Powder Coating for Pharmaceutical Solid Dosage Forms

During the past 50 years, pharmaceutical coating has gone through the transition from sugar coating to organic solvent coating and aqueous coating. Since the 1990s, aqueous coating has largely phased out organic solvent coating as the dominate coating method for pharmaceutical dosage forms due to the toxicity and environmental related concerns caused by the organic solvents. On the other hand, although prevailing, aqueous coating has other major limitations such as higher energy consumption and longer processing time than the solvent coating. Powder coating has the benefits of both, being environmentally friendly, energy efficient and short processing time, while equally effective in modifying drug release profiles. Consequently, powder coating is pointing to the future as the next breakthrough in pharmaceutical coating. Involving three steps including preheating of the dosage forms, electrostatic deposition of coating powders and film formation (curing), powder coating has been studied to tablet coating with easily coated polymers. The objectives of the present study are to expand powder coating to more difficult coating materials such as ethylcellulose and cellulose acetate, as well as more difficult dosage forms including small pellets, and in particular, osmotic controlled release tablets. As a water-insoluble polymer, ethylcellulose (EC) is a commonly used coating material for sustained drug release. However, it is very difficult to coat by powder coating due to its high glass transition temperature (Tg). The present study was successful to coat tablets with fine particles of EC in a rotatable pan coater. With the proper addition of plasticizers, a continuous and uniform coating film was formed. Pore forming agent was added in the coating formulation to adjust the permeability of the coating film to allow a more controlled drug release rate. Currently, small pellets are coated by solvent coating or aqueous coating in a fluidized bed with a larger amount of fluidizing hot air than a pan coater, leading to an extremely high energy consumption. The present study utilized the pan coater instead of a fluidized bed to coat small pellets with powder coating, leading to a significant energy savings by avoiding the use of large amount of fluidizing hot air required to fluidize those pellets and to evaporate the organic solvent or water. Three different coating formulations containing Eudragit® EPO, Eudragit® RS/RL and Acryl EZE were developed to achieve immediate release, sustained release and delayed release, respectively. Particularly, as the only oral drug delivery system capable of achieving constant drug release rate, osmotic drug delivery system (ODDS) is considered as the ultimate ideal drug delivery system. However, only organic solvent coating can be currently used to coat ODDS due to the high Tg of the coating materials. The present study successfully applied powder coating to coat ODDS with cellulose acetate (CA), resulting in a continuous, uniform and functionally acceptable coating film. Both elementary and porosity ODDS were achieved by using this powder coating. Following zero order drug release kinetics, drug release rate from powder coated ODDS varied with different coating levels while was independent with other factors such as drug delivery orifice diameter, pH vale of the release media and agitation speed. Considering that ODDS is the most promising controlled drug release system, the success of powder coating ODDS is a big breakthrough in pharma coating. To fully illustrate the powder coating process, in-depth characterization was carried out to investigate the coating powder deposition and film formation, and their influence factors. Positively related to the coating efficiency, powder deposition was found to be the key of the whole process, which can be promoted by preheating the dosage forms, spraying suitable liquid plasticizer and applying electrostatics for the powder spray. A curing step is necessary after coating powder adhesion to allow deposited particles to coalesce and form a continuous coating film. Finer coating powders, higher curing temperature and longer curing time would lead to a more uniform and smoother coating film