Enteric coating of pharmaceutical products
By Krisanin Chansanroj
Enteric coating formulations
Enteric coating is aimed to prevent the formulations from gastric fluid in the stomach and release the drug component in the intestinal region. Based on this approach, enteric coating is suitably applied for drugs which cause gastric irritation or are deteriorated by the gastric fluid or gastric enzyme.
Enteric coating polymer
With an acid-resistant property, enteric coating polymers generally possess free carboxylic acid groups on the polymer backbone. They are insoluble in acidic media but become deprotonated and dissolved in basic media at nearly neutral pH values (pH>5). Enteric coating polymers can be classified into 3 groups based on chemical compositions as listed below:
|Methacrylic acid/ethyl acrylate
|Cellulose acetate phthalate (CAP)
|Cellulose acetate trimellitate (CAT)
|Hydroxypropylmethylcellulose acetate succinate
|Polyvinyl acetate phthalate (PVAP)
Solubility of the polymers depends on the number of carboxylic acid groups varied in the composition. Commercial enteric coating polymers are available as powder, aqueous dispersion and organic solution.
Enteric coating formulations need special care of coating operation due to the constrain of drug release specified in the regulatory requirements. Enteric formulations should have less than 10% drug release after 2 hours in the acid stage. The completion of the drug release in the continuation testing in the buffer stage should take place within 45 min.
Organic solution and aqueous dispersion
Generally, enteric coating polymers dissolve well in organic solvents, giving a stable coating solution that facilitates faster coating processes due to easy evaporation of organic solvents. However, the practical use of organic solvents in pharmaceutical formulations has decreased since organic solvent residues in final products are restricted by the authorities. Flammability of organic solvents and their toxicity to operators, as well as their harmfulness to the environment are further reasons. These concerns encourage the use of aqueous dispersion systems with 30-40% wt. dry polymer dispersed in water systems, assisted by surfactants. The last years efforts have been made to develop ready to use dispersions which include all auxiliary components such as plasticizers, opacifiers, and antifoaming agents.
However, the film formation process based on organic solvents and aqueous dispersions is basically different. The polymer in the organic solutions undergoes sol to gel transitions during solvent evaporation whereas polymer particles in aqueous dispersions deposit layer by layer on the surfaces of the coating substrates. Whilst water evaporates, polymer particles approach each other, due to capillary force, and gradually fuse to a uniform layer . Therefore the size of polymer particles in the dispersion could influence film formation. The smaller the particles are, the larger the contact area between the polymer particles becomes. This accelerates polymer coalescence . By consequence a lower amount of dry polymer is required for the enteric protection .
Enteric coating based on aqueous dispersion systems has also some limitations. Coating processes take longer than with organic solvent systems as there is more energy required to evaporate water than for solvents. This could increase the deterioration of heat- and/or moisture-sensitive drugs during coating processes . Furthermore, the aqueous dispersion systems are generally susceptible to coagulation because of a number of factors, such as additions of fine powder pigments or wetting agents, high shear gradients during mixing and pH change. Therefore, the preparation of coating dispersion needs careful operations following the directions for use suggested by the producer.
Success of enteric coating efficiency mostly relies on the addition of plasticizers. Plasticizers are a group of auxiliary components that improve elasticity of the polymeric film which is generally rigid and breakable. Plasticizers reduce the minimum film forming temperature (MFFT) of the polymers, softening the polymeric film at lower temperature. This improves the spreadability of the polymer on the surface of the coating substrates and generates a smoother surface texture of the coating layer .
The type of plasticizer should be selected carefully as it influences the film brittleness , compatibility with the coating substrates  and product stability [3, 5]. Hydrophilic plasticizer, triethyl citrate, is reported to improve the property of Eudragit L 30 D-55 film in the soft gelatin capsule formulations regardless of the type of filled liquid whereas hydrophobic plasticizer, tributyl citrate, gives satisfactory enteric protection only with hydrophobic filled liquid . The latter plasticizer could migrate to the hydrophobic filled liquid upon storage, resulting in the reduction of the enteric protection.
Besides the plasticizer type, the amount of plasticizer is important for film flexibility. Insufficient amount of plasticizer causes the film blistering which could lead to a premature drug release in acidic media, as shown in Figure 1. However, high amount of plasticizer reduces the strength of the film and may accelerate the water uptake into the cores upon storage.
Fig. 1: Enteric coated tablets with insufficient plasticizer; (A) before dissolution test, (B) and (C) after dissolution test in the acid stage for 1 and 2 h, respectively.
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, i.e., HPMC, amylopectin, prior to enteric coating. This thin film layer impedes water penetration through the cores and thus prevents the premature drug release.
Subcoating is supportive in formulations which contain highly water-soluble drugs [6-8]. 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 [6, 8, 10-11].
Furthermore, 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 .
Minimum film forming temperature (MFFT)
Besides the knowledge of enteric coating liquids, the coating condition are important for coating efficiency. 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, see Figure 2.
Fig. 2: Effect of low product temperature during coating process; (A) uncoated pellets, (B) coated pellets with agglomeration.
Fig. 3: Effect of high product temperature during coating process; (A) orange peel surface, (B) air trapped under coating layer.
On the other hand, 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. Furthermore, 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, see Figure 3. 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 [13-15]. 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 [3, 16]. 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. Furthermore, final products coated with aqueous dispersion systems tend to be sintered upon storage if hydrophilic plasticizers are incorporated .
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Krisanin Chansanroj PhD, is currently a postdoctoral
fellow at Industrial Pharmacy Research Group, Dep. of Pharmaceutical Sciences, University of Basel. Her present works involve enteric coating formulations using fluid bed systems and direct compacted matrix tablets.