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Jun 19, 2018


As will be seen later, enteric polymers are designed to resist the acidic nature of the stomach contents,
yet dissolve readily in the duodenum.

1. Cellulose acetate phthalate (CAP)

Substituent groups: —CO—CH3, —CO—C6H4—COOH
This is the oldest and most widely used synthetic enteric coating polymer patented as an enteric agent by Eastman Kodak in 1940. It is manufactured by reacting a partial acetate ester of cellulose with
phthalic anhydride. In the resulting polymer, of the free hydroxyl groups contributed by each glucose unit of the cellulose chain, approximately half are acylated and one-quarter esterified with one of the two carboxylic acid groups of the phthalate moiety. The second carboxylic acid group being free to form salts and thus serves as the basis of its enteric character. CAP is a white free-flowing powder usually with a slightly odour of acetic acid. Among the pharmacopoeias it is found in the EP, JP and USNF. The USNF and JP impose specifications for the percentage content of the substituent groups. The JP has requirements for the content of acetyl and phthalyl to be respectively 17–22 and 30–40% while the USNF requires 21.5–26 and 30–36% respectively. The JP is alone in not specifying any viscosity control on a standard solution. All three pharmacopoeias require a maximum limit on the quantity of free acid (JP specifies phthalic acid) and loss on drying (EP specifies water content). The last two parameters are important as CAP is somewhat prone to hydrolysis. Of the generally accepted solvents used for tablet coating, CAP is insoluble in water, alcohols and chlorinated hydrocarbons. In the following solvents or solvent mixtures. A pseudolatex version of CAP is available (Aquateric) as a dry powder for reconstitution in water and offers the convenience of aqueous-based processing. Owing to their chemical constitution, most of the phthalate-based enteric coating agents are to a
greater or lesser degree unstable. This important aspect is dealt with in more detail in Chapter 14, along with the implications this has on the use of the materal in practice.

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