Choosing the right technique
PTE: One tool for bioavailability enhancement is to create amorphous solid dispersions through processes such as hot-melt
extrusion (HME) or spray drying. What factors come into play to decide whether to produce the amorphous solid dispersions
through HME or spray drying?
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Bend Research: Both spray drying and HME can be used to produce amorphous dispersions that enhance the bioavailability of poorly soluble
compounds. There are a number of factors that come into play when deciding to progress an amorphous dispersion. These include
performance, projected dose, stability and manufacturability. When choosing which technology to employ for optimising the
amorphous dispersion formulation's performance, two key factors are the physical-chemical properties of the API and the phase
of development, which influences the amount of API available for formulation development.
Important physical–chemical properties include the solubility of the API in either a solvent (for spray drying) or polymer
(for HME), the melting temperature of the API and the LogP value of the API. For spray drying, the solubility of the API in
the solvent is crucial to ensure a readily scalable and viable process, whereas for HME, the solubility of the API in the
polymer is crucial to ensure a thermodynamically stable system. The particle size of the API, which influences the dissolution
rate during processing, can also be crucial for complete dissolution into the polymer melt.
The processing temperature is important for HME because the API must either melt to form a dispersion or dissolve through
high shear forces into the molten polymer. If the processing temperature is too high, the compound or the polymer used in
the formulation can degrade. Typically, 200 °C to 225 °C is regarded as the upper processing-temperature limit for an effective
HME process. Although compounds can be extruded at higher processing temperatures, this physical situation often produces
a partially crystalline formulation instead of an amorphous dispersion.
The phase of development is also an important factor in process selection. For example, for early-stage or discovery-support
activities, API availability is often limited. This limited API availability tends to make spray drying the preferable process
because its feasibility can be determined with as little as 50 to 100 mg of API, whereas several grams of API are typically
required to develop an initial HME process. For APIs that are amenable to HME, typically after proof-of-concept clinical studies,
when hundreds of grams of API are available, an initial spray-drying process can be converted from spray drying to HME.
