Development and Evaluation of Gastroretentive Floating Tablets of an Antihypertensive Drug Using Hydrogenated Cottonseed Oil

The aim of the present work was to develop a gastroretentive floating tablet of Atenolol and investigate the effects of both hydrophilic and hydrophobic retardant on in vitro release. Atenolol is an antihypertensive drug with an oral bioavailability of only 50% because of its poor absorption from lower gastrointestinal tract. The floating tablets of Atenolol were prepared to increase the gastric retention, to extend the drug release, and to improve the bioavailability of the drug. The floating tablets were formulated using hydrophilic polymers as Hydroxy propyl methyl cellulose (HPMC K4M and HPMC K15M), hydrophobic retardant as a hydrogenated cottonseed oil (HCSO), and sodium bicarbonate as a gas generating agent to reduce floating lag time. The formulated tablets were evaluated for the quality control tests such as weight variation, hardness, friability, swelling index, floating lag time, and total floating time. The in vitro release study of the tablets was performed in 0.1 N HCl as a dissolution media. The results of the present study clearly indicates the promising potential of Atenolol floating system as an alternative to the conventional dosage and other sustained release formulations. The study also revealed the effectiveness of HCSO as retardant in combination with HPMC.


Introduction
The oral bioavailability of many drugs is limited by their unfavourable physicochemical characteristics or absorption in well-defined part of the gastrointestinal tract (GIT) referred as "absorption window" [1]. Prolonged gastric retention improves bioavailability, reduces drug waste, and improves the solubility for drugs that are less soluble in a high pH environment [2]. Various approaches have been investigated to increase the retention of oral dosage form in the stomach, including floating systems, swelling and expanding systems, bioadhesive systems, modified shape systems, high density systems, and other delayed gastric emptying devices [1].
Atenolol is a beta (1)-adrenergic antagonist or more commonly known as a beta-blocker used in the treatment of hypertension and angina pectoris. Chemical name of Atenolol is 4-[2-hydroxy-3-[(1-methyl ethyl)amino]propoxy] benzene acetamide. Molecular structure of Atenolol is as shown Figure 1.
Atenolol undergoes little or no hepatic first pass metabolism and its elimination half-life is 6 to 7 hours. The present modes of administration of Atenolol are oral and Parenteral. It is incompletely absorbed from the gastrointestinal tract and has an oral bioavailability of only 50%, while the remaining is excreted unchanged in faeces. Therefore, it is selected as a suitable drug for the design of a gastroretentive floating drug delivery system (GFDDS) with a view to improve its oral bioavailability.
Hydroxy propyl methyl cellulose (HPMC) is hydrophilic cellulose ether widely used as release retarding material. HPMC releases drug by diffusion mechanism. HCSO belongs ISRN Pharmaceutics to USP-NF type 1 consisting of triglycerides of hydroxy stearic acid widely used as a tablet lubricant [3]. In the present study, HCSO was investigated as hydrophobic matrix forming retardant as well as floating material.
The objective of the present study was to develop a gastroretentive floating drug delivery system (GFDDS) of Atenolol and to examine the effects of both hydrophilic and hydrophobic retardant on in vitro drug release. In the present study, Atenolol floating tablets were prepared by using hydrophilic polymer, HPMC K4M, HPMC K15M, and HCSO as a hydrophobic retardant, alone and in combination to study the release kinetics and find out the effects of both the retardants and their combinations.

Materials.
Atenolol was obtained as a gift sample from Kopran Pvt. Ltd., Mumbai. HPMC K4M and HPMC K15M were supplied by Colorcon Pvt. Ltd., Goa. Hydrogenated cottonseed oil was obtained as a gift sample from Lubritab, New York. All other chemicals and reagent used were of analytical grade.

Drug Excipients Compatibility Study.
Compatibility studies were carried out to know the possible interactions between Atenolol and excipients used in the formulation. Physical mixtures of drug and excipients in the ratio 1 : 1 were prepared to study the compatibility. Drug polymer compatibility studies were carried out using FTIR spectroscopy. The IR spectra's were recorded in between 500-4000 cm −1 .

