Anomalous Dissolution Behaviour of a Novel Amorphous Form of Efavirenz

This study evaluated the dissolution behaviour of a novel amorphous form (Form A) and the commercially preferred crystalline form (Form I) of efavirenz. Generally, amorphous forms tend to achieve a greater extent and rate of dissolution compared to their crystalline counterparts. The results showed that the dissolution of Form A to be significantly lower than that of Form I due to agglomeration. Factors which contributed to the agglomeration behaviour of Form A include: high surface free energy, a lower degree of wetting, and the low glass transition temperature of Form A which caused the sample to convert to the rubber phase which is stickier. The agglomeration increased the relative particle size thereby reducing the exposed surface area of Form A; ultimately reducing the rate and extent of dissolution. The dissolution behaviour of Form A was found to be dependent on sample size and surfactant (SLS) concentration. Scanning Electron Microscopy (SEM) was employed to investigate surface area properties which provided information supporting the powder dissolution results. The solubility and intrinsic behaviour of the two forms were found to be comparable. Upon further investigation it was found that Form A undergoes phase mediated transformation into Form I during the solubility and dissolution experiments and that this too contributed to the apparent dissolution and solubility behaviour of Form A.  It was found the nucleation rate of Form A was potentiated by higher SLS content in the dissolution medium. Keywords: polymorph, amorph, dissolution, solubility, efavirenz.