The Effect of Formulation on the Quality of Film Coated Tablets

proportion of the tablets produced around the world are film coated. Coating is applied for a variety of reasons such as aesthetic appearance; identification and branding; taste or odor masking; enhanced mechanical strength; and protection from moisture, light and/or air.

With the current awareness of health, safety and environmental problems, aqueous film coating is a process that is routinely employed in the preparation of pharmaceutical solid dosage forms. The success of this process is determined by three factors: formulation of the coating system, coating process parameters and tablet core. During the last 20 years, there has been significant research into coating formulations and processes. In focusing on these areas, less attention has been paid to issues that relate to the preparation of a suitable tablet or substrate. This article will highlight the importance of pharmaceutical formulation on the quality of a film coated tablet.

Tablet Formulation

The formulation of a robust tablet 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 and the applied coating are constantly subjected to mechanical stress. Tablet breakage and surface erosion are typically seen when the mechanical strength and friability of the tablet core are inadequate. The problem can worsen during scale-up because of the increased weight of the tablets charged into the coating pan. This situation may occur after product approval, because the use of ever-increasing pan sizes is not uncommon as product sales increase. Therefore, any product that is performing poorly with respect to mechanical strength should not even be exposed to the scale-up process.

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

A measure of equal or perhaps greater importance in terms of tablet robustness is friability. This test more accurately reflects the stresses that tablets will encounter when tumbling into a coating pan. Tablets to be film coated should have a maximum friability value of 0.3% and preferably less then 0.1%. This guideline should be adhered to regardless of tablet size or shape.

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

Adhesion of Film Coating to the Tablet Core

Good adhesion between a polymeric film and a tablet is a fundamental requirement to guarantee a good bond between the coating and surface of the core as the tablets tumble in the coating pan, and to maintain the clarity of logos. For the formation of an adequate and adhesive film coat, the atomized droplets have to spread completely over the surface of the core – and to a certain degree penetrate into a substrate. Some of the materials used in tablet formations, however, may interfere with the intermolecular bonding at the film-substrate interface and hinder adhesion of the film to the core.

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

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

Conclusion

Many of the ingredients chosen in initial tablet formulation development can have a significant impact on 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 ultimate appearance of the tablet are often left until later stages of development and can impact coating quality. To ensure success in the film coating process, formulation along with the design of the tablet should be considered early in the development.

About the Authors

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.

About Colorcon®

Colorcon® is a world leader in the development, supply and technical support of formulated coatings and excipients for the pharmaceutical industry. With Colorcon as their pharmaceutical product development partner, companies produce cost effective, high quality products with superior performance and appearance. To learn more, please visit http://www.colorcon.com/.