INTRODUCTION:

An enteric coating is a polymer barrier applied to oral medication that prevents its dissolution or disintegration in the environment this helps by enteric protecting drug from the acidity of stomach the stomach form the detrimental effect of the drug or to release the drug after the stomach (usually in the upper tract of the intestine). some drugs are unstable at the pH of gastric acid and need to be protected from degradation. Enteric coating is also an effective method to obtain drug targeting (such as gastro resistance drug). Other drugs such as some anthelmintic may need to reach high concentration in a specific part of intestine. Enteric coating may also be used during studying as a research tool to determine drug absorption.

Enteric coated medication pertains to the “delayed action” dosage from category. Tablets, pellets, and granules (usually filled into capsule shells) are the most common enteric coated dosage forms. An enteric coating, also known as gastro-resistant coating is a barrier applied to oral medications that controls the location in the digestive track where it is absorbed. The term “enteric” refers to the small intestine; therefore, enteric coating resist breakdown of medication before it reaches the small intestine. Enteric coatings are employed when the drug substance is inactivated or destroyed in the acid secretion of the stomach or are particularly irritating to the gastric mucosa or when bypass of the stomach substantially enhances drug absorption. Early approaches to preparing enteric dosage forms involved treating gelatin capsules with formation or coating tablets with shellac. Both of these approaches where unreliable since the solubility of the membrane (which is responsible for the enteric effect) can be unpredictable modern enteric coating are usually formulated with synthetic polymeric material often referred to as polyacis

Types of enteric coating: [6]

· Shellac (ester of aleurtic acid)

· Cellulose acetate phthalate (CAT)

· Poly (Methacrylic acid-co-methyl methacrylate)

· Cellulose acetate trimellitate (CAT)

· Poly (vinyl acetate phthalate) (PVAP)

· Hydroxpropyl methylcellulose phthalate (HPMCP)

Polymers used for enteric coating:

1. Cellulose acetate phthalate (CAT):

Cellulose acetate phthalate, also known as cellacefate is the oldest and most widely used synthetic enteric coating polymer. CAP is synthesized by reacting a particle acetate ester of cellulose with phthalic anhydride in the presence of the tertiary organic base such as pyridine, or a strong acid such as sulfuric acid.

2. Polyvinyl acetate phthalate (PVAP):

Polyvinyl acetate phthalate is a free-flowing white to off-white powder with a slight odour of acetic acid. The onset of aqueous dissolution of PVAP begins at a pH of about 5.0 allowing for enteric release as well as potential for targeted drug release to the proximal small intestine.

3. Hydroxpropyl methylcellulose phthalate (HPMCP):

HPMCP is a white to slightly off-white, free-flowing flakes or granular powder with a slightly acidic odour and a barely detectable taste. It is a derivative of Hydroxpropyl methylcellulose.

4. Cellulose acetate phthalate (CAT):

Chemically this polymer bears a strong resemblance to CAP. It is formed by the same synthesis process as CAP with trimellitic anhydride group in the place of phthalic anhydride. The pKa of CAT is between 4.1 and 4.3.

Tablet coating:

Coating is process by which an essentially dry, outer layer of coating material is applied to the surface of a dosage form in order to confer specific benefits that broadly ranges from facilitating product identification to modifying drug releases from the dosage form after making a good table, one must often coat it.

Coating may be applied to multiple range or oral solid dosage form, when coating composition is applied to a batch of tablets in a coating pan, the tablet surface becomes covered tacky polymeric film. Before the tablet surface dries the applied coating changes from a sticky liquid to a tacky semi-solid and eventually to a non-sticky surface pans. The enteric coating process is conducted in a series of mechanically operated acorn shaped coating pans of galvanized iron stainless steel or copper. The smaller pans are used for experimental, developmental and pilot plant operations, the larger pans for industrial production.

Primary components involve in tablet coating [6]:

· Tablet properties

· Coating process

· Coating equipment

· Parameters of the coating process

· Facility and ancillary equipment

· Automation in coating process.

Coated Pills:

The protective coating placed around pills are important in the treatment of illness as the chemical included in the pills. Pills contain a fulfill a variety of purpose, depending on the type of medication they are enhancing. In some circumstances, the coating is used to extend the useful life of the drugs by protecting it from exposure to temperature, moist or light. The coating also keeps the pills from being cracked or broken during handling. Some protective coatings mask the tastes of chemicals, making the pills easy to swallow. In other case coating acts as a protective barrier.

Capsule Coating:

Enteric coated dosage forms, such as coated tablets, sugar-coated tablets, soft and hard gelatin capsules, granules or pellets have their place in medical arsenal (1a, 2). An investigation of 181 ready-to-use enteric coated medicaments revealed that this sample comprise about 59% sugar-coated tablets (106 preparations), about 27% film-coated tablets (49 preparations) and about 49% of soft and hard gelatin capsules (25 preparations). However, this group investigated the preparations on German market.

Reasons for the application of enteric coatings:[2]

Enteric coatings are applied forms the following reasons:

1. To protect the acid-labile drug substances from the acidic pH gastric of acid. Such drug substances include erythromycin, pancreatin, omeprazole etc.

2. To prevent gastric distress/ulceration or nausea due to irritation caused by certain drugs such as aspirin and certain nonsteroidal anti-inflammatory compounds.

3. To deliver drug that are optimally absorb in small intestine to their primary absorption site in their most concentrated form.

4. To provide a delayed release component to repeat action tablet.

Ideal properties of enteric material: [2]

Some of the key attributes of enteric coating material include:

1. Resistance to gastric fluids.

2. Ready susceptibility or permeability to intestinal properties.

3. High compatibility with other film-coating additives and the tablet being coated.

4. Capacity to form continuous film with adequate mechanical properties.

5. Ease of application without special equipment.

6. Ability to be printed or to be allows film to be applied to debossed tablets.

7. Nontoxic with no pharmacological activity.

8. Stable alone and in coating solution.

9. The enteric-coated material should be stable on storage. Film coated on tablets or granules should not be subject to performance changes not on storage.

