Tablet cores 

1. Drug solubility
Higher solubility drug containing cores in compression-coated tablets provided shorter lag time than lower solubility drug containing cores.

2. Tablet core formulation
Drug release from compression-coated tablet containing a fast release core was faster than extended release core containing coated tablet when the same coating composition was used. The release behavior and the lag time were dependent on the type of excipient used in the core.
The influence of different composition of cores on drug release from spray-dried chitosan acetate/ HPMC compression-coated tablets. Soluble diluent and appropriate amount of super disintegrant in core tablets enhanced drug release while osmotic agent slightly retarded drug release.
Formulation and compression-coating technique

Compression-coating amount
Coating amount is the most important parameter to achieve a coating uniform for compression-coated tablets.
1. the compression-coating should about twice the weight of the core.
2. the volume must be greater than that of the core itself.
3. If the cores are comprised mainly of low density materials, such as fats and waxes, the amount or weight of coating must be even greater to assure a uniform volume of coating material for covering the core and adhesion of core and coating.

Position of core in Coated Layer 

The main drawback of this system is the centralization of core in the compression-coated tablets. The reproducibility of drug release from compression-coated tablet is questionable, since the faults of press-coating can happen. Examples of press-coating fault are unequal coating, cocking and off-center.

Compression force and Compressibility of Materials 

The compressibility of coated tablets is depended on the coating material. Thus, cohesiveness and plasticity of the powder coat are needed to obtain satisfactory mechanical strength of the coating. The cohesiveness indicates the continuity of the coating around the edge of the core, which depends on its strength and the plasticity responses for the expansion of the core after the final tablets are released from the die.

The final compression force applied to prepare compression-coated tablets has to be higher than the compression force which was applied to the core, to ensure the adhesion between core and coat. Tablets with adhesive coating can be applied as core to ensure adhesion of compression coat and core.

The Interaction Between Drug and Compression Coat
The interaction of drug and coating is needed to be considered when gellable compression coats are used for drug release control. Drug in compression-coated tablets diffuses through the swollen coat. This process might enhance some possible interaction between drug and coat.

Read more : Factors Affecting on Drug Release in Compression-Coated Tablet (Part I)