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During the live session and in the following days, we have received interesting questions about this innovative technology and its main applications. Our experts are preparing the answers to the most relevant questions, which we will post in this Q&A section. Stay tuned for periodical updates!
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The SSD is amorphous, and the re-crystallization of API during the storage of FDF has to be avoided. What is the typical ratio of API to polymer in the solid dispersion prepared by spray drying? Is it similar to ratios of about 1:3 as used in hot melt extrusion?
Stefano Console answers
When spray drying occurs, because the drying step is very quick, the API in solution does not have enough time to rearrange as a crystalline form and so it becomes solid in an amorphous form.
As physics teach us, the amorphous form tends to transform in a less energetic form and so rearrange in a more stable crystalline habit. Of course the kinetic of the transformation depends from other factors as storage temperature and residual moisture and/or solvents that have a relevant impact on such amorphous/crystalline transformations. Because these powders present a very large surface area, they are also very much open to water intake and this hygroscopicity control is one of the most critical factor in spray dried powder handling. In order to reduce as much as possible the amorphous instability special excipients have been – and currently are still – developed to stabilize the API amorphous form. These excipients, in general polymers, have a scope to physically block the possibility of the API to rearrange in a crystalline form and this effect is also amplified if a good microencapsulation effect is obtained.
In my experience I have seen different API/polymer ratios and so the initial polymers screening is a very delicate step. You must try to minimize the quantity of polymer needed and at the same time the product must show a suitable stability.