10. Adhesion between film and substrate should be storage.

Factors that can influence the behavior of enteric-coating polymers: [2,4]

The functionality of enteric-coating polymers can be greatly affected by factors, such as:

1. The nature of drug substance contained in the dosage form; this is especially true when drug substance is ionic in nature.

2. The quantity of coating applied; application of excessive coating can substantially delay drug release from the dosage from while insufficient coating can result in infective gastric resistance.

3. The presence of imperfection in the coating (e.g., cracks, “pick marks”, etc.) can destroy the integrity of the coating.

4. The dissolution pH and dissolution rate of polymer used in coating.

5. The influence of the in-vitro test conditions used (such as pH and ionic strength of the test solution; as well as the agitation rate used in the test).

6. The presence or absence of plasticizers.

7. Gastrointestinal physiological factors.

a. The pH of the stomach and intestinal contents

b. Gastric emptying

c. Enzyme activity of the gastrointestinal tract

Example of enteric-coated tablets:

Alophen-Bisacodly 5 mg USP (Unmark Laboratories Inc.)

Gastrointestinal pH and polymer performance: [1,5]

All the enteric polymers in current use posse ionizable acid group, usually a free carboxylic acid from a phthalyl moiety. The equilibrium unionized insoluble polymer and ionized soluble polymer will be determined by the pH of the medium and the pK_a of a polymer. The Henderson-Hasselbach equation can be used to predicate the ratio of the ionized to unionized polymer based on these two parameters, i.e.

Concetration ionized form

pH - pK_a= log --------------------------------------

Concetration unionized form

In order to perform adequately, an enteric-coated From Should Not allowed significant release of the drug in stomach, yet must provide rapid dissolution of the polymer and complete release of the active material once in the environment of intestine. It is a fact, however, that all of the enteric-coating polymers in the hydrated state in the stomach will be permeable to some degree to a given activate material.

Enteric coating material: [6]

Enteric coating work because they are selective insoluble substances they won’t dissolve in acidic juices of the stomach, but they will when they reach the higher pH of the small intestine.

Most enteric coatings won’t dissolve in solutions with a pH power lower than 5.5. Commonly-used enteric coated may be made from:

· Methacrylic acid copolymers

· Cellulose acetate (and its succinate and phthalate version)

· Polymethacrylic acid/acrylic acid copolymer

· Hydroxpropyl methyl cellulose phthalate

· Polyvinyl acetate phthalate

· Hydroxyethyl ethyl cellulose phthalate

· Cellulose acetate tetrahydrophtalate

· Acrylic resin

· Shellac

The earliest enteric coating utilized formalized gelatin; this was unreliable because of the polymerization of the gelatin could not be accurately controlled. Another was shellac, disadvantages was polymerization with time, resulting in poor dissolution of the coating.

The most extensively used polymers are CAP, PVAP. The mostly recently used polymers are HPMCP, Methacrylic acid copolymers.

Mechanism: [6]

ETP tablets are compound of three layers, a drug containing core tablets (rapid release function), the press coated swellable hydrophobic polymer layer (Hydroxy propyl cellulose layer (HPC), time release function) and an enteric coating layer (acid resistance function). The tablet does not release the drug in the stomach due to the acid resistance of the outer enteric coating layer The enteric coating layer dissolves after gastric emptying and the intestinal fluid begins to slowly erode the press coated polymers (HPC) layers. Rapid drug release occurs when the erosion front reaches the core tablets since the erosion process takes a long time as there is no drug release period (lag phage) after gastric emptying.

CONCLUSION:

From the above review, we can conclude the tablets are made enteric coated for avoiding the first pass metabolism, gastric irritation and degradation and to direct the drug to the target intestine. Enteric coated tablets could be used to treat streptococcal infection of throat (strep throat) and skin also be used in treating the lung infection (pneumonias) caused by streptococcal pneumonia. The choice of polymer and thickness of the coated layer are critical to control the pH solubility profile of the enteric coated dosage form. It is very convenient and easy to formulate, cost-effective and does not require high cost-equipment. For that reason, this dosage form it has been gaining so much attention nowadays.

REFERENCE:

1. Lachman L, Lieberman HA, Joseph LK. The Theory and Practice of Industrial Pharmacy. Varghese Publishing House; Mumbai; Third Edition: 297-321.

2. Aulton M. Pharmaceutics: The Science of Dosage Form Design. International Student Edition: 304-321, 347-668.

3. Hussan, S.D., Santanu, R., Verma, P. and Bhandari, V., 2012. A review on recent advances of enteric coating. IOSR Journal of Pharmacy, 2(6), pp.05-11.

4. Kumar Vinay. KV, Sivakumar T and Tamizh mani. T, Colon targeting drug delivery system: A review on recent approaches, International Journal of Pharmaceutical and Biomedical Science, 2, 2011, 11-19.

5. D. Raju, J. Padmavathy, V. Sai Saraswathi, D. Saravanan and I. Aparna Lakshmi, Formulation and development of enteric coated tablets of prednisolone as a colon targeted drug delivery, IJPSR, 2(3), 2011, 685-690.

6. Anil K. Philip and Betty Philip, Colon Targeted Drug Delivery Systems: A Review on Primary and Novel Approaches, Oman Medical Journal, 25(02), 2010, 70-78.

Received on 15.10.2020 Modified on 18.11.2020

Res. J. Pharma. Dosage Forms and Tech.2021; 13(1):82-85.

DOI: 10.5958/0975-4377.2021.00015.X