Optimization of Hot Air Coating Technique as a novel method for the spray-congealing

In previous works we showed that the spray-congealing method easily yields spherical enhanced or controlled release microparticles with a good encapsulation efficiency. However, this technique could present some drawbacks: the thermal degradation of the drug (for heat-sensitive drugs) during its permanence in the molten mixture before spraying and the difficulty to achieve a drug loading exceeding 25% w/w, due to the difficulty to atomize a so highly viscous system. To overcome these disadvantages, recently a technique denominated HAC (Hot Air Coating) has been proposed. The most significant advantages of this novel technique are: the use of solid samples as starting materials, the short time of exposure to high temperature minimizing the risk of thermal degradation and the possibility of processing formulations with a high drug/excipient ratio. However the apparatus showed some drawbacks related to the low amount of material which can be processed and to the difficulty to effectively coat granules with large particle size. The aim of this work was to optimize the apparatus and the method. The result showed that this novel spray-congealing technology is able to nebulize very viscous systems that are not processed by conventional apparatus. The drug loading is quite high for microspheres (about 40%), the yield approaches 90% and the drug release can be easily modified according to the nature of the used excipients. Finally, the optimized HAC technique enables to deeply decrease the processing time with respect to the previous technology.