Preparation of Tablets.
Floating tablets containing Atenolol were prepared by direct compression technique using varying concentrations of retardants (HPMC and HCSO) with sodium bicarbonate. All the powders were accurately weighed and passed through 40 mesh sieve. Then, except Magnesium stearate all other ingredients were mixed thoroughly for 15 minutes. After sufficient mixing of

Determination of Postcompression Parameters.
Consider the following.
(1) Hardness Test. Monsanto hardness tester was used for the determination of hardness of tablets [5].
(2) Friability. Twenty tablets were accurately weighed and placed in the friabilator (Roche's Friabilator) and operated for 100 revolutions. The tablets were dedusted and reweighed. The tablets that loose less than 1% weight were considered to be compliant [6]. The % friability was then calculated by % friability = (initial weight − final weight) initial weight * 100. (1) (3) Weight Variation. Twenty tablets were selected randomly from the lot and weighed individually to check for weight variation [7].
(4) Drug Content (Assay). Ten tablets were finely powdered; quantities of the powder equivalent to 50 mg of Atenolol were accurately weighed and transferred to a 100 mL of volumetric flask. The flask was filled with 0.1 N HCl (pH 1.2 buffers) solution and mixed thoroughly. The solution was made up to volume 100 mL and filtered. Dilute 1 mL of the resulting solution to 100 mL with 0.1 N HCl. The absorbance of the resulting solution was measured at 226 nm using a Shimadzu UVvisible spectrophotometer. The linearity equation obtained from calibration curve was used for estimation of Atenolol in the tablet formulations [8].
(5) In Vitro Buoyancy Studies. The tablets were placed in a 250 mL beaker, containing 200 mL of 0.1 N HCl. The time required for the tablet to rise to the surface and float was determined as floating lag time (FLT) and the time period up to which the tablet remained buoyant is determined as total floating time (TFT) [9].
(6) Swelling Study. The tablets were weighed individually (designated as 0 ) and placed separately in petri dish containing 5 mL of 0.1 N HCl and incubated at 37 ∘ C ± 1 ∘ C. At regular 2 h time intervals until 12 h, the tablets were removed from petri dish, and the excess surface liquid was removed carefully using the tissue paper [10]. The swollen floating tablets were then reweighed ( ) and % swelling index (SI) was calculated using the following formula: In Vitro Dissolution Studies. The in vitro dissolution of all the batches were carried out in 0.1 N HCl as the dissolution medium using USP Type II apparatus (TDT-08L, Electrolab) apparatus at 50 rpm. The temperature was maintained at 37 ± 0.5 ∘ C. The dissolution was carried out for 12 hours. The absorbances of the samples at different time intervals were carried out using UV visible spectrophotometer (UV 1800, Shimadzu) at max of 226 nm [11].
(8) Kinetics Study. The mechanism of Atenolol release from the floating tablets was studied by fitting the dissolution data of optimized formulation in following models: Zero order: = 0 − 0 ; First order: Log = Log 0 − /2.303; Higuchi square root law: = 1/2 ; Korsemeyer's model: where , , and are the amount of drug released at time , 0 , and 0 are total amount of drug, and 0 , , and are corresponding rate constant. In case of Korsemeyer's model / ∞ is the fractional drug release at time , is a constant incorporating the properties of the macromolecular polymeric systems and the drug, is a kinetic constant, which is used to characterize the transport mechanism. The value of for a cylinder is <0.5 for fickian release, 0.5 < < 1.0 for Anomalous transport (Nonfickian diffusion), 1.0 for Case-II transport, >1.0 for Super Case-II transport type release [12].
(9) Stability Studies. The optimized formulation of Atenolol were packed in amber color bottle and aluminum foil laminated on the upper part of the bottle and these packed formulation was stored in stability chamber maintained at 40 ∘ C ± 2 ∘ C and 75% ± 5% RH for 3 months. The samples were withdrawn periodically and evaluated for their drug content, in vitro buoyancy studies and for in vitro drug release [13].

Drug-Excipients Compatibility
Studies. The peaks obtained in the spectra of each formulation correlates with the peaks of drug spectrum. It does not show any welldefined interaction between Atenolol and excipients. This indicates that the drug is compatible with the formulation components. The spectra for pure drug, drug-excipients mixture and optimized formulation are shown in Figures 2, 3, 4, 5, and 6.

Precompression Parameters.
Results of the precompression parameters performed on the blend for batch F1 to F18 are tabulated in Table 2. The bulk density and the tapped density for all the formulations varied from 0.384 to 0.486 g/mL and 0.4809 to 0.5667 g/mL, respectively. The percentage compressibility of powder was determined using Carr's compressibility index. Carr's index lies within the range of 11.2 to 23.08%. All formulations show good compressibility. Hausner ratio was found to be in the range of 1.13 to 1.22. Angle of repose of all the formulations was found to be less than 30 ∘ , which indicates a good flow property of the powders.

