Pharmaceutical polymers

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Chapter: Pharmaceutical Drugs and Dosage: Pharmaceutical polymers

Polymers are widely used in pharmaceutical dosage forms. Based on the solubility of polymers in water, they can be classified as water-soluble polymers, water-insoluble polymers, and hydrogels.


Pharmaceutical polymers

Introduction

Polymers are widely used in pharmaceutical dosage forms. Based on the solubility of polymers in water, they can be classified as water-soluble polymers, water-insoluble polymers, and hydrogels.

Water-soluble polymers: These polymers are used in many different ways, so to increase the viscosity of the aqueous solutions; to improve and maintain the physical stability of suspensions; to promote the adhesion of solid particles of different types, leading to granulation in wet processes; to form a flexible film on tablets during the coating of tablets; as adhesives for buccal and bioadhesive drug delivery systems; as emulsifying agents; as flocculating agents; and as components of sustained and site-specific drug delivery systems. Water-soluble polymers used in the coating of tablets include hydroxypropyl methylcellulose (HPMC) or polyvinyl alcohol (PVA) as a film former, and polyethylene glycol (PEG) as a plasticizer. Water-soluble polymers used for the stabilization of suspensions and emulsions include carrageenan, hydroxypropyl cellulose (HPC), and xanthan gum.

Water-soluble polymers can be cross-linked to give hydrogels. For exam-ple, crospovidone is cross-linked polyvinylpyrrolidone (PVP), and croscar-mellose sodium is cross-linked carboxymethyl cellulose (CMC) sodium. Crospovidone and croscarmellose sodium are used as superdisintegrants in oral solid dosage forms. The rapid and high water absorption capability of these water-insoluble cross-linked polymers aids in the disintegration of compressed dosage forms.

Water-insoluble polymers: These polymers are used to form membranes and matrices for sustained-release and localized drug delivery systems. Being water-insoluble, these polymers help delay, slow, or sustain the rate of drug release. An example is the coating of a tablet with a sustained-release water-insoluble polymer. For such an application, water-insoluble polymer is mixed with a limited quantity of a water-soluble polymer, which dissolves in contact with aqueous fluid, leading to the formation of pores in the membrane through which the drug can diffuse out of the dosage form. Factors influencing drug release from these systems include membrane thickness, drug solubility in the membrane, and the porosity of the polymer matrix. Water-insoluble polymers used in the coating of tablets include polymers that dissolve at basic pH but not at acidic pH. Such polymers are called enteric polymers, and such coating on the tablet is called enteric coating. Polymers used for the enteric coating of tablets include cellulose acetate phthalate (CAP), hydroxypropyl methylcellulose phthalate (HPMCP), and methacrylic acid—methyl methacrylate copoly-mers (Eudragit®).

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