What Is Enteric Coating

The making of enteric coating is described in two sections:

• Composition
• Manufacturing

Composition of Enteric Coating

Commercially manufactured enteric coating material includes the following ingredients:

POLYMER

Synthetic as well as natural polymers are utilized in the preparation of enteric coating drugs. They are used to enhance the flexibility of the coating and protect the drug from undesirable breakage and cracking.

The common polymers used in the Enteric Coating substances are:

Cellulose Acetate Phthalate (CAP)

• Cellulose acetate phthalate or CAP is also considered cellacefate.
• This is one of the traditional and broadly used synthetic enteric coating polymers. It was patented as an enteric entity in 1940 by Eastman Kodak Company.
• CAP is insoluble in water, alcoholic solution, hydrocarbons, and chlorinated hydrocarbons.
• CAP dissolved rapidly in alkaline pH > 6 and is comparatively permeable to moisture and gastric juices.
• CAP is susceptible to hydrolysis under a high temperature and moisture.

Cellulose Acetate Trimellitate (CAT)

Chemically, CAT is strongly resembling CAP.

It is the most soluble cellulose derivative that can dissolve under pH ranges from 4.7 to 5.0. Thus, this is an ideal property of CAT which makes it most suitable for drug delivery at the proximal site of the small intestine.

Hydroxyl Propyl Methyl Cellulose Phthalate (HPMCP)

HPMCP was introduced in 1971 as a cellulose derivative used for enteric coating.

• It is soluble in an aqueous medium at pH ranging from 5.0 to 5.5.
• It is insoluble in the stomach although rapidly swells and is solubilized in the small intestine.

Hydroxy Propyl Methyl Cellulose Acetate Succinate (HPMCAS)

HPMCAS is also termed Hypromellose Acetate Succinate.

• Commercially available in the various grade pH-dependent releasing profiles. For instance, low pH 5.0, Medium pH 5.5, and high pH 6.5.
• Since it is a relatively as rigid polymer, hence plasticization is used to enhance the film’s flexibility and overcome the chances of cracking.

Polyvinyl Acetate Phthalate (PVAP)

PVAP is a free-flowing powder that is more stable than CAP. This is because of moisture permeability.

It begins to dissolve when reaches pH 5.0, this property makes it a targeted substance to release at the proximal site of the small intestine.

Methacrylic Acid Copolymers

This group is extensively used for enteric coating applications firstly introduced in the mid-1960s.

• It can easily dissolve at a pH above 5.5.
• It is suitable for certain drugs that have some specific target delivery sites.

SHELLAC

It is one of the main components used for enteric coating applications. Shellac is a secretion of an insect habitat in the tropic and subtropical regions. It is a natural bio adhesive polymer associated with several benefits.

• Shellac is reliable, cost economical and acid-proof substance that ensures a uniform coating.
• Shellac can dissolve at pH above the range of 7.0.

NATURAL WAXES

The implementation of waxes in pharmaceutical coating is well-recognized step as it helps in achieve polished and glossy tablets. Waxes like stearic acids, palmitic acids, etc are considered as a potential functional coating agent possessing enteric properties. An ideal percentage oof wax for enteric coating must be 25-43% to meet compendial criteria for modified release content.

SODIUM ALGINATE

It is a water-soluble polysaccharide extensively utilized in the pharmaceutical industries for enteric coating applications. Alginate is refined from brown seaweed. For a novel coating it is recommended to use Alginate with wax with ratio of 2:4 to 9:7.

Manufacturing of Enteric Coating

The manufacturing of enteric coating medication includes of the following steps:

STEP#1.  Manufacturing of Tablet Core

A tablet core is prepared by direct compression method where entire weighed ingredients are milled and passed through mesh no 60 are used. Before compression materials are blended using the mixer for half an hour and then compressed on a punching machine.
In the case of the capsule, an encapsulation machine is used to obtain gelatin capsules.

STEP#2. Pre-Coating/Sub-Coating/Undercoat

The formulation is now sub-coated with sealing coating using HPMC. It is prepared by using HPMC in a solvent mixture of isopropyl alcohol and dichloromethane (2:1).

STEP#3. Enteric Coating
The polymer is softened in volatile solvents to make a coating solution. The coating solution is sprayed over the tablet core by using s coating pan accompanied by continuous air. The medicines are allowed to be dried after being coated.

STEP#4. Color Coating
Approximately around 3% of coated tablets are applied with the same coating solution as described above. But in this step, a coating solution could be colored. This step gives more flexibility and prevents coated medicines to stick under high temperatures.

STEP#5.  Polishing
In this step, a coated tablet is polished by sprinkling 0.01% polishing wax over the surface of the rotating enteric-coated tablet bed.