Enteric coating is aimed to prevent the formulations from gastric fluid in the stomach and release the drug component in the intestinal region or once it has passed into the duodenum.
Enteric coated tablets resist the action of the acidic stomach fluids and pass through it before the coating can dissolve thus protecting the gastric mucosa from the irritating effects of the ingredients in the tablets e.g. aspirin. However, this coating dissolves in the neutral or alkaline milieu of the intestine and the active ingredients become available for absorption into the blood stream.

Basically, coating process of enteric coated tablet consist of several steps :
  1. Manufacturing of tablet core
  2. Pre-coating/ Subcoating/Undercoat
  3. Enteric Coating
  4. Color Coat
  5. Polishing
Subcoating (Pre-Coating)
The major concern in enteric coating formulations is a risk of premature drug release through the enteric coating film in acid media. This problem could be solved by an application of a subcoating layer where the coating substrates are subject to coating with a small amount of a soluble material prior to enteric coating. This thin film layer impedes water penetration through the cores and thus prevents the premature drug release.

Subcoating materials :
  • Hydroxypropylmethylcellulose
  • polyvinyl pyrrolidone
  • hydroxypropyl cellulose
  • polyethylene glycol 3350, 4500, 8000
  • methyl cellulose
  • pseudo ethylcellulose
  • amylopectin
Subcoating is supportive in formulations which contain highly water-soluble drugs. This is where premature drug release mostly occurres. On the contrary, subcoating could also enhance the release of acidic drugs in basic media. This causes a problem of acidic microenvironment at the interface between the core and the enteric film. The migration of diffused drug through the interface results in the delay of drug release in basic media.

Due to the restriction in the regulatory requirements, not only the prevention of premature drug release in acidic media should be taken into account, but also the accomplishment of rapid drug release in basic media. To cope with the latter constrain, a new concept of organic acids addition in coating substrates or subcoating layer is initiated in order to promote the basic microenvironment (pH 5-6) at the interface between the enteric film and the cores which could accelerate the polymer dissolution.

The subcoating layer reduces surface roughness of the coating substrate and improves adhesion of the enteric film on the substrate surface. This generates a robust film formation where a lower amount of enteric coating polymer may be required for enteric protection.

The solutions which will be used to prepare the overcoat or undercoat layers of the present tablets may also vary as to concentration of the non-enteric film forming polymer. Usually the latter will constitute from about 2% to about 8% by weight of said non-enteric film forming polymer based on the total weight of the coating solution with the preferred concentration being from about 5% to about 6% of said non-enteric film former on the same weight basis.

Enteric Coating
Aside from the enteric or non-enteric film formers the above described coating solution may also contain other ingredients which may aid in the application of the coating material to the tablet or to improve the character of the coating. These may be such ingredients as surfactants, plasticizers, antifoaming agents, solubilizing agents, coloring agents.

In general this will amount to about 5% to about 15% by weight of enteric polymer based on the total weight of the coating solution with the preferred range being from about 9% to about 12% on the same weight basis.

Color Coat
Approximately 3% film coating solids are applied to the tablet surface using the same film coating solution as described in Subcoating and applied in a similar manner. The coating solution in this step could be colored.

Application coating in this step prevents tablets from sticking to each other when stress tested at high temperature. It also gives flexibility when preparing this product in different colors.

The color coated tablets from the previous step are polished in the coating pan with exhaust turned off by sprinkling 0.01% powdered polishing wax onto the surface of the tablet bed while rotating slowly. The tablets are rolled in the coating pan until they start to slide. The exhaust is then turned on to remove the excess wax.

These finished enteric film coated tablets have approximately 12.5% total film coating solids applied.

Coating operation
Besides the knowledge of enteric coating liquids, the coating condition are important for coating efficiency.

Minimum Film Forming Temperature (MFFT)

Minimum Film-Forrmation Temperature (MFT) is an inherent problem to water emulsion polymer. MFT is the minimum temperature of the substrate at which the finish will form a strong continuous film.

Since film formation requires the coalescence of the polymer particles on the coating substrates' surface, product temperature should be set to about the polymer's MFFT. This temperature characterizes each polymer. It can be influenced by the type and amount of plasticizers. For enteric coating processes based on aqueous dispersion systems, product temperature is usually set to a range of 30-40°C, in practical operations.

The effect of product temperature becomes troublesome in enteric coating due to the hydrophilicity of enteric coating polymers. They tend to become sticky under humid conditions. The agglomeration of coated particles most likely occurs when the temperature is set too low. This problem becomes crucial in the case of pellet formulations as the growth of sticky pellets takes place in a very short time which could ruin the whole batch if the coating conditions cannot be adjusted in time.

If the product temperature is set too high, this accelerates the solvent/ aqueous evaporation, generating more viscous sprayed-liquid droplets which barely spread on the surface of the coating substrates. This leads to one kind of coating failure which is called 'orange peel appearance'. It results in an inconsistency of the coating layer. High temperature condition could accelerate the volume expansion of the air trapped under the coating layer, shown as the blow out of the film layer. High temperature and long time processing also accelerate the evaporation of some plasticizers, for example triethylcitrate, thus changing the enteric film property.

Coating Film Distribution

Coating uniformity is attributed to the distribution of sprayed liquid on the surface of the coating substrates. This correlates with the design of the equipment used. For example, in pan coating systems, pan speed has a significant influence on the quality of the film distribution through the mass variance of the moving tablets which determines the optimal amount of polymer for the enteric protection. In Wurster-type fluid bed systems, the coating uniformity depends on the mass of coating substrates passing through the spray zone. it is influenced by inlet air volume, spray shape, flow pattern of the substrates and the gap between the Wurster partition and the air distributing plate. The condition of low inlet air volume and low level of the partition tends to generate a dead zone, where the coating substrates cannot be uniformly coated.

Curing Process and Storage Condition

Some types of enteric coating polymers, such as HPMCAS, require a special curing process at an elevated temperature and high relative humidity to induce the polymer coalescence. CAP and CAT coatings present instability of the film upon storage especially at high temperatures. This is due to the hydrolysis of ester groups followed by the formation of insoluble cellulose acetate. Final products coated with aqueous dispersion systems tend to be sintered upon storage if hydrophilic plasticizers are incorporated.

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