Compression coating 

1. Polymer type
The pharmaceutical polymers used (single or combination) in compression coating are :
  • cellulose derivatives (e.g. hydroxypropylmethylcellulose acetate succinate, ethylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose and hydroxyethylcellulose),
  • polysaccharides (e.g. guar gum, sodium alginate and pectin)
  • water soluble polymer (polyethylene oxide)
  • wax (behenic acid)
  • methacrylate copolymers 
The coats containing these polymers could be divided into groups such as :
  • water insoluble (ethylcellulose)
  • erodible (low molecular weight hydroxypropylmethylcellulose, hydroxypropylcellulose, polyethylene oxide)
  • gellable or swellable (high molecular weight hydroxypropylmethylcellulose)
  • pH dependent soluble (hydroxypropyl-methylcellulose acetate succinate, methacylic acid copolymer)
  • waxy
  • bacterial digestible.

The release behavior from compression-coated tablets was controlled and modulated by type and molecular weight of the polymer used as shell. Drug release starts when the shell is completely eroded swollen or dissolved. A purely erodible coating is supposed to prevent drug release from the core until it is removed from by dissolution medium. The release behavior of the cores from compression-coated tablets containing erodible shell would not be modified by the erodible coating. Instead, drug release from compression-coated tablets which have a gellable coat would be delayed and altered the release performance. Smaller molecular weight of gellable coat would provide a faster release rate after lag time than higher molecular weight.

2. Particle size of polymer used 

Compression-coated tablets prepared with smaller particles sizes of ethylcellulose provided longer lag time. The smaller particle size of ethyl cellulose used in coat provided less porosity and higher tortuous path for medium infiltration, then the longer lag time of drug release was obtained. Lag time of compression-coated tablets containing ethylcellulose mixtures (granules and fine powder, 1:1) as coat was only slightly different from tablets containing fine powder as coat because fine ethylcellulose powder was filled the inter- and intraparticulate gaps of coarse ethylcellulose powder.

The influence of particle size of coating material in form of granules was more pronounced than in form of powder. Compression-coated tablets from granulated coat provided faster drug release and shorter lag time compared to tablets from fine powder coat.

3. Porosity or release modifier incorporated in coat 

When hydrophilic excipients are incorporated into an insoluble coating, they possibly act as a pore-forming agent for water penetration and the higher content of water soluble excipient in the coating results in shorter lag time. Different release behaviors from compression-coated tablets containing different hydrophilic excipients were resulted from different physiochemical properties.

The lag time from press-coated tablets containing an ethylcellulose/hydroxypropylmethylcellulose E4M shell was longer when compared with ethylcellulose/spray dried lactose shell due to higher water solubility of the latter. The viscous hydroxypropylmethylcellulose gel deposited within and on the surface of compression-coated tablet prolonged the lag time. The delay release after the lag time was found for compression-coated tablet containing a high content of HPMC in the compression coat. At the certain amount of pore former used, the larger particle size of pore former used, the larger pore size formed in the compression coat.

4. Core-coat ratio 

For the time controlled release system from compression-coated tablets, the amount of the outer shell is a key factor for controlling the lag time. Higher amount of the outer coating added would prolong the lag time of drug release.

5. Compression force 

When an insoluble coat is applied on a core with different compression forces, the lag time and drug release rate will be modified. The lag time of drug release increased and the release rate decreased when the compression force applied to the coating increased till a critical point. Their result could be explained by a decrease of coat porosity with higher compression forces leading to slower diffusion or lower permeability of water through the porous polymer matrix as compression coat.

Higher compression force applied in compression coating leading to lesser porosity in the coat results in longer lag time. When the applied compression force for the coating was higher than the critical point, absence of the compression parameter was shown due to no further reduction in porosity. The same relationship was found for erodible shells.

In case of an swellable shell, applied compression force to the coating showed less effect on drug release from compression-coated tablet compared to insoluble and erodible shell.
Different lag times from different hardness of compression-coated tablet could be attributed to the influence of excipients incorporated in the coat (HPMC, lactose and microcrystalline cellulose).

Continue to : Factors Affecting on Drug Release in Compression-Coated Tablet (Part II)