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The Effect Of Formulation On The Quality Of Film Coated Tablets

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proportion of the tabletsproduced around the world are film coated. Coating is applied for a variety ofreasons such as aesthetic appearance; identification and branding; taste orodor masking; enhanced mechanical strength; and protection from moisture, lightand/or air.

With the current awareness ofhealth, safety and environmental problems, aqueous film coating is a processthat is routinely employed in the preparation of pharmaceutical solid dosageforms. The success of this process is determined by three factors: formulationof the coating system, coating process parameters and tablet core. During thelast 20 years, there has been significant research into coating formulationsand processes. In focusing on these areas, less attention has been paid toissues that relate to the preparation of a suitable tablet or substrate. Thisarticle will highlight the importance of onthe quality of a film coated tablet.

Tablet Formulation

The formulation of a robusttablet has to be considered in terms of:

The ability of the core to withstand the mechanical stress of the process.
Maximized adhesion of the coating to the tablet surface, especially when a logo is present.
A smooth film coat with uniform thickness.
The stability of the final coated dosage form on storage.

Tablet Mechanical Strength and Friability

The tablets being coated andthe applied coating are constantly subjected to mechanical stress. Tablet breakageand surface erosion are typically seen when the mechanical strength andfriability of the tablet core are inadequate. The problem can worsen duringscale-up because of the increased weight of the tablets charged into thecoating pan. This situation may occur after product approval, because the useof ever-increasing pan sizes is not uncommon as product sales increase.Therefore, any product that is performing poorly with respect to mechanicalstrength should not even be exposed to the scale-up process.

It is extremely difficult tocreate generalized guidelines defining the physical attributes of a robusttablet that is acceptable for use in a coating process because so much dependson the materials that are being used. Tablets with relatively low breakingforce values should be dealt with much more carefully if they are to be coated,particularly during scale-up. Large tablets, such as multivitamin cores, mayneed to have greater mechanical strength values compared with smaller tablets.

A measure of equal or perhapsgreater importance in terms of tablet robustness is friability. This test moreaccurately reflects the stresses that tablets will encounter when tumbling intoa coating pan. Tablets to be film coated should have a maximum friability valueof 0.3% and preferably less then 0.1%. This guideline should be adhered toregardless of tablet size or shape.

Drug and excipient particlecharacteristics can also have a significant affect on the success of filmcoating. Variations in coating thickness may present a serious problem when theproperties of the film are dependent on its thinnest part, particularly in thecase of modified drug release coatings.

Adhesion of Film Coating to the Tablet Core

Good adhesion between apolymeric film and a tablet is a fundamental requirement to guarantee a goodbond between the coating and surface of the core as the tablets tumble in thecoating pan, and to maintain the clarity of logos. For the formation of anadequate and adhesive film coat, the atomized droplets have to spreadcompletely over the surface of the core ? and to a certain degree penetrateinto a substrate. Some of the materials used in tablet formations, however, mayinterfere with the intermolecular bonding at the film-substrate interface and hinderadhesion of the film to the core.

Lubricants are added totablet formulations to minimize both die-wall friction and punch adhesion. Bothof these requirements necessitate that the lubricants function at the tabletsurface, precisely where they are counterproductive in the adhesion processconsidering the inherent hydrophobicity of lubricants, such as metal stearates.

No excipient used in suchsmall quantities can have as detrimental effect on tablet quality thanmagnesium stearate. Magnesium stearate, although a very effective lubricant,can reduce the mechanical strength of the cores, decrease film adhesion andslow drug dissolution. Therefore, the quantity of magnesium stearate used in atablet formulation should be minimized. Self-lubricating products such asStarch 1500 can also be used to reduce the need for significant lubricantaddition.

Conclusion

Many of the ingredientschosen in initial tablet formulation development can have a significant impacton aqueous film coating quality. They may affect the physical, mechanical,adhesive, drug-release and stability properties of the coated dosage form.

Decisions as to the ultimateappearance of the tablet are often left until later stages of development andcan impact coating quality. To ensure success in the film coating process,formulation along with the design of the tablet should be considered early inthe development.

About the Authors

Marina Levina is the globaltechnical manager, MR applications at Colorcon Ltd., UK. Charles R. Cunningham is thepharmaceutical technical services manager, at Colorcon, USA.

About Colorcon?

Colorcon? is a world leaderin the development, supply and technical support of formulated coatings andexcipients for the pharmaceutical industry. With Colorcon as their partner,companies produce cost effective, high quality products with superiorperformance and appearance. To learn more, please visit .

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Marina has sinced written about articles on various topics from Medicine. Marina Levina is the global technical manager, MR applications at Colorcon Ltd., UK. Charles R. Cunningham is the pharmaceutical technical services manager, at Colorcon, USA.. Marina's top article . to your Favourites.

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