Postcompression Parameters.
The formulated tablets were subjected for post compressional evaluation such as thickness, hardness, weight variation, friability, drug content,     1% indicating that the tablets of all formulations are having good compactness and showing enough resistance to the mechanical shock and abrasion. The percent drug content of the tablets was found to be in between 97 to 103%. Table 3 shows the results of physicochemical characters of Atenolol tablets.

In Vitro Buoyancy Studies.
In vitro buoyancy of the tablets from each formulation F1 to F18 was evaluated and the results are mentioned in Table 4, where the highest and lowest floating lag time (FLT) were observed with the formulation hydrogenated cottonseed oil and HPMC, respectively.

Swelling Index.
The swelling index of the tablets from each formulation F1 to F18 was evaluated and the results are mentioned in Table 5 and plot of % swelling index versus time (hrs) is depicted in Figure 7, where the highest and lowest swelling was observed with the formulation F5 and F12 after 12 hrs, respectively. No significant swelling was observed with Formulation F1-F4 since they were prepared using HCSO. The swelling index was increased with concentration of HPMC since this polymer gradually absorbs buffer due to hydrophilic nature.     length of the drug. Its viscous nature also affects the diffusion coefficient of the drug.

In Vitro Dissolution
As a result reduction in drug release is obtained. Batches formulated with HPMC K15M and HPMC K4M showed similar release pattern till 4th hr. After 4th hr, release was retarded with formulations containing HPMC K15M to higher extent because of its high viscosity as compared to HPMC K4M. Thus, all HPMC K15M formulations exhibited sustained effect for more than 12 hrs.
It was observed that the type of polymer/retardant influences the drug release pattern. HCSO showed release of drug by erosion mechanism, while HPMC by diffusion mechanism. It was observed that as the concentration of polymer/retardant increased in formulations, the % drug release was decreased. Formulations F13 and F14 busted in 1 hr due to failure of matrix to entrap gas. Formulation F15 with HCSO, HPMC K4M, and HPMC K15M in ratio 2 : 1 : 1 showed release of 57.2% at end of 4th hr. While F16 with HCSO, HPMC K4M, and HPMC K15M in ratio 1 : 2 : 1 showed release of 51.13%, but complete drug release occurred at 11th hr. Formulation F17 was considered as optimized formulation because it showed 51.08% drug release at end of 4th hr and successful sustained effect up to 12 hrs. Formulation F18 with higher amount of hydrophilic polymers, HPMC K4M and HPMC K15M showed release more than 12 hrs. Thus, the optimum combination of hydrophilichydrophobic matrix forming material required in formulation to get buoyancy and release of drug over 12 hrs.

3.7.
Curve Fitting Analysis. The data obtained from in vitro dissolution studies were fitted to zero-order, first-order, Higuchi, and Korsemeyer-Peppas equations. The dissolution data obtained were plotted as time versus cumulative percent drug released as zero order, time versus log cumulative        model which indicate the drug release via diffusion mechanism. Zero-order rate equation, which describes the system where release rate is independent of the concentration of the dissolved species. The Korsemeyer-peppas equation is used to analyze the release of pharmaceutical polymeric dosage forms, when the release mechanism is not well known or when more than one type of release phenomena could be involved. The values of with regression coefficient of all the formulations are shown in Table 8. The value of obtained was in the range of 0.519 to 0.765, indicating nonfickian diffusion in case of tablets formulated with HPMC K4M only. While tablets of hydrogenated cotton seed oil and HPMC K15M followed fickian diffusion, matrix tablet of HPMC and hydrogenated cottonseed oil followed nonfickian diffusion. From the results, it was confirmed that all the formulations are following zero order models followed by higuchi model which indicate the drug release via diffusion mechanism. Formulation F17 gave 99.08% drug release at 12th hr fulfilling the aim of study and, hence, was selected as optimized batch.

Stability Studies.
The results of stability studies did not show any significant change in the physical appearance, drug content, in vitro buoyancy studies, and in vitro dissolution studies of above four formulations as shown in Table 9.

Conclusion
The results of the present research work demonstrates that the combination of both hydrophilic and hydrophobic polymers successfully employed for formulating the sustained release matrix tablets of Atenolol. It is observed that optimum concentration of each of the polymer in combination was able to produce desired formulation which releases complete drug in 12 hours. The mechanism of drug release has observed the combined effect of diffusion and erosion for sustained drug release. So, the combination of both hydrophilic and hydrophobic retardant was suitable to produce the matrix tablet rather than using a single type of polymer. The present study also revealed that HCSO can be used as a matrixforming agent for the preparation of floating tablets. Using HCSO also makes the formulation cost effective.