It is well known that, crystal hydrate is typically the most stable form of a drug substance, consequently it shows the lower solubility and dissolution rates than its anhydrous counterpart.
In the present case both olanzapine polymorphs, form, hydrates (dihydrate or more) during the solubility and dissolution studies, also elevated humidity condition. The Form I of the olanzapine is more stable in comparison with the Form II at 75% relative humidity conditions. As expected the hydrate conversion minimises the solubility and dissolution rates for the both forms. The unstable Form II shows slower dissolution rate the stable Form I.
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Article Role of moisture on the physical stability of polymorphic olanzapine