Polymers are macromolecules composed of a large number of repeating monomer units. Polymers are used as additives in the manufacturing of dosage forms containing pharmacologically active molecules. They play an important role in different pharmaceutical dosage forms as diluents, binders, disintegrants, gelling agents, thickening agents, emulsifying agents, suspending agents, sustained release agents, stabilizers and coating materials. Polymers affect the stability of the prepared formulation and can influence its physicochemical properties and the drug release . Polymers can be natural or synthetic, and are usually high molecular weight compounds with regularly arranged smaller units. Natural polymers are usually polysaccharides and are widely used as excipients in oral drug delivery dosage forms. These natural materials have advantages over synthetic polymers, since they are chemically inert, nontoxic, less expensive, biodegradable, widely available, have less chance of adverse effects and have better patient acceptance . Natural polymers like gums and mucilages are plant-based hydrocolloids abundant in Nature and common in higher plants. Gums are considered to be pathological products formed following an injury to the plant or as a result of some pathological condition, while mucilages are physiological and normal products of metabolism .
The tamarind tree, Tamarindus indica Linn. is a large, semi-green and dicotyledonous fruit-bearing tree which belongs to the Leguminosae family. It is common in Southeast Asia, India, Pakistan, Bangladesh, Myanmar and Sri Lanka . Tamarind seeds are rich in polysaccharides (~65%–72%), which contain glucose, xylose and galactose units. They are high molecular weight polysaccharides (720–880 kDa) , which form viscous solutions in water. Tamarind kernel powder is used as a thickening, stabilizing and gelling agent in the food industry . In view of its characteristics it was selected in the present study along with xanthan.
The oral drug administration route is considered as the most convenient, low cost and the safest route of drug delivery. The effectiveness of oral drug delivery depends on several factors such as gastric emptying rate, gastrointestinal transit time of the dosage form, the rate of drug release from the dosage form and the site of drug absorption . Factors such as short gastric residence time (GRT) and highly variable gastric emptying time of dosage forms can influence the drug bioavailability of pharmaceutical oral dosage forms .
Several gastroretentive drug delivery systems (GRDDS) aimed at improving the oral delivery of drugs with an absorption window in the upper part of the gastrointestinal tract, have been designed over the past three decades . The gastroretentive dosage forms can be divided into four main classes: (a) floating systems that cause buoyancy above gastric fluid; (b) expandable systems, which limit emptying through the pyloric sphincter due to expansion; (c) bioadhesive systems that cause bioadhesion to the stomach mucosa and (d) high density systems, retained at the bottom of the stomach . GRDDS improve bioavailability of poorly soluble drugs, increase therapeutic efficacy and patient compliance by reduction of the frequency of the drug intake and by maintaining a constant therapeutic level over a prolonged period of time. Floating drug delivery would be beneficial to drugs that; (a) act locally in the stomach; (b) are primarily absorbed in the stomach; (c) are poorly soluble in alkaline pH; (d) have a narrow absorption window and (e) are not stable in the intestinal or colonic environment [9,11].
Famotidine is an H2-receptor antagonist, commonly prescribed for treatment of gastric ulcers, Zollinger-Ellison syndrome, duodenal ulcers and gastro-esophageal reflux disease. Orally administrated famotidine is incompletely absorbed from the gastrointestinal tract . Low bioavailability (40%–45%) and short half-life (2.5–4 h) are the main limitations of the therapeutic effectiveness of famotidine. Due to its short biological half-life, multiple doses are needed to maintain a constant plasma concentration of the drug for a good therapeutic response. However, the increased frequency of drug intake is associated with an increased risk of adverse effects like constipation, fatigue, insomnia, bleeding, depression, nausea, confusion and irregular heart beat .
Matrix (monolithic) systems are widely used to sustain the drug release from dosage forms in which the drug is dispersed or dissolved homogenously throughout the polymer. Initially the drug is released from the outer surface of the dosage form, and later from the deeper regions of the dosage form. The drug release from deeper regions can be either by diffusion or by polymer degradation, depending on the polymer and the drug properties. Presently famotidine is dispensed in the market as an over-the-counter medication for treatment and prevention of occasional heartburn associated with excessive gastric acid. Famotidine is useful in promoting the healing of ulcers and in reducing ulcer-induced pain. It has been proven effective in preventing the recurrence of ulcers when given in low doses for prolonged periods of time. Thus the primary objective of the present study was to prepare a sustained release gastro- retentive famotidine dosage form for reducing gastric acid secretion in ulcers as well as for maintenance therapy.
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