

100

100

100

100

100

100

100

Calcium carbonate

20

20

20

20

20

20

20

20

20

20

20

20

20

20

Pectin

100

100

100

100

100

100

100

Carbopol 71G

40

60

80

40

60

80

Benecel K4M DC

40

60

80

40

60

80

Pullalum gum

80

80

Sodium bicarbonate

90

90

90

90

90

90

90

90

90

90

90

90

90

90

Talc

4

4

4

4

4

4

4

4

4

4

4

4

4

4

Magnesium stearate

4

4

4

4

4

4

4

4

4

4

4

4

4

4

Total weight

400

400

400

400

400

400

400

400

400

400

400

400

400

400

Table 2: Flow characterization of stavudine raft forming gastro retentive dosage forms

Formulation

Bulk density (g/ml)

Tapped density (g/ml)

Compressibility Index (%)

Hausner’s ratio

Angle of repose (θ)

F1

0.54

0.62

12.9

1.15

29.4

F2

0.57

0.63

9.5

1.10

30.8

F3

0.48

0.55

12.7

1.14

31.9

F4

0.46

0.53

13.2

1.15

30.1

F5

0.49

0.56

12.5

1.14

30.2

F6

0.39

0.45

13.3

1.15

30.7

F7

0.50

0.59

15.2

1.18

31.6

F8

0.46

0.54

14.8

1.17

29.9

F9

0.41

0.50

18.0

1.22

30.3

F10

0.52

0.61

14.7

1.17

30.5

F11

0.48

0.56

14.2

1.17

31.5

F12

0.42

0.51

17.6

1.21

31.8

F13

0.39

0.49

20.4

1.26

32.6

F14

0.36

0.45

20.0

1.25

31.7

Table 3: Physio-chemical characterization of stavudine raft forming gastro retentive dosage forms

Formulation

Average Weight (mg)

Thickness(mm)

Hardness(kp)

Friability (%)

Drug content (%)

F1

399.6 ± 1.07

4.32 ± 0.14

9.1 ± 0.5

0.2

99.4 ± 0.8

F2

398.4 ± 1.21

4.29 ± 0.23

9.3 ± 0.6

0.2

100.4 ± 1.3

F3

400.5 ± 1.32

4.38 ± 0.21

9.4 ± 0.6

0.1

100.2 ±1.5

F4

399.4 ± 1.28

4.37 ± 0.23

9.4 ± 0.6

0.2

98.9 ±1.6

F5

398.6 ± 2.14

4.36 ± 0.28

9.5 ± 0.6

0.2

101.7 ±1.8

F6

399.6 ± 1.47

4.33 ± 0.20

9.8 ± 0.5

0.1

100.5 ± 1.7

F7

401.2 ± 1.25

4.39 ± 0.13

9.7 ± 0.7

0.2

99.5 ± 0.8

F8

401.6 ± 2.08

4.28 ± 0.18

9.6 ± 0.4

0.2

100.3 ± 0.6

F9

402.4 ± 2.48

4.27 ± 0.24

9.2 ± 0.5

0.1

101.1 ± 1.9

F10

399.6 ± 2.41

4.32 ± 0.25

9.5 ± 0.4

0.1

100.3 ± 1.2

F11

400.6 ± 1.35

4.31 ± 0.19

9.8 ± 0.3

0.2

100.1 ± 0.7

F12

400.7 ± 1.53

4.30 ± 0.14

9.9 ± 0.6

0.1

99.8 ± 0.6

F13

402.4 ± 2.82

4.35 ± 0.19

9.6 ± 0.7

0.3

99.3 ± 2.6

F14

402.9 ± 2.75

4.37 ± 0.23

9.2 ± 0.8

0.4

102.1 ± 2.9