The physico-chemical properties of an active pharmaceutical ingredient (API) in a solid state are very important as they affect the choice and design of formulation as well as the performance of the dosage form. A robust solid state characterisation of the API at an early stage helps better understanding of its properties and the effect these characteristics have on the drug behaviour, especially on uniform distribution, solubility, stability and its in vitro dissolution and bioavailability.
Solid state manipulation is carried out in the final step of the API production and in subsequent steps. For small molecules, the final step is typically carried out as a recrystallisation of the drug substance in a non-toxic solvent (mostly water or alcohols), including a filtration step increasing the overall purity of the substance and removing any particles from upstream processing. The most important parameter to control in this step besides the actual purity is particle size distribution and polymorph crystallisation. This step can also be used to optimise drug substance characteristics for downstream processing of the drug product.
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